Influence of precursor concentration on printable mesoscopic perovskite solar cells

Abstract

Over the last decade, the power conversion efficiency of hybrid organic–inorganic perovskite solar cells (PSCs) has increased dramatically from 3.8% to 25.2%. This rapid progress has been possible due to the accurate control of the morphology and crystallinity of solution-processed perovskites, which are significantly affected by the concentration of the precursor used. This study explores the influence of precursor concentrations on the performance of printable hole-conductor-free mesoscopic PSCs via a simple one-step drop-coating method. The results reveal that lower concentrations lead to larger grains with inferior pore filling, while higher concentrations result in smaller grains with improved pore filling. Among concentrations ranging from 0.24–1.20 M, devices based on a moderate strength of 0.70 M were confirmed to exhibit the best efficiency at 16.32%.

Key words： printable perovskite solar cell (PSC); precursor concentration; crystallization; morphology

Introduction

Over the past decade, perovskite solar cells (PSCs) have been the focus of much research [¹–⁸]. The record-certified power conversion efficiency (PCE) of PSCs has increased rapidly to 25.2%, rivaling the optimum efficiency of silicon-based solar cells [²]. This rapid increase in efficiency is largely attributed to the improved control of the crystallinity and morphology of halide perovskites, which generally have the formula of ABX₃, where A is a monovalent cation, such as methylammonium (MA⁺); B is a divalent metal, such as lead (Pb²⁺); and X is a halogen anion, such as iodide (I⁻) [⁹–¹¹]. Generally, properties of good crystallinity, such as large grain size, along with well-developed orientation are required for inhibiting non-radiative recombination and promoting charge-carrier transport in halide perovskite films [¹²,¹³]. Additionally, morphological preferences, such as good uniformity and effective coverage, are essential for absorbing light and inhibiting shunt caused by undesired contact between the other layers in PSCs [¹²,¹³].

Since halide perovskites are solution-processable, most PSCs are currently prepared via solution-processed methods [²]. The composition of the precursor is fundamental in the processing of solutions [¹⁴,¹⁵]. For example, the precursor of MAPbI₃ is usually prepared by directly dissolving methylammonium iodide (MAI) and lead iodide (PbI₂) in selected solvents, such as dimethylformamide (DMF) or g-butyrolactone (GBL) [¹⁶–¹⁸]. An intensive study on the precursor of MAPbI₃ revealed that the precursor is not a pure solution; since a typical Tyndall effect is easily observed when a laser passes through the precursor, it can be defined as somewhat colloidal [¹⁹]. These colloidal precursor particles can be as large as 1 µm [¹⁹]. In the crystallization process of halide perovskites from their precursors, the existing colloidal particles function as nucleation sites during film preparation, thereby influencing the quality of the obtained films [¹²,¹³]. Additionally, the colloidal nature of the precursor is influenced by its concentration; a lower concentration can help to promote the precursor to a real solution [¹⁹]. Therefore, the concentration of the precursor is fundamental for controlling both the crystallinity and morphology of halide perovskites.

The influence of precursor concentrations has been demonstrated via the sequential deposition method in which PbI₂ is spin-coated and MAI is introduced after [²⁰]. Bi et al. confirmed that for the preparation of perovskite films with good crystallinity and morphology, a moderate PbI₂ concentration of 1.0 M is the most appropriate [²¹]. They found that a 1.0-M precursor resulted in an efficiency of 13.99%, while an efficiency of 1.4% was obtained at a concentration of 0.1 M. Zhang et al. discovered that a high MAI concentration is beneficial for obtaining high-quality perovskite films [²²]. In their study, MAI at a concentration of 8 mg/mL resulted in an efficiency of 7.46%, while a concentration of 15 mg/mL produced an efficiency of 12.76%. Furthermore, the influence of precursor concentration has also been demonstrated in the antisolvent method [¹⁷], in which an antisolvent is dripped during the spin-coating of the precursor. Wieghold et al. confirmed that larger and more oriented grains resulted from higher precursor concentrations [²³].

However, research is currently lacking on the impact of precursor concentration in a simple one-step drop-coating method [²⁴,²⁵]. This approach is generally applied in the fabrication of printable mesoscopic hole-conductor-free PSCs (PMPSCs). PMPSCs comprise a printed porous TiO₂/ZrO₂/C triple layer filled with halide perovskites and are a particular type of PSC [²⁶]. When used together with C electrodes, the application of these devices is seen as commercially promising in terms of low cost and improved stability [²⁷]. PMPSC efficiency is dependent on the control of the crystallinity and morphology of halide perovskites in the porous structure. The crystallization process in a simple one-step drop-coating method is different from both the sequential deposition approach and the antisolvent method since the simple drop-coating method involves a much slower crystallization process. Therefore, studying the influence of precursor concentrations in the simple one-step drop-coating method is important for the successful development of PMPSCs.

This study investigates the influence of precursor concentrations between 0.24–1.20 M on the crystallinity and morphology of drop-coated MAPbI₃ perovskites and explores their impact on the performance of printable mesoscopic hole-conductor-free PSCs. It is demonstrated that a lower concentration of 0.24 M leads to the formation of larger grains, while better coverage is obtained with a higher concentration of 1.20 M. In terms of device performance, a moderate concentration of 0.70 M produces the highest efficiency of 16.32%, 0.24 M leads to an efficiency of 15.38%, while a concentration of 1.20 M produces an efficiency of 15.89%.

Experimental section

Materials&chsp;PbI₂ and MAI were obtained from p-OLED, and 5-Ammonium valeric acid iodide (5-AVAI) was sourced from Shanghai MaterWin New Materials Co., Ltd. Sigma-Aldrich provided the GBL, and ethanol was obtained from Sinopharm Chemical Reagent Co., Ltd. The pastes for the mesoscopic layers were sourced from WonderSolar Co., Ltd. All materials were used as received without further purification.

Precursor preparation&chsp;The MAPbI₃ precursor with a concentration of 1.2 M and additional 5-AVAI was prepared by dissolving 1.5362-g MAI, 0.0828-g 5-AVAI, and 4.61-g PbI₂ in 8.3 mL of mixed solvent containing GBL and ethanol (9:1 volume ratio) [²⁸]. The other concentrations of 0.24, 0.41, 0.55, 0.70, 0.83, and 1.00 M were obtained by diluting 1.2 M of precursor with the mixed solvent.

Device fabrication&chsp;The fluorine-doped tin oxide (FTO) glasses were laser-etched with the desired patterns and were cleaned by ultrasonication in water with detergent, deionized water, and ethanol, respectively. A compact layer of TiO₂ (c-TiO₂) was then deposited on the FTO glass by spray pyrolysis at 450°C. This was followed by the screen printing and annealing of the porous TiO₂ (mp-TiO₂) layer (700 nm, annealed at 500°C), the ZrO₂ mp-TiO₂ layer (3 µm, annealed at 500°C), and the C layer (10 µm, annealed at 400°C). After cooling to room temperature, a simple one-step drop-coating method infiltrated the precursor into the porous TiO₂/ZrO₂/C scaffold. After slow drying at 50°C, printable hole-conductor-free mesoscopic PSCs were obtained.

Characterization &chsp;A field-emission scanning electron microscope (SEM) (Nova NanoSEM 450) was used to obtain both top-view SEM images of the perovskite-infiltrated porous ZrO₂ layers and cross-sectional images of the devices. The ultraviolet–visible (UV–Vis) spectra were measured using a PerkinElmer Lambda 950 spectrophotometer. Time-resolved photoluminescence (TRPL) decay transients were measured with a Horiba Jobin Yvon FluoroMax-4 fluorimeter. X-ray diffraction (XRD) measurements were obtained using an X’Pert PRO X-ray diffractometer. The incident photon conversion efficiency (IPCE) was measured using a 150-W xenon lamp (Oriel) fitted with a monochromator (Cornerstone 74004) as a monochromatic light source. Photocurrent density–voltage (J–V) curves of both reverse scan (1.2 to -0.2 V) and forward scan (-0.2 to 1.2 V) together with steady outputs were characterized with a Keithley 2400 source/meter and a Newport solar simulator providing light with a spectral distribution of AM 1.5 G. A black mask with a circular aperture of 0.1 cm² was applied on top of the cell.

Results and discussion

Figure 1 presents the top-view SEM images of drop-coated perovskite films formed from precursors of different concentrations on porous ZrO₂ layers. At a concentration as low as 0.24 M, the ZrO₂ layer is partially covered by perovskite films with distinguishable bare areas (Fig. 1(a)). Coverage improves as the concentration increases, and at concentrations of 0.7 M and above, complete coverage is achieved. SEM images with higher magnification of films obtained from precursors at concentrations of 0.24, 0.70, 0.83, and 1.20 M were also acquired (as shown in Figs. 1(i)–1(l); they reveal that the perovskite domains consist of small grains. The grain sizes in the films from the 0.24-M precursor are noticeably larger than those obtained from the 1.20-M precursor.

The perovskite’s crystallization process from the precursor in the porous substrate is complicated and involves three probable nucleation processes: the inducement of heterogeneous nucleation by the substrate, the inducement of heterogeneous nucleation by colloidal particles in the precursor, and the inducement of homogeneous nucleation by the real solution portion of the precursor. Since the crystallization process of the simple drop-coating method is quite slow, homogeneous nucleation is not dominant. Therefore, the crystallization process is determined by both the substrate and the colloidal properties of the precursor.

Considering that substrates are identical, the variations in coverage mainly result from different crystallization processes induced by varying precursor concentrations. During the annealing process, there is fast precipitation of large colloidal particles in precursors with higher concentrations that results in the uniform distribution of multiple nucleation site on the substrates. Consequently, the perovskite films are continuous and provide good substrate coverage. At low concentrations, there are too few nucleation sites to provide good substrate coverage, and the crystallization process is slower. The slow removal of solvent at 50°C during annealing results in larger-sized grains. This process differs from the spin-coating methods used for preparing perovskite films in which the crystallization process is generally quite fast, and the growth time of the grains is quite short. However, although the 0.24-M precursor results in larger grain sizes, they are not large enough to connect together to form a continuous perovskite film.

Consequently, the authors suggest that if the number of nucleation sites can be reduced, and the crystallization process can be optimized by inhibiting the colloidal nature of the precursor via appropriate control, the method of establishing continuous and uniform perovskite films with larger grain sizes by optimizing precursor concentrations is promising.

Fig.1 Top-view SEM images of drop-coated perovskite films on a porous ZrO₂ layer with different precursor concentrations at 10000´ magnification: (a) bare ZrO₂, (b) 0.24, (c) 0.41, (d) 0.55, (e) 0.70, (f) 0.83, (g) 1.00, and (h) 1.20 M concentrations. At 80000´ magnification: (i) 0.24, (j) 0.70, (k) 0.83, and (l) 1.20 M concentrations

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Figure 2(a) presents the XRD patterns of perovskite films prepared on porous ZrO₂ layers. At different precursor concentrations, the XRD patterns exhibit a similar form with strong diffraction peaks at 14.02°, 19.92°, 28.32°, 31.76°, 40.46°, and 43.02°; these can be accurately indexed as crystal faces at (110), (112), (220), (310), (224), and (314). However, the results for light absorption reveal significant differences. As shown in the UV–Vis absorption spectra in Fig. 2(b), the absorbance of the film prepared with the 0.24-M precursor is much lower than those films prepared with precursors at higher concentrations. As discussed in Fig. 1, those films prepared with the 0.24-M precursor contain many bare areas, which signifies insufficient light absorption. Conversely, those continuous films prepared with higher concentration precursors result in stronger absorption.

This study also examined the carrier lifetimes in different perovskite films using TRPL measurements (Fig. 2(c)). The carrier lifetime in films prepared from a precursor of 0.24 M is clearly longer than in those films prepared from a precursor of 1.20 M. This indicates that low precursor concentrations lead to perovskite films with fewer defects. This effect is likely to be attributed to the enlarged grain size.

Fig.2 Optical characterization of drop-coated perovskite films on a porous ZrO₂ layer: (a) XRD patterns, (b) UV–Vis spectra, and (c) TRPL spectra of drop-coated perovskite films on a porous ZrO₂ layer

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To clarify the influence of precursor concentrations on device performance, this study fabricated printable hole-conductor-free mesoscopic PSCs. Figure 3(a) presents the PMPSCs in schematic form. PMPSCs comprise an FTO glass and a printed porous TiO₂/ZrO₂/C triple layer. After drop coating, perovskite infiltrates the porous triple layer and grows inside. Figure 3(b) is a typical cross-sectional SEM image of a PMPSC. Generally, the porous TiO₂ layer is about 700 nm thick, the porous ZrO₂ layer is about 3 µm in thickness, and the thickness of the C layer is about 10 µm. Figure 3(c) shows the energy-level alignment in the device. TiO₂ serves as an electron collection and transportation layer, C serves as the back electrode and collects holes, and ZrO₂ functions as a spacer to inhibit recombination by separating TiO₂ and C. Perovskite serves as the light absorber. Since no additional hole-transport materials are applied in PMPSCs, perovskite also serves as the hole conductor.

Fig.3 (a) Schematic, (b) typical cross-sectional SEM image, (c) energy-level alignment of PMPSCs, (d) J–V curves, (e) steady-state current density output, and (f) IPCE of PMPSCs filled with perovskite precursors at different concentrations

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Figure 3(d) contains the current J–V curves of the reverse scan (from open circuit to short circuit) corresponding to the peak efficiency of PMPSCs filled with precursors of different concentrations. These curves are relatively concentrated, signifying minimal variation in peak efficiency. This is quite different from the reported results that indicate wide variations in the efficiency of PSCs prepared with low- and high-concentration precursors. This could be attributed to structural differences in devices or in perovskite preparation methods. Due to the ZrO₂ layer in PMPSCs, non-continuous perovskite will not allow direct contact between the top and bottom layers. Additionally, the thick porous layer also aids light absorption. Furthermore, the slow crystallization process can, to some extent, ensure the crystallinity and morphology of perovskite.

Detailed parameters of the J–V curves (reverse scan from the short to the open circuit) together with the forward scan (from the open to the short circuit) performance parameters are presented in Table 1. The efficiencies of the reverse scan for PMPSCs obtained from different precursor concentrations of 0.24, 0.41, 0.55, 0.70, 0.83, 1.00, and 1.20 M are 14.29%, 14.84%, 15.11%, 15.31%, 15.06%, 14.85%, and 14.83%, respectively. The highest efficiency is obtained at a moderate concentration of 0.70 M. The efficiencies from the forward scan are 15.38%, 15.67%, 16.00%, 16.32%, 16.12%, 16.09%, and 15.89%, respectively. The highest efficiency obtained is 16.32%. The efficiency of the forward scan is evidently larger than that of the reverse scan, indicating an abnormal hysteresis phenomenon [²⁹,³⁰]. Previous research has demonstrated that this phenomenon is related to both charge accumulation and imbalanced charge extraction. To obtain an accurate PCE under conditions of hysteresis, a steady-state current density output under a given voltage bias from a maximum power point was conducted. The steady-state current density output results of those PMPSCs prepared with precursors of different concentrations are shown in Fig. 3(e). The calculated steady efficiencies are 14.35%, 14.65%, 15.09%, 15.35%, 15.16%, 14.98%, and 14.82%, respectively, which are similar to the efficiency values obtained by the reverse scan. It is evident from the steady-state current density that a low concentration of 0.24 M and a high concentration of 1.20 M lead to a lower current density than the moderate concentration of 0.7 M. This finding is consistent with the trend shown by the IPCE results in Fig. 3(f). At a low concentration of 0.24 M, the IPCE is lower due to insufficient pore filling. At a high concentration of 1.20 M, the device demonstrates a quicker drop of photon conversion efficiency in the band of 550–800 nm. This is probably due to the inability of smaller grains induced by higher concentrations to effectively absorb long-wavelength photons.

Tab.1 J–V parameters and steady-state outputs of PMPSCs prepared with precursors of different concentrations

	scan direction	VOC/V	JSC/(mA·cm−2)	FF	PCE/%	steady-state current density/(mA·cm−2)	bias voltage/V	steady-state PCE/%
0.24 M	reverse	0.92	23.00	0.68	14.29	19.93	0.72	14.35
	forward	0.96	22.91	0.70	15.38
0.41 M	reverse	0.94	23.10	0.68	14.84	20.35	0.72	14.65
	forward	0.98	23.04	0.69	15.67
0.55 M	reverse	0.96	23.23	0.68	15.11	20.96	0.72	15.09
	forward	0.98	23.20	0.70	16.00
0.70 M	reverse	0.94	23.39	0.70	15.31	21.32	0.72	15.35
	forward	0.96	23.32	0.73	16.32
0.83 M	reverse	0.94	23.31	0.69	15.06	21.05	0.72	15.16
	forward	0.96	23.26	0.72	16.12
1.00 M	reverse	0.94	23.04	0.69	14.85	20.81	0.72	14.98
	forward	0.98	22.93	0.72	16.09
1.20 M	reverse	0.94	23.10	0.68	14.83	20.59	0.72	14.82
	forward	0.96	23.02	0.72	15.89

This study also fabricated batches of PMPSCs to further verify the results of the influence of concentration on PMPSC performance. The statistical results are presented in Fig. 4. The open circuit voltage (V_OC) trend varies little with concentration, except at 0.24 M. As the concentration increases, the short circuit current density (J_SC) initially increases and then decreases. A concentration of 0.70 M results in improved J_SC. Additionally, the fill factor (FF) also changes with concentration, and the optimum result is achieved at 0.70 M; consequently, the optimum efficiency is obtained with 0.70 M. The resulting statistical PCE trend is consistent with that determined from the peak efficiency.

Fig.4 Statistical results of the performance of PMPSCs filled with perovskite precursors of different concentrations: (a) V_OC, (b) J_SC, (c) FF, and (d) PCE

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Figures 5(a)–5(c) are the cross-sectional SEM images of PMPSCs filled with 0.24, 0.70, and 1.20-M perovskite precursors. The performance of PMPSCs is determined by the perovskite filling in the triple layer. As shown in the areas marked by dashed lines in Fig. 5(a), it is evident that the pores are not well filled, indicating the existence of non-continuous perovskite. Both light absorption and charge transport are restricted by these unfilled areas, resulting in lower J_SC and FF values. Insufficient pore filling could also result in poor contact between perovskite and C or TiO₂, which affects the V_OC. Therefore, the efficiency of devices filled with 0.24-M perovskite is relatively poor. The filling at a concentration of 1.20 M is much better than that at 0.24 M. However, there remain multiple areas that are poorly filled. This is likely to be attributed to the rapid speed of the crystallization process from high-concentration precursors, which results in some pores not being filled in time. These unfilled areas may also induce recombination and affect light absorption, further restricting performance. At a concentration of 0.70 M, the device has a fairly compact cross section, indicating effective pore filling. This adequate filling and the superior crystallinity of halide perovskites ensure sufficient light absorption and charge-carrier transport, resulting in PMPSCs with optimum efficiency.

Fig.5 Cross-sectional SEM images of PMPSCs filled with perovskite precursors of different concentrations: (a) 0.24, (b) 0.70, and (c) 1.20 M

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This study further applied the optimized concentration of 0.70 M to fabricate additional PMPSCs. By adjusting the thickness of the triple layer, devices with smaller hysteresis were obtained. A typical optimized device exhibited a reverse-scan efficiency of 15.86% with a V_OC of 0.94 V, a J_SC of 23.42 mA/cm², and an FF of 0.72; the device produced a forward scan efficiency of 15.99% with a V_OC of 0.96 V, a J_SC of 23.33 mA/cm², and an FF of 0.71 (Fig. 6). The authors’ previous study revealed that the thickness of the compact layer significantly influences the hysteresis of PMPSCs [²⁹]. Therefore, a detailed investigation of the influence of other layers on the hysteresis of PMPSCs would be an appropriate area for future research.

Fig.6 Reverse and forward scan J−V curves of the optimized PMPSC prepared with a precursor at a concentration of 0.70 M

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Conclusions

This study investigated the influence of precursor concentration on the crystallinity and morphology of halide perovskites prepared by a simple drop-coating method for the fabrication of printable mesoscopic PSCs. Precursors with lower concentrations were confirmed to result in larger grain sizes due to extended growth time, while higher concentrations led to the development of more continuous films due to sufficient nucleation sites from colloidal particles in the precursor. Among various concentrations from 0.24 to 1.20 M, a moderate concentration of 0.70 M was confirmed to result in the highest PMPSC efficiency of 16.32% due to more effective pore filling and better crystallinity. Furthermore, since the simple drop-coating and spin-coating methods involve different crystallization processes, an efficiency of 15.38% was still achieved at a low concentration of 0.24 M.

Acknowledgements

The authors acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 91733301, 51902117, and 21702069), the Fundamental Research Funds for the Central Universities, the Science and Technology Department of Hubei Province (No. 2017AAA190), the 111 Project (No. B07038), and the Program for Huazhong University of Science and Technology (HUST) Academic Frontier Youth Team (No. 2016QYTD06). We thank the Analytical and Testing Center of HUST for performing various characterization and measurements.

Conflicts of interest

There are no conflicts of interest to declare.

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