A Cytop Insulating Tunneling Layer for Efficient Perovskite Solar Cells

Improvement of the interface between the perovskite layer and the electrontransport layer (ETL) is critical toward the advancement of planar perovskite solar cells (PSCs). It is important to obtain a uniform and pinhole-free ETL that can minimize film defects and thus undesirable electron-hole recombination between the perovskite layer and cathode. However, this is extremely difficult because the rough perovskite surface causes charge-carrier recombination, facilitates large leakage currents, and inevitably deteriorates the electron-extraction efficiency. Here, fluorine-containing insulating polymers, poly(perfluorobutenylvinylether) (Cytop), are used as the tunneling layer in planar PSCs for the first time, resulting in a significant improvement in the power conversion efficiency (PCE) from 11.9% to over 14.5%. It is found that the Cytop layer not only fills the pinholes of the perovskite surface to decrease the trap concentration, but can also provide a strong electron-extraction ability to facilitate charge-transfer process and restrict charge recombination. In addition, improved water resistance is demonstrated using Cytop, which significantly extends the PSC lifetime (78% of initial PCE vs 22% for control after 1500 h) and increases the practical applicability.