High Stable Perovskite Solar Cells using Ionic-liquid Addition and Cesium Halides Intercalation Technology

The rapid development of perovskite solar cells with a certified power conversion efficiency (PCE) of 25.7% is already at the level of commercialization. Still, long-term operational stability has become a major concern owing perovskite’s intrinsically soft ionic crystal structures. Very recently, we used the ionic liquid (IL) aided-CH3NH3PbI3 (MAPbI3) perovskite nanoparticles (NPs) as a seeded-growth approach to fabricate high moisture-stable perovskite solar cells with a PCE of around 20%.1 It retained above 80% of its initial output even after 6000 hours of storage at ambiance with relative humidity (RH) range of 30−40% (non-encapsulated).<br/>In the first half, I will talk about the nanoscale investigation of with and without IL-based perovskites. In this study, we investigated the nanoscale degradation mechanisms of why the higher stability was achieved by high-speed atomic force microscopy (AFM). We found that IL-aided MAPbI3 NPs form in the grain boundary of the CsFAMA perovskite crystal domains. This implies that the embedding of IL-aided MAPbI3 NPs in the CsFAMA perovskite crystal domain showed increased hydrophobicity (water contact angle of 72.3°) than pristine CsFAMA (water contact angle of 54.1°) by repelling moisture and preventing drop water infiltration under humid conditions.<br/>In the second half, I will talk about cesium halides (CsX: CsCl, CsBr, CsI) intercalation technology for efficient and stable PSCs. Previously, we intercalated vacuum deposited cesium iodide (CsI) into solution processed host MAPbI3 perovskite framework and achieved a PCE of 18.43% and remained above 80% of their initial output after 6000 h storage in open air (non-encapsulated) for the first time.2 In this study, we introduced vacuum deposited CsX (CsCl, CsBr and CsI) thin layers into solution processed host MAPbI3 perovskite film from the up, down or both layers to promote precise intercalation, resulted in high-quality perovskite film for high stable PSCs. The use of CsX layer greatly altered the MAPbI3 morphology to produce large grain sizes, as a result of the precise intercalation of the CsX molecules into the host MAPbI3. We tested thermal stability at 85 °C on hot plate at RH range of 50−60% for more than 30 hours, Cs-containing perovskite films showed higher stability (no color changed, retained black) than pristine MAPbI3 film (color changed and degraded). The light-soaking of devices prepared with CsCl, CsBr and CsI intercalation remained 76.9%, 77.2% and 71.2%, respectively, of their initial PCEs, while the pristine MAPbI3 device showed only 4.3% of its initial PCE after 120 minutes of continuous light illumination (1 sun). More importantly, a similar trend with the CsCl, CsBr, and CsI-based PSCs were observed for the moisture stability for 3000 h storage at ambient with a RH range of 50–60% (non-encapsulated). This ILs and CsX intercalation technology will enable a significant improvement in the device operational stability.<br/>References<br/>(1) Shahiduzzaman, M.; Wang, L.; Fukaya, S.; Muslih, E. Y.; Kogo, A.; Nakano, M.; Karakawa, M.; Takahashi, K.; Tomita, K.; Nunzi, J.-M.; Miyasaka, T.; Taima, T., Ionic Liquid-Assisted MAPbI3 Nanoparticle-Seeded Growth for Efficient and Stable Perovskite Solar Cells. ACS Applied Materials & Interfaces 2021, 13, (18), 21194-21206.<br/>(2) Wang, L.; Shahiduzzaman, M.; Muslih, E. Y.; Nakano, M.; Karakawa, M.; Takahashi, K.; Tomita, K.; Nunzi, J. M.; Taima, T., Double-layer CsI intercalation into an MAPbI3 framework for efficient and stable perovskite solar cells. Nano Energy 2021, 86, 106135.