Comparative Study of the Structural, Electronic and Charge Transport Properties of Benzothiazole- and Indole-Based Squaraine Sensitizers

The donor organic sensitizers and acceptor inorganic part (e.g. TiO2 nanoparticles) gained significant attention in the field of photo-excitation, electrochemistry and hetero-junction solar cells. In the present study structural, electronic and charge transport parameters have been calculated with respected to benzothiazole- and indole-based squaraine dyes as donor while Si/TiO2 as acceptor then compared with previously studied hetero-junction solar cell materials. We have optimized the ground state geometries of benzothiazole- and indole-based squaraine dyes by density functional theory (DFT). The geometries of neutral, cation and anion species have been optimized by restricted and unrestricted B3LYP/6-31G** level of theories, respectively. The 2-3-(2-ethoxy-2- oxoethyl)benzo[d]thiazol-2(3H)-ylidene)methyl)-4-((3-(2-ethoxy-2-oxoethyl)benzo[d]-thiazol-3-ium-2-yl)methyl-ene)-3-oxocyclobut-1-enolate (BTSQD1) has been synthesized by our group. Moreover, we have designed derivatives of the benzothiazole-(BTSQD2 and BTSQD3 which have –COOH and –OCH3 at terminal –R positions, respectively) and indole-based squaraine dyes (ISQD1-ISQD3). The structures of the benzothiazole- and indole-based dyes investigated here are same except that in later ones the “sulfur” has been substituted by -C(CH3)2. We have compared the geometries, electronic properties, ionization potentials, electron affinities, reorganization energies, relationship between the energies of highest occupied molecular orbitals/lowest unoccupied molecular orbitals and open-circuit voltages (Voc), diagonal band gaps and short circuit current densities (Jsc), fill factors (FF) and factors affecting on the external quantum efficiency.