DOI: https://doi.org/10.29363/nanoge.inform.2019.038
Publication date: 8th January 2019
Electronic excitation plays a key role in the operation of organic-based semiconductor devices such as organic solar cells, photodetectors and light-emitting diodes. Thus, developing our understanding of the processes that dictate the fate of primary photoexcitations is essential in the design, optimization and longevity of such devices. In the field of organic solar cells, there has been an intense search for low energy band gap systems, based upon π-conjugated polymers, for more efficient collection of solar energy in the near IR. In our recent work,[1] three low energy bandgap polymers have been demonstrated to exemplify a significantly higher extinction coefficient compared to other conjugated polymer systems, either homopolymers or donor-acceptor polymers, to date, especially in higher molecular weights, due to their extended persistence length.
Using ultrafast Time-Resolved Infrared spectroscopy we have studied the excited state structural evolution in solution in these three polymers systems; two copolymers of diketopyrrolopyrrole and thiophene (DPP-3T) or thienothiophene (DPP-TT-T) and one a copolymer of indacenodithiophene with benzothiadiazole (IDTBT). In all three cases, vibrational features appear promptly after excitation. The vibrational features appear on a broad absorption background that decays, which is tentatively assigned to polaron absorption due to interchain interactions. For DPPTT-T, both the broad background and IR bands decay within 200 ps. However, for IDTBT we observe not only longer decay times but also significant differences in the kinetics between the broad background and IR bands, indicative of an initial intramolecular charge-transfer state formation which then evolves to the charge separated species that subsequently recombines. We will discuss the possible origin of these phenomena with the aid of calculations of vibrational spectra and models for excited state.
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