DFT Modeling of Novel Donor-Acceptor (D-A) Molecules Incorporating 3-hexylthiophene (3HT) for Bulk Heterojunction Solar Cells

Abstract

As potential donor materials for organic photovoltaic (OPV) devices, six conjugated small molecules with donor-acceptor configuration based on 3-hexylthiophene monomer (3HT) were designed and studied by means of density functional theory (DFT) and time-dependent DFT. Their geometric structure, electronic and optical properties were investigated at PBE0/6-31G(d) level. Amongst these times we selected the two most promising molecules (3HT)2-DPP)n=1 (1) and (3HT)2-DTP)n=1 (4), containing respectively the diketopyrrolo[3, 4-c]-pyrrole-1,4-dione (DPP) and 7,8-dihydroxy-thieno[3, 4-b]pyrazine (DTP) as acceptor units coupled with a core donor (3HT dimer (3HT)2), showing a novel strong absorption band in the visible range and spatially delocalized HOMO and LUMO states. These have been exploited as electron donors for a heterojunction with [6, 6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the electron acceptor. The charge transfer dynamics, including intermolecular charge transfer (inter-CT) and recombination (inter-CR) rates, were examined. The results reveal that the ratio kinter-CT/kinter-CR for the (3HT)2-DTP)n=1/PC61BM heterojunction is more than 109 times higher than that of the (3HT)2-DPP)n=1/PC61BM, suggesting that (3HT)2-DTP)n=1 is a promising candidate for OPVs.