The Role of the Conjugate Bridge in Electronic Structures and Related Properties of Tetrahydroquinoline for Dye Sensitized Solar Cells

To understand the role of the conjugate bridge in modifying the properties of organic dye sensitizers in solar cells, the computations of the geometries and electronic structures for 10 kinds of tetrahydroquinoline dyes were performed using density functional theory (DFT), and the electronic absorption and fluorescence properties were investigated via time dependent DFT. The population analysis, molecular orbital energies, radiative lifetimes, exciton binding energies (EBE), and light harvesting efficiencies (LHE), as well as the free energy changes of electron injection (Delta G(inject)) and dye regeneration (Delta G(dye)(regen)) were also addressed. The correlation of charge populations and experimental open-circuit voltage (V-oc) indicates that more charges populated in acceptor groups correspond to larger V-oc. The elongating of conjugate bridge by thiophene units generates the larger oscillator strength, higher LHE, larger absolute value of Delta G(inject), and longer relative radiative lifetime, but it induces the decreasing of EBE and Delta G(dye)(regen). So the extending of conjugate bridge with thiopene units in organic dye is an effective way to increase the harvest of solar light, and it is also favorable for electron injection due to their larger Delta G(inject). While the inversely correlated relationship between EBE and LHE implies that the dyes with lower EBE produce more efficient light harvesting.