The roles of thin dense metal oxide films in inverted hybrid solar cells

The wideband semiconducting metal oxide TiO2 and Nb2O5 thin films have been deposited onto FTO substrates by sol-gel method for the fabrication of inverted hybrid photovoltaics. A thin metal oxide film of approximately 10 nm in thickness between the electron collecting electrode and the photoactive blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) has been demonstrated to be necessary to promote the formation of continuous uniform PCBM film to block holes from being recombined with good power conversion efficiency of 2.8% and much enhanced stability. In spite the fact that the conduction band of Nb2O5 is higher than the LUMO of PCBM, a power conversion of 2.7% was achieved, a very small difference in comparison with TiO2 hybrid solar cells (2.8%). Increased thickness of dense metal oxide film leads to decreased fill factor, current density, and thus the power conversion efficiency. The above experimental results suggest that electrons from LOMO energy level of P3HT and PCBM can tunnel through dense metal oxide film to charge collecting FTO substrate.