RT-TDDFT Simulations of NLO-Polymers with Tunable Energy Levels

Polymer-based solar cells have attracted much attention due to their potential as cost-effective light harvesting devices. A new series of NLO-polymers has been recently developed\footnote{F. Huang \textit{et al.}, J. Am. Chem. Soc. {\bf131}, 13886 (2009).} where the HOMO-LUMO gap can be tuned to maximize the light absorption overlap with the solar spectrum, thus improving their photovoltaic efficiency. With the aim of predicting such HOMO-LUMO gaps, we have developed simplified models of these polymers. These models were optimized at B3LYP/6-31G(d,p) level and their UV-Vis spectra was calculated using our RT-TDDFT approach.\footnote{Y. Takimoto \textit{et al.}, J. Chem. Phys. {\bf127}, 154114 (2007).} Our results show that the absolute B3LYP HOMO-LUMO gaps follow the trend seen in the experiment, but are about 0.6 eV higher than those experimentally determined. When this systematic error, which is likely due to self-energy and local-field effects, is removed by referring the theoretical and experimental values to a single compound, the agreement between theory and experiment is remarkable.