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.

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Photoluminescence and transient absorption measurements of poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 (PCBM) blend films reveal that optical excitation of F8BT results in formation of the singlet and triplet excited states of PCBM. These observations are assigned to rapid (>1011s−1) singlet energy transfer from F8BT to PCBM, followed by intersystem crossing to the PCBM triplet state.

Transient and steady state optical spectroscopies were used to study thin films made from a series of polyfluorene polymers blended with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) in order to determine the influence of polymer ionisation potential on photoinduced charge separation. We find that the energy of the charge separated state DeltaE(CS), given by the energy difference between the ionisation potential of the polymer and the electron affinity of the fullerene, must be smaller than a threshold value of about 1.6 eV for charge separation to occur. When DeltaE(CS) is greater than this threshold, PCBM triplet formation is observed in preference to charge pair generation. If DeltaE(CS) is similar to the threshold value, both PCBM triplet formation and charge separation occur in the blend film, with a tendency for charge separation to dominate over PCBM triplet formation as PCBM concentration increases. The mechanism of triplet formation is believed to be energy transfer to the PCBM singlet state followed by intersystem crossing. The threshold value of DeltaE(CS) is found to be similar to the PCBM singlet energy.

Long-lived exciplex emission is observed in blend films of poly[9,9-dioctylfluorene-co-N-(4-methoxy-phenyl)diphenylamine] (TFMO) and the soluble silole derivative 2,5-bis-(2,2-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PyPySPyPy). The exciplex is characterized by a long-lived (approximately 40-90 ns) component in both the photoluminescence and electroluminescence spectra, which is red-shifted relative to the emission of the pristine materials. In addition to exciplex emission, delayed fluorescence from the TFMO singlet state is observed and is attributed to exciton regeneration through the interfacial exciplex state. Comparing blend films made using chlorobenzene and p-xylene solvents, we find that exciplex lifetime and exciton regeneration in the blend film are sensitive to the choice of solvent and the resulting morphology of the blend film. The exciplex emissive lifetime can be correlated to changes in photoluminescence quenching and efficiency of light-emitting diodes.