Engineering fluorinated-cation containing inverted perovskite solar cells with an efficiency of >21% and improved stability towards humidity

Perovskite solar cells, particularly three-dimensional (3D) organic-inorganic halide perovskites, have demonstrated remarkable power conversion efficiencies (PCEs). However, achieving high efficiency while maintaining long-term stability against moisture, heat, and light remains a significant challenge for commercial viability. Standard n-i-p architectures have yielded impressive results, but inverted p-i-n structures, while potentially simpler and more stable, often exhibit lower efficiencies. The pursuit of enhanced stability has led to the exploration of two-dimensional (2D) Ruddlesden-Popper (RP) perovskites, which offer improved hydrophobicity and thermal stability due to the incorporation of large organic spacer cations. While 2D perovskites show promise in stability, they often compromise efficiency due to quantum confinement effects, lower carrier mobility, and a narrower absorption window compared to 3D materials. Mixed 2D/3D hybrid perovskites aim to combine the advantages of both, but the optimal design criteria and the actual presence of a 2D/3D structure are not fully understood. Previous research using various large organic cations, including n-butylammonium and phenylethylammonium (PEA), has shown improvements in stability, reaching PCEs up to 20%. However, the search continues for new organic spacers to further enhance stability without sacrificing efficiency. Fluorinated organic spacer cations have emerged as a promising candidate due to their inherent hydrophobicity, potentially leading to improved stability. The inverted p-i-n architecture, offering advantages in simplicity and low-temperature processing, has seen fewer reports of 2D/3D PSCs, with efficiencies generally lower than their n-i-p counterparts. This study focuses on addressing these challenges by incorporating 2-(2,3,4,5,6-pentafluorophenyl)ethylammonium iodide (FEAI) into a 3D methylammonium lead iodide (MAPbI3) perovskite to achieve high efficiency and stability in a simple inverted solar cell design.

The literature extensively documents the advancements and limitations of various perovskite solar cell architectures and compositions. High-efficiency 3D perovskite solar cells often rely on complex multi-cation and multi-anion mixtures, hindering scalability and manufacturability. Efforts to improve stability have led to the exploration of 2D perovskites, which demonstrate enhanced moisture resistance due to the presence of large organic cations. However, these 2D materials often show reduced efficiency compared to their 3D counterparts. Mixed 2D/3D perovskites offer a potential solution, combining the stability of 2D with the efficiency of 3D. Several studies have explored different large organic cations to achieve this balance, resulting in improved stability but with varying degrees of success. Fluorinated organic spacer cations have gained attention recently due to their hydrophobic properties, leading to advancements in water resistance and stability. The inverted p-i-n configuration offers advantages in simpler fabrication and potentially reduced hysteresis, making it an attractive alternative to the standard n-i-p architecture. However, high-efficiency inverted 2D/3D devices are less common, underscoring the continued need for improvements in this area.

The study employed a solution-processing method to fabricate inverted perovskite solar cells. The active layer consisted of a mixture of methylammonium iodide (MAI) and varying mole percentages (mol%) of the fluorinated lead salt, 2-(2,3,4,5,6-pentafluorophenyl)ethylammonium iodide (FEAI). The precursor solution, containing PbI2, MAI, and FEAI in GBL:DMSO solvent, was spin-coated onto ITO-coated glass substrates. A two-step spin-coating process was used, with toluene added during the second step to aid in film formation. The resulting perovskite film was then annealed at 100 °C for 30 min. The device architecture was ITO/PTAA/PFN-P2/perovskite/C60/BCP/Cu. Various characterization techniques were used to investigate the properties of the perovskite films and devices. UV-Vis spectroscopy determined the absorption spectra, while photoluminescence (PL) spectroscopy, including temperature-dependent measurements and time-resolved PL, provided insights into the photophysical properties. Grazing incidence wide-angle X-ray scattering (GI-WAXS) and high-resolution grazing incidence X-ray diffraction (GIXRD) were used to analyze the crystal structure and orientation of the perovskite films. X-ray photoelectron spectroscopy (XPS) depth profiling determined the distribution of FEAI within the film. Scanning electron microscopy (SEM) examined the morphology of the layers, atomic force microscopy (AFM) and scanning Kelvin probe microscopy (SKPM) investigated surface topography and work function. Contact angle measurements assessed the hydrophobicity of the films. Finally, current-voltage (J-V) curves were measured under simulated AM 1.5G illumination to determine device performance, and long-term stability tests under thermal stress (85 °C in N2) and high humidity (80 ± 10% RH) were conducted to assess the stability of the unencapsulated devices. Maximum power point tracking (MPPT) stability tests were also carried out under continuous illumination.

The study found that incorporating 0.3 mol% FEAI into the MAPbI3 perovskite significantly enhanced the performance of inverted solar cells, achieving a champion efficiency of 21.1% without an antireflection coating. This represents a substantial improvement compared to devices made with neat MAPbI3 (19.1% efficiency). Interestingly, the high-efficiency devices showed no detectable 2D perovskite phase at FEAI concentrations up to 5 mol%. Instead, the FEAI was found to concentrate at the film-air interface (within 50 nm of the surface), leading to larger and more oriented surface perovskite crystals. XPS analysis confirmed the preferential accumulation of FEAI at the surface, with a fluorine concentration of 19 atom% near the surface and negligible concentration in the bulk. This surface enrichment resulted in increased hydrophobicity, as evidenced by a contact angle of 90° compared to 62° for neat MAPbI3. The increased hydrophobicity correlated with improved stability against humidity, with the 0.3 mol% FEAI devices showing significantly enhanced resistance to degradation under high-humidity conditions. While thermal stability was also modestly improved, the key contribution of FEAI appears to be in enhanced moisture resistance. The surface passivation by FEAI, indicated by higher PL efficiency and lifetime, likely contributed to the improved device performance and stability. Furthermore, the FEAI addition suppressed the temperature-dependent phase transition in MAPbI3, suggesting enhanced phase stability.

The findings demonstrate a novel approach to enhancing the performance and stability of perovskite solar cells. By incorporating a small amount of FEAI, the researchers achieved high efficiency without the need for complex multi-cation perovskites or layered 2D/3D structures. The preferential surface segregation of FEAI, resulting in surface passivation and enhanced hydrophobicity, offers a pathway to overcome the traditional trade-off between efficiency and stability in perovskite solar cells. This approach simplifies the device fabrication process while delivering superior performance and stability. The results highlight the potential of employing surface engineering strategies to improve perovskite solar cells' overall characteristics, moving closer to the goal of commercial-scale manufacturing.

This work presents a significant advancement in perovskite solar cell technology by demonstrating a simple, high-efficiency, and humidity-stable inverted device. The incorporation of 0.3 mol% FEAI leads to surface enrichment of the fluorinated salt, passivating defects and improving hydrophobicity. This results in a champion efficiency of 21.1% and remarkably enhanced stability against humidity, surpassing many previous results with more complex formulations. Future research could explore other fluorinated cations and optimization of the FEAI concentration to further enhance performance and long-term stability. Investigating the scalability of this method for large-area device fabrication is also crucial for its eventual commercial application.

The study primarily focuses on unencapsulated devices, limiting the assessment of long-term stability under real-world conditions. While the enhanced stability observed is promising, further investigation is needed to assess the long-term stability of encapsulated devices. The study also focuses on a specific inverted architecture; the applicability of this approach to other device architectures needs to be explored. The exact mechanisms of the FEAI-induced surface passivation and its influence on charge transport require more detailed investigation.