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

Abstract

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 TiO2 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 TiO2. 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 TiO2/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.