Variation in cell efficiency of organic solar cells by illumination properties of zno electron transport layers

Inverted organic solar cells based on poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM) were fabricated with ZnO electron-transport layers that were grown with various thicknesses (20~70 nm) by atomic layer deposition. The cell with 40-nm ZnO buffer thickness exhibited the best power conversion efficiency. The electrical properties of semiconducting ZnO layers were strongly affected by film thickness and ultraviolet (UV) absorption, because UV illumination enhanced the electrical conductivity of the ZnO layer due to the discharge of oxygen ions. The increase in thickness of the ZnO layer enhanced the photocurrent due to the increased ZnO volume, and led to more conductive ZnO film. The conductive ZnO layer induced a reduction in shunt resistance under the illumination due to increased tunneling events, resulting in poor cell performance.