Regenerative PbS and CdS quantum dot sensitized solar cells with a cobalt complex as hole mediator

Metal sulfide (PbS and CdS) quantum dots (QDs) were prepared over mesoporous TiO ₂ films by improved successive ionic layer adsorption and reaction (SILAR) processes. The as-prepared QD-sensitized electrodes were combined with a cobalt complex redox couple [Co(o-phen) ₃] ^{2 +/3+} to make a regenerative liquid-type photovoltaic cell. The optimized PbS QD-sensitized solar cells exhibited promising incident photon-to-current conversion efficiency (IPCE) of over 50% and an overall conversion efficiency of 2% at 0.1 sun in a regenerative mode. The overall photovoltaic performance of the PbS QD-sensitized cells was observed to be dependent on the final turn of the SILAR process, giving a better result when the final deposition was Pb ²⁺, not S ^2-. However, in the case of CdS QD-sensitized cells, S ^2- termination was better than that of Cd ²⁺. The cobalt complex herein used as a regenerative redox couple was found to be more efficient in generating photocurrents from PbS QD cells than the typical hole scavenger Na ₂S in a three-electrode configuration. The CdS-sensitized cell with this redox mediator also showed better defined current-voltage curves and an IPCE reaching 40%.