INTRODUCTION
The continuous increase in greenhouse gas emissions (CO2, N2O, CH4, etc.) from the burning of non-renewable energies is influencing the climate change and global warming. Besides total greenhouse gas emissions; depleting of fossil fuels, natural resource scarcity, and price hikes of fossil fuels are critical issues that require immediate attention.1 To solve these issues, alternative clean energy sources utilization are inevitable for a future clean and smart world.1 Among the clean renewable energy sources, solar energy is the most abundant, clean, smart, non-toxic, and non-environmental polluted source of energy. The solar energy harvesting into electricity is directly converted by photovoltaic (PV) device. Nonetheless, the electric power generation using this technology is still a challenge in terms of sufficient generation and storing of such clean source of energy. At present, the PV market is dominated by crystalline silicon (c-Si) with power conversion efficiency (PCE) exceeding 20%.2,3 Whereas amorphous-microcrystalline silicon (a-Si, m-Si), organic photovoltaics (OPVs), cadmium telluride (CdTe), copper indium gallium selenide (CIGS), dye-sensitized solar cells (DSSCs), and quantum dot sensitized solar cells (QDSSCs) are designed to reduce the fabrication cost and weight of the devices.2,3 However, these solar cells still have issues with mass production for commercialization because of their high cost, use of rare and/or toxic elements (one or more of them), and low efficiency. Therefore, researchers are exploring alternative materials that offer cost effectiveness, ease of device fabrication, and a higher energy payback.4 To this end, organic–inorganic hybrid perovskites in the form of ABX3 (where A is inorganic or organic cation = (methylammonium (MA) CH3NH3, formamidinium (FA) CH2(NH2)2, Cs, Rb), B is divalent metals = Pb2, Sn2, or Ge2, and X stands for halide = Cl−, Br−, I−, or their mixes and SCN)5–8 have gained enormous attention for solar cells applications because of the rapid improvement of PCE, which is comparable to that of conventional c-Si based solar cells.9–11 They also have special benefits including, low-cost, low-temperature processability, broad optical absorption, low non-radiative recombination, and so forth.9–11 Among ABX3, the MAPbI3 perovskite solar cell aroused a wide of variety research interest because of its aforementioned remarkable features and currently achieved an exceptional PCE of 25.5%.12 This solar cell was first reported in 2009 with PCE of 3.81% and climbed quickly to a certified PCE of 25.7%13–17 in 2022. These Pb-based compounds suffer a drawback for practical applications in large scale production because of toxicity and poor stability toward light, heat, and atmospheric moisture.8,9 Thus, for effective performance of the devices and to tune optical properties, mixing of halide composition is necessary for the enhancement of carrier transport, open circuit voltage (VOC) and stability of perovskite solar cells.18–23 To improve stability, many research groups have deposited halide-mixing perovskite materials.23,24 From the literature, it is found that mixed-halide perovskites MAPbI3–xClx and MAPbBr3–xClx exhibited PCE of 19.3%25 and 15.9%.26 Furthermore, these mixing-halides demonstrated wideband optical absorptions, large carrier diffusion length, small exciton binding energy, and higher electron mobility.18,27 It is reported that addition of Cl into pure halide, ABX3 impacts the film morphology and charge carrier transport, resulting in PCE of 14.3%28 and 10.9%.29 In our previous works, MAPbI3–xClx thin film was deposited using spray pyrolysis, chemical dip coating, and repeated dip coating methods, where the highest theoretical efficiency was obtained as 17.40%.30–32 It should be noted that the substitution of MA and Pb by FA and Sn, can further improve the stability.21–24 Importantly, removing toxic Pb from the perovskite structure is desired for commercialization.21–24 Thus, organic–inorganic Pb-free FASnI3-xClx perovskite is a potential candidate for PV applications in comparison with MAPbI3.33 By contrast, the development of Pb-free all inorganic halide or mixed-halide perovskites with a suitable band gap and improved PCE are of paramount importance. To these points of view, all inorganic Pb-free CsSbCl4 perovskite has attracted great research interest owing to improving stability.34–36 Hence, instead of MA, the FA-based Pb-free organic–inorganic, and all inorganic perovskites with mixing of halide ions would be alternative candidates for future light harvesting commercial devices, basically optoelectronic, photocatalytic, photodetector, photonic, and PV devices, and so forth.37 Yuqin et al. reported about the stability improvement of Sn-based perovskite device with PCE of 7.94%.38 Ke et al. reported FTO/CH3NH3PbI3–xClx/spiro-OMeTAD/Au planer device with PCE ~ 14.14%39 and ~ 16.07%.39 Pearson, et al.40 obtained PCE ~ 9.4% of MAPbI3-xClx solar cell. Shi et al. pointed out PCE ~12.67% of MAPbI3-xClx mesoporous solar cell.41 The unoptimized MAPbI3-xClx based solar cell showed PCE ~ 14.64% which was predicted as >26%.42 There are few reports on Pb-free Sn-based perovskite solar cells. Among them, FASnI3 is the most studied perovskite,43 but it can easily oxidized.44 A stable FASnI3 solar cell exhibits PCE 11.22%, and after 1000 h it shows over 95% of its initial efficiency.45 Seed layer growth of FASnI3 solar cells demonstrated improved efficiency from 5.37% to 7.32%.46 Theoretically, the efficiency of FASnI3 solar cell is reported as 23.11%.47 The FASnI3-xClx solar cell exhibits PCE ~ 5.30%48 and the post-treated device demonstrates PCE ~12.11%.49 The Pb-free Cs3Sb2Cl9 perovskite possesses both the trigonal and orthorhombic phases, and they showed indirect band gap of 2.86 and 2.89 eV,50–52 but the CsSbCl4 (Dion–Jacobson) perovskite showed a direct band gap of 1.41 eV.53 The TiO2/CsSbCl4/spiro-OMeTAD junction showed a PCE of 20.07%, which can further be enhanced by optimizing different solar cell parameters.53 Besides solar cell applications, perovskite materials are promising candidates for photocatalytic activity due to their outstanding properties of light absorption, optimal band gap, long electron–hole diffusion length, high extinction coefficient, high photoluminescence quantum yield, easy manufacturing process, and cheaper in cost.54,55 Perovskites in photocatalytic CO2 reduction reaction, pollutant degradation, and hydrogen evolution have been ringing out over the past few years.55 But pure halide perovskite faces problems of low efficiency and instability as photocatalysts. To improve the stability of halide perovskite in photocatalytic reactions, mixed halide and Pb-free all inorganic perovskites could be suitable candidates because the band gap plays a key role in the charge transport behavior and it can be tuned in between 1.20 and 3.16 eV. For a suitable photocatalytic reaction, the conduction band (CB) edge (or valence band (VB) edge) should be more negative (or positive) than the reduction (or oxidation) potential (Ered or Eox) levels, respectively. For these factors an ideal band gap of the semiconductors should be1.8–2.8 eV.56,57 Thus making composites, mixed-halide and Pb-free perovskites is an effective strategy to improve the photocatalytic performances.9–11,54,55
Perovskite thin films have been deposited using different deposition techniques.3–11,30–32 Among these, the spin coating is an easy and common technique that has been widely employed in depositing perovskite thin films in stringent Ar and N2 filled glove boxes at a temperature of about 100°C. Nevertheless, it is applicable to the fabrication of non-scalable solution-processed PSCs,58 but large substrates cannot be spun at a sufficiently high rate in order to allow the film to thin and for large area device applications. This practical bottleneck situation can be overcome by considering new scalable versatile chemical spray pyrolysis (CSP) technique. It is a cost-effective, easy-to-scale-up, and time-saving method that employs low-cost solvents of the precursor's solution for large-scale thin films deposition in atmospheric conditions.
In the present study, at first organic–inorganic Pb-based mixed-halide perovskite CH3NH3PbI3-xClx(MAPbI3-xClx) is deposited by a two-step simple, cheapest, and most commonly used CSP technique at a temperature of 125°C. Then, MA and Pb parts of MAPbI3-xClx are replaced by CH5N2 (FA) and Sn, respectively, to make Pb-free organic–inorganic perovskite FASnI3-xClx, and finally Pb-free all inorganic perovskite CsSbCl4 thin films under the same conditions. The synthesized temperature was chosen as 125°C, because the pure FAPbI3 phase is highly sensitive to temperature, and its phase transition takes place from the yellow nonperovskite phase (δ-phase) to the black perovskite phase (α-phase) at ≈125°C.59 Furthermore, it is highly sensitive to humidity because of the presence of two amino groups in the FA+ cation.60 It is reported that the replacement of MA from pure MAPbI3 by FA+ or Cs+, or any of the combinations showed tunable optoelectronic property and improved long-term stability.34,61–63 On the other hand, the replacement of toxic Pb by Sn, Sb can also impact crystal structure, optical, chemical potential, and defects, and so forth. properties, which consequently affect the device's performance. Thus, the role of Cs, FA, Sn, and Sb on the morphology, structural, optical, and so on properties is systematically investigated to provide potential applications in the PV and photocatalytic activity of mixed-halide, MAPbI3-xClx, FASnI3-xClx, and pure CsSbCl4 perovskites. Correlating investigations among the crystal structure, optical properties, and phases are performed by different characterization measurements. All the steps were performed under ambient conditions. Especially, structural, compositional effect, and band gap engineering as well as the effect of the thin films growth and their device performance have been inspected with the aim to improve the thin film quality for efficient optoelectronic, photocatalytic, and solar cells applications.
RESULTS AND DISCUSSION
Surface morphology
The morphology, crystallinity, and coverage of the film play crucial role in device performance in addition to the film thickness and phase control. Poor morphology and/or crystallinity reduce the device's performance. Figure 1A–C shows surface morphology of the as-deposited MAPbI3-xClx, FASnI3-xClx, and CsSbCl4 thin films, which are taken using a power optical metallurgical microscope with a 30-W halogen light source. From the surface morphology of the as-deposited thin films, it appears that all the deposited films well covered throughout the substrate surfaces. The films surface are continuous and uniformly distributed over the whole substrate surface, which indicates the high material yield. Although, the films are grown at the same deposition conditions, the change in surface morphology could be due to changes in film compositions. From Figure 1A of MAPbI3-xClx, it seems that the flat surface is composed of larger grains, but there are few large islands that develop on the film surface. This could be due to the aggregating of small grains into large one and/or the aggregating of the solution droplet, which is interconnected to the roughness. To clarify this phenomenon, we need reworking. When MA and Pb are replaced by FA and Sn, grains size changes, which indicates that they were incorporated into the film (Figure 1B). This nature of the film is in good agreement with the result of x-ray diffraction (XRD) pattern. From Figure 1C, it is obvious that the film surface also contains large grains, and the film surface seems to be less rough. This feature indicates that the constituent elements effect the grain growth of CsSbCl4.
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Figure 2A–C illustrates top-view scanning electron microscopy (SEM) images of the MAPbI3-xClx, FASnI3-xClx, and CsSbCl4 thin films. It is clearly seen that the MAPbI3-xClx thin film (Figure 2A) has different grain sizes insight a relatively rough surface. It is also clear that the surface of MAPbI3-xClx film contains large grains with an average grain size greater than 1.0 μm. This result supports that Cl was incorporated into MAPbI3 and favors the formation of large grains, which can reduce charge recombination at the grain boundaries. The obtained result is consistent with reported results.64,65 A tetragonal phase structure/grain is clearly observed, which is in good agreement with the XRD result. In the case of FASnI3-xClx, relatively flat and small-size grains distribution lies (Figure 2B) over the whole substrate surface. The small grains growth of FASnI3-xClx film may be due to the strong electrostatic interaction between the ionic size of FA (2.79 Å) and Sn (0.71 Å), which acts as a barrier for crystal initiation or growth. However, some cracks are developed on the FASnI3-xClx thin film surface, and the source of these cracks is thought to be relevant to the low film thickness. This occurrence insights inhomogeneous crystallization, including both nucleation and growth. From the surface morphology of Pb-free inorganic perovskite CsSbCl4 (Figure 2C), it can be seen that aggregating grains are generated on the film surface. Indeed, a significant improvement in microstructure is obvious which is coherent with the reported structure.66 The microstructure improvement of CsSbCl4 thin film could be due to the strong solubility of Sb associated with SbCl3 in dimethylformamide (DMF). Thus the high crystallinity of the films is confirmed by surface morphology (SEM images, Figure 2) and the appearance of sharp peaks in the XRD patterns (Figure 3A–C).
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X-ray diffraction
The crystallinity and phases of MAPbI3-xClx, Pb free FASnI3-xClx, and CsSbCl4 perovskite films are characterized by powder XRD. The XRD patterns also insight the orientation and morphology of the crystal. Figure 3A–C displays the XRD patterns of the films, which are well indexed with different JCPDS cards of MAPbI3-xClx, FASnI3-xClx, and CsSbCl4. Each pattern, contains a few Bragg reflection peaks conclusively indicating that they were formed of crystalline structure. Figure 3A represents the XRD pattern of MAPbI3-xClx, where a series of diffraction peaks appeared, which are in good agreement with the literature data of the tetragonal phase of MAPbI3-XClX. The peaks at (2θ) ~14.39°, and ~ 28.60° are due to (110) and (220) planes, which are the evident of tetragonal perovskite structure of MAPbI3-xClx.65 The intensity of the (110) peak is higher than other peaks, which indicates preferred growth orientation of MAPbI3-XClX along the crystallographic (110) plane. The diffraction peaks are located at 2θ ~ 24.88°, 35.35°, 39.88°, 40.87°, and 43.462° corresponding to reflection planes (202), (210), (211), (224), and (330) for the MAPbI3-xClx phase. These results support the previous reports.67–70 In addition, there are few low intensity peaks that appear corresponding to (001), (011), and (202) planes at 2θ ~13.04°, 26.30°, and 52.63°, which could be due to the PbI2 phase attribute to the decomposition of MAPbI3-xClx.71 The diffraction peaks are located at 2θ = 14.38° and 32.26° corresponding to (002) and (310) planes for the tetragonal phase of MAPbCl3.72,73 Importantly, the existence of the mixed-phases is observed in the XRD pattern due to dissociation of the precursor reagents materials. Dissociation occurred due to low growth temperature. The lattice constants (a, c) for the tetragonal structure are calculated using Formula (1) as:
The crystalline quality of the film can also be studied by calculating the crystallite size (D) of the film using Debye–Scherrer Formula (2) as:
TABLE 1 Lattice constants, crystallite size and volume of the as-deposited MAPbI3-xClx, FASn3-xClx, and CsSbCl4 thin films.
| Sample | Lattice constant (Å) | Grain size (nm) | Volume (Å3) | ||
| a | b | c | |||
| MAPbI3-xClx | 8.82 | - | 12.39 | 131.63 | 964.23 |
| FASnI3-xClx | 6.30 | 9.66 | 8.87 | 455.17 | 540.09 |
| CsSbCl4 | 8.02/8.00 | 13.06 | 18.53/9.26 | 99.83 | 1940.51 |
The peaks positions are located at 13.80°, 24.52°, 28.31°, 30.80°, 40.17°, 43.40°, and 44.08° can be assigned to (100), (102), (200), (122), (222), (213), and (300) planes for the orthorhombic structure of FASnI3-xClx (Figure 3B), which is consistent with the literature report of FASnI3.74 The peak arising at 26.80° is due to the complex structure of SnCl2*.75 But the peaks introducing at 47.17°, 51.70°, and 53.18° correspond to crystallographic (311), (222), and (023) planes for the cubic phase of FASnI3-xClx.76,77
The lattice constants (a, b, and c) for the orthorhombic structure and the crystallite size were calculated using the Formulas (3) and (2) as,
From Table 1 it is found that the obtained lattice constants are well agreed with the report of FASnI3 orthorhombic (Amm2) structure associated with lattice parameters, a = 6.3096, b = 8.9298, and c = 9.0622 Å.78
There are a few broad and less intense reflection peaks that appear in the pattern of CsSbCl4 (Figure 3C) thin film, which primarily correspond to both trigonal and orthorhombic phases.47–49 A careful observation analysis identifies the low intensity peaks at 11.31°, 18.87°, 30.95°, 38.11°, and 44.46° that correspond to (012), (013), (043), (226), and (020) planes of the orthorhombic phase (space group: a = 7.63, b = 13.079, c = 18.663, ICSD 2066).47–49 The isolated peaks arising at 17.80° and 27.42° correspond to (201) and (002) planes for pure trigonal phase (ICSD 22075; space group: H; a = 7.61, c = 9.32 Å).47–49 Importantly, the additional isolated reflection from the orthorhombic or trigonal phases is not observed rather than several diffraction overlapped peaks of orthorhombic phase along with trigonal phase presence at 21.83°, 28.88°, 32.92°, 45.20°, and 59.46° correspond to (110/130), (003/006), (022/044), (204/400), and (402/183) planes.47,48 It is reported that the trigonal phase is stable at high temperatures, whereas the orthorhombic phase is stable at low temperatures.49 On the other hand, a mixture of trigonal and orthorhombic phases is observed at a specific growth temperature range.49 Since the deposition temperature was 125°C, there is a possibility for forming mixture phases, which is in good agreement with the result reported by Pradhan et al., where the reaction temperature was kept between 85 and 130°C52 The crystallite size and the lattice constants for orthorhombic crystal structure were calculated using Formulas (2) and (3), and the obtained results (Table 1) are in well agreed with the orthorhombic phase (space group: a = 7.63, b = 13.079, c = 18.663 Å, ICSD 2066), respectively.48 The lattice constants were also calculated for the trigonal structure using Formula (4) as:
It is obvious that calculated lattice constants, a = 8.00 and c = 9.26 Å are also in good agreement with the report of CsSbCl4 (Table 1) for trigonal structure as, a = 7.61, c = 9.32 Å (ICSD 22075; space group: H).48
Optical properties
The absorption coefficient (α) and band gap (Eg) of a semiconductor determine its optical property. Optical characteristics are carried out using a UV–vis absorption spectrophotometer in the wavelength range from 300 to 900 nm. A good absorber for PV applications needs to have strong absorption over a wide range of spectrums to minimize the amount of material usage and reduce charge and energy loss during extraction to electron–hole pairs. Figure 4A shows the absorption spectra of the deposited films are obtained from the relation (T%), where A is the absorbance and T is transmittance. Relatively high absorption is observed for each film (Figure 4A) which indicates high material yield. The absorption tail of FASnI3-xClx film is in the lower wavelength region (indicated by the dotted arrow sign) than the other two perovskite thin films, which insights lower band gap of FASnI3-xClx. It is also evident that the absorption edge shifts toward lower energy for Pb free perovskites, revealing narrowing the band gap. Figure 4B shows the absorption coefficient of the films, which is obtained from the formula, where A is the absorbance and t is the thickness of the samples. The thickness of the samples was calculated using the formula , where ∆m is the weight difference of the substrate before and after the film deposition (g), which was measured by the gravimetric method by a sensitive electronic balance (OHAUS, Pioneer-028.02.00.0DO2) with four-digit sensitivity. Where A is the area of the film (cm2), ρm is the mass density. The thickness was found to be (t = 241 ± 9.64, 230 ± 9.2, and 240 ± 9.60) nm. It should be noted that the thickness cannot be measured accurately by this method. The obtained value of α is on the order of 105 cm−1 (Figure 4B), which is comparatively higher than other inorganic PV materials.79 The band gap was found out from the Tauc plot as:
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TABLE 2 Band gap values of the as-deposited MAPbI3-xClx, FASn3-xClx, and CsSbCl4 thin films.
| Sample | Value of band gap, Eg (eV) from curve | Reported band gap, Eg (eV) | |
| A2 versus hν | (αhν)2 versus hν | ||
| MAPbI3-xClx | 1.58 | 1.61 | 1.60,57 1.63,92 1.57,18,91 1.6493 |
| FASnI3-xClx | 1.44 | 1.48 | 1.41,88 1.50,18,91 1.36,48 1.1995 |
| CsSbCl4 | 1.51 | 1.52 | 1.4153 |
Solar cell performance
The solar cell performance parameters of the deposited films were calculated using the solar cell capacitance simulator (SCAPS) one dimensional (1D) 3.3.0885 software by collecting baseline parameters from reports,53,86–94 which are provided in Table 3. The proposed device architecture is shown in Figure 5A. Figure 5B–D shows the J-V characteristics under illumination, where the black line is for without effect of defects, series and shunt resistances, whereas the red line is shown for the effect of series and shunt resistances, respectively. In these solar cells, the optimized thickness of the buffer (CdS) and window (ZnO:Al) layers as well as the fluorine-doped tin oxide (FTO) substrate have been used. The performance parameters, that is, short-circuit current density (JSC), open circuit voltage (VOC), fill factor (FF), and the efficiency (η) under light illumination are presented in Table 4. From Table 4, we can see that the solar cells exhibit the device performance as: JSC = 17.92 mA/cm2, VOC = 1.104 V, FF = 82.62%, η = 16.34% for MAPbI3-xClx is good matched with the reports,29,94 but is smaller than the other reported values.26,30,39–42 The value of JSC = 21.28 mA/cm2, VOC = 0.683 V, FF = 68.32%, and η = 9.90% is obtained for FASnI3-xClx is in good matched with the report.49 On the other hand, JSC = 20.15 mA/cm2. VOC = 0.866 V, FF = 74.91%, and η = 13.08% are obtained for CsSbCl4, which is smaller than the literature report.53 Although, the obtained efficiency of the solar cells is smaller than some experimental and desired efficiency. It is noteworthy that the higher thickness of the perovskite layer exhibits higher efficiency, and most of the reported solar cells have been used with more than 1.0 μm thickness of perovskite layer. Thus, the efficiency can be further enhanced by optimizing parameters, such as absorber layer thickness, absorber layer defect density, interface defect density, band gap and the series-shunt resistances. The efficiency is obtained as 20.78%, 11.93%, and 18.02% for MAPbI3-xClx, FASnI3-xClx, and CsSbCl4 thin films based optimized solar cell devices (Table 5), respectively, which are well matched with the reports.26,30,49,53 These favorable characteristics of the deposited Pb-free perovskite thin films, basically FASnI3-xClx and CsSbCl4 could serve as potential alternatives to Pb-based halide perovskites in PV device applications.
TABLE 3 Baseline parameters of different layers for solar cell performance calculation collected from References.53,86–94
| General device properties | |||||||
| Thermionic emission/surface recombination velocity (cm/s) | Front | Back | |||||
| Se (cm/s) | 107 | 105 | |||||
| Sh (cm/s) | 105 | 107 | |||||
| Work function (eV) | 4.26 (Ag) | 5.10 (Au) | |||||
| Optimized layer properties | |||||||
| Parameters | FTO | ZnO:Al | CdS (n) | Spiro-MeOTAD (p) | MAPbI3-xClx | FASnI3-x Clx | CsSbCl4 |
| Thickness (μm) | 0.300 | 0.200 | 0.100 | 0.100 | 0.241 | 0.230 | 0.240 |
| Band gap (eV) | 3.50 | 3.3 | 2.42 | 3.0 | 1.59 | 1.46 | 1.51 |
| Electron affinity (eV) | 4.0 | 4.6 | 4.3 | 2.2 | 3.9 | 3.52 | 3.77 |
| Dielectric permittivity (relative) | 9 | 9 | 9.35 | 3 | 6.5 | 8.2 | 10 |
| CB effective density of states (1/cm3) | 2.2 × 1018 | 2.2 × 1018 | 2.2 × 1018 | 2.2 × 1018 | 2.2 × 1018 | 1.0 × 1018 | 1.0 × 1019 |
| VB effective density of states (1/cm3) | 1.8 × 1019 | 1.8 × 1019 | 1.8 × 1019 | 1.8 × 1019 | 1.8 × 1019 | 1.0 × 1018 | 6.16 × 1019 |
| Electron thermal velocity (cm/s) | 107 | 107 | 107 | 1 × 107 | 107 | 107 | 107 |
| Hole thermal velocity (cm/s) | 107 | 107 | 107 | 1 × 107 | 107 | 107 | 107 |
| Electron mobility (cm2/Vs) | 20 | 100 | 100 | 2.1 × 10−3 | 2 | 22 | 501 |
| Hole mobility (cm2/Vs) | 10 | 25 | 25 | 2.16 × 10−3 | 2 | 22 | 22 806 |
| Shallow uniform donor density ND (1/cm3) | 2 × 1019 | 1 × 1018 | 1 × 1017 | 0 | 0 | 0 | 0 |
| Shallow uniform acceptor density NA (1/cm3) | 0 | 0 | 0 | 1 × 1018 | 1 × 1013 | 7 × 1016 | 2.1 × 1017 |
| References | 87,91 | 88–90 | 88–90 | 53,91–94 | 86,91,94,97 | 92 | 53 |
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TABLE 4 Solar cell performance parameters of the as-deposited thin films obtained from SCAPS-1D software.
| Obtained solar cell parameters | References | ||||||
| Sample | Rs and Rsh (Ω-cm2) | Thickness (nm) | VOC (Volt) | JSC (mA/cm2) | FF% | η% | |
| MAPbI3-xClx | 0 | 241 | 1.104 | 17.92 | 82.62 | 16.34 | 19.3,26 14.3 and 10.9,30 17.40,30 14.14,39 12.67,41 14.64,42 10.895 |
| Rs = 1, Rsh = 103 | 1.100 | 17.91 | 77.14 | 15.19 | |||
| FASnI3-xClx | 0 | 230 | 0.683 | 21.28 | 68.32 | 9.90 | 5.30,48 12.1149 |
| Rs = 1, Rsh = 103 | 0.679 | 21.26 | 64.84 | 9.37 | |||
| CsSbCl4 | 0 | 240 | 0.866 | 20.15 | 74.91 | 13.08 | 20.0753 |
| Rs = 1, Rsh = 103 | 0.863 | 20.13 | 70.54 | 12.26 |
From Table 4 and the red line curve of Figure 5B–D, it can be clearly seen that as Rs = 1 and Rsh = 1000 Ω−cm2 are introduced in the structure, the solar cell performance deteriorates, implying that the series and shunt resistances affect the performance. The decrease of the solar cell parameters with adding Rs and RSh insights that leakage loss increases, hence the performance of the proposed solar cell device decreases.
It is well known that the light-absorbing material is the crucial part of a solar cell device. Absorber layer thickness is the major deciding factor for device efficiency.95,96 The effect of the thickness of the absorber layer on device performance was inspected by the variation of absorber thicknesses in the range of 100–2400 nm at the optimized thickness of electron transport layer (ETL) and hole transport layer (HTL). Figure 6a, a′, b, b′, c, c′ show the effect of the thickness variation of the absorber layer MAPbI3-xClx, FASnI3-xClx, and CsSbCl4 on the solar cell performance parameters. From Figure 6, it is evident that a significant variation of the efficiency is observed for all the absorber layers with an increasing layer thickness up to ~1.0 μm, after that, the efficiency is more or less constant. The variation of the value of JSC is similar to efficiency, which increases acceleratedly with an increase in absorber layer thickness and reached its saturated value around 1.2 μm. The obtained results provide insight that thicker perovskite absorbs a large number of incident photons, which can generate more carriers with a higher wavelength, resulting in the improvement of the performance of the device.97 A significant decrement of FF with an increase of the absorber thickness and a very little change in VOC is also observed with the variation of thickness due to increased recombination of electron–hole pairs, decreasing carrier lifetimes.
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TABLE 5 Optimized solar cells performance parameters of the as-deposited thin films obtained from SCAPS-1D software.
| Sample | Thickness (μm) | VOC (Volt) | JSC (mA/cm2) | FF% | η% | References |
| MAPbI3-xClx | 1.00 | 1.12 | 24.46 | 76.12 | 20.78 | 19.3,26 14.3 and 10.9,29 17.40,30 14.14,39 12.67,41 14.64,42 10.895 |
| FASnI3-xClx | 1.00 | 0.693 | 29.16 | 59.01 | 11.93 | 5.30,48 12.1149 |
| CsSbCl4 | 1.00 | 0.883 | 27.34 | 74.64 | 18.02 | 20.0753 |
On the other hand, the defects have a major effect on the performance of solar cell devices. Generally, the solar cell performance is typically evaluated based on its efficiency, but the actual efficiency of the device cannot be found due to the presence of different defects in the absorber layer. It is well known that the presence of high defect density in the absorber layer directly effects cell efficiency; hence, the relation between cell efficiency and the presence of neutral defects density in the perovskite layers needs to be evaluated. In this context, the solar cell performance parameters are shown in Figure 7A–C and Table 6 based on the existing neutral defect concentration in the absorber layers. From Figure 7 and Table 6, it is evident that the defect concentration is lower than 1014 cm−3 in the absorber layer, which does not show any significant change due to less scattering of the photogenerated carriers, and hence there is no or little effect on the device performance. The performance decreases gradually with increasing the defect concentration from 1014 to 1016 cm−3. Importantly, an abrupt deterioration of the efficiency and FF are observed (Figure 7A–C) for all proposed solar cell devices beyond 1016 cm−3 defect density could be due to an increase of the surface recombination rate. From the above phenomena, it can be summarized that the neutral defects in the absorber layers: MAPbI3-xClx, FASnI3-xClx, and CsSbCl4 strongly affect the PV performance of the perovskite solar cells. Therefore, the neutral defects in the absorber layer must be lessened down to least probable values so that superior PV efficiency can be achieved.
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TABLE 6 Performance parameters for different concentrations of acceptor type defects density induce in perovskite layers calculated by SCAPS-1D software.
| Sample name | Neutral defect, Nt (1/cm3) | VOC (Volt) | JSC (mA/cm2) | FF% | η% |
MAPbI3-xClx |
1 × 1013 | 1.104 | 17.92 | 82.61 | 16.35 |
| 1 × 1014 | 1.104 | 17.92 | 82.62 | 16.34 | |
| 1 × 1015 | 1.099 | 17.92 | 82.74 | 16.29 | |
| 1 × 1016 | 1.075 | 17.91 | 82.35 | 15.86 | |
| 1 × 1017 | 1.031 | 17.89 | 76.83 | 14.16 | |
| 1 × 1018 | 0.989 | 17.51 | 61.61 | 10.67 | |
| 1 × 1019 | 0.956 | 14.48 | 37.69 | 5.22 | |
| 1 × 1020 | 0.871 | 5.92 | 33.64 | 1.74 | |
FASnI3-xClx |
1 × 1013 | 0.680 | 21.28 | 68.32 | 9.90 |
| 1 × 1014 | 0.680 | 21.28 | 64.32 | 9.90 | |
| 1 × 1015 | 0.680 | 21.27 | 68.25 | 9.88 | |
| 1 × 1016 | 0.679 | 21.12 | 67.63 | 9.70 | |
| 1 × 1017 | 0.666 | 19.88 | 63.19 | 8.37 | |
| 1 × 1018 | 0.637 | 14.93 | 53.18 | 5.05 | |
| 1 × 1019 | 0.610 | 9.39 | 47.95 | 2.75 | |
| 1 × 1020 | 0.595 | 4.71 | 57.08 | 1.60 | |
CsSbCl4 |
1 × 1013 | 0.866 | 20.15 | 74.91 | 13.08 |
| 1 × 1014 | 0.866 | 20.15 | 74.91 | 13.08 | |
| 1 × 1015 | 0.866 | 20.15 | 74.91 | 13.08 | |
| 1 × 1016 | 0.866 | 20.14 | 74.87 | 13.06 | |
| 1 × 1017 | 0.862 | 20.02 | 74.52 | 12.86 | |
| 1 × 1018 | 0.843 | 18.95 | 71.48 | 11.41 | |
| 1 × 1019 | 0.804 | 14.14 | 62.45 | 7.10 | |
| 1 × 1020 | 0.767 | 8.41 | 53.08 | 3.43 |
Redox potentials
Use of the solar energy to split water and produce clean energy is a feasible and effective approach to solving energy and environmental problems. As excellent intriguing optoelectronic properties, perovskites received enormous attention for pollutant degradation and water splitting applications. Accordingly, the energy-level positions of CBM and VBM must satisfy the requirement for photocatalytic reactions with respect to water reduction potential of H+/H2 (0.0 eV) and the oxidation potential level of O2/H2O (1.23 eV). Theoretically, positions of CBM and VBM can be predicted as,98
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CONCLUSIONS
Pb-free perovskites are promising materials for solving the energy crisis and mitigating carbon emissions. In this work, firstly organic–inorganic Pb based mixed-halide perovskite MAPbI3-xClx, then its MA and Pb parts are replaced by FA and Sn, growth in the form of FASnI3-xClx and finally Pb-free all inorganic perovskite CsPbCl4 thin films are successfully deposited by CSP process in atmospheric conditions. The metallurgical optical microscope images exhibit that the films surface are compacted and the color of the precursor solution changes with stirring time. Large and void as well as defect less grains are developed on the films' surface. The existence of the mixed-phases is observed which is confirmed from the XRD analysis. The crystallinity and grains size of the films change with the compositions. The absorption coefficient is found to be high enough on the order of 105 cm−1 and the band gap can be tuned in the range of 1.46 to 1.59 eV which are very important to the solar cell application as a harvesting layer. Importantly, the nature of the films is important for photonic and optoelectronic applications. The solar cell efficiency is calculated and obtained as 16.34%, 9.90%, and 13.08% for the deposited films, and the efficiency of the optimized cells is 20.78%, 11.93%, and 18.02% for MAPbI3-xClx, FASnI3-xClx, and CsSbCl4-based solar cells, respectively. The neutral defect density in the perovskite layers affects the solar cells efficiency. Theoretical photocatalytic activity calculation shows that the deposited films can easily split H2O into pronounced active species O2 by the strong oxidation of the photogenerated holes under electromagnetic irradiation. However, they may be capable of HER reaction if co-catalyst is added with them, which can be beneficial for proper photocatalytic activity for overall water-splitting reactions performance and for experimental study. Therefore, the Pb-free perovskites thin films deposited by the CSP technique exhibit stable structure and excellent properties, which would be cost effective for optoelectronic and solar cell device applications.
MATERIALS AND METHODS
Materials
Methylammonium iodide, CH3NH3I (99.999%, Sigma-Aldrich, USA) Formamidinium Iodide, CH5N2I (99.999%, Sigma-Aldrich, USA), cesium iodide, CsI (99.999% Merck, Germany), cesium chloride, CsCl (99.999% Merck, Germany), antimony chloride, SbCl3 (99.999%, Sigma-Aldrich, USA), N,N-dimethylfomamide, DMF (Merck, Germany), tin chloride, SnCl4 (99.999% Merck, Germany), lead chloride, PbCl2 (99.999%, Merck, Germany), ethanol, C2H5OH (99.99%, Merck, Germany), acetone, C3H6O (99.99%, Merck, Germany), and so forth. Chemicals were purchased from different chemical sources and used without further purification.
Methodology
Substrates cleaning and thin films deposition
Before thin films deposition, the microscopic glass slides were cleaned as References102,103 Thereafter, the MAPbI3-xClx, FASnI3-xClx, and CsSbCl4 thin films were grown on these cleaned substrates using a two steps CSP technique in atmospheric conditions (as shown in Figure S1A,B).
Step-1: Precursor solution making process
Figure S2(X–Z) shows different stages of precursor solution making for MAPbI3-xClx, FASnI3-xClx, and CsSbCl4 thin films growth in an ambient atmosphere. First, MAI:PbCl2, FAI:SnCl2, and CsCl:SbCl3 (molar ratio 3:1) were dissolved in a beaker containing 100 mL DMF solvent, and the fresh mixed solution looked like turbid solution (Figure S2(X-Z) left first one of each image (a)). The turbid solution was continuously stirred with a constant speed, and its color started to change and slowly became transparent with increasing stirring time (Figure S2 (X-Z)b, middle of each image). At the end of 30 min of stirring, the solution became transparent (Figure S2(X-Z)c of each image). Interestingly, the solution completely became highly transparent at the end of 60 min stirring time, and an attractive colorful fine solution was obtained for thin film growth, as shown in Figure S2(X-Z)d of each image. This highly transparent fine solution insights that reacted was taken place, and crystallization could be occurred at room temperature. This fine solution was sprayed unto clean glass substrate at a constant temperature of 125°C to fabricate MAPbI3-xClx, FASnI3-xClx, and CsSbCl4 solid thin films.
Step-2: Thin film fabrication process
The fine solution was kept in a measuring cylinder ‘F’ fitted with the spray nozzle “A” (Figure S1A). The clean substrate with a suitable mask was put on the susceptor of the heater “H.” The distance between the tip of the nozzle and the surface of the glass substrate was kept ≤25 cm. Before supplying the compressed air, the substrate temperature “Ts” was kept at a level (128°C) slightly higher than the required substrate temperature (125°C) because, at the onset of spraying a slight fall of temperature is likely. The temperature of a substrate was controlled by controlling the heater power using a variac. The substrate temperature was measured by placing a copper constantan thermocouple on the substrate. When compressed air is passed through “P” at constant pressure of 0.50 bar, the fine mixed solution of MAI and PbCl2 was automatically carried to the reactor zone, where MAPbI3-xClx film is deposited on the glass substrate that was kept on a hot plate at a temperature of 125C. The solution was adjusted for 30 min at a flow rate of 0.2 mL/min for producing MAPbI3-xClx solid layer. Finally, an annealing step was carried out on the hot plate to the specified temperature (125°C) to achieve the best crystalline perovskite thin film. After that, the film was cooled down slowly to room temperature to remove residual organic solvents and to form a MAPbI3-xClx solid layer. For the deposition of CH5N2SnI3-xClx (FASnI3-xClx) and CsSbCl4 thin films, FAI:SnCl2, and CsCl:SbCl3 (molar ratio 3:1) reagents were mixed in the 100 mL DMF as before, and then followed by the same conditions. When the solution is sprayed on the heated substrates the following reactions may take place as:
Characterization techniques
After synthesized perovskite thin films, a few characterization techniques have been performed which are the prerequisites for good research and the fabrication of high quality materials to design and improve device performance. To this end, the microstructures of the fabricated thin films were characterized by Kern metallurgical optical microscope (OKM 172) under magnification of 400×. The surface morphological property of the deposited films is characterized by field emission SEM (EF-SEM). The XRD experiment was performed to identify crystal structure and the crystallinity using a Smart Lab diffractometer instrument with incident CuKα radiation (λ = 0.15418 nm). The scanning rate of 0.02°/s was employed in the 2θ range of 10° to 60°. The optical properties were investigated by the UV-2600 UV–VIS Spectrometer (Shimadzu, Japan). The solar cell performance of the films was inspected by theoretical calculations using SCAPS One Dimensional (1D) 3.3.08 software.85 The photocatalytic performance was demonstrated by theoretical calculations.
Numerical modeling and material parameters
Device structure
In this work, we proposed a perovskite solar cell as: glass supported-FTO/ZnO:Al/CdS/perovskite/Spiro-OMeTAD/back contact (Figure 5A) and the solar cell performance parameters of the deposited films were calculated using SCAPS-1D 3.3.08 software.85 The calculation was performed under AM 1.5G one sun solar spectrum with an intensity power of p = 1000 W/m2 at 300 K. Au and Ag were used as the back and front contacts interfaces with the work functions of 5.10 and 4.26 eV. The surface recombination velocity was set to 1 × 105 and 1 × 107 cm/s in the absorber/back contact-interface, whereas 1 × 107 and 1 × 105 cm/s in the window/front contact-interface for electron and hole, respectively. We consider the interface HTM/absorber and absorber/ETM defect type for acceptor capture cross section electrons/holes (cm2) 1 × 10−18, 1 × 10−19, and 1 × 10−17, 1 × 10−198. Al doped ZnO (AZO) layer was used to play both roles of window and ETL and Spiro-OMeTAD was used as HTL. The effect of series resistance Rs = 1 Ω.cm2 and shunt resistance Rsh = 103 Ω.cm2 was investigated while the sheet resistance was set to10−3 Ω.cm−2. The input data were collected from different published reports53,86–94 which are listed in Table 3, and the interface parameters are provide in Table 7. In this study, the obtained optical band gap values and the thicknesses of the as-deposited thin films were used to simulate the solar cell performance parameters.
TABLE 7 Parameters of interface layer.
| Interface | Defect type | Capture cross section electrons/holes (cm2) | Energetic distribution | Reference for defect energy level Et | Energy with respect to Reference (eV) | Total density (integrated over all energies) (1/cm2) |
| HTM/absorber | Acceptor | 1 × 10−18 1 × 10−19 |
Single | Above the highest EV | 0.60 | 1 × 1010 |
| Absorber/ETM | Acceptor | 1 × 10−17 1 × 10−18 |
Single | Above the highest EV | 0.60 | 1 × 1010 |
AUTHOR CONTRIBUTIONS
M. Kamruzzaman performed conceptualization, design the study, validation, interpreted the data, and wrote the manuscript. Md. Faruk Hossain, Juan Antonio Zapien, H. N. Das, and A. M. M. Tanveer Karim are collaborators who help to characterize the samples using SEM, XRD, and UV-vis spectroscopy measurements, and to study, data analysis and wrote the manuscript. M. A. Helal help to study, data analysis and curation, writing-review, and editing the manuscript.
ACKNOWLEDGMENTS
The authors would like to express their sincere thanks and heartiest gratitude to the Government of the People's Republic of Bangladesh, Ministry of Education, Bangladesh Bureau of Educational Information & Statistics (BANBEIS) for providing research fund under the “Grant for Advanced Research in Education (GARE)” in the fiscal year 2023/24. The author also would like to thanks materials science lab of the department of Physics, Begum Rokeya University, Rangpur allow for spray pyrolysis technique.
CONFLICT OF INTEREST STATEMENT
The authors declare no conflict of interest.
DATA AVAILABILITY STATEMENT
The data used and/or analyzed during the current study are available from the corresponding author (email:
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Abstract
MAPbI3 is the most attractive perovskite, but toxicity and instability issues hinder its commercial applications. Stability can be improved by halide mixing; however, Pb‐free perovskites are designed to alleviate the toxicity and to enable green photovoltaics (PVs). To this end, MAPbI3‐xClx, FASnI3‐xClx and CsSbCl4 films are deposited by spay pyrolysis technique in atmospheric conditions. SEM images demonstrated that through this process, high quality film fabrication is possible. Color of the precursor solutions changes with stirring time. High crystallinity and existence of mixed‐phases are confirmed by XRD analysis. Compositions greatly impact the morphology and optical properties. Value of α is larger than 105 cm−1 for all films. Band gaps of FASnI3‐xClx and CsSbCl4 are 1.46 eV and 1.52 eV, which are more suitable for PVs, optoelectronic applications than MAPbI3‐xClx (Eg = 1.59 eV). The efficiency was obtained as 16.34%, 9.90%, and 13.08% for deposited MAPbI3‐xClx, FASnI3‐xClx, and CsSbCl4 films. The lower efficiency can further be enhanced by optimizing parameters, and in this study it was found as 20.78%, 11.93%, and 18.02%. Theoretical calculations show the films can easily produce O2 by a strong oxidation process. Thus, the favorable characteristics of FASnI3‐xClx and CsSbCl4 make alternative Pb‐free perovskites for PV, electronic, and optoelectronic applications.
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Details
; Hossain, Md. Faruk 2 ; Zapien, J. Antonio 3 ; Karim, A. M. M. Tanveer 4 ; Das, H. N. 5 ; Helal, M. A. 1 1 Department of Physics, Begum Rokeya University, Rangpur, Bangladesh
2 Department of Electrical and Electronics Engineering, Rajshahi University of Engineering and Technology, Rajshahi, Bangladesh
3 Department of Materials Science and Engineering, Center of Super‐Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong SAR, People's Republic of China
4 Department of Physics, Rajshahi University of Engineering and Technology, Rajshahi, Bangladesh
5 Materials Science Division, Atomic energy center Dhaka, Dhaka, Bangladesh