A high-performance counter electrode based on poly(3,4-alkylenedioxythiophene) for dye-sensitized solar cells

A poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et2) counter electrode prepared by electrochemical polymerization on a fluorine-doped tin oxide (FTO) glass substrate was incorporated in a platinum-free dye-sensitized solar cell (DSSC). The surface roughness and I−/I3− redox reaction behaviors based on PProDOT-Et2, poly(3,4-propylenedioxythiophene) (PProDOT), poly(3,4-ethylenedioxythiophene) (PEDOT), and sputtered-Pt electrodes were characterized, and their performances as counter electrodes in DSSCs were compared. Cells fabricated with a PProDOT-Et2 counter electrode showed a higher conversion efficiency of 7.88% compared to cells fabricated with PEDOT (3.93%), PProDOT (7.08%), and sputtered-Pt (7.77%) electrodes. This enhancement was attributed to increases in the effective surface area and good catalytic properties for I3− reduction. In terms of the film thickness effect, the fill factor was strongly dependent on the deposition charge capacity of the PProDOT-Et2 layer, but the aggregation of PProDOT-Et2 in thicker layers (>80 mC cm−2) resulted in decreases in JSC and the cell conversion efficiency. The charge transfer resistances (Rct1) of the PProDOT-Et2 counter electrodes had the lowest value of ∼18 Ω at a deposition charge capacity of 40 mC cm−2. These results indicate that films with high conductivity, high active surface area, and good catalytic properties for I3− reduction can potentially be used as the counter electrode in a high-performance DSSC.