Density Functional Theory (DFT) Investigation of Fullerene and Non-Fullerene donor part of Organic Photovoltaic cell

Abstract

The transformation of solar energy into electrical energy via photovoltaic cells is presently regarded as one of the most exhilarating research endeavors. Organic photovoltaic (OPV) cells are in high demand for photovoltaic applications because of their mechanical flexibility, affordability, ease of production, and lightweight nature. The research has primarily focused on D-A-D (Donor-Acceptor-Donor) photovoltaic cells, extensively investigating the nature of Donor 1, Acceptor, and Donor 2 units. In the context of this research, Density Functional Theory (DFT) in conjunction with Total Deposition Spectroscopy (TD-SCF) calculations utilizing the selected hybrid B3LYP-6311(G) functional is utilized to explore the electronic structure and properties of the reference material, with a focus on the examination of the properties resulting from the attachment of both fullerene and non-fullerene units at the first donor part. The research begins with the involvement of one fullerene unit in a reference molecule and exploring their molecular geometries, electronic band structures, and charge transport properties. Then two fullerene units were attached to compare their properties with the previous one. Similarly, the effect of non-fullerene aromatic units (Pentacene, 2H-benzo[cd]pyrene) was also studied by attachment of one and two units respectively. At the end, electron-donating alkyl groups (Methyl, Ethyl and Propyl) were attached to the acceptor part of the reference molecule in the presence of aromatic (fullerene, non-fullerene) units at 1st donor. A total number of 16 geometries were made and studied accordingly. The results showed that when only one fullerene units was involved in the absence of alkyl groups, it possesses lowest energy band gap (0.31eV) compared to others and surpassed in exhibiting the best photovoltaic properties because a smaller energy gap is required for the easy excitation of electron from HOMO to LUMO and allows for a larger built-in potential across the device and leading to higher voltages across photovoltaic cell. Results also indicate that attachment of two fullerene units in the absence of alkyl groups shows maximum λ value with (λmax = 10982 nm) and lowest value of excitation energy (0.113eV), while single fullerene unit has wavelength (λ = 9960 nm) and value of excitation energy xvi (0.125eV). By further proceeding, the molecule where ethyl group is attached as acceptor following the two fullerene units at 1st donor have maximum value of Voc (~1.787eV.) In conclusion single C60 molecule with the donor part when no alkyl group is attached is more favorable than other molecules having non-fullerene aromatic compounds due to having very less amount of energy band gap.