Exploring 2D perovskite chemistry for advancing efficient and stable solar cells

Abstract

Perovskite-based photovoltaic devices have garnered significant interest owing to their remarkable performance in converting light into electricity. Recently, the focus in the field of perovskite solar cells (PSCs) has shifted towards enhancing their durability over extended periods. One promising strategy is the incorporation of two-dimensional (2D) perovskites, known for their ability to enhance stability due to the large organic cations that act as a barrier against moisture. However, the broad optical bandgap and limited charge transport properties of 2D perovskites hinder their efficiency, making them less suitable as the sole light-absorbing material when compared to their three-dimensional (3D) counterparts. An innovative approach involves using 2D perovskite structures to modify the surface properties of 3D perovskite. This hybrid approach, known as 2D/3D perovskites, while enhancing their performance. Beyond solar energy applications, 2D perovskites offer a flexible platform for chemical engineering, allowing for significant adjustments to crystal and thin-film configurations, bandgaps, and charge transport properties through the different organic ligands and halide mixtures. Despite these advantages, challenges remain in integration of 2D perovskites into solar cells without compromising device stability. This review encapsulates the latest developments in 2D perovskite research, focusing on their structural, optoelectronic, and stability attributes, while delving into the challenges and future potential of these materials.

Graphical abstract

Keywords

two-dimensional (2D) perovskites / crystal structure / thin film structure / optoelectronic properties / stability / perovskite solar cells (PSCs)

1 Introduction

The certified power conversion efficiency (PCE) of single-junction solar cells based on three-dimensional (3D) metal halide perovskites has soared to an impressive 26.7% [1], rivaling that of well-established photovoltaic technologies [2]. This achievement has sparked considerable interest in perovskite solar cells (PSCs) from both research and commercial sectors. Recently, the focus in PSC research has shifted towards improving the long-term durability of these devices [3]. A plethora of studies have delved into optimizing perovskite absorbers through compositional engineering [4,5], defect passivation [6], and controlling the crystallization process [7]. Additionally, advances in device architecture have been made, including improvements in contact layers [8,9], interface engineering [10], and the development of encapsulation techniques [11,12]. Among the various strategies, the incorporation of two-dimensional (2D) perovskites has emerged as a promising method for significantly bolstering the stability of PSCs [13–15].

Unlike the commonly-used cations in 3D perovskites (such as MA⁺, Cs⁺, and FA⁺), the substantial organic cations in 2D perovskites form a physical barrier that minimizes surface water absorption [16,17]. These sizable hydrophobic cations within the 2D perovskite framework significantly inhibit the ingress of moisture [18]. However, despite these advantages, 2D perovskites are usually not ideal for use as solar cell absorbers because of their broad optical bandgap and limited charge transport properties. Since the first report in 2014, which demonstrated a PCE of 4.73% for 2D perovskites-based solar cell absorbers [19], the PCE of 2D PSCs has seen an improvement to over 19.7% [20]. However, this efficiency remains significantly lower compared to 3D PSCs, which have exceeded 26%. Instead of using 2D perovskites as the primary solar cell absorbers, a strategic alternative is to employ 2D perovskite structures to refine the surface characteristics of 3D perovskite grains and films. This hybrid 2D/3D perovskite approach has demonstrated considerable potential for enhancing both the performance and stability of perovskite materials across a variety of compositions [21–25].

Beyond their role in solar cells, 2D perovskites offer a robust platform for chemical engineering, providing an opportunity to explore structure-property correlations. Unlike 3D perovskites, which are constrained by the Goldsmith tolerance factor in the selection of organic cations, 2D perovskites allow for extensive chemical engineering. A wide array of organic ligands has been utilized in 2D hybrid perovskites, and halide tuning has enabled the synthesis of inorganic variants like Cs₂PbCl₂I₂. This high level of chemical adjustability enables precise manipulation of crystal and thin-film structures, directly affecting their optoelectronic properties and overall device performance.

This review examines the latest advancements in 2D perovskites, focusing on their crystal and thin-film structures and the associated optoelectronic properties, including band structure, optical behavior, and charge transport mechanisms. The discussion highlights their unique features, such as enhanced stability and distinct optoelectronic properties in comparison to 3D perovskites. Special attention is given to the limitations of 2D perovskites, including potential challenges related to device stability when integrated into solar cells. These challenges remain a central focus of ongoing research. Finally, the review explores future opportunities and challenges for expanding the applications of 2D perovskites, evaluating how advancements in material design and processing could address existing shortcomings while enhancing the performance of optoelectronic devices. The implications for device performance are emphasized throughout the discussion, underscoring their relevance across multiple sections. This integrated approach highlights the potential of 2D perovskites to bridge the gap in both stability and performance, paving the way for next-generation applications.

2 Crystal structure

3D perovskite materials are generally following the ABX₃ framework, where six halide ions (at the X site, such as iodide, bromide, and chloride) encircle a divalent metal cation (at the B site, for instance, tin and lead), forming a BX₆ octahedral complex [26]. The vertices of these octahedra are connected by 12 monovalent cations (at the A site, such as MA⁺, FA⁺, and Cs⁺). The A, B, and X sites can incorporate multiple elements, allowing compositional flexibility for tailored properties. The structural stability of a perovskite for specific compositions is often accessed using the Goldschmidt tolerance factor (t) [27], calculated as

(1)t=\frac{r_{\mathrm{A}}+r_{\mathrm{X}}}{\sqrt{2}(r_{\mathrm{B}}+r_{\mathrm{X}})} .

This formula relies on the ionic radii of the elements involved, denoted as r_A, r_B, and r_X. Typically, a 3D perovskite structure is stabilized when t is between 0.8 and 1, while a lower-dimensional structure is more likely to form when t exceeds 1 [17].

2D perovskites are commonly denoted by the formula (A’)_m(A)_n–1B_nX_{3n + 1}, with A’ being either a divalent (m = 1) or monovalent (m = 2) cation, leading to the formation of double or single layers that interconnect the inorganic layers. The parameter n indicates the number of inorganic layers sandwiched between organic ones, as depicted in Fig.1 [28,29].

To date, most 2D perovskites have been synthesized with large organic cations serving as spacers between the inorganic layers to establish the 2D layered structure. Recent studies have shown that, in addition to modulating the A-site components, altering the size of X-site ions can also be used to create 2D perovskites. Examples include compounds like Cs₂PbCl₂I₂ and MA₂Pb(SCN)₂I₂.

2.1 2D perovskites based on bulky cations

The preparation of 2D perovskites often involves large cations with amino groups, which can either be terminal (monoamines) or present at both ends (diamines). The existence of van der Waals gaps in these materials is largely determined by the number of cations interposed between the inorganic layers, leading to the classification of 2D perovskites into three principal phases: Ruddlesden-Popper (RP), Dion-Jacobson (DJ), and alternating cations in the interlayer (ACI) phases [30].

Among these, the RP phase, with the formula (A’)₂(A)_n−1B_nX_3n+1, is the most prevalent structural type. As illustrated in Fig.2(a), this phase places monoammonium spacers between inorganic slabs, with their amine groups oriented towards adjacent octahedral cavities. Typical organic cations such as BA⁺ and phenethylammonium (PEA⁺) contain a single terminal amine. This configuration enables considerable conformational flexibility and positional disorder within the organic spacer layer, making it adaptable to a variety of aliphatic or aromatic ammonium species [31–37].

DJ phase, represented by the formula (A’)(A)_n−1B_nX_3n+1, incorporates single-layer cations with amino groups at both ends, linking adjacent inorganic layers as shown in Fig.2(b). In contrast to RP phase perovskites, DJ phase perovskites lack a van der Waals gap between the interlocking organic cations, forming a more robust barrier against moisture and heat, thus enhancing the rigidity and stability [38,39]. Organic cations like 1,3-propanediammionum (PDA²⁺) and 3-(aminomethyl)piperidinium (3-AMP²⁺) have been used in DJ phase perovskites, leading to improved device performance compared to RP phase perovskites. For example, Li et al. [39] showed that DJ phase 2D perovskites exhibited higher stability and efficiency in solar cell applications than their RP phase counterparts. They compared RP phase (PA)₂(MA)₃Pb₄I₁₃ (PA = propylamine) with DJ phase (PDA)(MA)₃Pb₄I₁₃, finding that the DJ phase-based perovskite solar cell had a superior PCE of 13.3%, compared to the 8.8% PCE of the RP phase device. DJ-based PSCs also demonstrated superior thermal and humidity stability, attributed to increased structural rigidity [39].

More recently, a novel class of 2D perovskites, known as ACI, has been identified, showing potential for optoelectronic applications. As shown in Fig.2(c), ACI perovskites uniquely combine the compositional attributes of RP phases with the layered architecture of DJ perovskites, incorporating two alternating cations within the interlayer region. For instance, Soe et al. [40] demonstrated an ACI system with the formula (GA)(MA)_nPb_nI_3n+1 (n = 1–3; GA⁺ = guanidinium), where the ordered GA⁺ and MA⁺ cation arrangements stabilize the framework. Similarly, Yan et al. [41] reported photovoltaic devices based on a quasi-2D ACI perovskite, (AcA)MA_nPb_nI_3n+1 (n = 4), using AcA⁺ and MA⁺ as dual spacers. In contrast to the conventional RP and DJ perovskites, ACI perovskites feature a distinct stacking configuration, enhanced structural symmetry, and marginally narrower bandgaps. These attributes correlate with their exceptional photovoltaic performance, with power conversion efficiencies (PCE) reaching as high as 22.26% [42].

2.2 2D perovskites based on (pseudo-)halides with distinct sizes

Beyond the use of large cations in the synthesis of 2D perovskites, another approach to enhance their properties involves manipulating the X-site. This technique restructures the lead halide octahedron into a 2D arrangement of [Pb(X)₄(X’)₂], thereby eliminating its corner-sharing properties. The introduction of an extra halide ion (X’) occupying the apical positions within the lead halide octahedron is responsible for this change.

Reports on X-site substitutions in 2D perovskites remain relatively scarce in the literature [43–46]. A key strategy in this field is the replacement of the axial atom in lead halide octahedra with a pseudohalide anion. In 2015, Daub and Hillebrecht [45] made pioneering contributions by documenting the first instance of X-site-substituted 2D perovskites, specifically (MA)₂Pb(SCN)₂I₂. Their research revealed that the pseudohalide SCN⁻ anion occupies the axial positions within the lead halide octahedron, forming a 2D [Pb(I)₄(SCN)₂] structure, attributed to the asymmetric electronic configuration and unique shape of the SCN⁻ anion. However, it was later found that this structure is unstable under ambient conditions and tends to decompose into PbI₂ and MASCN when exposed to moisture [47].

In a significant breakthrough in 2024, Liu et al. [48] introduced the cyanate ion (OCN⁻) as a new pseudohalide, with an effective ionic radius of 1.97 Å, comparable to the bromide ion (1.95 Å), suggesting its potential as a bromide substitute. Despite its promise, the integration of OCN⁻ into perovskites has proven challenging, with only a few studies exploring its incorporation into perovskites [49,50]

An approach to X-site engineering involves the solid-state synthesis of all-inorganic RP phase lead halide perovskites. In 2018, Kanatzidis and colleagues [46] reported the first all-inorganic 2D perovskite, Cs₂PbI₂Cl₂, single crystal, which formed a 2D [Pb(Cl)₄(I)₂] framework due to the size discrepancy between Cl⁻ and I⁻ anions. As depicted in Fig.3, the I⁻ anions are positioned axially, with the Cl⁻ anions located in the octahedral plane. The remarkable stability of Cs₂PbCl₂I₂ was confirmed through powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA) [46]. After four months of storage under ambient conditions, the PXRD pattern of Cs₂PbI₂Cl₂ showed no significant deviation from the fresh sample, and TGA results indicated stability up to 520 °C, highlighting its excellent environmental and thermal stability.

Recently, Zhao and colleagues [10] developed an all-inorganic 2D/3D perovskite structure by overlaying the 2D perovskite Cs₂PbCl₂I₂ on the 3D perovskite CsPbI₃. They found that the 2D capping layer effectively prevents ion migration in PSCs, significantly improving their operational stability. Accelerated aging tests predict an operational stability of over 5 years, the longest lifespan reported for PSCs to date (Fig.3).

In general, the X-site-substituted 2D perovskites reported in these advanced studies show a narrower interlayer spacing, typically ranging from 1.33 to 3.10 Å, compared to the larger gaps found in those with large organic cations, which typically range from 5.32 to 17.58 Å [51–53]. This reduced interlayer spacing is vital for decreasing the bandgap and enhancing the vertical charge transport within the plane, which is a critical factor for the efficiency of optoelectronic devices. Research into the use of X-site-substituted 2D perovskites for device applications is still in its early stages, and further developments in this field are eagerly anticipated.

2.3 Octahedral distortion

The dynamics of bonding, particularly hydrogen bonding, between organic ligands and [PbX₆]⁴⁻ octahedra are crucial for both 3D and 2D perovskites [17]. Most 2D perovskites typically incorporate ammonium-based modifiers with either single or paired cationic groups. These crystalline architectures often demonstrate distorted inorganic framework geometries, with [PbX₆]⁴⁻ octahedra characterized by sub-180° bond angles between lead and halide ions. This structural deformation stems from charge-mediated associations between halogen atoms in the lead halide coordination polyhedra and hydrogen-donor moieties within ammonium functionalities [17,54,55]. Given the direct correlation between octahedral network geometry and key electronic properties like energy gap and carrier mobility [54,56], substantial research focuses on optimizing structural alignment to strengthen orbital hybridization between lead’s s-orbital and halide p-orbitals [54,57,58].

The interfacial chemistry between organic components and [PbX₆]⁴⁻ octahedra significantly influences three critical factors: magnitude of structural tilting in octahedral units, interlayer separation distances, and resulting optoelectronic response characteristics. Together, these factors collectively determine device efficiency metrics [54,56,59–62]. These structure-property relationships necessitate a comprehensive evaluation of molecular engineering parameters, particularly in spacer group selection and their molecular interactions with the inorganic lattice. Current investigative priorities emphasize developing rational design principles that systematically address these multidimensional structure-function correlations in hybrid perovskite materials.

Bifunctional ligands, characterized by distinct chemical groups at their terminals, offer a promising approach for controlling molecular interactions and tailoring the structural properties of 2D perovskites, such as octahedral distortion and organic-inorganic interlayer spacing. These ligands typically have an ammonium terminus paired with a secondary functional group designed to enhance intermolecular connectivity within the organic layer. As shown in Fig.4(a), Zhao and colleagues [13] conducted structural investigations using three bifunctional ligands with hydrogen-bonding non-ammonium termini (–CN, –OH, or –COOH, highlighted in purple) to explore their influence on perovskite architectures. The inserts illustrating hydrogen bonding between proximate ligands spanning the organic layers; the green dashed ellipses denoting close interactions of the RH₂N⁺–H…R, withR = –OH for (COOH–PA)₂PbI₄, –OH for (OH–PA)₂PbI₄, and –CN for (CN–EA)₂PbI₄; the angles being Pb-I-Pb angles in each structure (Fig.4(b) and Fig.4(c)). The modulation of the bulk optoelectronic properties in 2D perovskites relies on the interactions between adjacent organic ligands, a phenomenon observed across various material systems. Both (OH–PA)₂PbI₄ and (CN–EA)₂PbI₄ form dimeric configurations mediated by organic-layer interactions. Structural studies indicate that (CN–EA)⁺ dimers adopt pseudohexagonal arrangements due to hydrogen bonding between ammonium termini and cyano groups, effectively bridging adjacent organic layers. In contrast, (OH–PA)⁺ ligands form pseudo-rectangular assemblies through ammonium-hydroxyl interactions.

For (CN–EA)⁺ dimerization, steric adjustments occur as the ammonium group tilts away from the inorganic plane, weakening equatorial iodide interactions while strengthening hydrogen bonds with axial halides. To quantify these effects, researchers introduced a charge separation descriptor (CSD), which correlates intermolecular forces with structural and electronic properties. Enhanced intermolecular interactions were shown to redistribute charge from inorganic layers, directly affecting Pb-I-Pb bond angles, bandgap energies, and charge transport efficiency. Systems with elevated CSD values display reduced octahedral distortion (Pb-I-Pb angles approaching 180°), narrower bandgaps, and improved in-plane carrier mobility. For instance, (CN–EA)₂PbI₄ exhibits the highest CSD and a near-180° Pb-I-Pb angle. In contrast, (CH₃–PA)₂PbI₄ showed the lowest CSD and a Pb-I-Pb angle of 145°. Linear correlations were observed between the Pb-I-Pb bond angle and both the band gap and the logarithm of hole mobility. Hole mobility measurements via space-charge limited current (SCLC) revealed substantial variations across the series: (CN–EA)₂PbI₄ demonstrated superior in-plane mobility (2.4 ± 0.4 cm²V⁻¹s⁻¹), while (CH₃–PA)₂PbI₄ exhibited the lowest value (0.5 ± 0.04 cm²V⁻¹s⁻¹). These structural-electronic correlations directly translate into solar cell performance enhancements, with optimized ligand interactions enabling improved charge extraction and device efficiency.

Investigations into Ge-based perovskites have indicated that deviations from the ideal cubic BX₆ octahedral geometry, characterized by octahedral distortions, are primarily due to the localization of Ge s² lone pair electrons [63,64]. These structural deformations observed in GeX₆ octahedral configurations manifest as asymmetric bond length variations, with three elongated Ge-X bonds and three contracted ones, a configuration consistent with second-order Jahn-Teller distortion characteristics [65,66]. This distortion mechanism is similar to that observed in Sn/Pb-containing perovskite analogs, where stereochemically active 5s²/6s² electrons in B-site cations induce comparable MI₆-octahedral deformations [67,68]. Experimental investigations by Knutson et al. [69] on SnI-based perovskite revealed that even minimal structural deviations from ideal cubic symmetry can significantly alter optoelectronic response characteristics. Quantitative analysis of structural parameters reveals an inverse proportionality between B-site cationic radii and in-phase rotational angles. Notably, comparative studies demonstrate that most 2D perovskite architectures exhibit greater octahedral tilting (3−5° increment) compared to their 3D counterparts, with the exception of GeI-based compounds. Furthermore, systematic bond angle variations within 8° tolerance emerge as a potential structural determinant for bandgap modulation in these materials, suggesting a crystallographic approach for engineering optoelectronic properties [70].

Lead (Pb) remains the most widely used material in perovskite research thanks to its larger atomic radius, which facilitates better lattice matching and stability, reducing lattice defects and distortions. Pb also exhibits greater stability in its + II oxidation state compared to Sn and Ge, which are more susceptible to oxidation, thereby compromising the long-term stability of perovskite materials [71].

3 Thin film structure of 2D perovskites

3.1 Phase purity

Incorporating organic ligands, such as A’, during the fabrication of 2D perovskite thin films can sometimes compromise phase purity, which is critical for the performance and longevity of the resulting solar cells. Achieving uniform and high phase purity is essential, but it is often challenging in practice. Due to variations in processing methods, a mixture of phases with different n values is commonly observed within 2D perovskite thin films. For example, when attempting to fabricate an n = 4 (BA)₂(MA)₃Pb₄I₁₃ 2D perovskite thin film through post-spin-coating and annealing, the film may contain a mix of other n-values, including the n = ∞ phase of MAPbI₃. The formation of MAPbI₃ is linked to the preferential evaporation of precursor solvents at the film surface during spin coating, leading to solution supersaturation and the crystallization of MAPbI₃ [72]. Additionally, the low formation energy of MAPbI₃ also significantly contributes to its formation. The presence of MAPbI₃ on the film surface increases the BA⁺ to MA⁺ ratio in the remaining film, resulting in a lower n-value at the base of the film compared to the target n-value [72].

In contrast, the absence of an organic ligand restricts the configurational possibilities, often leading to the formation of either MAPbI₃ or (BA)₂PbI₄ when only MA⁺ or BA⁺ is present. The formation of n = 1 2D perovskites with alternating BA⁺ and [PbI₆]⁴⁻ layers is due to the larger size of BA⁺ ions compared to the A-site cations they replace. A wide distribution of n-values, especially the presence of n = 1 or n = 2 phases, can prominently hinder charge carrier transport and influence charge recombination. Materials with a broad n-value distribution pose challenges in understanding their physical and chemical properties, making it more difficult to design materials with targeted properties [73,74]. Therefore, developing systematic approaches and strategies to obtain phase-pure 2D perovskites with predetermined compositions is vital. This is critical for elucidating structure-property correlations and enabling the development of 2D perovskites for use in optoelectronic devices and systems [75].

Researchers have realized the difficulties in achieving phase-pure 2D perovskite films and have made a concerted effort to develop efficient approaches with diverse chemical compositions. For example, by strategically employing n-butylamine acetate (BAAc) as the ammonium cation (BA⁺) precursor instead of conventional n-butylamine iodide (BAI) in 2D perovskite synthesis, Liang’s group successfully produced phase-pure 2D perovskite films (n > 1) (Fig.5) [76]. They found that the strong coordination between BAAc and Pb²⁺ led to the formation of an intermediate with a single n-value, resulting in a high phase purity 2D perovskite film with n = 5. PSCs using such pure-phase n = 5 2D perovskite films showed improved power conversion efficiency and stability compared to those using mixed-phase films. This suggests that achieving phase-pure 2D perovskite films with predetermined compositions could be an effective strategy for enhancing device performance.

Additionally, Mohite’s team developed a method to selectively control the phase of 2D perovskites by dissolving pure-phase single-crystal powders, enabling the production of phase-pure 2D perovskite thin films [77]. This research revealed the presence of sub-micron-sized seeds in the solution, which retained the high-purity characteristics of single crystals. Utilizing these seeds for crystallization and growth resulted in 2D perovskite films with high phase purity. Solar cells made from these phase-pure films achieved higher PCE and operational stability compared to those made with common 2D perovskite films containing a broad n-value range. This further supports the notion that obtaining phase-pure 2D perovskite films with specific n-values is an effective method for enhancing device performance. Notably, the authors confirmed that this strategy is effective in both RP and DJ phase 2D perovskite systems, highlighting its broad applicability across various 2D perovskite systems.

To capitalize on the advantages of both 3D and 2D perovskites, 2D-3D perovskite heterostructures (PHSs) are extensively utilized in PSCs to boost long-term stability without sacrificing their superior photoelectric properties. However, this strategy introduces new challenges, particularly in achieving phase-pure films. The standard approach for depositing the 2D layer in 2D-3D PHSs involves spin-coating an organic cation solution in isopropyl alcohol or chloroform onto a pre-existing 3D perovskite layer. However, the uncontrolled dissolution of the 3D perovskite during this process can result in a wide range of n-values in the 2D perovskite.

Sidhik and colleagues [78] addressed this challenge by presenting a solvent design principle for creating solution-processed 2D-3D PHSs. They discovered that employing acetonitrile (MeCN) to dissolve 2D perovskite seeds is effective for producing the desired 2D-3D PHSs, based on two indispensable properties of the processing solvents: the dielectric constant (ϵ_r) and the Gutmann donor number (D_N). As depicted in Fig.6(a), processing solvents with \epsilon_r > 30 and 5 < D_N < 18 can selectively dissolve 2D perovskite powders without damaging the underlying 3D perovskite layer. Based on this method, they successfully fabricated 2D-3D PHSs, with n values ranging from 1 to 4, showcasing remarkable phase purity in the 2D perovskite.

Furthermore, during the aging of PSCs, ion migration between the surface 2D and bulk 3D perovskites can threaten phase purity. To address this issue, Luo and colleagues [79] integrated a cross-linked polymer (CLP) between the 2D and 3D perovskite layers. The CLP, composed of polyhedral oligomeric silsesquioxane (POSS) and ethylene dimethacrylate (EDMA), as shown in Fig.6(b), acts as a barrier to ion migration. The incorporation of this CLP effectively inhibits ion migration between the surface 2D and bulk 3D perovskites, ensuring that the film maintains high phase purity even after aging at 100 °C for 120 min.

3.2 Crystal orientation

The conductivity and carrier mobility anisotropy in 2D perovskite thin films is a direct result of their layered architecture. The degree of this anisotropy depends on the alignment of the lamellar structure with the desired direction of charge carrier transport [80]. Due to variations in fabrication methods, the orientation of the layered structure in 2D perovskite films can be categorized into three types: parallel to the substrate (as shown in (Fig.7(a)), perpendicular to the substrate (as shown in Fig.7(c)), and randomly oriented (as shown in Fig.7(b)).

Measurements of conductivity and carrier mobility taken parallel to the layered structure tend to be significantly higher compared to those perpendicular to it. This is because the organic layers within the structure can hinder vertical charge transport through the layers. Consequently, the alignment of these layers greatly affects the performance of devices that rely on vertical charge transport, such as solar cells, light-emitting diodes (LEDs), and photodetectors.

Various deposition techniques have been developed to manipulate crystallization and ensure the vertical alignment of 2D perovskite thin films, including the hot casting method [81,82] and additive-assisted crystallization [83–92]. While these methods have proven effective for specific 2D perovskite chemical compositions, particularly those containing BA⁺ and PEA⁺, further investigation is needed to fully understand the broader formation mechanisms that lead to vertical orientation.

A recent study demonstrated that partial substitution of I with Cl in the equatorial position induces strain in the octahedra, elongating the unit cell along the vertical axes. This substitution is achieved by replacing a portion of methylammonium iodide (MAI) with methylammonium chloride (MACl), with an optimal ratio corresponding to 10% Cl substitution for I in the perovskite formulation, as depicted in Fig.8(a) [93]. The resulting vertical compression redirects the crystal orientation from horizontal to vertical.

In photovoltaic devices, the PSCs that use horizontally aligned, low-n 2D perovskites (n < 3) as the active layer demonstrate a PCE of 0.8%. On the contrary, PSCs with vertical orientation display a PCE of 9.4%, with a short-circuit current density (J_SC) of 10.6 mAcm⁻², a fill factor (FF) of 63.2%, and a V_OC of 1.40 V. To the authors’ knowledge, these represent the highest photoelectric parameters reported for 2 eV band gap 2D perovskites with low n-values. However, this effect appears to be limited to 2D perovskites with low n-values (n < 3), and further research is needed to identify more universally applicable mechanism for crystal orientation control.

The post-deposition solvent vapor annealing (SVA) method is another widely used technique that has successfully achieve strong vertical orientation in 2D perovskite films with diverse chemical compositions [15]. The SVA process increases the plasticity of the 2D perovskite film, enhances its crystallinity, and induces vertical orientation. This post-treatment also preserves phase purity, leading to significant improvements in the photovoltaic efficiency and stability of PSCs fabricated from 2D perovskite films treated with SVA.

Additionally, a SCM technique has been reported, where the substrate is exposed to a DMSO vapor environment during the spin-coating of the precursor, as shown in Fig.8(b) [94]. This process results in 2D perovskite films with vertical crystal orientation and high phase purity, with the n = 3 species accounting for 98% of the whole film. Incorporating highly vertically oriented and phase-pure 2D perovskites lead to exceptional photoelectric properties and enhanced stability.

4 Optoelectronic properties

The anisotropic optoelectronic characteristics of 2D perovskites offer distinct advantages and limitations in comparison to their 3D counterparts [95]. This section aims to highlight the unique optoelectronic attributes of 2D perovskites, focusing on their electronic configuration, exciton dynamics, and the impacts of quantum and dielectric confinement. The objective is to provide a comprehensive understanding of the unique traits of 2D perovskites, helping readers appreciate the nuanced differences that set these materials apart from traditional 3D perovskites.

4.1 Electronic structure and bandgap

The optoelectronic characteristics of 2D perovskites are profoundly influenced by the properties of the organic ligands integrated into their structure. Common organic ligands such as BA⁺, PEA⁺, and BDA²⁺ are generally electrically insulating. By electrically isolating the inorganic layers, these ligands form a ‘quantum well’ (QW) band structure that restricts orbital overlap, resulting in a blue shift in the absorption spectrum in comparison to 3D perovskites.

Recent research has been focused on developing 2D perovskites with electrically active organic ligands by incorporating conjugated systems that can interact with and modulate the optoelectronic properties of the inorganic layers, moving beyond their traditional role as insulators.

Fig.9 illustrates three distinct energy diagrams for 2D perovskites [17,96]. In Fig.9(a), a 2D perovskite with an insulating organic ligand is shown to have a large bandgap and feature a type I energy alignment. In this case, the conduction band (CB) of the organic layer is considerably higher than that of the inorganic layer, while the valence band also resides at a higher energy level. This large bandgap creates a barrier for charge transport, effectively confining charge carriers within a quantum well around the inorganic layer. For instance, (PEA)₂PbI₄, which shows strong excitonic emission, is a potential candidate for LED applications.

In contrast, 2D perovskites with organic ligands that have a smaller bandgap can display a different energy alignment, as depicted in Fig.9(b). Furthermore, when the bandgaps of the organic and inorganic layers are mismatched, a type II heterostructure is formed (Fig.9(c)). This donor−acceptor (D−A) arrangement facilitates low-energy absorption due to D−A interactions between the layers, significantly reducing the bandgap of the 2D perovskite. Such a configuration enhances charge transfer and minimizes recombination losses, rendering it appropriate for the use of solar cells.

Blum’s team explored the use of oligothiophene-based ligands (AE_nT²⁺, with n indicating the thiophene ring count) in 2D halide perovskites, which enhance hole transport. Their research quantitatively predicted that the electronic structure and carrier delocalization in these perovskites can be fine-tuned by altering the thiophene units in the organic ligands and the halides within the [PbX₆]⁴⁻ octahedra [97]. Similarly, Kumar and Vasudevan [98] performed both experimental and computational studies on the optical bandgap variations in linear alkyl mixed-halide perovskites, (C_n)₂PbCl_4(1−y)Br_4y and (C_n)₂PbBr_4(1−y)I_4y (n = 4, 8). Their findings showed a direct correlation between the optical bandgap and the halide composition.

Gao et al. [99] proposed a molecular design approach integrating fluorination and conjugation engineering, demonstrating that 2D DJ hybrid organic-inorganic perovskites (HOIPs) could achieve tunable bandgaps by substituting hydrogen with fluorine in organic cations with different conjugation extents. First-principles calculations revealed that the organic−inorganic interface could be systematically modified to achieve any type of energy level alignments between I or II, allowing for bandgap tunability from 2.06 to 2.68 eV [100]. These theoretical predictions aligned with experimental outcomes using AE₂T and AE₄T ligands [101,102], demonstrating the potential to manipulate electronic properties, such as charge separation and recombination, by adjusting the energy level alignment between inorganic and organic layers.

Dou and his team [103] experimentally demonstrated the design of 2D perovskites with distinct type I and type II energy level alignments by using tailored conjugated organic ligands. Photoluminescence (PL) studies of (2T)₂PbI₄ confirmed a type I heterojunction, while introducing a narrow-bandgap ligand, BTm, inverted the type I heterojunction, with PL emission originating from the organic layers in (BTm)₂PbI₄. Additionally, (4Tm)₂PbI₄ and (4TCNm)₂PbI₄ exhibited type II heterojunctions, with staggered HOMO-LUMO levels in the organic ligands and conduction and valence bands in the inorganic layers. Incorporating electron-withdrawing cyano groups further inverted the energy level offset, quenching 99.9% of emission from both organic and inorganic layers. This indicated efficient charge separation, occurring within 10 ps and leading to long-lived charge-separated states on the nanosecond scale [104]. Ou et al. [105] also investigated how electron-phonon coupling can induce a transition between type I and type II heterostructures in (2T)₂PbI₄, demonstrating the dynamic tunability of energy level alignments.

Together, these experimental and computational results highlight the feasibility of engineering energy and charge transfer in 2D halide perovskites through bandgap tailoring of conjugated organic ligands. This capability opens up new possibilities for discovering novel electronic structures and characteristics in these sophisticated materials.

Recently, Liu et al. [97] delved into how the conjugation of organic spacers affects the electronic structure of 2D perovskites They integrated three distinct conjugated spacers—PEA⁺, naphthylethanamine (NEA), and pyrenylbutanamine (PyBA)—into 2D/3D perovskite films, each with an increasing number of fused aromatic rings. The study discovered that the (PEA)₂PbI₄ perovskite has a quantum well electronic structure characterized by a type I band alignment, where both the VBM and CBM are predominantly situated within the inorganic regions (as depicted in Fig.10(a)). Utilizing NEA caused the VBM to partially shift into the organic spacers (Fig.10(b)), suggesting that the increasing conjugation from PEA to NEA progressively alters the type I band alignment. Conversely, the use of PyBA caused the VBM to be completely confined within the PyBA spacer layers (Fig.10(c)), resulting in a type II band alignment [106].

Doping plays a critical role in modulating the electronic and band structures of materials, and in 2D perovskites, B-site doping is particularly important for tailoring electronic properties and optoelectronic performance. Eperon et al. [107] synthesized 2D layered Sn–Pb perovskite solid solution thin films, (PEA)₂Sn_xPb_1−xI₄ (x = 0, 0.3, 0.5, 0.7, 1), with tunable crystallinity, morphology, band structures, and charge transport properties. They observed a bandgap bowing effect in these films, similar to that found in 3D perovskites, and noted that the valence band maximum (VBM) could be adjusted, making these materials promising for transistor applications. All films exhibit thermally activated semiconducting behavior. While (PEA)₂PbI₄ shows in-plane ion migration, (PEA)₂SnI₄ does not. Notably, the incorporation of both Sn and Pb effectively suppresses ion migration and reduces conductivity.

As a result, the (PEA)₂Sn_0.7Pb_0.3I₄ film exhibits reduced ion migration and low electronic conductivity. Increasing the Pb ratio further reduces hole depletion and weakens the current at zero gate voltage. Among all tested devices, (PEA)₂Sn_0.7Pb_0.3I₄ exhibits the most tremendous performance, with a hole mobility of 0.020 cm²V⁻¹s⁻¹, a threshold voltage of 8 V, and an on/off current ratio of 100 [108]. The incorporation of Sn into the perovskite structure introduced new Sn²⁺-derived valence and CBs, significantly altering the band structure of the material. This modification can affect the electronic properties of the perovskite, potentially improving the charge carrier dynamics and overall device performance [71].

4.2 Excitons and dielectric confinement

As previously discussed, 2D perovskites demonstrate strong quantum confinement because of the presence of insulating A-site cations. The notable dielectric contrast between the spacer layers and the inorganic layers further enhances the dielectric confinement effect. Together, these factors help stabilize excitons at room temperature. In contrast, 3D perovskites do not maintain stable excitons under illumination, as photoexcited electrons quickly transition to the CB as free charges. In 2D perovskites, however, excitonic characteristics remain significant even at ambient temperatures, thanks to their high exciton binding energy (greater than 100 meV), which is roughly four times that of 3D perovskites. This increase is attributed to both dielectric confinement and quantum effects.

To explore excitonic behavior, researchers measured the optical absorption spectrum of 2D perovskites at very low temperatures, as shown in Fig.11(a) [109]. The onset of the bandgap (E_g) appears as a step-like feature, typical of the 2D density of states. Furthermore, a distinct peak at lower energy indicates stable exciton absorption. The exciton binding energy (E_b) can be estimated by calculating the energy difference between the bandgap energy and the exciton peak, as shown in Fig.11(b). This method offers a detailed understanding of the excitonic properties and the factors contributing to the unique optoelectronic characteristics of 2D perovskites.

The Mott-Wannier exciton model offers a comprehensive explanation for the exciton dynamics observed in 2D perovskites. According to this model, excitons generated by photon absorption undergo a relaxation process, leading to the formation of excitonic states at different energy levels, labeled as s1, s2, s3, and so on. Among these states, the 1s excitons, which occupy the lowest energy state, are the most stable and essential to the performance of optoelectronic devices [110]. The stability of these 1s excitons, relative to the CB minimum, is directly related to the exciton binding energy. In 2D perovskite semiconductors, the electrostatic attraction between electron-hole pairs greatly influences the probabilities of exciton recombination and dissociation. This interaction plays a fundamental role in the strong excitonic behavior that is essential for the material’s optoelectronic characteristics.

Due to strong hole-lattice or electron-lattice coupling, 2D perovskites have a pronounced presence of self-trapped excitons (STEs). These STEs facilitate radiative recombination, making the emission of light energetically advantageous and rendering 2D perovskites as promising candidates for light-emitting applications [111]. Their layered structure further amplifies exciton localization and emission efficiency [112]. Interestingly, while 2D perovskites have been made as emissive materials for several decades, in solar cell applications, STEs can hinder charge transport by trapping excitons instead of allowing them to dissociate into free carriers. The PCE of solar cells is significantly affected by the exciton recombination rate and the charge transport barriers present between the inorganic layers in 2D perovskites [113]. As discussed in Section 2.2, achieving a long-range vertical orientation in 2D perovskites is crucial for reducing recombination rates and increasing carrier diffusion distance. Additionally, reducing the dielectric contrast between the inorganic and organic components can lower potential barriers, thus enhancing charge transport in solar cells that incorporate 2D perovskites.

Gao et al. [114] researched the effects of triple cations (Cs⁺, FA⁺, MA⁺) on 2D perovskites and PSCs. Compared to monocation-based perovskite layers, triple cation perovskite films demonstrated smoother and more compact surface morphologies, larger grain sizes, and fewer grain boundaries. These improvements led to extended carrier lifetimes and enhanced conductivity, resulting in significantly higher PCEs in PSCs using triple cations compared to those based on monocations or binary cations.

Beyond solar cells, recent experiments have highlighted the tremendous potential of 2D perovskites across a range of applications. These materials exhibit unique properties, including high PL quantum yield, tunable bandgap, and excellent charge transport characteristics, making them highly promising for optoelectronic devices like photodetectors. Their intrinsic defect tolerance further enhances their suitability for high-performance photodetection, providing features such as high responsivity and low dark current [115]. Notably, 2D layered perovskites often outperform their 3D counterparts in photodetector applications. For instance, Li et al. synthesized 2D lead-halide hybrid perovskite single crystals with the ACI phase, GAMA₂Pb₂I₇ (GA = C(NH₂)₃, MA = CH₃NH₃) using a simple cooling crystallization method. The fabricated photodetector, featuring an Au/GAMA₂Pb₂I₇/Au structure, demonstrated broad-spectrum light detection across the entire visible range. The photodetector demonstrated high photoresponsivity values of 1.56, 2.54, and 2.60 A/W under a −1.5 V bias for incident light at 405, 532, and 635 nm at 9.82 nW, respectively [116].

In addition to photodetectors, advances in nanotechnology have revealed the potential of 2D perovskites in LEDs. These materials benefit from enhanced radiative recombination, efficient energy transfer, superior stability, and highly tunable properties [117]. For example, the inherent quantum confinement of 2D layered perovskite (PEA)₂PbBr₄ enables PL at shorter wavelengths (410 nm) in comparison to its 3D counterpart, Liang et al. successfully employed the SVA method to fabricate room-temperature violet LEDs made from this 2D perovskite material. These LEDs emit at 410 nm with a narrow bandwidth (FWHM: 18 nm) and enhanced external quantum efficiency [118]. These findings underscore the versatility of 2D perovskites and their potential to revolutionize applications far beyond conventional PSCs. Coupled with ongoing innovations in material engineering, 2D perovskites hold great promise for unlocking new possibilities across various industries.

5 Stability

To achieve widespread implementation of perovskite devices, stability stands as a pivotal factor. In this regard, 2D perovskites show greater inherent stability compared to their 3D counterparts.

5.1 High formation energy

Section 2 highlights the crucial role of organic components in strengthening the crystal lattice structure of perovskites. The ionic and covalent bonds, along with intermolecular forces, contribute significant to the enhanced longevity of 2D perovskites. For instance, in the RP phase of 2D perovskites, the spacer layers are reinforced by interactions between spacer cations through van der Waals forces. This results in greater stability in 2D perovskites, as the stronger interactions among their larger spacer cations compared to the MA⁺ cations found in 3D perovskites raise the energy barrier for the decomposition of the 2D perovskite lattice [119].

Research by Sargent and his colleagues revealed that the energy required to transform MAI and PEAI from the solid phase to the gaseous phase within the perovskite structure is 2.15 and 2.51 eV, respectively. This finding implies that MAI can be more readily released from the perovskite structure compared to PEAI, making the [BX₆]⁴⁻ octahedral structure more prone to instability. Furthermore, density functional theory (DFT) computations demonstrate that as the n values of the phase increase, the formation energy diminishes. Consequently, the desorption frequency of spacer cations in PEA₂PbI₄ films drops by six orders of magnitude in contrast to MAPbI₃, leading to a decomposition rate of 2D perovskite films that is 1000 times slower than their 3D counterparts [119].

In the pursuit of higher formation energy, progress has been made in developing spacer cations, including fluorine-modified molecules, hydrogen-bonded structures, and diammonium-based compounds. For example, 2D PSCs with F-PEA demonstrate greater thermal stability than those with PEA [32]. In the DJ phase, the inorganic slab is connected via covalent interactions with diammonium cations, leading to a more robust lattice compared to the RP phase. However, some DJ phase perovskites exhibit poor environmental stability. The relevant theory on this is being continuously supplemented.

Liu et al. [120] proposed, based on comparative experiments, that perovskites with moderately rigid cations have higher structural stability, because appropriate cation rigidity assists in the geometric adjustment of organic diammonium and inorganic octahedra, fostering mutual adaptation. Yang et al. studied the stability and dimensionality reduction behavior of DJ perovskites composed of aromatic diammonium cations, which are closely related to the π-π stacking interactions of aromatic diammonium cations. The strength of π-π stacking interactions is mainly affected by the symmetry and polarity of aromatic diammonium cations. The study found that there is a strong π-π stacking between highly symmetric and highly polar benzene diammonium cations, which is prone to degradation, while there is no π-π stacking between non-symmetric and non-polar benzene diammonium cations, which can form stable DJ-type 2D perovskites [121].

While the research on the stability mechanism of 2D perovskites is ongoing, new experimental findings continue to emerge. For example, Zhang synthesized a series of novel slightly interlayer displaced DJ perovskite materials using flexible 1,4-cyclohexanediammonium. The solar cells fabricated through the doctor-blade process achieved a conversion efficiency of 19.11%. Furthermore, the unencapsulated cells, after operating at the maximum power point for more than 6000 h under continuous illumination of 100 mW·cm⁻² at 45 °C, had negligible efficiency loss [122].

5.2 Suppressed ions migration

Ion migration poses a substantial challenge to the stability of perovskite materials. Generally, 3D perovskites have a low energy barrier for defect formation [123–127], resulting in a high equilibrium defect concentration. Studies suggest that the major point defects in perovskite structures give rise to shallow traps, as the formation of deep traps is less likely [127–131]. The presence of these shallow traps near the conduction or valence band edges causes perovskites to self-dope in either n-type or p-type manners [129]. While these shallow traps do not contribute to non-radiative recombination processes, their ionic properties enable them to move or diffuse when an electric field is applied [132]. The redistribution of these mobile defects can cause band bending [133] and J‒V hysteresis in solar cells [134–139]. Moreover, it can lead to phase separation or undesirable chemical reactions with external electrodes [140].

Studies have demonstrated that the grain boundaries and surfaces of perovskite materials serve as main pathways for the migration of mobile ions [141], emphasizing the significance of defect passivation in controlling ion migration. It has been proposed that overlaying 2D perovskites on 3D layers could help passivate surface defects. Consistently, several studies have associated the improved stability of 2D/3D PSCs with the effective passivation of the photo-absorbing layers by 2D perovskite caps [142].

The migration of mobile ions towards the interfaces of perovskite with electron transport layers (ETLs) and hole transport layers (HTLs) can result in undesirable chemical reactions, contributing to device degradation [132,143,144]. Carrillo et al. [145] observed that I⁻ ions migrating to the perovskite-HTL interface reacted with oxidized Spiro-OMeTAD⁺, forming neutral Spiro-OMeTAD-iodine complexes. This irreversible interaction at the boundary between perovskite and Spiro-OMeTAD reduces the conductivity of the HTL, negatively impacting the efficiency of the solar cell. Furthermore, mobile iodine species can interact with organic HTLs and corrode certain metal electrodes, including silver [146,147]. Consequently, when using 2D layers to improve stability, it is crucial to prevent the migration of halogen ions and cations.

The introduction of bulky organic ligands into 2D perovskite capping layers has been demonstrated to suppress ion migration in 2D/3D heterostructured perovskite photovoltaics [148,149]. Mathew’s research demonstrates that incorporating PEA as aromatic spacer cations into perovskites enhances structural stability due to the higher rigidity of PEA, significantly mitigating halide segregation compared to BA-based systems [150]. Similarly, Zhang and colleagues introduced aromatic spacers with molecular conjugation into 2D perovskites between 3D perovskites and ETLs. The PyBA spacer significantly increased the resistance of 2D/3D PSCs to moisture and ion migration, thereby enhancing stability. Notably, PyBA-based devices retained over 90% of their initial PCE after 2000 h at 25 °C and 80% relative humidity, 1000 h at 85 °C and 85% humidity (Fig.12(a)), or 3000 h under continuous 1-sun illumination at 40 °C (Fig.12(b)), demonstrating exceptional stability compared to previous 2D/3D PSCs [106].

Conversely, ion segregation efficiency increases with the number of layers [150]. Cho and his team found that light-induced halide ion separation in RP type 2D lead halide perovskites decreases as the number of layers reduces. The efficiency dropped dramatically from 20% at n = 10 to less than 1% at n = 1 (Fig.12(c)), and this trend was fitted with a function (Fig.12(d)). They propose that exciton binding in 2D perovskites hinders charge separation, thereby slowing halide ion movement and separation. This finding underscores the importance of controlling 2D structure dimensions to limit halide ion migration [151].

For cation migration, commonly used PEA⁺ and BA⁺ easily react with 3D perovskite layers under high-temperature illumination, deprotonating to form amines, which limits the high-temperature photostability of the devices. In response to this known phenomenon, Wang’s team utilized PEAMA⁺ and BAMA⁺ to replace PEA⁺ or BA⁺ in forming 2D perovskites for surface passivation of 3D perovskites. The resulting solar cells maintained over 90% of their initial efficiency after being exposed to light for 1500 h at 90 °C and under open-circuit conditions. This approach presents a different path to enhance the high-temperature photostability of solar cells [152].

Cation migration in perovskite materials is a recognized issue, particularly with the commonly used PEA⁺ and BA⁺ cations, which can react with 3D perovskite layers under high-temperature illumination, resulting in deprotonation and the formation of amines, which negatively impact the high-temperature photostability of perovskite devices. To address this challenge, Huang’s group substituted PEA⁺ or BA⁺ with PEAMA⁺ and BAMA⁺ to create 2D perovskites for the surface passivation of 3D perovskites. The modified solar cells demonstrated remarkable stability, retaining over 90% of their initial efficiency after 1500 h of light exposure at 90 °C under open-circuit conditions. This innovative approach offers a promising solution to bolster the high-temperature photostability of PSCs [152].

5.3 Hydrophobicity

Among the environmental factors impacting the stability of perovskite devices, humidity and oxygen stand out as particularly ubiquitous challenges [153]. Understanding their contribution to the degradation of metal halide perovskite structures is essential. The ability of the device stack to resist external stressors determines the extent of packaging required, which in turn can impact manufacturing costs. Air-stable perovskite devices can be fabricated under normal environmental conditions, significantly reducing production expenses compared to the expensive, dry, inert atmospheres typically required for perovskite fabrication. This resistance to environmental stressors is especially important for the development of flexible perovskite devices [153].

While advancements in flexible electronics packaging have improved barriers materials and lowered costs, current encapsulation methods are often either too permeable to moisture and air or too expensive compared to the dense glass layers used in rigid packaging [154,155]. Even the best packaging methods can fail, and perovskite devices are often exposed to air as they move through the manufacturing processes before they are packaged. Therefore, the primary goal, regardless of whether the devices are rigid or flexible, is to strengthen the inherent resistance of perovskite devices to water and oxygen exposure [153].

The 2D/3D hybrid perovskite combines the advantages of both architectures: the 3D component provides high-efficiency and broad-spectrum light absorption for photovoltaic applications, while the 2D layer enhances resistance to environmental factors. The authors, using the solid-phase hot-press deposition method, created a uniform 2D perovskite layer on the surface of the 3D perovskite layer. The durability of this film was evaluated through XRD analysis, with both 3D and 2D/3D perovskite samples stored for 1000 h at room temperature and a relative humidity (RH) of 25%. As shown in Fig.13(a) and 13(b), a new peak appeared at 12.78° in the 3D perovskite samples, corresponding to PbI₂, which was hardly visible in the 2D/3D samples [156]. These findings demonstrate that the 2D passivation layer significantly improves the long-term stability of perovskite films.

Seok’s group systematically investigated alkylammonium-based 2D capping layers with varying hydrocarbon chain lengths, specifically comparing BAI, octylammonium (OAI), and dodecylammonium (DAI) iodide derivatives [157]. They found that progressively elongating the alkyl chains (BA → OA → DA) in 2D/3D perovskites enhanced moisture resistance, though extended hydrocarbon moieties exhibit a hydrophobicity-stability tradeoff. While long-chain alkylammonium modifications improve environmental resilience, their compromised interlayer charge transport limits photovoltaic performance enhancements. To reconcile this dichotomy, Liu’s team engineered a mixed-cation 2D/3D architecture by incorporating halogenated spacers—2-chloroethylammonium hydrochloride (CEA) and 2-bromoethylammonium hydrobromide (BEA)—into cesium/formamidinium 3D perovskite matrices. This molecular design strategy achieved several synergistic improvements: halogen-functionalized ammonium salts elevated film’s hydrophobicity (water contact angle analysis, as shown in Fig.13(d)‒Fig.13(f)), and the optimized 2D/3D heterostructure retained 92% of its initial PCE after 2400 h of aging under (50 ± 5)% relative humidity (Fig.13(c)), demonstrating an unprecedented humidity stability [158].

6 Summary and perspective

In summary, a comprehensive understanding of 2D perovskite chemistry is essential for advancing their application in optoelectronics. This review has systematically explored key aspects of 2D perovskites, including their crystal structure, thin-film properties, optoelectronic behavior, and stability. These insightful investigations confirm that 2D perovskites hold significant potential in addressing the long-term stability challenges typically encountered by perovskite devices. However, despite these advantages, several emerging challenges still remain that require further investigation for a deeper and more complete understanding of these materials.

1) Quantum and dielectric confinement effects: The insulating spacer layers in 2D perovskites cause quantum confinement and dielectric confinement effects, which hinder charge carrier mobility and elevate exciton binding energy, resulting in lower optoelectronic efficiency, which is different from 3D perovskites. To address these limitations, it is essential to develop spacer cations that strike a balance between hydrophobicity and conductivity. Designing spacer cations with π-conjugated segments offers a promising approach to overcoming these challenges. For example, the incorporation of aromatic compounds within spacer cations can reduce exciton binding energy, enhance charge delocalization, and improve conductivity, all while maintaining long-term stability. Studies on innovative organic spacers, such as those incorporating thiophene or pyridine rings, have demonstrated the potential to achieve better stability and performance comparable to 3D perovskites. These advancements highlight the importance of tailoring spacer chemistry to optimize the trade-off between stability and optoelectronic efficiency in 2D perovskite materials.

2) Crystallization kinetics and phase distribution: The crystallization kinetics and associated phase distribution in 2D perovskites remain elusive and represent a critical challenge for material optimization. Despite advancements in solvent engineering and component modulation to influence phase arrangement, achieving strictly pure-phase films continues to be a formidable task, primarily due to limited understanding of the crystallization process. The crystallization process involves complex interactions among solvents, precursors, and thermal treatments. However, the mechanisms determining phase purity remain unclear. Future research should prioritize uncovering the factors controlling phase purity and developing methodologies to optimize composition specificity across diverse perovskite chemistries. Additionally, it remains an open question whether pure-phase films unequivocally outperform aligned multi-phase architectures in optimizing device performance. For instance, in typical 2D perovskite BA₂MA₃Pb₄I1₃ (n = 4), both pure-phase and multi-phase structures offer distinct advantages. A homogeneous pure-phase (n = 4) film with vertical orientation facilitates efficient charge transport across the film, minimizes charge recombination, and enhances the open-circuit voltage (VOC) in solar cells. Conversely, aligned multi-phase structures, characterized by a gradient phase distribution, exhibit low n-value phases at the bottom transitioning to higher n-value phases at the top. This configuration improves JSC by enabling exciton dissociation, facilitating energy transfer, and broadening spectral absorption. Optimizing VOC and JSC for specific applications requires further studies. These studies should quantify performance differences between single-phase and multi-phase films under the same conditions. Comprehensive studies of phase formation dynamics, transitions, and their influence on device performance will be instrumental in advancing 2D perovskite solar cell technologies.

3) Photo-physics of 2D perovskites: The fundamental photo-physics of 2D perovskites is less studied than material processing, even though it plays a vital role in optoelectronic advancements. Future studies should prioritize the following aspects to deepen our understanding and improve device performance. In the first place, exciton-phonon couplings significantly impact charge transport, recombination, and energy loss mechanisms. Understanding these interactions can provide insights into optimizing energy transfer processes and reducing thermal losses, thereby improving device efficiency. Exploiting exciton-phonon dynamics can also guide strategies to mitigate issues such as non-radiative recombination and enhance charge carrier mobility. In the second place, investigating the mechanisms of charge carrier generation, separation, and recombination is crucial, as they are influenced by the unique layered architecture of 2D perovskites. By understanding these dynamics, researchers can utilize anisotropic charge transport properties. This insight will support the development of efficient and stable devices. Lastly, energy losses during energy transfer and charge recombination are key challenges that reduce device efficiency. Material design improvements and interface engineering strategies offer potential solutions to mitigate these losses. By optimizing interfaces and aligning energy levels, energy transfer can be made more efficient, reducing recombination rates. The layered structure of 2D perovskites enables efficient energy transfer and optoelectronic anisotropy, making them highly promising for applications in photodetectors, LEDs, and field-effect transistors (FETs). Understanding the photo-physics of 2D perovskites will enhance current technologies and enable new optoelectronic applications.

4) Environmental concerns and sustainability: The large-scale application of 2D perovskite materials may bring environmental challenges. In fact, lead halide perovskites are extensively researched because of their superior optoelectronic characteristics. However, lead, which is a toxic and hazardous material, will damage the human body and pollute the environment. More concerns have been raised over material disposal, leakage during device operation, device maintenance, and recycling processes. Perovskites without lead are under development, such as those based on Sn, Bi, or Sb. Unfortunately, compared to lead-based perovskites, these alternatives often have many disadvantages, such as instability, subpar electronic properties, and lower efficiency. Sustainable development and commercialization of 2D perovskite materials need to face these concerns. So proper materials design, device encapsulation, and device management are important to the future of 2D perovskite materials.

5) Large-scale fabrication and commercialization: Perovskite materials hold great promise for optoelectronics, however, challenges in large-scale fabrication and commercialization still need to be addressed. The preparation of perovskite materials often requires precise control under laboratory conditions to ensure the excellence of their crystal structure. However, there are still a number of problems with scaling this process to mass production, such as equipment standardization, batch consistency, and process precision. Additionally, although low-cost methods such as solution processing are employed in the current production of PSCs and other optoelectronic devices, the production speed remains insufficient to meet the demands of large-scale industrial applications. Accelerating the manufacturing process while maintaining high-quality output is a critical hurdle on the path to commercialization. Moreover, while perovskite materials boast performance advantages, they must also contend with competitive pressures from well-established technologies, such as silicon-based solar cells and other mature photovoltaic solutions, to secure a foothold in the market.

Addressing these challenges will be the key for realizing the full potential of 2D perovskites in advanced optoelectronic applications. Future research should focus on overcoming these barriers to maximize their performance, stability, and scalability. With continued advancements, 2D perovskite materials are expected to offer new insights and lead to groundbreaking discoveries in the field of optoelectronics.

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