The effects of Ta2O5-ZnO films as cathodic buffer layers in inverted polymer solar cells

Ta2O5-ZnO composite films with varied composition were fabricated by sol-gel processing and applied as cathodic buffer layers (CBLs) for inverted polymer solar cells, and demonstrated enhanced power conversion efficiency with excellent stability. Physical and surface properties of Ta2O5-ZnO CBL films were examined by XPS, AFM, UV-Vis absorption spectra, and goniometry. It was found that CBLs incorporated with Ta2O5 exert two competing impacts on the solar cell performances. On one hand, the presence of Ta2O5 is likely to induce more positive charges around the Zn atom and form Ta-O-Zn bonding; it can reduce the surface charge recombination between the bulk heterojunction (BHJ) active layer and the cathodic buffer layer (CBL), and result in high power conversion efficiency; however, on the other hand an excessive amount of Ta2O5 would block the pathways of charge transport and lead to a drastic reduction in power conversion efficiency.