Role of zinc oxide thickness on the photovoltaic performance of laminated organic bulk-heterojunction solar cells

A comprehensive study of zinc oxide (ZnO) film thickness and morphology on the electronic properties of inverted cells is reported. The complete conversion of zinc acetate precursor to 3–5 nm particles of ZnO with no residual acetate is obtained after 10 min at 300 °C. The work-function determined by Ultraviolet Photoelectron Spectroscopy (UPS) was 4.3 eV. and is independent of the thickness of the ZnO layer or with the planarization of the ITO surface topology. However, the efficiency varies from 0.6% to 1.7% as the ZnO thickness varies from 17 nm to 28 nm (assuming full density for the ZnO layer) in a laminated device, with both the shunt and series resistance showing a strong variation with ZnO thickness. A relatively thick, mixed phase ZnO/bulk hetero-junction in which the bulk hetero-junction penetrates into the porous ZnO layer is proposed to explain the observed performance trends.