Effect of polyphenyl-substituted ethylene end-capped groups in metal-free organic dyes on performance of dye-sensitized solar cells

A novel series of highly similar and comparable metal-free organic dyes have been designed and synthesized for dye-sensitized solar cells (DSSCs). The end-capped groups, namely, diphenylethylene, triphenylethylene, and tetraphenylethylene, have significant effects on the performance of DSSCs. The DSSC based on a triphenylethylene end-capped dye exhibits the highest power conversion efficiency (eta) of 6.29%, whereas that based on a tetraphenylethylene end-capped dye has the highest open-circuit photovoltage (V-oc) of 804 mV. The V-oc of the DSSCs can be greatly improved upon the incorporation of a twisted non-planar aggregation-induced emission (AIE) moiety in the molecular structure of the dyes. The triphenylethylene group with a moderately twisted non-planar structure is found to be suitable for the improvement of the power conversion efficiency. The dyes exhibit an aggregation-caused emission quenching effect, i.e., the dyes do not emit light after being adsorbed on TiO2 films, although they contain aggregation-induced emission triphenylethylene and tetraphenylethylene moieties.