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-C(61)-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.

A fluorine-doped tin oxide ͑FTO͒/zinc sulfide ͑ZnS͒/͓6,6͔-phenyl C 61 butyric acid methyl ester ͑PCBM͒: regioregular poly͑3-hexylthiophene͒ ͑P3HT͒/poly͑3,4-ethylenedioxylenethiophene͒: poly͑4-styrene sulfonic acid͒ ͑PEDOT:PSS͒/Au type organic solar cell ͑FTO/ZnS/PCBM:P3HT/ PEDOT:PSS/Au, ZnS cell͒ with a 1 cm 2 active area was first developed using transparent ZnS prepared on a FTO electrode by a chemical bath deposition method. The ZnS inserted solar cells were investigated by photocurrent-voltage ͑I-V͒ and ac impedance spectroscopy ͑IS͒ measurements. In photo I-V measurements, the ZnS cell exhibited scattered power conversion efficiencies ͑͒ of 0.7%-1.2% when the FTO/ZnS electrode was not immersed in various aqueous solutions before the fabrication of the ZnS cells. In contrast, the solar cells with a surface-modified FTO/ZnS electrode by immersing in 0.1M Na 2 S solutions at pH 7-9 containing hydrosulfide ions ͑HS − ͒ exhibited reproducible of 1.5%-1.7%. The electric resistance components in the cell consisting of five layers were separately estimated by the IS measurement. The larger of the solar cell using HS − modified ZnS was explained by the fact that the charge transport resistance at the ZnS/PCBM:P3HT interface became smaller due to this surface modification. When a continuous light irradiation was carried out for the modified cells, the resistance at the ZnS/PCBM:P3HT interface increased, being accompanied by the decrease in the . That is, the cell performance was controlled mainly by charge transport velocity at the ZnS/PCBM:P3HT interface.

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