Characterization of Inverted-Type Organic Solar Cells with a ZnO Layer as the Electron Collection Electrode by ac Impedance Spectroscopy

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Abstract
An inverted-type organic bulk-heterojunction solar cell inserting zinc oxide (ZnO) as an electron collection electrode, fluorine-doped tin oxide (FTO)/ZnO/[6,6]-phenyl-C61-butyric acid methyl ester:regioregular poly(3-hexylthiophene) (PCBM:P3HT)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS)/Au, was fabricated in air and characterized by an alternating current impedance spectroscopy (IS). In the IS measurement, we observed reproducibly the electric resistance and capacitance components originating from ZnO and organic active layers, and we found that the depletion layer functioning to take out the photocurrent to the external circuit was formed in both the ZnO and PCBM:P3HT layers at the ZnO/PCBM:P3HT interface. In this letter, we propose that this IS measurement is effective for evaluating the electric properties of several layers with capacitance components in organic thin-film solar cells.