Green Solvent-Enabled Sustainable Fabrication Route for High-Quality Halide Perovskite Devices

Perovskite-based optoelectronic devices are of great interest in a variety of areas including solar cells, memristors, light-emitting devices, detectors, etc., and have gained significant attention as a leading next-generation optoelectronic technology due to their high performance, ease of processing, and low processing cost. In addition to the well-documented environmental issues related to the usage of lead for the processing of halide perovskite devices, organic solvents like N, N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) pose a significant challenge related to their volatile and toxic nature. These concerns become increasingly significant in the large-scale fabrication of perovskite-based optoelectronic devices due to solvent volatilization, leading to major operational and environmental problems. The mixture of DMF and DMSO has been the standard solvent for producing high-performance devices due to its versatility and wide window of operation and crystallization control. Not only DMF: DMSO-based precursor inks are toxic, but they possess a limited storage life. The instability of the precursor inks comes from the potential hydrolysis of DMF, which can adversely affect the pH of the precursor solution impacting the efficiency and stability of the perovskite-based devices. That is why in laboratories, there is a common practice of preparing fresh DMF: DMSO-based precursor solution before use, but this is impractical for large-scale production of halide perovskite devices including solar cells. An environmentally friendly and sustainable fabrication route with a considerable operational and controlled crystallization window is the need of the hour for an attempt to commercialize halide perovskite devices. Inspired by this, here we report an environment-friendly green solvent system for sustainable solution-based fabrication of high-quality FAPbI₃ perovskite devices that will open green pathways toward the scalable fabrication of this emerging renewable technology. The novel green solvent-based perovskite inks showed remarkably high shelf life even in ambient environments and a broader operational range, compared to their conventional counterparts. Initially, the proposed green solvent-based perovskite inks remained stable for up to 192 hours without any signs of degradation, whereas the precursors based on DMF: DMSO degraded and changed to a deep orange color after being stored for 192 hours in ambient conditions (70-80% RH). We verified this phase transformation from the photoactive perovskite phase to a photo-inactive perovskite phase using X-ray diffraction (XRD) of the thin films made with fresh and 192 hours-aged inks. As equally important as ink stability, the moisture stability of the thin films needs to be considered because of the direct exposure of the as-fabricated perovskite film to ambient conditions. After exposure to harsh moisture conditions (70-80% RH) for just 24 hours, there was more than 50% conversion of the perovskite to thermodynamically stable yellow δ-FAPbI₃ observed as verified from the XRD for the thin films fabricated using DMF: DMSO-based solvents. On the other hand, no degradation was seen for the novel green solvent-based perovskite films at such elevated humidity conditions. In addition to the improved shelf life of the ink and the moisture stability of the thin films, our solar cells fabricated using the proposed solvent system deliver a stabilized power conversion efficiency as high as 18.59% with negligible hysteresis and tremendously improved thermal stability for over 350 hours retaining >96% of the original film composition, suggesting that this work will open doors to a sustainable and environment-friendly fabrication route for high-quality halide perovskite devices towards more scalable and practical applications.