Experimental Determination of Charge/Neutral Branching Ratio in $\pi $-Conjugated Polymers by Broad-band Ultrafast Spectroscopy$^{1}$

We demonstrate a reliable method of determining the branching ratio, $\eta $ of photogenerated charge (polarons) to neutral (excitons) photoexcitations in various $\pi $-conjugated polymer films and solutions using femtosecond ultrafast spectroscopy with broad spectral range from 0.14 to 2.7 eV. We found that both excitons and polarons are instantaneously photogenerated, but $\eta $ critically depends on the film nanomorphology, which, in turn controls the interchain coupling. In films, $\eta $ varies between 1{\%} for derivatives of poly(p-phenylene vinylene) casted from chloroform solution, to more than 30{\%} for regio-regular poly-3-hexyl thiophene. Our results show that charge photogeneration quantum efficiency in these materials is an interchain process; and this has ramifications for their use in solar cell applications. $^{1}$Supported in part by the DOE.