Tannic Acid as a Multifunctional Bridge Driving Structural Ordering and Surface Passivation of ZnO for Over 20% Efficiency Inverted Solar Cells with Improved Stability

Abstract

Inverted organic solar cells (OSCs) utilizing zinc oxide (ZnO) as the electron transport layer are promising candidates for commercial applications, yet their power conversion efficiency (PCE) lags behind conventional architectures due to structural disorder and surface vacancy defects in the ZnO layer. Herein, tannic acid (TA), a natural polyphenolic compound derived from renewable sources, is introduced to simultaneously tackle these challenges. The abundant ether, ester, and hydroxyl groups in TA enable strong multifunctional coordination with ZnO, ordering the internal structure and passivating surface vacancies, thereby enhancing electron extraction and transport while mitigating photocatalytic degradation at the interface. Consequently, the incorporation of TA into ZnO layers enhances device performance, achieving a record PCE of 20.1% for inverted binary OSCs. Furthermore, TA improves the operational stability of the devices by extending the T80 lifetime (defined as the time to 80% of the initial PCE) from 93 to 2,808 hours. These results demonstrate that natural green TA functions as an effective modifier for ZnO-based electron transport layers, offering robust support for advancing high-efficiency and stable inverted OSCs.