Vacancy Sealing with Pyridine Molecules Lowers Interfacial Trap Density and Accelerates Charge Extraction in Organic Photovoltaics

In organic photovoltaics (OPVs) using zinc oxide (ZnO) as the electron transport layer (ETL), radicals generated in the oxygen vacancies of ZnO interfere with the π-π stack structure of the upper organic photoactive layer. The disruption of π-π stacking by these radicals hinders charge transfer and increases recombination losses. If this disruption continues, it eventually impairs the operational stability of the device. To mitigate these effects, pyridine (Py)-based passivation is explored. Among various candidates, 4-tert-butylpyridine (4TPy) is most effective in preserving π-π stacking, enhancing interfacial charge extraction, and suppressing radical formation. Incorporation of 4TPy into the device architecture improved the fill factor (FF) from 74.1 ± 0.7% to 78.0 ± 0.9% and power conversion efficiency (PCE) from 15.7 ± 0.2% to 18.0 ± 0.1%, demonstrating excellent operational stability while retaining 90% of its initial efficiency after 1000 h under ambient conditions. Furthermore, a two-strip mini-module incorporating the same passivation strategy achieved a PCE of 15.6% and an FF of 74.8%, compared to 13.2% and 69.1% for its unpassivated counterpart. These results underscore radical-induced interfacial disruption as a key limiting factor in ZnO-based OPVs and establish molecular passivation as a scalable and effective route for enhancing performance, stability, and large-area applicability.