Light soaking effects on charge recombination and device performance in dye sensitized solar cells based on indoline–cyclopentadithiophene chromophores

The synthesis, characterization, electrochemical and photophysical properties of a novel D–π–A indoline organic dye, VCL01, are described. Its performance characteristics in dye sensitized solar cell (DSC) devices under standard AM 1.5G illumination are also investigated. VCL01 incorporates a cyclopentadithiophene unit as the π-bridge between the indoline donor and cyanoacetic acceptor units. In comparison with the reference dye LS-1 containing only one thiophene unit in the π-bridge, VCL01 shows a 40 nm red shift in adsorption, an increase in molar absorptivity and a 0.17 V lower oxidation potential, all consistent with the more conjugated nature of this sensitizer. The efficiencies of VCL01 and LS-1 DSC devices were 4.81% and 6.23%, respectively, which upon >100 min continuous light soaking under AM 1.5G illumination rose to 7.21% and 6.95%, representing an unprecedented 50% increase in efficiency for the VCL01 device. This increase is overwhelmingly due to an increase in photocurrent but, remarkably, Voc also increases by 50 mV upon illumination reflected in transient photovoltage data which indicate that electron lifetime increases considerably also. Time-correlated single photon counting data indicate that the light soaking effect can be partly attributed to improved TiO₂/dye interaction leading to enhanced electron injection.