Enhancing UV Stability of Perovskite Solar Cells with Transparent Fluorinated Polyimide

Ultraviolet (UV)-induced degradation is one of the major problems in the field of perovskite solar cells (PSCs). Therefore, exploring materials and techniques to prevent UV light from penetrating into the device is urgently necessary. Here, we developed a special transparent fluorinated polyimide (FPI) resin, which can be directly spin-coated on the front side of conventional indium-doped tin oxide substrates (glass/ITO). Most aromatic polyimides strongly absorb visible light and are colored. The FPI we designed and synthesized bears electron-acceptor CF3- groups, which reduces the intra-/intermolecular charge-transfer (CT) effect, enabling FPI to possess high transmittance in the visible range while completely blocking UV light. As a result, the FPI coating slightly pulls down the initial power conversion efficiency (PCE) (21.02% to 20.19%). Remarkably, the coating significantly improves the PSC UV stability. Upon an 8-h enhanced UV aging test in air, the FPI/glass/ITO-based PSC is able to retain 85.0% of its initial PCE. In contrast, the control device (glass/ITO-based PSC) only keeps 40.9% of its initial PCE. The protective effect of FPI is even more prominent in current popular 3D/2D high-performance PSCs because UV light can seriously damage the 2D layer. The unencapsulated 3D/2D device based on FPI/glass/ITO substrate has a very high PCE retention of up to 80% after 12-h enhanced UV aging test in air, comparing to 36% for the control 3D/2D device without FPI. This work demonstrates that FPI and its possible derivatives could provide a feasible avenue to handle UV-induced degradation for PSCs effectively.