Optimizing the Cosensitization Effect of SQ02 Dye on BP-2 Dye-Sensitized Solar Cells: A Computational Quantum Chemical Study

Cosensitization of the semiconducting electrode in dye-sensitized solar cells (DSCs), with two or more light-harvesting dyes, is a chemical fabrication method that aims to achieve a panchromatic absorption spectrum emulating that of the solar emission spectrum. In this paper, SQ02 and BP-2 cosensitizers have been investigated, as isolated monomers/dimer and adsorbed monomers/dimer on the TiO2 (101) anatase surface, by employing density functional theory (DFT) and time-dependent DFT calculations. Computed results showed that the dominant electron injection pathway is direct injection from each dye into the conduction band of TiO2. The almost complete spectral overlap between the simulated absorption spectrum of BP-2 and fluorescence emissions of SQ02 implies that excitation energy transfer occurs between cosensitizers via the trivial reabsorption mechanism. However, the results showed very limited unidirectional intermolecular charge transfer (CT) from SQ02 dye to BP-2 dye (0.04 |e-|). Therefore, this study also presents a stepwise molecular engineering of BP-2 dye, aiming at optimizing the cosensitization functionality. First, 14 redesigned dye candidates are reported to identify dyes with photophysical properties matching the requirements for efficient DSCs. Second, the four most promising dyes are shortlisted for testing as cosensitizers with the SQ02 dye. The molecular design factors of cosensitization that need validation are chemical compatibility, availability of CT between cosensitizers, and complementarity of the absorption spectra. This screening suggests the judicious choice of the modeled difluorenyl amine donor-based dye (BP-D4) as a very promising cosensitizer. In particular, the SQ02/BP-D4 dimer showed 10 times larger (0.53 |e-|) unidirectional CT than that of SQ02/BP-2 dimer, in addition to the maximum increased electron population of acceptor moieties upon photoexcitation.