Triarylene linked spacer effect for dye-sensitized solar cells

The effect of switching the phenylene and thiophenylene units in the triarylene bridge of organic donor-bridge-acceptor dyads on the performance of dye-sensitized solar cells is investigated. A thiophenylene group displays several distinctive advantages over those of a phenylene group. The electron-donating nature of thiophenlene elevates the electron energy level of the dyads and narrows down the energy gap of the electronic transition, therefore elongates the absorption wavelength. The presence of thiophenylene unit along the bridge also increases the planarity of the molecular geometry, therefore enhances the degree of π-delocalization; however, it also speeds up the rate of charge recombination. The multiple effects of thiophene group along the bridge are examined systematically on two types of dye derivatives, i.e., the T-series and the M-series dyes. Among all the dyes, the ones containing a phenylene-thiophenylene-thiophenylene bridge (T-PSS) showed the highest performance. A typical device made with T-PSS displayed the maximal monochromatic incident photon-to-current conversion efficiency of 65% in the wavelength region between 350 nm and 515 nm, a short-circuit photocurrent density 15.88 mA cm^{- 2}, an open-circuit photovoltage 0.64 V, and a fill factor 0.60, that corresponds to an overall conversion efficiency of 6.13%. The packing order of T-PSS can be further improved by adding deoxycholic acid to an overall conversion efficiency of 6.71%.