Theoretical study for tuning the HOMO level of the donor to increase the efficiency through open-circuit voltage of small molecule solar cells

Density Functional Theory (DFT), the method has been used for the approach of pre-synthesis of virtual screening of newly designed small molecules. The designed four small molecules have been done using the concept of ‘Medium donor-vinylene-strong acceptor’. Malononitrile is flanked on the donor moiety of benzo[1,2-b:6,5-b']dithiophene-4,5-dione(BDTD) connected with vinyl linked benzothiadiazole (BTD) moiety studied with strong electron withdrawing groups (EWG) such as F, CN, and NO 2 onto the benzothiadiazole unit. DFT and TD-DFT method provided results of studied small molecules with 1.79eV–2.06eV band-gap with simulated high energy open-circuit voltage of 1.38V–1.49V for designed small molecules. The calculation also indicates D1, which exhibits the highest power conversion efficiency (∼9%) among the four systems predicted by Schraber diagram. This manuscript describes the detailed investigation of this study.