Influence of polymer–metal interface on the photovoltaic properties and long-term stability of nc-TiO2-P3HT hybrid solar cells

The role played by the interface between light harvesting polymer poly(3-hexylthiophene) (P3HT) and thermally evaporated metal electrodes such as Au, Ag, and Cu on the photovoltaic performance and stability of nc-TiO2-P3HT (nc refers to nanocrystalline) ordered heterojunction hybrid solar cells has been systematically investigated. These hybrid solar cells with Cu films as positive electrode have been fabricated for the first time, and they exhibit improved open circuit voltage (Voc) and fill factor (FF) compared to those fabricated with Au and Ag electrodes. More importantly, the Cu-based cells show better stability during continuous operation and ageing in an inert atmosphere. The improved photovoltaic performance and stability of the cells fabricated with the Cu electrodes are attributed to the chemical bonding between Cu and the S atoms of P3HT to give copper–sulfur complex or copper sulfide like species as indicated by the photoelectron spectroscopic (XPS) data. The improvement in stability exhibited by the cells with Cu electrode is interesting in light of the fact that hybrid solar cells based on semiconductor oxides like TiO2 quickly decay in the absence of oxygen.