Neutral, Polaron, and Bipolaron States in PEDOT Prepared by Photoelectrochernical Polymerization and the Effect on Charge Generation Mechanism in the Solid-State Dye-Sensitized Solar Cell

We investigate dye-sensitized solar cells (DSSCs) based on PEDOT as hole conductor and prepared by photoelectrochemical polymer deposition at different light intensities. We specifically investigate the effect of light intensity on the PEDOT polymer and in turn the efficiency of the solar cells. We find that the PEDOT prepared by this method is largely oxidized and contains significant amounts of polarons and bipolarons and only a small fraction of neutral PEDOT. Photoelectrochemical polymer deposition under low light intensity leads to a particularly low fraction of neutral PEDOT and a high fraction of bipolarons as measured in the UV-vis spectra. The solar cells based on PEDOT as a hole conductor prepared under these conditions are the most efficient with a higher power conversion efficiency, which can be explained by a longer electron lifetime, faster charge transport, and higher transparency of the PEDOT. Interestingly, we conclude that in this type of solid-state DSSCs the mechanism of dye regeneration occurs from PEDOT polarons that then form bipolarons, which is different from the mechanism of dye regeneration proposed in standard solid-state DSSCs.