Design and effective synthesis methods for high-performance polymer semiconductors in organic field-effect transistors

Abstract

To date, versatile polymer semiconductors have been reported for field-effect transistors (FETs). And the third-generation donor–acceptor (D–A) polymers have been among the most intensively studied semiconductors. Meanwhile, there are a variety of methods adopted for the enhancement of performance. To the best of our knowledge, a p-type polymer semiconductor with a highest hole mobility of 52.7 cm² V⁻¹ s⁻¹, an n-type polymer semiconductor with a highest electron mobility of 8.5 cm² V⁻¹ s⁻¹ and a balanced ambipolar semiconductor with the highest both hole and electron mobility of over 4 cm² V⁻¹ s⁻¹ have been achieved. This review describes building block selection, backbone halogenation, side chain engineering and random copolymerization, which are the effective synthesis approaches applied in this field, affording assistance for developing high-performance polymer semiconductors in the future.

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Longxian Shi

Longxian Shi received his BS degree from Wuhan Textile University in 2014. Since September 2015, he has been a master’s student in the Institute of Chemistry, Chinese Academy of Sciences (ICCAS). His research interests focus on molecular materials and devices for organic field-effect transistors.

Yunlong Guo

Yunlong Guo received his PhD degree in physical chemistry from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 2010. In April 2016, he became a project associate professor at the Department of Chemistry, University of Tokyo. Now, he is a professor at ICCAS. His research interests include organic–inorganic hybrid perovskite electronics and functional organic field-effect transistors.

Wenping Hu

Wenping Hu is a professor of Tianjin University and the Institute of Chemistry, Chinese Academy of Sciences (ICCAS). He received his PhD from ICCAS in 1999. He then joined Osaka University as a research fellow of the Japan Society for the Promotion of Sciences and Stuttgart University as an Alexander von Humboldt fellow. In 2003 he worked for Nippon Telephone and Telegraph, and then returned to ICCAS. In 2016, he moved to Tianjin University. His research focuses on molecular electronics, and he has more than 390 refereed publications with over 11 000 citations.

Yunqi Liu

Yunqi Liu is a professor at the Institute of Chemistry, Chinese Academy of Sciences (ICCAS). He graduated from Nanjing University in 1975, and received a doctorate from the Tokyo Institute of Technology, Japan, in 1991. He was selected as an Academician of CAS in 2015. His research interests include molecular materials and devices, the synthesis and applications of carbon nanomaterials, and organic electronics. He has more than 500 refereed publications with over 22 000 citations.

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1. Introduction

Since Tsumura^1,2 and his co-workers fabricated the first polythiophene-based organic field-effect transistor (OFET), polymer semiconductors have been widely studied in organic field-effect transistors, as they are flexible, lightweight, transparent, solution-processable and modifiable. Compared to the inorganic semiconductors, the solution-processable polymer semiconductors have been recognized as promising candidates for commercial applications due to their low cost. Among the polymer semiconductors, the third-generation D–A conjugated polymers are more attractive than all donor and all acceptor conjugated polymers because of their relatively favourable charge transport with a low bandgap. In addition, several designs and synthesis means of these materials have been proposed, and these polymers showed excellent OFET behaviour when combined with device architectures (top-gate bottom-contact (TGBC), top-gate top-contact (TGTC), bottom-gate bottom-contact (BGBC) and bottom-gate bottom-contact (BGBC)). This review emphasizes the discussion of design and effective synthesis methods including building block selection, backbone halogenation, side chain engineering and random copolymerization, aiming to provide a valuable message for the development of high-performance polymer semiconductors. For building blocks, diketopyrrolopyrroles (DPPs) and isoindigo (IID) and other typically reported structures will be fully discussed. Then, the section on backbone halogenation will summarize the effect of fluorination and chlorination on the properties of the polymer including energy levels, molecular packing order and coplanarity. After that, the function of side chain engineering for controlling the solubility and condensed state structure of a rigid conjugated polymer will be unfolded. Finally, random copolymerization as an effective way to adjust the various properties of polymer semiconductors will be mentioned.

2. Building blocks

Generally, building blocks in a conjugated polymer can be classified as acceptors and donors according to their electron-donating or electron-withdrawing role in the conjugated system. Obviously, the rational design and introduction of building blocks is a basic and important step for the exploration of high-performance semiconductors in OFETs. Up to now, various kinds of molecules have been used as acceptors in conjugated polymers, such as DPPs, IID, imides, etc. Different acceptors energy matched by suitable donors can generate polymers with a favorable behavior. In this section, a series of common acceptors accompanied by different donors will be discussed.

2.1 Diketopyrrolopyrroles

DPPs are excellent acceptors, which possess a planar conjugated bicyclic lactam unit that induces low highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels as it is of high electron-withdrawing ability.³Scheme 1 shows a common route for DPP synthesis. It is worth mentioning that the reaction rate could be improved by enhancing the concentration of succinate and temperature. In addition, the elevated temperature and slow addition of succinic ester is needed.⁴ Different aromatic nitrile original materials give a DPP core with different adjacent aromatic structures, which could be a way to adjust the steric hindrance between the DPP core and the adjacent aromatic unit, the planarity of the molecule and the energy levels. Besides, the different steric hindrances also affect the yields.⁴

Scheme 1 A general method for symmetric DPP synthesis.

Studies of the DPP-based semiconductors were mainly focused on DPP1 (Fig. 1). As listed in Tables 1 and 2, most high-performance p-type (μ_h > 1 cm² V⁻¹ s⁻¹), and even n-type and ambipolar DPP-based polymer semiconductors use DPP1 as the charge acceptor, because DPP1 not only has the fine properties of the core that DPP derivatives possess but also the fine coplanarity between the core and flanked aryl units resulting from the existence of hydrogen⋯oxygen (H⋯O) non-covalent interactions. To the best of our knowledge, the first effective DPP-based polymer for OFETs was reported by Li²⁵ and co-workers in 2010 via DPP1 as the acceptor and thieno[3,2-b]thiophene (TT) as the donor. This fused ring structure polymer showed a p-type semiconductor behaviour with a hole mobility of 0.94 cm² V⁻¹ s⁻¹. Later on, Chen⁷et al. utilized (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) units as the donor monomer and DPP1 as the acceptor monomer to obtain the polymer PDPP3 (Fig. 2 and Table 1) with a high hole mobility of up to 8.20 cm² V⁻¹ s⁻¹.

Table 1 Electronic parameters for high-performance p-type (μ_h > 1 cm² V⁻¹ s⁻¹) DPP-based polymer semiconductors

Polymer	Acceptor	Donor	M n (kDa)/PDI	Device structure	HOMO/LUMO (eV)	Max. μh (cm^2 V^−1 s^−1)	I on/Ioff	Ref.
PDPP1	DPP1	BT2	63.7/2.36	BGBC	−5.02/−3.72	2.01	10^4–10^5	5
PDPP2	DPP1	3T	44.0/1.92	BGTC	−5.12/−3.55	3.46	10^8	6
PDPP3	DPP1	TVT	73.5/2.49	BGBC	−5.28/—	8.20	10^5–10^7	7
PDPP4	DPP1	SVS	35.8/1.62	BGTC	−5.27/—	12.04	>10^6	8
PDPP5	DPP1	TVS	91.0/1.61	BGTC	—	7.60	—	9
PDPP6	DPP1	FBVFB	80.2/2.46	BGBC	−5.40/−3.59	1.48	10^6–10^7	10
PDPP7	DPP1	BTVBT	39.0/2.10	BGTC	−5.07/−3.83	1.40	>10^7	11
PDPP8	DPP1	DTT	29.3/N/A	BGBC	−5.27/−3.88	3.16	3 × 10^4	12
PDPP9	DPP1	TTTT	58.1/2.95	BGTC	−5.21/−3.91	3.20	>10^6	13
PDPP10	DPP1	TTTT2	20.5/3.25	BGTC	−5.3/−3.95	2.10	3 × 10^6	14
PDPP11	DPP1	BDT	56.8/2.41	BGTC	−5.36/−4.02	1.31	>10^6	13
PDPP12	DPP1	DTNDT	56.9/2.93	BGTC	−5.15/−3.48	1.80	10^5	15
PDPP13	DPP1	DTBDT	28.9/2.60	BGTC	−5.39/−3.38	2.70	10^7	16
PDPP14	DPP1	TTZTZT	17.0/4.00	BGTC	−5.14/−3.50	1.23	10^5–10^6	17
PDPP15	DPP1	BS	19.3/3.25	BGTC	—	1.50	10^6	18
PDPP16	DPP1	TVT-O	10.1/2.54	BGTC	−5.10/−3.30	1.69	—	19
PDPP17	DPP4	TVT	63.8/3.26	BGBC	−5.34/−3.96	1.90	>10^6	20
PDPP18	DPP4	DTT	24.0/2.29	BGTC	—	4.10	10^2–10^3	21
PDPP19	DPP5	BT	114.0/2.41	BGBC	−5.18/−3.86	1.79	>10^7	22
PDPP20	DPP5	BS	217.3/3.21	BGBC	−5.16/−3.90	2.08	>10^6	22
PDPP21	DPP5	TVT	140.7/2.88	BGBC	−5.12/−3.79	4.15	>10^7	22
PDPP22	DPP5	SVS	347.0/3.10	BGBC	−5.10/−3.83	5.23	>10^7	22
PDPP23	DPP8	DTT	29.3/1.38	BGBC	−5.19/−3.85	5.27	4 × 10^3	12
PDPP24	DPP10	BT	75.4/1.57	BGBC	−5.48/−4.03	3.05	8 × 10^6	23
PDPP25	DPP11	T	205.9/1.93	BGBC	—	5.87	2 × 10^5	24
PDPP26	DPP12	T	138.8/1.65	BGBC	—	12.5	7 × 10^4	24
PDPP27	DPP12	DTT	34.4/1.41	BGBC	−5.25/−3.87	5.32	7 × 10^4	12

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Fig. 1 Chemical structures of DPP-based acceptors.

Fig. 2 Chemical structures of PDPP3, PDPP18, PDPP22, PDPP23, PDPP24, PDPP25, PDPP26 and PDPP27.

Table 2 Electronic parameters for high-performance n-type and ambipolar DPP-based polymer semiconductors

Polymer	Acceptor	Donor	M n (kDa)/PDI	Device structure	HOMO/LUMO (eV)	Max. μh (cm^2 V^−1 s^−1)	Max. μe (cm^2 V^−1 s^−1)	Ref.
PDPP28	DPP1	2CNT	23.3/2.52	BGTC	−5.48/−3.80	—	0.09	47
PDPP29	DPP1	2CNB	67.5/1.80	BGTC	−5.63/−3.71	—	0.18	47
PDPP30	DPP1	2Tz	64.0/3.60	BGTC	−5.88/−2.14	—	0.31	48
PDPP31	DPP2	T	93.3/3.05	TGBC	−5.63/−4.00	—	0.13	26
PDPP32	DPP1	V	48.4/2.00	TGBC	−5.28/−3.55	0.37	0.78	50
PDPP33	DPP1	TV	49.2/1.90	TGBC	−5.18/−3.39	2.96	0.36	50
PDPP34	DPP1	BTV	23.5/2.90	TGBC	−5.13/−3.23	1.88	0.04	50
PDPP35	DPP1	T	33.7/6.08	BGTC	−5.30/−4.00	1.57	0.18	51
PDPP36	DPP1	2FT	40.8/3.97	BGBC	—	0.13	0.12	39
PDPP37	DPP1	S	23.3/—	BGTC	−5.10/−3.49	8.84	4.34	52 and 53
PDPP38	DPP1	BTz	42.4/1.42	BGTC	−5.20/−4.00	0.35	0.40	54
PDPP39	DPP1	FBT	24.0/2.01	BGTC	−5.38/−4.18	0.21	0.42	55
PDPP40	DPP1	2FBT	25.0/2.34	BGTC	−5.48/−4.22	0.10	0.30	55
PDPP41	DPP1	PHI	25.5/2.15	BGTC	−5.30/−3.75	0.13	0.52	56
PDPP42	DPP1	V	56.2/2.74	BGBC	—	0.17	0.017	40
PDPP43	DPP1	BF	102/4.30	TGBC	−5.67/−4.24	0.36	0.41	57
PDPP44	DPP1	CNTVT	125.2/1.51	BGTC	−5.77/−3.92	0.749	7.00	58
PDPP45	DPP1	4FTVT	59.9/4.91	BGTC	−5.36/−3.50	3.40	5.86	59
PDPP46	DPP1	BTzVBTz	20.6/4.99	BGTC	−5.34/−3.43	0.32	0.13	60
PDPP47	DPP1	TAT	82.1/2.30	BGTC	−5.39/−3.89	2.19	0.38	49
PDPP48	DPP1	BTzABTz	11.0/1.81	BGTC	−5.31/−3.53	0.24	0.15	60
PDPP49	DPP1	TT	50/3.87	TGBC	−5.33/−4.07	1.60	2.00	61
PDPP50	BBT	DPP1	8.8/1.80	BGBC	−4.55/−3.90	1.17	1.32	62
PDPP51	DPP1	BTP	32.8/2.77	BGBC	−5.48/−3.98	1.43	0.13	63
PDPP52	DPP3	T	14/5.40	TGBC	−5.06/−3.68	1.95	0.071	29 and 30
PDPP53	DPP4	2FT	16.1/3.13	BGTC	−5.40/−4.03	0.26	0.12	35
PDPP54	DPP4	BTz	204.6/2.20	BGTC	−5.37/−3.74	0.20	0.56	33
PDPP55	DPP4	T4FBT	12.08/1.71	BGTC	−5.48/−4.11	0.40	0.12	34
PDPP56	DPP5	S	70/3.00	BGBC	−5.20/−4.02	0.1	0.1	38
PDPP57	DPP5	TT	100/2.50	BGBC	−5.10/−3.92	1.1	0.15	38
PDPP58	DPP7	BT	26.3/3.56	TGBC	−5.69/−4.33	2.78	6.30	41
PDPP59	DPP7	T2FBTT	58.4/1.64	BGBC	−5.66/−4.02	0.24	0.65	43

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After that, DPP derivatives DPP2,^26,27 DPP3,^28–30 DPP4,^{20,21,31–35} DPP5,^22,36–38 DPP6,^36,39,40 DPP7,^39,41–43 DPP8,¹² DPP9,⁴⁴ DPP10,²³ DPP11,²⁴ and DPP12^12,24 were also intensively investigated in a previous study and are shown in Fig. 2. Among them, DPP1–DPP9 are all symmetric units with thiophene, thiazole, TT, furan, selenophene, benzene, pyridine, 3-methylthiophene and benzothiophene, respectively, while DPP10–DPP12 are asymmetric units. The HOMO and LUMO levels changed following variation of the flanking aryl units. Besides, the HOMO delocalization and coplanarity of a molecule could also be affected by the flanking aryl units, eventually resulting in good or bad charge-carrier hopping between the molecules.³ DPP2 is not a good structure for charge transport as the structure of the two thiazole subunits destroyed the H⋯O non-covalent interaction. And, there are few reports regarding its use for OFETs except organic solar cells. For DPP3, the stronger electron-donating properties of TT than thiophene give DPP3 higher HOMO and LUMO levels, which is not beneficial for n-type and ambipolar performance of semiconductors. Additionally, the larger size of TT than thiophene also has a negative impact on the solubility of the polymer, which causes poorer molecular packing during the device fabrication process and eventually leads to undesirable electrical properties. Therefore, few DPP3-based conjugated polymers exhibit high OFET performance. It is worth mentioning that researchers have proved that DPP4 flanked by furan possesses better solubility in chlorine-free solution than analogous thiophene-based DPP1.⁴⁵ Chang²¹ and co-workers also synthesized a high-performance polymer PDPP18 (Fig. 2 and Table 1) with a hole mobility of up to 4.10 cm² V⁻¹ s⁻¹ based on the acceptor DPP4. Selenium, as an element in the same group as sulfur, possesses more electrons and is of a larger size than sulfur. Based on this, the corresponding HOMO–LUMO levels of DPP5 flanked by selenophene are also higher than DPP1 flanked by thiophene, which has also been confirmed by comparison of the energy levels of the analogues between the DPP1- and DPP5-based polymers listed in Tables 1 and 2. High OFET performance DPP5-based polymer semiconductors also emerge in an endless stream. For example, in 2015, Choi et al.²² obtained a high p-type polymer PDPP22 with a hole mobility of up to 5.23 cm² V⁻¹ s⁻¹. From the theoretical calculations, it was revealed that DPP6 flanked by two phenyl substituents had a larger band gap and a less coplanar structure, which is detrimental to its amibipolar properties and intermolecular charge transfer.⁴¹ So almost no high-performance DPP6-based polymer semiconductors have been reported. However, when the two pyridine units were adjacent to the DPP core instead of benzene or thiophene, low-lying HOMO–LUMO levels and fine coplanarity were observed from calculations and experiments,⁴¹ which benefited the electrical performance. More detailed discussion on such a structure will be displayed in the later content. For DPP8 and DPP9, the HOMO–LUMO levels are adjusted by methyl substitution or phenyl fusion. As they are new DPP-based building blocks, their OFET performance still needs to be investigated.

Different from polymers based on symmetric units, the asymmetric units DPP10–DPP12 can provide a conjugated polymer with some special properties such as good solubility. As the two different flanking aromatic substituents are adjacent to the DPP core, the synthesis method of asymmetric DPPs is different from the methods used for symmetric DPPs as shown in Scheme 1, which required two steps to obtain a high yield by linking two different kinds of aromatic nitriles adjacent to the molecular backbone (Scheme 2). Li’s group^12,23,24 found that the asymmetric conjugated structure of the DPP-based molecule imparted a less preferential order and good solubility in non-chlorinated solvents for the resulting polymer. For example, DPP12, as an acceptor monomer, with adjacent aromatic substituents of thiophene and 3-methylthiophene on each side, copolymerized with thiophene contributes a high hole mobility to the PDPP26 polymer (Fig. 2 and Table 1) at 12.5 cm² V⁻¹ s⁻¹. In addition, the hole mobility of asymmetric polymers PDPP24²³ (Fig. 2 and Table 1), PDPP25²⁴ (Fig. 2 and Table 1), and PDPP27¹² (Fig. 2 and Table 1) was lower than that of PDPP28, with the data of 3.05, 5.87 and 5.32 cm² V⁻¹ s⁻¹, respectively.

Scheme 2 Syntheses of asymmetric DPPs (DPP10, DPP11, and DPP12).

Compared with the p-type polymers, the high performance of n-type and ambipolar behaviour is less reported, resulting from the limited electron acceptors for polymer semiconductors to be of a LUMO energy level lower than −4 eV for the facile injection and efficient transport of electrons.^41,46 On the other hand, the present knowledge of n-type and ambipolar polymers constructed from DPP acceptors is still in its infancy. As is shown in Table 2, PDPP28–PDPP31^26,47,48 are n-type DPP-based polymers with a mobility lower than 1 cm² V⁻¹ s⁻¹. In one of the early studies, Sun et al.⁴¹ reported a DPP-based ambipolar polymer PDPP58 (Fig. 3 and Table 2) with a relatively high hole mobility (μ_h = 2.78 cm² V⁻¹ s⁻¹) and the highest electron mobility (μ_e = 6.30 cm² V⁻¹ s⁻¹). Furthermore, this is the first time that DPP7 as the acceptor monomer of PDPP58 was introduced into a polymer semiconductor. Notably, DPP7 has a highly coplanar structure because of the weak steric hindrance between the 2-pyridinyl substituents and the DPP core. Apart from that, pyridine is a relatively electron-deficient structure, as a result of which, the LUMO energy level could be reduced. Thus, PDPP59 (Table 2) reached a high electron mobility and excellent ambipolar properties. Besides, Yun⁴⁹ and co-workers introduced the donor 2-[2-(thiophen-2-yl)ethynyl]thiophene (TAT) containing a structure of acetylene into the polymer PDPP47 (Fig. 3 and Table 2). Compared with the polymer PDPP3 whose linkage is vinylene, PDPP47 showed ambipolar behaviour caused by the more electron-withdrawing property of acetylene and conformation-insensitive charge transport. Chen et al.⁶¹ optimized the device architecture, charge injection and processing of the polymer PDPP49 (Fig. 3 and Table 2) reported for the first time by Li et al.²⁵ After optimization, PDPP49 showed ambipolar behaviour with a hole mobility of up to 1.60 cm² V⁻¹ s⁻¹ and an electron mobility of up to 2.00 cm² V⁻¹ s⁻¹. Jonathan D. Yuen⁶² prepared the polymer PDPP50 (Fig. 3 and Table 2), the benzobisthiadiazole unit of which possesses a high electron-withdrawing ability that is even higher than that of DPP. As a result, DPP was a donor and benzobisthiadiazole was an acceptor in the polymer PDPP50, which presented an excellent fine ambipolar property with a hole mobility of 1.17 cm² V⁻¹ s⁻¹ and an electron mobility of 1.32 cm² V⁻¹ s⁻¹.

Fig. 3 Chemical structures of PDPP28, PDPP29, PDPP30, PDPP31, PDPP47, PDPP49, PDPP50 and PDPP58.

2.2 Isoindigo

In 2011, IID-based polymer semiconductors were first studied in OFETs.⁶⁴ They also exhibit a strong electron efficiency owing to the two lactam units similar to DPP in the molecular structure. As shown in Scheme 3, this is a common method for the synthesis of IID as 1 and 2 are commercially available. Such a reaction could afford a brominated IID monomer (3) in 86% yield.⁶⁴

Scheme 3 General method for the synthesis of a brominated IID monomer.

PIID1⁶⁵ (Fig. 4 and Table 3) is one of the IID-based polymers reported for OFETs with a hole mobility of up to 1.06 cm² V⁻¹ s⁻¹. A series of polymers PIID1 (PIID1–PIID-C6Si, Table 3), PIID2 (PIID2–PIID2-C11Si; Fig. 4 and Table 3), and PIID3 (Fig. 4 and Table 3) are high-performance p-type IID-based polymer semiconductors with a hole mobility over 1 cm² V⁻¹ s⁻¹. The PIID1 and PIID2 series exhibit side chain engineering, which will be discussed in detail later in this review. It is worth mentioning that the PIID4 polymer (Fig. 4 and Table 3) based on IID1 (Fig. 5) and (E)-2-(2-(selenoph-2-yl)vinyl)selenophene (SVS) exhibited a high hole mobility up to 5.83 cm² V⁻¹ s⁻¹,⁶⁶ to the best of our knowledge, which was the highest one based on IID1.

Fig. 4 Chemical structures of PIID1, PIID2, PIID3, and PIID4.

Fig. 5 Chemical structures of IID-based acceptors.

Table 3 Electronic parameters for high-performance p-type (μ_h > 1 cm² V⁻¹ s⁻¹) IID- and IID-derivative-based polymer semiconductors

Polymer	Acceptor	Donor	M n (kDa)/PDI	Device structure	HOMO/LUMO (eV)	Max. μh (cm^2 V^−1 s^−1)	I on/Ioff	Ref.
PIID1	IID1	BT	20.4/2.0	BGTC	−5.70/−3.70	1.06	>10^6	65
PIID1-C3	IID1	BT	39.2/3.2	BGTC	−5.52/−3.74	3.62	>10^6	65
PIID1-C4	IID1	BT	37.3/2.3	BGTC	−5.50/−3.74	1.76	>10^6	65
PIID1-C6Si	IID1	BT	138/3.29	BGTC	−5.20/−1.61	2.48	>10^6	70
PIID2	IID1	BF	55.9/2.08	BGTC	−5.32/—	1.1	≈10^5	71 and 72
PIID2-C5Si	IID1	BF	54.7/1.58	BGTC	−5.35/—	1.9	≈10^5	72
PIID2-C6Si	IID1	BF	50.4/1.74	BGTC	−5.30/—	3.2	≈10^6	72
PIID2-C7Si	IID1	BF	56.2/1.91	BGTC	−5.25/—	3.2	≈10^6	72
PIID2-C8Si	IID1	BF	63.2/1.81	BGTC	−5.25/—	3.0	≈10^6	72
PIID2-C9Si	IID1	BF	48.3/2.10	BGTC	−5.25/—	4.8	≈10^6	72
PIID2-C10Si	IID1	BF	34.1/1.37	BGTC	−5.25/—	2.0	≈10^5	72
PIID2-C11Si	IID1	BF	34.0/1.42	BGTC	−5.35/—	1.8	≈10^6	72
PIID3	IID1	TVT	98.8/3.31	BGTC	−5.54/−3.65	2.20	>10^6	66 and 73
PIID4	IID1	SVS	295/2.36	BGTC	−5.22/—	5.83	10^5	66
PIID5	IID2	D61	21.0/4.87	TGBC	−5.12/−3.49	14.4	—	68
PIID6	IID7	BT	—	TGBC	−5.40/−3.73	1.1	—	74
PIID7	IID8	TVT	61.1/3.00	BGBC	−5.66/−2.04	3.63	—	75
PIID7-C3	IID8	TVT	50.6/3.87	BGBC	−5.67/−3.64	7.28	—	75
PIID8	IID13	BF	37.6/2.38	BGTC	−5.32/−4.05	1.92	—	76

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As shown in Scheme 4, thienoisoindigo (IID2, Fig. 5) as a derivative of IID was synthesized for the first time in 2012.⁶⁷ The dihedral angle between the two subunits is minimized, and the planarity of the molecular backbone is also enhanced when compared with that of IID. The IID2-based polymer PIID5⁶⁸ (Fig. 6) showed a favourable hole mobility up to 5.8 cm² V⁻¹ s⁻¹ owing to its fine coplanarity and an ultrahigh mobility of 14.4 cm² V⁻¹ s⁻¹ when high-k gate dielectric poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) worked as an insulator for device fabrication.

Scheme 4 Synthesis of a brominated thienoisoindigo monomer.

Fig. 6 Chemical structures of PIID5, PIID7, PIID-C3 and PIID28.

IID3, IID4 and IID5 (Fig. 5) also exhibited a smaller dihedral angle than IID1. Polymer PIID28⁶⁹ (Fig. 6 and Table 4) based on IID3 exhibits a hole mobility of 0.4 cm² V⁻¹ s⁻¹ and an electron mobility of 0.7 cm² V⁻¹ s⁻¹. For IID3, the incorporation of thienothiophene extended the fused nature of the IID core and resulted in the increment of the π-orbital overlap, which is good for charge transport. As described in Meager's work,⁶⁹ the synthesis procedure of IID3 is similar to that of IID2. Besides, as a similar structure to IID3, IID4 has also been mentioned for a small molecule semiconductor but not a polymer. The OFETs based on it showed a hole mobility of 0.18 cm² V⁻¹ s⁻¹ when annealed under a temperature of 80 °C.⁷⁷ In order to reduce the electron density of thienoisoindigo and keep the fine planarity of its fine backbone, Yue et al.⁷⁸ combined the structure of IID and thienoisoindigo and synthesized IID5 (Scheme 5 and Fig. 5). The fusion of phenyl with TT not only lowered the dihedral angle between the linking repeat units to enhance the π–π stacking interaction between the polymer backbones but also increased the polaron delocalization, resulting in the promotion of efficient charge transport. But the experimental result was not good with a poor hole mobility below 1 cm² V⁻¹ s⁻¹, but with a fine organic photovoltaic (OPV) performance for the polymer based on the acceptor IID5 and the donor thiophene. Such a phenomenon may be caused by the molecular orientation packing, which will be discussed in detail in the later section. The author also demonstrated that a polymer with a high molecular weight has better charge transporting properties than that of a polymer with a low molecular weight.

Table 4 Electronic parameters for high-performance n-type and ambipolar IID- and IID-derivative-based polymer semiconductors

Polymer	Acceptor	Donor	M n (kDa)/PDI	Device structure	HOMO/LUMO (eV)	Max. μh (cm^2 V^−1 s^−1)	Max. μe (cm^2 V^−1 s^−1)	Ref.
PIID9	IID1	BTz	25.0/1.32	BGTC	−5.68/−3.54	—	0.22	88
PIID10	IID12	V	37.6/2.38	TGBC	−6.21/−4.24	—	1.1	83
PIID11	IID12	BT	77.2/3.00	TGBC	−5.72/−4.15	—	1.74	89
PIID12	IID12	TVT	28.07/1.72	BGTC	−5.43/−4.09	—	0.55	90
PIID13	IID12	TAT	23.95/1.83	BGTC	−5.60/−4.11	—	0.13	90
PIID14	IID12	TT	26.5/1.65	BGTC	−5.57/−4.10	—	0.65	90
PIID15	IID14	BT	51.6/2.62	TGBC	−5.80/−4.37	—	3.22	87
PIID16	IID19	BT	25.9/2.7	BGBC	−5.78/−4.09	—	0.18	91
PIID17	IID19	2Tz	14.3.2.5	BGBC	−5.99/−4.18	—	0.017	91
PIID18	IID19	TT	23.5/2.15	BGBC	−5.92/−3.98	—	0.14	92
PIID19	IID1	T	26/2.1	BGBC	−5.57/−3.86	0.04	0.1	93
PIID1	IID1	BT	72.8/1.95	TGBC	−5.31/−3.74	1.27	0.07	94
PIID20	IID1	BTzVBTz	49.6/2.53	BGTC	−5.72/−3.51	0.076	0.04	60
PIID21	IID1	BTzABTz	13.5/2.09	BGTC	−5.68/−3.49	0.03	0.086	60
PIID22	IID1	2Tz	27.2/2.4	TGBC	−5.86/−4.14	0.03	0.022	95
PIID23	IID2	BTz	40/2.25	TGBC	−4.86/−3.73	0.16	0.14	67
PIID24	IID2	TVT	23/1.84	BGTC	−5.38/−3.76	0.12	0.0015	96
PIID25	IID2	CNTVT	38.02/1.87	BGTC	−5.62/−3.82	0.07	0.19	96
PIID26	IID2	TAT	33.02/1.67	BGTC	−5.65/−3.78	0.43	0.03	96
PIID27	IID3	T	30/2.23	TGBC	−4.8/−3.7	0.2	0.2	69
PIID28	IID3	BTz	17/1.59	TGBC	−4.9/−3.9	0.4	0.7	69
PIID29	IID3	BT	20/2.05	TGBC	−4.8/−3.6	0.4	0.1	69
PIID7	IID8	BT	61.1/3.00	TGBC	−5.66/−2.04	0.45	0.47	75
PIID7-C3	IID8	BT	50.6/3.87	TGBC	−5.67/−3.64	2.33	0.78	75
PIID30	IID11	BT	49.8/1.76	BGBC	−5.6/−3.7	0.11	0.035	82
PIID31	IID12	BT3	17.4/1.85	BGTC	−5.63/−4.06	0.37	1.23	84 and 97
PIID32	IID12	BT4	19.4/1.98	BGTC	−5.73/−4.01	0.17	0.70	84
PIID33	IID12	BT5	10.2/1.48	BGTC	−5.68/−4.05	0.0018	0.175	84
PIID34	IID12	TT	20.9/2.86	TGBC	−5.70/−3.83	1.70	1.37	98
PIID35	IID12	BDT	13.9/2.14	TGBC	−5.78/−3.81	0.12	0.11	98
PIID36	IID12	BDS	19.2/2.56	TGBC	−5.74/−3.81	0.10	0.16	98
PIID37	IID13	BT3	10.98/1.86	BGTC	−5.18/−3.94	0.61	0.22	85
PIID38	IID13	TVT2	21.96/2.55	BGTC	−4.96/−3.83	0.45	0.11	86
PIID39	IID13	TVT3	13.69/2.11	BGTC	−5.10/−3.70	0.86	0.19	86
PIID40	IID15	BT	43.1/4.8	BGBC	−5.65/−3.84	0.51	0.50	99
PIID41	IID15	TT	27.6/4.9	BGBC	−5.76/−3.79	0.12	0.14	99
PIID42	IID16	BT	128/2.89	BGBC	−5.60/−3.71	0.19	0.088	100
PIID43	IID16	TVT	158/1.59	BGBC	−5.71/−3.70	0.10	0.075	100
PIID44	IID17	BT	82.1/2.07	TGBC	−5.16/−3.58	0.41	0.18	101
PIID45	IID17	TVT	46.2/5.31	TGBC	−5.24/−3.58	0.45	0.16	101
PIID45-C3	IID17	TVT	35.5/7.18	TGBC	−5.38/−3.71	0.32	0.071	101
PIID46	IID18	BT	21.0/2.3	BGBC	−5.60/−4.06	0.10	0.14	102

---

Scheme 5 Synthesis of a brominated IID5 monomer.

The brominated monomer of IID6 (Fig. 5), as an asymmetric IID-derivative acceptor, was prepared by the acid-promoted condensation of 6-bromo-oxindole and thieno-dioxypyrrole, followed by alkylation and bromination.⁷⁹ The polymer based on the acceptor IID6 and the donor bithiophene (BT) or 2,5-bis(thiophen-2-yl)thiophene (3T) exhibited a hole mobility below 1 cm² V⁻¹ s⁻¹, totally different from the high-performance behaviour of polymers based on the asymmetric DPP acceptors, which can be explained by the different packing orientation as well.

James et al.⁷⁴ coupled two IID6 units via benzene-coupling to obtain the new symmetric acceptor unit IID7 (Fig. 5). In other words, IID7 was a building block for the donor–acceptor based conjugated polymer, which is of a lower LUMO level than IID6. But the corresponding polymer PIID6 (Table 3) formed by the acceptor IID7 and a BT-based donor showed an unexceptional high hole mobility of 1.1 cm² V⁻¹ s⁻¹ when poly(trifluoroethylene) (PTrFE) acted as a dielectric layer for the OFET.

As shown in Scheme 6, 7,7′-diazaisoindigo⁷⁵ (IID8, Fig. 5) and 5,5′-diazaisoindigo⁸⁰ (IID9, Fig. 5) monomers were also synthesized for the acceptor unit of polymer semiconductors in OFETs. Replacement of the aromatic benzene ring with pyridine results in a lower lying LUMO to afford a more electron-deficient acceptor owing to the electron-deficient feature of the pyridinic nitrogen. In addition, the coplanarity of the backbone was enhanced as the dihedral angles between the indolone subunits of 7,7′-diazaisoindigo and 5,5′-diazaisoindigo were reduced. Such effects were proved in theoretical calculations and experiments.^75,80 The polymer PIID7-C3 (Fig. 6) based on the 7,7′-diazaisoindigo acceptor exhibited a favourable hole mobility up to 7.28 cm² V⁻¹ s⁻¹ since the ordered packing of the polymer was caused by the aforementioned fine coplanarity. However, the charge transfer performance of a polymer made from the 5,5′-diazaisoindigo acceptor (IID9) was poor (μ_h = 1.27 × 10⁻³ cm² V⁻¹ s⁻¹)⁸⁰ and even poorer than the IID-based polymer when copolymerized with the same donor, which still needs to be further studied.

Scheme 6 Syntheses of brominated (A) 7,7′-diazaisoindigo and (B) 5,5′-diazaisoindigo monomers.

Li’s research group creatively incorporated the ethane-1,2-diylidene moiety and the hydrazine-1,2-diylidene moiety into the two indolin-2-one subunits of IID.^81,82 The chemical structure and synthesis steps of them are shown in Fig. 7 (IID10 and IID11) and Scheme 7, respectively. The polymer based on IID10 and BT showed a p-type performance but not with a high charge mobility. However, the polymer based on IID11 and BT showed an ambipolar performance since the azine is a strong electron-withdrawing moiety to afford low LUMO levels.

Fig. 7 Chemical structures of IID10 and IID11.

Scheme 7 Syntheses of brominated (A) IID10 and (B) IID11 monomers.

In 2013, Jian Pei⁸³ incorporated the benzodifurandione moiety into the IID molecular backbone to obtain IID12 (Fig. 8, synthesis steps as shown in Scheme 8(A)), which was seen as a poly(p-phenylene vinylene) derivative. The introduction of carbonyl groups reduced the LUMO level of IID12, and more intramolecular hydrogen bonds were formed to “lock” the polymer backbone and ensure the fine molecular packing and crystallinity. As a result, an electron mobility over 1 cm² V⁻¹ s⁻¹ (PIID10 listed in Table 4) was firstly achieved under ambient conditions for polymer semiconductors. Besides, PIID32–PIID36 (Table 4) based on IID12 exhibited ambipolar properties and PIID31⁸⁴ (Fig. 9 and Table 4) exhibited a hole mobility of 0.37 cm² V⁻¹ s⁻¹ and an electron mobility of 1.23 cm² V⁻¹ s⁻¹ among them. Similar to IID12, IID13^85,86 (Fig. 8) and IID14⁸⁷ (Fig. 8) were also reported. The IID13 based polymers PIID37–PIID39^85,86 (Table 4) exhibited ambipolar characteristics. The IID14 based polymer PIID15⁸⁷ (Fig. 9) exhibited a unipolar n-type performance with a high electron mobility of 3.22 cm² V⁻¹ s⁻¹ under ambient conditions owing to the introduction of an electron-deficient sp²-nitrogen. Such an effect was proved in the aforementioned polymers PIID7 and PIID7-C3.

Fig. 8 Chemical structures of IID12−IID19.

Scheme 8 Syntheses of brominated (A) IID12, (B) IID15, (C) IID16, (D) IID17, (E) IID18 and (F) IID19 monomers.

Fig. 9 Chemical structures of PIID31 and PIID15.

Li et al.⁹⁹ substituted the naphthalene ring for a central benzene ring to obtain IID15 (Fig. 8, synthesis steps shown in Scheme 8(B)). The large size of naphthalene endows IID15 with a weaker electron-accepting ability than IID12, which is beneficial for the highly balanced ambipolar properties of the IID15-based polymers PIID40 (Fig. 10 and Table 4) and PIID41 (Fig. 10 and Table 4). Replacement of the central benzodifurandione ring by benzodipyrrole-dione^76,100 (IID16 as in Fig. 8), naphthodipyrroledione¹⁰¹ (IID17 as in Fig. 8) and benzodithiophenedione¹⁰² (IID18 as in Fig. 8) was also studied, and the synthesis steps are displayed in Scheme 8(C), (D) and (E), respectively. For IID16 and IID17 based polymers, the additional N-substituted alkyl chains afford the improvement of solubility of such conjugated polymers in solvent which is beneficial to the processing of the polymers. In 2016, the Li⁹² research group reported a quinoid-type IID derivative (IID19 as in Fig. 8, synthesis methods are shown in Scheme 8(F)). The IID19-based polymers PIID16–PIID18^91,92 (Table 4) exhibited a unipolar n-type semiconductor performance.

Fig. 10 Chemical structures of PIID40 and PIID41.

2.3 Others

Naphthalenedicarboximide (NDI) and perylenedicarboximide (PDI) based polymers were also studied for OFETs. NDI1,^46,103–107 NDI2,¹⁰⁸ NDI3,^109–111 NDI4,¹¹² PDI1,^{103,104,106,113} PDI2¹¹⁴ and PDI3¹¹⁵ (Fig. 11) have been reported as the acceptors. The six-membered dicarboxylic imide rings endow the rylene diimides with electron-withdrawing properties. Therefore, the polymers based on NDI and PDI generally show p-channel or ambipolar behaviour. The polymers based on NDI and PDI and their OFET performance are listed in Table 5. The record high electron mobility of 8.5 cm² V⁻¹ s⁻¹ was achieved for PNDI8¹⁰⁵ (Fig. 11).

Fig. 11 Chemical structures of NDI1–NDI4, PDI1–PDI3 and PNDI6–PNDI9.

Table 5 Electronic parameters for high-performance polymer (except DPPs and IID- and IID-derivative-based polymers) semiconductors

Polymer	Acceptor	Donor	M n (kDa)/PDI	Device structure	HOMO/LUMO (eV)	Max. μh (cm^2 V^−1 s^−1)	Max. μe (cm^2 V^−1 s^−1)	Ref.
PNDI1	NDI1	B	6.81/1.99	BGTC	−5.173/−3.15	—	0.8	103
PNDI2	NDI1	BTz	5.17/1.28	BGTC	−5.14/−3.03	—	0.2	103
PNDI3	NDI1	BT	47.8/5.53	BGTC	−5.36/−3.91	—	0.06	104
PNDI4	NDI1	BT	52.5/5.51	TGBC	—	—	0.45–0.85	46
PNDI5	NDI1	TVT	23.7/2.46	TGBC	−5.62/−3.90	0.3	1.57	107
PNDI6	NDI1	BTz5	36.7/4.05	TGBC	−5.90/−3.77	0.7	3.1	105
PNDI7	NDI1	T2FBTT	48.5/4.57	TGBC	−6.24/−3.85	—	2.2	105
PNDI8	NDI1	SBTzS	39.7/3.72	TGBC	−5.84/−3.81	1.7	8.5	105
PNDI9	NDI1	2FSBTzS	56.1/4.25	TGBC	−6.20/−3.88	—	3.5	105
PNDI10	NDI1	PTZ	9.97/1.53	BGTC	−5.85/−3.75	—	0.05	106
PNDI11	NDI2	2FT	13.6/1.53	BGTC	−5.86/−3.93	0.22	0.05	108
PNDI12	NDI3	BTz	14.4/2.93	BGTC	—/−4.1	—	0.1	109
PNDI13	NDI3	BT	27.1/3.3	BGTC	−5.6/−4.4	0.1	0.27	110
PNDI14	NDI3	TT	19.0/2.6	BGTC	−5.7/−4.1	0.046	0.26	111
PNDI15	NDI3	NTz	15.7/1.73	BGTC	−5.6/−4.2	—	0.21	109
PNDI16	NDI4	V	88.8/2.47	BGTC	−5.73/−3.86	—	0.015	112
PPDI1	PDI1	B	16.08/1.53	BGTC	−5.89/−3.49	—	0.04	103
PPDI2	PDI1	BTz	30.62/1.17	BGTC	−5.49/−3.08	—	0.032	103
PPDI3	PDI1	BT	11.0/2.9	BGTC	−5.61/−3.96	—	0.002	104
PPDI4	PDI1	DTT	—	BGBC	−5.57/−3.59	—	0.034	113
PPDI5	PDI1	ATTTA	15/1.1	BGBC	−5.73/−3.53	—	0.075	113
PPDI6	PDI1	PTZ	11.82/1.57	BGTC	−5.83/−3.79	—	0.05	106
PPDI7	PDI2	T	10.7/1.6	TGBC	−3.70/−5.56	0.04	0.30	114
PPDI8	PDI3	BTz5	5.3/3.4	BGTC	−5.54/−3.72	0.018	0.019	115
PTPD	TPD	2TTT	16/2.06	BGTC	−5.05/—	1.29	—	121
PBTPD1	BTPD	TVT	17.7/3.13	BGTC	−5.65/−3.90	0.087	—	123
PBTPD2	BTPD	TVS	14.5/3.74	BGTC	−5.61/−3.95	0.17	—	123
PBTPD3	BTPD	BTz5	62.5/2.89	BGTC	−5.72/−4.01	0.33	1.02	122
PBTPD4	BTPD	T2FBTT	61.3/4.38	BGTC	−5.85/−4.10	0.16	0.36	122
PBTI1	BTI	BTI	3.59/2.2	BG	−6.28/−3.47	—	>0.01	124
PBTI2	BTI	BT	1.81/1.38	BG	−5.88/−3.04	0.01	—	124
PPHI	PHI	BT2	207.5/—	BGBC	—	0.28	—	126
PNDI	NTDI	BTz5	20.1/2.9	BGBC	−5.58/−3.58	0.82	—	128
PBTz1	BTz1	TTTT	75.6/2.59	TGBC	−5.41/−3.7	8.7	—	131
PBTz2	BTz1	CDT	10.1/2.63	BC	—	0.17	—	159
PBTz3	BTz1	4TR	35/1.63	BGTC	−5.18/—	0.13–0.20	—	160
PBTz4	BTz2	T	27/1.5	BGTC	−5.55/−3.60	0.02	—	161
PBTz5	BTz2	TT	13/2.1	BGTC	−5.37/−3.60	0.26	—	161
PBTz6	BTz5	SiIDT	22.3/2.54	TGBC	−5.2/−3.4	0.28	—	162
PBTz7	BTz6(SF)	BT4	37.0/1.3	BGBC	−5.23/−3.50	0.39	—	163
PBTz8	BTz7(DF)	SiIDT	26.4/2.69	TGBC	−5.2/−3.2	0.19	—	162
PBTz9	BTz8	T	7/2.0	BGTC	−5.24/−3.37	0.007	—	164
PBTz10	BTz9	T	35/2.21	BGTC	−5.20/−3.23	0.019	—	164
PBTz11	BTz10(F)	T	7.5/1.19	BGTC	−5.36/−3.44	1.9	—	164
PNCz1-1	NCz1	BT	52.9/2.1	BGTC	−5.14/−3.46	0.23	—	132
PNCz1-2	NCz1	TNTR	67.3/1.3	BGBC	−5.40/−3.64	0.214	—	133
PNCz2	NCz2	BT	28.7/2.1	BGTC	−5.22/−3.55	0.040	—	132
PNCz3	NCz3	BT	49.7/4.8	BGTC	−5.48/−3.65	0.27	0.17	132
PCDTPT	TPT	CDT	50/—	BGBC	—	52.7	—	134
PBAI1	BAI1	BDT2	41.2/2.47	BGBC	−4.91/−3.63	1.5	0.41	135
PBAI2	BAI1	CZ	68.2/4.13	BGBC	−5.03/−3.65	0.14	0.09	135
PBAI3	BAI2	T	15.7/3.15	TGBC	−4.24/−3.02	0.23	0.48	165
PBAI4	BAI2	S	25/3.32	TGBC	−4.99/−3.77	0.20	0.99	166
PBAI5	BAI2	B	29/3.55	TGBC	−5.14/−3.74	0.002	0.028	166
PBAI6	BAI2	BTz	40/2.98	TGBC	−4.97/−3.74	0.52	3.11	166
PBPD1	BPD	T	37/2.4	BGBC	−5.06/−3.62	0.22	0.16	167
PBPD1-Si	BPD	T	28/2.4	BGBC	−5.01/−3.65	0.74	0.87	167
PBPD2	BPD	S	52/3.1	BGBC	−4.88/−3.65	0.22	0.05	167
PBPD2-Si	BPD	S	13/2.2	BGBC	−4.82/−3.69	0.52	0.35	167
PBPD3	BPD	BT	7.2/2.22	BGBC	−5.0/−3.7	1.24	0.82	137
PBPD4	BPD	TT	4.8/2.08	BGBC	−5.0/−3.6	1.37	—	137
PBPD5	BPD	TVT	7.4/2.32	BGBC	−3.62/1.14	0.245	0.095	168
PBPD6	BPD	TVT2	12.4/2.36	BGBC	−3.65/1.26	0.109	0.081	168
PP661	P661	V	35/2.68	TGBC	−5.74/−3.45	—	0.34	140
PP662	P662	V	20.4/2.62	TGBC	−5.80/−3.52	—	0.51	140
PNTDT	NTDT	BDT2	60.7/3.15	BGTC	−5.33/−3.67	0.0058	—	141
PBFIT	BFI	T	47.5/3.68	BGTC	−3.80/−5.45	—	0.3	130
PBFIBT	BFI	BT	—	BGTC	−3.74/−5.14	—	0.09	130
PBBT1	BBT	TT	15.1/1.94	BGBC	−4.36/−3.80	1.0	0.7	142
PBBT2	BBT	CDT	17/2.8	BGBC	−4.8/−4.0	0.13	0.1	143
PBBT3	BBT	SiDT	18/2.8	BGBC	−4.8/−4.1	0.0025	0.016	143
PBBT4	BBT	FL	11/2.8	BGBC	−5.1/−3.9	0.0017	0.007	143
PBBT5	BBT	PD	14/2.8	BGBC	−4.8/−4.2	0.011	0.011	143
PBBT2-C6	BBT	DPP1	8.7/1.5	BGBC	−4.55/−3.9	0.83	1.36	62
PDPP50	BBT	DPP1	8.8/1.8	BGBC	−4.55/−3.9	1.17	1.32	62
PSeS	SeS	TzFLTz	20.3/3.53	BGTC	−5.34/−3.86	0.0073	0.087	144
PSN	SN	TzFLTz	30.0/2.38	BGTC	−5.21/−3.48	0.65	—	144
PSeN	SeN	TzFLTz	12.6/2.21	BGTC	−5.17/−3.53	0.0017	0.0078	144
PBDTTQ-1	BDTTQ-1	BT3	37.3/2.43	BGTC	−5.50/−3.86	—	—	145
PBDTTQ-2	BDTTQ-2	BT3	11.8/1.66	BGTC	−5.48/−4.01	0.0012	0.0006	145
PBDTTQ-3	BDTTQ-3	BT	76.3/3.63	BGBC	—	0.22	0.21	148
PTTQ	TTQ	D110	17.7/2.21	BGBC	−5.40/−3.91	0.0018	0.0016	146
PPTQ-1	PTQ-1	D110	18.3/3.56	BGBC	−5.50/−3.96	0.028	0.042	146
PPTQ-2	PTQ-2	T	274/6.3	BGTC	−5.17/−4.21	0.03	0.014	147
PPTQ-3	PTQ-3	T	45/6.3	BGTC	−5.21/−4.29	0.0011	0.02	147
PAPhTQ	APhTQ	BT	100.9/6.49	BGBC	—	0.11	0.02	148
PBPP1	BPP	T	37.0/1.13	TGBC	−3.58/−5.34	0.012	0.03	169
PBPP2	BPP	BT	17/2.47	TGBC	−5.63/−4.05	—	0.001	169
PBPP3	BPP	TT	15/3.41	TGBC	−5.68/−4.07	—	0.001	169
PBPP3	BPP	TVT	20/2.28	TGBC	−5.55/−4.06	—	0.002	169
PBPP4	BPP	CDT	21/1.96	TGBC	−5.44/−3.95	—	0.01	169
PBPP5	BPP	BTz	54/1.25	TGBC	−5.84/−3.99	—	0.01	169
PBPP6	BPP	S	74.0/1.74	TGBC	−3.59/−5.32	0.017	0.022	170
PBPP7	BPP	Fu	47.0/1.64	TGBC	−3.60/−5.37	0.0048	0.00058	170
PBPP8	BPP	B	20/2.08	BGTC	−5.39/−3.35	—	0.0024	149
PBPT1	BPT	T	34/1.67	TGBC	−5.27/−4.24	0.2	0.1	150
PBPT2	BPT	BT	5/3.30	TGBC	−5.33/−4.16	0.08	0.01	150
PDPIDT-T	DPIDT	T	25/1.6	TGBC	−5.4/−3.8	0.05	0.04	152
PDPIDT-P	DPIDT	B	106/1.7	TGBC	−5.2/−3.5	0.08	—	152
PDPIDT-2T	DPIDT	BT	10/1.9	TGBC	−5.3/−3.7	0.09	0.04	152
PTPTQD1	TPTQD	BT	74.7/4.97	BGTC	−5.29/−3.55	0.15	—	153
PTPTQD2	TPTQD	TVT	76.9/2.28	BGTC	−5.14/−3.57	0.58	—	153
PPQ2TBT	PQ2T	BT	38.9/3.68	BGBC	—	0.0064	—	154
PQA2T	QA	BT	12/2.5	BGTC	−5.2/—	0.27	—	155
PQA3T	QA	3T	11/1.9	BGTC	−5.2/—	0.24	—	155
PQTE	QA	TVT	20.60/1.15	BGTC	−5.26/−3.38	0.67	—	156
PQB	QA	BTz5	16.2/2.5	BGTC	−5.26/−3.31	0.00577	—	157
PQBOC8	QA	BTz5-OR	40.4/1.8	BGTC	−5.31/−3.06	0.30	—	157
PEPD2T	EPD	BT	10.7/—	BGTC	−5.50/−3.42	0.40	—	158

---

Thieno[3,4-c]pyrrole-4,6-dione (TPD),^116–121 1,1′-bithieno[3,4-c]pyrrole-4,4′,6,6′-tetraone (BTPD),^122,123 bithiopheneimide (BTI),^116,124 thienoisoindoledione (TID),¹²⁵ phthalimide (PHI),^118,126 pyromellitic diimide (PMDI),¹²⁷ and naphthalene tetracarboxylic diimide¹²⁸ (NTDI or iso-NDI) were also reported as the arylene imides for polymer semiconductors. PMDI has the same carboxylic group numbers as NDI and PDI, but the OFET performances of the polymers based on it and a different number of thiophene rings were poor.¹²⁷ Meanwhile, PDI, TPD, BTI, PHI and TID have two fewer carboxylic groups resulting in lower electron-deficiency when compared with NDI and PDI, but they exhibit fine symmetry and planarity, and are rigidly fused, which is beneficial to the enhancement of intermolecular interactions.¹¹⁸ Polymer PTPD (Fig. 12 and Table 5) exhibited a hole mobility of 1.29 cm² V⁻¹ s⁻¹, while polymers based on BTI (PBTI1-2, Table 5) and PHI (PPHI, Table 5) did not show high mobility. TID-based polymer semiconductors have only been mentioned in organic solar cells.¹²⁵ BTPD and NTDI, as the dimeric forms of TPD and PHI, exhibit higher electron-withdrawing characteristics. In addition, the strong O⋯S non-covalent interaction between the TPD subunits endowed the anti-coplanar structure of TPD.¹²³ Among the polymers based on BTPD (PBTPD1–4, Table 5), the highest performance was achieved by PBTPD3 (Fig. 12) with an electron mobility of 1.02 cm² V⁻¹ s⁻¹ and a hole mobility of 0.33 cm² V⁻¹ s⁻¹, when the charge donor was benzothiadiazole (BTz). Tetraazabenzodifluoranthene diimides (BFI, Fig. 12), as the first examples of heterocyclic diimides, were synthesized by Li et al.¹²⁹ Later on, they also constructed the polymers PBFIT (Table 5) and PBFIBT (Table 5) based on BFI, among which, PBFIT exhibited a high electron mobility of 0.3 cm² V⁻¹ s⁻¹ (the highest value for unipolar n-type conjugated polymers before 2013) based on BGTC transistors.¹³⁰

Fig. 12 Chemical structures of TPD, BTI, PHI, BTPD, TID, NTDI, PTPD, BFI, PMDI and PBTPD3.

Benzothiadiazole also played the acceptor role in donor–acceptor polymers when a suitable co-monomer was chosen. BTz1–BTz10 (Fig. 13) are BTz derivatives. Zhang et al.¹³¹ prepared the BTz-based polymer PBTz1. Indacenodithieno-[3,2-b]thiophene of a largely conjugated structure and fine planarity was copolymerized with BTz. Linear alkyl chains connected to indacenodithieno[3,2-b]thiophene gave the polymer PBTz1 (Fig. 13 and Table 5) good solubility in tetrahydrofuran (THF), chloroform, toluene and chlorobenzene. Therefore, PBTz1-based OFETs showed a high-performance with a hole mobility of up to 6.6 cm² V⁻¹ s⁻¹ when annealed at 270 °C and the mobility further increased to 8.7 cm² V⁻¹ s⁻¹ when CuSCN was solution processed to be the electrode. Naphthobischalcogenadiazole (NCz)^132,133 was also studied. Kawashima et al.¹³² investigated the effect of the chalcogen atom on NCz-based conjugated polymers. The results showed that NCz2 (Fig. 13)-based polymer PNCz2 (Table 5) had a poor p-channel behavior caused by the defect of the polymer backbone (the repulsion between the large size of the selenium atom and the adjacent thiophene ring), while the NCz3 (Fig. 13)-based polymer PNCz3 (Table 5) exhibited a good ambipolar property which was even better than that of the naphthobisthiadiazole (NCz1 as in Fig. 13)-based polymer PNCz1 (Table 5). The polymer PCDTPT (Fig. 13 and Table 5) based on TPT (the benzene ring of BTz was replaced by pyridine, Fig. 13) was synthesized as the semiconductor layer for OFETs as reported by Luo and Heeger et al.¹³⁴ They adopted a novel processing method that regulates the self-assembly and arrangement of the polymer chain on nanogrooved substrates through capillary action. The fabricated corresponding PCDTPT based OFETs (BGBC) via such a method showed a hole mobility of 36.3 cm² V⁻¹ s⁻¹ for a channel length of 140 μm, while 52.7 cm² V⁻¹ s⁻¹ for a channel length of 160 μm.

Fig. 13 Chemical structures of BTz1–BTz10, NTz1–NTz3, TPT, PBTz1 and PCDTPT.

Recently, bay-annulated indigo (BAI), an old natural dye, was also studied as the electron-deficient building block. He et al.¹³⁵ firstly synthesized the polymers PBAI1 (Table 5) and PBAI2 (Table 5) based on BAI1 (Fig. 14), and a hole mobility of 1.5 cm² V⁻¹ s⁻¹ and an electron mobility of 0.41 cm² V⁻¹ s⁻¹ were shown for PBAI1 (Table 5). PBAI2–PBAI6 (Table 5) are also polymers based on BAI. Among them, PBAI6 (Fig. 14 and Table 5) exhibited a high electron mobility of up to 3.11 cm² V⁻¹ s⁻¹. Besides, Matthew A. Kolaczkowski et al.¹³⁶ also synthesized the desymmetrized BAI derivative BAI3 (Fig. 14), which was flanked by thiophene on one side and benzene on the other side. But polymers based on BAI3 or other desymmetrized BAI derivatives still have not been reported. As a side note, BAI may be a potential electron-deficient unit for high-performance polymer semiconductors.

Fig. 14 Chemical structures of BAI1–BAI3 and PBAI6.

Pechmann dye, as a planar and polar electron-deficient unit, was firstly incorporated into D–A conjugated polymers in 2014 by Zhang’s group.¹³⁷ The bipyrrolylidene-2,2′(1H,1′H)-dione (BPD, a Pechmann dye derivative) based polymer PBPD3 (Fig. 15 and Table 5) exhibited a narrow bandgap of 1.1 eV and ambipolar behaviour with a hole mobility of 1.24 cm² V⁻¹ s⁻¹ and an electron mobility of 0.82 cm² V⁻¹ s⁻¹. In addition, the polymer PBPD4 (Fig. 15 and Table 5) showed p-channel properties with mobility reaching 1.37 cm² V⁻¹ s⁻¹. Zhang’s group also synthesized polymers PBPD1, PBPD-SiPBPD2, PBPD2-Si, PBPD5, and PBPD6 (Table 5) for polymer FETs, which exhibit a mobility below 1 cm² V⁻¹ s⁻¹. Similar to BPD, BID¹³⁸ and APD¹³⁹ (Fig. 15) were investigated as well but still have not been studied for polymer FETs yet. Finally, Pei¹⁴⁰ and co-workers calculated the HOMO and LUMO levels of different kinds of Pechmann dyes via DFT. According to the calculation result that 6,6-endo-dilactones (P₆₆) is of a lower LUMO level than 5,5-exo-dilactams (N–P₅₅, the central part of BPD), they prepared charge acceptor monomers P₆₆1 (Fig. 15) and P₆₆2 (Fig. 15) and their corresponding polymers PP₆₆1 (Table 5) and PP₆₆2 (Table 5) successfully. Theory calculations were proved by the OFET characterization with a unipolar electron-transporting behaviour (μ_e(PP₆₆1) = 0.34 cm² V⁻¹ s⁻¹ and μ_e(PP₆₆2) = 0.51 cm² V⁻¹ s⁻¹). 3,7-Dithiophen-2-yl-1,5-dialkyl-1,5-naphthyridine-2,6-dione (NTDT as in Fig. 15), which is of a tightly similar structure to P₆₆1 and P₆₆2, was synthesized by Yoon et al.¹⁴¹ But the performance of the polymer PNTDT (Table 5) based on it was very poor. The synthesis steps of the brominated P₆₆1 monomer and the NTDT monomer are shown in Scheme 9.

Fig. 15 Chemical structures of BPD, BID, APD, P₆₆1, P₆₆2, NTDT, PBPD3 and PBPD4.

Scheme 9 Syntheses of brominated (A) P₆₆1 and (B) NTDT monomers.

As shown in Fig. 16, benzobisthiadiazole^62,142,143 (BBT) was studied as the charge acceptor of polymer semiconductors as well. As aforementioned, BBT is of strong electron-withdrawing ability, and it even acted as a charge acceptor when copolymerized with DPP (PBBT2-C6 and PDDP50 listed in Table 5). The BBT-based polymer PBBT1¹⁴² (Fig. 16 and Table 5) showed a nearly balanced ambipolar behaviour with a hole mobility of 1.0 cm² V⁻¹ s⁻¹ and an electron mobility of 0.7 cm² V⁻¹ s⁻¹ owing to the low bandgap caused by the strong D–A interaction. Besides, as the side alkyl chain added to neighboring BT heterocycles endows benzobisthiadiazole with processable properties, many other ambipolar polymers based on BBT were synthesized (PBBT2–PBBT5 as listed in Table 5). But their OFET performance still needs to be improved. Subsequently, Wang et al.¹⁴⁴ prepared a series of benzobisthiadiazole analogues including selenadiazolo-benzothiadiazole (SeS as in Fig. 16), thiadiazolobenzotriazole (SN as in Fig. 16) and selenadiazolobenzotriazole (SeN as in Fig. 16) via heteroatom substitution and copolymerized them with the same donor (TzFLTz listed in Table 5), respectively. Such polymers did not show promising OFET performance, but the charge polarity of the polymers was proved to be influenced as the polymers PseS (Table 5) and PseN (Table 5) exhibit ambipolar properties while PSN (Table 5) showed p-channel charge transporting behaviour.

Fig. 16 Chemical structures of BBT, SeS, SN, SeN, BDTTQ-1–BDTTQ-3, TTQ, PTQ-1–PTQ-3, APhTQ, and PBBT1.

Thiadizoloquinoxaline (TQ), as another structure containing thiadiazole, was also investigated. Compared to the BBT acceptor, TQ acceptors have more sites to be alkylated, which will influence the solubility, structure, and packing of the corresponding polymers. As shown in Fig. 16, BDTTQ-1, BDTTQ-2 and BDTTQ-3 are of the same core structures but of different side chain distributions and linking pathways for polymers. PBDTTQ-2¹⁴⁵ (Table 5) based on BDTTQ-2 exhibited ambipolar properties while PBDTTQ-1¹⁴⁵ (Table 5) based on BDTTQ-1 did not show any field-effect response. Besides, such a phenomenon was also observed for PTQ1–PTQ3 (Fig. 16) based polymers. More recent studies found that the polymer PPTQ1¹⁴⁶(Table 5) based on PTQ1 and the polymer PPTQ2¹⁴⁷ (Table 5) based on PTQ2 exhibited a balanced ambipolar property while PPTQ3¹⁴⁷ showed an accessible n-type behavior.

Baumgarten et al.¹⁴⁶ also made a comparison between the TTQ acceptor (Fig. 16) and the PTQ-1 acceptor (Fig. 16) when they copolymerized them with the same donor (thiophene derivatives). The results showed that the polymer PPTQ-1 (Table 5) based on PTQ-1 exhibited ambipolar properties, which were an order of magnitude larger than those of the polymer PTTQ (Table 5) based on TTQ. Apart from that, researchers also compared the polymer PBDTTQ-3¹⁴⁸ (Table 5) based on BDTTQ-3 (Fig. 16) with the polymer PAPhTQ (Table 5) based on AphTQ (Fig. 16). The results suggested that PBDTTQ-3 had the best OFET performance of the AQ-based polymers with a hole mobility up to 0.22 cm² V⁻¹ s⁻¹ and an electron mobility of 0.21 cm² V⁻¹ s⁻¹.

Benzodipyrrolidone (BPD), as the elongated DPP, has also been explored. In 2011, a phenyl-flanked benzodipyrrolidone (BPP in Fig. 17) based polymer for OFETs was synthesized by Cui et al.¹⁴⁹ Later on, a series of polymers (PBPP1–PBPP8, as listed in Table 5) based on BPP were investigated, but all of them exhibited poor performance. Similar to the studies in DPP, the polymers PBPT1 (Table 5) and PBPT2 (Table 5) based on thiophene-flanked benzodipyrrolidone (BPT, Fig. 17) were firstly introduced by Rumer et al.¹⁵⁰ Both of them showed ambipolar behaviour, and a hole mobility of 0.2 cm² V⁻¹ s⁻¹ and an electron mobility of 0.1 cm² V⁻¹ s⁻¹ were observed for the OFETs based on PBPT1. Meanwhile, dihydropyrroloindoledione (DPID), as another unit structurally related to DPP, was explored by Rumer et al.^151,152 as well. Thiophene-flanked DPID (DPIDT as in Fig. 17)-based polymers PDPIDT-T (Table 5) and PDPIDT-2T (Table 5) possessed ambipolar behaviour while the polymer PDPIDT-P (Table 5) showed p-type semiconductor characteristics. However, the TT-flanked DPID (DPIDTT, Fig. 17) based polymer (copolymerized with thiophene) was too insoluble to be further studied. Pei et al.¹⁵³ also synthesized a pentacyclic aromatic lactam, thieno[2′,3′:4,5]pyrido[2,3-g]-thieno[3,2-c]quinoline-4,10(5H,11H)-dione (TPTQD, Fig. 17), via Beckmann rearrangement. The polymers PTPTQD1 (Table 5) and PTPTQD2 (Table 5) exhibited p-channel semiconductor behaviour with a mobility of 0.15 cm² V⁻¹ s⁻¹ and 0.58 cm² V⁻¹ s⁻¹, respectively. Similarly, Li¹⁵⁴ and co-workers reported polymers consisting of a kind of aromatic lactam unit – pyrimido[4,5-g]quinazoline-4,9-dione (PQ). But the polymer PPQ2TBT (Table 5) based on PQ2T (Fig. 17) exhibited poor p-type behaviour with the mobility at a magnitude of 10⁻³ cm² V⁻¹ s⁻¹.

Fig. 17 Chemical structures of BPP, BPT, DPIDT, DPIDTT, TPTQD, QA, PQ-2T and EPQ.

Quinacridones (QAs, Fig. 17), as red-violet pigments, were investigated as well. Osaka¹⁵⁵ and co-workers firstly incorporated the quinacridone unit into polymers for OFETs. Researchers found that the orientational order of the polymer PQA2T was improved by the enhancement of molecular weight and a small π–π stacking distance of 3.6 Å was achieved. Nevertheless, the hole mobility of the polymer was not sensitive to the varying molecular weights, which may be caused by the relatively limited delocalization of the HOMO and π-conjugation. The polymer PQA2T (Table 5) exhibited a hole mobility of up to 0.27 cm² V⁻¹ s⁻¹ and PQA3T (Table 5) showed a hole mobility of up to 0.24 cm² V⁻¹ s⁻¹. Then, the polymers PQTE,¹⁵⁶ PQB¹⁵⁷ and PQBOC8,¹⁵⁷ as listed in Table 5, were also further studied. Among them, PQTE exhibited the highest hole mobility of 0.67 cm² V⁻¹ s⁻¹.

Epindolidione (EPD, Fig. 17), as an isomer of IID and of a similar structure to quinacridone, was firstly incorporated into conjugated polymers for OFETs by Pei et al.¹⁵⁸via the effective synthesis method (Scheme 10). The corresponding polymer PEPD2T (Table 5) showed p-type semiconductor features with a mobility of up to 0.40 cm² V⁻¹ s⁻¹.

Scheme 10 Syntheses of the brominated EPD monomer.

3. Backbone halogenation

3.1 Fluorination

Halogen, cyanogroup and diimide moieties are the commonly electron-deficient groups used for the n-type semiconductors for OFETs. Among them, fluorine substitution on the conjugated polymer has attracted more attention because of the special properties of the fluorine atom, e.g. a small atom size slightly larger than hydrogen (r_{van der Waals}(F) = 1.35 Å, r_{van der Waals}(H) = 1.2 Å) and the highest Pauling electronegativity of 4.0.¹⁷¹ Due to its small size, it does not cause steric hindrance enhancement but does affect the crystallinity and orientation of the polymer. As shown in Fig. 18, through comparing poly(3-alkyl-4-fluoro)thiophenes (F-P3ATs) and poly(3-alkyl)-thiophenes (P3ATs), Fei et al.¹⁷² found that the ionization potential of the polymer could be enhanced without a significant change in the optical band gap by backbone fluorination. Besides, because the polymer had a more co-planar backbone after fluorination, the average charge carrier mobility of the fluorinated polymer F-P3ATs was 5 orders of magnitude higher than the analogous non-fluorinated polymers P3ATs. Boufflet et al.¹⁷³ also theoretically and experimentally studied the effect of fluorination on the molecule backbone planarity and OFET performance via a comparison between poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT as in Fig. 18) and poly(2,5-bis(4-fluoro-3-hexadecyl-thiophen-2-yl)thieno[3,2-b]thiophene) (PFBTTT as in Fig. 18). They illustrated that the physical properties of the polymer were strongly affected by backbone fluorination, including a significant enhancement of the aggregation degree and melting temperature owing to the F⋯S interactions leading to an increase in backbone rigidity and planarity. In addition, the hole mobility in the OFET devices and ambient stability were also improved through backbone fluorination.

Fig. 18 Chemical structures of P3ATs, PBTTT, F-P3ATs and PFBTTT.

Following the above-mentioned fluorothiophene studies, difluorothiophene was also used as the building block for fluorinated conjugated polymers. In 2015, in order to explore the effect of fluorination of the co-monomer on the charge carrier mobility, optical properties, and crystallinity of different aryl-flanked DPP based polymers, Mueller et al.³⁹ prepared six polymers (PDPPT, PFDPPT, PPyDPPT, PFPyDPPT, PPhDPPT and PFPhDPPT, as shown in Fig. 19) based on DPP having varying flanking aryl units and thiophene or difluorothiophene as the co-monomer. They found that the crystallinity of the DPP-based polymers could be improved by fluorination irrespective of the different flanking aryl units. As shown in Fig. 19, it was also realized that electron transport of such polymers was deeply affected by fluorination while the bulk hole mobility was insensitive to such structural variations. This conclusion was also proved by Gao et al.⁵⁹ The fluorinated polymer PFDPPTVT (Fig. 19) exhibited a hole mobility of 3.4 cm² V⁻¹ s⁻¹ and an electron mobility of 5.86 cm² V⁻¹ s⁻¹, while the nonfluorinated analogue PDPPTVT only showed p-channel transport with a hole mobility of 3.05 cm² V⁻¹ s⁻¹. This can be explained by the fact that the LUMO level of polymer semiconductors can be reduced by fluorination contributing to easier electron injection from the electrode in OFET devices.¹⁷¹ To further check whether the effect of fluorination on DPP based polymers is positive or negative, Li et al.¹⁷⁴ synthesized another six polymers (PDPP4T, PFDPP4T, PDPP4T-M, PFDPP4T-M, PDPP4T-2M and PFDPP4T-2M, as shown in Fig. 19) with variation of the flanking aryl units and BT or difluorobithiophene as the co-monomer. The results showed that the fluorinated DPP polymers exhibit a much lower hole mobility than the nonfluorinated analogues caused by the more “face-on” orientation. Therefore, the influence of fluorination on DPP-based polymers still needs to be deeply studied.

Fig. 19 Chemical structures of DPP-based fluorinated polymers.

Prior to difluorothiophene as the fluorinated building block for polymer semiconductors, a fluorinated phenyl unit was also incorporated into the DPP-based polymer backbone. In 2013, Park et al.¹⁷¹ prepared fluorinated polymers using a fluorinated phenyl unit for n-type semiconductors, for the first time. As shown in Fig. 19 (PDPPPhF0–PDPPPhF4), as the number of fluorine substitutions on the phenylene varies from zero to four, the LUMO level of the corresponding polymers reduced from −3.56 eV to −3.64 eV and the charge transport behaviour slightly changed from p-channel to n-channel. Then PDPPPhF₄ showed a high n-type performance with an electron mobility of up to 2.00 cm² V⁻¹ s⁻¹, resulting from the effective charge hopping induced by the unique structural and electrochemical properties. Such a phenomenon was also observed in polymers PDPPTPT, PDPPTPF₂T and PDPPTPF₄T by Mueller et al.¹⁷⁵ Zhang et al.¹⁰ made a comparison between two series of DPP based polymers (P2DFEP-n and P3DFEP-n as in Fig. 19) with two different kinds of difluorodiphenylethene. The results showed that the diverse fluorination substitution positions on diphenylethene caused a different OFET performance of the polymers because of the distinct intra- and intermolecular interactions and backbone conformations. P2DFPE-n containing the F⋯H–C non-covalent interaction (conformation locks) showed a higher hole mobility than P3DFPE-n containing both F⋯H–C and F⋯S non-covalent interactions (conformation locks). There are also many other non-covalent interactions existing in polymers. Besides, P3DFPE-n exhibited a higher HOMO level than P2DFPE-n, which had a non-negligible impact on hole injection of the devices as well. Kim et al.¹⁷⁶ investigated the different effect of the O⋯S and F⋯S interactions on the DPP-based polymers through the introduction of the fluorine atoms or methoxy groups into the backbone. It was found that the fluorinated polymer P8DTB-F (Fig. 19) containing the F⋯S interaction exhibited a predominantly edge-on orientation with high crystallinity and a hole mobility of 1.32 cm² V⁻¹ s⁻¹ while P8DTB-M (Fig. 19) (contain the O⋯S interaction) showed principally a face-on orientation with low hole mobility which is even lower than the non-functionalized polymer P8DTB-H (Fig. 19) of bimodal orientation.

The above-mentioned fluorination process belongs to the charge donor fluorination. Besides, charge acceptor fluorination has also been reported. Compared to the charge donor fluorination, it may be more difficult to fluorinate the charge acceptors in terms of their complicated chemical structures. Fluorinated IID-based polymers were investigated in 2012. Ting Lei⁹⁴ observed the ambipolar transport behaviour of the IID-based polymer semiconductors for the first time via fluorinating the IID core to reduce the bandgap and HOMO–LUMO levels. The electron mobility of the IID-based polymers was improved from 0.07 cm² V⁻¹ s⁻¹ to 0.43 cm² V⁻¹ s⁻¹, while the hole mobility was maintained at the same level after fluorination (PII2T and PFII2T in Fig. 20). Characterization by grazing incident X-ray diffraction (GIXRD) and tapping-mode atomic force microscopy (AFM) also revealed the different interchain interactions and stronger crystalline tendencies caused by fluorination of the IID core. Additionally, there are also some donor fluorinations reported for IID-based polymers. Jo et al.¹⁷⁷ prepared two kinds of IID-based polymers with the fluorinated donor of difluorobenzothiadiazole (PIDT2FBT as in Fig. 20) and tetrafluorobenzene (PIDT4FBZ as in Fig. 20). But both of them did not exhibit high performance. Then, Hu et al.¹⁷⁸ made a comparison between fluorination on the donor unit and fluorination on the acceptor unit for the IID-quarterthiophene-based D–A semiconductor polymers (PID4T, PIDFF4T and PFFID4T as in Fig. 20). It was shown that fluorination on the donor unit resulted in stronger temperature-dependent aggregation than fluorination on the acceptor unit. But the OFET performance was not mentioned. Subsequently, Gao et al.¹⁷⁹ obtained six donor–acceptor conjugated polymers (PIDTVTF₀–PIDTVTF₆, PIDTVTF₆–C3 and PID2TF₆–C3 as in Fig. 20) based on the IID core through direct arylation polycondensation (DAP) and checked the impact of multifluorination on the molecular properties. It was demonstrated that more fluorine atoms were introduced into the backbone, and the IID-based polymers showed deeper HOMO (from −4.88 eV to −5.25 eV) and LUMO (from −2.96 eV to −3.36 eV) levels, which resulted in high-performance ambipolar (PIDTVTF₆: μ_h = 3.9 cm² V⁻¹ s⁻¹, μ_e = 3.5 cm² V⁻¹ s⁻¹) and unipolar n-type (PIDTVTF₆–C3: μ_e = 4.97 cm² V⁻¹ s⁻¹) semiconductors. Isoindigo-based polymers with different backbone conformations were also studied via different fluorination positions. As shown in Fig. 20, FBDPPV-1 and FBDPPV-2 of different backbone conformations were synthesized by Lei et al.¹⁸⁰ Both of them showed n-type ambient-stable behaviour owing to the low-lying LUMO levels. In comparison, FBDPPV-2 (E_HOMO = −6.22 eV, E_LUMO = −4.30 eV) showed slightly lower HOMO and LUMO levels than FBDPPV-1 (E_HOMO = −6.19 eV, E_LUMO = −4.26 eV). The electron mobility of FBDPPV-1 was higher than that of the nonfluorinated analogue FBDPPV while FBDPPV-2 exhibited a lower electron mobility than FBDPPV. Such a phenomenon could be attributed to the different backbone conformations leading to the different interchain interactions and film microstructures.

Fig. 20 Chemical structures of IID- and IID-derivative-based fluorinated polymers.

Other fluorinated polymers based on monofluorobenzothiadiazole (FBT) and difluorobenzothiadiazole (2FBT) were reported in numerous studies. As shown in Fig. 21, the FBT-based polymer PTh4F₁BT¹⁸¹ and the 2FBT-based polymers PTh4F₂BT-1¹⁸² and PTh4F₂BT-2¹⁸³ and the nonfluorinated analogue PTh4BT¹⁶⁰ with the same donor were explored. The results showed that the non-covalent interaction and the solid-state order were enhanced by fluorine substitution, eventually resulting in an improvement in OFET performance. Such an improvement was also observed in P(BDT-TT-FBT) (Fig. 21) when compared with the nonfluorinated analogue (P(BDT-TT-HBT) as in Fig. 21).¹⁸⁴ However, when the co-monomer was cyclopentadithiophene¹⁸⁵ (CDT, the corresponding polymers PBT, PDF, PRF and P2F as in Fig. 21), indacenodithiophene¹⁸⁶ (IDT, the corresponding polymers PIDTDTBT and PIDTFDTBT as in Fig. 21) or silaindacenodithiophene¹⁶² (SiIDT, the corresponding polymers SiIDT-BT, SiIDT-2FBT, SiIDT-DTBT and SiIDT-2FDTBT as in Fig. 21), the 2FBT-based polymer exhibited a lower hole mobility than the nonoflurinated analogues. This opposite phenomenon was due to the solid-state order enhancement and HOMO–LUMO level reduction caused by fluorination. Supposedly, for polymer fluorination, when the effect of the solid-state order enhancement is greater than the effect of the HOMO–LUMO level reduction, the hole mobility will be increased. In contrast, the hole transport is restricted as the reduction effect of the HOMO–LUMO levels is greater than the solid-state order enhancement effect. Additionally, a decrease in both hole mobility and electron mobility was also characterized for the fluorination of some polymers. As shown in Fig. 21, the hole mobility and electron mobility of the polymer PBTPDFDTBT¹²² were 0.16 and 0.36 cm² V⁻¹ s⁻¹, respectively, which were lower than the nonfluorinated polymer PBTPDDTBT¹²² with a hole mobility of 0.33 cm² V⁻¹ s⁻¹ and an electron mobility of 1.02 cm² V⁻¹ s⁻¹. And the high-performance polymers PNBT (Fig. 21) and PNBS (Fig. 21) based on NDI and BTz aforementioned exhibited a much lower performance after fluorination. In addition, high-performance with a hole mobility of 9.05 cm² V⁻¹ s⁻¹ (PDFDT, Fig. 21) and high bias-stress stability were approached when P(VDF-TrFE) containing fluorine was used as the dielectric layer of OFETs (TGBC) for 2FBT-based polymers.¹⁸⁷ This favourable hole mobility resulted from the manipulation of energy levels via fluorinated ferroelectric dielectrics and the fluorinated semiconductor PDFDT, which could reduce the Fermi level and the gap between the Fermi level and the HOMO.

Fig. 21 Chemical structures of fluorinated polymers.

3.2 Chlorination

In addition to fluorination, chlorination was also introduced into the polymer semiconductor backbone. Compared with the fluorine atom, the chlorine atom has a larger size which may cause steric hindrance effects severely. Thus, reports on the chlorination of the polymer semiconductor backbone are rarer than those on fluorination. In 2015, Pei et al.¹⁸⁸ compared the effects of fluorination and chlorination in IID-based polymers on the phase separation and molecular orientation. The results showed that the polymers containing the fluorinated IID core (FIIDT, Fig. 22) exhibited a higher OFET performance but a lower OPV performance than the polymers containing the chlorinated IID core as less crystallinity and face-on orientation were induced by the large size of chlorine and the large torsional angle of the backbone. Before that, Pei and co-workers first synthesized two polymers (PCII2T and PCII2Se as in Fig. 22) based on chlorinated IID in 2013.¹⁸⁹ They found that the phenylthienyl dihedral angles were larger after chlorination, whereas OFETs based on these types of polymers exhibit an ambipolar behaviour with an increased electron mobility and ambient stability. Later on, chlorinated polymers (PTIIT-T and PTII-TVT-8 as in Fig. 22) and small molecules (C20-DBTII and C20-DBFII as in Fig. 22) based on the IID derivative were also reported by Wan et al.^190,191 But their electrical properties were not good. Following the chlorination of polymers based on IID or its derivative, chlorinated polymers composed of NDI via the incorporation of dichlorothiophene were studied.¹⁹² As shown in Fig. 22 (P(NDI2OD-T2), P(NDI2HD-T2), P(NDI2OD-T2Cl2) and P(NDI2HD-T2Cl2)), chlorination of NDI-thiophene based polymers was detrimental to the OFET performance with a reduction in charge mobility. In summary, since few examples of chlorination of conjugated polymers have been investigated, the detailed effects of the introduction of chlorine still need to be further studied.

Fig. 22 Chemical structures of chlorinated polymers and small molecules C20-DBTII and C20-DBFII.

4. Side chain engineering

Due to the poor solubility following the rigid backbone of π-conjugated polymers, flexible side chains are always introduced into the polymers. Different side chains consisting of alkyl chains, oligo(ethylene glycol) chains, siloxane-terminated side chains, carbosilane chains, fluoroalkyl chains and so on lead to the different molecular packing distances, crystallinities and interchain interactions, eventually resulting in different OFET performances. Additionally, the form, length, density, distribution and branching position of side chains also affects the property of the polymers.

4.1 Alkyl chains

Alkyl chains are the most widely used side chains to improve the solubility of the conjugated polymers, which have two forms including linear chains and branched chains. In 2006, Park et al.¹⁹³ investigated the effect of different linear alkyl side chain lengths (–(CH₂)_nCH₃, n = 3, 5, and 7) on the molecular ordering and OFET behaviour of P3AT. According to the Fourier transform infrared (FT-IR) spectra, they found that more ordered side chains were observed following an increase in length, whereas the charge transport behaviour decreased as the alkyl side chain length was increased. Poly(3-butylthiophene) (P3BT) showed 20 times higher hole mobility than poly (3-octylthiophene) (P3OT). However, such a trend was suitable for untreated SiO₂ but not for octyltrichlorosilane-treated (OTS) SiO₂ transistor dielectric layers. For the latter, the mobility of P3AT with longer side chains could still maintain high values because of the weak interaction between the dielectric surface and side chains and better molecular order, which was confirmed by McCullough et al.¹⁹⁴ They demonstrated that the competing processes between backbone packing (π-stacking) and side-chain packing during conjugated polymer self-assembly would result in polymorphic behaviour and disorder. And the mobility of P3ATs with longer side chains was more easily affected by surface treatment than that of P3ATs with shorter side chains.

Compared with linear side chains, branched side chains have attracted more attention from researchers as they can provide better solubility for polymers. Reichmanis et al.¹⁹⁵ compared the properties of DPP-based polymers with linear chains and branched chains. As shown in Fig. 23, the polymers PTBTD-2DT and PTBTD-5DH(H) with branched side chains exhibited higher mobility than PTBTD-OD with a linear side chain because of the improved solubility.

Fig. 23 Chemical structures of PTBTD-OD, PTBTD-2DT, and PTBTD-5DH(H).

In 2013, Kim et al.⁸ firstly extended the branching point of branched side chains in the DPP-based polymer to achieve an unprecedented record high hole mobility of 12 cm² V⁻¹ s⁻¹ at that time. Subsequently, they systematically investigated the branched side chain effect on the OFET performance of DPP-SVS polymers (Fig. 24).¹⁹⁶ According to the result, the electrical performance of such polymers was sensitive to the branching point of the side chains (or lengths of spacer groups) and its odd–even characteristics. Polymers (25-DPP-SVS, 27-DPP-SVS and 29-DPP-SVS as in Fig. 24) with an even number of carbon atoms in the spacer group showed one order of magnitude higher mobility than the polymers (26-DPP-SVS and 28-DPP-SVS as in Fig. 24) with an odd number of carbon atoms in the spacer group when the number was below six. The highest mobility was observed for the 29-DPP-SVS with a hole mobility up to 13.90 cm² V⁻¹ s⁻¹, a lamellar distance of 26.4 Å and a π-stacking distance of 3.60 Å, whose carbon number of the spacer group was six. As shown in Fig. 24, similar research studies have been reported for polymers IIDTs,⁶⁵ BDPPVs,¹⁹⁷ PTTD4Ts¹⁹⁸ and PQA2Ts.¹⁹⁸ Bao et al.⁷⁰ firstly proved that reduction of the π-stacking distance could increase the charge mobility in conjugated polymers. And the π-stacking distance can be adjusted by the length of the spacer group. For IID and its derivative based polymers IIDTs and BDPPVs, the highest charge transport as well as the low π-stacking distance was observed when the number of carbon atoms in the spacer group was three. For thienothiophenedione- and QA-based polymers PTTD4Ts and PQA2Ts, the π-stacking distance reduced as the spacer group became longer, leading to suppressed steric hindrance. Therefore, there was a clear trend observed that the hole mobility of PQA2Ts increased with a longer group.

Fig. 24 Chemical structures of DPP-SVSs, IIDDTs, BDPPVs, PTTD4Ts and PQA2Ts with different branching position alkyl chains.

4.2 Oligo(ethylene glycol) chains

Differing from hydrophobic alkyl chains, oligo(ethylene glycol) (OEG) chains are hydrophilic and more flexible due to the ether moieties. They have also been reported for semiconductors as the side chains. In 2015, Wang et al.¹⁹⁹ compared the properties of polymers with alkyl side chains (PFDTOBT as in Fig. 25) and OEG side chains (PFDTOBT-O2, PFDTOBT-O3 and PFDTOBT-O4 as in Fig. 25). It was revealed that the π-stacking distance became shorter when OEG chains substituted for alkyl chains. Thus, PFDTOBT-O2, PFDTOBT-O3 and PFDTOBT-O4 showed a higher charge mobility, a narrower bandgap and a red-shifted absorption spectrum in a thin film than PFDTOBT. But the mobility decreased when the length of the OEG side chains was increased, whereas the π-stacking distance did not change. Such mobility reduction resulted from a more inert component following the ORG chain length growth. Meanwhile, the hybrid linear chain consisting of an alkyl chain segment and an OEG chain segment were explored by Cho and co-workers.²⁰⁰ As shown in Fig. 25, PNDI-OR and PNDI-RO were with different adjacent chain structures. The polymer PNDI-RO whose backbone was adjacent to the OEG moiety exhibited a high unipolar n-type property with a mobility up to 1.64 cm² V⁻¹ s⁻¹. They also demonstrated that the order of the microstructure was dependent on the adjacent side chain, but not the rigid conjugated backbone. Before that, Patil et al.²⁰¹ adopted another manner of hybrid chains. Two adjacent DPP units, among which one unit was attached to the alkyl side chains and the other unit was attached to the OEG side chains, were used for the synthesis of the conjugated polymer N-CS2DPP-OD-TEG (Fig. 25). Such a polymer showed a wider adsorption behavior (up to ∼1100 nm) and an n-channel semiconductor property with a mobility up to 3 cm² V⁻¹ s⁻¹. The introduction of OEG side chains not only afforded good solubility of the polymer but also induced the chain to be crystallized spontaneously.

Fig. 25 Chemical structures of PFDTOBTs, PNDI-OR, PNDI-RO and N-CS2DPP-OD-TEG.

4.3 Siloxane-terminated side chains

Siloxane-terminated side chain, as a novel hybrid solubilizing chain, was introduced into a conjugated polymer by Bao et al.⁷⁰ for the first time. The synthesis of a brominated IID monomer with siloxane-terminated side chains is shown in Fig. 26(A). Siloxane-terminated side chains are more flexible chains because of the longer length of the Si–O bond (L_Si–O = 1.64 Å) and the larger angle of the Si–O–Si bond (A_Si–O–Si = 143°) compared with the C–C bond (L_C–C = 1.53 Å) and the usual tetrahedral angle (A_C–C–C ≈ 110°).^70,202 As a result, the IID-based polymer PII2T-Si (Fig. 26(B)) with siloxane-terminated side chains was provided with better solubility, a lower π-stacking distance and a larger crystalline coherence length, eventually leading to a higher charge mobility than that of the polymer PII2T (Fig. 26(B)) with branched alkyl side chains. After that, Yang et al.⁵³ introduced siloxane-terminated side chains via N-alkyl. As shown in Fig. 26(B), the polymer PTDPPSe-Si with siloxane-terminated side chains was also accompanied by a closer π-stacking distance of 3.6 Å relative to the polymer PTDPPSe with the alkyl chain. The solution-sheared thin film of PTDPPSe-Si also exhibited a higher ambipolar behaviour with a hole mobility of 3.97 cm² V⁻¹ s⁻¹ and an electron mobility of 2.20 cm² V⁻¹ s⁻¹ in comparison with PTDPPSe. Yang et al.²⁰³ also synthesized a polymer with siloxane-terminated side chains based on DPP and TT, and a remarkably high stability and hole mobility of over 4.5 cm² V⁻¹ s⁻¹ were approached. Besides, the introduction of siloxane-terminated side chains into the BPD-based polymer resulted in an improved OFET performance.¹⁶⁷

Fig. 26 (A) Synthesis of the brominated IID monomer with siloxane-terminated side chains. (B) Chemical structures of PII2T, PII2T-Si, PTDPPSe and PTDPPSe-Si.

The length of the alkyl spacer between the bulky siloxane group and the conjugated backbone was studied as well for DPP-based polymers (PTDPPSe-Sis as in Fig. 27) and IID-based polymers (PII2F-Sis as in Fig. 27). Yang et al.⁵² suggested that PTDPPSe-SiC5, whose carbon number of the alkyl spacer was five, exhibited the best ambipolar performance with a hole mobility of up to 8.84 cm² V⁻¹ s⁻¹ and an electron mobility of up to 4.34 cm² V⁻¹ s⁻¹ through a solution-shearing method.

Fig. 27 Chemical structures of PTDPPSe-Sis and PII2F-Sis.

Bao et al.⁷² demonstrated that the polymer PII2F-SiC9, whose carbon number of the alkyl spacer was nine, showed the highest value for the bulky siloxane group for good solubility (better than PTDPPSe-SiC4) as well as dense microstructures (denser than PTDPPSe-SiC6). Additionally, for IID-based polymers, the thin-film microstructure, crystallinity and domain size but not the length of the spacers played a more important role for thin film charge transport characteristics when the carbon number of the alkyl spacer was three and over three.

4.4 Carbosilane side chains

Compared with siloxane-terminated side chains, carbosilane chains, as chains also containing the silicon element, have been rarely reported in conjugated polymers for OFETs. In 2016, Chen et al.²⁰⁴ synthesized high mobility, stretchable, and mechanically stable polymer semiconductors via introduction of carbosilane side chains into a IID-based polymer (Fig. 28). When the length of the carbosilane chains increased, the lamellar spacing of the polymer increased and the π-stacking distance decreased. Thus, the polymer SiC-PII2T-C8 achieved a high hole mobility of up to 8.06 cm² V⁻¹ s⁻¹. In addition, it still had a mobility over 1 cm² V⁻¹ s⁻¹ under 60% strain and could be simultaneously operated over 400 stretching/releasing cycles. Generally, these novel carbosilane side chains may be a potential choice for high-performance semiconductors in future studies.

Fig. 28 (A) Synthesis of carbosilane chains. (B) Chemical structures of SiC-PII2T-C6 and SiC-PII2T-C8.

4.5 Fluoroalkyl chains

As aforementioned, the fluorine element was positioned adjacent to the conjugated backbone directly with the aim of adjusting the conformation of the backbone and the energy levels. Since fluoroalkyl chains possess many special properties including rigidity, thermal stability, hydrophobicity, chemical and oxidative resistance and self-assembly ability, such chains were also studied as soluble side chains especially for n-type organic semiconductors.²⁰⁵ In 2013, Kim et al.²⁰⁶ utilized semifluorinated alkyl chains for the substitution of hydrocarbon alkyl chains of PQT12 (Fig. 29), which resulted in the self-assembly of the polymer SFA-PQT (Fig. 29) via the fluorophobic effect accompanied by the improvement of mobility without a thermal annealing process. Cho and co-workers²⁰⁵ introduced a semifluorinated alkyl chain into NDI-based polymers. The polymers PNDIF-2T (Fig. 29) and PNDIF-TVT (Fig. 29) with the semifluorinated alkyl side chain exhibited a well-ordered microstructure better than the analogues with hydrocarbon alkyl side chains as semifluorinated side chains induced rigid backbone organization of the conjugated polymer due to their strong self-organization. The results obtained from 1-chloronaphthalene(CN)-processed polymers PNDIF-2T and PNDIF-TVT exhibited a high electron mobility of up to 6.50 cm² V⁻¹ s⁻¹ and 5.64 cm² V⁻¹ s⁻¹, respectively, with a high on–off current ratio of 10⁵. Later on, the DPP unit with semifluorinated side chains was also investigated. As shown in Fig. 29, the polymer PFDPP-BT with semifluorinated side chains showed 3 times higher hole mobility than PDPP-BT with hydrocarbon alkyl chains. AFM and X-ray diffraction (XRD) characterization also confirmed the positive function of semifluorinated alky chains for the thin film morphology and crystallinity. As there are still few reports focused on semifluorinated alkyl chains or even on per-fluorinated side chains,²⁰⁷ which have been mentioned in OPVs, the effect of fluoroalkyl side chains on polymer semiconductors for OFETs stills needs to be further explored.

Fig. 29 Chemical structures of PQT12, SFA-PQT, PNDIF-2T, PNDIF-TVT, PDPP-BT and PFDPP-BT.

5. Random copolymers

Generally, Stille polycondensation,²⁰⁸ Suzuki polycondensation²⁰⁹ and DAP²¹⁰ are common methods used for the polymerization process of conjugated polymers (Scheme 11). In earlier studies, investigators concentrated on regular conjugated polymers with the aim to release the high charge transporting property. For regular conjugated copolymers, the HOMO and LUMO levels are often controlled by selecting different pairs of charge donors and acceptor monomers, which may be limited by the complexity and difficulty of designing monomers with different structures. Therefore, random copolymerization, as a facile method, has been utilized in the synthesis of polymer semiconductors to regulate the HOMO–LUMO energy levels, crystallinity, solubility, molecular packing, functionality, etc.

Scheme 11 Paths for polymerization of conjugated polymers including (A) Stille coupling reaction, (B) Suzuki–Miyaura coupling reaction, and (C) direct arylation reaction.

In 2011, Jenekhe et al.⁴⁰ synthesized the regular copolymers HD-PPTV (Fig. 30) and HD-PPPV (Fig. 30) and the random copolymer PPTPV (Fig. 30). The results showed that HD-PPTV exhibited an ambipolar behaviour and HD-PPPV exhibited a unipolar p-type behaviour. Besides, the random copolymer PPTPV exhibited ambipolar behaviour between HD-PPTV and HD-PPPV when it was composed of 50% HD-PPTV and 50% HD-PPPV. In addition, the optical absorption spectra and XRD patterns of PPTPV also showed a photophysical property and crystallinity behaviour between HD-PPTV and HD-PPPV. Later on, Kim et al.²¹¹ systematically explored the effect of the ratio between two monomers on the property of the random copolymer PDPP-Th-Se (Fig. 31). It was suggested that the crystallinity of PDPP-Th-Se could be systematically manipulated by adjusting the ratio between two different electron donors – selenophene and thiophene. The melting and crystallization temperatures as well as the crystallinity of PPTPV were improved following the enhancement of the Se content, which resulted in the enhancement of the OFET performance. Based on such an adjustment effect of random copolymers, their solubility could also be finely tuned. Kwon and co-workers achieved good solubility of the random copolymer PDPP-BTT-SVS (Fig. 31) in a non-chlorinated solvent and high OFET performance (μ_h = 6.51 cm² V⁻¹ s⁻¹) when 10% bithienothiophene (BTT) and 90% SVS were present.²¹² After that, a series of random copolymers (PDPP-BT-SVS,²¹³ PDPP-2T-TVT,²¹⁴ PDPP-TVT-SVS²¹⁵ and PDPP-CNTVT-SVS²¹⁶ as in Fig. 31) based on the thiophene-flanked DPP as the charge acceptor and two different charge donors were also reported. Among them, Kim and co-workers realized the modulation of PDPP-CNTVT-SVS from p-type (μ_h = 6.23 ± 0.4 cm² V⁻¹ s⁻¹, CNTVT/SVS = 1 : 9) to n-type (μ_e = 6.88 ± 1.01 cm² V⁻¹ s⁻¹, CNTVT/SVS = 9 : 1) as well as a balanced ambipolar (μ_h = 3.15 ± 0.2 cm² V⁻¹ s⁻¹, μ_e = 3.03 ± 0.15 cm² V⁻¹ s⁻¹, CNTVT/SVS = 1 : 1) semiconductor via precisely regulating the copolymerization ratio of 2,3-di(thiophen-2-yl)acrylonitrile (CNTVT)/SVS.²¹⁶

Fig. 30 Chemical structures of regular copolymers HD-PPTV and HD-PPPV and the random copolymer PPTPV.

Fig. 31 Chemical structures of PDPP-Th-Se, PDPP-BTT-SVS, PDPP-BT-SVS, PDPP-2T-TVT, PDPP-TVT-SVS and PDPP-CNTVT-SVS.

Some special polymer semiconductors based on random copolymers have also been reported. Bao et al.²¹⁷ incorporated the non-conjugated segment – 2,6-pyridine dicarboxamide (PDCA) – into a DPP and TVT-based polymer via random copolymerization (PDPP-TVT-TPDCAT, Fig. 32(A)). The introduction of PDCA provided enhanced mechanical properties via the formation of intra- and intermolecular hydrogen-bonding or variation of the film morphology (Fig. 32(B)). This kind of polymer exhibited intrinsically stretchable and healable properties. When the OFETs based on PDPP-TVT-TPDCAT were strained up to 100%, their charge mobility decreased linearly and slowly. And the mobility value could return to the initial one when the strain was released. Besides, the tolerance measurement of common movements (Fig. 32(C)) via mounting OFETs on human limbs revealed that the devices could maintain an average hole mobility over 0.1 cm² V⁻¹ s⁻¹ as well.

Fig. 32 (A) Chemical structure of a stretchable and healable semiconducting polymer based on DPP (renamed as PDPP-TVT-TPDCAT here); (B) description of variation of the polymer structure during stretching and releasing; (C) photographs of folded, twisted and stretched OFETs on the human skin. ((B and C) were reproduced with permission from Macmillan Publishers Limited, part of Springer Nature. Copyright 2016.)

Zhang et al.²¹⁸ also rationally utilized the random copolymerization strategy to control the hydrogen bonds. Different from Bao's work, Zhang et al. introduced the functional groups via the side chains instead of the main backbone. As is shown in Fig. 33, DPP-quaterthiophene-based units containing alkyl chains with urea groups were randomly copolymerized with those containing branching alkyl chains without urea groups. IR and ¹H NMR characterizations verified the existence of inter-chain hydrogen bonds. And the polymer PDPP4T-U (Fig. 33) exhibited the highest hole mobility of up to 13.1 cm² V⁻¹ s⁻¹ after thermal annealing at just 100 °C when (1 − x)/x = 1 : 10, which resulted from the ordered lamellar packing of alkyl chains and inter-chain π–π stacking.

Fig. 33 Chemical structure of a random copolymer containing a urea group (renamed as PDPP4T-U here).

In addition to random copolymerization occurring between two DPP-based monomers, other monomers were also reported for random copolymers. As shown in Fig. 34, Ajayaghosh et al.²¹⁹ utilized the random compolymerization method to explore how the fused chalcogenophene affected the molecular packing and charge carrier transport by varying the percentage composition of the fused chalcogenophene in the polymers TDPBTBT and BTDPBTBT. The results showed that the charge carrier mobility could be enhanced via increasing the percentage composition of fused chalcogenophene. Chen and co-workers prepared a novel charge donor di(thiophen-2-yl)thieno[3,2-b]thiophene (DTTT) and randomly copolymerized it with the DPP-based unit.²²⁰ It was indicated that the random copolymer PDTTT-T-DPP (Fig. 34) exhibited a higher hole mobility than the regular analogues formed by the direct polymerization between DTTT and DPP without thiophene as the linkage. Besides, as shown in Fig. 34, other random copolymers PDPP-BTD,²²¹ PBPD-T-DPP²²² and PDPP-T-ID²²³ containing commonly studied electron deficient units (BT, IID, and BPD) were also reported. Among them, the properties of PDPP-BTD and PBPD-T-DPP could be finely tuned to be suitable for either OPV devices or OFETs via adjusting the random copolymerization ratio ((1 − x)/x) of monomers. While the polymer PDPP-T-ID was only tested for OPV devices.

Fig. 34 Chemical structures of random copolymers TDPBTBT, BTDPBTBT, PDTTT-T-DPP, PDPP-BTD, PBPD-T-DPP and PDPP-T-ID.

In 2016, Fang and co-workers combined random copolymerization with side chain engineering for the synthesis of the IID-based polymer PIIT-BOCx (Fig. 35).²²⁴ The long polyisobutylene side-chains provided solubility for the polymer while t-Boc groups, as a thermal cleavable group, afforded a latent hydrogen bond for cross-linking when thermally treated. The crosslinked polymer PIIT-BOCx exhibited a favourable solvent-resistant property which was beneficial for application in solution-processable multiple-layer electronic devices. In addition to the traditional synthesis methods used for polycondensation of the IID-based polymer, Kiriy et al.²²⁵ adopted a novel method via zinc-activating the monomers. As shown in Fig. 35, the low-bandgap random copolymer PTIIT-TNDIT was prepared by the copolymerization of zinc-activated IID- and NDI-based monomers. Such a polymerization approach proceeds quickly at room temperature, which is an advantage compared to traditional polymerization methods. However, the OFET performance of the polymer PTIIT-TNDIT was not discussed in this article, and such a synthesis method still needs to be deeply studied for more semiconducting polymers. Other random copolymers PDTBDTFBT-DTBDII (Fig. 35) and PTTTTFBT-TTTTII (Fig. 35) based on IID and 2FBT were also reported but with poor OFET performance.²²⁶

Fig. 35 Chemical structures of random copolymers PIIT-BOCx, PTIlT-TNDIT, PDTBDTFBT-DTBDII (renamed here), PTTTTFBT-TTTTII (renamed here), NDI-r-OPPV/BBO, PNDI2OD-T2-PSx, PNDI2OD-T2, PNBI3T-PBI3T and the gradient copolymer PTNDIT-F2/6.

Furthermore, as shown in Fig. 35, imide-based random copolymers were reported as well. For NDI-r-OPPV/BBO²²⁷ (Fig. 35), oligo(p-phenylenevinylene) (OPPV) with side chains served to improve the solubility of the NDI-benzobisoxazole (BBO) based segment. NDI-r-OPPV/BBO was synthesized by the Horner–Wadsworth–Emmons polymerization method. OFETs of NDI-r-OPPV/BBO showed dominant n-type properties different from the balanced ambipolar performance of the regular polymer based on NDI and OPPV. In 2016, Bao et al.²²⁸ introduced polystyrene (PS) oligomer side chains into the NDI-based polymer and investigated their effect by means of tuning the monomer ratio in the random copolymer (PNDI2OD-T2-PSx, Fig. 35). The results showed that the ambient stability of the NDI-based polymer was obviously improved while keeping the favourable electron transporting performance when 20 mol% of PS side chains were present in the polymer. They assumed that PS side chains worked as the molecular encapsulation layer around the conjugated polymer backbone to relieve the electron traps. Additionally, PNDI2OD-T2²²⁹ (Fig. 35) and PNBI3T–PBI3T²³⁰ (Fig. 35), as random copolymers consisting of NDI and PDI, were also reported. But their OFET performance was not mentioned.

Gradient or even block-like copolymer PTNDIT-F2/6 (Fig. 35), distinguished from random copolymers, was also synthesized by the zinc-activating approach aforementioned.²³¹ Such a gradient or even block-like behaviour resulted from the faster polymerization of the fluorine segment than the NDI based segment, and was confirmed by NMR, gel permeation chromatography (GPC), AFM, and fluorescence quenching experiments. But the OFET properties of such a novel polymer were not measured in the literature.

6. Conclusions and outlook

This review summarized the versatile chemical strategies, consisting of building block selection, backbone halogenation, side chain engineering and random copolymerization, for high-performance semiconductor polymers in OFETs. Suitable building block selection for charge donor and acceptor energy level matching as well as facile charge carrier injection can increase the semiconductor performance of the polymer. Halogenation including fluorination and chlorination can subtly modify the energy levels, molecular packing order and coplanarity of polymers via electron-withdrawing properties and non-covalent bonds. Compared with chlorination, fluorination is more effective due to the smaller size of the fluorine atom. Side chain engineering is a favourable method to tune the solubility and condensed state structure of the rigid conjugated polymer backbone for increasing molecular weight and coplanarity, which will eventually result in the enhancement of OFET performance. What's more, random copolymerization is a facile and efficient method that can creatively adjust the energy levels, molecular orientations, solubilities, interchain interactions, charge transport properties, and so on.

To some extent, findings on new building blocks are limited, and they still need to be deeply studied. In addition, some new building blocks with high OFET performance as mentioned above, such as BAI, should be paid more attention in future research works. On the other hand, halogenation, side chain engineering and random copolymerization as remarkable tools for favorable and functional (e.g. stretchable and healable) polymer semiconductors may be the research focus in future works. Additionally, the device architecture is also essential for high-performance OFETs, which is not discussed in detail here. In summary, high-performance polymer based OFETs are expected when the chemical approaches mentioned above are combined with rational device architectures.

Acknowledgements

This work was financially supported by the National Key R&D Program of “Strategic Advanced Electronic Materials” (No. 2016YFB0401100), the National Natural Science Foundation of China (51233006 and 21633012), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030100).

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