We conducted a comprehensive Kelvin probe force microscopy (KPFM) study on a classical organic solar cell system consisting of MDMO-PPV/PCBM blends. The KPFM method yields the information of topography and local work function at the nanometer scale. Experiments were performed either in the dark or under cw laser illumination at 442 nm. We identified distinct differences in the energetics on the surface of chlorobenzene and toluene cast blend films. Together with high-resolution scanning electron microscopy (SEM) experiments we were able to interpret the KPFM results and to draw some conclusions for the electron transport toward the cathode in the solar cell configuration. The results suggest that surfaces of toluene cast films exhibit a morphologically controlled hindrance for electron propagation toward the cathode, which is usually evaporated on top of the films in the solar cell device configuration.

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The role of bathocuproine ͑BCP͒ buffer layer inserted between active layer and Al contact in photovoltaic cells based on phthalocyanine ͑Pc͒ and C 60 was investigated. Photoluminescence ͑PL͒ experiments show exciton quenching at the C 60 -Al interface to be strongly reduced by inserting BCP. Current-voltage characteristics of photovoltaic cells with planar geometry front electrode/ Pc/ C 60 / BCP/ Al show that BCP dramatically improves electron transport out of the C 60 film into the Al electrode. The tenfold increase in power conversion efficiency ͑͒ with BCP can mostly be attributed to the latter effect. BCP does not improve in photovoltaic cells with blend film geometry front electrode/Pc: C 60 / Al.

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