Evidence is presented for the formation of a weak ground-state charge-transfer complex in the blend films of poly[9,9-dioctylfluorene-co-N-(4- methoxyphenyl)diphenylamine] polymer (TFMO) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), using photothermal deflection spectroscopy (PDS) and photoluminescence (PL) spectroscopy. Comparison of this polymer blend with other polyfluorene polymer/PCBM blends shows that the appearance of this ground-state charge-transfer complex is correlated to the ionization potential of the polymer, but not to the optical gap of the polymer or the surface morphology of the blend film. Moreover, the polymer/PCBM blend films in which this charge-transfer complex is observed also exhibit efficient photocurrent generation in photovoltaic devices, suggesting that the charge-transfer complex may be involved in charge separation. Possible mechanisms for this charge-transfer state formation are discussed as well as the significance of this finding to the understanding and optimization of polymer blend solar cells. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.