Solution processed reduced graphene oxide/metal oxide hybrid electron transport layers for highly efficient polymer solar cells

We report new solution processable electron transport layers for organic photovoltaic devices based on composites of metal oxides and reduced graphene oxides. Low bandgap polymer cells fabricated using these nanohybrid transport layers display power conversion efficiencies in the range of 7.4-7.5% which is observed to be an improvement over conventional metal oxide or thermally evaporated electron transport layers. This efficiency enhancement is driven mainly by improvements in the short circuit current (from similar to 14.8 to similar to 15.0 mA cm(-2)) as well as the fill factor (similar to 65% to similar to 68%) upon the inclusion of reduced graphene oxide with the metal oxides. This is attributed to the reduced graphene oxide providing charge transfer pathways between the metal oxide nanoparticles. In addition, the metal oxide/reduced graphene oxide nanohybrids also lead to more balanced electron and hole mobilities which assist in the improvement of the fill factor of the device. The versatile nature of these nanohybrids is increased due to the wrapping of the graphene layers around the metal oxide nanoparticles, which leads to very smooth films with surface roughness of similar to 3 nm. The improvement observed in this study upon the incorporation of RGO as well as the solution processable nature of the interfacial layers brings the organic photovoltaic technology a step closer towards realising an all solution processed solar cell.