Efficient donor-acceptor (D-A) molecular scaffolds should be developed for the advancement of organic solar cells (OSCs). Density functional theory (DFT) and time-dependent density functional theory (TDDFT) studies provide an effective methodology to perform initial studies to design and investigate D-A molecular systems. Two fluorine-substituted bis-benzothiadiazoles (FBBTs) are designed and optimized using the DFT method. The results show better planarity for FBBT2, which is attributed to π-extension between the FBBT units. A series of D-A small molecules CB1-4 are designed utilizing FBBT2 to study the effect of systematically substituting carbazole donor and cyano-based acceptor groups on the optoelectronic properties of FBBT. DFT calculations are performed using the B3LYP functional. The designed D-A scaffolds exhibit systematic tuning of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), HOMO-LUMO gap (from 2.333 eV to 1.825 eV). The observed HOMO-LUMO gap follows the trend CB1 > CB2 > CB4 > CB3. The Voc (open-circuit voltage) and power conversion efficiency (PCE) for CB1-4 are presented with the PC71BM acceptor. The overall trend observed for the Voc follows the order CB1 < CB4 < CB2 < CB3. The PCE trend observed using the Scharber model follows the trend CB3 > CB4 > CB2 > CB1. The results show that end cap modeling of π-extended FBBT with cyano-based acceptor groups significantly improves the observed PCE and Voc.

Abstract
1. Introduction
2. Experimental Procedure
3. Results and Discussion
    3.1. Optimization of the geometry
    3.2. Frontier molecular orbitals and electronic properties
    3.3. Photovoltaic properties
    3.4. Optical Properties
4. Conclusion
References
Author Information

• Prabhat Gautam(Department of Chemistry, CMR Institute of Technology, Bengaluru, Karnataka 560037, India) Corresponding author
• Anurag Gautam(School of Sciences, Malla Reddy University, Dulapally, Hyderabad, Telangana 500100, India)
• Rekha Sharma(Prestige Institute of Engineering Management and Research Indore, Madhya Pradesh 452010, India)
• Sangram Keshari Das(School of Electrical & Electronics Engineering (SEEE), VIT Bhopal University, Bhopal, Madhya Pradesh-466114, India)
• Neeraj Kumar(Department of Metallurgical Engineering, Chemical Metallurgy Laboratory, School of Engineering, O.P. Jindal University, Raigarh 496109, India) Corresponding author