Simultaneous Passivation of Surface Vacancies and Enhancement in Charge Transfer Property of ZnO Electron Transport Layer for Inverted Organic Solar Cells

The design and development of a charge carrier transport material are decisive parameters for controlling the organic solar cell's power conversion efficiency (PCE). ZnO is one of the most suitable electron transport materials used in inverted bulk heterojunction polymer solar cells. However, the solution-processed ZnO has surface defects, which hinders the power conversation efficiency of the device. Herein, it is designed and demonstrated that the 2D NO(2)group functionalized reduced graphene oxide (rGN) sheet coated on top of the 1D ZnO nanoridges not only passivates the surface but also enhances the charge transport property of the electron transport layer (ETL), thereby improving the overall PCE by 31%. Atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), and optical measurements reveal that the highly transparent bilayer ZnO/rGN ETL has uniform film formation and, thereby, improved ohmic contact between the cathode and the photoactive layer. Due to the improved electron transport from the photoanode (PTB7-Th:PC71BM) to the buffer layer, a photoinduced current density of 20.05 mA cm(-2)is achieved. This interface modification by rGN can be an effective strategy to passivate the surface and retards the recombination rate to enhance the efficiency of organic photovoltaic cells.