Naphthalene-diimide-incorporated conjugated polyelectrolyte interfacial modifier for the efficient inverted-type polymer solar cells

A cationic conjugated polyelectrolyte (PFTNDI) containing electron-accepting naphthalene diimide moiety was designed and synthesized to improve the surface properties of the ZnO layer in the inverted polymer solar cells (PSCs). After the PFTNDI treatment of the ZnO surface, the uniformity and hydrophobicity of the surface was significantly enhanced, resulting in highly reduced surface tension. In addition, the permanent interfacial dipole at the interface of ZnO and PFTNDI induced a shift in the vacuum level, leading to the reduced work function (WF) of the ZnO layer. The inverted devices with an ITO/ZnO/PFTNDI/PTB7-Th:PC71BM/MoO3/Ag configuration were fabricated, and the photovoltaic properties were investigated. The reduced WF of the ZnO film resulted in slightly increased Voc values and highly reduced contact resistance. Combined with reduced surface tension, the PFTNDI-treated devices exhibited much higher power conversion efficiency (8.65%) than the bare ZnO devices (7.99%).