Solar cells based on p-conjugated donor-acceptor (D-A) organic molecular systems are a promising alternative to conventional electrical energy generation. D-A molecular systems, which have a triphenylamine (TPA) moiety linked with a benzothiadiazole (BTD) moiety, open the potential development of new small molecule donors for bulk heterojunction (BHJ) solar cells. Here, a series of donor-acceptor-π-acceptor (D-A-π-A) small molecule donors (SMD) derived from triphenylamine (TPA) donor and benzothiadiazole (BTD) acceptor building blocks, were designed for BHJ organic solar cells. The small molecule donors SMD1-4 were studied using density functional theory (DFT) and time dependent-DFT (TDDFT) methods, to understand the effect of cyano and fluorine group functionalization on their properties. The effect of structure alteration by cyano and fluorine group functionalization on the optoelectronic properties, the calculated highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) and the HOMO-LUMO gaps were theoretically explored. The Voc (open-circuit photovoltage) and fill factor (FF) for SMD1-4 were obtained with a PC71BM acceptor, which showed that these organic small molecules are potential small molecule donors for organic bulk heterojunction solar cells.

1. Introduction
2. Materials and Methods:Computational Investigation
3. Results and Discussion
    3.1. Frontier molecular orbitals and electronic properties
    3.2. Photovoltaic properties
    3.3. Absorption properties
4. Conclusion

• Prabhat Gautam(Professor (Assistant), Department of Chemistry, CMR Institute of Technology, Bengaluru 560037, India) Corresponding author
• Anurag Gautam(Professor (Associate), School of Sciences, Malla Reddy University, Hyderabad 500100, India) Corresponding author
• Neeraj Kumar(Professor (Sr. Assistant), Department of Metallurgical Engineering, School of Engineering, O.P. Jindal University, Raigarh 496109, India) Corresponding author