Tuning Optoelectronic Properties of Dithienopyrrole Donor Molecules for Organic Solar Cells

Theoretical analysis of physical properties of organic solar cells (OSCs) are important in order to reveal the correlation between power conversion efficiencies (PCE), structure and properties. Five new A-D-A type small molecules M-1, M-2, M-3, M-4, and M-5 were designed by using dithienopyrrole (DTP) as electron rich donor unit with different types of pi-spacers and end capped acceptor units. Functional MPW1PW91/6-31G(d,p) level of theory was used to optimize the geometry of all molecules. For excited state calculation TD-MPW1PW91/6-31G(d,p) level of theory was used. The geometries, electronic structures, dipole moment, open circuit voltage, reorganization energies and charge transport properties of designed molecules M-5) have been scrutinized comparing with the reported compound R. The results revealed that the HOMO energy levels of molecules M-1, M-2, M-3, and M-5 were lower while M-4 was of high energy level thus facilitate the donation of electron as compared to references molecule R. While LUMO energy level of all the molecules were slightly high energy due electron withdrawing effects of spacer and acceptor moiety. Highest energy gap of HOMO-LUMO was observed in M-1 which was 2.48 eV and M-3 showed low energy gap (2.11 eV) as compared to other designed molecules. All molecules showed low values for lambda(e), so they have high rate of electron transfer as compared to R. All designed molecules exhibited higher value of dipole moment as compared to reference molecule R except M-1. Higher value of dipole moment of donor molecules contrary to reference means good solubility towards organic solvents which is beneficial for further solar cell device fabrication. All designed molecules show higher V-oc values except M-4 which has comparable V-oc with respect to reference molecule R. In short, choice of appropriate electron withdrawing and donating groups is very important for improving power conversion efficiencies of OSCs.