Transient absorption spectroscopy of ultra-low band gap polymers for organic electronic applications

In this paper two extremely narrow band-gap polymers, based on naturally occurring indigo with high thin film crystallinity, have been examined using transient absorption spectroscopy. This was done in order to assess their charge photogeneration and recombination characteristics in blends with PC₇₁BM. Two charge photogeneration mechanisms are found to be operating, depending on which component of the blend is photoexcited. Despite virtually isoenergetic LUMO levels, photoexcitation of the polymer causes standard electron transfer, albeit with a relatively low efficiency of 17 %. Photoexcitation of the fullerene, however, produces an exceptionally slow nanosecond timescale hole transfer.