Chlorin e6 sensitized photovoltaic cells: effect of co-adsorbents on cell performance, charge transfer resistance, and charge recombination dynamics

The effect of dye-aggregation-preventing co-adsorbents, cholic acid and deoxycholic acid, on the performance of dye-sensitized solar cells constructed using a metal-free sensitizer, chlorin e6 adsorbed onto TiO 2 surface is investigated. Absorption and fluorescence studies of chlorin e6 provided the spectral coverage, whereas electrochemical studies allowed estimation of the free energy of charge injection. B3LYP/6-31G* studies were performed to visualize location of the Frontier orbitals and their contribution to the charge injection when they were surface-modified on TiO 2 . The concentration of the co-adsorbent and soaking time was optimized for improved cell performance. Better dye regeneration efficiency for co-adsorbed cells compared to the cells with no co-adsorbent was revealed by electrochemical impedance spectroscopy. Femtosecond transient absorption studies were performed to probe the kinetics of charge injection and charge recombination on the TiO 2 /chlorin e6/co-adsorbent electrodes. Such studies showed slower by an order of magnitude charge recombination rates for electrodes co-adsorbed either with cholic acid or deoxycholic acid while maintaining almost the same charge injection rates, thus rationalizing the importance of co-adsorbents on the overall cell performance.