The influence of methylammonium iodide concentration on the properties of perovskite solar cells

This study focuses on improving the quality of MAPbI3‐based perovskite films by adjusting the concentration ratios of the methylammonium iodide (MAI) precursor using a two‐step sequential deposition method. The primary objective is to explore how altering the MAI concentration influences the microstrain, dislocation density, perovskite film quality, and their subsequent impact on the performance of perovskite solar cells. The examined device configuration CdS/MAPbI3/Spiro‐OMeTAD demonstrates impressive power conversion efficiency of 12.05%, Voc of 1.02 V, Jsc of 16.2 mA cm−2, and fill factor of 0.73. X‐ray diffraction and scanning electron microscope analyses show improved crystal quality and surface characteristics with reduced microstrain, dislocation density, larger crystal grains, and minimized pinholes. The investigation of MAPbI3's optical and electrical characteristics provides in‐depth insights, facilitating the optimization of MAI precursor concentrations for improved perovskite film development and enhanced solar cell performance.

nature, and simplified manufacturing process. 2 Researchers have extensively explored the utilization of diverse perovskite materials-including MAPbI 3 , MAPbBr 3 , MASnI 3 , CsPbBr 3 , and FAPbI 3 -as active layers in perovskite solar cells (PSCs). 3,4Indeed, the quality of the perovskite material holds a substantial influence on the device's performance.Superior perovskite materials are crucial for maximizing light absorption, reducing charge recombination, and extending carrier diffusion length, all of which improve solar cell efficiency.
Undoubtedly, enhancing the quality of perovskite material stands as a key approach to boosting the performance of perovskite solar cells (PSCs).][7] Solution-processing techniques are predominantly used in preparing perovskite solar cells, making the composition of the precursor solution equally critical.Several solution-processing deposition methods are employed in perovskite fabrication one-step, two-step, sequential vapor deposition, two sources vapor deposition, and vapor-assisted solution deposition.Among various deposition techniques, two-step deposition method is preferred in perovskite fabrication due to several advantages over other deposition techniques: cost-effective, controlled nucleation, reduced defects, improved crystallinity, tunable properties, and enhanced stability.The electronic structure of MAPbI 3 exhibits high stoichiometric and surface defect tolerance factors during fabrication, rendering it insensitive to a wide range of compositional changes.However, the ideal quantity of methylammonium iodide (MAI) or PbI 2 precursor solution for optimal perovskite material performance remains uncertain.][10][11] It is imperative to further explore these mechanisms to gain deeper insights into the improvement of perovskite materials and, consequently, PSC efficiency.
The precursor solution's concentration ratio plays a vital role in governing the crystallinity, morphology, and colloidal properties of perovskite materials.The existing colloidal particles act as nucleation sites during the formation of perovskite films from the precursors, thereby influencing the overall film quality. 12Hong et al. conducted a study where they carefully adjusted the MAI and PbI 2 ratios under Pb-rich/I-rich conditions to create MAPbI 3 films.They found that solvent engineering and stoichiometric ratios had a significant impact on the efficiency, photostability, surface morphology, and coverage of MAPbI 3 films. 13In another approach, the addition of excess MAI to the precursor solution, combined with deposition using a Lewis acid-base adduct, effectively suppressed recombination at grain boundaries, leading to improved performance. 14Chen et al. demonstrated that releasing organic species during the annealing process allowed the presence of the PbI 2 phase in the grain boundaries of perovskite, resulting in enhanced carrier behavior and stability. 15Huang et al. found that using DMF as a solvent for MAI and PbI 2 was advantageous, as it controlled the crystallization rate, facilitating the formation of compact perovskite films. 16Wieghold et al. affirmed that higher precursor concentrations contributed to the formation of larger and more oriented grains in MAPbI 3 films. 17yung-Wook Park et al. showcased how the presence of excess lead iodide in the perovskite precursor solution played a crucial role in achieving PCEs exceeding 20% by reducing halide vacancies. 18Also, A mechanism elucidating the generation of distinct morphologies was postulated through the integration of in situ crystal growth analysis with X-ray diffraction (XRD) measurements.The thin film crystals produced at both low (60°C) and high (120°C) temperatures exhibit (110) and ( 200) orientations, respectively.This discrepant manner of crystal growth engenders substantially dissimilar film morphologies.In comparison to the spin-coating method, drop-casting demonstrates significantly heightened resilience against humidityinduced effects.Notably, PV cells based on MAPbI 3 fabricated under 88% humidity conditions yielded an impressive PCE of 18.17%.This achievement stands as the pinnacle PCE for perovskite solar cells manufactured in environments exceeding 70% humidity, all achieved without the utilization of antisolvent agents. 19urthermore, many findings have been reported on the approach of various precursor concentrations using a two-step sequential deposition technique in which PbI 2 is coated first and MAI is spin-coated later.1][22][23] Nevertheless, no detailed studies on the effect of microstrain and dislocation density on the MAPbI 3 perovskite film quality have been reported.
This article focuses on enhancing the quality of MAPbI 3 -based perovskite by varying the MAI precursor concentration ratios through a sequential deposition process using the spin coating method.The investigation delves into the impact of different MAI precursor concentration ratios on the film quality of the perovskite, specifically examining microstrain and dislocation density.Extensive analysis of the MAPbI 3 -based perovskite material and the influence of lead iodide (PbI 2 ) concentration on its properties is conducted.XRD measurements demonstrate that the perovskite film ELANGOVAN ET AL.
| 2005 quality experiences improvement, attaining the lowest microstrain and dislocation density with an optimal MAI (35 mg) PbI 2 (464 mg) precursor solution ratio.Scanning electron microscopy (SEM) reveals that the perovskite film with an optimal MAI (35 mg) ratio exhibits a larger grain size, reduced pinholes, and uniform surface coverage.The corresponding perovskite solar cell, with a device structure of FTO/CdS/MAI (35 mg) PbI 2 /Sprio-OMeTAD/Ag, achieves an impressive PCE of 12.05%.Additionally, the study delves into the intricate properties of the perovskite material, providing valuable insights for potential future development paths.By elucidating the optimal MAI precursor concentration ratio and its influence on film quality and solar cell performance, this research paves the way for further advancements in the field of perovskite-based solar cells.Here is a summary of the contribution's points for this article: • Enhance the quality of MAPbI 3 -based perovskite by varying the concentration ratios of the MAI precursor using a sequential deposition technique with spin coating.• The impact of different MAI precursor concentration ratios on film quality is investigated, focusing on microstrain and dislocation density in the perovskite material.• XRD measurements reveal that an optimal MAI (35 mg) PbI 2 (464 mg) precursor solution ratio results in improved perovskite film quality with the lowest microstrain and dislocation density.• The study also demonstrates that the perovskite film with the optimal MAI (35 mg) ratio exhibits larger grain size, reduced pinholes, and uniform surface coverage, leading to a perovskite solar cell with an impressive PCE of 12.05%.
The remaining sections of the article are organized as follows: Section 2 discusses the experimental method.Section 3 presents the fabrication and characterization.Section 4 presents the simulated results obtained from implementing the proposed methodology.Finally, Section 5 concludes the work based on the attained outcomes from the entire study.

| Device structure
The proposed MAPbI 3 -based perovskite solar cell has the following stack of layers: (1) FTO/CdS/MAI (15 mg) PbI 2(464 mg) /Sprio-OMeTAD/Ag, (2) FTO/CdS/MAI (25 mg) PbI 2(464 mg) /Sprio-OMeTAD/Ag, (3) FTO/CdS/MAI (35 mg) PbI 2(464 mg) /Sprio-OMeTAD/Ag.The effective shading mask area of 0.09 cm 2 with a well-defined aperture is fabricated using a laser cutting machine, according to the key device structure.The device's active area was 1 cm 2 .Figure 1 illustrates the device structure and energy level diagram of PSC.The overall thickness of the perovskite solar cell for Sample 1, Sample 2, and Sample 3 is measured to be around 390 nm ± 5 nm, 490 nm ± 5 nm, and 540 nm ± 5 nm, respectively, using atomic force microscopy (AFM) analysis.These thicknesses for each layer were calculated using AFM analysis and each layer is coated separately on an FTO glass with the same deposition time and RPM as mentioned for the overall device fabrication.Notably, the series resistance of the film is taken into consideration for analyzing the performance of the solar cells.

| Device fabrication
The unwanted FTO glass substrate was removed with HCl and zinc powder based on the device structure.The FTO glass substrate was cleaned for 15 min each with acetone, isopropyl alcohol, and deionized water using ultrasonication.Chemical bath deposition is used to fabricate the CdS layer on the FTO glass substrate.100 mL 0.25 M CdCl 2 was taken in a beaker and steered until 75°C.After reaching the desired temperature, 50 mL liquid NH 3 was added to the solution and steered for 25 min.The solution slowly changes from colorless to yellow after adding 2.286 g thiourea.All samples were removed from the CdS chemical bath after 10 min based on a previously reported study and rinsed in deionized water. 20The MAPbI 3 layer above the ETL was fabricated using a sequential deposition method.
To prepare the PbI 2 precursor solution, 464 mg PbI 2 was added to 1 mL of (7:3) (DMF + DMSO) solution and stirred for 2 h at 70°C.The precursor solution is then spin-coated for 30 s at 4000 rpm on the ETL before being annealed for 10 min at 100°C.Furthermore, various precursor solutions are prepared by adding 15, 25, and 35 mg of MAI to 1 mL of IPA solution and stirring continuously until the MAI is dissolved.The MAI precursor solutions are then spincoated at 4000 rpm for 30 s above the PbI 2 layer of Samples 1, 2, and 3, and annealed at 70°C for 10 min.The mole percentage ratio of MAI and PbI 2 for Sample 1 is 8.6% and 91.4%; Sample 2 is 13.5% and 86.5%; Sample 3 is 18.1% and 81.9%, respectively.Then, 1 mL of chlorobenzene was added to 40 mg of Spiro-OMeTAD precursor solution and it is stirred continuously till the solution gets dissolved.After preparing the solution, a Spiro-OMeTAD was spin-coated over the MAPbI 3 layer for 20 s at 4000 rpm.To make an electrical contact above the hole transport material, Ag was sputtered.Table 1 summarizes the sequential deposition of perovskite film with various MAI precursor solution ratios.Figure 2 illustrates the two-step fabrication process of perovskite thin film.

| Characterization
The surface morphology of the MAPbI 3 layers of Samples 1, 2, and 3 was examined using a scanning electron microscope (Inspect F50-FEI).The absorption spectrum of MAPbI 3 samples was examined via UV-visible spectroscopy (Perkin Elmer spectrometer).The sample structure and phase identification were investigated via XRD (XPert PRO-PAN analytical), the thickness of the sample was analyzed using AFM (XE 70 Atomic Force Microscopy-Park systems), and I-V measurements were performed in the presence of AM 1.5 conditions provided by the solar simulator (Keithley Electrometer).

| RESULTS AND DISCUSSIONS
The optical properties of a CH 3 NH 3 PbI 3 film made at different MAI precursor solution ratios of Samples 1, 2, and 3 were investigated using a UV-visible spectrometer, as shown in Figure 3.According to the findings, the absorption of perovskite films increases with increasing MAI precursor ratio over the long wavelength range.The study demonstrates that the CH 3 NH 3 PbI 3 layer possessed favorable light absorption characteristics ranging around 400-700 nm for perovskite film fabricated at Sample 3. The absorption spectrum of perovskite Sample 1 and Sample 2, on the other hand, is slightly reduced.The optical bandgap energy was determined to be 1.65, 1.69, and 1.72 eV (=1241/λ onset ) for Samples 1, 2, and 3, respectively, which is greater than the literature reported value of 1.55 eV.However, depending on the halide However, the presence of excess PbI 2 during the fabrication of the solar cell can introduce defects or grain boundaries in the perovskite film, which can act as sites for charge recombination or promote degradation under certain conditions.Also, PbI 2 may contribute to the migration of ions within the perovskite layer, altering the stability of the material and leading to the degradation of the film over time.
PSCs face crucial challenges regarding stability, especially the phase instability arising from the Perovskite crystal structure and device configuration.Perovskites when used in PSC devices are subject to external factors like environmental stability, thermal stability, and photo-stability, all of which have an impact on long-term stability.The most important factor contributing to environmental stability issues in PSCs is encapsulation.Encapsulation is an effective method for extending the life of solar cells, reducing photo instability, and controlling degradation issues by acting as an oxygen and moisture barrier.Heating Perovskite beyond 100°C leads to degraded performance due to increased PbI 2 and organic salt formation.
The crystallite size is calculated using Debye-Scherrer's law, where D is the mean crystallite size, λ is the wavelength of Cu, β is the full-width half maximum (FWHM), and θ is the scattering angle.The average crystallite size of Samples 1, 2, and 3 were estimated to be 23.88,24.54, and 32.5 nm.However, smaller crystalline sizes can introduce quantum confinement effects leading to changes in the bandgap and affecting the absorbed wavelengths (see Figure 3).Similarly, altering the halide content modifies the bandgap, enabling control over the absorption spectrum of the perovskite film for tailored optoelectronic performance.Furthermore, the microstrain (ε) and dislocation density (δ) were calculated from the XRD data using the following formulas, where n is a factor that is nearly equal to the minimum dislocation density.The calculated values of the microstrain and dislocation density of CH 3 NH 3 PbI 3 are summarized in Table 2. Figure 5 depicts the microstrain and dislocation density of a CH 3 NH 3 PbI 3 thin film fabricated at various MAI precursor solution ratios.
The presence of the maximum microstrain is observed in the ( 110), (112), and (220) peaks when compared to all other peaks of perovskite.The presence of strain, according to the studies, is capable of inducing phase transitions in a crystal structure and can completely shift a material's phase diagram.Smaller microstrain, on the other hand, is usually associated with a more stable crystal.In this case, the perovskite film fabricated at 35 mg of MAI/IPA solution (Sample 3) has very low microstrain when compared to the other samples.
When related to other peaks, the dislocation density of the perovskite film is greatest for (241) and (314) peaks.The presence of dislocation density indicates that the crystal is imperfect.Furthermore, decreasing the dislocation density will increase in electron lifetime.According to the results, the film fabricated using the two-step deposition method for Sample 3 has the lowest microstrain and dislocation density.However, both the microstrain and dislocation density for the fabricated Sample 3 of perovskite film is minimal when compared to the reported literature. 24According to the findings, the presence of maximum microstrain and dislocation density in the perovskite film may have a significant impact on the overall PV performance of the PSC while fabricating a device. 25,26canning electron microscopy is a powerful characterization technique that is used to investigate surface roughness, surface morphology, crystalline packing density, and other surface-oriented defects.shows an SEM image of a fabricated CH 3 NH 3 PbI 3 thin film.According to the results, Sample 3 film fabricated using the sequential deposition method has fewer observable pinholes, good crystalline quality, a large T A B L E 2 X-ray diffraction parameters of Samples 1, 2, and 3 of MAPbI 3 fabricated film using a two-step deposition method.

Sized (nm) d-spacing (Å)
hkl Ɛ × 10  grain size, and uniform surface coverage, which has the potential to reduce leakage current and suppress the recombination effect when applied to solar cell applications.However, the presence of pin-holes in Samples 1 and 2 may allow direct contact of the charge transport layers through the perovskite layer, resulting in a larger hysteresis effect and lower PCE.Surface homogeneity is important in the making of a good-quality solar cell device.The final solar cell device performance will be very poor when the surface has a large number of pinholes, disorders, and uneven grain formation.The strong and efficient perovskite solar cell device architecture is based mainly on the surface quality of the perovskite film.Whereas, the presence of surface porosity will lead to an immediate recombination effect and the photo-generated carrier will recombine near the defect area.Hence the device's final output will be very poor and in some cases, the device will not work.In addition, Samples 1, 2, and 3 films are subjected to AFM analysis to determine the sample's thickness.As shown in Figure 7, the film fabricated using the two-step deposition method for Samples 1, 2, and 3 has a thickness of 200 nm ± 50 nm, 300 nm ± 50 nm, and 350 nm ± 50 nm.According to previous studies, increasing the thickness of the active layer from 100 to 1000 nm shows an increase in the photogeneration of the electron-hole pair.][29] The effect of various MAI precursor concentration ratios prepared using the sequential deposition method on the performance of the Perovskite solar cell was investigated.
To analyze the PV performance of a perovskite solar cell, four key solar cell parameters, namely PCE, FF, V oc , and J sc , are summarized in Table 3.The FF and PCE of perovskite solar cells were used to evaluate their overall performance.
where V max represents the maximum photovoltage, I max represents the maximum photocurrent, P in represents the incident light, and J sc , and V oc represents the short-circuit current and open-circuit voltage.In this process, the device architecture of the PSC with device structure CdS/ MAI(35 mg)PbI 2 /Sprio-OMeTAD/Ag showed a very good PCE of 12.05%.According to the findings, perovskite material quality was the first major significant parameter used in determining the performance of PSCs.The series ELANGOVAN ET AL.
| 2011 resistance of a perovskite film fabricated for Sample 3 precursor solution ratio is R s = 6.28 Ω.cm 2 .Films made with Sample 1 and 2 precursor solutions, on the other hand, have series resistances of R s = 7.61 and 7.15 Ω.cm 2 , respectively.Increases in the MAI precursor solution ratios had a significant impact on increasing PCE.According to the findings, increasing the MAI precursor solution ratios results in increased film thickness, improved film quality, and decreased microstrain, dislocation density, and series resistance.1][32][33] Perovskite has a diffusion length ranging from 100 to 1000 nm, so increasing the thickness of the perovskite layer in relation to the diffusion length and overall device architecture results in improved generation of charge carriers.
F I G U R E 6 SEM top surface morphology of the fabricated two-step perovskite (A) 1 μm ×10,000 and (B) 1 μm ×20,000 of Sample 1, (C) 1 μm ×10,000 and (D) 1 μm ×20,000 of Sample 2, (E) 1 μm ×10,000 and (F) 1 μm ×20,000 of Sample 3. | 2013 Based on the observations from Figure 8, it is found that the initial behavior of the solar cell shows inhomogeneous characteristics for the various MAI precursor solution ratios of PSCs; it is due to the effect of series resistance.Also, the transportation of charges from the perovskite to the electron and hole transport layers is hampered by poor film quality, resulting in increased series resistance by causing a recombination effect in the device that upholds the inhomogeneous.This recombination effect declined the generation of photocurrent and efficiency of the perovskite solar cell.However, increasing the MAI precursor solution ratios results in better perovskite film quality for increasing the voltage for all the considered samples.On the other hand, a decrease in PCE could be caused by interfacial resistance at ETL/Perovskite or Perovskite/HTL interfaces.Moreover, the use of a nano-rod or nano-wire arrangement for the electron transport layer could result in a further decrease in interfacial resistance.As a result, the study shows that increasing the MAI precursor solution ratios causes a gradual increase in perovskite solar cell performance.In contrast to voltage, the short circuit current density of the PSC changes dramatically with the various precursor solution ratios.Relating the existing work notably reported by Jeong-Hyeok Im et al., 34 the proposed work extensively focuses on the effect of MAPbI 3 microstrain, dislocation density, device thickness, and series resistance that impact the overall performance of perovskite solar cells which were not considered by the existing work.

| CONCLUSIONS
According to the findings, the perovskite film fabricated using the optimal MAI precursor solution ratio of Sample 3 has no PbI 2 peaks and has the least amount of microstrain and dislocation density.SEM analysis, on the other hand, shows that the film fabricated using Sample 3 precursor solution ratio has fewer observable pinholes, which reduces leakage current and suppresses the recombination effect in the perovskite solar cell.The UV-visible, XRD, SEM, and AFM data from Samples 1, 2, and 3 indicate that the Sample 3 precursor solution ratio is the best for preparing MAPbI 3 -based perovskite films using a two-step spin coating deposition method.The highest PCE of 12.05%, V oc of 1.02 V, J sc 16.2 mA cm −2 , and FF of 0.728 were obtained for an optimum MAI precursor solution ratio of Sample 3 for the following device structure CdS/MAI (35 mg) PbI 2(464 mg)/ Spiro-OMeTAD.These findings suggest that increasing microstrain, dislocation density, and series resistance will have a significant impact on the performance of a PSC.Furthermore, the findings of this study will provide a quantitative understanding of the working mechanism of a PSC, allowing for further performance enhancement.

F
I G U R E 1 Structure and energy level diagram of perovskite solar cell (A-C).T A B L E 1 Summarization of two-step perovskite films with varying MAI precursor ratios.peaks are observed in theXRD spectra, average crystalline size is estimated as D = 23.88 nm.PbI 2 peaks were observed in the XRD spectra at 2θ 12.7°, indicating that PbI 2 has not been completely converted to perovskite.Surface defects and pin holes are observed.AFM thickness = 200 nm ± 50 nm.sharp perovskite XRD peaks are observed, and the average crystalline size is estimated as D = 24.54nm.The lower intensity of the PbI 2 peak at 2θ 12.7°was observed in the XRD spectra.Surface defects and pin holes are observed.AFM thickness = 300 nm ± sharp perovskite XRD peaks are observed, with average crystalline size D = 32.5 nm.No PbI 2 peak was observed in the XRD spectra.Low surface defects, fewer pinholes, and large grain sizes with smooth surfaces are observed.AFM thickness = 350 nm ± 50.size, this energy gap is within the 1.5-2.3eV range for halide perovskite nanomaterials.The XRD peaks of perovskite film Samples 1, 2, and 3 are depicted in Figure 4.During the investigation of the XRD spectra of CH 3 NH 3 PbI 3 , major diffraction peaks were observed for Samples 1 and 2 at 2θ 12.7°, 14.22°, 19.97°, 28.6°, 32.0°, 36.5°, and 43.2°which corresponds to (001), (110), (112), (220), (222), (241), and (314) peaks.However, PbI 2 peaks were observed at 2θ 12.7°which corresponds to (001) in the XRD spectra and 6.972 d-spacing, indicating that PbI2 has not been completely converted to perovskite.When compared to Sample 1, the Sample 2 film has a lower intensity of the PbI 2 peak.Furthermore, the presence of PbI 2 causes faster degradation of the MAPbI 3 film.Perovskite film fabricated with Sample 3 MAI precursor solution, on the other hand, appear to have major diffraction peaks at 2θ 14.22°, 19.97°, 28.6°, 32.0°, 36.5°, and 43.2°which correspond to (110), (112), (220), (222), (241), and (314) peaks of a tetragonal crystal structure.According to the data, no PbI 2 peaks were detected in the XRD spectra of Sample 3, indicating that PbI 2 has been completely converted to perovskite.On the other hand, perovskite inevitably has internal defects, such as Pb and I vacancy defects, particularly near the grain boundaries, which diminishes the stability and performance of PSC.When MAPbI 3 -based perovskite is subjected to humid circumstances, the hydrolysis reaction weakens hydrogen bonds in the crystal lattices, degrading PbI 2 and changing its color from dark brown to yellow.The dominant factors causing instability are H 2 O, O 2 , ultraviolet light, and heat.

F I G U R E 2
Schematic illustration of the two-step fabrication process of perovskite film.

F
I G U R E 3 (A) Absorption spectrum and (B) Tauc coordinates of CH 3 NH 3 PbI 3 material.F I G U R E 4 Sample 1, 2, and 3 XRD image of CH 3 NH 3 PbI 3 .

F I G U R E 5
Calculated microstrain and dislocation density of CH 3 NH 3 PbI 3 film fabricated at different MAI precursor ratios of Samples 1, 2, and 3.

F
I G U R E 7 AFM image of MAPbI 3 thin film (A) Sample 1, (B) Sample 2, and (C) Sample 3. ELANGOVAN ET AL.