Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 281 (2022) 121610 Available online 11 July 2022 1386-1425/© 2022 Elsevier B.V. All rights reserved. An extension of electron acceptor sites around Thiazolothiazole unit for evaluation of large power conversion effciency: A theoretical insight Usman Ali a, b, c, * , Faheem Abbas c, d a Beijing National Laboratories for Molecular Sciences, Key Laboratories of Organic Solids, Institute of Chemistry Chinese Academy of Sciences, Beijing 100190, PR China b University of Chinese Academy of Science, Beijing 100049, PR China c Department of Chemistry, University of Agriculture, Faisalabad 38040, Pakistan d Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, PR China HIGHLIGHTS G R A P H I C A L ABSTRACT • Extended conjugated acceptor sites around thiazolothiazole unit. • Effect of basis sets size on optoelectronic properties. • Estimated short circuit current density (J sc ) based on PCE. • Large power conversion effciencies at constant.J sc . • Improved fll factor (FF) with large voltages (V OC ). • Fast electron transport rate for effcient non-fullerene acceptors. A R T I C L E INFO Keywords: Thiazolothiazole (TT) Large acceptor sites Charge transport properties Photophysical characteristics DFT TD-DFT ABSTRACT Small organic solar cells containing thiazolothiazole unit as an electron acceptor for solution processed bulk heterojunction (BHJ) small donor-acceptor-donor (D-A-D) type materials have been designed and studied theoretically with state-of-the-art density functional theory and time-dependent density functional theory (TD- DFT) for reliable estimation of their excited state and charge transfer photophysical characteristics for estimating their power conversion effciencies. The suggested possible synthetic routes with complete reaction information have been also provided for synthesis. The electron acceptor sites around the thiazolothiazole unit have been enlarged by introducing different strong electron withdrawing groups and checked their effects on the voltages (V OC ) and fll factor (FF) which are the two main parameters directly infuences on power conversion effciencies. Out of fve theoretically studied molecules, the experimental reported data of TT-TTPA (Thiazolothiazole- thiaophene triphenyl amine) has been compared with four designed molecules and concluded that extension of acceptor sites signifcantly contributed towards the better charge transport properties of electron and hole. * Corresponding author. Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.journals.elsevier.com/spectrochimica-acta-part-a- molecular-and-biomolecular-spectroscopy https://doi.org/10.1016/j.saa.2022.121610 Received 1 March 2022; Received in revised form 6 July 2022; Accepted 7 July 2022