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-pi-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 pi-bridge between the indoline donor and cyanoacetic acceptor units. In comparison with the reference dye LS-1 containing only one thiophene unit in the pi-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, V-oc 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.