Manipulating the doping level via host-dopant synergism towards high performance n-type thermoelectric composites

Journal Pre-proofs Manipulating the Doping Level via Host-dopant Synergism towards High Performance n-type Thermoelectric Composites Xiaojun Yin, Fei Zhong, Zhanxiang Chen, Chunmei Gao, Guohua Xie, Lei Wang, Chuluo Yang PII: DOI: Reference:

S1385-8947(19)32227-2 https://doi.org/10.1016/j.cej.2019.122817 CEJ 122817

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

21 June 2019 11 August 2019 11 September 2019

Please cite this article as: X. Yin, F. Zhong, Z. Chen, C. Gao, G. Xie, L. Wang, C. Yang, Manipulating the Doping Level via Host-dopant Synergism towards High Performance n-type Thermoelectric Composites, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.122817

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Manipulating the Doping Level via Host-dopant Synergism towards High Performance n-type Thermoelectric Composites Xiaojun Yina,b, Fei Zhonga, Zhanxiang Chenb, Chunmei Gaoc, Guohua Xieb, Lei Wanga,* and Chuluo Yanga,b,* aShenzhen

Key Laboratory of Polymer Science and Technology, College of Materials Science

and Engineering, Shenzhen University, Shenzhen 518060, China. bHubei

Key Lab on Organic and Polymeric Optoelectronic Materials, Department of

Chemistry, Wuhan University, Wuhan, 430072, P. R. China. cCollege

of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, PR

China. E-mail addresses: [email protected] (L. Wang), [email protected] (C. Yang), Keywords: thermoelectric materials, n-doping, organic small molecular semiconductors, composites, carbon nanotubes

Abstract Precisely controlling the doping profile of organic semiconductors is critical for optimizing the Fermi levels (EFs) of the materials and thus models their thermoelectric (TE) performance. Herein, four new 4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene (IDT)-cored smallmolecular semiconductors with gradient changes of lowest unoccupied molecular orbital (LUMO) levels and synchronously decreased energy gaps are elaborately designed to adapt to single-walled carbon nanotube (SWNT)-incorporating TE composites. The doping levels can be manipulated by either two different n-doping mechanisms or the variation of IDT hosts. A mild doping process with a weak n-dopant results in synchronous improvements in the Seebeck coefficient and conductivity (σ), featuring a high power factor (PF) of 212.8 μW m-1 K-2 (from IDTT-CN/SWNT = 1:1). On the other hand, the hydride-transfer-induced n-doping mechanism achieved by using a powerful n-dopant leads to significantly different n-doping 1

behaviors among these IDTs, and a remarkably high σ of over 2500 S cm-1 is achieved by QIDT-CN/SWNT (1:1).

1. Introduction Organic thermoelectric materials (OTMs) including π-conjugated semiconductors have attracted extensive interest due to their distinct advantages of abundant structures, scalable processing methods and mechanical flexibility [1-6], which render them geometrically versatile for fully realizing the potential application of thermoelectric (TE) modules in wearable devices [7-10]. Since the performance of TE material is evaluated with a figure of merit (ZT = S2σT/κ) [11-15], efforts to promote either the Seebeck coefficient (S) or the electrical conductivity (σ) of the materials while restraining their thermal conductivity (κ) are critical [16-19]. In organic systems, κ is intrinsically low due to the van der Waals contact as well as structure disorder between molecules [20]; hence, the power factor (PF = S2σ) is a more significant parameter for TE performance optimization. Unfortunately, for intrinsic organic semiconductors, which always suffer from low electrical conductivity (σ) because of their low carrier concentrations and relatively wide bandgaps [21-26], chemical doping is essential required [27-31]. However, the fundamental mechanisms for chemical doping in organic semiconductor systems are still complicated [32-37], especially for n-doping, due to the additional challenges regarding the small number of n-dopants and electron trapping [3841]. Typically, a valid n-type doping process can be achieved through electron transfer according to the following three pathways [34]: ⅰ) direct electron-transfer mechanism (Scheme 1a), which requires the highest occupied molecular orbitals (HOMOs) of the dopants to be higher than or close to the lowest unoccupied molecular orbitals (LUMOs) of the host to enable effective electron transfer from the dopants to the hosts [42]; ⅱ) hydridetransfer-induced n-doping mechanism (Scheme 1b), which occurs between air-stable n2

dopants such as benzimidazole derivatives (e.g., N-DMBI) and hosts with LUMO levels below ~ -4.0 eV [43, 44]; ⅲ) photo-activation mechanism (Scheme 1c), i.e., the air-sensitive dopant radicals are stabilized by dimerization through a weak C-C σ bond, and effective ndoping can be realized by means of a photo-activation process [45]; however, the synthesis of the dimers requires extremely harsh conditions. Nevertheless, precise control of the doping profile of the OTMs to optimize the TE parameters remains a challenge. To date, the n-type OTMs predominantly used in adapting to TE applications have been mainly

limited

to

only

compounds

with

π-deficient

backbones,

for

instance,

diketopyrrolopyrrole, naphthalimide or perylenediimide [46-53]. Although individual seminal works have been reported in recent years, e.g., Di et al., reported a high PF of 236 μW m-1 K-2 and a ZT value over 0.2 from N-DMBI-doped A-DCV-DPPTT films [54], OTMs generally still suffer from inferior performances than the applicable inorganics (e.g., germanium telluride [55], lead telluride [56, 57], silicides [58], and half-heuslers [59]). For example, Koster et al. achieved a PF of 4.5 μW m-1 K-2 by tailoring the dihedral angle of the diketopyrrolopyrrole-containing polymers [60], and Pei et al., obtained an optimized PF of 4.65 μW m-1 K-2 via donor engineering of the polymer backbones [61]. In fact, improving the PFs in individual OTM systems remains a particular challenge due to the strong interdependence among σ, S and κ [62-68]; i.e., all these parameters relate to the carrier concentrations. Alternatively, a composite system by incorporating single-walled carbon nanotubes (SWNTs) into the π-conjugated organic systems is very promising due to the complementarity of high σ and mechanical robustness of the SWNTs towards the OTMs [6973]. In addition, the coupling between OTMs and SWNTs can provide a synchronous improvement in the S and σ, while the internal interfaces introduce phonon-boundary scattering to reduce the lattice κ [74-78]. To highlight the current interest in manipulating the doping levels of the materials, four new 4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene (IDT)-cored acceptors with different 3

end-capping substituents and LUMO levels (Scheme 1d) were designed for incorporation into SWNT-containing composites. Herein, control of the doping levels of the composites can be achieved upon the adjustment of the LUMO levels of the matrix and the intensity of the ndopants, which enter as phenomenological parameters to model the Fermi level (EF) position in determining the PFs of the composites. Both the density functional theory (DFT) simulations and cyclic voltammetry measurements revealed gradually decreased LUMO levels and energy gaps from IDT-CN and IDTT-CN to IDT-4F and then to Q-IDT-CN, which reflected a progressively easier n-doping process. As expected, the SWNTs could be readily doped with even a very weak n-dopant, N1,N1,N6,N6-tetramethylhexane-1,6-diamine (TMHDA), that was distinct from the IDTs. In this way, the reduced SWNTs could serve as sensitizers to perform n-type doping of the composites, and the doping levels were regulated with the LUMO levels of the IDTs. Therefore, in contrast to those of the individual SWNTs, the S and σ of the composites were simultaneously increased, which enabled IDTTCN/SWNT (1:1) to achieve the highest PF of 212.8 μW m-1 K-2. To our best knowledge, this was one of the highest value for n-type SWNTs-containing composites with definite hybrid ratio to date (Table S2). On the other hand, a more vibrant n-dopant, 1,3-dimethyl-2-(2,4,6trimethoxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole (TM-DMBI), was also employed [43], and this dopant can not only bring the SWNTs to a greater doping level but also act as a screen that exerts diverse doping effects on these IDTs since the valid n-doping based on the hydride transfer mechanism (Figure S1) occurs on only the LUMOs of the host below ~ -4.0 eV. The UV-vis-NIR and electron paramagnetic resonance (EPR) measurements indicated that only Q-IDT-CN could produce abundant organic radicals when doped with the TMDMBI solution. Therefore, a remarkably high σ of over 2500 S cm-1 was obtained with QIDT-CN/SWNT (1:1) due to the high doping level of the composite, which could be attributed to the significant shift in EFs from the point of view of the Kelvin probe (KP) results. Meanwhile, a comparably high PF of 199.3 μW m-1 K-2 was achieved. 4

LUMO

a)

Dopant

e

LUMO

N

-H N H

HOMO

Host

N

(eV)

e

Direct electrontransfer mechanism

LUMO

LUMO

SOMO

Hydride-transfer mechanism

e

Dimer Dopant

Acceptor

Acceptor

HOMO

HOMO -2.0

N

HOMO

HOMO

HOMO Photo-activation mechanism

d) N

-3.0

LUMO -3.56

-4.0

N

-3.64

C6H13

C6H13

C6H13 NC

CN

C6H13

NC

CN C6H13

C6H13

CN

NC

C6H13

S

NC

IDTT-CN

NC NC

F

F

C6H13

C6H13

-6.0

HOMO IDT-CN -7.0

F

O O

IDT-4F

S

O

N CN

N

-5.15 TMHDA TM-DMBI

C6H13

Q-IDT-CN

3

-4.33

CN

S

C6H13

SOMO (-2.31)

O

C6H13

CN

S

O

C6H13

N

-4.34 C6H13

O CN

S

F

C6H13

CN

S

S

S NC

NC

S

S

-5.0

C6H13

O

O N H

-4.11

c)

LUMO

b)

N

N

SWNT

Scheme 1. Simplified model of the typical n-type doping process for a) direct electrontransfer mechanism, b) hydride-transfer-induced mechanism, and c) photo-activation mechanism. d) schematic diagram of manipulating the doping levels by means of dopant-host synergism. 2. Results and discussion 2.1 Concept of molecular design and doping Recently, IDT-cored molecules end-capped with strongly electron-deficient moieties (e.g., malononitrile derivatives) have attracted considerable interest as acceptors in organic solar cells due to the favorable electron transport property and controllable LUMO energy levels [79-82], which are desirable to tailor the host-dopant matching model for TE applications. To extract the primary factors of LUMO levels in determining the n-doping process, characterization of the molecular conformations and orbital distributions of these IDTs should be carried out first, and thus, DFT simulations of these IDTs were performed. As Figure 1 shows, all these IDTs revealed homologous HOMO/LUMO distributions; i.e., both the HOMOs and LUMOs were mainly distributed along the π-delocalization of the central IDT backbones. Meanwhile, all four IDTs sharing the same side chains and similar central 5

skeletons exhibit negligible geometry differences (Figure 1), which are supposed to produce parallel contacts among different IDTs/SWNTs surfaces when under the same hybrid ratio. Notably, the calculated LUMO levels and energy gaps gradually decreased from IDT-CN and IDTT-CN to IDT-4F, and then to Q-IDT-CN, which reflected a progressively easier n-doping process. On the other hand, the incorporation of SWNTs can significantly improve the σ of the TE films, but more importantly, the low-lying LUMO levels of the SWNTs will facilitate the n-doping process of the composites from weak n-dopants, which is a prerequisite for airstable n-doping. In general, the doping level of the composites can be regulated with the LUMO levels of the IDTs or the intensity of the n-dopants, subsequently affording desirable TE performance of the materials.

Figure 1. The optimized molecular conformations and calculated orbital distributions (LUMO+1, LUMO, HOMO, and HOMO-1) of these IDT-based organic small-molecule semiconductors with the long alkyl chain simplified to methyl groups. 6

2.2 Electrochemical properties To identify the energy levels of these studied compounds, cyclic voltammetry measurements were carried out. The reduction processes of these IDTs were performed in acetonitrile solution and revealed gradually enhanced onset potentials ranging from -1.24 to -0.46 eV (vs Fc/Fc+, Figure S2). Correspondingly, their LUMO energy levels can be calculated as -3.56 eV (IDT-CN), -3.64 eV (IDTT-CN), -4.11 eV (IDT-4F), and -4.34 eV (Q-IDT-CN). The oxidation processes of the dopants were carried out in dichloromethane and revealed HOMO levels of -5.15 eV for TMHDA and -4.33 for TM-DMBI. Notably, the controllable LUMO levels of the hosts can be doped in a targeted manner with either TMHDA or TM-DMBI in

900 1200 1500 Wavelength (nm)

3480

300

1800

e)

3510

3465

IDT-CN

3480 3495 3510 3525 Magnetic Field (Gauss)

600

900 1200 1500 Wavelength (nm)

A B C D E

3480

300

1800

f)

3510

3465

IDTT-CN

Pristine Doped with TMHDA Doped with TM-DMBI

IDT-4F

600

3480

1800

d)

300

g)

3510

3465

3480 3495 3510 3525 Magnetic Field (Gauss)

900 1200 1500 Wavelength (nm)

A B C D E

Normalized Abs. Intensity (a.u.)

IDTT-CN

c)

Intensity (a.u.)

600

A B C D E

Pristine Doped with TMHDA Doped with TM-DMBI

Normalized Abs. Intensity (a.u.)

IDT-CN

b)

Intensity (a.u.)

Intensity (a.u.)

300

Pristine Doped with TMHDA Doped with TM-DMBI

Normalized Abs. Intensity (a.u.)

a)

Intensity (a.u.)

Normalized Abs. Intensity (a.u.)

order to optimize the TE performance.

IDT-4F

3480 3495 3510 3525 Magnetic Field (Gauss)

Pristine Doped with TMHDA Doped with TM-DMBI

Q-IDT-CN

600 A C E

900 1200 1500 Wavelength (nm) B D

3480 3500 3520

3465

1800

h)

Q-IDT-CN

3480 3495 3510 3525 Magnetic Field (Gauss)

Figure 2. The UV-vis-NIR absorption spectra and EPR spectra of the IDTs in dichloromethane solution, a)/e) IDT-CN, b)/f) IDTT-CN, c)/g) IDT-4F, and d)/h) Q-IDT-CN (For 2e-2h, line A is for pristine material, line B is for material doped with TM-DMBI, and line C, D, and E for pristine material, material doped with TM-DMBI, and material doped with TMHDA, respectively, but with the participation of radical scavenger TEMPO). 2.3 Photophysical properties and EPR studies To investigate the n-type doping features of these IDT derivatives with different n-dopants, their UV-vis-NIR absorption spectra and EPR measurements were recorded. Notably, whether n-type doping proceeded through either the “direct electron-transfer” (from TMHDA to the 7

IDTs) or “hydride-transfer” mechanisms (from TM-DMBI to the IDTs) was largely dependent on the LUMO levels of the matrix. In response, no significant absorption changes were observed in IDT-CN and IDTT-CN when doped with TMHDA or TM-DMBI (Figure 2a and 2b) due to the high-lying LUMOs of the matrix, while obvious polaron/bipolaron absorption (ca. 1200-1800 nm) could be discerned with the synchronous reduction in the intramolecular charge-transfer (ICT) state (approximately 652 nm), indicating possible n-doping of Q-IDTCN (Figure 2d and S6) [53]. Intriguingly, although IDT-4F possesses a valid deep LUMO energy level (-4.11 eV), no characteristic polaron/bipolaron absorption could be identified from the UV-vis-NIR spectra (Figure 2c and S5). There was a possibility that the n-doping occurred with IDT-4F as well; however, the generated radicals were kinetically unstable since the ICT absorption peaks (at approximately 656 nm) were remarkably decreased after the addition of n-dopants (Figure 2c). To this end, EPR measurements were executed to trace the formation of radicals. As Figure 2e-2g shows, all three compounds (IDT-CN, IDTT-CN and IDT-4F) exhibited an initial negative EPR signal (line A), but Q-IDT-CN displayed a tiny positive EPR signal (the insert of Figure 2h), which could be attributed to the tautomerism between the diradical resonance state and the intrinsic quinoid conformation [54]. After doping with the powerful n-dopant TM-DMBI, only Q-IDT-CN responded with a significantly enhanced EPR intensity, revealing that effective n-doping occurred. To exclude the quenching effect of the radicals, equimolar quantities of a free radical scavenger (TEMPO) were added to capture the in situ-generated radicals (line C of Figure 2e-2h and Figure S7) [83]. As expected, for Q-IDT-CN, the intrinsic EPR signal of TEMPO vanished immediately after loading TM-DMBI (line D of Figure 2h), while the EPR signals in the other three cases were still maintained, revealing that no obvious radicals were produced with the addition of TM-DMBI. Furthermore, all four compounds exhibited no EPR response due to the weak n-type doping ability of TMHDA (line E of Figure 2e-2h). Noticeably, the appropriate LUMO energy levels and molecular structures of the hosts mixed 8

with suitable dopants could effectively regulate the doping level of the matrix and thus determine their TE performances.

Table 1. Photophysical and electrochemical properties of the studied IDTs. λabs,maxa)

Egb)

HOMO-1C)

HOMOC)

LUMOc)

LUMO+1c)

HOMOd)

LUMOd)

[nm]

[eV]

[eV]

[eV]

[eV]

[eV]

[eV]

[eV]

IDT-CN

527

2.22

-6.79

-6.05

-3.48

-2.86

-5.90

-3.56

IDTT-CN

549

2.11

-6.54

-5.81

-3.41

-2.97

-5.85

-3.64

IDT-4F

656

1.77

-6.67

-5.96

-3.75

-3.36

-5.83

-4.11

Q-IDT-CN

652

1.56

-6.93

-5.73

-4.26

-2.52

-5.58

-4.34

Hosts

a)Measured

in dilute dichloromethane solution, b)estimated from the optical absorption edge, c)obtained from the DFT results, d)determined from the onset of the oxidation/reduction potential. 2.4 Thermoelectric properties To evaluate the doping level effect on TE properties, the Seebeck coefficients and conductivities of these IDT/SWNT hybrid films doped with either TM-DMBI or TMHDA were investigated. As Figure S9 and Table S1 show, all these composites initially exhibited positive S values, which can be ascribed to the p-type doping of SWNTs under ambient conditions. As expected, the S values of the composites were effectively converted to negative after doping with TMHDA (Figure 3a), which reveals that effective electron transfer from TMHDA to the SWNTs occurs, thus featuring n-type doping. Herein, despite invalid n-doping occurring between the IDTs and TMHDA, the doped SWNTs could act as sensitizers to enable an effective n-channel for electron transport among the neighboring LUMOs. Notably, all four IDTs exhibited similar variation trends in S with hybrid ratios ranging from 5:1 to 1:9 regardless of the changes in the LUMO, which can be ascribed to the invalid n-doping occurring between TMHDA and the IDTs; moreover, the doping levels were largely dependent on the hybrid ratios. The variations in σ were desynchronized with each other, presumably due to the energy level difference between the EFs and LUMOs of the composites 9

(Figure 3b). In general, the incorporated IDTs essentially served as binders or linkers for IDT/SWNT composites to weave continuous conductive networks, and therefore, the mostly IDT-based composites demonstrated an even better σ than pure SWNTs [69]. In this way, the optimized PFs achieved were 159.6, 154.6, 65.5 and 62.9 μW m-1 K-2 for IDT-CN/SWNT (1:1), IDTT-CN/SWNT (1:1), IDT-4F/SWNT (2:1) and Q-IDT-CN/SWNT (1:9), respectively (Figure 3c).

IDT-CN IDTT-CN IDT-4F Q-IDT-CN

b)

600 500

-30 -15

IDT-CN IDTT-CN IDT-4F Q-IDT-CN

0

400 300 200

2:1 1:1 1:2 Hybrid Ratio (OSC/SWCNT)

0

1:9

900

-60 d)

150

5:1

2:1 1:1 1:2 Hybrid Ratio (OSC/SWCNT)

1:9

5:1

e)

IDT-CN IDTT-CN IDT-4F Q-IDT-CN SWCNT

IDT-CN IDT-4F SWCNT

IDTT-CN Q-IDT-CN

-30 -25 -20

330

360 390 420 Temperature (K)

450

480

-2

-1

100 IDT-CN IDTT-CN IDT-4F Q-IDT-CN SWCNT

75 50 25 0

325 350 375 400 425 450 475 500 Temperature (K)

325 350 375 400 425 450 475 500 Temperature (K)

2500 h)

210 i)

2000

180

-1

1500 1000 IDT-CN IDT-4F SWCNT

500

300

125

IDTT-CN Q-IDT-CN

-1

 (S cm )

-35

150

-2

g)

IDT-CN IDT-4F SWCNT

150

-1

S (V K )

-40

450 300

325 350 375 400 425 450 475 500 Temperature (K)

1:9

175

600

0

2:1 1:1 1:2 Hybrid Ratio (OSC/SWCNT)

200 f)

PF (W m K )

-20

60

0

PF (W m K )

 (S cm-1)

-1

S (V K )

-40

-30

90

30

750

-50

IDT-CN IDTT-CN IDT-4F Q-IDT-CN

120

100

15 5:1

-1

-1

 (S cm )

-1

S (V K )

-45

180 c) -2

700

a)

PF (W m K )

-60

300

330

360 390 420 Temperature (K)

IDTT-CN Q-IDT-CN

450

480

150

IDT-CN IDTT-CN IDT-4F Q-IDT-CN SWCNT

120 90 60 300

330

360 390 420 Temperature (K)

450

480

Figure 3. The a) S, b) σ, and c) PF of these composites versus hybrid ratio, and the temperature dependency of the d)/g) S, e)/h) σ, and f)/i) PF of the composites (for 3d-3f, doped with TMHDA and the optimized hybrid ratio of IDTs/SWNTs were 1:1, 1:1, 2:1, and 1:9 for IDT-CN, IDTT-CN, IDT-4F and Q-IDT-CN, respectively, for 3g-3i, doped with TMDMBI and uniform hybrid ratio of 1:1). Additionally, the temperature dependences of the TE performance of these composites (doped with TMHDA) with optimized hybrid ratios were investigated (Figure 3d-3f). 10

Notably, both IDTT-CN/SWNT (1:1) and the Q-IDT-CN/SWNT (1:9) exhibited slightly enhanced S with increasing temperature (r.t. to ≃ 420K), consistent with the thermally assisted hopping mechanism (Figure 3d) [23]. However, for IDT-CN/SWNT (1:1) and IDT4F/SWNT (2:1), their S values indicated a descending tendency first and a subsequent increase, consistent with the pure SWNTs, which may be due to the relatively weaker interaction between the SWNTs and the IDT-CN or IDT-4F. All these hybrid films showed typical thermal activation semiconducting behavior, with the highest σ reaching up to 890.3 S cm-1 (IDT-CN/SWNT, 1:1), which was superior to that of pure SWNTs (714.1 S cm-1) (Figure 3e). Therefore, compared to pure SWNTs, all these IDT/SWNT composites (except IDT-4F) showed preferable PFs, and the optimal PFs (212.8 μW m-1 K-2) were achieved by the IDTT-CN/SWNT (1:1) composite at 378 K (Figure 3f). On the other hand, a more active dopant, TM-DMBI, was introduced to distinguish the doping level of these composites (under a uniform hybrid ratio of 1:1, other ratios see Figure S11). Remarkably, in contrast to the TMHDA-doped system (Figure 3a), all these composites exhibited a significantly reduced S (Figure 3g), especially for Q-IDT-CN, which could be attributed to the stronger doping ability of TM-DMBI than TMHDA; i.e., a higher doping level would result in an enhanced carrier concentration and then decrease the S. Correspondingly, an obviously increased σ can be reasonably observed in all the cases (Figure 3h), and an extremely high σ of over 2500 S cm-1 was achieved by Q-IDT-CN/SWNT (1:1), which was two times higher than that of the pure SWNT film (1168.5 S cm-1). All these composites exhibited a descending tendency first and subsequent increase in S with increasing temperature, while the σ exhibited the exact opposite trend, which was closely related to the carrier concentration. Notably, the σs decreased with the concomitant increase of Ss at high temperature region (> 430 K), which may due to the de-doping process of these composite films (Figure 3g, 3h and S8). Overall, the PFs of all these composites delivered a continuously increasing trend from 300 K to 480 K, and comparably high PFs approaching 200 μW m-1 K-2 can be achieved by Q-IDT-CN/SWNT 11

(1:1). To assess the air stability of these n-type composites, the TE properties of IDTTCN/SWNT (1:1) and Q-IDT-CN/SWNT (1:1) doped with either TMHDA or TM-DMBI under different exposure time in air were collected. As revealed in Figure S10, all these composites exhibited good air stabilities and almost negligible roll-offs can be maintained within 10 hours.

Table 2. Key TE data of the composite films (in-plane). S (μV K-1)

Samples

σ (S cm-1)

PF (μW m-1 K-2)

r.t.

Maximum

r.t.

Maximum

r.t.

Maximum

IDT-CN/SWNT (1:1)a)

-50.7

-56.3

621.0

890.3

159.6

197.9

IDTT-CN/SWNT (1:1)a)

-53.8

-56.7

533.4

676.3

154.6

212.8

IDT-4F/SWNT (2:1)a)

-50.0

-53.3

262.1

403.3

65.5

101.7

Q-IDT-CN/SWNT (1:9)a)

-38.3

-58.9

428.9

648.3

62.9

204.0

SWNTa)

-42.7

-50.6

388.4

714.1

70.7

156.5

IDT-CN/SWNT (1:1)b)

-33.1

-33.1

645.3

1642.4

70.7

136.1

IDTT-CN/SWNT (1:1)b)

-33.8

-33.8

705.8

2450.0

80.6

174.1

IDT-4F/SWNT (1:1)b)

-36.8

-36.8

361.1

1308.0

48.9

107.3

Q-IDT-CN/SWNT (1:1)b)

-29.2

-29.7

1102.3

2500.2

94.0

199.3

SWNTb)

-35.7

-35.7

413.4

1168.5

52.7

96.3

with TMHDA for 2 min, b)doped with TM-DMBI for 2 min.

Normalized Counts (a.u.)

Pristine Doped with TMHDA Doped with TM-DMBI

Normalized Intensity (a.u.)

1.0

a)

4.80

b)

0.8 0.6 0.4 0.2

Hybrid ratio (wt%) IDTT-CN:SWNT

5:1 2:1 1:1 1:2 0:1

17

16 15 Binding Energy (eV)

14

19

c)

Doped with TMHDA

18 17 16 Binding Energy (eV)

4.56

Doped with TMHDA

4.44 Doped with TM-DMBI

4.32 4.20

0.0 18

Pristine

4.68

Work Function

a)Doped

15

SWNT

IDT-CN IDTT-CN IDT-4F Q-IDT-CN /SWNT /SWNT /SWNT /SWNT

Figure 4. The ultraviolet photoelectron spectroscopy (UPS) spectra of the a) pure SWNTs and and b) IDTT-CN/SWNT hybrid films (doped with either TMHDA or TM-DMBI). c) the WFs of these composites with the uniform hybrid ratio of 1:1. 2.5 UPS/Kelvin probe and hall effect measurements 12

To elucidate the host-dopant synergistic effect on their doping levels, the EFs and carrier concentrations (n) of these composites were explored. Noticeably, the EFs of the pure SWNTs were significantly shifted from -4.55 eV (pristine) to -4.40 eV (doped by TMHDA), and then -4.12 eV (doped with TM-DMBI) (Figure 4a), indicating that the doping levels can be availably regulated by the intensity of n-dopants. In addition to the n-dopants, the hybrid ratios (e.g. IDTT-CN/SWNTs in Figure 4b) and energy leves of the IDT hosts (Figure 4c) were profoundly determining the EFs of the composites. Herein, mild loadings of the reduced SWNTs (doped with TMHDA) can act as a sensitizer to enable n-doping of the IDTT-CN matrix, which results in a high EF of -3.71 eV near the LUMO level of IDTT-CN (Figure 4b). Additionally, the EFs of the IDTT-CN/SWNT composites gradually shifted to the SWNT side with increasing SWNT content, which can be attributed to the intrinsically low EF of pure SWNTs (-4.40 eV) upon the n-doping of TMHDA. To distinguish the doping levels of the composites (with the same hybrid ratio of IDTs/SWNTs = 1:1) upon diverse IDTs with different LUMOs, in situ measurements of the Kelvin probe response to different n-dopants were carried out. Notably, all these hybrid films exhibited significantly decreased WFs after doping with TMHDA, which reflects the upward shift in the EFs of the composites (Figure 4c and Table 3) [51]. The WFs of the TMHDA-doped composites gradually increased from IDT-CN/SWNT (4.439 eV) to Q-IDT-CN/SWNT (4.534 eV), revealing a stepwise decrease in EFs from IDT-CN/SWNT to Q-IDT-CN/SWNT, which was in line with the gradually decreasing LUMO levels of the IDTs (Scheme 1). In comparison with TMHDA, TM-DMBI exhibited a stronger n-doping feature that could modify the EF of the SWNTs to a greater level (Figure 4c). Correspondingly, all these composites demonstrated lower WFs and higher ns than the TMHDA-doped system (Figure 4c and Table 3). Notably, the variation trends in the WFs of the TM-DMBI-doped materials were similar to those of the TMHDA-doped systems except for the Q-IDT-CN/SWNT films. As expected from the aforementioned inference, Q-IDT-CN could be effectively doped with TM-DMBI, thus rendering the doping 13

level of these composites higher than the others, which contributes to the low EF of the QIDT-CN/SWNT composite. In general, the transportation of charge and energy in energetically disordered organic semiconductors is described as thermally activated hoping [84]. According to percolation theory, the σ of composites is determined with the characteristic hop, which takes place 1

+∞

between the EF and so-called ETr (𝐸𝑇𝑟: = 𝜎∫ ―∞𝐸𝜎′(𝐸)𝑑𝐸, defined as the averaged energy of the charge carriers contributing to the conductivity) [17]. In this regard, the S of the composite can be simplified to S = (EF-ETr)/eT (T is the temperature). Since the charge transport in SWNT hybrid system was predominantly determined by the SWNTs, precisely controlling the EFs was essential to optimize the Ss. According to the Fermi-Dirac distribution, n was strongly related to the EFs and the LUMOs of the matrix (Table 3) and thus determined the σ of the samples. Therefore, a reasonable relationship between the key TE parameters and the doping levels can be described in this model, while the doping levels of the samples can be regulated by means of host-dopant synergism.

Table 3. The WFs and carrier concentrations of different composites with a uniform hybrid ratio of 1:1. WFa)

na)

WFb)

nb)

WFc)

nc)

[eV]

[cm-3]

[eV]

[cm-3]

[eV]

[cm-3]

Pure SWNT

4.669 ± 0.024

3.2 × 1021

4.458 ± 0.023

3.4 × 1021

4.300 ± 0.022

7.5 × 1021

IDT-CN/SWNT

4.671 ± 0.027

7.5 × 1019

4.439 ± 0.018

3.8 × 1021

4.215 ± 0.002

3.9 × 1021

IDTT-CN/SWNT

4.644 ± 0.025

9.4 × 1018

4.459 ± 0.017

1.3 × 1021

4.327 ± 0.021

1.5 × 1021

IDT-4F/SWNT

4.681 ± 0.025

2.5 × 1021

4.477 ± 0.021

1.8 × 1021

4.355 ± 0.019

2.0 × 1021

Q-IDT-CN/SWNT

4.733 ± 0.037

1.9 × 1022

4.534 ± 0.018

3.4 × 1021

4.313 ± 0.025

1.6 × 1022

Samples

The WFs (obtained from Kelvin probe measurements) and the ns (obtained from Hall Effect measurements) of these composite films for a) pristine samples, b) doped with TMHDA and c) doped with TM-DMBI. 2.6 SEM image studies 14

SEM images provided direct insight into the morphologies of the IDT/SWNT composites differing in their skeletons, hybrid ratios, or doping procedures since the morphologies are crucial to determine the σs other than the doping levels [85]. As Figure S12 shows, no obvious difference in morphologies can be observed among the IDT-CN/SWNT, IDTTCN/SWNT and IDT-4F/SWNT composites under the same hybrid ratio, which can be attributed to the similar configurations of the incorporated molecules. For Q-IDT-CN, the quinone skeleton enabled a more rigid structure and crystallinity than the other three compounds, which resulted in relatively poor morphologies of the Q-IDT-CN/SWNT composite in low SWNTs loading. For example, a visually crystalline block can be discerned at the boundary of Q-IDT-CN/SWNT at a high hybrid ratio of 5:1 (Figure 5k), while the other three exhibited only mild surface inhomogeneities (Figure 5i, 5j, and 5l); therefore, compared to the other composites the Q-IDT-CN/SWNT composites delivered an inferior conductivity (Figure 3b). On the other hand, all the composites revealed a gradually increased homogeneity at the beginning and then decreased with the continuous loading of SWNTs. For instance, the surface homogeneity of the IDTT-CN/SWNT gradually improved as the hybrid ratio range from 5:1 to 1:2 and then deteriorated from 1:2 to pure SWNT, which agreed with the similar trend in σ (Figure 3b and S9b). In comparison with the pristine films (e.g., Figure 5c for IDTT/SWNT = 1:1), the films doped with either TMHDA (Figure 5g and S13) or TM-DMBI (Figure 5h and S13) exhibited no significant changes in morphologies due to the appreciable corrosion resistance ability of the composites in both ethanol and acetonitrile; therefore, the TE parameters of the composites were largely dependent on the doping levels rather than the morphologies under the same hybrid ratio.

2.7 Raman and Fourier transform infrared spectroscopy studies Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR) of these IDT/SWNT composites were performed to reflect the interactions between the IDTs and the 15

SWNTs. As Figure S14-S15 demonstrates, compared to the pure SWNT sample, all of these IDT/SWNT composites revealed a slightly redshift in the characteristic graphite-like G band (at ~ 1593 cm-1) [85]. which could be ascribed to the π-π interactions between the IDTs and the SWNTs. Correspondingly, the π-π interactions can be observed in the FT-IR spectra of the composites; for example, in contrast to the individual IDTs, all the IDT/SWNT composites exhibited varying degrees of attenuation at approximately 2220 cm-1 (assigned to the typical stretching vibration peaks of “C≡N”) (Figure S16-S17). Notably, all these composite films exhibited small D/G ratios comparable to that of the pure SWNTs, indicating that negligible structural defects were introduced during the film-forming process because the identified D band of the SWNTs (at ~ 1346 cm-1) was associated with defect-induced resonant scattering.

a)

b)

c)

d)

IDTT-CN/SWCNT (5:1)

IDTT-CN/SWCNT (2:1)

IDTT-CN/SWCNT (1:1)

IDTT-CN/SWCNT (1:2)

i)

j)

g)

e)

Q-IDT-CN/SWCNT (5:1)

IDTT-CN/SWCNT (1:9)

IDT-4F/SWCNT (5:1)

IDTT-CN/SWCNT (5:1)

k)

TMHDA

l)

h)

TM-DMBI

f)

SWCNT

IDT-CN/SWCNT (5:1)

10 μm

Figure 5. SEM images of IDT/SWNT composites. a)-f) IDTT-CN/SWNT with hybrid ratios of 5:1, 2:1, 1:1, 1:2, 1:9 and pure SWNTs, respectively; g) and h) IDTT-CN/SWNT doped with TMHDA and TM-DMBI, respectively; i)-l) images of the boundary of the presented composites.

3. Conclusion

16

In conclusion, four new IDT-cored small-molecule semiconductors with gradient decreasing LUMOs levels and energy gaps, which indicated a progressively easier n-doping process, were designed. In this way, the doping levels of IDT/SWNT hybrid films can be manipulated by means of host-dopant synergism, which delivered significantly different TE performances ranging from the minimum PF of 1.3 ± 0.1 μW m-1 K-2 to the maximum PF of 159.6 ± 1.4 μW m-1 K-2 under ambient conditions. Notably, the remarkably high σ of over 2500 S cm-1 and high PF of over 212 μW m-1 K-2 achieved in this work are among the best results for n-type organic TE composites. In addition, this doping-controlling strategy described in this work offers a valid method for structural optimization of either the hosts or the dopants, which benefit the further progress of TE materials.

4. Materials and methods 4.1 Materials synthesis and characterization The Knoevenagel condensation reactions with aldehyde precursors and the corresponding malononitrile derivatives gave the target molecules IDT-CN, IDT-4F, and IDTT-CN (Scheme S1). The quinone structure of Q-IDT-CN was synthesized with a bromine precursor and malononitrile via a palladium-catalyzed coupling reaction and a subsequent oxidation reaction (Scheme S1). All these final products were fully characterized with 1H NMR and 13C NMR spectroscopy, MALDI-TOF and elemental analysis (see supporting information). Notably, all these target products exhibited fair solubility in commonly used solvents, such as chlorobenzene and chloroform, which is desirable for the drop-casting process.

4.2 TE composite film preparation First, 30 mg of SWNTs were well dispersed in 30 mL of dry chlorobenzene using a probe sonicator (JY99-IIDN, Scientz, China). Afterwards, different amounts of IDTs were added to the dispersions to give diverse hybrid ratios of IDTs/SWNTs, and the hybrid systems were 17

stirred continuously. The glass substrates (1.5 cm × 1.5 cm) were successively sonicated with deionized water, acetone and isopropanol, and then the suspensions were drop-cast on the glass substrates under ambient conditions to give the composite films. The n-doping processes of these composites were carried out by immersing the samples into TMHDA solutions (TMHDA/ethanol = 1:4, v/v) or TM-DMBI solutions (10 mg/mL in acetonitrile) for 2 mins.

4.3 Seebeck coefficient and conductivity measurements The Seebeck coefficients and electrical conductivities of the studied composite films were synchronously tested on a thin film TE parameter test system (MRS, JouleYacht, China) under vacuum, wherein the Seebeck coefficients and electrical conductivities were assessed by using dynamic method and four-probe method, respectively [75]. The dynamic method can be described as follows: a continuous change in temperature was applied to both sides of the sample, and the temperature difference (ΔT = Thot – Tcold) was controlled; the PID curve was sent by intervals, and the setting principle of interval temperature followed ΔT =By-Ay (Ay was the initial temperature, and By was the next temperature to be measured). Then, the electrical signals were collected. The Seebeck coefficient can be calculated according to the definition of S =-∆V⁄ΔT, wherein the slope was the Seebeck coefficient of the sample. The Seebeck coefficient estimated in this system was verified by a nickel standard, and the measured S value of -16.2 ± 0.3 μV K-1 was in line with the reference value of -15 μV K-1. The thicknesses of these samples were ascertained by a profile system (ET-4000M, Kosaka Laboratory Ltd., Japan), and σ can be obtained.

4.4 Theoretical calculations The DFT simulations of these IDTs and dopants were executed by using a suite of Gaussian 09 programs. The conformation of these studied molecules were optimized by means of B3LYP-GD3 with the atomic basis set of 6-311G (d, p). The orbital energy levels of these 18

molecules were simulated at the tHCTHhyb/6-311þþG(d, p) level, and visualization of the orbitals was achieved by using GaussView 5.0.

Acknowledgements X. Yin and F. Zhong contribution equally to this work. We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Project Nos. 51773118, 51803124, and 61575146), Shenzhen Science and Technology Research Grant (JCYJ20170818143831242,), Shenzhen Peacock Plan (KQTD20170330110107046).

Declarations of interest None.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at XXX.

References [1]. Q. Zhang, Y. Sun, W. Xu & D. Zhu, Organic thermoelectric materials: emerging green energy materials converting heat to electricity directly and efficiently, Adv. Mater. 26 (2014) 6829-6851. [2]. X. Ren, F. Yang, X. Gao, S. Cheng, X. Zhang, H. Dong & W. Hu, Organic Field-Effect Transistor for Energy-Related Applications: Low-Power-Consumption Devices, Near-Infrared Phototransistors, and Organic Thermoelectric Devices, Adv. Energy Mater. 8 (2018) 1801003. [3]. A.D. Avery, B.H. Zhou, J. Lee, E.-S. Lee, E.M. Miller, R. Ihly, D. Wesenberg, K.S. Mistry, S.L. Guillot, B.L. Zink, Y.-H. Kim, J.L. Blackburn & A.J. Ferguson, Tailored semiconducting carbon nanotube networks with enhanced thermoelectric properties, Nat. Energy 1 (2016) 16033. [4]. W. Jin, L. Liu, T. Yang, H. Shen, J. Zhu, W. Xu, S. Li, Q. Li, L. Chi, C.A. Di & D. Zhu, Exploring Peltier effect in organic thermoelectric films, Nat. Commun. 9 (2018) 3586. [5]. S. Wang, H. Sun, T. Erdmann, G. Wang, D. Fazzi, U. Lappan, Y. Puttisong, Z. Chen, M. Berggren, X. Crispin, A. Kiriy, B. Voit, T.J. Marks, S. Fabiano & A. Facchetti, A Chemically Doped Naphthalenediimide-Bithiazole Polymer for n-Type Organic Thermoelectrics, Adv. Mater. 30 (2018) 1801898. [6]. Z. Zhu, C. Liu, F. Jiang, J. Xu & E. Liu, Effective treatment methods on PEDOT:PSS to enhance its thermoelectric performance, Synth. Met. 225 (2017) 31-40. [7]. B. Russ, A. Glaudell, J.J. Urban, M.L. Chabinyc & R.A. Segalman, Organic thermoelectric materials for energy harvesting and temperature control, Nat. Rev. Mater. 1 (2016) 16050. [8]. J. Liu, L. Qiu, R. Alessandri, X. Qiu, G. Portale, J. Dong, W. Talsma, G. Ye, A.A. Sengrian, P.C.T. Souza, M.A. Loi, R.C. Chiechi, S.J. Marrink, J.C. Hummelen & L.J.A. Koster, Enhancing Molecular nType Doping of Donor-Acceptor Copolymers by Tailoring Side Chains, Adv. Mater. 30 (2018) 1704630.

19

[9]. E.M. Thomas, B.C. Popere, H. Fang, M.L. Chabinyc & R.A. Segalman, Role of Disorder Induced by Doping on the Thermoelectric Properties of Semiconducting Polymers, Chem. Mater. 30 (2018) 29652972. [10]. Y. Sun, C.A. Di, W. Xu & D. Zhu, Advances in n‐Type Organic Thermoelectric Materials and Devices, Adv. Electron. Mater. 5 (2019) 1800825. [11]. Z. Fan, D. Du, X. Guan & J. Ouyang, Polymer films with ultrahigh thermoelectric properties arising from significant seebeck coefficient enhancement by ion accumulation on surface, Nano Energy 51 (2018) 481-488. [12]. H. Wang, D. Zhao, Z.U. Khan, S. Puzinas, M.P. Jonsson, M. Berggren & X. Crispin, Ionic Thermoelectric Figure of Merit for Charging of Supercapacitors, Adv. Electron. Mater. 3 (2017) 1700013. [13]. W. Shi, G. Wu, K. Hippalgaonkar, J.S. Wang, J. Xu & S.W. Yang, Poly(nickel-ethylenetetrathiolate) and Its Analogs: Theoretical Prediction of High-Performance Doping-Free Thermoelectric Polymers, J. Am. Chem. Soc. 140 (2018) 13200-13204. [14]. J. Lee, J. Kim, T.L. Nguyen, M. Kim, J. Park, Y. Lee, S. Hwang, Y.-W. Kwon, J. Kwak & H.Y. Woo, A Planar Cyclopentadithiophene–Benzothiadiazole-Based Copolymer with sp2-Hybridized Bis(alkylsulfanyl)methylene Substituents for Organic Thermoelectric Devices, Macromolecules 51 (2018) 3360-3368. [15]. Y. Joo, L. Huang, N. Eedugurala, A.E. London, A. Kumar, B.M. Wong, B.W. Boudouris & J.D. Azoulay, Thermoelectric Performance of an Open-Shell Donor–Acceptor Conjugated Polymer Doped with a Radical-Containing Small Molecule, Macromolecules 51 (2018) 3886-3894. [16]. S.A. Gregory, A.K. Menon, S. Ye, D.S. Seferos, J.R. Reynolds & S.K. Yee, Effect of Heteroatom and Doping on the Thermoelectric Properties of Poly(3-alkylchalcogenophenes), Adv. Energy Mater. 8 (2018) 1802419. [17]. G. Zuo, X. Liu, M. Fahlman & M. Kemerink, High Seebeck Coefficient in Mixtures of Conjugated Polymers, Adv. Funct. Mater. 28 (2018) 1703280. [18]. G. Zuo, Z. Li, E. Wang & M. Kemerink, High Seebeck Coefficient and Power Factor in n-Type Organic Thermoelectrics, Adv. Electron. Mater. 4 (2018) 1700501. [19]. J.A. Cooley, P. Promkhan, S. Gangopadhyay, D. Donadio, W.E. Pickett, B.R. Ortiz, E.S. Toberer & S.M. Kauzlarich, High Seebeck Coefficient and Unusually Low Thermal Conductivity Near Ambient Temperatures in Layered Compound Yb2–xEuxCdSb2, Chem. Mater. 30 (2018) 484-493. [20]. R.A. Schlitz, F.G. Brunetti, A.M. Glaudell, P.L. Miller, M.A. Brady, C.J. Takacs, C.J. Hawker & M.L. Chabinyc, Solubility-limited extrinsic n-type doping of a high electron mobility polymer for thermoelectric applications, Adv. Mater. 26 (2014) 2825-2830. [21]. L. Qiu, J. Liu, R. Alessandri, X. Qiu, M. Koopmans, Remco W.A. Havenith, S.J. Marrink, R.C. Chiechi, L.J. Anton Koster & J.C. Hummelen, Enhancing doping efficiency by improving host-dopant miscibility for fullerene-based n-type thermoelectrics, J. Mater. Chem. A 5 (2017) 21234-21241. [22]. J.R. Bertram, A. Penn, M.J. Nee & H. Rathnayake, A Novel n-Type Organosilane-Metal Ion Hybrid of Rhodamine B and Copper Cation for Low-Temperature Thermoelectric Materials, ACS Appl. Mater. Interfaces 9 (2017) 10946-10954. [23]. Y. Sun, L. Qiu, L. Tang, H. Geng, H. Wang, F. Zhang, D. Huang, W. Xu, P. Yue, Y.S. Guan, F. Jiao, Y. Sun, D. Tang, C.A. Di, Y. Yi & D. Zhu, Flexible n-Type High-Performance Thermoelectric Thin Films of Poly(nickel-ethylenetetrathiolate) Prepared by an Electrochemical Method, Adv. Mater. 28 (2016) 3351-3358. [24]. W. Ma, K. Shi, Y. Wu, Z.Y. Lu, H.Y. Liu, J.Y. Wang & J. Pei, Enhanced Molecular Packing of a Conjugated Polymer with High Organic Thermoelectric Power Factor, ACS Appl. Mater. Interfaces 8 (2016) 24737-24743. [25]. E.H. Suh, Y.J. Jeong, J.G. Oh, K. Lee, J. Jung, Y.S. Kang & J. Jang, Doping of donor-acceptor polymers with long side chains via solution mixing for advancing thermoelectric properties, Nano Energy 58 (2019) 585-595. [26]. W. Shi, D. Wang & Z. Shuai, High‐Performance Organic Thermoelectric Materials: Theoretical Insights and Computational Design, Adv. Electron. Mater. 5 (2019) 1800882. [27]. X. Yin, G. Xie, Y. Peng, B. Wang, T. Chen, S. Li, W. Zhang, L. Wang & C. Yang, Self-Doping Cathode Interfacial Material Simultaneously Enabling High Electron Mobility and Powerful Work Function Tunability for High-Efficiency All-Solution-Processed Polymer Light-Emitting Diodes, Adv. Funct. Mater. 27 (2017) 1700695. [28]. H. Zhu, C. Xiao & Y. Xie, Design of High-Efficient Thermoelectric Materials: Tailoring ReciprocalSpace Properties by Real-Space Modification, Adv. Mater. 30 (2018) 1802000.

20

[29]. G. Samsonidze & B. Kozinsky, Accelerated Screening of Thermoelectric Materials by First-Principles Computations of Electron-Phonon Scattering, Adv. Energy Mater. 8 (2018) 1800246. [30]. B. Nell, K. Ortstein, O.V. Boltalina & K. Vandewal, Influence of Dopant–Host Energy Level Offset on Thermoelectric Properties of Doped Organic Semiconductors, J. Phys. Chem. C. 122 (2018) 1173011735. [31]. X. Yin, X. Liu, Y. Peng, W. Zeng, C. Zhong, G. Xie, L. Wang, J. Fang & C. Yang, Multichannel Strategies to Produce Stabilized Azaphenalene Diradicals: A Predictable Model to Generate Self-Doped Cathode Interfacial Layers for Organic Photovoltaics, Adv. Funct. Mater. 29 (2019) 1806125. [32]. Y. Xu, H. Sun, A. Liu, H.H. Zhu, W. Li, Y.F. Lin & Y.Y. Noh, Doping: A Key Enabler for Organic Transistors, Adv. Mater. 30 (2018) 1801830. [33]. Z. Wang, Y. Zou, W. Chen, Y. Huang, C. Yao & Q. Zhang, The Role of Weak Molecular Dopants in Enhancing the Performance of Solution-Processed Organic Field-Effect Transistors, Adv. Electron. Mater. 4 (2018) 1800547. [34]. B. Lussem, C.M. Keum, D. Kasemann, B. Naab, Z. Bao & K. Leo, Doped Organic Transistors, Chem. Rev. 116 (2016) 13714-13751. [35]. I. Salzmann, G. Heimel, M. Oehzelt, S. Winkler & N. Koch, Molecular Electrical Doping of Organic Semiconductors: Fundamental Mechanisms and Emerging Dopant Design Rules, Acc. Chem. Res. 49 (2016) 370-378. [36]. Z. Liang, Y. Zhang, M. Souri, X. Luo, Alex M. Boehm, R. Li, Y. Zhang, T. Wang, D.-Y. Kim, J. Mei, S.R. Marder & K.R. Graham, Influence of dopant size and electron affinity on the electrical conductivity and thermoelectric properties of a series of conjugated polymers, J. Mater. Chem. A. 6 (2018) 16495-16505. [37]. A. Jha, H.G. Duan, V. Tiwari, M. Thorwart & R.J.D. Miller, Origin of poor doping efficiency in solution processed organic semiconductors, Chem. Sci. 9 (2018) 4468-4476. [38]. Z. Bin, Z. Liu, Y. Qiu & L. Duan, Efficient n-Dopants and Their Roles in Organic Electronics, Adv. Opt. Mater. 6 (2018) 1800536. [39]. B.R. Lüssem, M. and Leo, K., Doping of organic semiconductors, Phys. Status Solidi A 210 (2013) 943. [40]. X. Yin, D. Chen, Q. Peng, Y. Xiang, G. Xie, Z. Zhu, C. Zhong, F. Li, S. Su & C. Yang, Rational utilization of intramolecular and intermolecular hydrogen bonds to achieve desirable electron transporting materials with high mobility and high triplet energy, J. Mater. Chem. C 4 (2016) 14821489. [41]. N. Toshima, Recent progress of organic and hybrid thermoelectric materials, Synth. Met. 225 (2017) 3-21. [42]. C. Gaul, S. Hutsch, M. Schwarze, K.S. Schellhammer, F. Bussolotti, S. Kera, G. Cuniberti, K. Leo & F. Ortmann, Insight into doping efficiency of organic semiconductors from the analysis of the density of states in n-doped C60 and ZnPc, Nat. Mater. 17 (2018) 439-444. [43]. Z. Bin, J. Li, L. Wang & L. Duan, Efficient n-type dopants with extremely low doping ratios for high performance inverted perovskite solar cells, Energy Environ. Sci. 9 (2016) 3424-3428. [44]. P. Wei, T. Menke, B.D. Naab, K. Leo, M. Riede & Z. Bao, 2-(2-Methoxyphenyl)-1,3-dimethyl-1Hbenzoimidazol-3-ium iodide as a new air-stable n-type dopant for vacuum-processed organic semiconductor thin films, J. Am. Chem. Soc. 134 (2012) 3999-4002. [45]. X. Lin, B. Wegner, K.M. Lee, M.A. Fusella, F. Zhang, K. Moudgil, B.P. Rand, S. Barlow, S.R. Marder, N. Koch & A. Kahn, Beating the thermodynamic limit with photo-activation of n-doping in organic semiconductors, Nat. Mater. 16 (2017) 1209-1215. [46]. J. Liu, L. Qiu, G. Portale, S. Torabi, M.C.A. Stuart, X. Qiu, M. Koopmans, R.C. Chiechi, J.C. Hummelen & L.J. Anton Koster, Side-chain effects on N-type organic thermoelectrics: A case study of fullerene derivatives, Nano Energy 52 (2018) 183-191. [47]. D. Kiefer, A. Giovannitti, H. Sun, T. Biskup, A. Hofmann, M. Koopmans, C. Cendra, S. Weber, L.J. Anton Koster, E. Olsson, J. Rivnay, S. Fabiano, I. McCulloch & C. Muller, Enhanced n-Doping Efficiency of a Naphthalenediimide-Based Copolymer through Polar Side Chains for Organic Thermoelectrics, ACS Energy Lett 3 (2018) 278-285. [48]. X. Zhao, D. Madan, Y. Cheng, J. Zhou, H. Li, S.M. Thon, A.E. Bragg, M.E. DeCoster, P.E. Hopkins & H.E. Katz, High Conductivity and Electron-Transfer Validation in an n-Type Fluoride-Anion-Doped Polymer for Thermoelectrics in Air, Adv. Mater. 29 (2017) 1606928. [49]. D. Yuan, D. Huang, C. Zhang, Y. Zou, C.A. Di, X. Zhu & D. Zhu, Efficient Solution-Processed nType Small-Molecule Thermoelectric Materials Achieved by Precisely Regulating Energy Level of Organic Dopants, ACS Appl. Mater. Interfaces 9 (2017) 28795-28801.

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[50]. Y. Wang, M. Nakano, T. Michinobu, Y. Kiyota, T. Mori & K. Takimiya, Naphthodithiophenediimide–Benzobisthiadiazole-Based Polymers: Versatile n-Type Materials for Field-Effect Transistors and Thermoelectric Devices, Macromolecules 50 (2017) 857-864. [51]. J. Liu, L. Qiu, G. Portale, M. Koopmans, G. Ten Brink, J.C. Hummelen & L.J.A. Koster, N-Type Organic Thermoelectrics: Improved Power Factor by Tailoring Host-Dopant Miscibility, Adv. Mater. 29 (2017) 1701641. [52]. S. Wang, H. Sun, U. Ail, M. Vagin, P.O. Persson, J.W. Andreasen, W. Thiel, M. Berggren, X. Crispin, D. Fazzi & S. Fabiano, Thermoelectric Properties of Solution-Processed n-Doped Ladder-Type Conducting Polymers, Adv. Mater. 28 (2016) 10764-10771. [53]. K. Shi, F. Zhang, C.A. Di, T.W. Yan, Y. Zou, X. Zhou, D. Zhu, J.Y. Wang & J. Pei, Toward High Performance n-Type Thermoelectric Materials by Rational Modification of BDPPV Backbones, J. Am. Chem. Soc. 137 (2015) 6979-6982. [54]. D. Huang, H. Yao, Y. Cui, Y. Zou, F. Zhang, C. Wang, H. Shen, W. Jin, J. Zhu, Y. Diao, W. Xu, C.A. Di & D. Zhu, Conjugated-Backbone Effect of Organic Small Molecules for n-Type Thermoelectric Materials with ZT over 0.2, J. Am. Chem. Soc. 139 (2017) 13013-13023 [55]. E. Hazan, N. Madar, M. Parag, V. Casian, O. Ben-Yehuda & Y. Gelbstein, Effective Electronic Mechanisms for Optimizing the Thermoelectric Properties of GeTe-Rich Alloys, Adv. Electron. Mater. 1 (2015) 1500228. [56]. G.M. Guttmann, D. Dadon & Y. Gelbstein, Electronic tuning of the transport properties of offstoichiometric PbxSn1−xTe thermoelectric alloys by Bi2Te3 doping, J. Appl. Phys. 118 (2015) 065102. [57]. I. Cohen, M. Kaller, G. Komisarchik, D. Fuks & Y. Gelbstein, Enhancement of the thermoelectric properties of n-type PbTe by Na and Cl co-doping, J. Mater. Chem. C. 3 (2015) 9559-9564. [58]. Y. Sadia, Z. Aminov, D. Mogilyansky & Y. Gelbstein, Texture anisotropy of higher manganese silicide following arc-melting and hot-pressing, Intermetallics 68 (2016) 71-77. [59]. O. Appel & Y. Gelbstein, A Comparison Between the Effects of Sb and Bi Doping on the Thermoelectric Properties of the Ti0.3Zr0.35Hf0.35NiSn Half-Heusler Alloy, J. Electron. Mater. 43 (2014) 1976-1982. [60]. J. Liu, G. Ye, B.V. Zee, J. Dong, X. Qiu, Y. Liu, G. Portale, R.C. Chiechi & L.J.A. Koster, N-Type Organic Thermoelectrics of Donor-Acceptor Copolymers: Improved Power Factor by Molecular Tailoring of the Density of States, Adv. Mater. 30 (2018) 1804290. [61]. C.-Y. Yang, W.-L. Jin, J. Wang, Y.-F. Ding, S. Nong, K. Shi, Y. Lu, Y.-Z. Dai, F.-D. Zhuang, T. Lei, C.-A. Di, D. Zhu, J.-Y. Wang & J. Pei, Enhancing the n-Type Conductivity and Thermoelectric Performance of Donor-Acceptor Copolymers through Donor Engineering, Adv. Mater. 30 (2018) 1802850. [62]. T. Zhu, Y. Liu, C. Fu, J.P. Heremans, J.G. Snyder & X. Zhao, Compromise and Synergy in HighEfficiency Thermoelectric Materials, Adv. Mater. 29 (2017) 1605884. [63]. H.S. Kim, W. Liu & Z. Ren, The bridge between the materials and devices of thermoelectric power generators, Energy Environ. Sci. 10 (2017) 69-85. [64]. X. Shi & L. Chen, Thermoelectric materials step up, Nat. Mater. 15 (2016) 691-692. [65]. J.-Y. Hwang, J. Kim, H.-S. Kim, S.-I. Kim, K.H. Lee & S.W. Kim, Effect of Dislocation Arrays at Grain Boundaries on Electronic Transport Properties of Bismuth Antimony Telluride: Unified Strategy for High Thermoelectric Performance, Adv. Energy Mater. 8 (2018) 1800065. [66]. C. Chang, M. Wu, D. He, Y. Pei, C.-F. Wu, X. Wu, H. Yu, F. Zhu, K. Wang, Y. Chen, L. Huang, J.-F. Li, J. He & L.-D. Zhao, 3D charge and 2D phonon transports leading to high out-of-plane ZT in n-type SnSe crystals, Science 360 (2018) 778-783. [67]. X. Zhou, Y. Yan, X. Lu, H. Zhu, X. Han, G. Chen & Z. Ren, Routes for high-performance thermoelectric materials, Mater. Today 21 (2018) 974-988. [68]. L. Yang, Z.-G. Chen, M.S. Dargusch & J. Zou, High Performance Thermoelectric Materials: Progress and Their Applications, Adv. Energy Mater. 8 (2018) 1701797. [69]. J.L. Blackburn, A.J. Ferguson, C. Cho & J.C. Grunlan, Carbon-Nanotube-Based Thermoelectric Materials and Devices, Adv. Mater. 30 (2018) 1704386. [70]. Q. Jin, S. Jiang, Y. Zhao, D. Wang, J. Qiu, D.M. Tang, J. Tan, D.M. Sun, P.X. Hou, X.Q. Chen, K. Tai, N. Gao, C. Liu, H.M. Cheng & X. Jiang, Flexible layer-structured Bi2Te3 thermoelectric on a carbon nanotube scaffold, Nat. Mater. 18 (2019) 62-68. [71]. K. Yusupov, S. Stumpf, S. You, A. Bogach, P.M. Martinez, A. Zakhidov, U.S. Schubert, V. Khovaylo & A. Vomiero, Flexible Thermoelectric Polymer Composites Based on a Carbon Nanotubes Forest, Adv. Funct. Mater. 28 (2018) 1801246.

22

[72]. Y. Nonoguchi, K. Kojiyama & T. Kawai, Electrochemical n-type doping of carbon nanotube films by using supramolecular electrolytes, J. Mater. Chem. A. 6 (2018) 21896-21900. [73]. S. Qiu, K. Wu, B. Gao, L. Li, H. Jin & Q. Li, Solution-Processing of High-Purity Semiconducting Single-Walled Carbon Nanotubes for Electronics Devices, Adv. Mater. 30 (2018) 1800750. [74]. Q. Jiang, X. Lan, C. Liu, H. Shi, Z. Zhu, F. Zhao, J. Xu & F. Jiang, High-performance hybrid organic thermoelectric SWNTs/PEDOT:PSS thin-films for energy harvesting, Mater. Chem. Front. 2 (2018) 679-685. [75]. C. Gao, Y. Liu, Y. Gao, Y. Zhou, X. Zhou, X. Yin, C. Pan, C. Yang, H. Wang, G. Chen & L. Wang, High-performance n-type thermoelectric composites of acridones with tethered tertiary amines and carbon nanotubes, J. Mater. Chem. A. 6 (2018) 20161-20169. [76]. K. Choi, S.L. Kim, S.I. Yi, J.H. Hsu & C. Yu, Promoting Dual Electronic and Ionic Transport in PEDOT by Embedding Carbon Nanotubes for Large Thermoelectric Responses, ACS Appl. Mater. Interfaces 10 (2018) 23891-23899. [77]. W. Zhou, Q. Fan, Q. Zhang, L. Cai, K. Li, X. Gu, F. Yang, N. Zhang, Y. Wang, H. Liu, W. Zhou & S. Xie, High-performance and compact-designed flexible thermoelectric modules enabled by a reticulate carbon nanotube architecture, Nat. Commun. 8 (2017) 14886. [78]. T. Kodama, M. Ohnishi, W. Park, T. Shiga, J. Park, T. Shimada, H. Shinohara, J. Shiomi & K.E. Goodson, Modulation of thermal and thermoelectric transport in individual carbon nanotubes by fullerene encapsulation, Nat. Mater. 16 (2017) 892-897. [79]. G. Zhang, J. Zhao, P.C.Y. Chow, K. Jiang, J. Zhang, Z. Zhu, J. Zhang, F. Huang & H. Yan, Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells, Chem. Rev. 118 (2018) 3447-3507. [80]. H.B. Naveed & W. Ma, Miscibility-Driven Optimization of Nanostructures in Ternary Organic Solar Cells Using Non-fullerene Acceptors, Joule 2 (2018) 621-641. [81]. J. Hou, O. Inganas, R.H. Friend & F. Gao, Organic solar cells based on non-fullerene acceptors, Nat. Mater. 17 (2018) 119-128. [82]. P. Cheng, G. Li, X. Zhan & Y. Yang, Next-generation organic photovoltaics based on non-fullerene acceptors, Nat. Photonics 12 (2018) 131-142. [83]. V.A. Yongho Joo, Seung Hyun Sung, Brett M. Savoie, Bryan W. Boudouris, A nonconjugated radical polymer glass with high electrical conductivity, Science 359 (2018) 1391-1395. [84]. H. Abdalla, G. Zuo & M. Kemerink, Range and energetics of charge hopping in organic semiconductors, Phys. Rev. B 96 (2017) 241202. [85]. X. Yin, Y. Peng, J. Luo, X. Zhou, C. Gao, L. Wang & C. Yang, Tailoring the framework of organic small molecule semiconductors towards high-performance thermoelectric composites via conglutinated carbon nanotube webs, J. Mater. Chem. A. 6 (2018) 8323-8330.

Graphical abstract

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Highlights

1) Four new IDT-cored semiconductors with gradient changes of LUMO and Egs were elaborately designed. 2) Precisely controlling the doping profile of the thermoelectric composites can be regulated by host-dopant synergism strategy. 3) A high power factor of over 212 μW m-1 K-2 and a remarkably high conductivity of over 2500 S cm-1 were achieved. 4) A visual relationship between doping levels and thermoelectric parameters is well discussed.

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