Photoexcitation Processes in Oligomethine Cyanine Dyes for Dye-Sensitized Solar Cells—Synthesis and Computational Study

Abstract

We report density functional theory (DFT) calculations of three newly synthesized oligomethine cyanine-based dyes as potential TiO2-sensitizers in dye-sensitized solar cells. The three dyes have π-symmetry and the same acceptor side, terminating in the carboxylic anchor, but they differ through the π-bridge and the donor groups. We perform DFT and time-dependent DFT studies and present the electronic structure and optical properties of the dyes alone as well as adsorbed to the TiO2 nanocluster, to provide some predictions on the photovoltaic performance of the system. We analyze theoretically the factors that can influence the short circuit current and the open circuit voltage of the dye-sensitized solar cells. We examine the matching of the absorption spectra of the dye and dye-nanocluster system with the solar irradiation spectrum. We display the energy level diagrams and discuss the alignment between the excited state of the dyes and the conduction band edge of the oxide as well as between the redox level of the electrolyte and the ground state of the dyes. We determine the electron density of the key molecular orbitals and analyze comparatively the electron transfer from the dye to the semiconducting substrate. To put our findings in the right perspective we compare the results of our calculations with those obtained for a coumarin-based dye used in fabricating and testing actual devices, for which experimental data regarding the photovoltaic performance are available.

Unspecified verticality of Franck–Condon transitions, absorption and emission spectra of cyanine dyes, and a classically inspired approximation

“…[17][18][19] Both THIA and INDO dyes are subjects of study for use in DSSCs. 20,21 We will refer to these dyes and their analogues with H or methyl substituted for ethyl on the nitrogen atoms according to abbreviations given in Fig. 2.…”

Reference Excitation Energies of Increasingly Large Molecules: A QMC Study of Cyanine Dyes

“…Cyanine dyes are a family of charged π-conjugated molecules which are employed in very diverse applications ranging from dye-synthesized solar cells to the labeling of biomolecules. − Their characteristic structure consists of a chain of an odd number of carbons with two amine groups at the ends. While their photophysical properties are strongly regulated by the length of the carbon chain, the lowest bright state of the cyanines always maintains a π → π* character and can be predominantly described as a HOMO to LUMO (HL) transition.…”

Guest-Host Interactions in Symmetrical Carboxy Heptamethine Cyanine Dyes-Titanium Dioxide Systems: Synthesis, Theoretical Calculations, Aggregation Properties, and Application in Dye-Sensitized Solar Cells

“…Another way to obtain the energy levels is by computing the excited state (corresponding to the LUMO energy) as a sum between the ground state energy (HOMO energy) and the optical gap (Δ opt ) according to the following expression [41,42]:…”