Theoretical investigation on the photophysical properties of low-band-gap copolymers for photovoltaic devices

Geometrical parameters, electronic structures and photophysical properties of some new [2,1,3]-benzothiadiazole based alternating copolymers, have been investigated through Density Functional Theory (DFT) calculations.The theoretical results including structural characteristics and photovoltaic properties of these compounds are in good agreement with the available experimental data extracted from bibliography. The Time-Dependent Density Functional Theory (TDDFT) level is employed to investigate the excited singlet states in donor-acceptor and donor-π spacer-acceptor structures.From these results, the correlation structure-properties is better understood. Furthermore, the benzothiadiazole-based composite, which is blended with fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), seems to be a good active layer in bulk heterojunction solar cells.

We successfully used the Density Functional Theory DFT/B3LYP method to investigate the photophysical properties of some benzothiadiazole-derivatives with alternate D-A repeating units. The obtained theoretical results are in good agreement with the experimental available data showing an effective charge transfert between donor and acceptor units. A model energy band diagram was proposed for resulting optimized bulk heterojuncton (BHJ) structure simulating the energy behavior of this active layer where an efficiency conversion of about 5% can be reached in optimum conditions.Highlights► Density Functional Theory (DFT) study of new benzothiadiazole (BT)-based alternating copolymers. ► Conformational and structural study of the ground state. ► Molecular orbital, excited-state energies and vibrational analysis. ► Photophysical properties of the active layer in bulk heterojunction (BHJ) photovoltaic cells. ► The proposed band diagram, simulating the energy behavior of P3HT2BTCz:PCBM active layer.