Enhancing the Performance and Stability of Perovskite Solar Cells via Morpholinium Tetrafluoroborate Additive Engineering: Insights and Implications

Abstract

Perovskite solar cells (PSCs), since their inception in 2009, have experienced a meteoric rise in power conversion efficiencies (PCEs), challenging established photovoltaic technologies. However, their commercial deployment is hindered by stability and performance issues related to the presence of defects at the perovskite surface and grain boundaries. This study focused on the exploration of Morpholinium tetrafluoroborate (MOT) as a post-treatment additive to mitigate these challenges. Comprehensive characterization techniques revealed that the synergistic action of Morpholine and BF₄⁻ ions in MOT substantially improved the quality of the perovskite films and passivates surface and bulk defects, yielding notable enhancements in device PCE and stability. MOT-doped PSCs exhibited a PCE of 23.83% and retain 92% of the initial PCE after 2000 h of continuous illumination under one sun condition. The findings underscore the significance of additive engineering in advancing perovskite solar cell technology, opening up prospects for high-performing and durable perovskite photovoltaic devices.

1. Introduction

Since their advent in 2009, organic–inorganic perovskite solar cells (PSCs) have emerged as a prominent player in the field of photovoltaics, captivating the scientific community with their high power conversion efficiencies (PCEs), tunable band gaps, solution-processability, and cost-effectiveness [1,2,3,4,5,6,7,8,9,10]. Over the past decade, the PCEs of PSCs have experienced an extraordinary surge from a mere 3.8% to over 25%, posing a significant challenge to established solar technologies such as crystalline silicon and thin-film solar cells [11,12,13,14]. However, despite these remarkable advancements, the path to commercializing PSCs is fraught with critical challenges, primarily centered around device performance and stability. Defects at the perovskite surface and grain boundaries have emerged as major contributors to non-radiative recombination and ion migration. Moreover, the ubiquitous solution spin-coating method for perovskite fabrication tends to introduce numerous surface and bulk defects due to the presence of non-coordinated lead iodide octahedra in the solution. As a consequence, the fabricated perovskite films often exhibit numerous iodine vacancies, thereby undermining both device performance and longevity [15,16,17]. In the pursuit of overcoming these hurdles, intensive research efforts have been directed towards devising strategies to alleviate these defects. One promising approach lies in the post-treatment of the prepared perovskite films with various molecular additives. Such additives have demonstrated their potential in controlling crystal growth, passivating defects, and modulating electronic properties of the perovskite materials [18,19,20,21,22]. The introduction of additives such as Phenethylammonium iodide (PEAI), which in situ forms films on mixed cation films to improve the quality of perovskite films, has successfully boosted the PCE of PSCs up to 23.32% [23]. Furthermore, multifunctional ionic liquids, possessing a broad range of desirable properties, have been employed as efficient passivating materials to elevate the performance of PSCs, evidenced by the augmentation in PCE to 23.25% and 9.92% upon the integration of 1,3-dimethyl-3-imidazolium hexafluorophosphate (DMIMPF6) and 1-butyl-2,3-dimethylimidazolium chloride ([BMMIm]Cl), respectively [24,25]. In light of these advancements, MOT, due to its unique structure that allows for strong bonding with lead ions and potential replacement of missing iodide ions, has been identified as a promising additive [26,27,28]. The interactions facilitated by MOT can profoundly influence the crystal growth mechanism and defect passivation, thereby enhancing both the performance and stability of PSCs. However, empirical insights into the role of MOT as a dopant in PSCs are relatively scant in existing literature.

Building upon the existing advancements in the field of perovskite solar cells (PSCs), our study introduced a novel aspect of employing MOT as an overlayer on the perovskite surface, a method not extensively explored in the current literature. The primary objectives of this research were to elucidate the multifaceted influence of MOT in tailoring the morphology, crystallinity, electrical properties, and stability of perovskite films. In the context of this study, MOT was employed as an overlayer on the perovskite surface to mitigate challenges at the interface between the perovskite and Spiro-OMeTAD layers. Although MOT may function as an additive in some applications, in this specific configuration, it is grown directly on the perovskite surface. This unique approach sets our work apart from existing strategies and allows us to unravel new pathways in enhancing both the performance and stability of PSCs. Our findings, marked by a remarkable PCE of 23.83% in MOT-modified perovskite film, not only contribute to a deeper understanding of the perovskite modification techniques but also signify a significant leap in the ongoing pursuit of highly efficient and stable PSCs. This study, therefore, stands as a valuable addition to the scientific community, bridging the gap between theoretical potential and practical implementation of MOT in the realm of PSCs.

2. Results and Discussion

2.1. Structure and Morphology

The synthesis process of the perovskite devices is shown in Figure 1a. Initially, an electron transport layer was fabricated on an FTO substrate through spin coating of SnO₂ solution. Subsequently, a perovskite precursor solution was drop cast onto the FTO/SnO₂ substrate. The prepared substrate was then annealed on a hot plate at 150 °C for 15 min, resulting in an FTO/SnO₂/Perovskite film. Different concentrations of MOT molecules were then dissolved in isopropyl alcohol (IPA) solution and drop cast onto the prepared FTO/SnO₂/perovskite film. This was followed by a second crystallization step involving spin coating. After this step, the films were again annealed at 150 °C for 1 min to solidify the structure. For comparison, control devices were fabricated following the same procedure, but without the addition of MOT molecules on the surface of the perovskite film. The results of these experiments should offer valuable insights into the effects of MOT molecule concentration and annealing time on the performance and stability of perovskite devices. Figure 2b presents a detailed depiction of the three-dimensional structure of MOT molecules. The image illustrates the three-dimensional arrangement of the Morpholine Tetrafluoroborate (MOT) molecule. The MOT molecule consists of a central morpholine ring, which is a six-membered heterocyclic ring containing four carbon atoms and two nitrogen atoms. Attached to the morpholine ring are four fluorine atoms, one at each position where a hydrogen atom would typically be present. The fluorine atoms are positioned tetrahedrally around the morpholine ring, with each fluorine atom bonded to one of the carbon atoms. The tetrafluoroborate anion, BF₄⁻, and these elements to interact with the perovskite surface, leading to a reduction in surface defects. Concurrently, the tetrafluoroborate moiety demonstrated its utility in effectively compensating for the deficiency of iodine elements within the perovskite structure, thereby ameliorating associated defects. These observations imply that the structural attributes of MOT molecules may contribute significantly to the optimization of perovskite device performance. Figure 1c showcases the Ultraviolet-Visible (UV-Vis) absorption spectra for perovskite films, comparing samples that were optimized with MOT molecules to those that have not. The UV-Vis absorption spectra revealed that the perovskite films, post-MOT optimization, exhibited enhanced absorption, notably within the 300 to 600 nm wavelength range. This heightened absorption was likely attributable to the introduction of uncoordinated BF₄⁻ ions, which resulted in the coordination of perovskite crystals, potentially leading to the formation of the fully coordinated Pb(I + BF₄)X complex. These findings suggest that the implementation of MOT optimization strategies can significantly influence the photophysical properties of perovskite films, thus enhancing their performance. Figure 1d–g provides the comprehensive X-ray Photoelectron Spectroscopy (XPS) spectra and narrow scans for O 1 s, C 1 s, B 1 s, and F 1 s of perovskite films, encompassing both KPF-modified and unmodified samples. The XPS data revealed that the perovskite films subjected to MOT ionic liquid modification exhibited distinctive XPS peaks. Specifically, an O 1 s peak was associated with the C-OH bond, and C 1 s spectra presented peaks corresponding to C-C, C-O-C, and C-N bonds, which was indicative of the MOT molecular structure’s influence. Moreover, the emergence of B 1 s and F 1 s peaks asserted the successful integration of MOT into the perovskite film structure, a feature absent in the unmodified samples. To better contrast the chemical alterations instigated by MOT optimization, Figure S1 illustrates the complete XPS spectra for perovskite films both before and after MOT modification, clearly highlighting the absence of B and F elemental intensities in the non-optimized films. These results underscore the substantial impact of MOT modification on the chemical properties of perovskite films.

Figure 2a and Figure S2a delineate the chemical structure of MOT, utilized for the surface treatment of perovskite in this investigation. Comprising two components—morpholine and tetrafluoroborate anion—the ionic liquid exhibited ease of diffusion and surface coverage owing to its small size, enabling the extensive passivation of perovskite defects. To capture the intricacies of atomic-level interactions between perovskite and MOT, simulations were performed focusing on the action of morpholine and the tetrafluoroborate anion on typical defects (F_A vacancies (V_FA), I interstitial atoms (I_i), and point defect I vacancies (V_I) present on an FAI-terminated FAPbI₃ perovskite surface with defects. Employing Density Functional Theory (DFT) and MOT as the basic unit, we observed electronic redistribution following bond or atom interactions, as represented by blue areas. Conversely, yellow areas highlighted electrons that were captured by pertinent atoms. Charge transfer, which was witnessed between I atoms on the perovskite surface and oxygen atoms within morpholine, and between the tetrafluoroborate anion and Pb atoms, is suggestive of strong interactions. These interactions restricted the migration and oscillation of I-, induced I vacancies, and upon the introduction of the tetrafluoroborate anion, rectified the lack of I ions, resulting in lattice deformations and structural distortions. Further interactions were observed between FA in perovskite and the oxygen and the tetrafluoroborate anion in MOT, which limited FA migration. To expound the implications of morpholine and tetrafluoroborate passivation on perovskite material properties, we conducted measurements of the partial density of states (PDOS) of perovskite films, both untreated and treated with the ionic modifier (refer to Figure 1d–f and Figure S2c). We identified the introduction of trap states by V_FA, V_I, and I_i at the perovskite surface, serving as non-radiative recombination centers and causing energy loss. DFT predictions suggested a possible reduction in or suppression of these defects within the bandgap following morpholine and tetrafluoroborate passivation, implying that these components acted by compensating for electronic defects at perovskite lattice defect sites. As shown in Figure 2g, the X-ray photoelectron spectra (XPS) of control and MOT-treated perovskite films underscored the effective interaction of MOT, as evidenced by the upshift of Pb²⁺ binding energy peak in the Pb 4f spectra of the treated perovskite films. These results align well with the experimental findings from the XPS study and serve to corroborate the theoretical projections made regarding the interactions within the perovskite films. To analyze the influence of MOT treatment on the optical and photovoltaic properties of perovskite films, we evaluated the steady-state photoluminescence (PL) spectra of these films. Owing to enhanced crystallinity and a reduction in film defects, we observed an augmentation in the PL intensity of the perovskite films post-MOT treatment. As depicted in Figure 2h, the treated perovskite films manifested superior emission intensity at approximately 765 nm. Time-resolved photoluminescence (TRPL) spectra (Figure 2i) further illustrated that the average carrier lifetime (τ) of the MOT-treated perovskite films was notably longer at 50.48 ns compared to the control films, which had a τ value of 13.63 ns. This observation was consistent with decay times τ1 and τ2, as detailed in the Supporting Information, Table S1. The escalation in PL intensity and carrier lifetime underscores that defects within the perovskite films can be effectively passivated by MOT molecules. The data presented herein corroborate the promising potential of employing MOT treatment to significantly improve the photophysical and optoelectronic properties of perovskite films, thereby positively impacting their performance in photovoltaic applications.

Figure 3a presents the chemical structure of MOT and a mechanistic explanation for its role in passivating the perovskite. Comprising morpholine and tetrafluoroborate groups, MOT functioned as a multi-faceted agent to improve the quality and stability of perovskite films. The morpholine component promoted superior crystal growth and defect passivation through its strong binding with lead. Concurrently, the tetrafluoroborate group effectively substituted missing iodine ions and restricts the escape of FA, thereby elevating the overall stability of the perovskite. These combined actions significantly boosted the moisture stability of perovskite films, positioning MOT as a promising candidate for enhancing both the power conversion efficiency (PCE) and the durability of perovskite-based photovoltaics. As such, the deployment of MOT provided a vital pathway towards achieving high-performance and long-lasting perovskite solar cells. Figure 3b–e presents SEM (Scanning Electron Microscopy) images of perovskite films that underwent treatment with varying concentrations of MOT. Notably, the discrepancies observed in the film morphology were directly attributed to alterations in the MOT additive concentration, assuming all other parameters involved in film fabrication remained constant. In Figure 3b, the virgin film manifested a rather uneven coverage of the perovskite layer, characterized by tiny pores and remnants of unreacted PbI₂. The grain size of the perovskite material was determined through the analysis of SEM images, allowing us to evaluate the microstructural characteristics of the material. There was no calculation of crystal size using XRD in this particular study. A noticeable enhancement in uniformity and crystalline growth was observable in the MOT-treated film (as shown in Figure 3c), despite the persistence of some PbI₂ residues. Increasing the concentration of MOT led to more noticeable advancements in film morphology (Figure 3d). The absence of discernible pinholes or fissures, the augmentation of grain size, and the elimination of PbI2 residues affirmed the beneficial impact of MOT. However, an excessive concentration of MOT, demonstrated in Figure 3e, negatively impacted the surface coverage of the film. Therefore, these results indicate that the introduction of an optimized amount of MOT significantly fosters grain growth and enhances film quality. An overabundance of MOT, however, impairs the quality of the perovskite layer, emphasizing the importance of finding the balance in additive concentration. The most superior film attributes were demonstrated when the MOT concentration was kept at 1 mg/mL, which is indicative of the essential role of carefully controlled additive concentrations in creating high-performance perovskite solar cells. In order to further substantiate our conclusions, we conducted X-ray diffraction (XRD) analysis of the perovskite thin films treated with different concentrations of MOT (see Figure 3f). Notably, perovskite films modified by various concentrations of MOT showed a distinctive peak at around 11 degrees. This peak is presumably indicative of a new crystalline phase resulting from the binding of MOT onto the perovskite surface, which appeared to enhance the perovskite’s overall stability. Moreover, we analyzed the full width at half maximum (FWHM) of the XRD peaks corresponding to the different MOT concentrations (Figure 3g). The smallest FWHM, signifying the highest crystallinity, was observed for the film with the MOT concentration at 1 mg/mL. Interestingly, the film with an MOT concentration of 2 mg/mL also exhibited a relatively small FWHM, suggesting good crystallinity. However, SEM images for this sample revealed a surface layer comprised of a new phase. A statistical box plot analysis of grain sizes for the perovskite films prepared with varying concentrations of MOT has been presented in Figure 3h. It was observed that the perovskite film exhibited an optimal grain size, and, and the average grain size reached 500 nm when treated with an MOT concentration of 1 mg/mL. This optimal grain size can be attributed to the efficacious role of MOT in regulating the growth kinetics of the perovskite crystals, thereby leading to improved crystallinity. These results substantiate that the addition of MOT can notably refine the microstructure of perovskite thin films, potentially enhancing the overall optoelectronic performance of perovskite-based devices.

2.2. Photovoltaic Performance of the PSC

The Kelvin Probe Force Microscopy (KPFM) was employed to further dissect the surface characteristics and electrical properties of perovskite films post Morpholine Tetrafluoroborate (MOT) modification. KPFM is known to offer reliable insights into the local surface potential, impacted by the contact potential difference (CPD)—the disparity in the work function between the microscope tip and sample surface. Our study showed a significant shift in CPD maps (Figure 4a,b) post MOT treatment, providing clues towards understanding the improved open-circuit voltage (V_OC) and work function. Figure 4c depicts the local potential distributions from random KPFM scans, where a substantial increase in the average surface potential, from 200.31 mV to 297.63 mV, was observed. This reinforces the idea that MOT modification successfully reduces perovskite defects, fostering enhanced carrier transport within the film and, consequently, a superior Voc. Collectively, our study underscores the promising role of MOT modification in perovskite films to enhance their electrical performance parameters and interface characteristics, as validated by KPFM analysis. This revelation could provide valuable guidance for the future development and optimization of perovskite-based devices. In a quest to delve deeper into the impact of Morpholine Tetrafluoroborate (MOT) on the surface characteristics of perovskite films, we present images acquired from the 3D height sensor Atomic Force Microscopy (AFM) in Figure 4d,e. The MOT-treated perovskite film showed a marked reduction in root mean square roughness, down to 45 nm from the 84 nm observed in the control film. This decrease in surface roughness fosters enhanced contact between the hole transport layer and the perovskite layer, thereby facilitating efficient carrier extraction. Most critically, MOT treatment led to a decrease in film roughness, improved uniformity, mitigation of cracks, and an enhancement in the crystallinity of the perovskite film. These findings collectively underscore the positive influence of MOT in optimizing the surface morphology and overall quality of perovskite films. Such improvements hold significant implications for elevating the performance and stability of perovskite materials in optoelectronic devices, among other applications. To elucidate the reduction in trap-assisted recombination due to MOT modification, we examined the electron trap density (N_t) of perovskite films via JV curves from pure electronic devices. This evaluation was undertaken to quantify the effectiveness of defect passivation (refer to Figure 4f). The J-V curves conventionally encompass three distinct regions: the ohmic, the trap-filled limit (TFL), and space-charge-limited current (SCLC). During low-voltage conditions, ohmic contact transpired within the device. In the TFL region, as the applied bias exceeded the trap-filled limit voltage (V_TFL)—equivalent to the inflection voltage—a marked escalation in the current signals the culmination of defect filling. Consequently, a diminished fill voltage indicates a reduced defect quantity in the film. The data exhibited V_TFL values of 0.32 V for the control and 0.24 V for the MOT-modified perovskite films. This discrepancy underscores the ability of MOT to effectively mitigate defects within the perovskite film, which is consistent with the improvement in carrier lifetime. This work thus highlights the crucial role of MOT in enhancing the quality and stability of perovskite-based devices by attenuating defect-related recombinations.

As represented in Figure 5a and

Table 1. Photovoltaic characteristics of best devices prepared without and with MOT treatment at forward (FS) and reverse (RS) scans.

Samples	Scanning Direction	VOC (V)	JSC (mA·cm−2)	FF (%)	PCE (%)
Control	RS	1.10	24.52	81.15	22.06
	FS	1.09	24.05	75.64	19.83
With MOT	RS	1.17	24.78	82.52	23.83
	FS	1.17	24.68	78.50	22.67

, the current-voltage (JV) characteristics of the control and MOT-optimized devices manifested distinctive enhancements in photovoltaic performance. The superior device from the control group demonstrated a power conversion efficiency (PCE) of 19.83% in forward scanning (FS), with short-circuit current density (J_SC) of 24.05 mA cm⁻², open-circuit voltage (V_OC) of 1.09 V, and fill factor (FF) of 75.64%. In the reverse scan (RS), it exhibited a PCE of 22.06%, with corresponding J_SC, V_OC, and FF values of 24.52 mA cm⁻², 1.10 V, and 81.15%, respectively. Conversely, the MOT-treated device exhibited substantial enhancements in the RS, achieving a PCE of 23.83% with J_SC, V_OC, and FF values of 24.78 mA cm⁻², 1.17 V, and 82.52%. In FS, the PCE reached 22.67%, with corresponding J_SC, V_OC, and FF values of 24.68 mA cm⁻², 1.17 V, and 78.50%. A significant reduction in hysteresis was evident in the MOT-optimized devices. This is likely attributed to the effective passivation of defects at the perovskite film surface and grain boundaries and to the suppression of ion migration within these regions. These findings underline the influential role of MOT treatment in enhancing perovskite photovoltaic performance. To elucidate the carrier transport dynamics of the perovskite solar cells (PSCs), the devices’ Mott–Schottky curves were meticulously examined utilizing an electrochemical workstation (refer to Figure 5b). As a result, the optimized devices with MOT molecular enhancements exhibited a significant improvement in PCE, correlated with the enhancement in Voc, FF, and Jsc, as evidenced by the corresponding external quantum efficiency spectra (Figure S3). This underscores the pronounced enhancement in device performance following the optimization of MOT molecules. The findings depict that the MOT-modified device manifested a steeper gradient in comparison to its control counterpart. This suggests a substantial diminution in trap state density, consequently leading to a reduction in non-radiative charge recombination. In addition, the built-in potential, determined via the intersection point of the Mott–Schottky graph’s linear state with the x-axis, was found to be 0.87 V for the control device and was enhanced to 0.99 V for the MOT-treated device. An elevated built-in potential invariably benefits the efficiency of carrier separation, transportation, and subsequent extraction, thus underlining the significance of the MOT modification in improving perovskite solar cell performance. Figure 5c presents the Nyquist plots for the perovskite solar cells. Notably, the MOT-treated device showcased a lesser charge transfer resistance relative to the control device. This could be attributed to the elevation in the crystallinity of the perovskite thin film, which ostensibly had a positive impact on the fill factor (FF). Furthermore, as a result of proficient defect passivation, a marked increase was observed in the recombination resistance following the application of MOT. This mirrors a diminished defect density, which, in turn, was expected to contribute favorably towards the amplification of the open-circuit voltage (V_OC). Figure 5d portrays the function between different light intensities and the open-circuit voltage (V_OC) to further explicate alterations in electron-hole recombination, incited by defects under varying light intensities in PSCs. It is noteworthy that the slope pertaining to the MOT-treated PSCs was inferior to that of the control devices, which suggests an effective mitigation of the trap-induced carrier recombination. These electrical characterizations provide substantial evidence for the diminishment of trap states associated with carrier recombination following the introduction of MOT, contributing positively towards charge extraction efficiency.

In a bid to investigate the ramifications of MOT optimization on the enduring stability of devices, we documented the X-ray diffraction (XRD) patterns of unencapsulated perovskite films under stress testing with high relative humidity (RH) at 80 ± 10% (Figure 6c). Contrasted with MOT-optimized films, the control films manifested an intensified PbI₂ peak after a 12-day aging period, signifying the decomposition of the perovskite film. Conversely, the perovskite peaks persisted dominantly in the MOT-optimized films, suggesting the preservation of the perovskite structure post-aging. The control 3D perovskite film exhibited a relatively low water contact angle of 71.2°, whereas the MOT-optimized film displayed a higher angle of 78.3° (Figure 6a,b), an implication of the MOT overlayer functioning as a barrier to water ingress into the perovskite lattice, thereby bolstering moisture stability. Further, after 2000 h at 40% RH, MOT-modified devices evinced no noticeable decomposition, retaining 92% of their original PCE. In our investigation, we identified MOT as a significant agent in enhancing the stability of perovskite materials. By employing MOT as an overlayer on the perovskite surface, we observed improvements in both hydrophobicity and resilience under high humidity conditions. MOT’s unique structure, which allows for strong bonding with lead ions and potential replacement of missing iodide ions, plays a vital role in this enhancement. Through its influence on crystal growth and defect passivation, MOT mitigates non-radiative recombination and ion migration, contributing to increased material stability. In contrast, the control PSCs demonstrated a PCE reduction to 45% of their original value (Figure 6d). The superior quality and enduring stability render our advanced devices highly promising for practical implementations.

3. Conclusions

Building upon the comprehensive findings of our investigation, we conclude that MOT provides substantial advancements in the optimization of both performance and stability of perovskite solar cells (PSCs). In our experimental paradigm, we observed that MOT, as an effective dopant, not only mediated the growth of perovskite crystals but also passivated defects within the perovskite stratum, which is substantiated by the pronounced morphological transitions and improvements in crystallography evidenced by SEM and XRD analyses, respectively. A specific concentration of MOT, precisely at 1 mg/mL, emerged as the most optimal, inducing the most substantial positive transformations in both film properties and crystallinity. An assessment of the photovoltaic properties of the device highlighted an appreciable escalation in the power conversion efficiency (PCE) of PSCs post-MOT treatment in comparison to the untreated controls, with a substantial mitigation in hysteresis. The substantial decrease in trap states and an enhancement in charge carrier separation and transport in MOT-incorporated devices was particularly noticeable, contributing to their superior performance. Furthermore, the experiment substantiated the pivotal role of MOT in augmenting the longevity and resilience of PSCs, especially under challenging environmental conditions. A significant enhancement in hydrophobicity was observed, evidenced by an increased water contact angle, along with a decreased rate of degradation under high humidity stress tests for the MOT-incorporated films. In light of these findings, our study demonstrates the potential for the integration of MOT in perovskite solar cell fabrication as a strategic approach towards improved device efficiency and stability. These findings pave the way for more practical applications of PSCs and highlight the need for continued exploration and refinement of MOT and analogous dopants to facilitate the development of superior, high-efficiency PSCs.

4. Experiment Section

4.1. DFT Simulation

DFT calculations utilized the VASP package with a 420 eV energy cutoff for plane wave basis sets. The Monkhorst–Pack k-point mesh, measuring 2 × 2 × 1, was selected for Brillouin zone integration. Perovskite surface models employed (2 × 2 × 2) supercells with a 15 Å vacuum separation for periodic images. Structure relaxation continued until total energy reached a convergence of 1.0 × 10⁻⁵ eV/atom.

4.2. Materials

Chemicals, such as Cesium iodide (CsI), Lead iodide (PbI₂), Lead bromide (PbBr₂), FAI, MAI, MABr, MOT, Dimethylsulfoxide (DMSO), N,N-Dimethylformaldehyde (DMF), isopropanol (IPA), SnO₂, FTO glass, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and Spiro-OMeTAD, were sourced from various reputable suppliers and utilized as received.

4.3. Device Fabrication

Our process commenced with the preparation of Fluorine-doped tin oxide (FTO) glass substrates. These substrates were subjected to ultrasonic cleaning in a specific sequence of FTO cleaning solution, deionized water, and ethanol, each for a duration of 20 min. To improve the surface wettability, the cleaned substrates were exposed to UV/ozone for an additional 30 min. Following substrate preparation, we established the electron transport layer using a SnO₂ precursor solution (2.67% concentration in water). This solution was spin-coated onto the FTO substrates at a speed of 3500 rpm, lasting for 30 s, and was subsequently annealed at 150 °C for half an hour on a hotplate. In parallel, we prepared the Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ precursor solution. This involved dissolving certain quantities of FAI, MABr, PbI₂, and PbBr₂ into a DMF/DMSO solution at a volume ratio of 4:1. Separately, we dissolved CsI in DMSO and integrated this into the precursor solution. This complex perovskite solution was then spin-coated onto the FTO/SnO₂ substrate using two spin speeds and dropwise addition of 8 microliters of chlorobenzene as anti-solvent, and subsequently, samples were exposed to MOT in a nitrogen-filled glovebox. MOT, dissolved in IPA, was then spin-coated onto the perovskite surface at 3000 rpm for 15 s. We then prepared the Spiro-OMeTAD solution (consisting of Spiro-MeOTAD, 4-tert-butylpyridine, and a lithiumbis-(trifluoromethylsulfonyl) imide solution in chlorobenzene) and deposited it onto the perovskite layer. This was achieved through spin-coating at 3000 rpm for 30 s in a nitrogen-rich environment. The final step involved the thermal evaporation of a 100 nm thick Au electrode onto the structure, resulting in a fully formed FTO/SnO₂/Perovskite/MOT/Spiro-MeOTAD/Au device. As a comparative reference, an FTO/SnO₂/Perovskite/MOT/PCBM/Au device was also fabricated using a solution of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in chlorobenzene, serving to estimate electron trap-state densities.

4.4. Device Characterizations

We initiated our examinations with an analysis of the surface morphology of the various perovskite films and a cross-sectional inspection of the PSCs. These investigations were facilitated by the advanced JEOF 7610F emission scanning electron microscope (SEM) (JEOL, Tokyo, Japan). For our X-ray diffraction (XRD) analyses of the perovskite films, we used a cutting-edge Rigaku SmartLab(9Kw)D X-ray diffractometer (Rigaku, Tokyo, Japan). This high-tech instrument utilized a monochromatized Cu Kα target radiation source, running at a scanning rate of 8°/min. We conducted our electrochemical impedance spectroscopy (EIS) tests with the reliable CHI660 electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) allowing us to inspect the impedance characteristics of the perovskite films. The absorption spectra were compiled using a UV-1900 spectrometer from the renowned company (Shimadzu, Kyoto, Japan). Photoluminescence (PL) characterizations were realized through a state-of-the-art fluorescence luminescence spectrometer, enabling us to study the light-emitting properties of the perovskite films. Current-voltage (J-V) characteristics were evaluated under AM 1.5 G irradiation with the aid of a customized 71S type solar simulator system from SOFN Instruments Co., Ltd. (Beijing, China), while a Keithley 2400 served as a highly precise digital sourcemeter. We ensured the light intensity was properly calibrated through a standard silicon cell. We utilized an Edinburgh Instruments spectrometer (FLS980) (Edinburgh Instruments Ltd., Livingston, UK) to capture both time-resolved PL (TRPL) spectra and steady-state PL spectra under an excitation wavelength of 480 nm. Both the control perovskite film and the N-MOT-treated perovskite film were thoroughly analyzed using an AFM and KPFM (Keysight 5500 scanning probe microscope, Keysight, Santa Rosa, CA, USA). This was carried out in tapping mode for AFM and in contact mode with a bias voltage of 1 V for KPFM. Lastly, our examination encompassed X-ray photoelectron spectroscopy (XPS) conducted on a Thermo ESCALAB 250XI (Thermo Fisher, Waltham, MA, USA), enabling us to study the electronic structure of the perovskite films.