Bulk-heterojunction (BHJ) photovoltaic cells have plenty of interest in both industry and academia due to their unique perks, e.g., mechanical flexibility, easy preparation, multitudinous raw materials, large fabrication area, tunable absorption range, lightweight, as well as low cost [[1], [2], [3], [4], [5], [6], [7]]. These devices can be used for multiple purposes [8,9]. To supply renewable green energy, organic photovoltaic cells are one of the most encouraging technologies [10,11]. Organic photovoltaic devices show a meaningful concern in improving the power conversion efficiency (PCE) because they have tunable optoelectronic properties [[12], [13], [14], [15]]. Regarding the better working efficiency of organic solar cells (OSCs), the electron-donor fragment and the electron-acceptor entity significantly contribute. Among BHJ-type OSCs, the bound electron and hole pairs are produced by incident light absorption. Then, at the neighboring acceptor-donor interface, these electron and hole pairs detach into electrons and holes and move toward their particular electrodes [[16], [17], [18]].

Fullerene electron-acceptors (FEAs) and their derivatives, including PC61BM and PC71BM, are prominent acceptors with many advantages, e.g., isotropic charge transportation, high electron mobility, and high electron affinity [19,20]. Despite these advantages, fullerene-based electron acceptors have some drawbacks, e.g., limited energy level variability, morphological instability, and lower absorption in the visible region [[21], [22], [23], [24]]. Because fullerene free electron-acceptors exhibit robust and wide visible area absorption ranges combined with acceptable amounts of energy [25], non-fullerene electron acceptors (NFEAs) can be possible compared to the extensively utilized FEAs [26,27]. In history, the working efficiency of the fullerene-free electron-acceptor-based OSCs has lagged behind the performance of FEAs-based devices [28]. In contradiction to the FEAs-based counterparts, NFEAs-based organic solar cells can easily tune the electronic-energy levels and the optical properties, better photochemical and thermal stability, extended lifespan of the device, and greater PCE [[29], [30], [31], [32], [33]].

In the last few years, a series of indacenodithiophene (IDT) and indacenodithiophene[3,2-b] thiophene (IDTT) based electron acceptors were designed with different electron-withdrawing moieties [[34], [35], [36], [37]]. The prevention of rotational disorder and conformational change, reduction of the reorganization energy, and enhancement of charge carrier mobility can be possible using the rigid extended fused ring structure of the IDT and IDTT [38,39]. By introducing the various electron-capturing acceptor moieties, the absorption spectra and energy levels of molecules can easily be tuned [40,41].

Recently, a non-fullerene small acceptor molecule, BTA2 has been synthesized by researchers. For the first time, introducing thiazolidine-2,4-dione (TD) electron-deficient moiety as end-capped A2 (acceptor) develops brand-new tiny non-fullerene acceptor of the A2-A1-D-A1-A2 type small non-fullerene acceptor was constructed and modified. The BTA2 reveals a −3.38 eV value for the lowest unoccupied molecular orbital (LUMO) and a −5.43 eV value for the highest occupied molecular orbital (HOMO) with a 2.00 eV optical band-gap. The P3HT: BTA2-based non-fullerene OSCs showed higher open circuit voltage (VOC) (1.22 eV) with 4.50 % PCE [42].

BTA2 has shown remarkable VOC and band gap values but relatively low PCE. However, the potential of the core of this molecule is not yet explored. So, the PCE can be improved by tuning this molecule's energy levels and optoelectronic properties using an end-capped modification approach. Moreover, enhancing OSC devices' fill factor (FF) is also very important. Till now, FF values of 81 % have been marked [43]. Therefore, we aim to boost the optoelectronic features of this molecule using different types of end-group acceptors at the terminals of this core.

In recent eras, computational studies have paved a path for improving different parameters necessary to update the efficiency of photovoltaic cells. For example, Akram et al. suggested four molecules in a study that can improve the Voc [44]. Similarly, in another attempt, a molecule named T5 was explored which showed great potential for improved Voc value [45]. Moreover, using DFT and TD-DFT approaches, some molecules have also been reported that can be used to develop devices with low band gaps [46].

In the existing study, we have formulated A2-A1-D-A1-A2 type eight new acceptor molecules named BT1, BT2, BT3, BT4, BT5, BT6, BT7, and BT8 by introducing eight different acceptor moieties at the terminus of model molecule BTR (Fig. 1). The BTR and the modified structures contain a common indacenodithiophene (IDT) donor core and bridge acceptor 2-methyl-2H-benzo[d] [[1], [2], [3]]triazole (A1) and different terminal acceptor moieties. Herein we replaced 3-ethyl-5-methylenethiazolidine-2,4-dione end capped acceptor (A2) of BTR with 2-(5,6-difluoro-2-methylene-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile in BT1, 1-(dicyanomethylene)-2-methylene-3-oxo-2,3-dihydro-1H-indene-5,6-dicarbonitrile in BT2, methyl 1-(dicyanomethylene)-6-fluoro-2-methylene-3-oxo-2,3-dihydro-1H-indene-5-carboxylate in BT3, dimethyl 1-(dicyanomethylene)-2-methylene-3-oxo-2,3-dihydro-1H-indene-5,6-dicarboxylate in BT4, 2-(2-methylene-3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene)malononitrile in BT5, 1-chloro-6-(dinitromethylene)-5-methylene-1,5,6,6a-tetrahydro-4H-cyclopenta[c]thiophen-4-one compound with methane (1:1) in BT6, 2-(5,6-difluoro-8H-indeno[2,1-b]thiophen-8-ylidene)malononitrile in BT7 and 2-(5,6-difluoro-8H-indeno[1,2-d]thiazol-8-ylidene)malononitrile in BT8. The new molecules are designed to boost the photovoltaic properties of the OSCs. These acceptors have already been synthesized using safe techniques and are being used. So, they are not toxic in nature. The opto-electronic properties were calculated via the DFT and TD-SCF approach and contrasted with BTR.