Compositional Engineering of FAPbI₃ Perovskite Added MACl with MAPbBr₃ or FAPbBr₃

Abstract

Many attempts have been made to stabilize α-phase formamidinium lead iodide (α-FAPbI₃) using mixed cations or anions with MA⁺, FA⁺, Br⁻ and I⁻. A representative method is to stably produce α-FAPbI₃ by adding methylammonium lead (MAPbBr₃) to the light absorption layer of a perovskite solar cell and using methylammonium chloride (MACl) as an additive. However, in the perovskite containing MA⁺ and Br⁻, the current density is lowered due to an unwanted increase in the bandgap; phase separation occurs due to the mixing of halides, and thermal stability is lowered. Therefore, in this study, in order to minimize the decrease in the composition ratio of FAPbI₃ and to reduce MA⁺, the addition amount of MACl was first optimized. Thereafter, a new attempt was made to fabricate FAPbI₃ perovskite by using formamidinium lead bromide (FAPbBr₃) and MACl together as phase stabilizers instead of MAPbBr₃. As for the FAPbI₃-MAPbBr₃ solar cell, the (FAPbI₃)_0.93(MAPbBr₃)_0.07 device showed the highest efficiency. On the other hand, in the case of the FAPbI₃-FAPbBr₃ solar cell, the (FAPbI₃)_0.99(FAPbBr₃)_0.01 solar cell with a very small FAPbBr₃ composition ratio showed the highest efficiency with fast photovoltaic performance improvement and high crystallinity. In addition, the FAPbI₃-FAPbBr₃ solar cell showed a higher performance than the FAPbI₃-MAPbBr₃ solar cell, suggesting that FAPbBr₃ can sufficiently replace MAPbBr₃.

1. Introduction

Perovskite solar cells (PSCs) are still one of the most popular fields and within a short period of time since their advent, they have achieved high power conversion efficiencies (PCEs) exceeding 25% with broader solar-light absorption through narrower bandgaps. Formamidinium lead iodide (FAPbI₃) has the narrowest bandgap (1.45–1.51 eV) among lead halide perovskites and improved thermal stability compared to methylammonium lead iodide [1,2]. However, α-FAPbI₃ (the FAPbI₃ perovskite) is prone to phase change to δ-FAPbI₃ (non-perovskite, hexagonal), which is thermodynamically more stable at room temperature. Yellow δ-FAPbI₃ reduces the crystallinity of the FAPbI₃ film, disrupting electron transport and reducing the performance of PSCs [3,4]. The first of two representative methods to overcome the phase transformation problem of α-FAPbI₃ is the use of methylammonium chloride (MACl) as an additive in the perovskite precursor solution. MACl induces the growth of the (001) plane of α-FAPbI₃ and improves the crystallinity of the perovskite [5,6]. Moreover, MACl can be removed by heating above 140 °C, which is essential for α-FAPbI₃ synthesis [7]. Therefore, a MA⁺-free perovskite film can be produced using this method. The second method is to stably synthesize α-FAPbI₃ by adding methylammonium lead bromide (MAPbBr₃) with a cation smaller than FA⁺ to the perovskite composition [8]. Researchers have focused primarily on mixed cations or anions in an effort to improve the stability of α-FAPbI₃. Therefore, FAPbI₃-MAPbBr₃ has been studied the most among all the processes for enhancing the phase stability of FAPbI₃ and exhibited a higher PCE than the first method of adding MACl. In addition, MACl has been used together in the manufacture of FAPbI₃-MAPbBr₃ perovskite, and here MACl has been mainly used as ‘a mediator for high-crystallinity’. However, this method has problems such as reduced light absorption, increased bandgap due to MAPbBr₃, and reduced thermal stability owing to MA⁺ ions, resulting in a low current density [9]. Although we attempted to stabilize the α-FAPbI₃ phase without MA⁺ using Rb⁺ and Cs⁺, the resulting PCE was still low when compared to the PCE obtained using FA⁺ and MA⁺ [10,11]. Therefore, to further improve the performance of PSCs, a novel configuration capable of stabilizing α-FAPbI₃ without MA⁺ while controlling the bandgap increase inherent in FAPbI₃ is required.

We focused on the composition of mono-cation FAPbI₃-FAPbBr₃ perovskite, which can help improve α-FAPbI₃ phase stability. However, δ-FAPbI₃ was still found in the FAPbI₃-FAPbBr₃ film, resulting in poor solar cell performance. In this study, a new attempt was made to solve this problem by using MACl as an additive to increase the phase stability of α-FAPbI₃ together with FAPbBr₃. The combination of cations and anions added to the perovskite is important to improve the stability of FAPbI₃ [12]. Therefore, FAPbI₃-FAPbBr₃ with MACl added stably produced α-FAPbI₃ by combining MA⁺ and Br^-. In addition, α-FAPbI₃ films are stably fabricated by the formation of metastable two-dimensional MAFAPbI₃Cl perovskite intermediates with high free energy due to the Cl- in the precursor solution [5]. The amount of MACl was first optimized and applied to the FAPbI₃ film, and then MAPbBr₃ or FAPbBr₃ was used in the perovskite composition. MACl added FAPbI₃-FAPbBr₃ films showed a dramatic improvement in photovoltaic performance even with very small amounts of FAPbBr₃. In addition, FAPbI₃-FAPbBr₃ PSCs showed a relatively high current density based on a smaller increase in FAPbI₃ intrinsic bandgap than FAPbI₃-MAPbBr₃ PSCs, suggesting that FAPbBr₃ could be used as a sufficient replacement for MAPbBr₃.

2. Materials and Methods

2.1. Materials

FTO glass (7 Ω sq-1, Wooyang GMS), titanium diisopropoxide bis(acetylacetone) (75 wt.% in isopropanol, Sigma-Aldrich, St. Louis, MO, USA), 1-butyl alcohol (99%, Sigma-Aldrich, St. Louis, MO, USA), TiO₂ paste (18 NR-T, Greatcell solar, Queanbeyan, Australia), N,N-dimethylformamide (DMF, 99.8%, Sigma-Aldrich, St. Louis, MO, USA), dimethyl sulfoxide (DMSO, ≥99.9%, Sigma-Aldrich, St. Louis, MO, USA), ethyl alcohol (≥99.5, Sigma-Aldrich, St. Louis, MO, USA), chlorobenzene (99.8%, Sigma-Aldrich, St. Louis, MO, USA), lead(II) iodide (99.999% trace metals basic, Sigma-Aldrich, St. Louis, MO, USA), lead(II) bromide (99.999% trace metals basic, Sigma-Aldrich, St. Louis, MO, USA), formamidinium iodide (FAI, greatcellsolar, Queanbeyan, Australia), methylammonium bromide (MABr, greatcellsolar, Queanbeyan, Australia), formamidinium bromide (FABr, greatcellsolar, Queanbeyan, Australia), methylammonium hydrochloride (MACl, greatcellsolar, Queanbeyan, Australia), toluene (99.9%, Sigma-Aldrich, St. Louis, MO, USA), 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD, 99%, Sigma-Aldrich, St. Louis, MO, USA), bis(trifluoromethane)-sulfonimide lithium salt (Li-TSFI; ≥99.0%, Sigma-Aldrich, St. Louis, MO, USA), acetonitrile (99.93%, Sigma-Aldrich, St. Louis, MO, USA), 4-tertbutylpyridine (98%, Sigma-Aldrich, St. Louis, MO, USA) were used. All reagents were used as received without further purification.

2.2. Device Preparation

FTO glass was used as the substrate for fabricating the device. The substrates were sequentially washed with acetone, ethanol, and deionized water for 15 min each in an ultrasonic bath. To deposit the compact-TiO₂ (c-TiO₂) layer, 55 mL of a titanium diisopropoxide bis (acetyl acetonate)/1-butyl alcohol (1:10 v/v) solution was spin-coated. The substrate was then heated at 120 °C for 15 min. On top of the c-TiO₂ layer, a mesoporous TiO₂ (mp-TiO₂) layer was spin-coated. TiO₂ paste with an average nanoparticle size of 20 nm was dispersed in ethyl alcohol (1:6 w/w). The prepared FTO/c-TiO₂/mp-TiO₂ substrates were calcined at 500 °C for 1 h and then cooled to room temperature. To fabricate the perovskite layer, 1.4 mol of perovskite solution was prepared in a mixture of DMF and DMSO (8:1 v/v). MACl was then added to the prepared precursor solution. Each sample was spin-coated onto the mp-TiO₂ layer at 4000 rpm for 20 s. During the spin coating, 200 μL of toluene was added dropwise using a pipette after spinning for 10 s. The film was heated on a hot plate at 150 °C for 10 min. The hole transport layer was prepared using spiro-OMeTAD in chlorobenzene (72.3 mg/mL), and 28.8 μL 4-tert-butyl pyridine and 17.5 μL Li-bis solution (520 mg Li-TFSI/1 mL acetonitrile) were added. Finally, a 60 nm thick gold electrode was deposited using a thermal evaporation system.

2.3. Characterization and Device Measurement

UV-vis absorption spectra were measured using an Agilent 8453 UV-vis spectrophotometer (Agilent 8453, Agilent Technologies, Santa Clara, CA, USA) at a scan rate of 494.95 [nm/min] in the wavelength range 200–1000 nm. Phases of the perovskite films formed on FTO/TiO₂ were analyzed using an XRD Rigaku DMAX 2200 system (Rigaku, Tokyo, Japan) with Cu Kα radiation (λ = 0.1542 nm). XRD patterns were analyzed in five step sizes in the range 10–60° 2θ. The surfaces of the perovskite layer on the FTO/TiO₂ and the cross-sections of the FTO/TiO₂/perovskite/spiro-OMeTAD. were obtained using field-emission SEM (Hitachi S-4700, Tokyo, Japan). All SEM images were sputter coated with gold for conductivity and measured at an acceleration voltage of 15 kV and a probe current of 10 μA. All surface images were measured at 30k magnification at distances of 12.2 mm and 12.3 mm, and cross-sectional images were measured at 50k magnification at distances of 15.2 mm. J-V curves of the PSCs were measured using a solar simulator (Polaromix K201, Solar simulator LAB 50, McScience K3000, McScience, Gyeonggi-do, Korea) under one sun illumination (AM1.5G, 100 mWcm⁻²). The active area of the PSCs was calculated using an area of 0.053 cm⁻².

3. Results and Discussion

3.1. FAPbI₃ Perovskite Solar Cells with MACl

To determine the optimal composition ratio of MAPbBr₃ and FAPbBr₃, five devices were prepared under various MACl conditions (0–50 mol%, or 0–50-MACl). Here, we designed a PSC with a fluorine-doped tin oxide (FTO)/TiO₂/perovskite/spiro-OMeTAD/Au structure. Figure 1a shows the current density-voltage (J-V) curves for the perovskite device for different amounts of MACl. A summary of the photovoltaic properties of 0–50-MACl PSCs is presented in

Table 1. Photovoltaic parameters of the best-performing FAPbI₃ PSCs with various amounts of MACl.

Sample	VOC	JSC	FF	PCE (%)	Rs(Ω)
0-MACl	0.831	16.696	40.105	5.567	331.839
20-MACl	0.938	22.550	68.982	14.596	119.269
30-MACl	0.929	22.599	70.791	14.859	104.548
40-MACl	0.908	24.181	70.037	15.379	109.747
50-MACl	0.869	23.074	64.768	12.989	135.155

. The device without MACl exhibited an open-circuit voltage (V_OC) of 0.831 V, a short-circuit current density (J_SC) of 16.696 mAcm⁻², a fill factor (FF) of 40.105%, and a PCE of 5.567%. The addition of MACl to the α-FAPbI₃ film increased the overall efficiency of all devices by stabilizing α-FAPbI₃ and improving the crystallinity [13]. The 40-MACl perovskite film exhibited the highest PCE of 15.379% with a V_OC of 0.908 V, a J_SC of 24.181 mAcm⁻², and an FF of 70.037%, indicating that the optimal MACl addition amount was 40 mol% (Table 1). However, as the MACl concentration increased to 50 mol%, the PCE decreased to 12.989%. The XRD pattern of the α-FAPbI₃ film (Figure 1b) by MACl concentration shows two characteristic peaks of α-FAPbI₃ at 13.95°, 24.26° and 28.12°, which are attributed to the (001) (111) (002) plane, along with one peak at 11.8° that corresponds to δ-FAPbI₃. In addition, the peak observed at 12.63° and 26.50° corresponds to lead iodide (PbI₂) residue due to incomplete reaction between PbI₂ and formamidinium iodide (FAI) in a perovskite precursor solution prepared by a stoichiometric method. The noticeable difference in the 2θ peak intensity of the α-FAPbI₃ (001) (002) plane shows an improvement of the crystallinity of the perovskite, which sharply increases with the increase of the MACl. In addition, the peak corresponding to δ-FAPbI₃ almost disappeared with the addition of MACl.

3.2. FAPbI₃-MAPbBr₃ Perovskite Solar Cells

Using the optimized MACl condition, 12 devices were fabricated with various composition ratios of MAPbBr₃ and FAPbBr₃, which were used as light absorption layers to improve the phase stability of α-FAPbI₃ and improve the performance of PSCs. Figure 2a shows the J-V curves of the (FAPbI₃)_1−X(MAPbBr₃)_X perovskite (renamed X-MAPbBr₃) device. A summary of the photovoltaic properties of the PSCs according to the composition ratio of MAPbBr₃ is presented in

Table 2. Photovoltaic parameters of the best-performing X-MAPbBr₃ PSCs.

Sample	VOC	JSC	FF	PCE (%)	Rs(Ω)
0.01-MAPbBr3	1.000	22.538	69.594	15.691	120.472
0.03-MAPbBr3	0.986	22.729	70.188	15.730	113.333
0.05-MAPbBr3	0.965	23.407	70.878	16.014	109.144
0.07-MAPbBr3	1.017	22.196	72.176	16.301	116.721
0.10-MAPbBr3	1.002	21.785	70.414	15.372	122.829
0.15-MAPbBr3	1.025	21.651	67.764	14.826	147.174

. The 0.07-MAPbBr₃ film exhibited the highest PCE of 16.301% with a V_OC of 1.017 V, a J_SC of 22.196 mAcm⁻², and an FF of 72.176%. In contrast, the PCE of the commonly used composition ratios, namely, 0.10-MAPbBr₃ and 0.15-MAPbBr₃, decrease gradually to 15.372% and 14.826%, respectively, which is attributed to the high series resistance (R_S) [14,15,16,17]. The existence of α-FAPbI₃ in the produced perovskite films was confirmed from the XRD pattern of the X-MAPbBr₃ film (Figure 2b), which also showed that the crystallinity of the perovskite film improved with increasing the amount of MAPbBr₃. However, as the composition ratio of MAPbBr₃ increased to 0.10 and 0.15, the perovskite crystallinity decreased. In addition, the α-FAPbI₃ peak shifted to a higher degree of diffraction with increasing amount of MAPbBr₃. Figure 2c shows the absorbance of the X-MAPbBr₃ perovskite film. The light absorption coefficient improved with the increasing amount of MAPbBr₃ in the X-MAPbBr₃ perovskite film. In particular, 0.07-MAPbBr₃ showed the highest absorbance. On the other hand, 0.10- and 0.15-MAPbBr₃ showed lower absorbance. When increasing the amount of MAPbBr₃, which has a wider bandgap than FAPbI₃ in the X-MAPbBr₃ perovskite film, the absorbance gradually blue-shifted in the 750–850 nm wavelength region [18,19].

3.3. FAPbI₃-FAPbBr₃ Perovskite Solar Cells

Figure 3a shows the J-V curve of the (FAPbI₃)_1−X(FAPbBr₃)_X perovskite (renamed X-FAPbBr₃) device with 40 mol% MACl. A summary of the photovoltaic properties of X-FAPbBr₃ is presented in

Table 3. Photovoltaic parameters of the best-performing X-FAPbBr₃ PSCs.

Sample	VOC	JSC	FF	PCE (%)	Rs (Ω)
0.01-FAPbBr3	1.016	23.413	69.622	16.569	117.628
0.03-FAPbBr3	1.025	23.221	68.781	16.364	115.223
0.05-FAPbBr3	1.003	23.481	66.948	15.766	123.569
0.07-FAPbBr3	1.045	21.938	67.446	15.468	126.702
0.10-FAPbBr3	0.965	22.275	62.861	13.508	153.506
0.15-FAPbBr3	1.027	18.055	60.814	11.274	231.911

. Surprisingly, despite the very small composition ratio of FAPbBr₃, the 0.01-FAPbBr₃ film exhibited an outstanding PCE of 16.569% with a V_OC of 1.016 V, a J_SC of 23.413 mA·cm⁻², and an FF of 69.622%. The enhanced V_OC of 1–15-FAPbBr₃ can be attributed to the increase in the bandgap and the decrease in the electron-hole recombination at the interface between the perovskite film and the hole transport layer and electron transport layer [20,21]. Figure 3b shows the XRD pattern of the X-FAPbBr₃ perovskite film. The 2θ peaks at 13.95°, 26.50°, and 28.12° in the XRD patterns confirm the existence of α-FAPbI₃ in the fabricated perovskite film. The XRD pattern confirmed that the 0.01-FAPbBr₃ film had the highest crystallinity. Thereafter, when increasing the amount of FAPbBr₃, the intensity of the perovskite peak decreased. As with X-MAPbBr₃, it was confirmed that as the composition ratio of FAPbBr₃ increased, the diffraction peak of the (001) plane shifted to a larger angle [22]. Figure 3c shows the absorbance of the X-FAPbBr₃ perovskite film. The 0.01-FAPbBr₃ film had the highest absorbance. Subsequently, the absorbance gradually decreased as the FAPbBr₃ content of the X-FAPbBr₃ film increased. These results are responsible for the progressive decrease in J_SC with an increasing concentration of FAPbBr₃ (Table 2) [23]. In the 750–850 nm wavelength range, the absorbance of the X-FAPbBr₃ film blue-shifted with increasing FAPbBr₃, which has a wider bandgap similar to X-MAPbBr₃.

3.4. Comparison of FAPbI₃-FAPbBr₃ and FAPbI₃-MAPbBr₃ Perovskite Solar Cells

In

Table 4. Photovoltaic parameters of the best-performing X-FAPbBr₃ PSCs.

Sample	Sweep Direction	VOC	JSC	FF	PCE (%)	RS(Ω)	HI
0.01-FAPbBr3	FS	1.001	23.425	66.797	15.656	132.024	0.055
	RS	1.016	23.413	69.622	16.569	117.628
0.07-MAPbBr3	FS	0.983	21.833	68.702	14.745	143.729	0.095
	RS	1.017	22.196	72.176	16.301	116.721

, the difference between R_S and FF of the optimized composition of FAPbI₃-MAPbBr₃ and FAPbI₃-FAPbBr₃ perovskite solar cells was not significant, indicating that the manufactured cells had similar stability [24,25]. Figure 4a,b shows the surface image of the perovskite films. The average grain sizes of 0.01-FAPbBr₃ and 0.07-MAPbBr₃ were approximately 769 nm and 652 nm, respectively. The difference in grain size between 0.01-FAPbBr₃ and 0.07-MAPbBr₃ affects J_SC based on the difference in light absorption. [26]. Figure 4c is a cross-sectional SEM image of PSCs without the top electrode. The thickness of TiO₂/perovskite/Spiro-OMeTAD is 240 nm, 348 nm, and 244 nm, respectively. As shown in Figure 5a, the J_SC difference between 0.01-FAPbBr₃ and 0.07-MAPbBr₃ is clearly visible. This is considered to be due to the difference in the grain size, as mentioned above [27]. In addition, 0.01-FAPbBr₃ shows reduced hysteresis compared to 0.07-MAPbBr₃. The hysteresis index (HI, listed in Table 4) was extracted using the equation in [28]. The 0.07-MAPbBr₃ device showed a significant PCE difference between 14.745% (forward) and 16.301% (reverse). The 0.01-FAPbBr₃ device has a low hysteresis effect with PCEs of 16.569% and 15.656% for the reverse and forward directions, respectively. That is, the HI decreased from 0.095 to 0.055. The normal distribution model was applied to the histogram shown in Figure 5b. Both histograms show that the 0.01-FAPbBr₃-based device exhibited improved solar cell performance compared to the 0.07-MAPbBr₃-based device.

4. Conclusions

In this study, the addition amount of MACl was first optimized to reduce the dependence of MAPbBr₃ or FAPbBr₃ on the phase stability enhancement of FAPbI₃. The 40-MACl device had a champion PCE of 15.379%, and 40 mol% MACl addition was adopted for the FAPbI₃-MAPbBr₃ and FAPbI₃-FAPbBr₃ films. MAPbBr₃ or FAPbBr₃ not only accelerated the δ to α phase transformation process of the FAPbI₃ perovskite film, but also improved the crystallinity and formed a uniform perovskite film. Among the FAPbI₃-MAPbBr₃ PSCs, the 0.07-MAPbBr₃ device had the highest PCE of 16.301% and higher photovoltaic performance than the commonly used 0.10- and 0.15-MAPbBr₃ devices. Among the FAPbI₃-FAPbBr₃ devices, 0.01-FAPbBr₃ showed the PCE of 16.569% even with the lowest FAPbBr₃. Interestingly, the 0.01-FAPbBr₃ device, which was not adopted due to its low performance, was more efficient than the 0.07-MAPbBr₃ device and exhibited a suppressed hysteresis effect.