Performance Comparisons of Polymer Semiconductors Synthesized by Direct (Hetero)Arylation Polymerization (DHAP) and Conventional Methods for Organic Thin Film Transistors and Organic Photovoltaics

C-C bond forming reactions are central to the construction of π-conjugated polymers. Classical C-C bond forming reactions such as the Stille and Suzuki coupling reactions have been widely used in the past for this purpose. More recently, direct (hetero)arylation polymerization (DHAP) has earned a place in the spotlight with an increasing number of π-conjugated polymers being produced using this atom-economic and more sustainable chemistry. As semiconductors in organic electronics, the device performances of the polymers made by DHAP are of great interest and importance. This review compares the device performances of some representative π-conjugated polymers made using the DHAP method with those made using the conventional C-C bond forming reactions when they are used as semiconductors in organic thin film transistors (OTFTs) and organic photovoltaics (OPVs).


Introduction
Organic electronics enabled by polymer semiconductors is an exciting field of research that promises to pave the way for low-cost, flexible electronic devices such as organic thin film transistors (OTFTs) [1,2] and organic photovoltaics (OPVs) [3,4]. The performances of these devices have improved greatly in the past few years and has now become commercially viable, i.e., the field effect mobilities and the power conversion efficiencies for the polymer based OTFTs and OPVs have exceeded 10 cm 2 V −1 s −1 [5][6][7][8] and 10% [9][10][11], respectively. While studies in this field have been more performance driven, the scale-up synthesis of polymer semiconductors has been attracting increased attention. In particular, novel synthetic methodologies that can produce high performance polymer materials in a more environmentally friendly way, i.e., using "green chemistry" [12][13][14][15], with increased atom economy and reduced production costs are highly desirable.
Central to the construction of polymer semiconductors is the C-C bond forming reaction that links the monomeric units. Common methods for the C-C bond formation, such as Stille and Suzuki coupling reactions require an aryl halide and an aromatic compound with a reactive directing group, e.g., a boronic acid (or ester) for the Suzuki coupling and an organostannyl group for the Stille coupling. These reactions, while highly effective in C-C bond formation, require additional steps to install the directing groups, which increases the production cost and generates stoichiometric amounts www.mdpi.com/journal/molecules of by-products that are potential health and environmental hazards. Specifically, the organotin compounds formed from the Stille coupling reactions are known to be highly toxic [16], while the boronic acid derivatives used in the Suzuki coupling reactions, which were previously assumed to be less harmful, have been recently found to be potential genotoxic hazards [17].
To address these issues associated with the common synthetic methods used to prepare polymer semiconductors, a novel C-C bond forming methodology, the so-called direct (hetero) arylation polymerization (DHAP) has been explored recently (Figure 1) [12]. The DHAP method eliminates the need for adding a directing group. Instead, the carbon atom with the most "active hydrogen" in the monomer is able to couple with the halogenated carbon atom in another (or the same) monomer. However, many monomer compounds have multiple C-H bonds with close dissociation energies, which can potentially be activated and react with a C-halogen bond. Furthermore, two Pd(II) complex intermediates bearing equal (hetero)aryl groups may undergo a disproportionation reaction, resulting in a homocoupling defect [18][19][20]. These side reactions may impede the formation of soluble (in the case of crosslinking side reaction) or high molecular weight (in the case of homocoupling side reaction) polymer products. Even for the polymers with good solubility and high molecular weights made by DHAP, a certain amount of branching, crosslinking, and/or homocoupling defects are frequently observed [21][22][23][24][25][26]. Figure 1 shows the formation of these defects in the DHAP of 3-alkyl-2-bromo-thiophene to poly (3-alkylthiophene). In the past few years, rigorous studies have been conducted to optimize the synthetic conditions to minimize or eliminate these side reactions. With a better understanding of the DHAP mechanism, a number of high-quality polymer semiconductors with fewer structural defects have been synthesized using the DHAP method [27].
An important question or concern from the organic electronics community is: are the performances of the polymers made by DHAP comparable to those of the polymers made by the conventional synthetic methods? In this review, we will provide a preliminary answer to this question by judiciously selecting some representative polymer semiconductors that were made by both the DHAP method and the conventional methods and compare their performances in OTFTs and OPVs. For the recent and significant progress made in clarifying the mechanism and optimizing reaction conditions of DHAP to improve the yield and molecular weight and reduce/eliminate structural defects of the resulting polymers, readers are directed to several recent review articles [12,[28][29][30].
Molecules 2018, 23, x FOR PEER REVIEW  2 of 17 install the directing groups, which increases the production cost and generates stoichiometric amounts of by-products that are potential health and environmental hazards. Specifically, the organotin compounds formed from the Stille coupling reactions are known to be highly toxic [16], while the boronic acid derivatives used in the Suzuki coupling reactions, which were previously assumed to be less harmful, have been recently found to be potential genotoxic hazards [17].
To address these issues associated with the common synthetic methods used to prepare polymer semiconductors, a novel C-C bond forming methodology, the so-called direct (hetero) arylation polymerization (DHAP) has been explored recently ( Figure 1) [12]. The DHAP method eliminates the need for adding a directing group. Instead, the carbon atom with the most "active hydrogen" in the monomer is able to couple with the halogenated carbon atom in another (or the same) monomer. However, many monomer compounds have multiple C-H bonds with close dissociation energies, which can potentially be activated and react with a C-halogen bond. Furthermore, two Pd(II) complex intermediates bearing equal (hetero)aryl groups may undergo a disproportionation reaction, resulting in a homocoupling defect [18][19][20]. These side reactions may impede the formation of soluble (in the case of crosslinking side reaction) or high molecular weight (in the case of homocoupling side reaction) polymer products. Even for the polymers with good solubility and high molecular weights made by DHAP, a certain amount of branching, crosslinking, and/or homocoupling defects are frequently observed [21][22][23][24][25][26]. Figure 1 shows the formation of these defects in the DHAP of 3-alkyl-2bromo-thiophene to poly(3-alkylthiophene). In the past few years, rigorous studies have been conducted to optimize the synthetic conditions to minimize or eliminate these side reactions. With a better understanding of the DHAP mechanism, a number of high-quality polymer semiconductors with fewer structural defects have been synthesized using the DHAP method [27].
An important question or concern from the organic electronics community is: are the performances of the polymers made by DHAP comparable to those of the polymers made by the conventional synthetic methods? In this review, we will provide a preliminary answer to this question by judiciously selecting some representative polymer semiconductors that were made by both the DHAP method and the conventional methods and compare their performances in OTFTs and OPVs. For the recent and significant progress made in clarifying the mechanism and optimizing reaction conditions of DHAP to improve the yield and molecular weight and reduce/eliminate structural defects of the resulting polymers, readers are directed to several recent review articles [12,[28][29][30].

Polymer Semiconductors for Organic Thin Film Transistors
Poly(3-hexyl-thiophene-2,5-diyl) (P1, Figure 2), also known as P3HT, is one of the most widely studied polymer semiconductors to date. Due to its widespread use in the organic electronics

Polymer Semiconductors for Organic Thin Film Transistors
Poly(3-hexyl-thiophene-2,5-diyl) (P1, Figure 2), also known as P3HT, is one of the most widely studied polymer semiconductors to date. Due to its widespread use in the organic electronics community, it has become a standard, to which new materials can be compared [31]. The monomer unit, 3-hexylthiophene-diyl, is unsymmetrical, which means that three different types of dyads, heat-to-head (HH), head-to-tail (HT), and tail-to-tail (TT) can be formed depending on the positions of the hexyl substituents on the two thiophene units. In an HH dyad, the two closely positioned hexyl substituents would cause severe twisting of the two thiophene rings, which would disrupt the π-conjugation pathway along the polymer backbone. The TT dyad is highly coplanar, but the formation of each TT dyad is accompanied with the formation of one HH dyad during the polymerization. On the other hand, the HT dyad is highly coplanar and a P3HT comprising of only HT dyads is a 'regioregular' HT P3HT or commonly rr-P3HT. The first P3HT was made using an FeCl 3 -mediated oxidative coupling polymerization, which contained a significant amount of HH dyads with an HT mol % or regioregularity of~70-80% [32][33][34][35] and thus had a rather twisted backbone. P3HT made in this way has poor crystallinity, which leads to low charge carrier mobilities of~10 −4 cm 2 V −1 s −1 in OTFT devices [36].
The aforementioned synthetic routes to rr-P3HT require additional synthetic steps to add a directing group and/or produce stoichiometric harmful by-products. Thus, it was of interest to researchers to explore alternative synthetic pathways for forming highly regioregular P3HT without the above drawbacks.
In 1998, Lemaire and co-workers reported a new method to synthesize polythiophenes using 2-iodothiophenes as the monomers and Pd(OAc) 2 as the catalyst [46]. The authors initially thought the polymerization was a Heck-type reaction, but it was later found to be a direct (hetero) arylation reaction, which involves a direct coupling of an active aromatic C-H bond with an aromatic C-X bond (X is halide) to form a C-C bonding motif [47]. In 2010, Wang et al. conducted a DHAP of 2-bromo-3-hexylthiophene with Herrmann's catalyst to make an rr-P3HT [45]. High number average molecular weights (M n ) of up to 31 kDa were achieved, which are much higher than those of the polythiophenes (~10 3 Da) through DHAP reported previously by Lemaire and coworkers [46]. A very high degree of regioregularity (>98%) of this rr-P3HT was confirmed by the 1 H NMR spectroscopic analysis, indicating that electronic grade P3HT was attainable using DHAP synthetic methods.
Pouliot et al. [48] prepared a sample of P1 with a very high regioregularity of >99.5% and an M n of 33 kDa by optimizing the synthesis of the 2-bromo-3-hexylthiophene monomer and DHAP conditions (P1 DHAP1 , Table 1). They compared the OTFT performance of this polymer with other rr-P3HT polymers made by GRIM and Rieke's methods using the same device configuration and testing conditions (P1 GRIM and P1 Rieke , Table 1). It was found that high hole mobilities of up to 0.19 cm 2 V −1 s −1 were achieved by the polymer made by DHAP, which are much higher than those of P1 GRIM (0.11 cm 2 V −1 s −1 ) and P1 Rieke (up to 0.02 cm 2 V −1 s −1 ). The mobility trend can be explained by their regioregularity order: P1 DHAP1 (HT mol % > 99.5%) > P1 GRIM (HT mol % = 98.0%) > P1 Rieke (HT mol % = 95.5%). The results from this study show that the performance of rr-P3HT made by DHAP in OTFTs is improved compared to those of the rr-P3HT polymers made by the Rieke and GRIM methods.  Figure 3) also known as N2200, is considered to be the most extensively investigated n-type polymer for OTFTs [49][50][51] and OPVs [52][53][54][55][56][57]. P2 has commonly been synthesized using the Stille coupling polymerization [58], so the ability to synthesize it using atom-economical methods is of interest. In 2015, the DHAP synthesis of N2200 was reported by Sommer et al. [50] (P2 DHAP , Table 1). Optimized DHAP conditions gave the polymer with an M n of up to 31 kDa and a 'perfect' NDI-bithiophene alternating structure, with no homocoupling, branching or cross-linking, according to 1 H NMR spectroscopic analysis. Importantly, the molecular weight of the polymer was readily controlled via in situ solvent (toluene) end capping by adjusting the monomer concentration. Another important observation was that the optical and thermal properties of the DHAP polymers reach their maxima at M n =~20 kDa. This is in excellent agreement with the sample produced via the Stille coupling (P2 Stille, Table 1). In OTFTs, the mobilities of P2 DHAP (2.9 cm 2 V −1 s −1 ) and P2 Stille (3.2 cm 2 V −1 s −1 ) with similar M n were very close, which are among the highest values reported for n-type polymers to date (OTFT transfer curves for P2 DHAP and P2 Stille are shown in Figure 4).
Another copolymer of DPP with tetrafluorobenzene, PDPPTh2F4 (P5), was synthesized by DHAP [66], which has not been approached by other synthetic methods (P5 DHAP , Table 1). Due to the strong electron withdrawing effect of the tetrafluorobenzene units, this polymer showed n-type electron transport semiconductor behavior, opposed to the majority of other DPP polymers that showed either p-type or ambipolar charge transport performance. High electron mobility of up to 0.60 cm 2 V −1 s −1 was achieved in OTFTs.

Polymer Semiconductors for Organic Photovoltaics
The use of rr-P3HT (P1) synthesized by DHAP in solar cells was first reported by Thompson et al. [25]. A sample of rr-P3HT (P1 DHAP2 ) with M n = 19 kDa, M w /M n = 2.0, and HT mol % = 90% and a control polymer P1 Stille1 with M n = 19 kDa, M w /M n = 2.7, and HT mol % = 93% were synthesized by DHAP and Stille coupling methods, respectively (Table 2). Interestingly, it was shown that P1 DHAP2 had~0.75% β-defects, but still showed even better solar cell performance compared to P1 Stille1 , which had no β-defects (PCE: 2.70% for P1 DHAP2 vs. 2.30% for P1 Stille1 ). The authors made another sample using DHAP, having 1.41% β-defect concentration, and found that the performance was poorer due to formation of unfavorable morphology when blended with fullerene and the low space-charge-limited current (SCLC) mobility, which resulted in low J SC . The authors concluded that rr-P3HT synthesized by DHAP with β-defect concentrations below 1% were able to achieve similar or better performance in solar cell devices when compared with their counterparts made by the Stille coupling method.
Another recent study by Thompson et al. compared the properties of rr-P3HT synthesized using a "green solvent assisted" DHAP (P1 DHAP3 ) with a sample prepared by the traditional Stille coupling method (P1 Stille2 , Table 2) [69]. P1 DHAP3 had M n = 20 kDa, M w /M n = 2.1, and HT mol % = 96.2%, while P1 Stille2 had M n = 18 kDa, M w /M n = 2.4, and HT mol % = 92.9%. These authors found that the use of a bulky carboxylic acid additive, neodecanoic acid, and a "green" solvent, 2-methyl-tetrahydrofuran, was critical in completely avoiding the formation of β-defects. In UV-Visible absorption spectra, the β-defect free P1 DHAP3 displayed a higher extinction coefficient at the wavelength of maximum absorbance (λ max ) and stronger vibrational fine structure, indicating more structural ordering in the solid state. P1 DHAP3 also had larger crystalline coherence lengths, as confirmed by X-ray diffraction, which were correlated to the increased SCLC hole mobility for this polymer. When evaluated as a donor with PC 61 BM as the acceptor in solar cells, P1 DHAP3 outperformed P1 Stille2 (PCE: 3.28% vs. 2.86%, Table 2). This is in agreement with the previous results by the same group, implying that a rr-P3HT sample with negligible β-defects produced by DHAP can outperform its counterpart made by the Stille coupling [25].
Kanbara et al. synthesized copolymers of fluorene and EDOT (P8, Figure 5) using both DHAP and Suzuki coupling methods (P8 DHAP and P8 Suzuki , Table 2) [70]. P8 DHAP made by DHAP with a microwave reactor had a much higher M n of 150 kDa than that of P8 Suzuki made by Suzuki coupling, which had an M n of 17 kDa. As a donor, P8 DHAP demonstrated a high PCE of 4.08%, while P8 Suzuki exhibited a very low PCE of 0.480%. The large discrepancy in the OPV performance of these two polymers is ascribed mainly to their different hole mobilities. In OTFTs, P8 DHAP showed an average hole mobility of 1.2 × 10 −3 cm 2 V −1 s −1 , while the average hole mobility of P8 Suzuki is about two orders of magnitude lower at 3.2 × 10 −5 cm 2 V −1 s −1 . Another P8 DHAP with a lower M n of 48 kDa showed a lower PCE of 2.55%, which was believed to be due to the presence of terminal Br groups and residual Pd impurities in the polymers, causing a decrease in the hole mobility (7.7 × 10 −4 cm 2 V −1 s −1 ). In a later study by the same group, P8 DHAP showed further improved PCE of up to 4.60% after the terminal Br groups were removed through a post-polymerization treatment of the polymer with sodium N,N-diethyldithiocarbamate [71].
D-A polymers are among the most popular materials for use in OPVs and synthesis of high-quality D-A polymers by DHAP was therefore explored. Russel et al. reported on two D-A polymers based on DPP, DPP-co-phenylene (PDPP-TPT, P9, Figure 5) and DPP-co-thiophene, (PDPP3T, P10), which were synthesized by DHAP (P9 DHAP and P10 DHAP , Table 2) [22]. Optimized DHAP conditions provided a sample of P9 DHAP with an M n of 14 kDa and an M w /M n of 1.8. P9 DHAP produced via DHAP in this work was compared with one produced by Janssen's group using a Suzuki coupling protocol (P9 Suzuki1 , Table 2) [72], which showed a bimodal GPC trace with two peak molecular weights of 65 kDa and 10 kDa due to the gelation and aggregation. As a donor, P9 DHAP showed slightly lower PCE in OPVs than P9 Suzuki1 when PC 61 BM was used as the acceptor (4.37 vs. 5.50%, Table 2). Later, optimization of the Suzuki coupling reaction gave a sample of P9 Suzuki2 with a higher M n reaching 72 kDa and an M w /M n of 1.98. Solar cells using P9 Suzuki2 gave PCE of up to 7.40% [73]. For the second polymer reported in this work [22], PDPP3T, optimized DHAP conditions gave a sample of P10 DHAP with M n 29 kDa and an M w /M n of 3.8. A previous paper by Janssen et al. in 2009 [74] reported on the synthesis of PDPP3T using a Suzuki coupling protocol, which gave P10 Suzuki with an M n of 54 kDa, nearly twice that of P10 DHAP reported in this work. In OPVs, P10 DHAP was also outperformed by P10 Suzuki (4.01% vs. 4.69%, Table 2). A later work by Janssen et al. improved the synthesis of P10 via Stille coupling [73] (P10 Stille , Table 2), achieving an improved M n of 150 kDa and an M w /M n of 2.72. Solar cells made using P10 Stille gave much better PCE of up to 7.10% when used as the donor material in OPVs. 1 H NMR spectroscopy was unable to determine the presence of homocoupling defects in P9 DHAP and P10 DHAP due to the tendency to aggregate in solution; however, the authors suggested that some homocouplings (<5%) may be present in these samples because a broadening of the low energy absorption shoulders was seen, a feature which is often attributed to minor homocoupling defects [75]. Both the presence of the homocoupling defects as well as the lower molecular weights might account for the lower mobilities observed for P9 DHAP and P10 DHAP compared with their counterparts made by the Suzuki and Stille couplings, P9 Suzuki1 , P9 Suzuki2 , P10 Suzuki and P10 Stille which contained lesser amounts of defects and had higher molecular weights.  P11) is a high performance polymer semiconductor, used commonly as a donor in OPVs [76][77][78], and a p-type semiconductor in OTFTs [79]. In 2012, Horie et al. synthesized PCPDTBT using both DHAP and Suzuki methods (P11 DHAP , P11 Suzuki , Table 2) for a direct comparison [80]. P11 DHAP had an M n of 72 kDa with an M w /M n of 4.52, while P11 Suzuki had a much lower M n of 15 kDa with an M w /M n of 2.1. By using MALDI-TOF and 1 H NMR spectroscopic analysis, it was found that P11 DHAP contained both homocoupling and branching defects, while P11 Suzuki did not. The β-defect concentration was found to be 12-13% for P11 DHAP , which is well above the "threshold" value of~1% for rr-P3HT for achieving good OPV performance observed by Thompson et al. [25]. Despite such high β-defect concentration, P11 DHAP exhibited higher PCE in solar cells using PC 71 BM as the acceptor under the same processing and testing conditions compared to P11 Suzuki (PCE: 3.98% vs. 3.74%. respectively) [80]. P11 Stille synthesized by the Stille coupling previously, which had an M n of 28 kDa, achieved a lower PCE of 3.50% [77] (P11 Stille , Table 2). These results suggest that the PCPDTBT system has a high tolerance for β-defects, which might be due to the beneficial effect of the branching structure on the morphology of the P11 DHAP :PC 71 BM blend films [80] as well as the much higher intrinsic hole mobility of this polymer [79] compared to P3HT. The fact that P11 DHAP showed the best PCE is most likely due to its highest molecular weight, which is a more dominant factor than the β-defect concentration in the OPV performance. Given that the rigorous optimization of device fabrication for commercial samples of P11 afforded an improved PCE of up to 6% when paired with PC 71 BM [76,78], it is possible that P11 DHAP may also exhibit a higher PCE when the active layer formation is further optimized.
Thompson et al. synthesized benzothiadiazole-co-benzodithiophene polymers (P12) using DHAP and Stille methods (P12 DHAP , P12 Stille2-LMW , P12 Stille1-HMW , Table 2) [81]. The optimized DHAP protocol afforded a 'defect-free' PPDTBT sample P12 DHAP with an M n of 15 kDa and an M w /M n of 2.1 (P12 DHAP , Table 2). The use of THF as a solvent and the bulky neodecanoic acid as an additive was found to be essential in minimizing structural defects during DHAP based on the 1 H NMR spectroscopic analysis [69]. Two PPDTBT samples having M n of 59 and 16 kDa and M w /M n of 3.3 and 2.1, respectively, were synthesized by the Stille method for comparison (P12 Stille1-HMW vs. P12 Stille2-LMW , Table 2). P12 DHAP showed a better PCE (3.40%) compared with the counterpart P12 Stille2-LMW made by Stille (PCE = 2.90%), which has a similar M n and M w /M n . On the other hand, polymer P12 Stille1-HMW with a high molecular weight made by the Stille method was able to achieve a higher PCE of 3.80%. These results illustrate, once again, the importance of the polymer molecular weight on device performance. It can be concluded that the quality of the polymer P12 made by DHAP is similar to that made from Stille coupling if the molecular weight can be matched.
In  [23]. Samples of P13 were prepared via DHAP and also by Stille coupling polymerization for direct comparison. DHAP yielded a sample (P13 DHAP ) with a low M n of only 10 kDa compared to the M n of 20 kDa obtained for P13 Stille synthesized via Stille coupling. In addition, the M w /M n was much larger for the DHAP synthesized sample compared to the one made using a Stille coupling (M w /M n : 7.6 for P13 DHAP vs. 3.1 for P13 Stille , Table 2). A hypsochromic shift in the absorption spectra is noticed for the DHAP synthesized polymer, indicative of a lower effective conjugation length. 1 H NMR analysis revealed the presence of β-defects for P13 DHAP . When the two polymers were tested in solar cells with PC 71 BM as the acceptor, P13 DHAP gave a lower PCE of 2.80% compared to a value of 4.80% for P13 Stille . Such a large difference in performance between P13 DHAP and P13 Stille can be directly ascribed to the lower molecular weight and higher defect concentration in the former. Optimization of the catalytic systems for DHAP such as the use of bulky carboxylic acid additives and specialized catalysts [27,69,82] may provide a sample of P13 with a lower concentration of defects and thus a higher effective conjugation length, which would improve its performance to be on par with P13 Stille.
In 2016, Marks et al. reported a comprehensive study to compare the physical and electronic properties of a series of D-A polymers made using both DHAP and Stille methods [83]. The first polymer is PBDTT-FTTE (P14), a well-known high performance p-type polymer that has been used in record setting OPVs [84,85]. It was found that increasing the steric bulkiness of the carboxylic acid additive, i.e., the use of 2,2-diethylhexanoic acid instead of pivalic acid, in the DHAP system was critical in forming high molecular weight, defect-free polymers, which is in agreement with other recent reports [69,81]. Optimization of DHAP conditions gave P14 DHAP with M n = 25 and M w /M n = 2.2, a very close match with P14 Stille made by the Stille coupling (M n = 25 kDa and M w /M n = 2.2). When evaluated in solar cell devices with PC 71 BM as the acceptor, P14 DHAP gave an almost same PCE compared with P14 Stille (PCE: 8.36% vs. 8.40%, Table 2 and Figure 6a). In addition, the two polymers showed very similar film morphologies when blended with PC 71 BM (Figure 6b). Two other pairs of D-A polymers based on thieno [3,4-c]pyrrole-4,6-dione (TPD), namely PBDTT-TPD (P15 DHAP and P15 Stille , Table 2) and PTPD3T (P16 DHAP and P16 Stille Table 2), were also made using both DHAP and Stille methods [83]. It was found that P15 DHAP has a higher M n than that of P15 Stille (M n : 30 kDa vs. 15 kDa), while P16 DHAP had a lower M n than that of P16 Stille (M n : 19 kDa vs. 30 kDa). 1 H NMR spectroscopy confirmed that the structural regularity of both polymers made by DHAP was similar to that of the ones made by the Stille method. In addition, the absorption spectra of each pair of polymers are nearly identical. In OPVs, P15 DHAP was found to outperform P15 Stille (PCE: 5.84% vs. 5.20%), while P16 DHAP performed slightly poorer than P16 Stille (PCE: 7.20% vs. 7.38%). Since P15 DHAP and P16 DHAP did not contain noticeable structural defects, the differences in their OPV performance with their counterparts P15 Stille and P16 Stille are likely related to their different molecular weights, with the higher molecular weight ones showing higher OPV performance.
PBDTTPD (P17) is another high performance donor polymer for OPVs [86]. Farinola et al. conducted a study comparing the optoelectronic properties and device performance of PBDTTPD made by both DHAP and Stille methods (P17 DHAP and P17 Stille , Table 2) [87]. Optimized DHAP conditions produced P17 DHAP with a low M n of 12 kDa, while P17 Stille synthesized via the Stille coupling protocol had an even lower M n of 10 kDa. The slightly higher M n of P17 DHAP resulted in a red-shifted UV-Visible absorption profile as well a clearer vibronic structure in the low-energy region compared to P17 Stille . In addition, the aggregation propensity of P17 DHAP in solution was noted to be higher. Furthermore, I t was found that P17 Stille allowed the formation of large micro-sized domains of PC 71 BM in the bulk-heterojunction blend, while P17 DHAP did not. OPV devices using these two polymers as the donors and PC 71 BM as the acceptor were fabricated and compared. PCEs of up to 5.31% and 4.82% were obtained for P17 DHAP and P17 Stille , respectively, which is in good agreement with the disparity in their molecular weights. conditions produced P17DHAP with a low Mn of 12 kDa, while P17Stille synthesized via the Stille coupling protocol had an even lower Mn of 10 kDa. The slightly higher Mn of P17DHAP resulted in a red-shifted UV-Visible absorption profile as well a clearer vibronic structure in the low-energy region compared to P17Stille. In addition, the aggregation propensity of P17DHAP in solution was noted to be higher. Furthermore, I t was found that P17Stille allowed the formation of large micro-sized domains of PC71BM in the bulk-heterojunction blend, while P17DHAP did not. OPV devices using these two polymers as the donors and PC71BM as the acceptor were fabricated and compared. PCEs of up to 5.31% and 4.82% were obtained for P17DHAP and P17Stille, respectively, which is in good agreement with the disparity in their molecular weights.  Table 2) [19]. The molecular weight is much higher than that (Mn = 2.5 kDa) of the same polymer made by DHAP by Harris et al. using DMAc as the solvent, where severe homocoupling occurred, resulting in the lower Mn [80]. On the other hand, the 1 H NMR and MALDI-TOF-MS analysis of P18DHAP made in toluene in this work showed no detectable homocoupling defects. A high PCE of up to 6.80% was achieved when P18DHAP was used as a donor in OPVs, which is higher than that (PCE = 5.20%) of P18Stille prepared by the Stille coupling, which had a lower Mn of 15 kDa [88].
Ahmed et al. reported on another series of polymers based on [3,4-c]pyrrole-4,6-dione derivatives with sulfur, oxygen or selenium heteroatom substitution, made by the DHAP method [89]. The champion polymer from their study, P19DHAP, was based on a seleno[3,4-c]pyrrole-4,6-dione acceptor and a dithienosilole donor and was obtained with an Mn of 29 kDa and an Mw/Mn of 1.6. When paired with PC61BM in inverted organic solar cells, a high PCE of 7.13% was achieved. No  Table 2) [19]. The molecular weight is much higher than that (M n = 2.5 kDa) of the same polymer made by DHAP by Harris et al. using DMAc as the solvent, where severe homocoupling occurred, resulting in the lower M n [80]. On the other hand, the 1 H NMR and MALDI-TOF-MS analysis of P18 DHAP made in toluene in this work showed no detectable homocoupling defects. A high PCE of up to 6.80% was achieved when P18 DHAP was used as a donor in OPVs, which is higher than that (PCE = 5.20%) of P18 Stille prepared by the Stille coupling, which had a lower M n of 15 kDa [88].
Ahmed et al. reported on another series of polymers based on [3,4-c]pyrrole-4,6-dione derivatives with sulfur, oxygen or selenium heteroatom substitution, made by the DHAP method [89]. The champion polymer from their study, P19 DHAP , was based on a seleno [3,4-c]pyrrole-4,6-dione acceptor and a dithienosilole donor and was obtained with an M n of 29 kDa and an M w /M n of 1.6. When paired with PC 61 BM in inverted organic solar cells, a high PCE of 7.13% was achieved. No equivalent polymer to P19 DHAP made by a conventional coupling method was reported for comparison.
A recent report by Li et al. compared the properties of indacenodithiophene (IDT) and thiophene-quinoxaline-thiophene (TQ) derived polymers (P20) made using DHAP and Stille coupling methods (P20 DHAP and P20 Stille , Table 2) [21]. P20 DHAP had an M n of 27 kDa with an M w /M n of 1.6, while P20 Stille had an M n of 23 kDa and an M w /M n of 1.5. The close number average molecular weights of the two samples allow for a fair comparison between the two polymers. 1 H NMR spectroscopy, MALDI-TOF MS, and elemental analysis revealed that P20 Stille contained a certain amount of TQ−TQ homocoupling defects, while P20 DHAP had a very well-defined IDT-TQ alternating structure with no detectable structural defects. Interestingly, SCLC experiments showed a weak correlation between their hole mobility and structural regularity. Furthermore, grazing incidence X-ray diffraction (GIXRD) studies indicated no significant differences in the film crystallinity between two polymers. P20 DHAP and P20 Stille achieved PCE of 5.10% and 4.82%, respectively. It was found that the larger number of defects in the backbone of P20 Stille had influenced the morphological properties and SCLC mobilities, which caused severe recombination and thus the lower PCE.

Conclusions
Recently, direct (hetero)arylation polymerization (DHAP) of a halogenated (hetero)aryl compound with another (hetero)aryl compound possessing activated C-H bonds has emerged as an alternative approach to the conventional C-C bond formation methods such as Stille and Suzuki coupling polymerization for the synthesis of linear π-conjugated polymers. Since DHAP does not require the use of a (hetero)aryl compound possessing a reactive directing group such as an organostannyl group in the case of a Stille coupling polymerization, fewer steps are needed for DHAP, which can improve the atom economy and reduce the cost for the synthesis of conjugated polymers. Furthermore, the by-product of a DHAP is a hydrogen halide (HX), which is much easier to treat compared with the toxic wastes generated from the Stille and Suzuki coupling polymerization. Therefore, DHAP has been considered a more cost-effective and "greener" technique for the mass production of conjugated polymers. However, since many (hetero)aryl compounds have more activated C-H bonds than required for the formation of a linear polymer, side reactions at the unwanted C-H bonds result in branching or cross-linking structural defects, often leading to degradation of the optoelectronic properties or even the formation of unprocessable insoluble products. In addition, homocoupling between the halogenated monomer molecules produces homocoupling defects in the polymer backbone and also results in the formation of lower molecular weight polymers. The presence of these structural defects in the polymers made by DHAP potentially have impacts on the performances of these polymers in organic electronic devices.
This review presented some exemplary conjugated polymer semiconductors made by DHAP and compared their performances in OTFTs and OPVs with their counterparts synthesized by conventional synthetic methods, such as the Stille and Suzuki coupling polymerization. It can be seen that by minimizing the structural defects and improving the molecular weight of the polymers through optimization of the DHAP conditions, similar or sometimes better OTFT and OPV performances could be achieved for the polymers made by DHAP in comparison to the ones prepared by the conventional methods. With the rapid progress made in the development of novel catalyst systems and optimization of reaction conditions (solvent, temperature, reaction time, etc.), it is expected that more and more high-performance polymer semiconductors, which have been synthesized and cannot be approached by conventional methods, will be developed. Further study of the atom-economic, cost-effective, and "greener" DHAP to enable the mass production of top-performing polymer semiconductors will propel the commercialization and wide-spread applications of organic electronics.