Design of new low band gap conjugated small molecules based on thiophene and diphenylamine units as organic dye sensitized solar cell materials

Three conjugated small molecules based on thiophene and diphenylamine units were designed and studied by quantum chemical method using the density functional theory (dft). to examine the ability of these studied compounds to be employed in organic dye sensitized solar cells (dssc) adsorbed on tio2,geometric and electronic properties were investigated at b3lyp/6-31g(d) level and absorption spectra were simulated by time dependent density functional theory (td-dft) at the same level. the designed molecules have two cyanoacrylic acid groups which are used as electron acceptor groups for the attachment of the dye on the tio2 surface. different π-conjugated groups (thiadiazolothienopyrazine (ttp),benzothiadiazole-pyrazine (btp) and benzopyrazine-pyrazine (bpp)) were introduced as π-spacers in one branch of the molecules to investigate their effects on the geometric,electronic and optical properties. the highest occupied molecular orbital (homo) and the lowest unoccupied molecular orbital (lumo) levels are calculated and compared to conducting band of tio2 (anatase). calculated band gap energies for the new dyes are comparable to the one of poly(3-hexylthiophene) which is considered as one of the best performing polymer solar cells to date. simulated absorption spectra showed that these dyes can harvest light in the visible spectrum. the examination of the frontier orbital density surfaces show that (btp and bpp) are the best π-conjugated groups promoting the intramolecular charge transfer to the one of the anchored group to the surface of semiconductor (tio2).