Revisiting the origin of open circuit voltage in nanocrystalline-TiO2/polymer heterojunction solar cells

Abstract

Recent reports suggest that the open circuit voltage, $V_{\text{OC}}$ , in polymeric heterojunction solar cells is determined by the energy difference between the highest occupied molecular orbital of the electron donor and the lowest unoccupied molecular orbital of the electron acceptor. Here we show that in solar cells formed from nanocrystalline titanium dioxide $\left({\text{nc-TiO}}_2\right)$ and poly(3-hexylthiophene) (P3HT), $V_{\text{OC}}$ may arise from a depletion region at the ${\text{nc-TiO}}_2/\text{P}3\text{HT}$ interface, which only forms in the presence of oxygen. Under illumination in vacuum, photovoltaic behavior is swamped by a significant increase in photoconduction enabled by reduced electron trapping in the absence of oxygen.