Enhancing Perovskite Solar Cell Stability by TCO Layer Presence Beneath MACl-Doped Perovskites

Abstract

Perovskite solar cells (PSCs) have emerged as a promising photovoltaic technology, yet their stability under environmental stressors remains a critical challenge. This study examines the substrate-dependent degradation mechanisms of perovskite films and evaluates the role of methylammonium chloride (MACl) incorporation. Devices fabricated on ITO and glass substrates exhibited markedly different stability behaviors under high-humidity conditions. ITO substrates delayed the phase transition from the black α-phase to the yellow δ-phase due to stronger substrate–film interactions and reduced defect densities, while glass substrates facilitated rapid degradation through moisture infiltration and grain boundary instability. MACl incorporation enhanced the initial crystallinity and optoelectronic properties of the perovskite films, as evidenced by superior power conversion efficiency and photon absorption. However, residual MACl under humid conditions introduced structural instability, particularly on glass substrates. To address these challenges, a fully coated ITO structure, referred to as the Island Type design, was proposed. This structure eliminates exposed glass regions, leveraging the stabilizing properties of ITO to suppress moisture infiltration and prolong device durability. The findings provide a comprehensive understanding of the interplay between substrate properties and material composition in PSC stability and highlight the potential of structural optimizations to balance efficiency and durability for commercial applications.

1. Introduction

Perovskite solar cells (PSCs) have emerged as a transformative technology in next-generation photovoltaics, achieving power conversion efficiencies (PCEs) exceeding 25%, comparable to traditional silicon-based solar cells [1,2]. Their appeal lies in their exceptional optoelectronic properties, such as high absorption coefficients, long charge carrier diffusion lengths, and defect tolerance, which have enabled rapid advancements in efficiency [3,4]. Despite these advantages, the commercial adoption of PSCs is hindered by their intrinsic instability under environmental stressors, including moisture, oxygen, and thermal fluctuations, which lead to phase degradation and shorter operational lifetimes [5,6]. These environmental sensitivities present significant challenges that must be addressed for PSCs to transition from research to real-world applications [7,8].

To address the stability challenges of PSCs, various strategies have been developed, with one promising approach being the incorporation of additives into perovskite precursor solutions [9,10]. Additives play a crucial role in enhancing crystallinity, optimizing grain growth, and reducing surface defects, thereby improving film quality and structural stability [11,12]. Among these, methylammonium chloride (MACl) has emerged as a particularly effective additive, known for promoting larger grain sizes and better crystallinity in perovskite films [13,14,15]. In conventional PSC fabrication, MACl is expected to volatilize during high-temperature annealing, leaving minimal residue in the perovskite layer [16,17]. However, under low-temperature annealing conditions, our study observed that residual MACl remains trapped within the perovskite layer, potentially interacting with other device components. This finding highlights the dualistic role of MACl: while it enhances initial crystallization and device performance, the persistence of residual MACl can create degradation pathways, particularly under environmental stress [18,19]. These observations call for a deeper understanding of the balance between MACl’s benefits and its potential drawbacks in PSC fabrication [20,21].

In addition to additives, interlayers have become essential for stabilizing PSCs by enhancing charge transport and reducing recombination [22]. The commonly employed n-i-p structure in PSCs positions the perovskite layer between an electron transport layer (ETL) and a hole transport layer (HTL), forming an efficient pathway for charge extraction [23]. Recently, cyclohexylmethylammonium iodide (CHMAI) has attracted significant attention as an interlayer material positioned between the perovskite and HTL [24]. CHMAI has been shown to effectively facilitate charge extraction, reduce recombination losses, and improve overall device efficiency [24,25,26]. Interlayers such as CHMAI are crucial for achieving high PCEs, as they passivate surface defects on the perovskite, improve film morphology, and enhance device durability [24,27,28]. Initially, phenethylamine-based interlayers were widely adopted in PSCs, but recent studies suggest that replacing the phenyl group with a hexyl group, as in CHMAI, leads to superior stability and efficiency [24,29]. This advancement demonstrates the critical role of interlayer composition in optimizing PSC performance and demonstrates the potential of tailored interlayers to address both efficiency and stability challenges [30].

While previous studies have extensively explored the influence of substrate layers on the quality and stability of perovskite films, the interaction between substrate type and interlayer materials remains underexplored. Existing literature highlights how different underlying substrates can affect the crystallinity, defect density, and overall film morphology of perovskite layers [31]. However, the consequential effects of substrate choice on the chemical and physical interactions between perovskite and interlayers, such as CHMAI, have not been thoroughly investigated. Our research distinguishes itself by systematically examining how varying substrate materials not only influence the intrinsic properties of perovskite films but also modulate the interactions with interlayer components. This dual focus provides a more comprehensive understanding of PSC architecture, revealing that substrate type can significantly alter the efficacy of interlayers in enhancing device performance and stability. By exploring these complex interactions, our study fills a critical gap in the current body of knowledge and offers insights for the design of more robust and efficient PSCs.

Despite the general expectation that interlayers enhance perovskite stability, our study reveals a complex interaction between residual MACl and the interlayer material CHMAI [32,33]. Specifically, residual MACl was found to accelerate the degradation of the perovskite layer in the presence of CHMAI, driving a rapid phase transition from the black α-phase to the yellow δ-phase [6]. Under typical conditions, interlayers are understood to stabilize perovskite films by passivating surface defects and improving the crystalline structure [34,35]. However, our findings suggest that residual MACl disrupts this stabilizing effect, triggering detrimental interactions that compromise both the structural and optoelectronic properties of the film [36]. This unexpected behavior challenges the conventional role of interlayers as stabilizing agents in PSCs and highlights the dualistic nature of MACl [37,38]. While MACl promotes crystallization and grain growth, its residual presence under specific interlayer conditions can introduce significant instability [6,39].

Herein, we systematically compare the effects of etched and non-etched ITO substrates on the degradation behavior of perovskite films in the presence of residual MACl. While previous studies primarily focused on the crystallinity, defect density, or simple degradation mechanisms of perovskite films, our research examines deeper into how the substrate’s surface structure interacts with interlayer materials and residual MACl, influencing the chemical stability and mechanical integrity of the film. In particular, the partial degradation patterns observed on etched ITO substrates reveal that glass regions negatively impact the quality of the perovskite film compared to ITO-covered regions, highlighting the complex interactions between the substrate, film, and interlayer, which can critically affect device efficiency and stability [40,41]. Conversely, non-etched ITO substrates provide a continuous physical barrier, suppressing the reactivity of residual MACl and preserving the black α-phase of the film even under conditions that typically promote degradation. The use of non-etched ITO substrates is particularly effective in mitigating the reactivity of residual MACl and maintaining the black α-phase, ensuring better stability of the perovskite film. This finding highlights the overlooked chemical role of the ITO layer and its importance in maintaining film integrity. Thus, this study highlights not only the importance of substrate choice but also the intricate interplay between substrates, residual MACl, and interlayer materials, underscoring the critical role of substrate selection in PSC design and fabrication while offering new perspectives for achieving more stable and efficient devices.

To evaluate these findings in greater detail, we systematically compared the current-voltage characteristics and long-term stability of PSCs fabricated on non-etched and etched ITO substrates, specifically when using MACl-containing precursors and CHMAI interlayers. Our results demonstrate that non-etched ITO substrates result in higher efficiency and improved stability, suggesting that continuous ITO layers are more effective at maintaining the integrity of the perovskite layer.

2. Materials and Methods

2.1. Reagents Information

Perovskite precursor solutions were prepared using formamidinium iodide (FAI, >99.0%, Greatcell Solar Materials, Queanbeyan, Australia), lead(II) iodide (PbI₂, >98.0%, TCI Chemicals, Tokyo, Japan), cesium bromide (CsBr, >99.9%, Sigma-Aldrich, St. Louis, MO, USA), and methylammonium chloride (MACl, ≥99.5%, Lumtec, Taiwan). Dimethylformamide (DMF, ≥99.5%, TCI Chemicals, Tokyo, Japan) and 1-methyl-2-pyrrolidone (NMP, ≥99.0%, TCI Chemicals, Tokyo, Japan) were used as solvents for the perovskite solution. Cyclohexylmethylammonium iodide (CHMAI, Greatcell Solar Materials, Queanbeyan, Australia) dissolved in isopropanol (IPA, 99.5%, Sigma-Aldrich, St. Louis, MO, USA) was employed as an interlayer material. 2,2′,7,7′-tetrakis(N,N-di(4-methoxyphenyl)amino)-9,9′-spirobifluorene (Spiro-OMeTAD, >99.8%, Lumtec, Taiwan) was doped with lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI, >99.0%, TCI Chemicals, Tokyo, Japan) dissolved in acetonitrile (CH₃CN, >99.5%, TCI Chemicals, Tokyo, Japan) and 4-tert-butylpyridine (tBP, 98.0%, Sigma-Aldrich, St. Louis, MO, USA) to prepare the HTL solution. Chlorobenzene (CB, ≥99.5%, Alfa Aesar, Haverhill, MA, USA) was used as the solvent for Spiro-OMeTAD. Tin(IV) oxide (SnO₂, 15% H₂O colloidal dispersion, Alfa Aesar, Haverhill, MA, USA) was the ETL solution.

2.2. Preparation of Solution

The perovskite precursor solution, with a composition of (FA_0.97Cs_0.03)Pb(I_0.99Br_0.01)₃, was prepared at a total concentration of 1.35 M by dissolving 8.62 mg of CsBr, 225.19 mg of FAI, and 622.36 mg of PbI₂ in a molar ratio of 3:97:100, along with 22.79 mg of MACl, in a mixture of 900 µL of DMF and 100 µL of NMP in a 9:1 volume ratio. Depending on the experimental conditions, the solution was prepared with or without MACl. The precursor solutions were mixed using a vortex mixer to ensure thorough blending of all components [42,43].

A SnO₂ solution was prepared by mixing 500 µL of SnO₂ with 1500 µL of deionized water, maintaining a 1:3 ratio.

The HTL was prepared by first dissolving 72.30 mg of Spiro-OMeTAD in 1000 µL of chlorobenzene using a vortex mixer to ensure complete dissolution. Subsequently, 26.60 µL of Li-TFSI and 28.80 µL of tBP were doped into the Spiro-OMeTAD solution.

The interlayer was prepared by first dissolving 12 mg of CHMAI in 2000 µL of IPA using a vortex mixer to ensure complete dissolution.

2.3. Device Fabrication

ITO-coated glass substrates with etched regions were cleaned through sequential ultrasonication in deionized water, acetone, and isopropanol, each for 15 min. The substrates were then treated with UV-ozone for 15 min to enhance surface wettability. The SnO₂ solution, as prepared in Section 2.2, was statically dispensed onto the substrates at 100 µL and spin-coated at 5000 rpm for 30 s [44,45]. To ensure proper adhesion and stabilize the film, the SnO₂ layer was subsequently annealed at 150 °C for at least 15 min, a step crucial for the complete evaporation of deionized water. Following annealing, the substrates were subjected to an additional UV-ozone treatment for 10 min.

For perovskite film spin-coating, 50 µL of the precursor solution was statically dispensed onto the SnO₂-coated substrates, which had undergone UV-ozone treatment. The spin-coating process was performed at 3000 rpm for 30 s. During spin-coating, 30 µL of chlorobenzene was dropped onto the rotating substrate at 12–15 s to induce anti-solvent crystallization. The coated films were then annealed at 100 °C for 1 h to form the perovskite layer.

An interlayer of CHMAI was spin-coated on top of the perovskite layer by statically dispensing 40 µL of a CHMAI solution in isopropanol onto the substrate once the spin coater reached 5000 rpm, maintaining the spin-coating process for 30 s. No additional annealing step was performed after the coating.

The HTL was statically dispensed onto the CHMAI-coated perovskite films when the spin coater reached 2500 rpm, and the spin-coating process was maintained for 30 s to complete the HTL.

Finally, gold electrodes were deposited on top of the HTL using a thermal evaporator under high vacuum (<3.0 × 10⁻⁶ Torr) with a patterned metal mask.

The configuration of the device is glass/ITO/SnO₂/(FA_0.97Cs_0.03)Pb(I_0.99Br_0.01)₃/CHMAI/Spiro-OMeTAD/Au. Additionally, the thickness of each layer has been provided in the SEM data and included in Figure S1 of the Supporting Information.

2.4. Measurement Condition

The current density–voltage (J-V) characteristics were measured under 1 sun equivalent illumination using an Ossila Solar Cell I-V Test System equipped with a LumiSun-50™ LED Solar Simulator (Innovations in Optics, Woburn, MA, USA). The LumiSun-50™ provides a Class A+A+A+ illumination, closely replicating the AM1.5G solar spectrum with exceptional spectral match and uniformity, ensuring highly accurate photovoltaic performance measurements under standard testing conditions. X-ray diffraction (XRD) analysis was conducted with a Rigaku SmartLab instrument (Rigaku, Tokyo, Japan), employing Cu Kα radiation and recording data at 0.02° intervals. Photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements were performed using a time-correlated single-photon counting (TCSPC) system (FLS1000, Edinburgh Instruments, Scotland, UK) with a pulsed laser source at 450 nm. PL and TRPL measurements were conducted in a dry room with a relative humidity (RH) below 10%. Scanning electron microscopy (SEM) images, both cross-sectional and planar, were captured with a Hitachi SU8600, while atomic force microscopy (AFM) data were obtained using an MFP-3D Origin+ system (Oxford Instruments, Oxfordshire, UK). XRD, PL, SEM, and AFM analyses were performed at the Central Laboratory of Hankyong National University on samples prepared using ITO-coated substrates. The substrates were etched and sequentially coated with SnO₂, perovskite, and CHMAI.

3. Results and Discussion

The investigation began with the fabrication of devices utilizing the etched ITO structure depicted in Figure 1a. This design allowed a direct comparison of perovskite film stability between ITO-covered and glass-exposed regions, creating distinct chemical and physical environments. Two types of devices were fabricated for comparison: MACl_X, where no MACl was added to the perovskite precursor solution, and MACl_O, where MACl was included. This distinction was crucial for evaluating the impact of residual MACl on the long-term stability of the perovskite films.

During experiments conducted under a relative humidity exceeding 40%, a striking visual difference was observed over time, as shown in Figure 1b. On Day 1, there was no significant visual difference between the MACl_O condition on ITO and glass substrates. The perovskite films on the ITO-covered regions retained the black α-phase, associated with the photoactive state, even after several days. However, as time progressed, distinct differences emerged; by Day 5 and Day 8, the films on the glass-exposed regions rapidly transitioned to the yellow δ-phase, a non-photoactive state indicative of structural degradation. In contrast, films on the glass-exposed regions rapidly transitioned to the yellow δ-phase, a non-photoactive state indicative of structural degradation. These visual changes were absent when the relative humidity was maintained below 30%, indicating that moisture infiltration plays a critical role in the degradation process. Furthermore, the transition to the δ-phase was more significant in MACl_O devices, suggesting that the presence of residual MACl exacerbates degradation.

The normalized performance data in Figure 1c supports these observations. Under the MACl_O condition, device performance declined rapidly, whereas devices under the MACl_X condition maintained stable performance over time with minimal changes in efficiency. This suggests that residual MACl contributes to performance degradation. Furthermore, these results emphasize the critical role of the ITO surface in reducing moisture-induced degradation.

To further investigate the substrate-dependent degradation behavior observed in Figure 1, XRD analysis was conducted. The results, presented in Figure 2a, reveal a characteristic phase transformation from the black α-phase to the yellow δ-phase in perovskite films under the MACl_O condition. On Day 1, all conditions exhibited a prominent α-phase peak near 14.1°, corresponding to the (110) plane of the black-phase perovskite. However, in the MACl_O condition, the intensity of the α-phase peak gradually decreased over time, accompanied by the emergence of the δ-phase peak near 11.9°, corresponding to the (100) plane of the yellow-phase perovskite.

In contrast, the MACl_X condition exhibited a distinct 2D perovskite (CHMA₂PbI₄) (002) plane peak near 5.2° from Day 1. This peak showed consistently higher intensity compared to the MACl_O condition, indicating that the 2D perovskite structure was more prominent under the MACl_X condition. While the MACl_O condition also displayed the same 2D perovskite peak near 5.2°, its lower intensity suggests a difference in the degree of 2D phase formation between the two conditions.

To quantitatively evaluate this phase transformation, the intensity ratio ($I_r$) was defined as follows:

$$I_r={\displaystyle \frac{I_{\delta }}{I_{\alpha }}}$$

(1)

where $I_{\delta }$ is the intensity of the δ-phase peak at 11.9° and $I_{\alpha }$ is the intensity of the α-phase peak at 14.1°. This metric provides a reliable means of tracking phase stability and comparing the degradation rates between different substrates.

As shown in Figure 2b, the $I_r$ increased more rapidly for perovskite films on glass substrates than on ITO substrates. This observation demonstrates that the transition from the α-phase to the δ-phase occurs significantly faster on glass substrates, highlighting their greater susceptibility to structural degradation. In contrast, perovskite films on ITO substrates exhibited a much slower increase in $I_r$, indicating their enhanced ability to stabilize the α-phase and delay the phase transformation, even under high-humidity conditions. The actual intensity changes of the δ-phase (11.9°) and α-phase (14.1°) peaks for glass and ITO substrates under the MACl_O condition are shown in Figure S2, providing additional insight into this phase transformation.

PL and TRPL measurements were conducted to investigate substrate-dependent differences in perovskite film quality and stability.

Generally, in the case of the ITO region, PL quenching occurs, leading to more active charge transport, resulting in lower PL intensity compared to glass [46,47]. The PL spectra in Figure 3a exhibit this behavior under the MACl_X condition, indicating efficient charge extraction at the ITO interface. However, the PL spectra in Figure 3b show that under the MACl_O condition, the difference in PL intensity between the ITO and glass substrates is relatively smaller.

We can evaluate the quality of the perovskite film based on the PL intensity in the glass condition and assess the efficiency of charge transfer using the PL intensity in the ITO condition. Comparing the MACl_X and MACl_O conditions, the addition of MACl is expected to enhance perovskite crystallinity, leading to an increase in PL intensity. However, this suggests that the PL intensity of the glass condition in the MACl_O case is lower than expected. Consequently, this indicates that the perovskite film on the glass substrate undergoes structural and electronic degradation in the presence of MACl.

We analyzed the PL peak, and both glass and ITO exhibited a blue shift upon MACl addition. This shift occurs due to the slight bandgap adjustment caused by the substitution of MA⁺ cations [13,48]. Additionally, as shown in

Table 1. PL peak FWHM for MACl_X_Glass, MACl_X_ITO, MACl_O_Glass, and MACl_O_ITO.

	KWHM (nm)
MACl_X_Glass	51 nm
MACl_X_ITO	54 nm
MACl_O_Glass	56 nm
MACl_O_ITO	52 nm

, the FWHM values were calculated. Interestingly, the FWHM data reveal an opposite trend between glass and ITO upon MACl addition. Generally, a smaller FWHM indicates better crystallinity, and MACl should enhance the uniformity of the crystal structure, leading to a reduced FWHM. While the FWHM decreased with MACl addition in ITO, it increased in glass. This suggests that the crystallinity of the perovskite film in the MACl_O_Glass condition is relatively poor.

The TRPL measurement results shown in Figure 3c,d exhibit a trend similar to the PL data. Generally, due to quenching and superior charge transport ability, the carrier lifetime in the ITO condition should be shorter than that in the glass condition. Under the MACl_X condition, as observed in Figure 3c, the films on ITO substrates exhibit faster decay rates, indicating efficient charge quenching.

Conversely, the TRPL data for the MACl_O condition in Figure 3d show that the carrier lifetime on ITO is actually longer than that on glass. Comparing Figure 3c,d, it is evident that the carrier lifetime in the MACl_O glass condition is significantly reduced. Ultimately, the results presented in Figure 3 confirm that the perovskite film quality in the MACl_O glass condition is deteriorated.

To investigate substrate-dependent differences in surface structure and degradation, SEM analyses were performed. The SEM images, shown in Figure 4a,b, illustrate the surface morphology of perovskite films on ITO and glass substrates on Day 1. At this stage, both substrates exhibited uniform grain distribution and well-defined grain boundaries, with no significant differences observed in geometry.

By Day 8, as depicted in Figure 4c,d, morphological changes were observed over time, likely resulting from the transformation of the α-phase to the δ-phase. However, SEM analysis revealed no substantial geometric differences between the ITO and glass substrates, indicating that the phase transition did not significantly alter the overall surface structure of either substrate. The SEM cross-sectional images for the ITO and glass conditions on Day 1 and Day 8 have been added to Figure S3 in the Supporting Information.

AFM measurements, as presented in Figure 5a–d and

Table 2. RMS roughness values of perovskite films on glass and ITO substrates on Day 1 and Day 8.

Day	Glass	ITO
Day 1	38.55 nm	39.59 nm
Day 8	41.99 nm	33.35 nm

, provide a quantitative analysis of surface roughness for perovskite films on glass and ITO substrates under the MACl_O condition. These results capture the temporal evolution of film morphology on both substrates. Figure 5a–d presents 3D AFM images of perovskite films, while the corresponding 2D AFM images are provided in Figure S4 of the Supporting Information for further reference.

On Day 1 (Figure 5a,b), the RMS roughness values show that perovskite films on ITO substrates exhibit a slightly higher surface roughness (39.59 nm) compared to those on glass substrates (38.55 nm), as summarized in Table 2. This difference arises from the inherently rougher morphology of the ITO surface, which influences the initial film formation.

By Day 8 (Figure 5c,d), the trend reverses, with the RMS roughness of glass substrates increasing sharply to 41.99 nm, while that of ITO substrates decreases to 33.35 nm. This result indicates that the initially smoother glass substrates experience significant surface degradation over time, likely driven by external environmental factors. In contrast, the ITO substrates exhibit improved surface smoothness, suggesting greater resistance to roughness evolution.

To better explain the degradation mechanism, a schematic representation of moisture infiltration is provided in Figure 5e. This schematic illustrates how the smoother glass substrate promotes deeper moisture penetration through grain boundary gaps, leading to localized degradation and destabilization [38]. In contrast, the initially rougher ITO substrate reduces grain boundary voids, resulting in a denser perovskite layer that limits pathways for moisture ingress. The AFM images and RMS roughness data for Glass and ITO are provided in Figure S5 and Table S1. This may make ITO substrates relatively more stable compared to glass, which contains more voids that facilitate moisture infiltration. Additionally, the hydrophilic nature of CHMAI and its interaction with residual moisture and unreacted MACl likely accelerate water penetration into the perovskite layer.

The J-V characteristics shown in Figure 6a and summarized in

Table 3. Summary of photovoltaic parameters for perovskite solar cells under MACl_X and MACl_O conditions.

Condition	Type	PCE (%)	Voc (V)	Jsc (mA/cm2)	FF (%)	HI
MACl_X	Forward	17.36	1.12	23.72	0.65	0.04
	Reverse	17.99	1.13	23.56	0.68
MACl_O	Forward	20.83	1.14	24.28	0.75	0.03
	Reverse	21.51	1.15	24.31	0.77

reveal that devices fabricated under the MACl_O condition achieved higher initial power conversion efficiencies (PCE) compared to those fabricated under the MACl_X condition. The measurements were conducted under AM1.5, 1 kW/m² (1 sun) conditions. Specifically, the reverse scan PCE of MACl_O devices reached 21.51%, surpassing the 17.99% observed for MACl_X devices. This enhanced performance in MACl_O devices can be attributed to the role of MACl in promoting larger grain sizes, improved crystallinity, and reduced defect densities during perovskite film formation. The box plot data for PCE, J_sc, V_oc, FF from Figure 6a and Table 3 can be found in Figure S6.

To quantify the extent of charge recombination, the hysteresis index (HI) was calculated using Equation (2) [49]:

$$\mathrm{H}\mathrm{I}={\displaystyle \frac{{\mathrm{P}\mathrm{C}\mathrm{E}}_{\mathrm{r}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{e}}-{\mathrm{P}\mathrm{C}\mathrm{E}}_{\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{w}\mathrm{a}\mathrm{r}\mathrm{d}}}{{\mathrm{P}\mathrm{C}\mathrm{E}}_{\mathrm{r}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{e}}}}$$

(2)

where PCE_reverse and PCE_forward represent the power conversion efficiencies measured in reverse and forward scans, respectively. Higher HI values indicate greater discrepancies between the two scan directions, reflecting increased charge recombination and interfacial instabilities. However, the HI values for MACl_O devices (0.03) and MACl_X devices (0.04) showed no significant difference.

In contrast, devices fabricated under the MACl_X condition demonstrated lower initial PCE values, with a reverse scan efficiency of 17.99%, as shown in Table 3. However, these devices exhibited significantly greater stability over time. In the MACl_X condition, the absence of residual MACl reduced the moisture-induced degradation pathway observed in MACl_O devices, specifically the formation of the δ-phase. These findings highlight the dual role of MACl in perovskite solar cells, where its ability to enhance crystallization and optoelectronic performance must be carefully balanced against its detrimental impact on stability.

The EQE spectra presented in Figure 6b further validate the trends observed in the I-V measurements. Devices under the MACl_O condition consistently exhibited higher external quantum efficiency (EQE) across the 400–800 nm wavelength range compared to MACl_X devices. This superior EQE performance highlights the enhanced photon absorption and charge collection efficiency achieved through MACl incorporation.

To address the stability challenges observed in MACl_O devices, a structural optimization is proposed, as shown in Figure 6c,d. This design, referred to as the “Island Type” structure, eliminates the exposed glass regions identified as primary sites of moisture infiltration and degradation. As shown in Figure 6c, for devices without MACl incorporation (MACl_X), both the etched type and Island Type structures exhibited good stability over time, as indicated by the minimal decrease in normalized PCE. However, for devices with MACl incorporation (MACl_O), the Island Type structure demonstrated significantly better stability compared to the etched type, which showed a rapid decline in performance. By employing a fully coated ITO layer, the Island Type structure leverages the stabilizing properties of ITO substrates, including their rough surface morphology and strong interfacial interactions. This continuous ITO layer minimizes grain boundary gaps, reduces pathways for moisture ingress, and enhances the structural integrity of the perovskite layer. Such an optimized design offers a practical solution to balance the high efficiency provided by MACl incorporation with the need for long-term stability in perovskite solar cells.

The Island Type structure is beneficial because it fully coats the ITO layer, leveraging its superior moisture resistance and strong interfacial interactions to enhance device stability. This design minimizes the exposure of moisture-prone regions, such as glass, and addresses the vulnerabilities of conventional structures, offering a balanced approach to improving both efficiency and long-term durability in perovskite solar cells.

4. Conclusions

This study investigated the effects of MACl incorporation, substrate selection, and device architecture on the performance and stability of perovskite solar cells. While MACl improved initial crystallinity and efficiency by increasing grain sizes and reducing defect densities, its residual presence under humid conditions caused significant stability issues, especially on smoother glass substrates. In contrast, ITO substrates offered better stability due to their rougher surface and stronger interfacial interactions, which reduced moisture infiltration and structural degradation.

The Island Type structure addresses these challenges by eliminating vulnerable glass-exposed regions and leveraging the stabilizing properties of ITO, achieving a balance between efficiency and durability. These findings emphasize the importance of combining material optimization and structural engineering to create stable, high-performance perovskite solar cells, while pointing to future work on refining interlayer materials, scalable fabrication, and real-world testing.