Chemically Controlled Reversible and Irreversible Extraction Barriers Via Stable Interface Modification of Zinc Oxide Electron Collection Layer in Polycarbazole-based Organic Solar Cells

A spin-cast method is presented for the formation of phosphonic acid functionalized small molecule layers on solution-processed ZnO substrates for use as electron collecting interlayers in organic photovoltaics. Phosphonic acid interlayers modify the ZnO work function and the charge carrier injection barrier at its interface, resulting in systematic control of V-OC in inverted bulk heterojunction solar cells. Surface modification is shown to moderate the need for UV light-soaking of the ZnO contact layers. Lifetime studies (30 days) indicate stable and improved OPV performance over the unmodified ZnO contact, which show significant increases in charge extraction barriers and series resistance. Results suggest that enhanced stability using small molecule modifiers is due to partial passivation of the oxide surface to molecular oxygen adsorption. Surface passivation while maintaining work function control of a selective interlayer can be employed to improve net efficiency and lifetime of organic photovoltaic devices. The modified cathode work function modulates V-OC via static energetic barriers and modulates contact conductivity by creating reversible and irreversible S-shape current-voltage characteristics as a result of kinetic barriers to charge transport.