Photocurrent generation by charge-transfer (CT) absorption is detected in a range of conjugated polymer: [6,6]-phenyl C 61 butyric acid methyl ester (PCBM) based solar cells. The low intensity CT absorption bands are observed using a highly sensitive measurement of the external quantum efficiency (EQE) spectrum by means of Fourier-transform photocurrent spectroscopy (FTPS). The presence of these CT bands implies the formation of weak groundstate charge-transfer complexes in the studied polymer:fullerene blends. The effective band gap (E g ) of the material blends used in these photovoltaic devices is determined from the energetic onset of the photocurrent generated by CT absorption. It is shown that for all

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