Role of ZnO Electron-Selective Layers in Regular and Inverted Bulk Heterojunction Solar Cells

Here the role of metal oxide (ZnO) electron-selective layers in the operating mechanisms of bulk-heterojunction polymer-fullerene solar cells is addressed. Inverted as well as regular structures containing ZnO layers at the cathode contact have been analyzed using capacitance methods in the dark and impedance spectroscopy under illumination. We systematically observed that the open-circuit voltage V-oc at 1 sun illumination results higher for inverted cells than that achieved by regular structures in Delta V-oc approximate to 30-50 mV. This shift correlates with the displacement of the flat-band potential V-fb extracted from Mott-Schottky capacitance analysis. A coherent picture is provided that states the hole Fermi level of the polymer highest occupied molecular orbital as an energy reference for both Delta V-oc and V-fb. The study connects the position of the hole Fermi level to the p-doping character of the active layer that is influenced by the film morphology through vertical phase segregation.