Charge transport in isoindigo-dithiophenepyrrole based D-A type oligomers: A DFT/TD-DFT study for the fabrication of fullerene-free organic solar cells.

In this paper, we have designed a series of isoindigo-dithiophenepyrrole based oligomers with donor-acceptor architecture. The donor and acceptor units are joined by a thiophene linkage. We have substituted the 5,5'-positions of the isoindigo acceptor unit with different +I groups, viz., -CH3, -NH2, -SH, -OH, -OCH3, and -CH=CH2, and -I groups, viz., -F, -NO2, -CN, -COCH3, -COOH, and -CF3. We have studied the structural, charge injection, and transport properties employing the density functional theory (DFT) formalism. Our study explores that the presence of bulky substituents adversely affects these properties. Values of frontier orbital energies, ionization potentials, and electron affinities are calculated for each compound to predict the ease of charge injection from metal electrodes to these compounds. Most of our compounds show the ease of hole injection ability and show a large electron injection barrier. Computation of reorganization energies followed by the charge transfer integral and charge transfer rate have also been performed. Our findings reveal that compounds substituted with +I groups possess larger hole mobilities than the compounds with -I groups. Substitution of a dimer of compound 9 with -NO2 leads to the highest hole and electron mobility. Dipole moment values have also been calculated to study the charge transport properties. We have also computed the absorption properties of the compounds using the time-dependent DFT method. Our study indicates that absorption properties are changed by the attachment of substituents and can be tuned according to the requirements. Among the studied compounds, the -OCH3 substituted dimer (dimer 6) exhibits the largest bathochromic shift with a λ max of 554 nm. From this study, we can infer that our designed compounds are promising candidates for fabrication of optoelectronic devices.