Perovskite solar cells (PSCs) have demonstrated remarkable progress in power conversion efficiency (PCE), reaching 25.7% for single-junction cells and 32.5% for perovskite-silicon tandem cells. Scalability is also improving, with certified PCEs of 23.1%, 22.7%, and 20.5% achieved on 1, 24, and 64 cm² cells, respectively. However, long-term stability remains a significant challenge hindering commercialization. Performance loss and degradation in PSCs are initiated at grain boundaries and interfaces, where defects and mobile ions accumulate under stress such as continuous illumination, humidity, and elevated temperature. Therefore, suppressing surface defects and inhibiting mobile ion migration is crucial for achieving long-term stability. Various molecular species, including ammonium-based salts, small organic molecules, polymers, and inorganic salts, have been explored for surface passivation, aiming to enhance stability. Aprotic sulfonium-based molecules show promise but remain largely unexplored. This work focuses on the synthesis and application of dimethylphenethylsulfonium iodide (DMPESI) as a surface passivation agent for FAPbI₃-based PSCs, leveraging the enhanced chemical and humidity stability of sulfonium-based cations and the potential for close packing on the perovskite surface due to the aromatic moiety of DMPESI.

Previous research has explored various strategies to improve the stability of perovskite solar cells. Ammonium-based salts, small organic molecules, polymers, and inorganic salts have been used as passivation agents to suppress defect formation and ion migration. While some success has been achieved, long-term stability under various operating conditions remains a major hurdle. Aromatic sulfonium salts have been synthesized, displaying elevated chemical stability, such as trimethylsulfonium lead triiodide and butyl-dimethylsulfonium iodide. However, their poor solubility in common solvents has limited their application. The current study aims to address these shortcomings by exploring the potential of DMPESI, a sulfonium salt with enhanced solubility and stability.

The study involved the synthesis of DMPESI, characterized using ¹H NMR and ¹³C NMR. FAPbI₃ perovskite films were prepared using a spin-coating method, followed by surface treatment with varying concentrations of DMPESI (1–10 mg ml⁻¹). The stability of DMPESI and treated perovskite films was assessed through ¹H NMR, visual observation, X-ray diffraction (XRD), and contact angle measurements under ambient and high-humidity conditions. Light-soaking stability was investigated using hyperspectral photoluminescence (PL) microscopy. Density functional theory (DFT) calculations explored the interactions between DMPESI and the perovskite surface. Scanning electron diffraction (SED) measurements examined the microstructure of the perovskite films. Solid-state magic angle spinning (MAS) NMR (¹H and ¹³C NMR) was used to probe the microscopic interactions between DMPESI and FAPbI₃. PSC devices were fabricated using an n-i-p architecture (FTO/cp-TiO₂/mp-TiO₂/3D perovskite/DMPESI/spiro-OMETAD/Au), and their optoelectronic properties were characterized using time-resolved PL, J-V curves, incident photon-to-current conversion efficiency, and impedance spectroscopy. Long-term stability tests included shelf-life stability, thermal cycling, damp heat tests, and operational stability under MPPT (maximum power point tracking) with continuous 1-sun illumination. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) was employed to analyze ion migration.

DMPESI demonstrated exceptional stability under ambient conditions, maintaining its black α-phase for two years without encapsulation. The treated perovskite films displayed enhanced hydrophobicity, increased contact angles, and resistance to phase transitions under high humidity. Hyperspectral PL mapping revealed that DMPESI treatment significantly improved light-soaking stability, preventing the formation of new phases and maintaining PL intensity. DFT calculations showed strong interaction between DMPESI and the perovskite surface, suggesting effective passivation. SED measurements indicated octahedral tilting in the perovskite grains, potentially contributing to stability. Solid-state NMR confirmed the interaction of DMPESI with FAPbI₃. PSC devices treated with an optimal concentration (3 mg ml⁻¹) of DMPESI achieved a PCE of 23.32%, showing negligible PCE loss (<1%) after over 4,500 h of MPPT testing. The devices also exhibited excellent stability under thermal cycling and damp heat tests, retaining >96% of their initial PCE after 400 h at 60 °C and <30% RH and passing the ISOS stability protocol. ToF-SIMS analysis indicated effective suppression of ion migration (iodide and gold) in DMPESI-treated devices. The findings suggested that DMPESI passivates surface states, reduces non-radiative recombination, and inhibits ion migration, leading to long-term operational stability.

The superior stability of DMPESI-treated PSCs addresses the critical challenge of long-term stability in perovskite photovoltaics. The multifunctional nature of DMPESI, acting as a surface passivator, hydrophobic layer, and inhibitor of ion migration, is responsible for its effectiveness. The combination of experimental techniques (NMR, PL, DFT, SED, ToF-SIMS) provided a comprehensive understanding of the mechanism of action. The excellent stability under various stress conditions demonstrates the potential of DMPESI as a promising surface treatment for commercial PSCs. The results provide valuable insights for designing and developing advanced passivation strategies for enhancing the stability and performance of perovskite-based optoelectronic devices.

This study demonstrates the effectiveness of a simple surface treatment using the sulfonium salt DMPESI in stabilizing FAPbI₃-based perovskite solar cells. The results showcase a remarkable improvement in long-term operational stability, with minimal efficiency loss under various conditions, including extensive light soaking, thermal cycling, and high humidity. The mechanistic understanding of DMPESI's interaction with the perovskite surface opens up new possibilities in material science and engineering for further stabilization of perovskite-based devices. Future studies could explore the use of DMPESI with other perovskite compositions and different device architectures to further optimize performance and stability.

While the study demonstrates the effectiveness of DMPESI in enhancing PSC stability, some limitations exist. The optimal concentration of DMPESI was determined empirically; further investigation could optimize this parameter through high-throughput screening. The long-term stability tests were performed under controlled conditions; further studies are needed to evaluate the stability under diverse and real-world environments. The scale-up of this surface treatment for mass production needs to be investigated. The exact composition and structure of the interaction products formed between DMPESI and FAPbI₃ remain to be further elucidated.