Structure-property relationships in 2D/quasi-2D lead-free halide double perovskites for optoelectronic devices

Abstract

Two-dimensional (2D) and quasi-2D lead-free halide double perovskites (LFHDPs) have gained significant attention as promising alternatives to conventional lead-based perovskites due to their superior environmental stability, reduced toxicity and versatile structural chemistry. This review provides a comprehensive overview of recent advances in the structural design, scalable synthesis strategies, and optoelectronic functionalities, with particular emphasis on bandgap tunability, strong excitonic effects, and photoluminescence quantum yields of 2D LFHDPs. We examine emerging trends in dimensional engineering, particularly the development of Ruddlesden–Popper and Dion–Jacobson phases incorporating monovalent and trivalent cations, which enable enhanced moisture resistance and tailored quantum confinement. Key challenges remain, including indirect bandgaps, limited carrier mobilities, defect-related trap states, and difficulties in large-area processing. To address these issues, innovative approaches such as compositional alloying, halide mixing, defect passivation, and hybrid device architectures are discussed. Furthermore, we discuss challenges such as limited light absorption, poor carrier mobility, and defect tolerance, and explore innovative strategies to overcome these restrictions. Finally, we outline future research directions that couple computational screening with experimental synthesis, integrate LFHDPs into flexible and neuromorphic devices, and pursue sustainable, low-cost compositions, emphasizing their potential in sustainable optoelectronic technologies.

Graphical abstract

Keywords

2D lead-free halide double perovskite / Structure-properties relationship / Bandgap engineering / Exciton / Processing / Optoelectronic device

1 Introduction

A diverse class of materials known as halide perovskites (HPs) has recently driven significant progress in high-performance optoelectronic devices, including solar cells, light-emitting diodes (LEDs), and photodetectors. This rapid development is primarily attributed to their outstanding optoelectronic properties, which have enabled substantial improvements in device efficiency, particularly in photovoltaic applications [1,2]. Unlike a single-component system, HPs comprise a wide range of compositions and crystal structures, offering considerable flexibility in tailoring their electronic and optical characteristics [3–5]. Their intrinsic characteristics including high absorption coefficients (~10⁵ cm⁻¹), long carrier diffusion lengths (often exceeding 1 µm), and defect tolerance, have made them particularly attractive for applications in photovoltaics, LEDs, and photodetectors [6,7]. In addition, solution-processable fabrication techniques enable low-cost and scalable production compared to conventional semiconductors such as silicon or GaAs.

From a structural standpoint, HPs typically adopt an ABX₃ framework, where A represents a monovalent cation, B a divalent metal cation, and X a halide anion. Among these, lead halide perovskites (LHPs), with Pb²⁺ occupying the B-site, have been extensively studied due to their superior optoelectronic performance, including efficient charge transport, high carrier mobility, and strong light absorption [8,9]. Nevertheless, several limitations hinder their widespread application. In particular, their susceptibility to degradation under environmental stressors such as moisture, heat, and illumination remains a critical issue [10]. In addition, the toxicity associated with lead-based compositions raises environmental and regulatory concerns, thereby motivating the development of safer material alternatives [11]. These challenges have prompted extensive efforts toward compositional modification, structural stabilization, and the exploration of lead-free systems.

An alternative strategy involves heterovalent substitution, where Pb²⁺ ions are replaced by either trivalent or monovalent cations possessing a 6s² lone-pair configuration analogous to Pb. Representative trivalent substituents include Bi³⁺ and Sb³⁺, yielding crystal structures of A₃B₂X₉ type [12]. Similarly, monovalent substitutions can produce reduced-dimensional structures such as A₃B₂X₅ or AB₂X₃, with Cu⁺ being a notable example. A particularly important development in this context is the formation of double perovskites, in which two Pb²⁺ ions are simultaneously substituted by a combination of monovalent (B⁺) and trivalent (B³⁺) cation, leading to an ordered two-dimensional (2D) and quasi-2D lead-free halide double perovskites (LFHDPs), A₂B(I)B(III)X₆ crystal structure [13–17]. This design not only preserves the fundamental perovskite framework but also enhances chemical stability while reducing toxicity. Representative systems such as Ag⁺/Bi³⁺-based compounds have demonstrated improved robustness; however, their optoelectronic performance remains limited by factors such as indirect bandgaps and relatively low carrier mobility [18,19].

This structural flexibility of these materials provides extensive opportunities for engineering their electronic properties through approaches such as alloying, mixed-halide systems, and compositional modulation. These strategies enable precise control over bandgap characteristics, charge transport behavior, and defect dynamics, which are critical for optimizing device performance. As a result, LFHDPs offer a unique combination of environmentally benign composition and tunable optoelectronic functionality, positioning them as promising candidates for next-generation sustainable devices.

In this work, we present a comprehensive overview of recent developments in 2D/quasi-2D LFHDPs, focusing on their structural features, synthesis routes, and optoelectronic properties, with particular emphasis on emerging 2D architectures. The review is organized as follows: Section 2 discusses the structural design principles; Section 3 examines synthesis and processing approaches; Section 4 analyzes optical and electronic properties; Section 5 establishes structure-property-device relationships; Section 6 evaluates device performances across photovoltaics, LEDs, and photodetectors; Section 7 identifies current challenges; and Section 8 outlines future perspectives. By integrating insights from both experimental studies and theoretical investigations, this review aims to provide a unified framework for understanding and advancing LFHDP-based optoelectronic technologies.

2 Structural design of 2D/quasi-2D LFHDPs

The structural design of 2D/quasi-2D LFHDPs forms the foundation for tailoring their optoelectronic properties. In contrast to three-dimensional perovskites, low-dimensional LFHDPs provide additional structural degrees of freedom through dimensional reduction, heterovalent B-site ordering, and organic spacer engineering. These structural parameters collectively determine lattice stability, electronic structure, excitonic behavior, and ultimately device performance.

2.1 (100)-oriented layered structures: RP, DJ, and ACI phases

Ideal HDPs adopt a lattice of corner-sharing octahedra, where A-site cations occupy cub octahedral cavities. The B-site cations reside in octahedral sites coordinated by halide anions and the rock-salt ordering is typically characterized by the changeable arrangement of B-site cations driven by charge differences within the octahedral framework. The (100)-oriented classes of 2D/quasi-2D HPs, namely Ruddlesden–Popper (RP), Dion–Jacobson (DJ) and Alternating Cations in Interlayer space (ACI) phases are distinguished by variations in A-site alternating cations arrangements of A₂B(I)B(III)X₆, arising from differences in the organic spacer cations and the consequent interlayer interactions, thereby representing the predominant structural motifs of layered HPs. RP phases are constructed using long-chain monovalent organic cations such as methylammonium (MA⁺), n-butylammonium (BA⁺), phenylethylammonium (PEA⁺), and 5-ammoniumvaleric acid (5-AVA⁺). which generate weak van der Waals (vdW) interactions between adjacent perovskite slabs, thereby introducing relatively larger interlayer spacing. In contrast, DJ phases employ divalent spacer cations such as ethylenediammonium (EDA²⁺), 1,4-butyldiammonium (BDA²⁺), and 4-aminomethylpiperidinium (4-AMP²⁺). which strengthen hydrogen bonds with adjacent inorganic frameworks, reducing the vdW gap and yielding stronger interlayer coupling. This structural difference is crucial, as dimensional confinement in these systems introduces both quantum and dielectric confinement effects that directly impact charge transport, exciton binding energies, and optical absorption spectra in Fig. 1 [20]. However, the relationship between the spacer cation selection and the resulting optoelectronic properties remains insufficiently understood, highlighting the need for systematic studies in both materials design and applications. These structural distinctions are directly reflected in device performance. Spacer type and arrangement govern interlayer distance and electronic coupling. The vdW interactions and hydrogen bonding between adjacent layers enhance structural stability and durability, as shown in Table 1. As reported in a study, DJ-phase LFHP-based solar cells have demonstrated promising photovoltaic performance alongside improved environmental stability. A homologous series of DJ perovskite, incorporating diammonium cations, reveals the layered single crystal (BEA)FA₂Sn₃I₁₀. The obtained PV parameters are as follows: an open-circuit voltage (V_oc): 0.7 V, a short-circuit current density (J_sc): 25.645 mA/cm², a fill factor (FF): 61.50%, and maximum power conversion efficiency (PCE): 11.05%. Remarkably, more efficient DJ-based devices exhibited enhancing energy generation and feasibility [21]. Another single crystal, (4-AP)₂AgBiBr₈ (1,4-AP = 4-amidinopyridine), as a DJ-LFDP, exhibits an ultra-narrow interlayer spacing of 3.0 Å, long-term photo-response retention (> 94% after 90 days), high sensitivity, is constructed by employing π-conjugated aromatic spacers, which improve structural stability [22]. The enhanced stability and efficiency of DJ phases are attributed to the reinforced hydrogen bonding between diammonium spacers and adjacent perovskite layers, which reduces lattice distortions, promotes tighter stacking and improved crystallinity, and suppresses phase segregation. These structural advantages also lead to higher charge-carrier mobility, reduced trap densities, and improved phase purity relative to RP analogs.

Although RP, DJ, and ACI structures were originally established for lead-based layered perovskites, these structural concepts have recently been extended to lead-free double perovskites through heterovalent B-site substitution. Unlike conventional 2D lead halide perovskites, halide layered double perovskites (HLDPs) exhibit multiple stacking motifs arising from the ordered arrangement of heterovalent B(I) and B(III) octahedra. Depending on the orientation of the inorganic slabs ((100), (110), or (111)) and the connectivity of the octahedral framework, RP-, DJ-, and ACI-type structures can be formed, each exhibiting distinct interlayer interactions and electronic coupling. In particular, Ag-Bi layered double perovskites with a (111)-oriented structure have been reported to consist of wrinkled inorganic sheets separated by bulky organic spacers, resulting in strong dielectric confinement and unique excitonic properties [23].

2.2 Structural stability: tolerance factor and octahedral factor

Organic–inorganic hybrid perovskite materials possess excellent electrical and optical properties as light absorbers, enabling perovskite solar cells based on these materials to achieve remarkable progress within a short period and to gain significant. However, the ammonium salt-based hybrid perovskites are highly sensitive to moisture and heat, and the presence of Pb poses a serious risk of neurotoxicity, thereby limiting their commercialization potential. To address these issues, research on inorganic-based, lead-free perovskites (LFPs) has become increasingly important. Such studies, often coupled with advanced quantum computational approaches, are being actively extended to a wide range of applications, through precise material design and performance optimization. This approach enables greater structural and electronic tunability while potentially improving chemical robustness. In particular, the study emphasized the complementary roles of the inorganic framework and organic components in hybrid perovskite optoelectronic devices: the inorganic lattice facilitates efficient charge transport and light absorption or emission, whereas the organic moieties provide mechanical flexibility, processability, and environmental protection. In addition, they highlighted low-temperature, low-cost fabrication routes for quasi-3D AGeI₃ perovskites (A = Cs⁺, CH₃NH₃⁺, HC(NH₂)₂⁺), which exhibit promising multiferroic and nonlinear optical properties [24].

From a fundamental perspective, two representative factors must be considered to predict the crystallographic stability of LFHDPs without distortion, Goldshumidt tolerance factor (t) (Eq. (1)) and the octahedral factor (μ) (Eq. (2)):

(1)t=\frac{(R_{\text{A}}+R_{\text{B}})}{\sqrt{2(R_{\text{B}}+R_{\text{X}})}},

(2)\mu =\frac{R_{\text{B}}}{R_{\text{X}}},

where R_A, R_B, and X represent the ionic radii of corresponding atoms. For an ideal structure, the t value should be close to 1.0, ensuring a high stable phase. Typically, stable architectures are obtained in the range of 0.81 < t < 1.1 and 0.4 < μ < 0.9. A smaller t value might transform into tetragonal or orthorhombic phases with reduced symmetry, while, a larger t value distorts the perovskite crystallographic structure from the 3D network to a low-dimensional frameworks. Actually, MA₂AgBiI₆ presents t and μ values of 0.86 and 0.50, which are close to the corresponding values of Pb (t = 0.83 and μ = 0.54) in range of stable HPs structure. Ultimately, the phase stability is predicted by these factors, is highly governed by the mismatch in ionic radii of the constituent atoms. Beyond such semiqualitative equations, recent studies of structural stability and optoelectrical properties of LFHDPs have demonstrated by bandgap engineering with first-principles density functional theory (DFT) calculations from Fig. 2 [25]. Collectively, RP, DJ and ACI phases are distinguished by their spacer cations and interlayer interactions. RP phases rely on monovalent cations that remain valuable for fundamental studies of low-dimensional confinement, whereas DJ phases use divalent cations that strengthen hydrogen bonding, leading to picking spacer cations, enhanced stacking, crystallinity, and charge transport. As noted above, DJ perovskites may offer comparatively improved efficiency and stability, with devices demonstrating the ability to retain performance under prolonged stress conditions. Beyond this and very recently, the phase stability of layered LFHDPs can be semi-empirically assessed by the Goldschmidt tolerance factor (t) and octahedral factor (μ), although recent machine learning and first-principles DFT approaches provide more reliable predictions beyond the other descriptors. Ongoing research on cation doping, spacer engineering, synthetic methods and process optimization is expected to further elevate the performance of 2D/quasi-2D LFHDPs. Recent advances in DFT calculations have provided fundamental insights into the structural chemistry and electronic behavior of LFHDPs. The stability and optoelectronic functionality of these materials are primarily governed by the BX₆ octahedral framework, where the interaction between B-site cations and halide X anions determines lattice distortion, defect tolerance, and electronic band dispersion [26]. Structural analyses based on tolerance factors, bonding environments, and lattice energetics indicate that variations in B-site composition, heterovalent ordering, and halide coordination strongly influence structural stability and carrier localization. In particular, the balance between covalent B-X bonding, ionic interactions, and hydrogen bonding involving organic spacer cations plays a crucial role in stabilizing the lattice while modulating electronic structure and excitonic effects. The fundamental design principles for vacancy-ordered double perovskite halides demonstrate how cation selection, vacancy ordering, and orbital interactions govern electronic structure, defect tolerance, and ultimately optoelectronic functionality [27]. Straus and Kagan elucidate how the interplay among electrons, excitons, and phonons in 2D hybrid perovskites, comprising stoichiometrically alternating inorganic metal-halide sheets and organoammonium cation layers, governs their optical and electronic responses, establishing a direct link between electronic structure, carrier dynamics, and charge transport properties [28]. Kanatzidis et al. review the critical role of spacer cations in governing the structure, optical properties, and device performance of 2D halide perovskites, providing systematic design, synthesis and film fabrication strategies to enable rational spacer principles beyond trial-and-error approaches for high-performance optoelectronic applications [29,30]. Tisdale and Stupp et al. demonstrate that naphthalene-based organic layers with tetrachloro-1,2-benzoquinone (TCBQ) doping in 2D hybrid perovskites enables independent tuning of exciton binding energy via enhanced electrostatic screening from donor-acceptor interactions, without significantly altering the inorganic lattice or bandgap [31]. Electronic-structure calculations, including band structures, density of states, and charge density distributions, further demonstrate that defect states, antisite disorder, and vacancy formation significantly affect bandgap characteristics and radiative recombination pathways. These theoretical insights are especially important for 2D/quasi-2D LFHDPs, where dimensional confinement, octahedral tilting, and spacer-cation engineering introduce additional structural distortions and dielectric confinement effects [32]. Although the Goldschmidt tolerance factor and octahedral factor provide convenient first-order indicators of structural stability, they remain semi-empirical descriptors that cannot fully capture the complex structural chemistry of LFHDPs. Consequently, recent studies increasingly combine first-principles DFT, high-throughput screening, and machine-learning approaches to predict phase stability, defect formation, and electronic structures with significantly improved accuracy.

2.3 Thermodynamic and structural principles

Thermodynamic driving forces of layered structures governing phase formation: the formation of layered LFHDPs is thermodynamically driven by the incompatibility between bulky organic spacer cations and the 3D corner-sharing octahedral framework. When the spacer size exceeds the tolerance permitted by the Goldschmidt factor, the crystal minimizes lattice strain by periodically terminating the inorganic framework, thereby forming energetically favorable two-dimensional quantum-well structures. Consequently, the formation of RP-, DJ-, and ACI-type phases represents a balance among spacer-cation size, steric effects, lattice strain, electrostatic interactions, and configurational entropy.

Hydrogen-bonding networks and structural stability: Hydrogen bonding constitutes one of the principal stabilizing interactions in layered LFHDPs. The strength and orientation of hydrogen bonds regulate octahedral tilting, lattice distortion, and interlayer spacing, thereby modifying electronic coupling between adjacent inorganic slabs. Stronger hydrogen-bonding networks generally suppress structural fluctuations while simultaneously reducing defect formation and enhancing environmental stability.

Spacer geometry and quantum/dielectric confinement: The geometric characteristics of spacer cations, including molecular length, rigidity, aromaticity, dielectric constant, and dipole moment, determine the width of inorganic quantum wells and the dielectric contrast between organic and inorganic layers. Reduced interlayer distances increase orbital overlap and facilitate carrier transport, whereas bulky low-dielectric spacers strengthen dielectric confinement, increasing exciton binding energies and promoting self-trapped exciton formation. Consequently, spacer geometry serves as a fundamental structural parameter governing both optical and electronic properties. Mechanistic comparison of RP, DJ and ACI was summarized in Table 1.

Quantum and dielectric confinement mechanisms: The layered architecture of 2D/quasi-2D LFHDPs naturally forms multiple quantum-well structures, in which semiconducting inorganic slabs are separated by insulating organic spacer layers. As the inorganic layer thickness (n) decreases, quantum confinement becomes stronger, leading to widened bandgaps, enhanced carrier localization, and modified band dispersion. These changes directly influence charge transport and optical absorption. In parallel, the large dielectric mismatch between the inorganic framework and organic spacers gives rise to dielectric confinement, which strengthens the Coulomb interaction between electrons and holes and increases the exciton binding energy. The magnitude of this effect is governed by the dielectric constant, molecular geometry, and packing density of the spacer cations, providing an effective strategy for tuning excitonic behavior without substantially altering the inorganic lattice [23,25,26,32]. In conjunction, these structural design principles establish the fundamental framework governing the structure-property relationships of layered LFHDPs. By regulating lattice stability, interlayer electronic coupling, and quantum/dielectric confinement through spacer engineering, the optoelectronic properties of layered double perovskites can be rationally tailored for high-performance devices.

2.4 (111)-oriented layered double perovskites (LDPs)

The (111)-oriented LDPs differ fundamentally from the conventional (100)-derived RP and DJ phases in that they feature an ordered A₂B(I)B(III)X₆ framework, where heterovalent cations are arranged in a highly ordered manner. This structural motif gives rise to strong dielectric confinement and distinctive orbital interactions, which in turn broaden the stability window and endow these materials with unusual photoluminescence behavior. In such (111)-aligned systems, cation ordering markedly reshapes the electronic structure through orbital hybridization, thereby influencing band-edge directness, carrier effective mass, and exciton localization. As a result, these changes may open additional radiative recombination channels; however, they can also favor self-trapped exciton formation or trap-assisted nonradiative recombination, both of which may compromise emission efficiency. Carrier mobility likewise depends sensitively on orientation, with the (100) direction generally offering more favorable transport owing to its lower trap density and reduced interfacial disorder, whereas the (111) orientation may suffer from stronger confinement and a higher surface atomic density, leading to comparatively reduced mobility. In Ag–Bi-based systems, for example, the relatively flat conduction band and weakly indirect bandgap can constrain charge transport, yet they simultaneously provide an advantageous platform for self-trapped exciton emission and structural diversification. Accordingly, the following Section 5.1 will further examine electronic-structure modulation and the prospects of (111)-oriented layered perovskites for device-oriented applications.

3 Synthesis and processing approaches

3.1 Design considerations for synthesis

The successful synthesis of 2D/quasi-2D LFHDPs relies on carefully balancing precursor chemistry, solvent properties, and crystallization kinetics, all of which govern phase purity, crystal orientation, and defect formation. The incorporation of bulky organic spacer cations and heterovalent metal ions introduces additional complexity to the crystallization process, making precise control over precursor coordination and nucleation behavior essential. Consequently, the choice of synthesis route should not only consider fabrication simplicity and scalability but also its influence on crystal growth dynamics and the resulting optoelectronic properties as shown in Tables 2 and 3.

Among the key factors, precursor coordination chemistry determines the stability of metal-halide complexes in solution and directly influences nucleation kinetics. Strong coordination between metal cations and polar aprotic solvents, such as dimethylformamide (DMF) and dimethylsulphoxide (DMSO), retards precursor conversion and promotes controlled crystal growth, whereas weaker coordination generally accelerates nucleation, often producing smaller crystallites with higher defect densities. Likewise, solvent polarity and boiling point play critical roles in regulating precursor solubility, solvent evaporation rate, and supersaturation during film formation. Solvents with high polarity facilitate homogeneous precursor dissolution, while those with higher boiling points prolong solvent evaporation, allowing sufficient time for crystal growth and grain coalescence. These parameters ultimately affect crystallization kinetics, grain size, preferred crystal orientation, and film compactness.

The quality of the final thin film is further determined by the interplay between nucleation rate and crystal growth during deposition. Rapid nucleation generally leads to a high density of small crystallites and grain boundaries, whereas slower nucleation combined with controlled crystal growth favors larger grains, reduced defect densities, and improved carrier transport. Therefore, deposition parameters, including precursor concentration, deposition temperature, spin-coating conditions, antisolvent treatment, vapor pressure, and post-annealing, must be carefully optimized according to the selected synthesis method. These design considerations establish a fundamental framework for understanding the strengths and limitations of solution-based, vapor-phase, and green synthetic strategies. As collectively viewed in Fig. 3, the synthesis of 2D/quasi-2D LFHDPs can be accomplished via a variety of routes, which are broadly classified into solution-based, vapor-phase, and environmentally friendly “green” approaches [33–41].

3.2 Solution processing

Solution-based synthesis, such as hot-injection, spin-coating, and antisolvent-assisted crystallization in Fig. 3a [33,37], remains the most widely adopted route due to its simplicity, low cost, and compatibility with large-area deposition. Spin-coating enables the fabrication of uniform thin films with controlled thickness, but its scalability is limited by solvent volatility and film inhomogeneity. Antisolvent-assisted crystallization has been shown to improve film compactness and surface coverage, but optimization of solvent–antisolvent dynamics is required for reproducibility. Ligand-assisted reprecipitation (LARP) offers a facile route to nanocrystals (NCs) under ambient conditions, though poor size uniformity and ligand-induced surface instability often limit device applicability [14,36,37]. In contrast, hot-injection methods can provide highly crystalline NCs with precise size control [38]; however, their reliance on high-temperature reactions, inert-atmosphere handling, and specialized equipment introduces barriers to reproducibility and industrial scalability [39–43].

The following considerations should be taken into account during the solution process.

3.2.1 Precursor characteristics

The characteristics of metal halide precursors play a crucial role in determining the crystallization behavior and film quality of 2D/quasi-2D LFHDPs. Common precursors, including CsBr, AgBr, BiBr₃, SnBr₂, and SbBr₃, exhibit distinct solubility, coordination behavior, and chemical stability, which influence nucleation and crystal growth. CsBr generally provides good solubility in polar aprotic solvents, whereas AgBr has relatively limited solubility and stronger coordination with Lewis-base solvents. BiBr₃ and SbBr₃ readily form intermediate coordination complexes with solvents such as DMF and DMSO, enabling controlled crystallization and improved film uniformity. In contrast, SnBr₂ is highly susceptible to oxidation from Sn²⁺ to Sn⁴⁺, leading to defect formation and reduced material stability unless oxidation is effectively suppressed. Therefore, careful selection of precursor composition and chemistry is essential for controlling crystallization kinetics, defect density, and the structural quality of LFHDP thin films.

3.2.2 Solvent selection

Solvent selection plays a pivotal role in determining precursor dissolution, coordination chemistry, and crystallization behavior during the solution processing of 2D/quasi-2D LFHDPs. Polar aprotic solvents such as DMF and DMSO are widely employed because of their excellent ability to dissolve metal halide precursors. DMSO, a strong Lewis-base solvent, coordinates with metal cations, forming intermediate complexes that retard nucleation and promote controlled crystal growth, whereas DMF provides weaker coordination, enabling faster precursor conversion while maintaining good solubility. Consequently, mixed DMF/DMSO solvent systems are frequently used to balance nucleation and grain growth. Solvent boiling point also influences evaporation kinetics during deposition; high-boiling-point solvents allow prolonged crystal growth and improved grain coalescence, whereas low-boiling-point solvents accelerate supersaturation, often leading to smaller grains and higher defect densities. Although environmentally benign solvents such as ethanol and water have emerged as sustainable alternatives, their limited precursor solubility and weaker coordination generally make it more challenging to achieve uniform, highly crystalline films. Therefore, rational solvent selection is essential for optimizing crystallization kinetics, film morphology, and defect density, ultimately determining the optoelectronic performance of LFHDP thin films.

3.2.3 Processing parameters

Processing parameters critically determine the crystallization behavior and microstructure of 2D/quasi-2D LFHDP thin films. In solution processing, variables such as precursor concentration, spin-coating speed, antisolvent dripping timing, and annealing temperature regulate supersaturation, solvent evaporation, and nucleation kinetics. Optimizing these parameters enables precise control over film thickness, grain size, and preferred crystal orientation, while minimizing pinholes and grain-boundary defects. Similarly, in nanocrystal synthesis, reaction temperature, injection temperature, and growth time govern precursor conversion and crystal growth, thereby influencing particle size distribution, crystallinity, and phase purity. Since these structural characteristics directly affect defect density, charge transport, and non-radiative recombination, careful optimization of processing conditions is essential for producing uniform, highly crystalline LFHDP films with improved optoelectronic performance.

3.2.4 Film quality

The quality of LFHDP films strongly depends on the selected synthesis method, as each processing route produces distinct microstructural characteristics. Hot-injection synthesis typically yields highly crystalline nanocrystals with narrow size distributions, making it suitable for fundamental studies; however, its high-temperature operation and batch processing limit scalability. Spin coating remains the most widely used method for thin-film fabrication because it produces uniform and compact films with controllable thickness, although rapid solvent evaporation can induce non-uniform crystallization and defects such as the coffee-ring effect. In contrast, LARP enables low-temperature synthesis of small nanocrystals under ambient conditions, but excessive surface ligands may introduce insulating interfaces and surface defects that hinder charge transport. Therefore, the choice of synthesis method requires balancing crystallinity, film uniformity, defect density, and scalability according to the targeted optoelectronic application.

3.3 Vapor processing

While spin-coating has achieved high efficiency in small-area laboratory-scale perovskite solar cells (PSCs), it suffers from non-uniformity and challenges in multi-layer coating when scaling up to commercial-sized modules, limiting mass production.

3.3.1 Advantages of vapor deposition

In contrast, vapor deposition utilizes vacuum states or gas flows to deposit materials, offering three powerful advantages: 1) superior film uniformity over large areas, 2) structural freedom since it is a solvent-free process that does not dissolve underlying layers, and 3) excellent compatibility with existing silicon solar cell mass-production lines (semiconductor processes) [44]. Beyond solution-based approaches, vapor-phase processing provides an alternative route for achieving superior crystallinity, film compact morphology, and compositional uniformity, all of which are crucial for high-performance optoelectronic devices. Vapor-phase processing provides precise control over film growth by regulating precursor volatility, deposition temperature, and vacuum conditions. The vapor pressures of metal halide precursors largely determine evaporation behavior and compositional uniformity, making careful precursor selection essential for stoichiometric film growth. During deposition, substrate temperature governs precursor diffusion, nucleation, and crystal growth, while vacuum pressure influences the mean free path of vapor species and deposition rate, thereby affecting film density and compositional homogeneity. Appropriate optimization of these parameters promotes compact films with high crystallinity, uniform morphology, and low defect density, which are beneficial for efficient charge transport and enhanced device stability.

3.3.2 Major vapor deposition methods

The paper classifies and compares various deposition techniques used to fabricate perovskite thin films:

Co-evaporation: Organic sources (e.g., MAI, FAI) and inorganic sources (e.g., PbI₂) are simultaneously vaporized in a vacuum chamber, allowing precise control over the composition ratio and film thickness. In the sequential evaporation/vapor treatment method, the inorganic layer is initially deposited and subsequently exposed to organic vapor to promote perovskite crystallization, providing a relatively simple and effective means of controlling the crystal structure

Vapor-assisted solution growth (VASG) and chemical vapor deposition (CVD): VASG improves grain size and homogeneity but is still constrained by the need for detailed vapor-solution equilibrium in Fig. 3b [34]. Sequential vacuum evaporation allows specific layer-by-layer control and is readily integrated into established semiconductor processes; nevertheless, it is costly and requires high-vacuum systems, making it less attractive for mass production. CVD enables the growth of stoichiometrically controlled films, as demonstrated for Cs₃Sb₂Br₉ using CsBr and SbBr₃ precursors, although it generally requires high deposition temperatures. Together with VASG and sequential vacuum evaporation, vapor-phase techniques provide superior thickness control, compositional uniformity, reproducibility, and compatibility with semiconductor manufacturing. To overcome disparities in precursor vapor pressures, continuous flash sublimation (CFS) has been developed to enable the deposition of large-area homogeneous films. Nevertheless, the practical implementation of vapor-phase processing remains constrained by high equipment costs, vacuum-system requirements, complex process optimization, and relatively low throughput for large-scale production [33,34,44–47].

Vapor deposition is a promising route for commercial perovskite solar cells because it offers uniform thin-film growth and strong compatibility with silicon-perovskite tandem architectures, but its wider adoption still depends on better control of stoichiometry, defect reduction, and lower manufacturing costs. Recent work also points to additive engineering and automated large-area deposition as key steps toward scalable, industrial production.

3.4 Green synthesis: scalable manufacturing and future processing strategies

Parallel to these approaches, “green” synthesis routes-including aqueous solution processing, mechanochemical ball-milling, and bio-inspired templating-approaches have gained traction as sustainable alternatives that minimize toxicity and environmental impact. The use of ethanol and other green solvents avoids the hazards of traditional aprotic solvents like DMF or DMSO, although issues related to precursor solubility and perovskite crystallinity still need to be addressed. Solvent-free techniques, such as mechanochemical ball milling, eliminate solvent use altogether, making them highly attractive for eco-friendly production; however, this approach generally yields powders rather than device-quality films and require additional post-processing. Furthermore, encapsulation strategies using bulky ligands or protective shell layers have demonstrated significant improvements in environmental and moisture stability, but often at the cost of reduced carrier mobility and impaired charge transport due to insulating organic interfaces [48–51].

Despite these trade-offs, all synthesis strategies generally involve precursor preparation, vaporization or dissolution, controlled deposition, and crystallization. Representative materials, such as bismuth-based Cs₃Sb₂Br₉ and tin-based halide perovskites, have been synthesized using both solution and vapor-phase techniques, illustrating the versatility of these approaches. Importantly, these methods have also been successfully extended to the fabrication of LFDHPs (A₂B(I)B(III)X₆: e.g., Cs₂AgBiBr₆), which combine structural robustness with reduced toxicity compared with single-cation halide perovskites [52]. Solution-based and vapor-phase approaches have both been successfully adapted to these compounds: solution routes enable rapid nucleation and cost-effective film deposition, while vapor-phase synthesis offers precise stoichiometric control and enhanced crystallinity [53].

The double perovskite framework provides broad compositional flexibility for tuning electronic and optical properties. Among these, Cs₂AgBiBr₆ has been extensively investigated for its intrinsic stability and innocuous composition, although its indirect bandgap limits photovoltaic efficiency [54]. To mitigate these limitations, compositional engineering, halide mixing, and defect passivation have been employed, while thin-film fabrication methods, including solution processing, vapor-assisted deposition, and hot-casting, have further enabled enhanced film uniformity and improved device performance [55]. Roll-to-Roll (R2R) processes reduce manufacturing time and cost for flexible PSCs (fPSCs). While efficiencies up to 13.5% have been reported using R2R methods; however, devices with higher efficiencies do not employ R2R processing for all fabrication steps [56].

Taken together, these synthetic strategies underscore the need to balance material quality, scalability, and environmental sustainability in the fabrication of 2D/quasi-2D LFHDPs. Beyond laboratory-scale synthesis, scalable techniques such as blade coating, slot-die coating, inkjet printing, spray coating, and vapor-assisted deposition have emerged as promising routes for large-area manufacturing, offering improved material utilization, film uniformity, and cost-effectiveness compared with conventional spin coating. Simultaneously, optimization of precursor chemistry, solvent engineering, crystallization dynamics, and post-treatment processes remains essential for controlling crystal orientation, defect density, and interfacial quality. Consequently, scalable processing should be viewed not merely as a manufacturing approach but as a critical process–structure engineering strategy that governs the structural evolution and optoelectronic performance of 2D/quasi-2D LFHDPs. These processing–structure–property relationships provide the conceptual framework for understanding the optical, electronic, and optoelectronic properties discussed in the following section, as well as for advancing LFHDPs toward practical device applications [57].

4 Optical and electronic properties

HPs, encompassing organic-inorganic systems, have emerged as exceptional light-harvesting materials due to their superior charge transport, tunable bandgaps, and strong optical absorption. Such remarkable optoelectronic properties have enabled PSCs to achieve rapid progress and to stand out as leading contenders for next-generation photovoltaic technologies [58–70]. These investigations have collectively underscored the potential of LFHPs to deliver environmentally benign device architectures while simultaneously confronting fundamental challenges associated with carrier transport, interfacial stability, and large-scale manufacturability. Accordingly, sustained research efforts that integrate rational materials design, advanced interface engineering, and device-level optimization are regarded as indispensable for realizing the full potential of LFHDP-based technologies in next-generation optoelectronic applications [71]. The overall optoelectrical properties of 2D/quasi-2D LFHDPs are summarized in Table 4. Research on 2D/quasi-2D LFHDP has evolved systematically, beginning with early demonstrations in perovskite-based solar cells and gradually broadening into diverse optoelectronic applications such as light-emitting devices (exploiting high exciton binding energies and self-trapped exciton emission), photodetectors (utilizing high absorption coefficients and low dark currents), and photocatalysts (leveraging wide bandgaps and efficient charge separation). The specific performances of LFHDPs for PSCs, LEDs, and Photodetectors are summarized in Tables 5, 6, and 7, respectively. 2D/quasi-2D LFHDPs, encompassing organic-inorganic layered systems, have emerged as promising optoelectronic materials owing to their tunable bandgaps, strong optical absorption, large exciton binding energies, and favorable charge-transport characteristics. These distinctive optical and electronic properties have enabled progress in perovskite-based solar cells and have positioned 2D/quasi-2D LFHDPs as attractive candidates for next-generation photovoltaic and optoelectronic technologies. Nevertheless, despite their advantages in structural stability and environmentally benign device design, fundamental challenges associated with carrier transport, interfacial recombination, and the limited optical absorption near the band edge continue to hinder further efficiency gains. Accordingly, sustained efforts in rational materials design, interface engineering, and device-level optimization are essential to fully exploit the optoelectronic potential of 2D/quasi-2D LFHDPs in photovoltaics, light-emitting devices, and photodetectors.

4.1 Electronic band structure and charge transport

A defining challenge of 2D/quasi-2D LFHDPs lies in their electronic band structure, which fundamentally differs from that of Pb-based counterparts. The prototypical double perovskite Cs₂AgBiBr₆ crystallizes in the cubic elpasolite-type structure (space group Fm3̄m) and possesses an indirect bandgap of approximately 1.95–2.19 eV, arising from the electronic mismatch between Ag-4d/Br-4p orbital hybridization at the valence band maximum (VBM, located at the Γ-point) and Bi-6p-dominated states at the conduction band minimum (CBM, located at the L-point) [72].

More advanced quasiparticle calculations using the GW approximation generally predict wider bandgaps than conventional DFT, while GW-Bethe-Salpeter equation (GW-BSE) calculations accurately reproduce excitonic transitions and optical absorption spectra. These theoretical approaches therefore provide complementary insight into the electronic structure beyond experimental characterization. Band structure engineering through cation substitution has emerged as an effective strategy for overcoming parity-forbidden transitions. For example, Ga incorporation into Cs₂AgBiBr₆ narrows the bandgap and improves carrier transport, whereas In-containing double perovskites such as Cs₂InSbCl₆ are theoretically predicted to exhibit more favorable direct-transition characteristics with significantly enhanced optical absorption. DFT calculations consistently show that the VBM is predominantly composed of Ag-d and halide-p orbitals, whereas the CBM mainly originates from Bi-p, Sb-p, or In-s/p states [25].

This indirect nature requires phonon-assisted transitions, which significantly suppress the absorption coefficient and carrier recombination efficiency relative to direct-gap absorbers such as MAPbI₃. Bandgap engineering strategies - including cation alloying with In³⁺ (yielding a direct bandgap of ~1.85 eV in Cs₂AgBi_0.25In_0.75Br₆) or controlled order-to-disorder transformation via Ag⁺/Bi³⁺ antisite defects (shifting the bandgap from 2.04 eV indirect to 1.59 eV direct) - have been proposed to convert the indirect character to a direct one, substantially improving photon absorption [57]. Deep-level defect states constitute another critical bottleneck for charge transport in LFHDPs. Specifically, Bi, Ag antisite defects and halide vacancies (V-X) in Cs₂AgBiBr₆ introduce electron trap levels that reduce carrier mobility and shorten carrier lifetimes. Similarly, In Ag antisite defects in In³⁺-based double perovskites act as deep electron traps, elevating the dark current density in photodetector configurations. In contrast to Pb-based perovskites - where the antibonding character of B-site/halide interactions pushes intrinsic defects toward shallow states - LFHDPs typically exhibit a higher density of deep-level traps, making defect passivation strategies such as Lewis-base molecular incorporation (e.g., thiourea) or surface modification essential for device optimization [73]. In 2D/quasi-2D LFHDP systems, charge transport is inherently anisotropic due to the layered crystal architecture. High in-plane carrier mobility within the inorganic [M(I)M(III)X₆] octahedral sheets - reaching up to ~16 cm² V⁻¹ s⁻¹ as measured by short-range THz spectroscopy is fundamentally limited in the out-of-plane direction by the electronically insulating organic spacer layers. This anisotropy is a central design consideration for vertical device architectures, where charge carriers must traverse across the crystallographic planes. Kober-Czerny et al. demonstrated that thin films exhibit notably high long-range in-plane mobilities comparable to those of 3D perovskite counterparts, attributing the observed performance gap in devices primarily to large exciton binding energies (e.g., ~230 meV for PEA₂PbI₄) rather than intrinsically low mobility [74]. Dimensionality engineering, by increasing the number of inorganic octahedral layers n in (A')₂A_n−1B_nX_3n+1 quasi-2D structures, progressively reduces the exciton binding energy and bandgap, promoting free carrier formation and enhancing charge extraction efficiency toward values approaching 3D analogs.

The effective masses of charge carriers further govern transport performance and device figures of merit. In Cs₂AgBiI₆, DFT calculations reveal that the valence band exhibits highly dispersive character near the Fermi level, predicting relatively high hole mobility; the conduction band is dominated by Bi-6p states with considerably larger effective electron masses, imposing an asymmetry in electron and hole transport. For vacancy-ordered double perovskite, the absence of alternating M(I)/M(III) rock-salt ordering leads to distinct band edge characters and, in some cases, improved carrier effective masses relative to elpasolite-type structures. Collectively, carrier mobility values reported for LFHDP thin films and single crystals remain substantially below those of Pb-based perovskites (Cs₂AgBiBr₆: ~10 cm² V⁻¹ s⁻¹ vs. MAPbI₃: ~35 cm² V⁻¹ s⁻¹), underscoring the need for material and interface engineering to close this performance gap [75].

4.2 Optical properties and excitonic characteristics

The reduced dimensionality of layered LFHDPs enhances both quantum and dielectric confinement, resulting in substantially larger exciton binding energies than 3D perovskites. Strong Coulomb interactions between electrons and holes promote exciton localization and radiative recombination, making low-dimensional LFHDPs particularly attractive for light-emitting applications. The optical properties of 2D/quasi-2D LFHDPs are governed by a combination of quantum confinement, dielectric mismatch, and orbital selection rules that collectively determine absorption onset, emission characteristics, and photon-to-carrier conversion efficiency. In the n = 1 RP phase limit, quantum and dielectric confinement effects produce exciton binding energies on the order of 100−230 meV, far exceeding those of 3D analogs (~2–50 meV) and necessitating exciton dissociation strategies at charge-extraction interfaces for efficient photovoltaic operation [74]. As n increases in quasi-2D structures, the binding energy decreases progressively and the bandgap redshifts toward the 3D limit, providing a composition-dependent optical tuning handle. A particularly important optical phenomenon in LFHDPs is the formation of self-trapped excitons (STEs), driven by strong electron-phonon coupling in the soft, polarizable inorganic lattice. In 2D hybrid silver-bismuth double perovskites such as (4FPEA)₄AgBiX₈ (X = Cl, Br, I), optical-pump terahertz-probe (OPTP) measurements reveal ultrafast charge-carrier localization following photoexcitation, consistent with small-polaron or STE formation. This STE emission manifests as characteristically broadband, strongly Stokes-shifted photoluminescence, and a property exploited in warm white-light LED applications. The halide identity modulates this behavior: the growing degeneracy between the organic HOMO and the inorganic valence band from Cl to Br to I influences both the band dispersion and the degree of self-trapping. Parity-forbidden optical transitions represent a distinct optical challenge in several LFHDP compositions. For example, Cs₂AgInCl₆, Cs₂AgTlCl₆, and Cs₂NaInCl₆ possess direct electronic bandgaps yet exhibit severely suppressed absorption near the bandgap onset due to inversion-symmetry-enforced Laporte selection rules. Tight binding model analysis reveals that the squared momentum matrix elements (P²) for VBM-to-CBM transitions in Cs₂AgInCl₆ vanish along the Γ-X direction due to the identical parities of valence and conduction band edges, giving rise to a measurable difference between the electronic bandgap and the effective optical bandgap [76,77]. Strategies to circumvent these forbidden transitions include: (1) alloying Na⁺ into Cs₂AgInCl₆, which introduces new allowed states and restores optical activity; (2) cation intermixing (e.g., Cs₂AgIn_xBi_1–xCl₆), where increasing In³⁺ concentration progressively promotes direct and strong optical transitions; and (3) dimensional reduction to 2D structures such as (4FPEA)₄AgBiBr₈, where the symmetry lowering upon layer separation creates direct bandgaps and enables optically active transitions.

Halide substitution provides an additional, experimentally accessible lever for optical property tuning across the visible spectrum. Substituting Br with Cl in Cs₂AgBiBr₆ progressively blue-shifts the absorption onset, while I-substitution red-shifts it; the bandgap of Cs₂AgBiI₆ is computed to be approximately 0.85 eV, significantly narrower than the Br-analog. The vacancy-ordered double perovskites exhibit an optical absorption coefficient reaching ~3 × 10⁵ cm⁻¹ in the visible range, making it a competitive absorber candidate despite its indirect bandgap. The dielectric function and refractive index of Cs₂AgBiI₆ show a large static value near ~2 eV photon energy - consistent with strong interband absorption - followed by a gradual decrease at higher energies, supporting its application in solar cells and quantum-dot LEDs. Together, these optical characteristics including modulated by composition, dimensionality, defect engineering, and structural symmetry, define the operating window of LFHDP-based optoelectronic devices and motivate continued efforts toward compositions that simultaneously achieve direct, parity-allowed transitions with low defect densities.

The optical response strongly depends on the electronic structure and electron-phonon coupling. Ag-Bi double perovskites generally exhibit weak photoluminescence because of indirect bandgaps and parity-forbidden transitions, whereas Bi³⁺- or Mn²⁺-doped In-based double perovskites display efficient broadband emission mediated by STEs. Defect passivation, alloy engineering, and surface modification have further increased photoluminescence quantum yields (PLQY) to values exceeding 80% in several Bi-based nanocrystal systems, demonstrating that exciton dynamics can be effectively tailored through compositional engineering. A quantitative comparison of experimentally measured PLQY, exciton binding energy, emission lifetime, and corresponding theoretical predictions is summarized in Tables 4 and 6, highlighting the strong dependence of optical performance on crystal structure and electronic configuration.

4.3 Defect physics

Charge transport in LFHDPs is primarily determined by carrier mobility, effective mass, and defect chemistry. Compared with Pb-based perovskites, many LFHDPs exhibit relatively flat conduction bands, resulting in large electron effective masses and lower carrier mobilities. Moreover, intrinsic point defects, including halide vacancies, Ag vacancies, and antisite defects, can introduce trap states that facilitate nonradiative recombination and reduce carrier diffusion lengths. Experimental techniques such as time-resolved photoluminescence (TRPL), space-charge-limited current (SCLC), and Hall-effect measurements have been widely employed to quantify carrier lifetime, mobility, and trap density, while first-principles calculations provide valuable insight into defect formation energies and electronic transition levels. Integrating experimental and theoretical approaches has demonstrated that defect passivation and interface engineering effectively suppress trap-assisted recombination, thereby enhancing carrier extraction and improving device performance. These results demonstrate that defect passivation, interface modification, and compositional engineering substantially improve carrier transport by suppressing trap-assisted recombination and reducing carrier scattering. Such improvements are directly reflected in the photovoltaic, photodetector, and versatile device performances discussed in Section 6.

4.4 Structure-property-devices correlation

The electronic and optical properties of LFHDPs are governed by the interplay among band structure, exciton physics, and defect chemistry. Bandgap engineering through cation substitution directly modifies optical absorption and carrier generation, whereas quantum and dielectric confinement regulate exciton localization and radiative recombination. Simultaneously, defect passivation and improved crystallinity enhance carrier transport by reducing trap-assisted losses. Therefore, integrating experimental characterization with first-principles calculations establishes a quantitative framework linking crystal structure to electronic structure and ultimately to optoelectronic functionality.

4.4.1 Fundamental electronic structure and bandgap engineering

From a fundamental perspective, 2D/quasi-2D LFHDPs exhibit relatively wide bandgaps (~2.0−3.5 eV), strong excitonic absorption, and high PLQYs, all of which stem from enhanced quantum and dielectric confinement. Nevertheless, their intrinsic exciton binding energies are large, leading to anisotropic charge transport and lower carrier mobilities compared to their 3D analogs. The incorporation of mixed halides or heterovalent alloying can fine-tune the bandgap and improve defect tolerance. Furthermore, their dielectric properties are highly dependent on the spacer cation and layer thickness (n-values), influencing charge separation efficiency in devices. The compositional tuning strategies have been employed to modulate bandgaps, improve defect tolerance, and enhance dielectric screening. These fundamental electronic characteristics provide the basis for understanding the structure-property relationships governing the optoelectronic performance of representative LFHDPs.

4.4.2 Inorganic double perovskites: Cs₂AgBiBr₆ and related systems

The following are recently reports published in the LFHDP field. Among inorganic systems, Cs₂AgBiBr₆ is the exemplary LFHDP, noted for its excellent thermal and humidity stability. Using single-crystal/powder X-ray diffraction and neutron powder diffraction, a structural phase transition in Cs₂AgBiBr₆ has verified at a critical temperature of T_s ≈ 122 K, corresponding to a change from a room temperature cubic phase to a low-temperature tetragonal phase. The peak exciton energy E_ex ≈ 2.85 eV near the direct gap has shifted proportionally to the tetragonal strain via reflectivity detection [72]. However, its indirect bandgap in cubic phase results in suboptimal absorption and electron trap states. To address this, both crystallization control and targeted doping strategies have been explored [52]. For instance, the VBM of Cs₂AgInCl₆ originates from the Ag-4d and Cl-3p orbitals hybridization, whereas the CBM is dominated by the antibonding In-s orbital. Transition-metal ions doping introduces the higher 3d orbital state in the VBM than the Ag-4d orbital without affecting the CBM, thereby reducing the bandgap and altering photoluminescence (PL) characteristics. Similarly, surface chemistry plays a critical role: the bandgap value and the PL intensity and lifetime are observed to decrease. The emission intensity of Cs₂AgBiBr₆ perovskite NCs is enhanced with increasing oleic acid (OA) ligand concentrations during growth, as OA suppresses surface defects, eliminates trap-related emission, and enhances band-edge luminescence. Ligand-free NCs show an exciton peak at 440 nm with a long absorption tail up to 700 nm, caused by surface defects. Adding OA suppresses these defects. Without ligands, double emission peaks appear at 465 nm (band-edge) and 510 nm (trap states). Increasing OA, the peak at 510 nm reduced while the peak at 465 nm enhanced and the bandgap value decreases from In- to Sb-based LFHDPs due to a transition from direct to indirect bandgaps.

4.4.3 Defect passivation and composition engineering

Defect passivation and compositional engineering have emerged as effective strategies for overcoming the intrinsic limitations of LFHDPs, including indirect bandgaps, deep trap states, and limited carrier transport. By regulating defect chemistry, modifying the electronic structure, and optimizing crystallization behavior, these approaches simultaneously improve charge-carrier dynamics and optoelectronic device performance. The Organic bromide additives containing appropriate cations (such as a volatile organic salt, methylammonium ion, MA⁺) were added to the precursor solution to suppress trap formation, achieving a PCE of 2.53% [78]. A Cs₂AgBiBr₆-based optoelectronic memristor (Ag/PMMA/Cs₂AgBiBr₆/ITO) shows low-voltage operation (1 V), a high ON/OFF ratio, and long retention times (> 6000 s) with unique optical RESET capability, enabling simultaneous light sensing and image storage. Integrated with a PSC, it also demonstrates an all-optical logic gate, underscoring its potential for optoelectrical neuromorphic systems by integrating visual sensing and memory, simplifying circuitry, lowering power consumption, and efficiently handling dynamic visual data [79]. A-site cation engineering in Ag-based LFPs (e.g., Cs₂AgSb₂I₉) provides an effective route to modulate electronic structure and trap states, enabling ultralow-power optoelectronic synaptic behavior with an energy consumption of 6.18 × 10⁻¹⁴ J per event and a high image recognition accuracy of 97.5% [80]. In photovoltaic applications, cationic substitution has proven effective for overcoming the intrinsic indirect bandgap (~2.0 eV) and limited carrier mobility of Cs₂AgBiBr₆. Gallium incorporation (Cs₂Ag_0.95Ga_0.05BiBr₆) narrows the bandgap, leading to an improved PCE of 4.52% compared to 3.51% for pristine films (J_sc = 6.01 mA cm⁻², V_oc = 0.94 V, FF = 0.80) [81]. Similarly, graphdiyne (GD) doping in Cs₂Ag_0.95Ga_0.05BiBr₆ lowers the bandgap to ~1.85 eV, boosting photovoltaic performance to a PCE of 3.74% [82]. Rare-earth and transition-metal doping schemes further expanded the optoelectronic versatility of LFHDPs.

4.4.4 Functional doping for optical and electronic modulation

Functional doping has emerged as a powerful strategy for tailoring the optical and electronic properties of LFHDPs by modulating the electronic structure, exciton dynamics, defect states, and spin-dependent interactions. Depending on the dopant species, these modifications can improve charge separation, enhance radiative recombination, and expand the functionality of LFHDPs for optoelectronic applications. Cobalt (Co) doping in Cs₄MnBi₂Cl₁₂ explored via DFT and Kelvin probe force microscopy (KPFM), induced spin polarization that promoted charge separation and prolonged carrier lifetime [83]. Likewise, Manna and coworkers further demonstrated the optoelectrical properties via a hot-injection preparation of both undoped and Mn-doped Cs₂AgInCl₆ NCs [84], while Nag and colleagues reported Yb-doped Cs₂AgInCl₆, expanding the family of emissive double perovskite NCs in the near-infrared (NIR) exhibition by modified hot-injection [85]. Cs₂AgInCl₆ possesses a direct bandgap and is distinguished by STE emission. While its intrinsic PL quantum yield remains low, dopants such as Mn²⁺ or Bi³⁺ significantly enhance luminescence efficiency, making it a promising host material for light-emitting and scintillation applications. More refined schemes, light-stable Na⁺/Bi³⁺ co-doping in Cs₂Ag_0.4Na_0.6In_0.8Bi_0.2Cl₆ achieves broadband warm-yellow emission with a quantum yield of 66.38%, maintaining 94.3% of its initial emission after five thermal cycles and 68.1% after 720 h of UV irradiation. Compared to its Bi analog, it shows enhanced visible absorption, suggesting potential in tandem solar architectures as well as in photocatalytic applications [86].

4.4.5 Emerging LFHDPs

Beyond the extensively investigated Cs₂AgBiBr₆ system, a growing family of emerging LFHDPs has been developed to expand the compositional landscape and diversify their optoelectronic functionalities. By introducing alternative monovalent or trivalent cations, these materials exhibit distinct electronic structures, optical responses, and carrier dynamics, offering new opportunities for device optimization. Cs₂AgSbCl₆ exhibits an indirect bandgap of ~2.6–3.0 eV along with strong absorption in the UV-visible range [87]. MA₂AgBiBr₆, a hybrid organic–inorganic double perovskite, offers a narrower bandgap (~2.02 eV) and superior film processability relative to all-inorganic variants. Although its long-term stability is lower, its optical and electronic characteristics point to potential applications in solar cells and light-emitting devices. Cs₂NaBiCl₆ features an indirect bandgap of ~3.41 eV and strong UV absorption. Its broadband PL can be significantly improved by Mn²⁺ doping, yielding orange–red emission with high quantum yields and strong resistance to thermal cycling and UV degradation. Collectively, these studies highlight how precisely tailored doping strategies can overcome intrinsic curbs and unlock multifunctionality in LFHDPs.

4.4.6 Layered LFHDPs and theoretical insights

Beyond compositional tuning, layered structural design provides an additional degree of freedom for tailoring the electronic structure and optoelectronic properties of LFHDPs. Variations in stacking sequence, interlayer coupling, and spacer chemistry modify quantum confinement, dielectric screening, and carrier transport, thereby influencing the band structure and optical responses. For example, in RP-phase LFHDPs, increasing film thickness of (C₅H₁₂N)₄AgBiI₈ (CAB-1) and (C₆H₁₄N)₄AgBiI₈ (CAB-2) leads to bandgap narrowing and enhanced photocurrent, with 1 µm-thick films maintaining excellent stability for over 90 days under humid conditions [88]. DFT studies of A₂B(I)B(III)X₆ structure reveal that the CBM is dominated by B(III)-p and X-p orbitals, while the VBM arises from hybridized B(I)-d and X-p states, producing unfavorable orbital overlap for transitions. In addition, the relatively flat conduction bands indicate large electron effective masses and limited carrier transport, consistent with the experimentally observed bandgap characteristics. More advanced theoretical approaches, including GW-BSE calculations, further demonstrate that strong excitonic effects and dielectric confinement dominate the optical response of layered LFHDPs, providing a quantitative framework for interpreting their absorption, photoluminescence, and charge-transport behavior. Various approaches have been proposed to mitigate these drawbacks, such as breaking inversion symmetry via alloying (e.g., partial substitution of Bi³⁺ with Sb³⁺), inducing structural distortions, or introducing defect states to relax forbidden transitions. The vacancy formation energy and halide ion migration barrier in LFHDPs such as CsAgBiX₆ (X = Cl, Br), Cs₂AgSbCl₆ and Cs₂AgInCl₆ are calculated based on DFT [89]. In halide double perovskites, ion migration and vacancy formation play a crucial role in determining their optoelectronic behavior. These processes can induce lattice distortions and modify the local electronic structure, resulting in bandgap variations that influence optical absorption, carrier recombination kinetics, and emission properties. Such effects often lead to photo-instability and gradual photodegradation under prolonged illumination. Nevertheless, when properly controlled, vacancy engineering and ion migration can be strategically utilized to achieve tunable optical bandgaps and dynamic optical switching, highlighting their dual impact as both a challenge and an opportunity in the development of functional optoelectronic materials. Wu and colleagues optimized of 2D layered RP type LFHDPs, Cs_n+1In_n/2Sb_n/2I_3n+1 (n = 3) and Cs_n+1In_n/2Sb_n/2Cl_3n+1/Cs_m+1Cu_m/2Bi_m/2Cl_3m+1 (n = 3, m = 1) and investigated the direct bandgap range of 1.29 eV to 1.65 eV and carrier mobility based on DFT and GW-BSE calculations [90]. The band gap transition of Cs₂AgBiBr₆ presented from 2.04 eV to 1.59 eV in the disordered system following the disordering of Ag⁺/Bi³⁺ cations using DFT to verify the suitability of its photovoltaic and optoelectronic applications [91,92]. The optoelectronics of the other LFHDPs, such as Cs₂KMI₆ (M = Ga, In) were performed with DFT [93]. Dong and colleagues in situ synthesized Sb³⁺/Mn²⁺ co-doped Cs₂KInCl₆ perovskite/PVDF films with tunable cold white light emission (PLQY 86.98%) and the corresponding International Commission on Illumination (CIE) coordinates of (0.31, 0.33) [94]. The energy transfer to the triplet self-trapped exciton around Sb³⁺ in the charge-transfer band and subsequently to the ferromagnetic coupled Mn²⁺ pair 3d state was confirmed by the temperature-dependent PL spectra and DFT calculations, supporting the single-component LEDs and anti-counterfeiting applications [19,91].

To summarize, these advances underscore how compositional doping, structural engineering, and interface design can overcome intrinsic curbs of 2D/quasi-2D LFHDPs, enabling multifunctional applications ranging from photovoltaics and LED to scintillators and photocatalysts. The optical and electronic properties of 2D/quasi-2D LFHDPs originate from the intricate interplay among electronic band structure, exciton physics, carrier transport, and defect chemistry. Quantitative comparison of experimental measurements with theoretical predictions reveals that compositional engineering, dimensional modulation, and defect passivation collectively govern the fundamental optoelectronic parameters, including bandgap, exciton binding energy, photoluminescence efficiency, carrier mobility, and trap-state density. Moving forward, coupling experimental synthesis with predictive computational modeling is expected to accelerate the rational design of 2D/quasi-2D LFHDPs with tailored band structures, optimized excitonic dynamics, and enhanced environmental stability. These intrinsic material properties ultimately dictate the performance of photovoltaic devices, LEDs, photodetectors, and neuromorphic electronics, as discussed in the following section.

5 Structural engineering strategies

While significant progress has been made in understanding the structural and optoelectronic properties of 2D/quasi-2D LFHDPs, establishing a direct correlation between these intrinsic material characteristics and device-level performance remains a critical challenge. In contrast to conventional semiconductors, where carrier transport dominates device operation, LFHDPs exhibit a complex interplay between quantum confinement, excitonic effects, and defect-mediated processes, all of which must be considered simultaneously to understand device functionality.

At the structural level, key parameters such as layer thickness (n-value), spacer cation chemistry, and B-site cation ordering critically determine the dielectric environment and orbital interactions within the perovskite lattice. These structural features directly influence fundamental optoelectronic properties, including bandgap nature (direct vs. indirect), exciton binding energy, and carrier mobility. For example, reduced dimensionality enhances dielectric confinement, leading to large exciton binding energies that favor radiative recombination but hinder efficient charge separation [84,88]. This duality highlights an intrinsic trade-off between light-emitting and photovoltaic applications. From an electronic structure perspective, the orbital hybridization between B(I)-d, B(III)-s, and halide p orbitals governs the formation of the VBM and CBM. In many LFHDPs, symmetry-imposed parity restrictions result in indirect or weakly allowed optical transitions, limiting absorption coefficients and reducing photocurrent generation in solar cells. Consequently, structural modifications that break inversion symmetry or alter orbital overlap can significantly impact device-relevant figures of merit [87]. Defect physics further plays a decisive role in determining device performance. Unlike lead-based perovskites, many LFHDPs exhibit reduced defect tolerance, where deep-level trap states act as nonradiative recombination centers. The trap density (N_t) directly affects carrier lifetime (τ) and open-circuit voltage (V_oc), particularly in photovoltaic devices, while also influencing noise characteristics and detectivity in photodetectors. In addition, ion migration and vacancy formation can dynamically modify the local electronic structure, leading to hysteresis effects and long-term instability under operation.

These coupled structure-property relationships ultimately manifest in device-level performance metrics. In solar cells, the bandgap nature and carrier mobility determine J_sc and FF, while recombination losses dictate V_oc and overall PCE. In LEDs, strong excitonic confinement enhances radiative recombination efficiency and PLQY, making low-dimensional LFHDPs particularly advantageous. In photodetectors, low dark current and high responsivity are closely linked to suppressed defect states and optimized charge transport pathways.

5.1 Structural diversity of (111)-oriented layered double perovskites

Conventional low-dimensional HPs are predominantly derived from the (100)-oriented slicing of the 3D ABX₃ framework, resulting in RP, DJ, and ACI structures. These layered architectures have been extensively investigated owing to their superior moisture stability, tunable quantum confinement, and versatile compositional flexibility. However, another important structural family, namely (111)-oriented layered double perovskites, has recently emerged as a promising platform for expanding the structural and functional diversity of LFHDPs. Unlike (100)-oriented structures composed of corner-sharing BX₆ octahedral sheets (111)-oriented layered double perovskites are constructed from alternating B(I)X₆ and B(III)X₆ octahedra along the <111> crystallographic direction, forming hexagonally packed inorganic layers separated by organic spacer cations. Representative compositions are generally described by the formula A₂B(I)B(III)X₆, where Ag⁺, Cu⁺, In⁺, Bi³⁺, Sb³⁺, and rare-earth cations occupy ordered octahedral sites. This unique octahedral connectivity introduces distinct crystal symmetries, stronger dielectric confinement, and modified orbital interactions compared with conventional RP or DJ analogs. As shown in Figs. 4a and 4b, (C_nH_{(2n + 1)}NH₃)₄AgBiI₈ (n = 10, 12, and 14) with different aliphatic spacers showed XRD patterns and optical bandgaps of approximately 2.0−2.1 eV and only broad emission was observed at photoluminescence was at low temperatures, suggesting strong carrier trapping or weakly indirect bandgap characteristics. DFT calculations revealed that the VBM is mainly derived functional derived from Ag 4d/I 5p orbitals, whereas the CBM originates from Bi 6p/I 5p orbitals. The conduction band consists of multiple nearly degenerate and relatively flat minima, resulting in large electron effective masses and limited electron transport. Although the calculated band structure exhibits a slightly indirect bandgap, the energy difference between direct and indirect transitions is only ~0.01 eV, making direct optical transitions thermally accessible at room temperature from Figs. 4c and 4e. Spin–orbit coupling (SOC) in Figs. 4d and 4f, splits the Bi 6p conduction-band states but does not alter the fundamental orbital composition or the flat conduction-band characteristics, confirming that the intrinsic electronic structure is primarily governed by the Ag-Bi inorganic framework. Spacer engineering provided a desirable strategy for tuning optoelectronics [14]. From an electronic-structure perspective, the (111)-oriented framework substantially alters orbital overlap between the B-site cations and halide p orbitals, thereby modifying the VBM, CBM, band dispersion, and carrier effective masses. The ordered arrangement of heterovalent cations further affects parity selection rules, defect formation energies, and exciton localization. Consequently, these materials often exhibit enhanced excitonic characteristics, broadband self-trapped exciton (STE) emission, and improved environmental stability, although relatively flat electronic bands may limit carrier mobility and long-range charge transport. Despite these distinctive advantages, the application of (111)-oriented layered LFHDPs in optoelectronic devices remains comparatively underexplored. Most reported studies have focused on crystal growth, optical spectroscopy, and photoluminescence properties, whereas systematic investigations linking crystal orientation to photovoltaic, light-emitting, or photodetection performance remain scarce. Future research should therefore integrate structural engineering with band structure modulation and interface optimization to exploit the full potential of (111)-oriented layered double perovskites in next-generation optoelectronic devices.

5.2 Spacer engineering: from structural modulation to device performance

Organic spacer cations represent one of the most powerful design parameters in 2D/quasi-2D LFHDPs because they simultaneously regulate crystal structure, electronic interactions, and device functionality. Unlike 3D perovskites, where the A-site cation primarily stabilizes the lattice, organic spacers in layered structures actively determine the interlayer distance, octahedral distortion, dielectric environment, crystal orientation, and quantum-well architecture. Consequently, spacer engineering provides a direct route to tailoring the structure-property-device performance relationships that govern optoelectronic applications [23,29,30,32].

The molecular characteristics of spacer cations, including chain length, aromaticity, rigidity, dipole moment, hydrogen-bonding capability, and dielectric constant, collectively govern the degree of electronic coupling between adjacent inorganic layers. Short, rigid, or π-conjugated spacer cations reduce the interlayer spacing, resulting in stronger orbital overlap and electronic coupling between neighboring inorganic layers. These structural changes facilitate charge transport and lower the exciton binding energy, ultimately improving the optoelectronic performance of layered LFHDPs. Conversely, bulky insulating spacers increase dielectric confinement, resulting in stronger exciton localization and radiative recombination, which are advantageous for high-color-purity and high-PLQY LEDs. Spacer engineering can tune dielectric screening and interfacial band alignment, thereby balancing efficient charge transport with strong optical emission. Thus, spacer engineering has emerged as a key strategy for simultaneously optimizing carrier transport, exciton dynamics, environmental stability, and overall device performance. Spacer chemistry also plays a critical role in defect regulation and interfacial stability. Overall, spacer engineering should not be regarded merely as a structural design strategy but rather as a comprehensive approach for simultaneously tuning crystal structure, electronic structure, exciton dynamics, defect chemistry, and ultimately device performance. Establishing these explicit structure-property-device relationships provides a rational framework for designing high-performance 2D/quasi-2D LFHDP optoelectronic devices.

5.3 Composition engineering

Composition engineering provides another effective strategy for tailoring the electronic structure and optoelectronic properties of LFHDPs. Partial substitution of metal cations, heterovalent alloying, mixed-halide compositions, and rare-earth or transition-metal doping enable systematic modulation of band structures, defect tolerance, and carrier transport. Representative examples include Ga-alloyed Cs₂AgBiBr₆, which exhibits bandgap narrowing and enhanced photovoltaic performance, and Bi-, Mn-, or Yb-doped Cs₂AgInCl₆, where dopant-induced electronic states significantly improve photoluminescence quantum yields through efficient radiative recombination [32,86,87,89,92]. Likewise, mixed-halide and heterovalent substitution strategies modify orbital hybridization, dielectric screening, and carrier localization, thereby extending the functionality of LFHDPs toward photovoltaics, LEDs, photodetectors, and neuromorphic devices. Composition engineering can modify band alignment and dielectric screening, thereby regulating exciton dissociation and charge extraction at heterointerfaces. Overall, composition engineering complements structural engineering by directly tuning the electronic structure while maintaining the intrinsic advantages of layered architectures. Through rational selection of composition, it is therefore possible to balance the competing requirements of efficient carrier transport and strong optical emission. This strategy is becoming increasingly important for realizing multifunctional optoelectronic devices that simultaneously require high carrier mobility, defect tolerance, environmental stability, and efficient light–matter interactions.

5.4 Interface and defect engineering

Besides structural and compositional optimization, interface and defect engineering play decisive roles in determining device performance. Native defects, including halide vacancies, antisite defects, and undercoordinated surface atoms, introduce trap states that accelerate Shockley–Read–Hall (SRH) non-radiative recombination and shorten carrier lifetimes. Various passivation strategies have therefore been developed to suppress trap-assisted recombination.

Functional groups capable of hydrogen bonding or Lewis acid–base interactions can effectively passivate undercoordinated halide ions and metal vacancies, thereby reducing defect densities, suppressing trap-assisted nonradiative recombination and improving carrier lifetime. At the device level, interface engineering further improves charge extraction by optimizing energy-level alignment between LFHDP absorbers and charge-transport layers while suppressing interfacial recombination. Furthermore, hydrophobic aromatic spacers protect inorganic slabs from moisture penetration while simultaneously improving thermal and operational stability. These strategies are particularly beneficial for photovoltaic and photodetector devices, where reduced trap density, lower dark current, suppressed hysteresis, and enhanced carrier collection directly translate into higher power conversion efficiency, improved detectivity, and long-term operational stability.

Therefore, achieving high-performance optoelectronic devices based on 2D/quasi-2D LFHDPs requires a holistic design strategy that simultaneously considers structural engineering, electronic structure modulation, and defect control. Establishing such multiscale correlations is essential for transitioning from empirical material optimization to predictive device design. Among various strategies, symmetry breaking is particularly important, as it directly links crystal symmetry to optical transition probability and device-level performance. This highlights the necessity of incorporating symmetry considerations into the design of high-performance LFHDP-based optoelectronic devices.

6 Device applications

Building upon the structure-property-device relationships discussed in Section 5, we now examine how these fundamental mechanisms translate into practical optoelectronic device performance. To ensure a comprehensive optoelectronic perspective, it is essential to systematically categorize device applications based on their governing physical mechanisms and performance metrics. In this context, layered 2D/quasi-2D LFHDPs have been explored across three primary optoelectronic platforms: photovoltaics, LEDs, and photodetectors, each requiring distinct optimization strategies in terms of band structure, exciton dynamics, and charge transport. A wide variety of LFHDPs, A₂B(I)B(III)X₆ with diverse bandgaps have been reported that the band structures are primarily determined by the B(I)-, B(III)-, and X-site atoms. The cation-transmutation strategy provides greater flexibility in selecting B-site cations. Notably, rock-salt-ordered structure has attracted considerable attention in photovoltaics, owing to its intrinsic stability and non-toxicity. Although numerous HDP compositions are theoretically possible, their realizable forms are constrained by Goldschmidt’s tolerance rule and thermodynamic stability. The structural and electronic characteristics of representative 2D/quasi-2D LFHDP devices by device are summarized in Table 8. The versatile applicability of LFHDPs in advancing optoelectronic devices underscores their potential for achieving commercial viability, enhanced chemical stability, tolerance to moisture, flexible cation chemistry, and environmentally benign technologies with the expectation of further improvement through tunable bandgap optimization.

6.1 Perovskite solar cells (PSCs)

2D/quasi-2D LFHDPs have garnered considerable attention in photovoltaic research due to their inherent structural and environmental stability, particularly under prolonged illumination and high-humidity conditions. Despite the current PCEs being lower than those of their 3D perovskite counterparts, significant progress has been achieved through several strategies. Mixed-dimensional engineering, which combines 2D layers with 3D frameworks, enhances vertical charge transport while maintaining the structural robustness of layered perovskites. Compositional tuning, including partial cation or halide substitution, allows fine control of the band gap and defect states, thereby optimizing light absorption and carrier dynamics. Optimized interfacial design, such as the incorporation of electron- and hole-transport layers (ETL/HTL) with tailored energy-level alignment, further improves charge extraction and suppresses non-radiative recombination, collectively narrowing the efficiency gap [9,52,95]. The solar cells performance of 2D/quasi-2D LFHDP-based device architectures was systematically evaluated using Cs₂AgBiBr₆, Cs₂AgSbBr₆, Cs₂InBiCl₆, Cs₂InSbCl₆ as a representative system. Among them, representative materials such as Cs₂AgBiBr₆ exhibit indirect bandgaps and parity-forbidden transitions arising from the ordered Ag(I)/Bi(III) framework, resulting in relatively small absorption coefficients and incomplete photocarrier generation. Consequently, J_sc remains significantly lower than that of lead-based perovskites despite their excellent thermal and environmental stability in Fig. 5. The current density–voltage (J–V) characteristics verified the suppression of hysteresis, balanced charge transfer, and superior photovoltaic performance, as evidenced by the high PCE and FF [54,96]. These wide-band-gap materials can serve as complementary absorbers in multi-junction architectures, enhancing overall solar-to-electric energy conversion. Recent advances in band-structure engineering, hydrogenation, and defect passivation have substantially improved the photovoltaic performance of LFHDP solar cells, while theoretical device optimization predicts that power conversion efficiencies exceeding 20% are achievable, underscoring the considerable potential of environmentally benign double perovskite photovoltaics, as summarized in Table 5. Experimentally, the highest reported efficiencies of LFHDP solar cells remain below 10%, with hydrogenated Cs₂AgBiBr₆ representing one of the best-performing systems. In contrast, first-principles calculations and SCAPS simulations predict that newly designed double perovskites, including Cs₂InBiCl₆, Cs₂InSbCl₆, and Cs₂CuSbCl₆, could achieve theoretical PCEs exceeding 24%–27%, highlighting the considerable room for future materials optimization [97,98]. In photovoltaic devices, the performance of layered 2D/quasi-2D LFHDPs is fundamentally governed by the interplay between bandgap nature, carrier diffusion length, and defect-assisted recombination. For instance, indirect bandgaps in materials such as Cs₂AgBiBr₆ limit the absorption coefficient and reduce J_sc. Furthermore, strong excitonic binding energies in low-dimensional systems hinder efficient charge separation, thereby reducing the internal quantum efficiency (IQE). Strategies such as cation disordering, alloying (e.g., Sb³⁺ substitution), and dimensional engineering have been shown to induce partial direct-gap character and enhance absorption coefficients, leading to measurable improvements in PCE. Optimized defect management, bandgap tuning, and charge transport engineering in Cs₂SnI₆ can theoretically enable high power conversion efficiencies approaching ~23%, highlighting its potential as a stable, non-toxic alternative to lead-based perovskites [99]. In addition to intrinsic materials engineering, interface engineering plays a decisive role in determining the photovoltaic performance of layered LFHDP solar cells by regulating charge extraction, interfacial recombination, and energy-level alignment. An ideal electron transport material (ETM) should possess high electron mobility, appropriate CBM alignment with the perovskite absorber, excellent optical transparency, and minimal interfacial trap states to enable rapid electron extraction while effectively blocking holes. Metal oxides such as TiO₂, SnO₂, and ZnO have therefore been widely employed because of their favorable band alignment, chemical robustness, and excellent electron selectivity. Conversely, an efficient hole transport material (HTM) requires high hole mobility, a suitable highest occupied molecular orbital (HOMO) energy level matched to the perovskite VBM, high optical transparency, and strong resistance to moisture, oxygen, and thermal degradation. Organic HTMs such as Spiro-OMeTAD, PTAA, and Poly-TPD, as well as inorganic HTMs including NiO_x and CuSCN, have been extensively investigated to facilitate efficient hole extraction while suppressing electron back-transfer. Beyond charge-selective transport, both ETMs and HTMs should exhibit low interfacial defect densities, good surface wettability, and chemical compatibility with the perovskite absorber to minimize SRH non-radiative recombination and ion migration. Accordingly, interface engineering strategies, including surface passivation, self-assembled monolayers (SAMs), ultrathin interlayers, defect-healing molecules, and energy-level matching, have become indispensable for reducing carrier recombination, improving charge collection efficiency, and enhancing the V_oc, FF, and long-term operational stability. Consequently, surface passivation, energy-level matching, and chemically stable transport layers have emerged as essential strategies for minimizing interfacial recombination and improving carrier collection efficiency. Although these strategies have been extensively established in lead-halide perovskite solar cells, they provide valuable design principles for next-generation LFHDP photovoltaics, where efficient charge extraction and defect suppression remain key bottlenecks limiting device performance [57]. Table 9 summarizes the representative interfacial materials and their functions in device-integrated 2D/quasi-2D LFHDP layers. These results clearly demonstrate that band structure engineering directly governs key photovoltaic figures of merit, including V_oc, J_sc, FF, and PCE. Bandgap engineering via cation and anion substitution tunes optical absorption and electronic structure, while interface engineering minimizes interfacial losses using organic and inorganic layers, and defect passivation suppresses trap states to enhance charge transport [100]. Together, these developments underscore the growing feasibility of LF 2D and quasi-2D perovskites as competitive candidates in high-performance renewable energy technologies.

6.2 LEDs

The sharp PL emission peaks and high PLQYs of layered 2D/quasi-2D LFHDPs make them promising emitters for next-generation LEDs. As discussed in Section 4, strong quantum and dielectric confinement enhances exciton binding energies, thereby promoting efficient radiative recombination while suppressing exciton dissociation. The sharp PL emission peaks and high PLQYs of 2D/quasi-2D LFHDPs underscore their promise as color-tunable and stable LED devices even in the presence of moderate defect densities. Their reduced dimensionality and associated dielectric confinement enhance exciton binding energies (Eb), resulting in significantly strong radiative recombination efficiency and narrowband emission with high spectral purity. Importantly, the external quantum efficiency (EQE) is strongly correlated with exciton localization (self-trapped excitons, STEs), electron–phonon coupling strength and suppression of non-radiative SRH recombination. For LFHDP-based LEDs, interfacial engineering is equally important because electroluminescence efficiency depends not only on radiative recombination within the emissive layer but also on balanced electron and hole injection. Interfacial energy-level alignment, suppression of exciton quenching at transport-layer interfaces, and defect passivation collectively enhance EQE and operational stability. Therefore, optimization of carrier-selective transport layers and interface chemistry represents a key strategy for translating the excellent photoluminescence properties of LFHDPs into efficient electroluminescent devices [57].

6.2.1 Emission mechanism: STE versus dopant-mediated luminescence

The emission behavior of layered LFHDPs is generally categorized into intrinsic STE emission and extrinsic dopant-mediated luminescence. In pristine layered Ag-Bi double perovskites, strong electron-phonon coupling induces lattice distortion following photoexcitation, resulting in exciton self-trapping and broadband emission with large Stokes shifts. Although STE emission provides excellent spectral stability and broad color tunability, the associated lattice relaxation often increases the radiative lifetime and limits emission efficiency.

6.2.2 Strategies for improving PLQY

The photoluminescence efficiency of layered LFHDPs is strongly affected by crystal quality, defect density, and exciton localization. Several material-engineering strategies have therefore been developed to suppress nonradiative recombination while enhancing radiative decay. Defect passivation through compositional engineering effectively suppresses halide vacancies and antisite defects that serve as nonradiative recombination centers. Alloying and heterovalent doping further optimize the electronic structure by introducing radiative states without significantly disrupting the crystal lattice. Surface ligand engineering in nanocrystals additionally minimizes surface trap states while improving colloidal stability. In low-dimensional LFHDPs, spacer engineering provides another unique approach to controlling emission properties. By modifying the dielectric constant, molecular rigidity, and interlayer interactions of organic spacer cations, dielectric confinement and exciton localization can be systematically tuned, thereby enabling optimization of both PLQY and emission wavelength. Collectively, these approaches have enabled PLQYs exceeding 80%–90% in several Bi-doped double-perovskite nanocrystals.

6.2.3 LED device architectures

The device architecture plays a crucial role in determining charge injection, exciton formation, and electroluminescence efficiency. Most reported layered LFHDP LEDs employ multilayer architectures consisting of transparent conducting electrodes, hole-transport layers, emissive LFHDP films, electron-transport layers, and metal electrodes. Proper energy-level alignment between adjacent layers minimizes injection barriers while promoting balanced electron and hole transport toward the emissive layer. In addition to conventional planar structures, nanocrystal-based emissive layers prepared via hot-injection or LARP have significantly improved film uniformity and defect passivation, resulting in enhanced electroluminescence stability. Continued optimization of interface engineering and charge-transport layers remains essential for achieving high EQE and long operational lifetimes.

6.2.4 LEDs performances

The performance of layered LFHDP-based LEDs is fundamentally governed by the interplay between crystal structure, exciton dynamics, not free carrier transport, and defect chemistry. Strong dielectric and quantum confinement promote exciton localization, while defect passivation suppresses nonradiative recombination. Meanwhile, dopant engineering modifies the electronic structure and introduces efficient radiative centers, substantially improving PLQY and electroluminescence efficiency. Consequently, the key performance metrics, including PLQY, EQE, emission stability, and operational lifetime, are directly correlated with the extent of exciton localization, electron–phonon coupling, and defect passivation.

For example, Sb³⁺, Mn²⁺ or Bi³⁺ doping enhances STE-mediated emission in Cs₂AgNaInCl₆ system, enabling broadband and high-efficiency luminescence. Bi-doped Cs₂AgInCl₆ and Cs₂AgNaInCl₆ exhibit efficient broadband white emission through Bi-induced radiative recombination pathways and significantly enhanced PLQYs, whereas Mn²⁺ doping produces stable orange emission through efficient d–d transitions with long emission lifetimes. These results demonstrate that emission in LFHDP LEDs is predominantly determined by exciton physics rather than free-carrier transport [86,87]. Notably, emissions in the green to blue spectral range have demonstrated remarkable brightness and excellent color coordinates, which are essential for achieving high-resolution displays and energy-efficient solid-state lighting [33]. PL decay measurements reveal prolonged carrier lifetimes in Cs₂AgBiBr₆ based films, indicative of suppressed nonradiative recombination and favorable exciton dynamics in Fig. 5a. EQE spectra, along with the integrated current density curves, confirm efficient photo-to-current conversion in mesoporous Cs₂AgBiBr₆ based cells increasing the EQE significantly over nearly the whole spectrum in Fig. 5b [54]. Consequently, the layered perovskite architecture imparts enhanced environmental and operational stability compared with conventional 3D halide perovskites, suppressing ion migration and phase segregation that typically degrade emission quality over time [47,48,101].

Overall, the performance of LFHDP LEDs is dictated by the interplay between crystal structure, exciton localization, defect chemistry, and interface engineering. Continued advances in dopant engineering, surface passivation, and layered heterostructure design are therefore expected to simultaneously improve PLQY, EQE, operational lifetime, and color stability, paving the way toward efficient lead-free LED. By integrating compositional engineering, surface passivation, and heterostructure design, 2D LFHDP-based LEDs are expected to achieve not only superior color tunability but also long-term device durability, thereby positioning them as compelling candidates for advanced optoelectronic platforms in both display and lighting technologies.

6.3 Photodetectors

Photodetectors, which transduce incident photonic signals into measurable electrical responses, form the cornerstone of numerous modern technologies, including high-speed optical communications, biomedical imaging, non-destructive inspection, and machine vision systems. The performance of such devices is critically determined by a combination of strong optical absorption, low intrinsic noise, and fast response dynamics. In this context, 2D/quasi-2D LFHDPs have recently emerged as promising materials due to their unique optoelectronic properties and environmentally benign composition. Their layered architectures enhance excitonic effects and facilitate broad optical absorption, extending from the ultraviolet (UV) to the visible region, while simultaneously suppressing leakage pathways that typically elevate the dark current. Notably, reported devices demonstrate an impressively low dark current of ~10 pA at 5 V bias, ensuring high sensitivity and excellent signal fidelity [49]. Furthermore, their photodetectors exhibit rapid photo-response times and high signal-to-noise ratios, both of which are essential for detecting weak optical stimuli with precision. For photodetectors, key performance metrics, including responsivity (R), detectivity (D*), and response time (τ) are strongly influenced by trap-state density and carrier mobility. The layered structure of 2D LFHDPs suppresses dark current by limiting leakage pathways, resulting in high signal-to-noise ratios. However, hysteresis and trap-assisted recombination can prolong response times. And, defect passivation and interface engineering significantly improve both response speed and sensitivity, establishing a direct link between defect physics and photodetector performance metrics [102–104]. The combination of high responsivity, low detection limits, and environmental stability underscores the suitability of 2D/quasi-2D LFHDP-based devices for advanced optical sensing applications. Moving forward, integrating compositional engineering, defect passivation, and device-level optimization is expected to further enhance responsivity, extend detection bandwidth, and improve long-term operational stability, thereby solidifying 2D/quasi-2D LFHDPs as a versatile materials platform for next-generation photodetection technologies. The introduction of cyclopentylamine into the layered lead-free double perovskite (CPA)₄AgBiI₈ enables a self-powered X-ray detector, measuring 88.8 μC Gy_air⁻¹ cm⁻² below zero voltage bias, with low detection limit of 235 nGy_air s⁻¹ and high sensitivity [105]. Beyond solar cells, LFHDPs has also been employed in photodetector architectures, where conventional device layouts provide a platform for efficient light sensing in Fig. 6a. Specifically, Au/Cs₂AgBiBr₆/Au thin-film photodetectors display high photoresponsivity that scales with illumination power, accompanied by long-term operational stability and robust time-dependent photoresponse behavior in Figs. 6b−6d. These results collectively highlight the versatility of 2D/quasi-2D LFHDPs in delivering reliable photovoltaic and photodetection performance, while also emphasizing their promise as environmentally benign alternatives to lead-based perovskite optoelectronics [10,11,54].

6.4 Flexible devices

The solution-processable nature and mechanical flexibility of 2D/quasi-2D LFHDPs allow their seamless integration into bendable and wearable electronics, as well as tandem device architectures. These features not only facilitate the fabrication of mechanically robust optoelectronic systems but also support conformal device operation under bending and stretching conditions, thereby addressing the growing demand for flexible electronics [65,106–110]. Beyond their utility in flexible photovoltaics, 2D LFHDPs have also been investigated in neuromorphic and memory-related applications, where their tunable electronic properties and structural stability under ambient conditions are highly advantageous. For instance, air-stable artificial synaptic films have been demonstrated [110], and solid-state electronic synapses based on LFHDPs have exhibited promising nonlinear transmission characteristics suitable for brain-inspired computing [111].

In addition, comprehensive reviews of LFHPs- and LFHDPs-based resistive switching memories highlight their potential as artificial synapses, underscoring their multifunctionality across both memory and neuromorphic domains [112,113]. Complementarily, progress in flexible photovoltaic devices incorporating LFHDPs demonstrates that such materials can deliver stable performance under mechanical deformation, further solidifying their relevance for next-generation flexible energy-harvesting platforms [114].

6.5 Emerging applications

Ferroelectricity in LFHDPs remains challenging to achieve due to their predominantly centrosymmetric crystal structures, which inherently suppress spontaneous polarization. Nevertheless, both theoretical predictions and preliminary experimental studies suggest that subtle structural distortions, octahedral tilting, or asymmetric displacements of ions could locally break inversion symmetry, potentially enabling polar behavior, e.g., (PA)₂CsAgBiBr₇ and (BA)₂CsAgBiBr₇. Experimental confirmation remains limited, but these insights provide guidance for designing ferroelectric LFHDPs. Figure 7a demonstrates switchable polarization (P) and current response (J) under an external electric field, while the temperature-dependent magnetic hysteresis in Fig. 7b indicates soft magnetic behavior, suggesting potential relevance for spintronic applications. Similarly, intrinsic magnetism is typically absent in these materials because of their closed-shell electronic configurations and wide bandgaps. However, introducing magnetic ions through doping or substituting B-site cations offers a promising route to induce magnetic ordering and magneto-optical properties, e.g., alloyed Cs₂Ag[Bi:Fe]Br₆ [54]. Taken together, these findings point to the potential of LFHDPs not only as efficient optoelectronic materials but also as multifunctional systems where ferroelectric and magnetic functionalities could be combined, opening new avenues for device applications.

Overall, these results underscore the versatility of 2D/quasi-2D LFHDPs for stable and efficient optoelectronic applications. Their intrinsic structural stability, environmental friendliness, and tunable optical properties make them promising candidates for next-generation solar cells, light emitters, and radiation detection. Recent advances in chiral 2D/quasi-2D double perovskites further highlight their potential, as chirality-induced polar photovoltages have been demonstrated for self-powered detectors. These materials incorporate organic spacer cations between perovskite layers, forming natural quantum wells that enhance excitonic effects and improve resistance to moisture and oxygen. 2D/quasi-2D LFDPs have demonstrated superior stability and optoelectronic performance in solar cell applications [13]. Their layered structure enables strong light absorption and efficient charge transport while maintaining excellent environmental stability. Furthermore, chiral 2D/quasi-2D double perovskites have recently been shown to exhibit unique photo-responses, such as chirality-induced polar photovoltage for self-powered X-ray detection [18,21,79,80].

7 Challenges

2D LFHDPs has demonstrated promising optical, electronic, and device-level properties; however, several key challenges remain that could hinder their broader implementation. In particular, relatively low carrier mobilities, indirect bandgaps in certain compositions, and the persistence of defect-related trap states limit charge transport efficiency and overall device performance. Furthermore, ensuring long-term operational stability under continuous illumination and environmental stress, while simultaneously developing scalable and reproducible synthesis techniques, remains a nontrivial task. These challenges, though significant, are not insurmountable, and continued progress in rational materials design, defect passivation, and advanced fabrication methods is expected to progressively overcome these barriers, thereby accelerating the transition of LFHDPs from laboratory prototypes to practical optoelectronic technologies.

7.1 Low charge-carrier mobility

The strong exciton binding energies and anisotropic transport in layered 2D structures generally result in lower carrier mobilities compared to 3D perovskites. This can reduce charge extraction efficiency, limit photovoltaic and photodetector performance [54,115,116]. Sometimes doping with another ion or adding conducting materials during preparation may result in an increase in the PLQY of LFHDPs to some extent by suppressing the surface defect.

7.2 Indirect bandgap in some compositions

Several lead-free compositions, such as Cs₂AgBiBr₆, Cs₂AgSbCl₆ and Cs₂NaBiCl₆, exhibit indirect bandgaps (typically 1.9–2.2 eV) and suffer from parity-forbidden transitions at the band edges due to centrosymmetric ordering, which leads to weak absorption coefficients and poor photovoltaic performance. Bandgap engineering strategies are essential to overcome this curb. Interestingly, a direct transition energy of Cs₂AgBiBr₆ is lower than that of MAPbBr₃ (2.3 eV). The involvement of Ag 4d electrons contributes to the bandgap reduction and is partially responsible for the indirect feature of Cs₂AgBiX₆ (X = Cl, Br, and I). Consequently, extensive efforts have been devoted to addressing the disparity in bandgap values of LFHDPs by performing state-of-the-art approaches [25,54,108,117].

7.3 Limited long-term stability under stress

Although 2D/quasi-2D LFHDPs generally display improved stability over 3D Pb-based perovskites, hybrid organic–inorganic variants (e.g., MA₂AgBiBr₆) remain susceptible to thermal, moisture, and UV-induced degradation over extended periods [48].

7.4 Low PCE

Current 2D/quasi-2D LFHDP-based solar cells exhibit lower PCEs relative to both lead-based 3D perovskites and some 2D analogs. Optimization of layer thickness, defect passivation, and interfacial engineering is still required to achieve competitive efficiencies.

7.5 Synthetic and scalability challenges

High-quality, large-area 2D/quasi-2D LFHDP films are difficult to prepare reproducibly. Solution-processing methods may result in poor crystallinity or defect formation, whereas vapor-assisted techniques can be complex and costly. In other words, while the morphology (size and shape) of HPs could be tuned to some extent, LFHDPs are difficult to control the morphology of double perovskite easily.

7.6 Confined material diversity

Although compositions such as Cs₂AgBiBr₆, Cs₂AgInCl₆, Cs₂AgSbCl₆, MA₂AgBiBr₆, and Cs₂NaBiCl₆ have been explored, the overall material palette remains narrow, restricting the ability to fully tune optoelectronic properties across the visible spectrum [43,45,104].

Addressing these challenges through compositional engineering, defect passivation, scalable fabrication, and theoretical-guided material design will be essential for realizing the full potential of 2D/quasi-2D LFHDPs in optoelectronic applications. Despite some restrictions, double perovskites remain an attractive Pb-free platform is expected to expand their potential in optoelectronic applications beyond photovoltaics. Future research on LFHDPs should prioritize stabilizing Sn- and Ge-based materials against oxidation, engineering double perovskite bandgaps to overcome indirect transitions, developing scalable and reproducible film fabrication techniques, and integrating computational screening with synthesis to efficiently identify promising Pb-free candidates.

8 Perspectives

This highlights an inherent trade-off between different optoelectronic applications. The continued development of 2D/quasi-2D LFHDPs is poised to address both environmental concerns and technological challenges in optoelectronic devices. Due to their innocuous nature, optimal stability, and durability, Sn-based LFHDP solar cells have achieved a PCE of more than 15% [43,118]. By addressing these directions, 2D/quasi-2D LFHDPs can evolve from promising laboratory-scale materials to practical, environmentally friendly alternatives for next-generation optoelectronic technologies [11,37,99,119,120].

Despite progress, 2D/quasi-2D LFHDPs still face fundamental limitations that prevent their practical deployment in high-performance optoelectronic devices. Unlike Pb-based perovskites, the absence of ns² lone-pair-induced antibonding character in many lead-free systems results in reduced defect tolerance and inferior carrier transport properties. Moreover, the intrinsic indirect bandgap in many double perovskites is not merely a materials issue but a consequence of symmetry-imposed parity-forbidden transitions, which cannot be easily eliminated without fundamentally altering the crystal structure. In addition, the strong excitonic nature of 2D systems–while beneficial for LEDs–poses a critical bottleneck for photovoltaic applications, where efficient charge separation is required. Breaking inversion symmetry through lattice distortion, alloying, or structural asymmetry relaxes parity selection rules and induces partially allowed direct transitions, thereby significantly enhancing optical absorption. Future research must move beyond incremental compositional tuning and instead focus on more fundamental design strategies, including symmetry breaking, orbital engineering, and hybrid dimensional architectures. Importantly, these approaches are not isolated concepts but are directly reflected in the following key research directions. Specifically, symmetry breaking strategies are closely associated with bandgap engineering and inversion symmetry control (Section 8.1), orbital engineering underpins electronic structure optimization and defect tolerance (Sections 8.1, 8.4, and 8.5), while hybrid dimensional architectures are realized through device integration and scalable fabrication strategies (Sections 8.3 and 8.4). Collectively, these approaches provide a unified framework for overcoming the intrinsic limitations of 2D/quasi-2D LFHDPs.

Future research directions include:

8.1 Bandgap and electronic structure engineering

Precise control over the bandgap and electronic structure of LFHDPs is critical for tailoring their optoelectronic functionalities. Advanced compositional tuning at both the B(I)/B(III) and X sites enables systematic modification of orbital hybridization, thereby adjusting the conduction and valence band edges to achieve optimal alignment with desired redox or device operation potentials. Heterovalent alloying strategies, such as partial substitution of Bi³⁺ with Sb³⁺ or Ag⁺ with Na⁺, offer pathways to break inversion symmetry and mitigate parity-forbidden transitions, thereby enhancing absorption coefficients and radiative recombination rates [38,54,72,89,99]. In low-dimensional perovskites, structural asymmetry induced by organic spacer layers inherently breaks inversion symmetry, offering a natural pathway to overcome parity-forbidden transitions. Similarly, mixed-halide configurations (Cl⁻/Br⁻/I⁻) allow fine-tuning of exciton binding energies, dielectric screening, and carrier mobilities, while also enabling bandgap engineering across the visible spectrum. Beyond empirical optimization, first-principles computational screening integrated with high-throughput experimental synthesis provides a powerful framework to rapidly identify stable, defect-tolerant compositions with favorable carrier effective masses and suppressed nonradiative recombination pathways. Although significant progress has been achieved through compositional and structural engineering, interface engineering remains relatively underexplored in LFHDP-based optoelectronic devices. Future improvements are expected to rely on simultaneous optimization of absorber materials and interfacial layers. Rational selection of ETMs and HTMs, surface passivation molecules, self-assembled monolayers, and 2D interlayers can reduce non-radiative recombination, suppress ion migration, improve energy-level alignment, and enhance long-term operational stability [57]. Such synergistic approaches not only accelerate the discovery of high-performance LFHDPs but also establish rational design principles for next-generation optoelectronic and photocatalytic devices.

8.2 Stability enhancement

The long-term operational stability of hybrid 2D and quasi-2D LFHDPs remains a critical challenge for their practical application in optoelectronic devices. These materials are inherently susceptible to thermal stress, moisture, and UV-induced degradation, which can trigger ion migration, phase segregation, and interfacial instability, ultimately compromising device performance. To address these issues, a multifaceted strategy has been adopted. Surface passivation using inorganic or organic capping layers can reduce defect densities and suppress nonradiative recombination, while encapsulation techniques provide a physical barrier against environmental factors such as oxygen and water. Additionally, the careful design of spacer cations, including hydrophobic or rigid organic linkers, enhances interlayer interactions and limits moisture infiltration. Compositional engineering, such as halide mixing or partial cation substitution, further stabilizes the lattice and mitigates photochemical degradation under prolonged illumination [9,24,52,88,109]. Collectively, these approaches not only improve thermal and chemical resilience but also suppress undesirable ion migration pathways, thereby extending the operational lifetimes of LFHDP-based devices. Such comprehensive stabilization strategies are pivotal for advancing the practical deployment of lead free 2D perovskites in high-performance, environmentally benign optoelectronic applications.

8.3 Scalable fabrication techniques

For the commercialization of large-area, high-quality 2D/quasi-2D LFHDPs, the ability to fabricate large-area, uniform, and defect-minimized thin films is of paramount importance. Conventional laboratory-scale approaches, while effective for proof-of-concept demonstrations, often suffer from poor reproducibility and limited scalability. To address these challenges, advanced deposition methods such as vapor-assisted crystallization can be employed to improve crystallinity, reduce grain-boundary defects, and enhance carrier lifetimes. In parallel, solution-based approaches including blade coating, slot-die coating, and inkjet printing offer opportunities for cost-effective, continuous film deposition with precise thickness control, while roll-to-roll processing provides a viable route toward industrial-scale production of flexible optoelectronic devices. Optimization of precursor chemistry, solvent engineering, and post-deposition annealing are further critical to achieving phase-pure, stable films with minimal trap densities. The integration of in situ diagnostics and real-time process monitoring can accelerate the establishment of reproducible fabrication protocols [29,33,50,54,57]. Ultimately, combining these scalable methodologies with defect passivation strategies and interface engineering will be key to translating 2D LFHDPs from laboratory prototypes into commercially viable technologies.

8.4 Device integration and architecture innovation

The integration of 2D/quasi-2D LFHDPs into advanced device architectures offers substantial opportunities to broaden their application space across photovoltaics, LEDs, photodetectors, and emerging wearable optoelectronics. By exploiting their structural anisotropy, mechanical flexibility, and inherent chemical stability, LFHDPs can be integrated with flexible substrates to realize bendable and lightweight optoelectronic devices [33,87,108]. In addition, tandem architectures enable spectral complementarity with narrow-bandgap absorbers, improving overall light harvesting and charge extraction, and thereby allowing PCEs to exceed the Shockley−Queisser limit, the theoretical maximum solar conversion efficiency of 33.16% for a single-junction solar cell under standard illumination with a bandgap of 1.34 eV, dictated by radiative recombination and thermodynamic considerations. Such strategies highlight the potential of LFHDPs for next-generation high-performance and mechanically versatile photovoltaic systems. Hybrid heterostructures that integrate LFHDP layers with oxide transport layers or other perovskite absorbers further optimize interfacial energetics, enhance carrier extraction, and suppress recombination losses. For instance, the optimized device structure FTO/TiO₂/Cs₂AgBi_0.75Sb_0.25Br₆/FASnI₃/Cu₂O demonstrated the synergistic advantages of compositional engineering and tandem stacking, achieving a remarkable 28.22% PCE [121]. Complementarily, first-principles DFT calculations predict that Cs₂AuBiCl₆, with a direct bandgap of ~1.09 eV, exhibits favorable band alignment with optimized ETL/HTL, which facilitates efficient charge separation and significantly improves photovoltaic device performance. These findings underscore the critical importance of architectural innovation, wherein rational device design-incorporating tandem configurations, flexible integration, and optimized transport interfaces-can unlock the full optoelectronic potential of 2D/quasi-2D LFHDPs [122].

8.5 Exploring novel physical phenomena

The layered structure of 2D/quasi-2D LFHDPs provides a fertile ground for uncovering exotic physical behaviors that are inaccessible in their 3D counterparts. The inherent quantum confinement and reduced dielectric screening in these low-dimensional systems intensify Coulomb interactions, giving rise to strongly bound excitons with large binding energies that are highly sensitive to lattice symmetry and external perturbations. Moreover, the absence of inversion symmetry in certain layered polymorphs enables chirality-induced bulk photovoltaic effects, where the shift current mechanism can yield anomalously large photovoltages under polarized light. The incorporation of heavy elements at the B(I)- and B(III)-sites enhances spin-orbit coupling, which not only modifies the band dispersion and spin-valley texture but also facilitates Rashba-type spin splitting. This phenomenon, originating from the interplay between strong spin–orbit interaction and broken inversion symmetry, induces momentum-dependent spin polarization of electronic states and lifts spin degeneracy in the conduction or valence bands. Such Rashba effects are of particular importance for spintronic and valleytronic applications, as they enable efficient spin manipulation without the need for external magnetic fields and promote long spin coherence times. In the context of HDPs, tailoring the degree of Rashba splitting through compositional engineering and structural control provides a unique avenue to couple optoelectronic performance with spin-dependent functionalities, thereby broadening their potential in next-generation multifunctional device architectures. Coupled with these effects, the anisotropic electronic transport along in-plane versus out-of-plane directions allows for the exploration of low-dimensional charge and exciton dynamics, which could be further tuned by external fields, strain engineering, or interfacial coupling in vdW heterostructures. Together, these attributes highlight 2D/quasi-2D LFHDPs as a versatile quantum material platform for realizing unconventional optoelectronic, spintronic, and multifunctional devices that extend far beyond the paradigm of conventional perovskite photovoltaics.

8.6 Environmentally benign and economical material design

The sustainable development of 2D/quasi-2D LFHDPs requires systematic exploration of earth-abundant, innocuous, and economically viable elemental compositions, particularly at the B-site and X-site positions that critically define the electronic band structure and stability. Substituting Pb with benign cations such as Bi³⁺, Sb³⁺, or In³⁺ in combination with monovalent cations like Ag⁺, Na⁺, or K⁺ not only alleviates toxicity concerns but also provides a rich design space for tailoring orbital hybridization and band alignment. Likewise, judicious selection of halide anions such as Cl⁻, Br⁻, I⁻, or mixed-halide configurations, allows fine-tuning of the band gap, dielectric constant, and exciton binding energy, thereby optimizing light absorption and carrier transport properties. From an economic perspective, prioritizing elements with large global reserves and stable supply chains reduces material scarcity risks and ensures scalability for industrial applications. Furthermore, life-cycle assessments emphasize the importance of designing LFHDPs that minimize hazardous by-products during synthesis and decomposition, while maintaining compatibility with low-temperature, solution-processable fabrication routes. By integrating these criteria-abundance, safety, cost-effectiveness, and high optoelectronic performance-2D/quasi-2D LFHDPs can advance toward truly sustainable commercialization in photovoltaics, light-emitting devices, and neuromorphic systems.

8.7 Photocatalytic technologies

The characterization of LFHDPs has revealed their environmentally benign attributes and compositional diversity, which make them attractive candidates for photocatalytic energy conversion processes. Their tunable band structures, derived from flexible B-site and X-site substitutions, provide suitable band edge alignments with respect to the redox potentials of water and carbon dioxide, enabling efficient charge transfer during photocatalysis [123,124]. Moreover, the strong optical absorption coefficients and long carrier diffusion lengths inherent to these compounds enhance photoexcited charge utilization, thereby facilitating selective hydrogen evolution and CO₂ reduction pathways. Recent theoretical investigations further support their promise; for example, Hossian and colleagues employed DFT to show that Cs₂TeI₆ possesses an optimal band gap (~1.9–2.0 eV) and appropriate CBM position to drive hydrogen production via solar water splitting, while maintaining chemical stability under aqueous conditions [125]. Collectively, these findings underscore the potential of 2D/quasi-2D LFHDPs not only as sustainable and innocuous alternatives to Pb-based perovskites but also as versatile photocatalysts for integrated solar fuel production.

Schematic illustration summarizing the key scientific challenges (indirect bandgaps, strong excitonic effects, defect formation, limited carrier mobility, stability, and scalability), corresponding materials engineering strategies (spacer engineering, composition engineering, crystal engineering, interface engineering, defect passivation, and device architecture optimization), and future research directions toward high-performance layered LFHDP-based optoelectronic devices. Emerging approaches, including AI-assisted materials discovery, high-throughput first-principles calculations, machine learning, green synthesis, multidimensional heterostructures, and industrial-scale fabrication, are highlighted as promising routes for accelerating the commercialization of sustainable lead-free perovskite technologies, as shown in Fig. 8.

9 Conclusion

This review has comprehensively examined the structure-property-device relationships in 2D/quasi-2D lead-free halide double perovskites, demonstrating how dimensional engineering, spacer cation design, and compositional tuning collectively govern their optoelectronic functionality. The layered architectures of RP, DJ, and ACI phases provide intrinsic environmental stability, while strategies such as symmetry breaking, defect passivation, and bandgap engineering offer pathways to overcome intrinsic limitations including indirect bandgaps and low carrier mobilities. Looking forward, the convergence of high-throughput computational screening with experimental synthesis, combined with the integration of LFHDPs into tandem, flexible, and neuromorphic architectures, positions these materials as compelling candidates for sustainable next-generation optoelectronics. Realizing this potential will require sustained progress in scalable fabrication, long-term stability enhancement, and the exploration of earth-abundant, non-toxic chemistries.

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