Dyes for use in dye sensitized solar cells (DSSCs)
Over the past 10 years, many metal-free organic dye sensitizers have been developed for application in DSSCs, with the typically structure based on a push-pull system (i.e. donor-spacer-acceptor system). Specifically, double bond based spacers have been frequently employed to give red-shifted absorption spectra aimed at high solar energy capture. However, widely employed C=C double bonds as spacers can limit DSSC efficiency, resulting in adverse effects such as self-quenching of generated electrons arising from cis-trans isomerization or dye aggregation. While the photovoltaic performance of a furan-based sensitizer has often been investigated with regard to recombination kinetics, fewer studies involving other heterocyclic ring systems as a spacer are reported. Hence, our contributions in this arena have involved 1) the synthesis of novel metal-free sensitizers by systematic variation of the spacer unit (cf. Fig. 5) and 2) determination of intrinsic factors affecting photovoltaic performance through use of the resultant dyes in DSSCs.
Based on current-potential curves, current density increased in the order DP-TZ < DP-P < DP-F < DP-B < DP-T The highest plateau of quantum efficiency from IPCE spectra was obtained from DP-B, indicating a relationship between decreased (or increased) aggregation behavior of dyes on a TiO2 surface and higher (or lower) electron transfer yield. The dependence of open-circuit voltage on electron lifetime was evident, resulting in enhanced Voc in the order of devices based on DP-TZ< DP-F < DP-T < DP-B < DP-P. The lower polarizabilities of the molecules contributed to increased lifetimes but this benefit was weakened when efficient surface blocking was reduced, resulting in an unexpected drop in open-circuit voltage for DP-F. The DSSC device based N-methyl pyrrole (DP-P) showed good kinetic properties, resulting in high Voc and making the pyrrole unit a logical spacer for further molecular design studies towards high efficiency DSSCs. Given the similar maximum quantum efficiency (from IPCE data) of the device based on DP-P compared to DP-T, an enhanced absorption spectral shift would give high current densities needed for large solar energy capture. It should be added that molecular engineering of sensitizers having an N-methyl pyrrole unit should be careful to reduce the dihedral angle between spacers and neighboring group. Such experiments should be augmented by molecular modeling studies designed to examine the behavior of dye sensitizers on TiO2 in the presence of additives.
Fig. 5. Target molecular structures based on different spacers in a D-π-A system.