Fearless of humid air：Enhanced Stability and Performance of α-FAPbI3 Perovskite Solar Cells via Confined-Space low temperature Annealing

Perovskite solar cells (PSCs) is emerging as a revolutionary technology and promises to advance the efficiency and reduce the cost of solar energy conversion. Since their first introduction (more than 10 years ago), PSCs have shown remarkable improvements, with power conversion efficiencies (PCEs) surpassing 26%, which is (very close) to that achieved with traditional monocrystalline silicon solar cells. Moreover, the many advantages of perovskite materials such as exceptional optoelectronic properties including strong light absorption, high charge-carrier mobilities, and long diffusion lengths, have enabled their rapid adoption and development in a wide array of applications such as photodetectors, light-emitting diodes (LEDs), integrated energy conversion and storage devices, and solar-driven water-splitting systems. However, still the commercialization and broader application of PSCs face significant challenges because they are highly sensitive to environmental factors such as moisture, oxygen, heat, and light, which can lead to rapid degradation and phase transitions that compromise device performance.

The hydrophilicity and black-phase instability of formamidinium lead iodide (FAPbI₃) perovskites under humid conditions present substantial hurdles. FAPbI₃, with its narrow bandgap of approximately 1.48 eV and excellent thermal stability, is a promising candidate for PSCs. However, it exists in two crystalline phases: the photoactive cubic α- FAPbI₃ and the photoinactive hexagonal δ- FAPbI₃. The α-phase is metastable at room temperature and readily converts to the δ-phase in the presence of moisture, leading to significant stability issues. To address these challenges, various strategies have been explored, including compositional engineering, additive engineering, surface energy manipulation, and intermediate phase engineering. Among these, additive engineering showed some promise due to its simplicity and effectiveness in enhancing film quality and stability. Chloride-based additives, particularly methylammonium chloride (MACl), have been widely studied for their ability to improve the structural and electronic properties of perovskite films by inhibiting defect formation and enhancing crystallinity. However, the application of these additives under humid conditions remains problematic, as the direct addition of MACl to the precursor solution can lead to undesirable volatilization and pinhole formation, which deteriorates the film quality and stability.

In light of these challenges, new study published in Dalton Transaction and led by Hao Gao from the from the School of Mechanical and Electronic Engineering, Jingdezhen Ceramic University alongside Minghui Zhang, Zicong Xu, Yichuan Chen, Yuehui Hu, Zhijie Yi, Jiayu Huang and Hua Zhu, developed a novel method for fabricating high-quality α-FAPbI₃ films under high humidity conditions that can overcome the limitations of conventional additive engineering approaches and achieve stable perovskite films suitable for air-processed PSCs. In the first set of experiments the authors tried to fabricate perovskite films using different (preparation methods). The team prepared FAPbI₃ films via a one-step anti-solvent method in humid air. They studied various additive strategies, such as the conventional method of adding MACI directly to the precursor solution. This initial approach, labeled as the CA method, revealed that films formed at high annealing temperatures (above 150°C) quickly degraded when cooled in humid air, with MACl-induced pinholes exacerbating this degradation. To address these issues, the researchers developed the confined-space annealing (CSA) strategy. In this method, a low concentration of MACl was spin-coated onto FTO/mesoporous TiO2 substrates to create MACl covers. These covers were then placed face down on freshly deposited FAPbI3 precursor films to form a confined space, retarding the volatilization of MACl and blocking moisture ingress during the annealing process. This innovative approach aimed to achieve smooth, pinhole-free films with excellent crystallinity at lower annealing temperatures.

The authors then compared the effects of different additives namely MAI (methylammonium iodide), FACl (formamidinium chloride), and MACl on the film quality and stability and found that only the combination of MA and Cl resulted in pure α-FAPbI3 films. Moreover, the X-ray diffraction (XRD) patterns showed that films fabricated with just MAI or FACl had significant amounts of the undesirable δ-phase, whereas films treated with MACl displayed strong α-phase peaks, indicating superior crystallinity. Fourier transform infrared spectra and X-ray photoelectron spectroscopy further confirmed the presence of stronger N–H⋯Cl hydrogen bonds, which are essential for stabilizing the α-phase. Additionally, the authors investigated using the new CSA method the impact of different annealing temperatures (80°C, 100°C, and 120°C) on film quality and found that at 100°C, the films exhibited the best balance of crystallinity and structural integrity while films annealed at 80°C showed insufficient crystallization and those annealed at 120°C had smaller grains and signs of PbI₂ segregation. They also tested the photovoltaic performance of the PSCs fabricated with these films and found that devices with the MACl-CSA films annealed at 100°C achieved a champion PCE of 17.27%, which outperformed those fabricated with the conventional method in a nitrogen-filled glove box, which had a PCE of 15.67%. According to the authors, the significant improvement was due to the superior crystallinity and reduced defect density of the MACl-CSA films. Furthermore, the researchers performed aging tests to evaluate the stability of the PSCs in humid air and observed the MACl-CSA films PSCs retained over 90% of their initial PCE after 480 hours of exposure to high humidity, and therefore highly stable.

In conclusion, the research work of Dr. Gao and colleagues demonstrated that their novel CSA method significantly improved the stability of α-FAPbI₃ films under high humidity conditions and they also advanced our scientific knowledge on the roles of methylammonium and chloride in the formation and stabilization of α-FAPbI₃ films. Their proposed mechanism of MA-assisted nucleation and Cl-induced diffusion recrystallization clarifies how these additives synergistically enhance film quality and stability. The ability to fabricate high-quality perovskite films in air, without the need for stringent environmental controls as reported by Dr. Gao and team, simplifies the manufacturing process and reduces costs which could accelerate the commercialization of PSC technology in the solar energy market, and contribute to more sustainable and affordable renewable energy solutions.