Indian Institute of Technology (Indian School of Mines) Dhanbad, Jharkhand-826004, India.
Organic solar cells (OSCs) comprising conjugated polymers (CPs) has attracted enormous interests in recent years depending on its many advantages such as low cost, light weight and mechanical flexibility, easy fabrication over the conventional inorganic solar cells. However, to achieve high power conversion efficiency (PCE) low energy band gap with deeper HOMO/LUMO energy is required. Merging electron rich unit (Donor) and electron poor unit (Acceptor) in a conjugated polymer backbone is the simplest and effective strategy for tailoring the energy band gap. Another effective way to deal with the above issues is to vary the strength of the acceptor unit of the donor-acceptor (D-A) CPs by incorporating electron withdrawing groups (EWGs) in the acceptor unit. In our study, different D-A systems containing thiophene-based donor and isoindigo (IID) based acceptor units were investigated using density functional theory (DFT). The effect of EWGs (-F, -CN) substitution in the IID unit was also studied. It was found that the energy band gap followed the order of -CN substituted D-A copolymer < -F substituted D-A copolymer < bare D-A copolymer. The most stable HOMO/LUMO energy was observed for –CN substituted D-A copolymers. Moreover, the –CN substituted D-A copolymers exhibited the highest power conversion efficiency among all the designed D-A copolymers. Hence, substitution of EWGs is proved to be an effective strategy to tune the photovoltaic properties.
 Scharber, M. C., Muhlbacher, D., Koppe, M., Denk, P., Waldauf, C., Heeger, A. J. and Brabec, C. J. Adv. Mater. 2006, 18, 789-794.
 Havinga, E. E., ten Hoeve, W. and Wynberg, H. Synth. Met. 1993, 55, 299-306.
 Pandey, L., Risko, C., Norton, J. E. and Bredas, J. -L. Macromolecules 2012, 45, 6405-6414.
 Kim, B. -G., X, Ma., Chen, C., Le, Y., Coir, E. W., Hashemi, H., Aso, Y., Green, P. F., Kieffer, J. and Kim, J. Adv. Funct. Mater. 2013, 23, 439-445.
 Liu, X., Li, M., He, R. and Shen, W. Phys. Chem. Chem. Phys. 2014, 16, 311-323.