Resonant Molecular Modification for Energy Level Alignment in Perovskite Solar Cells

The energy level alignment at interfaces of perovskite solar cells (PSCs) is an essential factor determining their efficiency and stability. Therefore, an interface capable of appropriate energy band bending, minimal defects, and good contacts is the key issue for obtaining high-performing and stable PSCs. Herein, we present an innovative interface engineering approach employing an ortho-squaraine derivative with zwitterionic resonance structure, which can change the charge state of the perovskite surface to tune the energy level structure and can form pi-pi stacking with the hole-transporting material to shorten the carrier transmission distance and facilitate charge transfer. By the synergistic action of the double functionalization, the PSCs containing 3,4-Bis(4-bromophenyl)cyclobut-3-ene-1,2-dione achieved power conversion efficiency (PCE) of 23.82%, and the stability of the unencapsulated devices are greatly improved. For perovskite interfacial engineering, use of the resonant squaraine zwitterion is a promising chemical design element to enhance PSCs' efficiency and stability.