Strategies To Enhance the Conductivity of n-Type Polymer

Review pubs.acs.org/cm

Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

Strategies To Enhance the Conductivity of n‑Type Polymer Thermoelectric Materials Yang Lu, Jie-Yu Wang, and Jian Pei*

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Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center of Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ABSTRACT: In the past several decades, conducting polymers have achieved remarkable progress and have been widely applied as the active materials for optoelectronics. So far, p-type conducting polymers exhibit high conductivities over 1000 S cm−1 and thermoelectric performance comparable to that of inorganic materials; however, only a few n-type conducting polymers showed conductivities over 1 S cm−1 after doping. The low conductivity of n-type conducting polymers is considered as the major barrier for further enhancing their thermoelectric performances. In this perspective, we highlight the scientific and engineering challenges to enhance the conductivity of n-type polymer thermoelectric materials, including n-doping efficiency in n-type polymers, factors influencing charge carrier mobilities after doping, and stability of n-type conducting polymers. Recent development and strategies to address these issues and enhance the conductivity of n-type conjugated polymers are summarized and discussed, providing materials and device engineering guidelines for the future high-performance polymer thermoelectric materials research and development. enhancing the power factors (PF = S2σ) may be an effective direction toward high ZT values in OTEs. Unfortunately, it is still very difficult to modulate the Seebeck coefficient in conducting polymers. At the current stage, electrical conductivity seems to be a more important issue than Seebeck coefficient to realize a higher ZT value in polymer materials. High-performance complementary p-type and n-type materials are required to realize highly efficient thermoelectric generator modules.12−15 Up to now, many p-type conducting polymers exhibit excellent performance with conductivity over 1000 S cm−1;9,16−19 however, only a few n-type polymers achieved conductivity over 1 S cm−1. The unbalanced development between p-type and n-type polymers has an adverse effect on high-performance OTEs (Figure 1b).13,20 In addition, a high electrical conductivity is also critical to achieve high power output for a practical thermoelectric device.21 Therefore, we focus on the electrical conductivity issue of n-type conjugated polymers in this review. Lately, considerable efforts have been devoted to enhance electrical conductivity and significantly improve thermoelectric performances. In this review, we highlight and identify three main challenges to realize high conductivity for n-type conducting polymers: (1) low n-doping efficiency that restricts the charge carrier density;

1. INTRODUCTION The major breakthrough of conducting polymers occurred in 1977 when redox doping was performed to a “plastic”, polyacetylene, which could display electrical conductivities that rival those of metals such as aluminum and copper.1,2 After considerable investigations in the past several decades, conducting polymers have shown great potential in organic optoelectronics, especially as transparent conductive thin films for advanced devices.3−5 Most recently, conducting polymers make remarkable achievements as active modules for organic thermoelectric (OTE) generators, which triggered a new wave of research on conducting polymers (Figure 1a).6−8 Thermoelectric conversion is a kind of clean energy technology that uses thermoelectric materials to convert electric energy and thermal energy to each other. OTEs have attracted increasing interests due to their low-cost, nontoxic, scalable fabrication, and broad applications in portable and wearable electric generating devices.9,10 The maximum power conversion efficiency of a thermoelectric device relates to the figure of merit of thermoelectric material ZT =

S2σ T κ

(1) Special Issue: Jean-Luc Bredas Festschrift

where σ, S, and κ are the electrical conductivity, Seebeck coefficient, and thermal conductivity, respectively. Most conducting polymers exhibited much lower thermal conductivity than inorganic thermoelectric materials.11 Therefore, © XXXX American Chemical Society

Received: April 10, 2019 Revised: June 11, 2019 Published: June 12, 2019 A

DOI: 10.1021/acs.chemmater.9b01422 Chem. Mater. XXXX, XXX, XXX−XXX

Review

Chemistry of Materials

Figure 1. (a) Publication report on the topic of “conducting polymer”. Data from Thomson Reuters Web of Science, by 31 Dec. 2018. (b) Unbalanced development of solution-processable conducting polymers. The performance of n-type conducting polymers lags far behind that of p-type ones. The specific data is summarized in Table 1.

Table 1. Summary of Thermoelectric Properties of n-Type Conducting Polymers polymer P(NDI2OD-T2)

dopant N-DMBI

pristine mobility (cm2 V−1 s−1) N/A

conductivity (S cm−1)

Seebeck coefficient (μV K−1)

power factor (μW m−1 K−2)

−3

−850 ± 90

0.6

−3

8 × 10

P(NDI2OD-T2)

N-DPBI

N/A

4 × 10

−770 ± 80

0.2

P(NDI2OD-T2) P(NDI2TEG-T2)

TDAE N-DMBI

0.2 2.2 × 10−4

0.003 0.17

−200 −130

0.012 0.4

P(gNDI-gT2)

N-DMBI

10−5

0.3

−190

0.4

P(NDI2OD-Tz2) P(NDI2TEG-Tz2)

TDAE N-DMBI

0.1 1.2 × 10−3

0.09 1.8

−447 ± 15 −159 ± 8

1.5 4.6 ± 0.2

2S-transP(NDI2OD-T2) FBDPPV

N-DPBI

0.05

5.9 × 10−3

−90 ± 4

0.049

N-DMBI

1.1

14

−210 ± 20

25.5 ± 2.5

FBDPPV

DPDHP

1.1

7.1

N/A

N/A

ClBDPPV

N-DMBI

0.5

7.0

−220 ± 20

16.5 ± 1.4

ClBDPPV

TBAF

0.104

0.62

−99 ± 9.2

0.4 ± 0.23

BDPPV

N-DMBI

0.5

0.26

−320

1.6 ± 0.11

PDPF

N-DMBI

1.19

1.3

−235

4.65

PDPH

N-DMBI

1.34

1.01 × 10−3

−235

5.1 × 10−4

P(BTP-DPP) PNDTI-BBT-DP

(RuCp*mes)2 N-DMBI

3 × 10−3 0.31

0.45 5

N/A −169

N/A 14

PNDTI-BBT-DT

N-DMBI

0.096

0.18

−56

0.6

P(PymPh) BBL

NaNap TDAE

N/A 2.3 × 10−3

18 2.4

−16.4 −40

0.49 0.43

(2) low charge carrier mobilities after doping, which directly limit intra- and interchain charge transport; and (3) poor air and operation stabilities that have an adverse impact on practical applications. We summarize recent advances and engineering

processing method solution-mixed and spin cast solution-mixed and spin cast doped in vapor solution-mixed and spin cast solution-mixed and spin cast doped in vapor solution-mixed and spin cast solution-mixed and spin cast solution-mixed and spin cast solution-mixed and spin cast solution-mixed and spin cast solution-mixed and drop cast solution-mixed and spin cast solution-mixed and spin cast solution-mixed and spin cast sequential casting solution-mixed and spin cast solution-mixed and spin cast immersing doped in vapor

ref 45 45 60 51 52 55 54 83 48 64 48 84 48 49 49 56 77 77 53 60

strategies to overcome these limiting factors and enhance the electrical conductivity of n-type conjugated polymers, further facilitating the design of next-generation n-type polymer thermoelectric materials. B


Review


Figure 2. Three main challenges for high conductivity n-type polymers: (1) charge carrier density related to n-doping efficiency; (2) charge transport mobility related to molecular structure and microstructure in the film; and (3) stability including air stability, dopants diffusion, and thermal stability.

2. FACTORS INFLUENCING THE CONDUCTIVITY OF n-Type CONJUGATED POLYMERS AFTER DOPING 2.1. Introducing Charge Carriers: Doping. Unipolar electrical conductivity is determined by the carrier charge q, charge carrier density n, and carrier mobility μ as σ = qnμ (2)

result, metallic transport may be observed at room temperature in conducting polymers (Figure 3c). According to the recent studies in p-type conducting polymers, the form of charge carriers plays an important role in charge and energy transfer. Ordered bipolaron networks exhibited significantly larger Seebeck coefficients than polaron networks, similar to semimetals.18 The major carriers for n-type conducting polymers are electrons. The primary carrier type can be known by testing its Seebeck coefficient (or thermopower) (S), an important parameter in OTE materials:

wherein the charge carrier generation and transport dominate the final conductivity. For a semiconducting polymer, doping is an efficient method to enhance its charge carrier density n. Unlike classical inorganic semiconductors doped by atomic substitution, conjugated polymers were doped by intermolecular charge transfer, which involves a redox chemical reaction between the dopant and the polymer backbone.22 To obtain effective n-doping in these polymers, the dopant transfers electrons to the lowest unoccupied molecular orbital (LUMO) of the n-type polymers (Figure 2). Besides electron transfer, a hydride (H−) or an anion can also be transferred to realize doping.7 The n-doping process generates two kinds of carriers in conducting polymers, namely, polaron (radical anion) and bipolaron (dianion) (Figure 3). Both quasi-particles are

S=−

yz kB ijj E T − E F + A zzz jj j z q k kB T {

(3)

where kB is the Boltzmann constant, q is the carrier charge, and T is the absolute temperature. Therefore, the Seebeck coefficient is determined by the relative position of the transport level (ET) and the Fermi level (EF) (eq 3). For n-type thermoelectric materials, a negative Seebeck coefficient is required and vice versa. To optimize the molecular doping process in organic materials, the structure design of the polymer and dopant, as well as the regulation of the interaction between the two components, can be considered as effective directions. 2.2. Charge Carrier Transport in Doped Polymers. After the generation of charge carriers by the doping process, charge transport is another key factor measured by the charge carrier mobility μ (Figure 2). The transport mechanism of the polymer can be described as a combination of ultrafast intrachain wavelike motion created by (bi)polaron wave function overlap on the same polymer chain and rate-determined hopping motion from site to site created by the interactions of (bi)polarons on adjacent chains.26−28 Unlike charge transport in neutral conjugated polymers, movement of carriers in doped conducting polymers requires consideration of the effects of counterions. At low doping levels, (bi)polarons get localized by the isolated traps created by attractive Coulomb potentials of the counterions. Because the dielectric constant of organic molecules is low, the Coulomb traps are large in size and suppressed the intra- and interchain transport of charge carriers. As the doping level increases, these traps begin to overlap; hence, the hopping barrier reduces and mobility increases. At a higher doping level, the Coulomb traps have fully overlapped and formed a “band” to boost the mobility, in theroy.6 At the same time, interchain transport of carriers is strongly affected by the morphology and microstructure in film states (Figure 4). In general, conjugated polymers have long πconjugated backbones which caused strong interchain π−π

Figure 3. N-doping process of conjugated polymer: (a) neutral polymer; (b) polaron states; and (c) bipolaron states.

composed of charges associated with the lattice distortion. Chemically, lattice distortion can be understood as the emergence of quinoid structures with more planar conformation. As shown in Figure 3b, the lattice distortion leads to a local downward shift of the LUMO level and upward shift of the highest occupied molecular orbital (HOMO) level, resulting in the localized electronic states (polaron states) in the band gap after doping. And the bipolaron states are fully occupied, which are spinless and further away from the band edges than the polaron states due to the significant electron−phonon coupling (Figure 3c).23 As the doping level increases, the wave functions of the near (bi)polarons which lied in the same polymer backbones or adjacent backbones gradually overlap and generate the (bi)polaron bands.24,25 Meanwhile, the width of (bi)polaron bands is gradually increased, resulting in a small band gap. As a C


Review


Figure 4. Research route of charge transport in conducting polymers: (1) tailoring molecular structure, including polymer (e.g., P(NDI2OD-T2)) and dopant structures (e.g., N-DMBI); modulating (2) solution aggregates; and (3) film microstructures. Molecular structures dominate the intrachain charge transfer and influence the solution aggregates and morphology; film microstructures dominate the interchain charge transfer which inherit the features of the solution aggregates.

the accurate testing of mobility after doping. Usually, once the ordered film morphology is destroyed, the carrier transfer efficiency is significantly reduced. Based on the above analysis, modulating and optimizing the charge transport in conducting polymers can be considered from these three aspects: molecular structures (polymer and dopant), solution aggregates, and film microstructures. 2.3. Device Stability. Device stability of n-type polymers can be divided into air stability and operation stability. Compared with p-type polymers, most n-type conducting polymers are only stable under nitrogen atmosphere, which strongly restricted their practical applications. The air stability is mainly restricted by the instability of organic anions, especially for carbanions. When the n-doped polymers were exposed to air, the charge-carrying electrons would be quenched by the redox reaction with oxygen and water as follows

stacking and formed aggregates in common organic solvents. The film microstructures might inherit the features of the solution-state micro-/nanoassembly structures.29−31 Many high-mobility conjugated polymers have been proved to form unique supramolecular structures in solution, such as poly(3hexylthiophene-2,5-diyl) (P3HT) and poly{[benzodifurandione-based oligo(p-phenylenevinylene)]-alt5,5′-(2,2′-bithiophene)} (BDOPV-2T).32−35 For instance, in the case of BDOPV-2T, an n-type D−A conjugated polymer formed 1D rod-like structures in good solvent 1,2-dichlrobenzene (oDCB) and 2D lamellar structures in toluene. The great difference of solution aggregates directly influences the morphology and the device performances. By tuning the solution-state structure, films deposited from cosolvent with high crystallinity and good interdomain connectivity were obtained, leading to higher mobilities.36 Therefore, solution aggregation states of conjugated polymers have significant influence on the microstructures and morphology of films. The films of conducting polymers comprise crystalline domains connected by amorphous chains and possess microscale heterogeneity (Figure 4).37,38 According to the solid-state aggregation behaviors and paracrystallinity disorder, conjugated polymers can be classified into three types: semicrystalline, disorder aggregated, and completely amorphous polymers.27 For semicrystalline polymers, such as P3HT, their films contain a large volume ratio of ordered regions with crystallites or ordered aggregates. For disorder aggregated polymers (e.g., donor−acceptor polymers), the short-range order region was observed in these polymer films, which may be attributed to their large conjugated planes, close π−π distance, and strong electronic coupling between two repeating units. Hence, efficient charge transport occurs through an interconnected network of ordered regions. In contrast, most amorphous polymers show high disorder in the whole film, which exhibits low mobility. Besides, there are still some exceptions, such as indacenodithiophene-co-benzothiadiazole (IDTBT), which exhibits fewer crystalline microstructures than semicrystalline polymers in film but high mobility.39 After doping, the introduction of the dopants has effects on film morphology and carrier transport in conducting polymers. In addition, polymers with different types may have different tolerances to the doping process. But at this stage, this research is limited by

2H 2O + 2e− ↔ H 2 + 2OH−

(4)

2H 2O ↔ O2 + 4H+ + 4e−

(5)

which caused a rapid decrease of the electrical conductivity of the n-doped polymers.40,41 To avoid this decay process, the LUMO level of n-type polymers needs to below −4.7 eV, which is extremely challenging.40 The operation stability was limited for two reasons: dopant diffusion and dopant escaping. For example, alkali-metal ions tend to diffuse during operation.42 This migration leads to the formation of spatially dedoped regions, hence influencing the electrical properties of polymer layers drastically. The poor operation stability of conducting polymers severely limits their application in OTE devices. Therefore, for OTEs that are expected to go to market, the issue of stability must be considered seriously.

3. STRATEGIES TO ENHANCE THE n-Doping EFFICIENCY To optimize conductivity, it is therefore necessary to maximize the doping efficiency, namely, the charge carrier introduced per dopant. The p-type conducting polymers have achieved ultrahigh doping efficiencies, even close to 100%, by a combination of tetracyanoquinodimethane (TCNQ)-based dopants with high electron affinity (EA) and a series of D


Review

Chemistry of Materials polymers with low ionization energy (IE).19,43,44 However, ndoping always suffers from a low ratio of charges transferred between dopant and polymer. For example, only 1% dimethylbenzoimidazol (DMBI)-based dopant mixed with poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) P(NDI2OD-T2) ultimately contributed to the mobile charge carrier.45 Therefore, improving the efficiency of n-doping is extremely urgent for enhancing conductivity. 3.1. Lowering the LUMO Level of Polymers. The dopant−polymer energy level offset strongly impacts the electron transfer doping efficiency. Thermodynamically, lowering the LUMO level of n-type polymers is regarded as an effective strategy to enhance n-doping efficiency in the case of the limited choice of n-type dopants.10,46,47 Introducing electron-deficient atoms or groups to the backbone to lower the LUMO level of polymers has been widely applied in the molecular design of high-mobility n-type polymers.46 Pei et al. investigated the effects of halogen atoms on thermoelectric properties in three n-type poly(p-phenylenevinylene) (PPV) derivatives, BDPPV and its halogen substituted polymers FBDPPV and ClBDPPV (Figure 5). The LUMO levels of ClBDPPV and FBDPPV reached −4.30 and −4.17 eV, 0.29 and 0.16 eV lower than that of BDPPV.48 The doping levels were characterized by UV−vis−NIR spectra, ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spec-

troscopy (XPS), and all results suggested the higher doping efficiency for FBDPPV and ClBDPPV. N-DMBI-doped FBDPPV achieved a high conductivity up to 14 S cm−1, which is attributed to the high n-doping level and the high intrinsic carrier mobility. Thus, lowering the LUMO level is an effective approach to improve n-doping efficiency. 3.2. Improving the Miscibility between Dopant and Polymer. In the molecular doping process, the phase segregation between polymer and dopant will severely hinder sufficient loading of dopant and thus prevent the enhancement of charge carrier density. This conclusion was confirmed in ndoped conducting polymer systems.49 For the (4-(1,3dimethyl2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) doped P(NDI2OD-T2), Chabinyc et al. pointed out that the doping level was strongly limited by the poor miscibility between the dopant and the host polymer. The dopant aggregates appeared on top surface of the doped film and generated phase separation. 45 To overcome this issue, introducing polar functional groups to the polymer or blending polar chemicals to the doped films were employed.50 Liu et al. demonstrated a 200-fold enhancement of electrical conductivity in P(NDI2TEG-T2) by replacing the traditional alkyl side chains of the polymer P(NDI2OD-T2) with polar triethylene glycol-based side chains (P(NDI2TEG-T2) in Figure 5).51 Although the pristine P(NDI2TEG-T2) exhibited a lower electron mobility than P(NDI2OD-T2), its charge carrier density and doping efficiency were much higher than P(NDI2OD-T2) after N-DMBI doping. The molecular dynamic simulation result was consistent with the authors’ concept that the polar side chains could significantly favor the dispersion of dopants into the host matrix compared to the alkyl chains (Figure 6a,b). The free energy required to move an NDMBI molecule from the alkyl phase to the TEG phase was

Figure 6. Representative snapshots of coarse-grained molecular dynamics simulations of N-DMBI molecules dissolved in (a) a pure P(NDI2OD-T2) side chain phase (2-octyl-dodecane) and (b) a pure P(NDI2TEG-T2) side chain phase (triethylene glycol dimethyl ether). Adapted with permission from ref 51. Copyright 2017 Wiley-VCH Verlag GmbH & Co. Schematic of the proposed molecular packing in the (c) PDPH films and (d) PDPF films. PDPF shows improved miscibility with the N-DMBI dopant. Adapted with permission from ref 49. Copyright 2018 Wiley-VCH Verlag GmbH & Co.

Figure 5. Molecular structures of (a) conjugated polymers and (b) ndopants. E


Review

Chemistry of Materials calculated to be −16 kJ mol−1, which may be attributed to the polar structure of the N-DMBI cation. Similar methods were also applied by other groups, suggesting the effectiveness of this strategy.52 Weakening the electron donating ability of the donor part in a D−A polymer is another way to improve miscibility, due to the decrease of interchain interactions.49 Two n-type DPP-based D−A polymers, PDPH and PDPF, were designed and synthesized (Figure 5). The latter one has four fluorine atoms on its donor moiety. For N-DMBI doped PDPF, a high electrical conductivity of 1.30 S cm−1 was obtained, which is over 1000 times higher than that of PDPH. Further characterizations proved that the electron-withdrawing modification of the donor moiety enhanced the electron affinity of the polymer and changed the polymer packing orientation, leading to much better miscibility with dopants and a significantly improved doping efficiency (Figure 6c,d). In N-DMBI doped PDPH films, phase separation between two components and higher surface roughness at high dopant concentration were observed. PDPH doped with 9 wt % N-DMBI exhibited a negative Seebeck coefficient of −87 μV K−1, while further increasing the dopant ratio leads to positive Seebeck coefficients of 71−124 μV K−1. Interestingly, this unusual polar switching behavior was also observed in other n-doped systems.53,54 The possible mechanisms of this behavior include: (1) Fermi level across the transport level (eq 2) and (2) carrier conduction through charge transfer complex (CTC) states.53,54 For the n-doped PDPH system, the authors performed conductive atomic force microscope (C-AFM) measurements. The results suggested that PDPH might form several highly conductive islands separated from dominated low conductivity regions in the doped film. These locally heavily doped regions were caused by the phase separation, which is largely due to the poor miscibility between polymer and dopant. In these “local doping” regions, the Fermi level passes across the transport energy level, thereby leading to positive Seebeck coefficients (eq 3). These results suggested that the Seebeck coefficient in organic thermoelectric materials is still not well understood. Another “donor engineering” strategy, twisted donor, was also applied to improve the miscibility. P(BTP-DPP) (Figure 5), an ambipolar D−A polymer with nonplanar donor, was doped by (RuCp*mes)2 and achieved a high conductivity of 0.45 S cm−1 through a sequential casting procedure.56 However, via the same doping procedure, P(NDI2OD-T2) only showed a conductivity lower than 1 × 10−6 S cm−1. The steric space created by the nonplanar donor parts in the backbone can improve its miscibility with the large size dopants, whereas the strong molecular packing of P(NDI2OD-T2) prevented the dopants from infiltrating the film. Another study based on a “kinked donor” copolymer also demonstrated this point.57 It should be mentioned that this strategy tends to severely reduce the mobility of the pristine polymer which is unfavorable for maximizing conductivity. 3.3. Reinforcing the Doping Efficacy of n-Dopants. Compared to various p-type dopants, the design of n-type dopants is more difficult owing to the requirement of high-lying HOMO levels of dopants, which makes n-type dopants unstable against oxygen. For instance, alkali-metal naphthalides are too sensitive to air and moisture to be used as an effective n-dopant. To solve the dilemma, using air-stable precursor molecules which can be converted to active intermediates for n-doping after thermal- or photoactivation turned out to be a promising strategy.22 In recent years, high efficacy n-dopants represented

by organic hydride and organic dimers have shown high doping efficacy and good solution processability and have greatly accelerated the development of n-type conducting polymers. Benzimidazole derivatives were proven to be strong ndopants, which could dope a variety of organic semiconductors, such as C60 and phenyl-C61-butyric acid methyl ester (PCBM).47,58 N-DMBI and their derivatives have been widely used as dopants for n-type conducting polymers.45,48,49,51,52,55,59−61 However, according to the results from cyclic voltammetry (CV) and DFT calculation, the HOMO levels of N-DMBI were estimated to be −4.45 eV and −4.4 eV, which is lower than the LUMO level of most n-type polymers (∼−4.0 eV).10,62 Then, one question was raised: why can N-DMBI exhibit such high n-doping efficacy? In the NDMBI doped PCBM system, a possible doping mechanism was proposed, which involves a bimolecular hydride transfer from the dopant to PCBM in the rate-determining step, although a hydrogen-atom transfer with polar character cannot be definitively ruled out.47 More importantly, the efficacy of doping is inadequately described by the offset between the ionization energy of the DMBI-H donor and the electron affinity of the host material, because DMBI-H dopants act first as hydride/ hydrogen-atom donors (Scheme 1a).63 It has been demonScheme 1. N-Doping Pathways of (a) DMBI-H Derivatives and (b) Organic Dimers as Dopants and (c) Electrochemical Redox Potentials of the Dopantsa

a

DMBI-H (N-DMBI) energy levels were calculated by DFT10 and D2 ((RuCp*mes)2) energy levels were measured by CV.68. F


Review

Chemistry of Materials strated that N-DMBI underwent different doping mechanisms when doped organic molecules with different LUMO energy levels.10 Multiple doping mechanisms of N-DMBI may be one of the possible reasons for its high reactivity. A 1,4-dihydropyridine derivative, 4-(4-methoxyphenyl)-1methyl-2,6-diphenyl-1,4 dihydropyridine (DPDHP), was developed for efficient n-doping (Figure 5).64 As an organic hydride donor, DPDHP exhibited good solubility in many organic solvents and excellent chemical stability in air. The kinetics study of the DPDHP doping reaction demonstrated that the ratedetermining step is the hydride transfer process from DPDHP to the host materials. It is worth noting that DPDHP doping is a simple solution process without further thermal or photochemical treatment. The obtained conductivities of polymers doped by DPDHP were comparable with those of N-DMBI and its derivatives. Dimerization is also an effective method to improve the ndoping efficacy and air stability of organometallic species, as reported by Marder et al. (Figure 5).65,66 A number of dimers formed by certain 19-electron organometallic sandwich compounds such as (RuCp*)2 and (RuCp*mes)2 were broadly used to dope organic semiconductors with EAs as low as 2.8 eV (Scheme 1c).67 (RuCp*mes)2 doped P(BTP-DPP) exhibited a high conductivity of 0.45 S cm−1 by sequential casting processing.56 The dimerization strategy was extended to obtain dimers of DMBI derivatives by Bao and Marder et al. (Figure 5).65−67,69 Compared with previously reported N-DMBI and oMeO−DMBI-I dopants, the dimers ((DMBI)2) exhibited stronger doping effect in a more diverse array of materials. The high reactivity of organic dimers may be ascribed to the particular doping mechanism: the doping reaction can occur either via a reversible endergonic cleavage of the dimer followed by a rapid exergonic electron transfer or an endergonic electron transfer followed by a rapid cleavage of the dimer cation and a second electron-transfer reaction (Scheme 1b).68,69 These dopants have been proven to have the high n-doping efficacy and were broadly applied in OTEs. It should be emphasized that, when designing new dopants, not only the doping efficacy but also the influence on the polymer, such as morphology, Coulombic interactions, etc., should be considered.

Figure 7. Molecular structures of n-type conjugated polymers.

polymers have long polaron delocalization lengths as characterized by UV−vis−NIR absorption spectroscopy and photothermal deflection spectroscopy (PDS), because of their the minimal torsional strain between repeating units. A classical ladder-type conjugated polymer, polybenzimidazobenzophenanthroline (BBL) (Figure 7), was vapor-doped by tetrakis(dimethylamino)ethylene (TDAE), which achieved a conductivity up to 2.4 S cm−1.60 Compared with P(NDI2ODT2), a more extended spin (polaron) delocalization was observed in BBL due to its nearly torsion-free backbone (Figure 8a). A similar strategy of “backbone planarization” was also used to develop a series of NDI-based n-type conducting polymers by replacing the thiophene rings with more electron-deficient thiazole units.54,55 4.2. Tailoring the Branching Position of the Alkyl Side Chains. The interchain charge transport is mainly influenced by the film morphology and microstructures. Actually, considerable research efforts including molecular design strategies and fabrication techniques have been devoted to optimizing the polymer films to achieve a “more ordered” state, such as alkyl chain engineering, conformation-locking, solution shearing, and dip-coating, etc.35,72−74 Among these methods, alkyl chain engineering is a simple and effective method that can modulate the polymer packing without changing the conjugated backbones.75 Pei et al. found that interchain π−π stacking distance decreases with moving the branching position of alkyl chains away from polymer backbones. The side chain branching position also influences the crystallinity of polymer, degree of disorder in thin film, and polymer packing conformation, which ultimately affects the device performance.75,76 Takimiya et al. used the branching position tuning strategy to synthesize two conjugated polymers with different branching positions, PNDTI-BBT-DP and PNDTI-BBT-DT, which consist of naphtho[2,3-b:6,7-b′]dithiophenediimide (NDTI) and benzo[1,2-c:4,5-c′]-bis[1,2,5]thiadiazole (BBT) units (Figure 7).77 PNDTI-BBT-DP with farther branching position exhibited stronger molecular packing both in pristine and in doped films, which plays a critical role in enhancing charge carrier transport. After N-DMBI doping, PNDTI-BBT-DP achieved a conductivity 20 times higher than that of PNDTI-BBT-DT, reaching 5.0 S cm−1. Compared with the development of novel π-conjugated

4. STRATEGIES TO ENHANCE THE CHARGE CARRIER MOBILITY 4.1. Conjugated Backbone Planarization. In conducting polymers, the wave function overlap of near (bi)polarons generates intrachain or interchain (bi)polaron bands.23 Therefore, long polaron delocalization can enhance the intrachain charge carrier transport and interchain band coupling, which ultimately affect the carrier mobility and conductivity of polymers. However, most high-performance n-type conjugated polymers have donor−acceptor structures which exhibit large torsion angles between two building blocks. Theoretical and experimental studies reveal that the HOMO and LUMO orbital coefficients of most D−A polymers localized on the donor and acceptor fragments, respectively.70 These results lead to the localization of polaron in the backbone which may restrict the intrachain transport.70 Bao et al. systematically investigated a series of perylenediimide- and naphthalenediimide-based polymers and found that host polymers lacking donor−acceptor character can achieve high conductivities by solution n-doping.71 Higher conductivities up to 0.45 S cm−1 were found for the ethynylene-linked polymers (P(PDI2OD-A)) (Figure 7). The ethynylene-linked G


Review


Figure 8. Strategies to enhance charge transfer: (a) spin (α and β) density distributions of the BBL (n = 8, top) and P(NDI2OD-T2) (n = 5, bottom) oligomers. Adapted with permission from ref 60. Copyright 2016 Wiley-VCH Verlag GmbH & Co. (b) Scattering profiles in the in-plane and the outof-plane directions for FBDPPV pure films and with 1, 3, 5, 7, 15, 20, and 50 wt % dopant N-DMBI. Adapted with permission from ref 59. Copyright 2016 American Chemical Society. Schematic of the two doping methods: (c) mixed solution doping method and (d) sequential doping method. Adapted with permission from ref 80. Copyright 2016 The Royal Society of Chemistry.

the interchain charge transport is not the increase of polymer crystallinity but the increase of their tolerance to an inevitable large amount of trap and disorder brought by the doping process. 4.4. Influence of Different Device Fabrication Methods. Optimizing the device fabrication is another method to enhance charge carrier mobility of doped films. Usually, the doped polymer film is deposited from a solution containing both polymer semiconductor and dopant in a desired ratio (Figure 8c). The mixed solution can be used to quickly fabricate conducting films by spin coating or drop casting. However, the large number of dopants negatively affects the morphology and conductivity of the host polymers, especially at a high doping level.80 Another doping process called “sequential casting” was developed recently. The pristine polymer film is first deposited from a solution with high quality. The molecular dopant is subsequently deposited from a semiorthogonal solvent. Compared with the solvent used for dissolving polymers, the semiorthogonal solvent dissolves the dopant and swells the polymer films.80,81 In this case, dopants can penetrate into the polymer film and then dope the films.80 The swellability of a polymer film can be modulated by depositing from different solvents, and the doping level can be modulated by different dopant concentrations.56 According to the X-ray diffraction results, the dopants might be inclined to lie in the side chain

structures, this strategy provides an easier approach to optimize the performance of conducting polymers. 4.3. Morphology and Microstructures Tuning. Another important issue is the effect of the dopants on the morphology of conducting polymer films. Ma et al. systematically investigated the impact of dopants on the morphology of the n-type conducting polymer.59 To quantitatively analyze the molecular packing structure of FBDPPV, the coherence lengths of the FBDPPV film were characterized through (100) (Q ∼ 0.18 Å−1) and (010) (Q ∼ 1.7 Å−1) peaks. The out-of-plane scattering (010) peak decreased at the very beginning and then increased to the maximum when 5−7 wt % dopant was added, corresponding to the highest conductivity. At higher doping levels, dopants disrupt the molecular packing or crystallinity, thus further reducing the mobility and conductivity (Figure 8b).78,79 These results indicated that a small amount of dopant induces more ordered face-on molecular packing of FBDPPV, which might be beneficial to interchain charge transport. This conclusion is not general since it depends on selection of the pair of dopant and conjugated polymer. In other dopant/polymer systems, we also observed that the doping process does not appear to significantly change the molecular packing, possibly due to better miscibility.51 At high dopant loading level (>50 mol %), polymer microstructures can be severely damaged, especially for those doping systems that use large size dopants.49,55,56 These results suggest that the key to enhance H


Review


0.1 S cm−1 after one week of air exposure. The energy level modulating strategy for enhancing air stability has also been demonstrated in other n-doped systems, such as NDI-based polymers and organic small molecules.62,83 The key to decreasing the diffusion of dopants is enhancing the interaction between dopants and polymers. According to recent studies, large size dopants exhibited weaker Coulomb interactions with host materials, which may result in easier drift after applying electrical fields.85,86 To strengthen the interactions between polymers and dopants, introducing polar functional groups onto polymers is an effective way, such as introducing polar side chains.50,87 On the other hand, the thermal stability of the doped polymer films is also important for their thermoelectric applications. The reasons of thermal instability may be attributed to the sublimation and volatilization of dopants. In F4TCNQ doped P3HT films, the dopants could easily overcome the Coulombic, dipolar, and van der Waals binding forces to sublimate at high temperature, which caused the dedoping of the films and restricted their applications.87,88 For TDAE doped P(NDI2OD-T2) films, the UV−vis−NIR spectra showed that the absorption band of radical anion gradually decreased as soon as the samples were removed from the TDAE vapor. This might be explained by the low boiling point (∼60 °C) of TDAE, and the radical anion is reoxidized to the neutral polymer.89 On the other hand, an irreversible doping reaction may significantly enhance the thermal stability, which means the counterions cannot convert into the neutral species. Hence, search of less volatile n-dopants and an irreversible doping process are presumably the key for improving thermal stability of n-type thermoelectrics. Although the dopant diffusion and thermal stability of p-type conductive films have been extensively studied, little attention has been paid to n-type materials.

regions and amorphous regions of the films without significantly disrupting the π−π stacking of conjugated backbones.80 Meanwhile, the Raman spectra also proved that the sequentially doped polymer (such as P3HT) films could increase the backbone planarity and conjugation length of the polymer.82 Using this doping method, the doping levels and conductivities can be obviously improved in some polymer systems.56 Recently, the solid-state diffusion doping (vapor infiltration doping) where the molecular dopants evaporated or sublimated on top of a polymer layer could significantly enhance the electrical conductivity and power factors of p-type polymers.19,44 This doping method could retain a high level of microstructural order, such as unperturbed π−π stacking and long-range orientational correlation length of domains.19 These factors enabled observation of 2D coherent charge transport. The vapor doped poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) achieved a high Hall mobility up to 1.8 cm2 V−1 s−1.44 Although sequential casting and solid-state diffusion doping exhibit advantages in improvement of electrical conductivity, they are only suitable for thin film (

Strategies To Enhance the Conductivity of n-Type Polymer

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