Tuning Optoelectronic Properties of Dithienopyrrole Donor Molecules for Organic Solar Cells
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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 M1, M2, M3, M4, and M5 were designed by using dithienopyrrole (DTP) as electron rich donor unit with different types of π-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 (M1\( - \)M5) have been scrutinized comparing with the reported compound R. The results revealed that the HOMO energy levels of molecules M1, M2, M3, and M5 were lower while M4 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 M1 which was 2.48 eV and M3 showed low energy gap (2.11 eV) as compared to other designed molecules. All molecules showed low values for λe, 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 M1. 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 Voc values except M4 which has comparable Voc 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.
Keywords:Dithienopyrrole open circuit voltages density of states density functional theory reorganization energy
The computations/simulations/SIMILAR were performed on resources provided by the Swedish National Infrastructure for ComputinG(SNIC) at Umeå University, 901 87, Umeå, Sweden. The authors acknowledge the financial and technical support from Punjab Bio-energy Institute (PBI), University of Agriculture Faisalabad (UAF).
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