Consequently, the evolution of novel photovoltaic materials with a narrower optical band gap (E\({}_{\text{g}}\)), a deep highest occupied molecular orbital (HOMO) level and the lowest unoccupied molecular orbital (LUMO) level is critical for extra enhancement of the PCEs of the PSCs [14; 15; 16]. Attaining an optimal BHJ morphology that can balance exciton dissociation and charge transport is essential for attaining high-performance PSCs [17]. Dithienosilole (DTS) is a suitable donor unit for constructing low band gap conjugated polymers with an acceptor unit. DTS has a similar design as the CPDT donor unit except for having a bridging silicon atom instead of a carbon. In the DTS unit, the C-Si bond is longer than the carbon-carbon bond in the CPDT unit. Consequently, the steric impediment between the backbones of the conjugated polymer and solubilizing side chains is decreased. This results in greater \(\pi\)-\(\pi\) stacking between polymer backbones as well as increased crystallinity and mobility of the charge carriers of the conjugated polymers containing the DTS unit [18]. The first copolymer, PDTSBT based on DTS, and BT, as a donor and acceptor units were investigated by [PERSON] et al. In their system, hole mobility was observed to be three times greater than that in the PCPDTBT. Photovoltaic devices fabricated from PDTSBT: PC\({}_{71}\)BM gave a power conversion efficiency (PCE) of 5.1% [19]. [PERSON] and co-workers developed a related copolymer, PDTSBT12, which has \(n\)-dodecyl chains on the silicon atom. The copolymer was synthesized via Stille-coupling polymerization and yielded a high molecular weight (\(M_{\text{n}}=\) 34,000 g mol\({}^{-1}\)). The PSC devices based on PDTSBT12:PC\({}_{71}\)BM exhibited a PCE of 5.9% [20]. Furthermore, [PERSON] et al. reported the use of DTBT acceptor units in three copolymers: PDTS\({}_{\rm H}\)DTBT, PDTS\({}_{\rm EH}\)DTBT and PDTS\({}_{12}\)DTBT. The first copolymer showed reduced solubility in typical organic solvents and could not be processed in BHJ solar cells. PDTS\({}_{\rm EH}\)DTBT, blended with PC\({}_{71}\)BM in the BHJ photovoltaic cell, provided a PCE of 2.95%; however, PDTS\({}_{12}\)DTBT gave a PCE of 3.43%. Consequently, the short-circuit current (\(J_{\rm sc}\)) and fill factor (\(FF\)) were improved [21]. This was likely because the \(n\)-dodecyl side chains have less steric hindrance than do the 2-ethylhexyl chains. These copolymers performed more effectively than their PCPDTDBT analogues [22]. [PERSON] et al. designed and synthesized a new type of copolymer, PDTSTPD, which used TPD as the acceptor unit [23]. The PDTSTPD that fabricated with PC\({}_{71}\)BM exhibited a very impressive PCE of 7.3%. When a higher molecular weight polymer (\(M_{\rm n}=\) 31,000 g mol\({}^{-1}\)) was used, a higher PCE of 7.5% was reported [24]. For the high molecular weight polymers, achieving a high PCE could be due to higher hole mobility, which lowered series resistance and raised the \(FF\). In contrast with its PCPDTTPD analogue, PDTSTPD had a higher PCE [25; 26]. [PERSON] et al. synthesized a novel PDTSDTBTz copolymer with bithiazole (BTz) as an acceptor and two thiophene units between the BTz and DTS units. PDTSDTBTz displayed a PCE of 2.86%, which was blended with PC\({}_{71}\)BM [27]. They developed two new copolymers, PDTSDTTz-3 and PDTSDTTTz-4 with a more rigid and more coplanar geometry. Thiazolo [5,4-d]thiazole (TTz) was used as an acceptor unit. The locations of the solubilizing side chains on the thiophene units between DTS and TTz were altered. Increased backbone planarity and stronger intermolecular interactions facilitated the hole mobility of PDTSDTTz-3 more than that of its PDTSDTBTz analogue. A PCE of 5.59% was achieved from blending PDTSDTTz-3 with PC\({}_{71}\)BM, while the PCE for PDTSDTTTz-4-PC\({}_{71}\)BM improved to yield 5.88% with a considerable \(FF\) of 71.6%. This was due to the more planar structure of PDTSDTTTz-4, which had a higher hole mobility than its PDTSDTTTz-3 analogue [28].