Photophysical Properties of Phenyl- or Thiophene-Cored Branched Molecules with Thiophene and/or Thienylenevinylene Arms toward Broad Absorption Spectra for Solar Cells: A Theoretical Study

The aim of this work is to provide an in-depth study of the optical and electronic properties for branched molecules possessing thiophene- or phenyl-core with two-, three-, or four-branches, where the branch varies from thiophene to thienylenevinylene, for solar cells. Equilibrium ground state geometry configurations and their relevant electronic properties of investigated molecules for photovoltaic applications were optimized at the PBE0/6-31G(d) level. The absorption spectra were evaluated by the TD-PBE0/6-31+G(d,p) method. For the molecules possessing the thiophene core with thiophene and thienylenevinylene branches, the red shift is observed in absorption spectra, relative to ones with the phenyl-core and the thiophene branch. Moreover, the ortho- or para-substituted phenyl-cored (2,3- or 2,5-substituted thiophene-core) molecule contributes a red shift for the absorption spectra compared with the meta-substituted (2,4-substituted thiophene-core) one. The calculated reorganization energies of electrons and holes for the investigated molecules indicated them to be potential ambipolar charge transport materials under the proper operating conditions. We have also predicted the mobility of the recommended molecule possessing 2,3,5-subsititued thiophene-core and thiophene and thienylenevinylene side fragments with better performance in two different space groups.