Nanoscale roughness control of electron transport layer in organic solar cells fabricated by sol-gel method

ZnO films are widely used in the electronic transport layers of organic solar cells (OSCs) because of thier electron transport properties and high transparency. In this study, the surface structure of zinc oxide (ZnO) films was controlled using two organic solvents with different boiling points, and this was applied to inverted polymer solar cells. Two types of ZnO film were created through the sol-gel method. One had a fiber like rough surface and the other had a smooth surface structure. The resulting surface structure of a ZnO film is dependent on the relationship between the annealing temperature and boiling point of the organic solvent in the sol-gel solution. This study showed that the conversion efficiency of inverted organic solar cells (IOSCs) with a rough ZnO film was 60% higher than that with a smooth ZnO film. Judging from the current-voltage (I-V) characteristics of the IOSCs, there appears to be a positive correlation between the roughness of the surface structure of ZnO films and their conversion efficiency. In other words, the conversion efficiency increases with surface roughness.