Enhancing Photovoltaic Performance by Cathode Interfacial Modification with Inorganic/Organic Gradient Diffusion Structures

The modification of the interfacial contacts between the active layer and the electrode is of great importance in achieving high-performance organic solar cells (OSCs). Herein, a composite film with gradient diffusion structure based on zinc oxide (ZnO) and nonfullerene organic semiconductor of ITIC (G-ZnO/ITIC) is constructed by a convenient one-step solution-processing method for the interfacial modification of OSCs. The facilely constructed G-ZnO/ITIC composite films show enhanced surface hydrophobicity and comparatively smooth morphology, contributing to the improved interfacial contact between the inorganic interfacial layer and organic photoactive layer. Meanwhile, the cascade energy level established inside the bulk G-ZnO/ITIC cathode interfacial layer (CIL) would further assist in the electron-transporting process for efficient charge extraction. Therefore, G-ZnO/ITIC-based PTB7-Th:PC71BM OSCs exhibit power conversion efficiency (PCE) up to 8.73%, which is remarkably larger than these of conventional ZnO-based devices (7.88% for pure ZnO CIL device, 7.27% for ZnO/ITIC bilayer CIL device, and 6.93% for ZnO/ITIC blends CIL device). This gradient diffusion structure is also effective in the PTB7-Th:ITIC-based nonfullerene OSCs, showing improved PCE values from 6.63% to 7.29%. The facilely prepared ZnO/organic semiconductor composite films with gradient diffusion structures and improved device performance would significantly broaden the types of interfacial layers with minimized boundaries among various functional layers, representing an important concept advance in constructing high-performance CILs for OSCs.