Charge-transfer complexes of conjugated polymers as intermediates in charge photogeneration for organic photovoltaics

Ultrafast visible-pump/IR-probe spectroscopy is applied to study the wavelength dependence of charge photogeneration in materials based on donor–acceptor charge-transfer complexes (CTCs) of the conjugated polymer MEH-PPV. In binary polymer–acceptor blends, photoexcitation in the absorption band of either CTC or polymer results in similar dynamics of the charge-associated transient absorption. Likewise, in polymer/CTC–acceptor/fullerene ternary blends, where charge separation occurs via a two-step pathway, the photophysics is also independent of excitation wavelength. These similarities in charge dynamics indicate that CTC excited states serve as an intermediate for charge photogeneration. The conclusions of the ultrafast study are supported by photocurrent spectroscopy.