Enhanced Efficiency of Organic Dye-Sensitized Solar Cells: Triphenylamine Derivatives

A general TDDFT procedure has been set up that accurately evaluates the UV/vis absorption spectra of a series of new conjugated metal-free organic dyes based on the triphenylamine (TPA) moiety, which have recently been developed for dye-sensitized solar cells (DSSCs). It turns out that the BHandH functional, combined with the 6-311+G(2d,2p) basis set, gives reliable auxochromic shifts when the bulk solvation effects are included in the model. Indeed, the theoretical procedure provides λmax with a mean absolute deviation limited to ∼0.1 eV only. In addition, we give insights into the geometrical and electronic structures of the dyes, and we unravel the structural modifications allowing to optimize the properties of TPA-based DSSCs. This investigation aims at improving the electron-injection process, as well as the light-harvesting efficiency (LHE) of the dyes. To this purpose, we considered a set of about 20 new dyes, and starting from the TPC-1 structure, the following modifications help to get better electron injection and light-harvesting properties: (i) the extension of the bridging group by addition of an ethylene subunit between the two phenyl groups (TPC-14); (ii) the 16-COOH, 15-OMe, 1a,6-diCN functionalization (TPC-18); (iii) moving the terminal cyano acceptor from the 16 to the 15 position, while introducing two −OMe functions in 11 and 13 positions and/or grafting two −CN groups in 1a and 6 positions on the TPA moiety (TPC-20). These specific modifications induce a maximal increase of the LHE and a more exoenergic free enthalpy of injection (−2.20 eV compared to −1.84 eV for TPC-1). Finally, TPC-23 (which results from the TPC-14/TPC-20 combination) shows an improvement of both the spectroscopic and energetic parameters. Moreover, the molecular topology analysis demonstrates that the coplanarity between the anchoring and the bridging unit is broken, that is, the positive charge is not directly in contact with the TiO2 surface, and the recombination reaction is therefore inhibited.