Inverted ITO- and PEDOT:PSS-free polymer solar cells with high power conversion efficiency

Highest published power conversion efficiencies of organic solar cells have mostly been achieved on substrates bearing a transparent indium tin oxide (ITO) electrode. However, the incorporation of ITO is not suited for future industrial production processes of organic solar cells, which will rely on a high-throughput of flexible substrates in order to achieve low cost of the final product. In this manuscript we present an alternative transparent electrode consisting of a layer stack of aluminum doped zinc oxide and a thin silver layer. Substrates with these electrodes have a transparency of above 75% in the wavelength range in which the photoactive layer absorbs light. Solar cells with a bulk-heterojunction of PTB7 and PC71BM in an inverted device architecture achieved a power conversion efficiency of 6.1%, which is the highest reported value for polymer solar cells free from both ITO and PEDOT:PSS. The sheet resistance of the novel electrodes increased only marginally after repeated bending which shows their full compatibility with future reel-to-reel processes or flexible products.