The first BT-polymer,
P1, consisting of the BT units co-polymerized with thiophenepyrrole-thiophene, was reported in 2001 by Janssen and coworkers. Due to the mismatch of energy levels,
P1: PC61BM only showed a PCE of 0.34% [
165].
P2 was synthesized to match with P3HT, but the PCE based on such blend was only of
ca. 0.02%, due to the poor solubility of the BT based polymer [
166]. To improve the overall solubility, a fluorene unit with two soluble alkyl side-chains was co-polymerized with the BT in 2003 to afford polymer
P3. As a result, PCE was significantly increased to 2.41% [
167]. Besides, in 2007 [
168], Brabec and coworkers combined BT with the cyclopentylthiophene co-monomer to afford
P4, characterized by an improved solubility and decent hole transporting properties. The device made from a blend with PCBM were thus able to achieve a PCE of 3.5%. In 2015, Kim and coworkers synthesized
P5 (Mn = 40 kg/mol), to study the relationship between molecular weight and photovoltaic properties. After
P5 was combined with P(NDI2OD-T2), promising PCEs of 3.59% were achieved with such all polymer solar cells [
169]. Besides, Leclerc and coworkers reported
P6, a polymer based on a 2, 7-carbazole and BT, that was found to improve the solubility, the planarity and conjugation within the backbone. With better optoelectronic and probably charge transport properties, PCEs were thus further improved to
ca. 3.6% [
170]. Then a PCE of 3.71% were achieved by Bo and coworkers through the preparation and use of the BT polymer
P7 as donor in combination with the Ni-based molecule NI-AA-NI. Note that despite modest efficiencies, impressive high
VOC of
ca. 1.07 V were measured [
171]. In 2008, Yang and coworkers synthesized the BT-polymer
P8 by introducing the silicon-contained fused bithiophene as the electron-rich co-monomer, resulting in light absorption in the near infra-red region since characterized by an onset at
ca. 800 nm. Once blended with C70 as acceptor, a promising PCE of 5.1% was successfully achieved [
172]. Then, in 2009, Bo and coworkers reported the polymer
P9 in which the BT units was functionalized with two alkoxy side chains, resulting in improved solubility and probably ideal morphology once processed with PC71BM since the successful use of additives 1, 8-diiodooctane (DIO) led to a PCE of 5.4% [
173]. In 2017, Kim and coworkers had reported a series of BT based polymers (
P10, P11 and
P12) differentiated by the number of fluorine atoms on the BT unit [
174]. They found out that the latter polymers exhibited enhanced dipole moments difference (Δ
μ) between the ground and excited states upon successive addition of fluorine atoms. When combined with P(NDI2HD-T2 as the acceptor, the large Δ
μ of
P12 was found to facilitate exciton dissociation while suppressing charge recombination in the corresponding OSCs. Therefore, while device made with this polymer successfully achieved a promising PCE of 6.24%, devices prepared with its fluorene-free analogue (
P10) reached PCE only around 3% (max 2.65%). On the other hand, Liu and coworkers also introduced different numbers of fluorine atoms into BT based polymers to tune their crystallinity and self-assembly properties. Hence,
P13, functionalized with four fluorine atoms per monomer reached a PCE of 6.45% when blended with RR-PBN [
175]. Other example of successful use of fluorine functionalized BT monomer was reported by Zhou and coworkers in the preparation of
P14. Due to its strong electron absorption capability in the visible region, PCEs of 7.2% were thus achieved in fullerene based OSCs [
176]. The versatility of the BT unit, from an organic chemistry point of view, allowed numerous structural modifications and functionalization. For instance, Bo and coworkers explored the concept of asymmetric BT monomers either functionalized by an alkoxy (
P15) or thioalkoxy group (
P16). It turned oud that the nature of the heteroatom strongly affected the photovoltaic performance since PCE of 7.2% were achieved with the oxygen based polymer (
P15) while PCE of only 1.55% was reached for its sulfur analogue, namely
P16 [
177]. On the other hand, to improve the overall solubility, Chen and coworkers developed, in 2014, the
P17, by functionalizing a thiophene co-monomer on its position 3 with long branched alkyl side chains. Inverted solar cells were then prepared and PCEs of 7.64% were achieved for 230 nm thick active layers [
178]. Further improvement (PCE = 7.9%) were reported in 2013, through the preparation of
P18 in which an oxygen atom was introduced into the cyclopentadithiophene (CPDT) based co-monomer [
179]. A significant jump of efficiencies was then achieved by Guo and coworkers since PCEs of 9.76% were reached with devices made from
P19 and the PC71BM [
180]. This symbolic step was overcome in 2018 by Yan and coworkers with
P20 that was successfully combined with the NFA entitled O-IDTBR (PCE of 10.4%) [
181]. In 2016, Yan and coworkers reported a series of OSCs embedding BT-based polymers
P21 used that was combined with perylene bisimide (PBI) derivatives. Beyond the promising PCEs of 10.5%, nearly 90% of internal quantum efficiency was demonstrated despite a low voltage loss of 0.61 V [
182]. A slight improvement was reported by Zhan and coworkers (11.03%) with their polymer
P22, consisting in an alkoxy functionalized BT copolymerized with a benzodithiophene (BDT) decorated with lateral fluorobenzene units [
183]. On the other hand, Huang
et al. developed a design strategy based on random copolymer (polymer
P23) for large-area OSCs. The device based on
P23: IEICO-4F exhibited the promising PCE of 12.1% with active layer's thickness over 300 nm [
184]. The 13.07% of PCE were then reached by Zhu and coworkers with their simple polymer
P24, but this time blended with ZITI-N-EH [
185]. Recently, Ding
et al. prepared the BT-based donor polymer
P25 with high planarity, which was fond to be favorable for charge transport since high hole mobility of 1.59 × 10
−3 cm
2 V
−1 s
−1 were measured. When combined with a Y6-based acceptors, impressive PCE of 18.22% were achieved in single junction fullerene free OSCs [
18] (
Table 2).