TCO-free conducting polymers/carbon cloths as the flexible electro-catalytic counter electrodes for dye-sensitized solar cells

Three kinds of water-soluble conducting polymers (CPs), including polypyrrole-block-poly(caprolactone) (PPy:PCL), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and sulfonated poly(thiophene-3-[2-(2-methoxyethoxy) ethoxy]-2,5-diyl) (s-PT), were used to decorate on the surface of fluorine-doped tin oxide (FTO) or carbon cloth (CC) separately; they were denoted as PPy:PCL/FTO, PEDOT:PSS/FTO, s-PT/FTO, PPy:PCL/CC, PEDOT: PSS/CC, and s-PT/CC and were used as the electrocatalytic counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). When a DSSC is coupled with a CP-coated CC as a CE, a higher solar-to-electricity conversion efficiency (eta) can be obtained than that of the cell with the same CP-coated FTO as the CE. This is due to the fact that the networked carbon fibers in a CC substrate provided large surface areas, high conductivity, and directional electron transfer pathways simultaneously. Therefore, the networked carbon fibers in a CC substrate worked as the conductive cores, while the coated CPs played the important role as the electro-catalytic shell for triggering reduction of I-3(-). Among all the electrodes, the s-PT/CC renders the best eta of 8.45% to its DSSC, even better than that of the cell with the traditional Pt/FTO CE, due to (1) the good intrinsic electro-catalytic ability of s-PT toward I-3(-) reduction, (2) the good conductivity of s-PT in the absence of any non-conducting component (e.g., the PCL in PPy:PCL or the PSS in PEDOT: PSS), and (3) the large surface area and orientated electron transfer pathways provided by the CC substrate. It was concluded that the low-cost, highly efficient, and flexible s-PT/CC electrode was an attractive substitution for the expensive Pt/FTO electrode. Moreover, the extra low-weight s-PT/CC electrode was very suitable for the large-scale roll-to-roll process, which benefits the future industrialization of the DSSCs.