Abstract

The exploitation of fossil resources to meet humanity’s energy needs is the root cause of the climate warming phenomenon facing the planet. In this context, non-carbon-based energies, such as photovoltaic energy, are identified as crucial solutions. Organic perovskites MAPbI₃ and FAPbI₃, characterized by their abundance, low cost, and ease of synthesis, are emerging as candidates for study to enhance their competitiveness. It is within this framework that this article presents a comparative analysis of the performances of MAPbI₃ and FAPbI₃ perovskites in the context of photovoltaic devices. The analysis focuses on the optoelectronic characteristics and stability of these high-potential materials. The optical properties of perovskites are rigorously evaluated, including band gaps, photoluminescence, and light absorption, using UV-Vis spectroscopy and photoluminescence techniques. The crystal structure is characterized by X-ray diffraction, while film morphology is examined through scanning electron microscopy. The results reveal significant variations between the two types of perovskites, directly impacting the performance of resulting solar devices. Simultaneously, the stability of perovskites is subjected to a thorough study, exposing the materials to various environmental conditions, highlighting key determinants of their durability. Films of MAPbI₃ and FAPbI₃ demonstrate distinct differences in terms of topography, optical performance, and stability. Research has unveiled that planar perovskite solar cells based on FAPbI₃ offer higher photoelectric conversion efficiency, surpassing their MAPbI₃-based counterparts in terms of performance. These advancements aim to overcome stability constraints and enhance the long-term durability of perovskites, ultimately aiming for practical application of these materials. This comprehensive comparative analysis provides an enlightened understanding of the optoelectronic performance and stability of MAPbI₃ and FAPbI₃ perovskites, which is critically important to guide future research and development of solar devices that are both more efficient and sustainable.

Keywords

Perovskites, FAPbI₃, MAPbI₃, Optoelectronic Properties, Performance

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1. Introduction

In recent years, tremendous progress has been made in the field of perovskite-based solar cells [1] . Perovskites are crystalline compounds with an ABX₃ chemical structure, where A represents a monovalent cation, B represents a divalent cation, and X represents a halide anion [2] . Within just a decade, their conversion efficiencies have increased from 6% to 25% [3] . The ongoing investigations regarding these materials aim to achieve a yield rate of 30% in the coming years [4] . These advancements are attributed to the multiple advantages offered by perovskites, including their broad optical absorption [5] , making them promising candidates for solar energy harvesting [6] . Furthermore, perovskites stand out due to their ease of synthesis from abundant and low-cost materials [7] , paving the way for large-scale production. Among the various perovskite variants, MAPbI₃ (methylammonium lead triiodide) and FAPbI₃ (formamidinium lead triiodide) compounds have demonstrated particularly promising properties for photovoltaic applications [8] . So far, the methylammonium (MA) cation has been the most studied, but MA-based perovskites generally exhibit band gaps greater than 1.55 eV [9] , limiting their optical absorption range and photoelectrical conversion efficiency [10] . To address this issue, the formamidinium (FA) cation has been developed as a more efficient absorber, with a narrower band gap of 1.47 eV [11] compared to MAPbI₃. Current research focuses on improving the stability of these materials to make them more suitable for large-scale utilization [12] . Materials and processes have been developed to address this challenge, and the use of FA as a replacement for MA has contributed to reducing the band gap towards a more ideal range [13] . Additionally, hybrid organic/inorganic perovskites have been explored, featuring high absorption coefficients, appropriate band gaps, and long carrier lifetimes with high mobility [14] [15] . Another crucial aspect is enhancing the stability of perovskites as it represents a major hurdle for their long-term utilization [16] . In this regard, a comprehensive comparative analysis of the optoelectronic performance and stability of MAPbI₃ and FAPbI₃ perovskites has been conducted. This analysis encompassed several key aspects such as the optical properties of perovskites, the crystal structure of perovskite films characterized by X-ray diffraction (XRD), the film morphology analyzed using scanning electron microscopy (SEM). The main objective was to identify significant differences between MAPbI₃ and FAPbI₃ in terms of optoelectronic performance and stability and determine the key factors influencing the durability of these materials. Additionally, resistivity and conductivity measurements were performed on the films. The obtained results demonstrated good crystallinity as well as higher efficiency and stability of the FAPbI₃ film compared to the MAPbI₃ film. These findings will provide valuable insights to guide research and development of novel strategies to enhance the efficiency and stability of perovskite-based solar cells.

2. Materials and Experimental Procedure

2.1. Materials

The necessary reagents for the synthesis of MAPbI₃ and FAPbI₃ perovskites were purchased from Sigma Aldrich, St. Louis, MO, USA, without requiring prior purification. The films were synthesized using perovskite solutions prepared from the following precursors: methylammonium iodide (MAI) with a concentration of 99.99%, formamidinium iodide (FAI) with a concentration of 99.99%, and lead (II) iodide (PbI₂) with a concentration of 99.99%. The preparation of perovskite solutions was carried out in small, very dark brown glass vials to prevent any undesirable reaction with light. For the preparation of FAPbI₃, the PbI₂ and FAI precursors were dissolved in a solvent mixture consisting of 90% anhydrous N,N-dimethylformamide (DMF) and 10% dimethyl sulfoxide (DMSO). As for the preparation of MAPbI₃, the PbI₂ and MAI precursors were dissolved in the same solvent mixture. Chlorobenzene (or toluene) was used as an antisolvent during the deposition of the perovskite films.

2.2. Production of the Films

Perovskite films were fabricated on pre-cleaned FTO glass substrates. The substrates were washed with Hell Max soap solution in distilled water, followed by rinsing with ethanol and acetone using an ultrasonic bath. Subsequently, any residual organic matter was removed through UV-Ozone treatment. To prepare PbI₂ solutions, 0.461 g of PbI₂ was dissolved in 1 ml of a solvent mixture consisting of 90% DMF and 10% DMSO. This dissolution was carried out in a small dark brown bottle. The resulting solutions were heated at 60˚C for two hours on a hot plate. To obtain perovskite solutions, each bottle containing MAI and FAI received 1 ml of PbI₂ solution. The obtained solutions were heated at 60˚C for two hours. For deposition, a volume of 100 μL of perovskite solution was used to coat the FTO glass substrate through centrifugation at a speed of 4000 revolutions per minute for 20 seconds using a spin-coater. The thickness of the deposited thin film depended on the spin-coater rotation time [17] . During this process, a few drops of chlorobenzene or toluene were deposited on the still wet perovskite films. Finally, the obtained deposits underwent thermal annealing. MAPbI₃ was annealed at 150˚C [18] for 20 minutes on a hot plate, while FAPbI₃ was annealed at 200˚C [19] for 20 minutes on a hot plate as shown in Figure 1.

2.3. Lattice Parameters

The parameters were calculated using the following equations:

$\frac{1}{d^2}=\frac{h^2+k^2}{a^2}+\frac{l^2}{c^2}$ (1)

and $n\lambda =d2\sin \left(\theta \right)$ (2)

2θ: Difraction angle of the corresponding plane.

λ: Wavelength of the CuK α radiation (0.154 nm),

h, k, l: Miller indices and a and c are lattice constants, d: Inter-planar-spacing, a, c: Lattice constants,

Octahedral factor

In the ABX₃ frame work perovskites, the stability of the BX₃ octahedron is predicted by the octahedral factor: µ

$\mu =\frac{r_B}{r_X}$ Or ${\mu }_n=\frac{{\displaystyle {\sum }_{i=1}^n{r}_{B^i}}}{{\displaystyle {\sum }_{i=1}^n{r}_{X^i}}}$ (3)

If 0.41 < µ < 0.73, the coordination is octahedral.

Goldschmidt tolerance factor

The Goldschmidt tolerance factor t is established by the relationship [20] :

$t=\frac{r_A+r_X}{\sqrt{2}\left(r_B+r_X\right)}$ Or $t=\frac{{\displaystyle {\sum }_{i=1}^n\left(r_{A^i}+r_{X^i}\right)}}{\sqrt{2}{\displaystyle {\sum }_{i=1}^n\left(r_{B^i}+r_{X^i}\right)}}$ (4)

Grain size and effective lattice strain

The calculation of the effective grating deformation gives an overview of the defects and distortions of the grains at the level of the films, for the calculation we use the equation:

$\cos \theta {\beta }_r=4\epsilon \sin \theta +\frac{k\lambda }{D}$ (5)

k: Constant whose value is 0.94, λ: 0.15406 nm wavelength of the X ray source,

D: Crystallite size or half-width (FWHM), ε: Deformation,

θ: Position of the peak in radians where is the Bragg angle.

Dislocation density

The dislocation density of the crystal was evaluated using the formula;

$\gamma =\frac{1}{D^2}$ (6)

Absorption coefficient

Perovskites are direct band gap semiconductors. The energy band gap is calculated from an estimate of the trace (αhν)² with respect to hν.

${\left(\alpha h\nu \right)}^2=B\left(h\nu -Eg\right)$ (7)

$\alpha =\frac{1}{t}\ln \left(\frac{1}{T}\right)$ (8)

α: Absorption coefficient,

h: Planck constant,

Eg: Forbidden band energy,

t: Thickness of the layers,

B: Constant.

Degradation rate (D)

$D=\frac{A_{\circ }-A}{A_{\circ }}$ (9)

or $A_{\circ }$ absorption of fresh sample A old sample absorption

3. Results and Discussions

Figure 2 shows the curves of the octahedral factor (µ) and the Goldschmidt tolerance factor (t). The calculated octahedral factors have a constant value of 0.4454 (Table 1), which corresponds to a range of values of 0.41 < µ < 0.73. According to Pauling’s rules for cation/anion geometry, this indicates octahedral coordination [21] [22] . The calculated values of the Goldschmidt tolerance factors are listed in Table 1. The t factor for FAPbI₃ (1.0517) is significantly higher than that of MAPbI₃ (0.9717). However, the calculated values of the Goldschmidt tolerance factors range between 0.813 and 1.107, confirming the formation of halide perovskites [23] [24] .

Figure 3 shows XRD diffraction spectra of MAPbI₃ and FAPbI₃ films. In the MAPbI₃ spectrum, the 2θ value of 12.80˚ represents unreacted residual PbI₂ [25] . The corresponding peak for FTO is located around 37.83˚ [26] . Characteristic peaks in the MAPbI₃ spectrum are found at 2θ coordinates of 14.24˚ and 28.57˚, corresponding to the planar orientations of (110) and (220) that represent parallel planes in the perovskite structure [27] . These peaks indicate preferential growth of this layer in the tetragonal direction [28] . Both of these characteristic

peaks have higher intensities compared to the other peaks. Similarly, medium intensity peaks are observed at positions 20.17˚, 23.83˚, 24.73˚, 31.54˚, 40.74˚, and 43.74˚, corresponding to the planar orientations of (112), (211), (202), (314), (214), and (330). These peaks coincide with the reference peaks [29] . In the FAPbI₃ spectrum, there are the desired alpha (α) phase and the gamma (δ) phase. The characteristic peak intensities for the alpha (α)-FAPbI₃ phases are located at positions 14.06˚ and 28.17˚, corresponding to the (001) and (002) orientations, which are parallel planes within the structure. For the same alpha (α)-FAPbI₃ phase, peaks are observed at 20.07˚, 24.10˚, 28.17˚, 31.95˚, 40.28˚, and 42.78˚, which correspond to the (110), (111), (002), (202), (202), and (100) orientations. These peaks coincide with the reference peaks [30] . The gamma (δ)-FAPbI₃ phase exhibits peaks at 2θ values of 11.34˚ (001), 25.93˚ (201), and 32.53˚ (202). It can be observed that FAPbI₃ undergoes nearly complete conversion, with distinct peaks that match the reference peaks and almost no residual PbI₂ peaks.

The data in the Table 2 allows us to plot the curves of the FWHM, D the grain size, d the interplanar spacing, the γ dislocation density and the ε deformation: Figure 4.

The FWHM data represents the (110) and (220) peaks of FAPbI₃ and MAPbI₃ perovskites. Around the 14.0˚ peak, the FWHM value of MAPbI₃ is higher than that of FAPbI₃, while around the 28˚ peak, the FWHM value of FAPbI₃ is higher than that of MAPbI₃. Structural parameters were calculated using the Williamson-Hall (WH) plot method in Table 2 based on XRD analysis data. The inter-planar spacing (d) value of FAPbI₃ is larger than that of MAPbI₃. Similarly, the grain size of FAPbI₃ is larger than that of MAPbI₃. Moreover, the average dislocation density (γ) and strain (ε) values are higher for FAPbI₃ compared to MAPbI₃. XRD analysis of the data reveals significant differences between FAPbI₃ and MAPbI₃.

The SEM images shown in Figure 5(a) & Figure 5(b) display the FAPbI₃ film (a) and the MAPbI₃ film (b). It can be observed that they have good adhesion to the substrate and are relatively rough. The surface roughness is correlated with the presence of grains, and we note that the surface of the FAPbI₃ film is rougher with more pores compared to the MAPbI₃ film. The multiplicity of pores and surface roughness allow the films to trap more light [31] [32] .

Optical properties

The analysis of optical properties of thin perovskite films was conducted in

the wavelength range of 400 to 900 nm. The absorbance, transmission, and energy curves are shown in Figure 6 & Figure 7. The films exhibit strong absorption in the near-infrared-visible region. Additionally, a significant increase in film absorption is observed across the entire 400 - 900 nm range, likely due to an abundance of electronic transitions at vibrational or rotational energy levels available in the absorbing materials [33] . However, a noticeable elevation of the

absorption edges in FAPbI₃ compared to MAPbI₃ is observed, with a maximum value of 2.5 (a.u.). The increased absorption in the samples can be attributed to improved crystallinity and film roughness [34] . Transmission curves have also been plotted, with the lowest value for MAPbI₃ at around 13% and the lowest transmission for the FAPbI₃ film reaching a minimum of 2%. This is in line with the XRD and SEM analyses. The film’s rigidity optimizes the trapping of incident light [35] .

The energy curves of the perovskites display the optical band gaps of FAPbI₃ and MAPbI₃. The band gap of FAPbI₃ is significantly lower than that of MAPbI₃. This difference could be attributed to the larger grain size of FAPbI₃ compared to MAPbI₃ [36] . The disparity in the band gaps of the films reflects a difference in lattice parameters [37] .

The photoluminescence measurement spectrum allows determining the band gaps of perovskite films, which can then be compared to the band gaps obtained from energy spectra.

Figure 8 shows the photoluminescence (PL) emission spectra of FAPbI₃ and MAPbI₃ films. The FAPbI₃ film exhibits the highest photoluminescence intensity, indicating strong emission. On the other hand, the MAPbI₃ film shows weak PL emission and appears darker. The reduced PL emission of MAPbI₃ can be attributed to a lower density of surface trap states, resulting in a decrease in non radiative recombination pathways [38] [39] . This suggests that recombination predominantly occurs through radiative processes [40] . The observed PL spectra are consistent with the XRD and SEM results.

Table 3 summarizes the values of optical band gap (Eg) of FAPbI₃ and MAPbI₃, extracted from UV and PL measurements. A small difference is observed

between these values. The Eg values for FAPbI₃ are 1.46 eV (UV) and 1.54 eV (PL), while those for MAPbI₃ are 1.60 eV (UV) and 1.61 eV (PL).

The electrical resistivity of the films was measured using the four-point probe method based on the Hall effect [41] . To determine the resistivity of the perovskites, deposits were made by spin coating on simple glass substrates without FTO. Thicknesses on the order of 500 nm were found. The electrical resistivity ρ is obtained by applying the equation: ρ = Rs × t, where t is the thickness and Rs is the resistivity of the thin film. Rs = 4.5324 × V/I represents the surface resistance of the film, and 4.532 is the correction factor (Figure 9).

The resistance of perovskites is measured using a low current intensity, in the nano-volt range, to avoid any alteration of the film structure due to prolonged exposure to the electrodes [42] . The films exhibit high resistivity values, primarily due to their organic nature, for both FAPbI₃ and MAPbI₃ [43] [44] . The lowest resistivity is observed for FAPbI₃. The resistivity and conductivity values of the charge carriers are listed in Table 4. The resistivities are very close, as are the conductivities. To facilitate the comparison of resistivities, their values have been reduced by 224, and the conductivity values have also been reduced by 438.

4. Degradation Study

The images of the aged films are obtained using the scanning electron microscopy

(SEM) technique and are presented in Figure 10. The degradation process has highlighted the presence of multiple pinholes as well as significant alterations in the surface morphology of the films. The image corresponding to the most deteriorated sample pertains to the MAPbI₃ that has been exposed to two weeks of aging in a humid environment. These findings are in line with the results obtained through X-ray diffraction (XRD) and absorption. Thus, they confirm the higher intrinsic stability of the FAPbI₃ film compared to the MAPbI₃ film [45] .

The degradation mechanism of the films has undergone analysis through photoluminescence (PL) for the two aged samples presented in Figure 11(a) & Figure 11(b). The films underwent a degradation process, and aging had a direct impact on the amplitude of the PL curves [46] . The collective PL curves of the aged films revealed a reduction in their intensity. However, it is noteworthy that the degradation curve associated with FAPbI₃ exhibited a less pronounced decrease compared to that of MAPbI₃, which lost its photoluminescence properties

[48] [49] [50] [51] . Additionally, a shift in the bandgap of FAPbI₃ was observed, transitioning from 1.548 eV for the initial sample to 1.556 eV for the degraded sample. The results from the PL measurements of the aged films align with absorption assessments, which also indicated a considerable decrease in absorption amplitudes.

Figure 12 shows the absorption spectra of both fresh and aged perovskite films. These absorption spectra reveal a difference between the films before and after aging. A decrease in absorption can be observed in all aged samples. The aged film of FAPbI₃ exhibits the highest absorption compared to the aged film of MAPbI₃.

The degradation rate curves of the films are also depicted in Figure 13. These curves illustrate the degradation speed of each film [47] . It is observed that the

degradation curve of FAPbI₃ is significantly lower than that of MAPbI₃. The FAPbI₃ film undergoes degradation at a slower pace compared to the MAPbI₃ film, and this observation aligns with the analyses conducted through scanning electron microscopy (SEM) of the degraded films.

5. Conclusion

In this study, we analyzed the structures of FAPbI₃ and MAPbI₃ perovskites. The structural, optical, and morphological measurements have shown that the FAPbI₃ film exhibits improved characteristics compared to the MAPbI₃ film. FAPbI₃ demonstrates a homogeneous and rough surface, allowing for maximum trapping of incident light, unlike MAPbI₃. XRD results indicated the presence of residual PbI₂ in the MAPbI₃ film. SEM images revealed relatively rough surfaces for all thin films. The band gap of FAPbI₃ is lower than that of MAPbI₃, and conversely, the absorption curves of FAPbI₃ are significantly higher than those of MAPbI₃. The band gaps obtained from photoluminescence (PL) spectroscopy analysis closely match the band gaps obtained from absorption measurements. Additionally, it was observed that the FAPbI₃ film experiences less degradation compared to the MAPbI₃ film, which exhibits a yellowish coloration on the aged film, indicating the presence of the PbI₂ precursor. These results contribute to the fundamental understanding of the degradation mechanism of perovskites, providing strategies for designing stable and efficient perovskite-based devices.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

References