Tuning the HOMO Energy of the Triarylamine Molecules with Orthogonal HOMO and LUMO Using Functional Groups

Tuning the HOMO and LUMO energy strategically of triphenylamine derivatives plays an important role towards understanding and design of high performance organic photovoltaics. To improve on the charge separation and electron transfer properties, twenty one D-pi-A organic dyes with nitrostyryl triphenylamine-based structures (NTPAs) were designed and computationally studied, in which triphenylamine, carbazole, or indoline structures were used as donor and NO2 group as acceptor. The LUMOs of these dyes are more localized at the nitrostyryl moiety with almost the same energy and the HOMOs, which are orthogonal to the LUMOs, can be conveniently tuned by changing the donor structure and using different functional groups substituted on the donor. Compared with triphenylamine molecules, carbazole molecules decrease the HOMO energy of NTPA, whereas cis N-phenylindoline structure increase the HOMO energy. In addition, the exciton sizes generated by these dyes are also tuned in a range of 7.0-12.7 A. These results show that introduction of dimethylamino group on the triphenylamine moiety greatly increases the HOMO energy and prolongs the maximum absorption wavelength as well as increases the exciton size, indicating that NTPA dyes containing strong electron donating dimethylamino group on the donor part are promising candidates as sensitizers in the dye-sensitized solar cell applications. The results of the structural and spectral properties show good correlation with the experimental results, which confirms that the DFT method employed in this work is an effective way to design organic small molecule based sensitizers.