Defect-Passivated ZnO via Carboxyl-Functionalized Imides for Efficient and Stable Inverted Organic Solar Cells

Recently, conventional organic solar cells (OSCs) using PEDOT:PSS or self-assembled monolayers (SAMs) as hole transport layers have achieved power conversion efficiency (PCE) exceeding 20%, but their insufficient stability limits further commercialization. Inverted OSCs based on inorganic oxides, such as zinc oxide (ZnO), have attracted increasing attention due to their superior stability. ZnO offers high electron mobility and good transparency but suffers from many surface defects and photocatalytic degradation of active layers, which negatively affect the device performance. To address these challenges, we design and synthesize two carboxyl-functionalized imides (NDI-1C and NDI-2C) for passivating ZnO electron transport layers (ETLs). Systematic investigations confirm that NDI-2C-modified ZnO ETLs exhibit reduced trap state density, enhanced charge extraction/transport, and improved interfacial contact. The inverted device based on NDI-2C-treated ZnO showed an outstanding PCE of 18.9% compared to the control device based on ZnO (17.6%). Furthermore, the device based on NDI-2C-treated ZnO not only maintained excellent thermal stability, but also demonstrated significantly improved photostability with a T₈₀ lifetime of 2300 h in comparison with a T₈₀ lifetime of 200 h for the control device. This study provides an effective interfacial modification strategy for simultaneously improving the efficiency and stability of ZnO-based inverted OSCs.