Synergistic modification of end groups in Quinoxaline fused core-based acceptor molecule to enhance its photovoltaic characteristics for superior organic solar cells

Lately, organic solar cells (OSCs) have garnered a lot of interest due to low cost of fabrication, translucency, flexibility, and short energy payback time. Switching from fullerene and moving towards non-fullerene acceptors is considered as a promising method for bringing improvement in percentage conversion efficiency (PCE) of OSCs because such sort of replacement has overcome most of the drawbacks linked with fullerene-based acceptors such as difficult tunability and poor solubility [1,2]. NFAs have easily tunable energy levels and flexibility. Additionally, they exhibited wider absorption bands in spectral range between visible and near-infrared parts. The A-D-A type NFA-based molecules containing fused rings electron acceptors have contributed significantly to boost efficiency up to 20% for OSCs [3,4]. Currently, the efficiency of OSCs is not yet at a level where they can be widely used for commercial purposes worldwide. But the efficiency of OSCs can be improved significantly by improving the factors that remarkably effect the overall efficiency of OSCs such as open circuit voltage (Voc), bandgap, absorption range, reorganization energy, and fill factor [[5], [6], [7]].

In order for the OSCs to work even more efficiently, broader absorption spectrum, small reorganization energy values and higher Voc values play a decisive role [8]. So, to gain such sort of results, Yanan and coworkers synthesized a new molecule named Qx-2 in 2022 to boost the OSCs efficacy. Qx-2 was developed by replacing the benzothiadiazole core with easily synthesizable quinoxaline core by molecular modeling of Y-type non fused acceptor. Qx-2 is highly conjugated molecule which possesses A-D-A type confirmation, contains a fused dihydrodibenzo-thienothienopyrrolo-thienothienopyrrolophenazine (Quinoxaline) subunit in the center and 2 (5, 6 difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-ylidene) malonitrile components at peripheries. It possesses relatively low reorganization energies, which led it to have improved charge transport, diffusion length, and exciton lifetime. It addition, it exhibited a reduction in energy loss of 0.48 eV during the exciton dissociation, in comparison to the 0.56 eV of the conventional Y6 acceptor molecule. Further, its Voc, short circuit current (Jsc) and PCE with PM6 donor molecule have attained the values of 0.93V, 26.5 mAcm−2, and 18.02%, respectively, significantly outperforming the 0.848V, 25.6mAcm−2, and 16.6% of Y6 molecule [9]. The solubility of Qx-2 in chloroform is 13.9 mg/ml, which indicates that it has good solubility in common solvents, that is good for device fabrication. It also has such HOMO and LUMO energy values that they work to increase the electron-donating ability between them [10,11]. Similarly, Shen et al. worked on this molecule and demonstrated its PCE to be 10.56% with a Voc of 0.932V and a Jsc of 16.91mAcm−2, when fabricated on glass/ITO substrates through spin-coating method and o-xylene solvent [12].

One key advantage of quinoxaline is that it can be easily modified to get improved OSC molecules. Taking into account all its exceptional attributes and modifiability, the Qx-2 molecule has been used as the reference of our study. There are various chemical and computational techniques to bring about suitable modifications in the molecules to tune the energy levels and get the desired molecules [13,14]. End group modification technique is utilized in this work to further enhance the absorption spectrum, improve the Voc and to narrow the band gap of Qx-2 molecule. The end group acceptor modification approach has been utilized by many scientists to improve different parameters of organic solar cells [7,15,16]. OSCs with efficiency more than 17% were developed by Gouping and coworkers by fluorination at their end acceptors. They produced two novel molecules, BTFM and BTF. In BTFM, three fluorine atoms were substituted at the terminals of acceptors, and in BTF, four fluorine atoms were substituted. They observed that BTF showed better PCE than BTFM because of increased amount of fluorine atoms [17]. Results indicated that molecules with greater electron withdrawing groups at the peripheries exhibited greater PCE compared to others. Thus, it may be inferred that optoelectronic characteristics of OSCs can be improved through introduction of various suitable changes in acceptor subunits [18].

In this reported study, seven novel molecules were designed through replacement of terminal groups of Qx-2 with various acceptors. These new molecules were analyzed using different computational approaches and different parameters were studied. New molecules were named as Q1, Q2, Q3, Q4, Q5, Q6 and Q7. As the newly designed molecules differ based on acceptors, Q1 contains 1-Dicyanomethylene-2-methylene-3-oxo-indan-5, 6-dicarbonitrile, Q2 contains 6-Cyano-3-dicyanomethylene-2-methylene-1-oxo-indan-5-carboxylic acid methyl ester, whereas, Q3 has 2-(6,7-Difluoro-2-methylene-3-oxo-2,3-dihydro-cyclopenta[b]naphthalen-1-ylidene)-malonitrile, Q4 has 2-(5-Methylene-1-methylsulfanyl-6-oxo-5,6-dihydro-cyclopenta[c]thiophen-4-ylidene)-malonitrile, Q5 having 2-(2-Bromo-6-methylene-7-oxo-6,7-dihydro-1-thia-s-indacen-5-ylidene)-malonitrile, Q6 contains 2-(2-Methylene-3-oxo-2,3-dihydro-cyclopenta[b]naphthalen-1-ylidene)-malonitrile and Q7 has 1-Dicyanomethylene-2-methylene-3-oxo-indan-5,6- dicarboxylic acid dimethyl ester. Different important photovoltaic parameters such as optical band gap, light absorption and Voc of molecules can be heightened significantly by means of strong electron withdrawing groups at the peripheries of OSC molecules.

In comparison to our recent study on BDD core based molecules, these proposed molecules might perform better in experimental work. In this work, Q1 molecule shows the highest absorption at a peak of 779 nm with a bandgap of only 1.90 eV, while the W1 molecule in our previously published work had these values at 748 nm and 2.15eV, respectively. Similarly, upon comparing the highest open-circuit voltage and fill factor values between W7 (1.51eV and 0.9144) and Q4 (1.55 eV and 0.9162), it's clear that these acceptors are better than the previously reported BDD cored based ones [19].

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