Mitigating Illumination Sensitive Dark Current in Inverted Organic Photodiodes by ZnO Defect Passivation

Organic photodiodes (OPDs) are a compelling alternative to their inorganic counterparts due to their tunable spectral response and compatibility with solution-processable fabrication. However, the performance of high-performance inverted OPDs incorporating a zinc oxide (ZnO) electron transport layer (ETL) is significantly hampered by the light-soaking effect. This phenomenon, triggered by exposure to light (particularly ultraviolet light), causes a pronounced and reversible increase in dark current, leading to instability in the signal-to-noise ratio and compromising device reliability. To address this fundamental issue, we introduce a targeted interfacial passivation strategy by depositing a thin conjugated polyelectrolyte interlayer, PNDIT-F3N-Br, at the ZnO/bulk heterojunction (BHJ) interface. Comprehensive characterization, including X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and trap density analysis, reveals the suppression of oxygen adsorption, resulting in an increase in shunt resistance, and a reduction in trap density. Finally, OPDs with the modified ETL exhibit significantly enhanced photostability, with minimal variation in dark current upon illumination. This work provides a critical insight into overcoming a key limitation of ZnO-based optoelectronics, paving the way for the development of stable and high-performance OPDs.