The number density effect of N-substituted dyes on the TiO2 surface in dye sensitized solar cells: a theoretical study

A series of organic dyes, containing N-substituted carbazole, diarylamine-naphthalene, and diarylamine-fluorene donor for Dye1, Dye2, and Dye3, respectively, and cyanoacrylic acid acceptor bridged by a thiophene fragment for Dye Sensitized Solar Cell (DSCs) applications, is successfully investigated by DFT and TDDFT. Among these dyes, Dye3, with the strongest electron donating ability, shows the most red shift in the UV-Vis absorption spectrum. Moreover, the Dye3@(TiO2)(38) complex shows stronger adsorption energy of -19.54 kcal mol(-1). Nevertheless, Dye2 has shown the best photovoltaic performance. We measured the molecular volume and molecular width based on the geometries from the PBE functional, together with the double-numerical with polarization performed in the DMol(3) program, to investigate the effect of N-substituted donors on the number density of the adsorbed dye, on the TiO2 surface. We found that the bulkiness of the N-substituted donor in Dye3 can dramatically reduce the number density of the adsorbed on the surface. The molecular width and projected area of Dye3 are calculated to be 15.980 angstrom and 214 angstrom(2), respectively, indicating the more bulky structure compared to Dye2 (molecular width = 14.505 angstrom and projected area = 180 angstrom(2)) which corresponds to the dye uptake of Dye3 (1.38 x 10(17) molecule per cm(2)) being significantly less than of Dye2 (4.55 x 10(17) molecule per cm(2)). Finally, Dye3 with an extra bulky donor exhibits poorer energy conversion efficiency of 3.91% compared to 5.45% of Dye2, under simulated AM 1.5 irradiation (100 mW cm(-2)).