## 1 Introduction  
The solar energy resource is considered as one of the most useful renewable energy resources to replace fossil fuels and to reimburse a high global energy demand [1; 2; 3; 4; 5; 6; 7]. For more than two decades, extensive research efforts have been focused on bulk-heterojunction organic solar cells (BH)-OSCs), owing to their distinct features such as ease of processability, small production cost, light weight, mechanical flexibility, and solution-processable manufacturing capability [8; 9; 10; 11; 12]. Progressive research has shown the remarkably high power conversion efficiency (PCE) of >18% for BH)-OSCs [13; 14; 15; 16]. BH)-OSCs fabricated with small organic molecules (SOMs) are more attractive due to their well-defined molecular weight, less batch-to-batch variation, higher purity, easy purification, and reproducible synthesis over its polymer counterparts [17; 18]. Organic semiconductors with high exciton binding energy, short life times in the range of \(\sim\)10-100 ps, and exciton diffusion length of 5-10 nm range are typically used as the photoactive layer, forming a bulk heterojunction of donor and acceptor materials by ensuring the efficient exciton to dissociate at the D/A interface due to the difference between their frontier molecular orbitals [19; 20]. The electron mobility in OSCs largely depends on the film morphology and molecular architectures. Fullerene derivatives such as [6; 6]-phenyl-C\({}_{61}\) butyric acid methyl ester (PCBM) are n-type semiconductors with rationally high electron concentrations of 10\({}^{17}\)-10\({}^{18}\) cm\({}^{-3}\) and are frequently applied to fabricate BHJ-OSCs by virtue of their excellent electron accepting and transporting capabilities [21; 22]. Moreover, PCBMs have long-life time polarons, originating from the coherent electron transfer between fullerene molecules and hence, reduce the energy barriers for electron transfer [23]. In organic photovoltaics (OPVs), the most general method for enhancing the photocurrent density (Jsc) and the power conversion efficiency of an organic solar cell is to design and synthesize low band gap materials which match well with the solar spectrum without sacrificing open circuit voltage (Voc). It is suggested that halogenation of an electron accepting unit can enhance the intramolecular charge transfer (ICT) effects and, hence, can exhibit an appreciable reduction in the band gap which is one of the most effective molecular designing strategies for investigating new and effective small organic and polymer semiconductor materials [24; 25; 26; 27]. In particular, the fluorine atom, due to its small size, high electronegativity with comparatively small Van der Walls radius is proficient at altering the HOMO and LUMO energy levels, extending the absorption, and increasing the electron mobility without undesirable steric hindrances [28]. Moreover, fluorine substitution also induces a rigid and planar backbone in the molecules due to intramolecular hydrogen bonds [29]. To achieve high performance BHJ-OSCs, the active organic layer must have materials with a broad absorption spectrum, high molar absorption coefficient, an appropriate alignment of energy levels between D/A layers, large charge mobility and thin film network with nanoscale phase separation [30]. The suitable phase separation difference between D/A interface surfaces through an internal driving force can effectively decrease Voc losses by suppressing recombination [31; 32]. Thiophene-based molecular semiconductors are commonly used materials for organic photovoltaics owing to their remarkable thermal, optical and electronic properties. The thiophene ring provides a prominent contribution to its derivatives towards enhanced charge transport and electron donating properties [33]. Recently, [PERSON] et al. reported a D-A shaped chromophore based on hexyl bithiophene donor and isatin unit as an electron acceptor in the BHJ-OSC device and obtained a PCE of \(\sim\)4.10% [34]. In another report, [PERSON] et al.