Pure-phase two-dimensional perovskite capping layer enables high-performance and durable carbon-based photovoltaics

Carbon-based perovskite solar cells (C-PSCs) have been considered to be promising candidates towards commercialization, due to their relatively low-production cost and excellent stability. However, the defects of the perovskite film and the mismatched energy level arrangement between perovskite layer and the carbon electrode (CE) severely limit the power conversion efficiency (PCE) of C-PSCs. Here, we incorporated pure-phase twodimensional (2D) perovskite crystal (OA)2PbI4 2 PbI 4 (OA=n-octylammonium) n-octylammonium) as a passivation layer atop of 3D perovskite layer in C-PSCs. The modified films show significantly reduced defect state density, optimized energy level arrangement, and enhanced hydrophobicity. As a result, a champion PCE of 20.5 % is achieved for the target devices, in conjunction with a composite hole-transport layer consisting of poly(3-hexylthiophenyl) (P3HT) and carboxylated multi-walled carbon nanotube (c-MWCNT). This PCE is among the top efficiencies reported for planar PSCs based on doctor-coating carbon electrodes. More importantly, the encapsulated target devices also exhibit excellent photo-thermal stability, maintaining more than 80 % of initial PCE after 1000 h under AM 1.5G illumination at 85 degrees C, following the ISOS-L-2. This work demonstrates a useful approach for the development of high-performance and durable C-PSCs.