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

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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.

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