Mechanistic and Time Resolved Single-Photon Counting Analysis for Light Harvesting Characteristics Depending on the Adsorption Mode of Organic Sensitizers in DSSCs

This study investigated the origin of the improved photovoltaic performance of bibranched phenothiazine derivatives in DSSCs, in terms of the adsorption modes of the π-bridge dyes as sensitizers. Two novel twin acceptor type organic dyes with various π-bridge systems were designed and synthesized. We mainly focused on the binding mode of the dye on the TiO₂ surface and investigated the effect of the adsorption mode on the characteristics of solar cells. A stronger chemical bonding and a fast electron injection were found to originate from the bridge bidentate mode. This adsorption mode was also found to have the fastest electron transfer bonding of a π-conjugation anchoring group for organic dyes with 3,4-ethylenedioxythiophene (EDOT). Meanwhile, using thiophene as a π-conjugated anchoring group with a monodentate adsorption mode was found to produce slower electron transfer. The structural differences due to the π-conjugated anchoring groups affect their electron injection/recombination properties. Further, we found that the DSSCs based on organic sensitizers containing EDOT exhibited significantly enhanced long-term stability.