Additive Engineering of Imidazolium-based cation for defect passivation with enhanced performance and stability of carbon-based perovskite solar cells

Due to the rapid increase in the power conversion efficiency (PCE) of perovskite solar cells (PSCs) and approaching the theoretical maximum level in recent years, the main drawback of their future commercialization is to improve their long-term stability under existing environmental conditions. One of the problems with this technology lies in the absorbing layer, specifically the perovskite. In particular, when producing a perovskite film under ambient conditions, it is almost impossible to avoid film defects caused by environmental conditions (such as moisture, oxygen, and temperature), which could randomly change during the preparation process. Therefore, the fabrication of high-quality perovskite films is an urgent process that has a direct impact on the overall performance of the solar cells. The use of ionic liquids (ILs) as additives in perovskite precursor solutions is a promising strategy to improve not only the power conversion efficiency (PCE) but also the long-term stability of photovoltaic devices by controlling nucleation and crystal growth of perovskite film that avoids defects and produces high-quality perovskite layers. Here, the critical role of the ionic liquid 1-methyl-3-propylimidazolium iodide (1-MPII) in different perovskite precursor solutions such as MAPbI3 (I), MAPbI3-PbBr2 (B) and MAPbI3-PbCl2 (CL) on carbon-based perovskite solar cells (C–PSCs) under ambient conditions is extensively studied. The results show that 1-MPII ionic liquid activates the surface morphology of all perovskite films and improves solar cells' efficiency due to the passivation of the uncoordinated Pb2+ defects. Furthermore, stability measurements show that the unencapsulated IL-modified devices for perovskites MAPbI3 (IIL), MAPbI3-PbBr2 (BIL), and MAPbI3-PbCl2 (CLIL) retained at 84 %, 86 %, and 80 % of their original PCE for over 1100 h under ambient conditions with 45 % relative humidity (RH), compared to the IL-free PSCs. Moreover, the IL-modified PSCs exhibited an increased electrical performance ranging from 16.5 % to 33 % depending on the type of perovskite it was used, so that long-term stability also was reinforced.