Intriguing Photochemistry of the Additives in the Dye-Sensitized Solar Cells

Over the years numerous mixes of chemical compounds have been tried in the electrolytes of dye-sensitized solar cells in efforts to improve their efficiency. How these chemicals interact with each other and the photoelectrode has received surprisingly little attention. Here we report results from a systematic study of two I–/I3– electrolytes and their additives using infrared and Raman spectroscopy together with quantum chemical calculations. In the LiI electrolyte competing interactions between lithium cation and the solvent MPN and the additives TBP, NMBI, and GuSCN were identified. These interactions could inhibit the interaction of lithium ions with the TiO2 surface. It was found that under Raman excitation of PMII solution in contact with the photoelectrode, efficient generation of I3– takes place. For LiI solution, in addition, a Dye–I2 complex is formed. The results could be explained by diffusion-limited buildup of I3– and depletion of I– concentrations in the focal area of the excitation beam and by reduction of I3– via conduction band electrons of TiO2 beyond the focal region. To explain the formation of Dye–I2 complexes in the LiI electrolyte solutions a multistep regeneration mechanism is proposed. It was found that GuSCN reduces the I3– concentration in the electrolyte solutions studied; in the LiI electrolyte in addition it binds to lithium ions and nearly depletes the Dye–I2 complexes. From infrared spectra it became clear that preventing water from entering the DSCs during the preparation stages in ambient air is a demanding task. The identified interactions paint an intriguing new photochemical landscape of the function of the dye-sensitized solar cells giving guidelines for further development of the devices.