How to design more efficient organic dyes for dye-sensitized solar cells? Adding more sp²-hybridized nitrogen in the triphenylamine donor

One of the most significant aspects in the development of dye-sensitized solar cells (DSCs) is the exploration and design of high-efficiency and low-cost dyes. In the present paper, we have reported a theoretical design of potential high-efficiency organic dyes with modified triphenylamine donors, using time-dependent density functional theory with the CAM-B3LYP method. The CAM-B3LYP method is first validated to have very good performance in the descriptions of spectral properties of C214 and C216 dyes. With C214 as a prototype, molecular modifications are then made and a scheme, using NH groups to connect neighboring phenyls in the triphenylamine donor, has been demonstrated to be successful to significantly red-shift the absorption maximum wavelength, extend the lifetime of the first excited state, and decrease the energy gap between HOMO and LUMO. In particular, the change amounts of these properties are illustrated to be dependent on the number of the added nitrogens, a significant finding that may perhaps make it possible to quantificationally regulate properties of organic sensitizers to match diverse requirements in the building of a high-efficiency DSC. The complementary nitrogens have been characterized to be sp2-hybridized and shown to play an important role in assisting in charge transfer.