ITO-free inverted polymer/fullerene solar cells: Interface effects and comparison of different semi-transparent front contacts

Polymer/fullerene solar cells with an inverted layer sequence and free from indium tin oxide (ITO) are presented in this study. We concentrate on critical interface effects in inverted devices and compare different semi-transparent front contacts, such as ultra-thin Au films and Au grid structures. The solvent of the absorber blend was found to have a distinct impact on the stability of the initially deposited cathode layer. Furthermore, we did wettability studies of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) on the active layer and determined the surface energy of polymer/fullerene blends. Ultra-thin Au films with varying thickness were investigated in terms of sheet resistance and transmittance. The optimal thickness was achieved via simulation of organic solar cells. Finally, we present a comparison of inverted solar cells with an active area of about 1 cm² both with Au films and Au grid structures as front contact in terms of photovoltaic performance, reproducibility, and degradation. Notably better results were achieved when using Au grid structures as front contact, even with a not optimized grid geometry.