Unraveling the Pivotal Impacts of Electron-Acceptors on Light Absorption and Carrier Photogeneration in Perylene Dye Sensitized Solar Cells

Understanding the ultrafast dynamics of excited state evolution and carrier generation at the complicated titania/dye/electrolyte interface is crucial for the development of narrow energy-gap organic dyes to enhance the performance of dye-sensitized solar cells (DSCs). We herein modulate the energy-levels of <i>N</i>-annulated perylene dyes in significant measure by use of benzothiadiazole-benzoic acid (BTBA) and pyridothiadiazole-benzoic acid (PTBA) segments as the electron-acceptors. Based on the model of cold vs hot excited states for electron injection in DSCs, we have perceived from femtosecond transient absorption measurements that a driving force diminution does not necessarily cause a deceleration of electron injection from the hot excited state to titania, suggesting a crucial role of electron coupling on interfacial charge transfer kinetics. Moreover, the electron-accetpor variation from BTBA to PTBA brings forth a fast relaxation of the hot excited state as well as a rapid deactivation of the cold excited state and a sluggish electron injection from the cold excited state to titania. These adverse multichannel kinetics jointly result in a remarkable diminishment of overall electron injection yield and, thus, lower external quantum efficiency of cell with the PTBA dye. Our studies have highlighted the importance of considering the excited state topology on the future design of low energy-gap photovoltaic materials