⁃ The MA(1-X)TBAX PbI3 thin film crystallinity was enhanced with TBA incorporation.

To begin with, nowadays, the Perovskites with formula ABX₃ (A = cation (Formamidinium (FA), Methylammonium (MA).), B = metal cation and X is a halogen anion VII halides (Br⁻, Cl⁻, I⁻)), have been demonstrated good absorbers properties for solar cells with higher photovoltaic conversion efficiency (PCE) (Fakharuddin et al., 2019; Weidman et al., 2019). Methylammonium lead triiodide (MAPbI₃) solar cells show good optoelectronic properties with a bandgap between 1.4 and 1.5 eV (Wang et al., 2019), a high absorption coefficient around 10⁵cm⁻¹ (Im et al., 2011; De Roo et al., 2016) with amazing PCE = 25% (Xiao et al., 2014; Park, 2015). To manufacture MAPbI₃ Solar cells, low-cost and simple techniques have been used to deposit the MAPbI₃ film: thermal vapor deposition, two-step vapor-assisted deposition (Chen et al., 2013), two-step solution deposition, and one-step solution deposition (Yantara et al., 2015; Patel et al., 2017). Despite the outstanding PCE reported of MAPbI₃ solar cells, the problem of MAPbI₃ degradation by the loss of MAI and formation of lead iodide PbI₂ under humid conditions and higher temperatures makes use of MAPbI₃ difficult (Ko et al., 2015). Concerning to improve the stability of MAPbI₃, MAPbI₃ was doped by cesium and showed the potential to enhance the stability of MaPbI₃ solar cell under UV irradiance conditions (Niu et al., 2016); also on the road for stable MAPbI₃, the divalent anion Se²⁻ was incorporated in MAPbI₃ structure to increase the atomic interactions between the inorganic and the organic cations (Gong et al., 2018; Li et al., 2018). Considering the ongoing discussion, some researchers have demonstrated that the MAPbBr₃ absorber is more stable than MAPbI_3; the MAPbBr₃ optical absorption is not appropriate for solar cell production (Ahmad et al., 2019).

In this research work, the tetrabutylammonium iodide (TBA) was incorporated into MAPbI₃ solution in the form of MA_(1-X)TBA_(X) PbI₃ thin film to study the effect on the structure-property of the MAPbI₃ when TBA was incorporated in different percentages. As a result, significant improvement was found in the crystallinity, morphology, optical properties, and stability of the MA_(1-X)TBA_(X) PbI₃ thin film. The obtained results are hoped to help delay the degradation and to enhance the performance of the MAPbI₃ thin film by doping TBA cations under ambient conditions.

Methylammonium iodide (MAI), Lead (II) iodide (PbI₂), Chlorobenzene (CBZ), Methylammonium iodide (MAI) and Tetrabutylammonium iodide (TBAI), anhydrous N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). All compounds were purchased from Sigma-Aldrich, and they were used without any additional purification. The MA_1−xTBA_xPbI₃ solution was prepared in a glovebox by dissolving PbI_2, TBAI, and MAI in the solvent of DMF and DMSO; the solution was agitated at 60°C for 3 h.

X-ray diffraction (XRD) was employed to characterize the crystallinity of the films using RIGAKU Ultima IV diffractometer the range of 2θ = 10°–60° using CuKa radiation (λ = 1.5418 Å) at room temperature. The morphology characteristic of thin films was constantly observed by scanning electron microscopy (SEM) model (Quanta 200–FEI) under 1.5 kV accelerated potential in several magnifications (Stewart et al., 2021). In addition, the films were examined by atomic force microscopy (AFM) with 0.5Hz a scan rate. The fringes crystallinity was confirmed by Transmission electron microscopy with 2.5 KV. Also, UV-VIS and PL spectra were characterized using Ocean Optics HR4000 spectrophotometer in the range of 300–850 nm and He-Cd laser source Si-CCD detector Hamamatsu for PL analysis, respectively.

In the present work, thin films perovskite MA_1−xTBA_xPbI₃ were successfully deposited on FTO back contact using a simple spin-coating technique. Figure 1 shows the XRD results of the MA_1−xTBA_xPbI₃ where X = (0, 1,2.5, 5, 10). The characteristic peaks located at 14° and 28° match to XRD planes of (110) and (220) respectively in agreement with MAPbI₃ structure (Ono et al., 2015; Abdelmageed et al., 2016) without any PbI₂ binary phase. Thus, a significant increase was found in the main XRD peaks (110) and (220) intensities and crystallinity by incorporating the TBA amount. Figure 2 represents the zoomed XRD peak in the range of 13°–15°; the small shift was observed towards a lower 2θ degree after the incorporation of TBA compared to the MAPbI₃ thin film. Furthermore, the increase in the crystal lattice could be related to a bigger radius TBA (4.70 Å) than a small radius of MA (1.8 Å).

The MAPbI₃ thin film with a tetragonal structure where a = b = 8.90 Å, c = 11.12 Å and with insignificant changes due to TBA incorporation where a = b = 8.91 Å, c = 11.11 Å. The crystal parameters details of MA_1−xTBA_xPbI₃ and the grain size were calculated using the Pawley method summarized in Table 1. Moreover, FWHM of peaks (110) and (220) values gradually decrease an agreement of good crystallinity of the films with increasing the amount of TBA from 0 to 5% in Figure 1C. In this respect, the stability of the MAPbI₃ main (110) peak remains excellent after incorporating TBA amounts. Especially many studies have been reported that the inorganized Pb atoms affect the surface imperfections of the film, which leads to a decrease in the performance of perovskite devices. In the current work, the TBA arguably shows a significant improvement of MAPbI₃ stability that degrades quickly into PbI₂ in external conditions (Yu et al., 2018).

Here, the Raman spectroscopy analysis was used to verify the phase identification of pure MAPbI₃ and 5% doped TBA Figure 1D. The dominants two vibrational modes were identified for MAPbI₃ approximately at 68 cm⁻¹ and 142 cm⁻¹. The intensities of the same vibrational mode increased after incorporating 5% TBA content. The 68 and 142 cm⁻¹ bands matched with the obtained results from XRD that confirmed the incorporation of TBA, which plays a vital role in the formation of crystallinity (Fateev et al., 2018).

Figure 3 illustrates the SEM images of MA_1−xTBA_xPbI₃ where X % (0%, 1%, 2.5%, 5%, 10%). The undoped MAPbI₃ shows a small grain size with a smooth surface and good distribution of lead and iodide (Chen et al., 2018). In the same way, the TBA incorporation increase gradually grain size of 210 nm for pure MAPbI₃, 290 nm for the doped MAPbI₃ with 1.0% TBA, 490 nm for 2.50% TBA as well as with a higher value of 500 nm for 5.0% TBA figure (a–d). This improvement of the grain size of doped MAPbI₃ can be due to the crystal growth by decreasing crystal nucleation and lead to good surface coverage and higher crystallinity, as shown by the XRD analysis (Banerjee and Chattopadhyay, 2018; Liu et al., 2020). In this connection, the enhanced crystallinity and grain size could be attributed to fewer defects in the trap state, decreasing the non-radiative recombination in the MAPbI₃ surface (Guo et al., 2019).

The specimen preparation technique used for perovskite samples prepared is tripod polishing for perovskite for high-resolution TEM investigation, scratching the perovskite film prepared and putting it in the special grid of aluminum to characterize the samples with TEM analysis, Figure 4 displays TEM analysis of pure MAPbI₃ and doped with 5% TBA expose the scattered through lattice fringes spacing of 13.10 Å correspond to [110] crystallographical plan as well as 7.60 Å match with [220] crystallographical respectively. The selected portion of the electron diffraction spectrum shows that pure and 5% TBA doped MAPbI₃ films are polycrystalline, where is in good agreement with XRD results (Giesbrecht et al., 2018; Jones et al., 2019).

Figure 5 shows AFM images with 2 and 3 Dimension of undoped and doped MAPbI₃, where the measured roughness is varying, that offers a change compared to doped and undoped MAPbI₃ where RMS= (147.7, 198, and 168 nm) calculated for the x% TBA where x = (0, 5 and 10) respectively which is measured by the root-mean-square (RMS) (Table 1). Moreover, the RMS value of 10% TBA doped MAPbI₃ shows a slight decrease than the incorporation of 5% of TBA, showing the optimum level for large grain size and high roughness in the AFM analysis (Tombe et al., 2018).

This experiment performed the optical absorption and photoluminescence measurements for MAPbI3 thin film doped TBA to analyze the optoelectronic properties. Here, Figure 6A illustrates the optical absorption of MAPbI₃ pure and doped TBA from 400 to 900 nm wavelength, where the optical bandgap was estimated around 1.55–1.59 eV. Upon monitoring carefully, the variation in the optical bandgap was observed by incorporating the TBA amount (Smith et al., 2019). This significant improvement could be related to pinhole-free TBA doped MAPbI₃ films, as shown in SEM analysis (Sun et al., 2017).

Besides, Figure 7 shows the photoluminescence spectrum of undoped MAPbI₃ and doped TBA, where the FWHM intensity progressively increases with the increase of TBA content. Doped 5% TBA represents a significantly higher red emission around 55 nm. This emission is three times higher than undoped MAPbI₃ thin film. The results could be attributed to reducing trap density states with decreased charge recombination, which improved the thin film’s optoelectronic properties (Brennan et al., 2017; Ngo et al., 2018).

In this context, one of the essential parameters of semiconductor materials is the stokes shift. This shift was observed between the optical absorption edge and PL peak. The comparison of stoke shift values is summarized in Table 2. The stoke shift describes the reduction of lattice parameters in the crystals. The low value of the stoke shift indicates the good photophysical properties of MAPbI₃ (Bouich et al., 2021a; Bouich et al., 2021b).

The degradation of pure MAPbI₃ and 5% doped TBA samples were examined under a dark relative 60% humidity environment where samples were stored for 15 days. Consequently, a significant transformation from the black color to the yellow color was observed for the pure MAPbI₃ aged sample after 2 weeks which indicated the dissociation of MAPbI₃ to PbI₂ confirmed by yellow color compared to 5% doped TBA aged sample was less affected (Figure 9).

Figure 8 illustrates the SEM images of the MAPbI₃ surface were affected by humidity and the water molecules over the grain boundaries, which led to the degradation of MAPbI₃ and the formation of PbI₂ and MAI. The TBA cation reduced the grain boundaries to prevent the access of humidity into the film. Furthermore, the 5% TBA doped MAPbI₃ has shown a slight crystal structure distortion than the pure MAPbI₃ (Huang et al., 2017; Kundu and Timothy, 2020).

Figure 9 displays the XRD patterns and the structural variations through the degradation of the doped and undoped MAPbI₃ with the characteristic peak (110) of pure MAPbI₃ shows dramatically reduced; however, the 5% TBA doped MAPbI₃ thin film observed less affected by the environment as compared to pure MAPbI₃.

Furthermore, the environmental effect was studied from the UV-Visible analysis of the 5% TBA doped MAPbI₃; Figure 10 shows a slow-down variation in the absorption edge, and the color changed from dark to brown of the sample after 2 weeks in relative humidity (60%). The obtained results confirm that incorporating TBA into the MAPbI₃ could decrease the degradation of the methylammonium lead triiodide absorber for photovoltaic application.

To sum up, from the preceding discussion, it appears that the doped with a small amount of TBA significantly increases the morphology and stability of MAPbI₃ thin film for photovoltaic applications. The XRD analysis revealed that the crystallinity of MAPbI₃ thin film enhanced with TBA, the TBA affects leading MAPbI₃ film with a homogenous, highly rough surface and large grain size, which could cause trap more light in the surface. Similarly, the MA_(1-X)TBA_XPbI₃ thin film shows better stability in a relative humidity of ∼60% after 15 days than pure MAPbI₃ thin film. The obtained results are hoped to help delay the degradation and to enhance the performance of the MAPbI₃ thin film by doping TBA cations under ambient conditions.