Bin Meng ,Jun Liu ,b,＊,Lixing Wng ,b

a State Key Laboratory of Polymer Physics and Chemistry,Changchun Institute of Applied Chemistry,Chi nese Academy of Sciences,Changchun,130022,PR China

b University of Science and Technology of China,Hefei,230026,PR China

Keywords:Polymer thermoelectrics n-type polymers n-doping Conductivity Power facto

ABSTRACT Thermoelectric(TE)materials based on conjugated polymers have received much attention due to their great advantages of solution processibility,light weight,flexibility,and low thermal conductivity.These advantages make them potential candidates for large-area,low-cost and low-power TE applications.Both efficient p-type and n-type conjugated polymers with high and comparable thermoelectric performance are required for practical TE applications.However,due to the inefficient n-doping efficiency and unstable electron transport of most n-type conjugated polymers,the TE performance of n-type polymers is much poorer than that of their p-type counterparts,impeding the development of polymer TE materials.Great efforts have been made to address the low ndoping efficiency and TE performance of n-type polymers,including the chemical modification of traditional ntype polymers,the design of new n-type conjugated polymers,and the development of more efficient n-dopants,as well as doping engineering.Nowadays,the TE performance of n-type polymers has been greatly improved,indicating a bright future for polymer TE materials.In this review,we summarize the recent progress made on ntype polymer TE materials,mainly focusing on the structure-performance relationships based on promising n-type polymers for TE applications.This review aims to provide some guidelines for future material design.

1.Introduction

Thermoelectric(TE)technology,which converts thermal energy to electric energy and electric energy to thermal energy with no fluids or moving parts,is considered one of the most promising clean energy technologies[1–8].Based on their chemical composition,TE materials include inorganic TE materials,organic small-molecule/polymer TE materials,metal-organic coordination polymer TE materials and organic-inorganic hybrid TE materials[3–5].Among them,polymer TE materials based on conjugated polymers have received much attention due to their great advantages of solution processibility,light weight,flexibility,and inherent low thermal conductivity[6–8].These advantages make them potential candidates for large-area,low-cost and low-power TE applications.In recent years,great progress has been made in the researchfield of polymer TE technology,and the TE performance of p-type polymers has been comparable to that of inorganic TE materials[6–8].

The energy conversion efficiency of TE materials is generally determined by a dimensionlessfigure of meritZT,which is defined asZT=S2σTκ-1,whereSis the Seebeck coefficient,σis the electrical conductivity,κis the thermal conductivity,andTis the absolute temperature[3].Since the thermal conductivity(κ)of thinfilm materials is difficult to measure exactly,as well as the fact that most organic small-molecule/polymer TE materials possess thermal conductivity values from approximately 0.3 to 0.5 Wm-1K-1,the TE performance of polymer TE materials can also be evaluated by the power factorPF,which is defined asPF=S2σ[3,4].

Both efficient p-type and n-type conjugated polymers with high and comparable thermoelectric performance are required for practical TE applications[1,2].However,due to the inefficient n-doping and unstable electron transport of most n-type conjugated polymers,the TE performance of n-type polymers is much poorer than that of their p-type counterparts[2].p-Type polymer TE materials have achieved electrical conductivities of over 1000 S cm-1and power factors of about 500μW m-1K-2.In contrast,very few n-type polymer TE materials exhibit electrical conductivities of over 10 S cm-1and power factors of over 10μW m-1K-2[1,2].The inferior TE performance of n-type polymers compared to the p-type impedes the development of thefield of polymer TE technology.

The recent research on n-type polymer TE materials starts from doping some traditional n-type polymers used for organic solar cells(OSCs)or organicfield effect transistors(OFETs)for enhanced TE performance[1,2,9].However,the usually inefficient n-doping and unstable electron transport of most n-type polymers retard the development of n-type polymer TE materials[1,2].Great efforts have been made to address these issues,including the chemical modification of traditional n-type polymers[10,11],the design of new n-type conjugated polymers[12,13],and the development of more efficient n-dopants[14,15],as well as doping engineering[16,17].Up to now,the TE performance of n-type polymers has been greatly improved,and is expected to catch up with that of the p-type soon,indicating a bright future for polymer TE applications[12,18,19].In this review,we summarize the recent progress on n-type polymer TE materials,focusing mainly on the structure-performance relationships based on promising n-type polymers for TE applications.This review aims to provide some guidelines for future material design.

2.Factors affecting the TE performance of n-type polymers

The electrical conductivities of n-type polymers are much lower than those of p-type ones.This is the key problem restricting TE performance[1,2].According to the definition of electrical conductivity,σ=qnμ(whereqis the elementary charge,nis the charge carrier density,μis the charge carrier mobility),high conductivity could be achieved via n-doping for enhanced electron density,and via improving the solid stacking of conductive polymers for enhanced charge carrier mobility[2].Several factors affecting the conductivity and TE performance of n-type polymers are schematically illustrated in Fig.1 and summarized below.

2.1.LUMO energy level of n-type polymers

LUMO energy levels of n-type polymers play an important role in ndoping efficiency[1,2].Lowering the LUMO energy level of conjugated polymers could enlarge the energy offset between the polymer and the n-dopant,thermodynamically facilitating electron transfer,thereby leading to efficient n-doping(Fig.1a)[2].Conductive polymers with a deep LUMO energy level could stabilize the electron transport without being quenched by water,oxygen and associated trap states(Fig.1a)[20,21].As a result,an extremely low LUMO energy level is considered to be a prerequisite for efficient n-type polymer TE materials.

2.2.Miscibility between polymer and dopants

Miscibility between the polymer and the dopants greatly affects the phase separation of the dopedfilm and thus the optimal loading of the dopants[10,22].Good miscibility enables the dopants to distribute uniformly in the polymer matrix without aggregating to an additional large domain(Fig.1b),which is beneficial for improving the n-doping efficiency without sacrificing the charge transport too much[22].On the contrary,poor miscibility would cause severe aggregation of dopants,which is harmful for the n-doping and the charge transport.As a result,improving the miscibility between the polymer and the dopants is considered an effective strategy for the enhanced conductivity and TE performance of n-type polymers[23].

2.3.Electronic structure of polymer backbone

Fig.1.Schematic illustration of the factors affecting the conductivity and TE performance of n-type polymers,(a)LUMO energy levels of polymers,(b)miscibility between polymer and dopants,(c)electronic structures of polymer backbone,and(d)solid stacking of polymer backbones.

Fig.2.Chemical structures of some representative n-type polymer TE materials and some commonly used n-dopants.

Polymer doping generates(bi)polarons with different delocalization lengths,which is partially determined by the electronic structure of the polymer backbone[24,25].For example,the donor-acceptor(D-A)characteristic of polymers always inhibits the delocalization of(bi)polarons after n-doping,while acceptor-acceptor(A-A)type polymers would exhibit facilitated charge delocalization after n-doping(Fig.1c)[24].The much delocalized(bi)polarons would result in the overlap of wave functions between adjacent(bi)polarons and thus would promote charge transport in both intra-chain and inter-chain directions[2].Some researchers point out that enhanced charge delocalization might facilitate the dissociation into free charges of the charge-transfer complex between the polymer and the dopant by overcoming the Coulomb interaction[17].All in all,the electronic structure of the polymer backbone plays an important role in charge generation and transport.

2.4.Solid stacking of polymer backbones

Compact stacking of polymer backbones with long-range order is beneficial for achieving an ideal doped morphology with molecular dopants preferentially distributing in the amorphous region for electron transition,leaving a pure crystalline region for charge transport,thereby largely maintaining the charge carrier mobility of the polymers(Fig.1d)[5,26].Some researchers have proposed that mixed face-on and edge-on stacking of polymer backbones would facilitate the accommodation of dopants,which is helpful for doping efficiency and charge transport[19,22].Therefore,the solid stacking of polymer backbones should befine-tuned for the enhanced conductivity and TE performance of n-type polymers.

3.Recent progress of n-type polymer TE materials

The recent research on n-type polymer TE materials starts from doping the state-of-the-art n-type polymer,poly{N,N′-bis(2-octyldodecyl)-1,4,5,8-napthalenedicarboximide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}(N2200,also named P(NDI2OD-T2))[27],which is well known for its high electron mobility and is commercially available[28].Although the preliminary attempt to use N2200 in TE applications resulted in only moderate performance,it attracted much attention to the enhancement of TE performance and opened the door to n-type polymer TE materials[10,29–31].To date,a series of efficient n-type polymer TE materials has been successfully developed via the chemical modification of traditional n-type polymers and the design of new n-type polymer systems[10–13].Their TE performance has been greatly improved[12,18,19].Generally speaking,these new n-type polymer TE materials can be divided into seven categories with specific properties for each:N2200 derivatives[10,29–31],poly(p-phenylene vinylene)(PPV)derivatives[11,18,32],benzimidazo-based ladder-type polymer [25],naphthodithiophene diimide (NDTI)-based copolymers [13,19],bithiazolothienyl-tetracarboxydiimide (DTzTI)-based homopolymer[17],diketopyrrolopyrrole (DPP)-based copolymers [12,22] and emerging organoboron polymers[33].The chemical structures of some representative n-type polymer TE materials and some commonly used n-dopants are summarized in Fig.2.The TE performance of these polymers is listed in Table 1.

Table 1 Summary of TE performance of representative n-type polymers.

3.1.N2200 derivatives

In the initial investigation of N2200 doped with 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)-N,N-dimethylaniline(N-DMBI),Chabinyc et al.found that only 1%of theN-DMBI molecules were active,and pointed out that the poor miscibility between N2200 andN-DMBI greatly hampered the doping efficiency and thus led to inferior TE performance[27].To address this issue,Liu et al.replaced the nonpolar alkyl side chains of N2200 with polar triethylene glycol(TEG)side chains to synthesize TEG-N2200(Fig.2)[10].Molecular dynamic simulation suggested that the polar side chains could significantly facilitate uniform dispersion of the dopants into the polymer matrix compared to the alkyl side chains(Fig.3a and b).This implied much enhanced miscibility between TEG-N2200 andN-DMBI,and led to greatly improved doping efficiency.As a result,the conductivity of TEG-N2200 after n-doping was enhanced by ca.200 times compared with that of N2200,reaching to 0.17 S cm-1.Müller et al.also introduced oligo(ethylene glycol)(OEG)type side chains to the backbone of N2200 to synthesize p(gNDI-gT2)for enhanced doping efficiency(Fig.2)[29].p(gNDI-gT2)doped withN-DMBI exhibited the maximum conductivity of 0.3 S cm-1and aPFof 0.4μW K-2m-1.Moreover,the authors found that doped p(gNDI-gT2)showed an enhanced air stability compared to the pristine polymer,indicating promising functions of OEG type side chains for efficient and stable n-type polymer TE materials[34].

Facchetti et al.reported a polymer,P(NDI2OD-Tz2),by replacing the bithiophene unit of N2200 with a bithiazole unit(Fig.2)[30].Due to the reduced steric repulsions of bithiazole compared with bithiophene,P(NDI2OD-Tz2)possessed a planar backbone and thus improved close π-πstacking.In addition,the electron-deficient character of bithiazole compared with bithiophene downshifted the LUMO energy level and weakened the D-A character of P(NDI2OD-Tz2).As a result,after doping with tetrakis(dimethylamino)ethylene(TDAE)vapor,P(NDI2OD-Tz2)exhibited a maximum conductivity of 0.1 S cm-1and aPFof 1.5μW m-1K-2,which were much higher than those of N2200 under similar treatment(σ=0.003 S cm-1,PF=0.012μW m-1K-2).

Fig.3.Representative snapshots of coarse-grained molecular dynamics simulations of N-DMBI molecules dissolved in(a)a pure N2200 side chain phase and(b)a pure TEG-N2200 side chain phase.Reproduced from ref.10 with permission from WILEY-VCH,copyright 2018.(c)Chemical structures of PNDI2TEG-2T and PNDI2TEG-2Tz,DFToptimized configuration for model molecules of PNDI2TEG-2T and PNDI2TEG-2Tz. (d) The measured DOS functions of PNDI2TEG-2T and PNDI2TEG-2Tz in the pristine and in the doped state.Reproduced from ref.31 with permission from WILEY-VCH,copyright 2018.

Liu et al.also reported a polymer based on an alternating naphthalenediimide(NDI)and bithiazole backbone,PNDI2TEG-2Tz,with triethylene glycol side chains on an NDI unit and alkoxyl side chains on a bithiazole unit(Fig.2)[31].Compared with the reference polymer with a bithiophene unit,PNDI2TEG-2T,PNDI2TEG-2Tz exhibited a planar configuration(Fig.3c),which was beneficial for theπ-πstacking of the polymer backbones.GIWAXS patterns indicated a decreasedπ-πstacking distance from 4.0?to 3.6?.The closeπ-πoverlap and preferential edge-on orientation of PNDI2TEG-2Tz were believed to help achieve a narrow and dense density of states(DOS)energy distribution(Fig.3d).As a result,PNDI2TEG-2Tz doped withN-DMBI exhibited a maximum conductivity of 1.8 S cm-1and aPFof 4.5μWm-1K-2,which were more than 1000 times higher than those of the reference polymer.

3.2.PPV derivatives

Pei et al.had previously developed a novel n-type polymer,benzodifurandione-based PPV(BDPPV),showing a low LUMO energy level of-4.0 eV,a strong aggregation tendency in solution and a high electron mobility of over 1 cm2V-1s-1[35].Inspired by these attractive n-type characteristics,they introduced chlorine and fluorine atoms onto the backbone of the BDPPV to synthesize ClBDPPV and FBDPPV(Fig.2),respectively,and then they investigated the TE behaviors of the three polymers[11].Due to the electron-withdrawing character of the halogen atoms,ClBDPPV and FBDPPV exhibited much downshifted LUMOenergy levels of-4.30 eV and-4.17 eV,respectively,which were believed to enhance the n-doping efficiency.The much stronger polaron absorption band for both conductive ClBDPPV and FBDPPVfilms confirmed their enhanced doping level(Fig.4a).In addition,the introduction of fluorine atoms improved the mobility of the FBDPPV by over two times compared with that of BDPPV.After doping withN-DMBI,BDPPV exhibited a maximum conductivity of 0.3 S cm-1and aPFof 1.7μW m-1K-2.The introduction of halogen atoms greatly enhanced the TE performance of the resultant polymers,with conductivities of up to 14.0 S cm-1and power factors of up to 28.0μWm-1K-2for FBDPPV.ThePFof FBDPPV was further improved to 51μWm-1K-2by the authors via employing a newly-developed n-dopant,trisaminomethane(TAM)[14].They further modified FBDPPV by replacing one alkyl chain with an OEG chain to synthesize UFBDPPV for enhanced doping efficiency(Fig.2)[18].After doping with TAM,UFBDPPV achieved the highest TE performance reported for solution processed n-type polymer TE materials,with a conductivity of up to 25.0 S cm-1and aPFof up to 92.0μW m-1K-2.

Pei et al.reported another PPV derivative,LPPV-1,with a rigid and coplanar backbone(Fig.2)[32].The carbon-carbon double bond in the main chain and the intra-molecular hydrogen bonds endowed the polymer with a nearly torsion-free backbone(Fig.4b).The rigid and coplanar backbone with the A-A character of LPPV-1 greatly extended the(bi)polaron delocalization length (Fig.4c),which was beneficial for intra-chain charge transport.Owing to the dense electron-withdrawing groups embedded in the backbone(Fig.4b),LPPV-1 exhibited an extremely low LUMO energy level of-4.49 eV,which was beneficial for the n-doping.LPPV-1 doped withN-DMBI exhibited a maximum conductivity of 1.1 S cm-1and aPFof 1.96μW m-1K-2.Moreover,the doped LPPV-1 was very stable,with only 2%degradation of thePFafter 7-days exposure to air.The TE performance of LPPV-1 was further improved via doping with TAM,giving a conductivity of up to 4.0 S cm-1and aPFof up to 34.8μW m-1K-2[18].

3.3.Benzimidazo-based ladder-type polymer

Ladder-type polymers for TE applications have attracted much attention due to their completely coplanar and torsion-free backbone with diminished D-A character,which would facilitate delocalizing(bi)polarons along the polymer backbones of conductive polymers and thereby improve the generation and transport of free charges[25].Fabiano et al.investigated the TE performance of a typical n-type ladder polymer,polybenzimidazobenzophenanthroline(BBL)(Fig.2).They also investigated the TE behavior of N2200 for comparison[25].DFT calculations revealed that the spin density distributions of the N2200 oligomer covered only one repeating unit(Fig.5a),which was ascribed to the much distorted backbone and the remarkable D-A character of N2200.On the contrary,the spin density distributions of BBL oligomer extended over three repeating units(Fig.5b),which was attributed to the rigid and planar backbone without the D-A character of BBL.Compared with N2200,the relatively long(bi)polaron delocalization length of BBL was considered to be beneficial for charge transport.After doping with TDAE,BBL exhibited a maximum conductivity of 2.4 S cm-1,which was almost 1000 times higher than that of N2200(Fig.5c).By optimizing the doping process,thePFof BBL was enhanced to 0.43μW m-1K-2.

Fig.5.Spin density distributions of(a)N2200 oligomer(n=5)and(b)BBL oligomer(n=8).(c)Electrical conductivities of BBL and N2200 doped with TDAE vapor.Reproduced from ref.25 with permission from WILEY-VCH,copyright 2016.

3.4.NDTI-based copolymers

Conjugated polymers with A-A character could conveniently achieve low LUMO energy levels and delocalized charges,which are beneficial for TE performance[24].Takimiya et al.copolymerized two strong electron-deficient units of NDTI and benzo[1,2-c:4,5-c′]-bis[1,2,5]thiadiazole(BBT)to synthesize two A-A copolymers,PNDTI-BBT-DT and PNDTI-BBT-DP(Fig.2)[13].The former linked 2-decyltetradecyl as the side chains and the latter linked 3-decylpentadecyl as the side chains.The two polymers exhibited extremely deep LUMO energy levels of about-4.4 eV,which would facilitate n-doping and stabilize electron transport.PNDTI-BBT-DP with a farther branching point of the side chains exhibited enhanced crystallinity in thinfilm compared with PNDTI-BBT-DT, which was beneficial for charge transport.PNDTI-BBT-DP doped byN-DMBI exhibited a maximum conductivity of 5.0 S cm-1and aPFof 14μW m-1K-2,which were much higher than those of PNDTI-BBT-DT(σ=0.18 S cm-1,PF=0.6μW m-1K-2).This work highlighted the importance of side chain engineering for optimized n-type polymer TE materials.

Fig.4.(a)Absorption spectra of BDPPV derivatives in thinfilm under pristine and doping conditions.Reproduced from ref.11 with permission from American Chemical Society,copyright 2015.(b)Schematic illustration of molecule design for LPPV-1,and DFT-optimized configuration for model molecule of LPPV-1.(c)Calculated spin density distributions of LPPV oligomers(n=6).Reproduced from ref.32 with permission from WILEY-VCH,copyright 2019.

Recently,Takimiya et al.reported another A-A polymer,pNB(Fig.2),by copolymerizing NDTI with electron-deficient bithiopheneimide(BTI)[19].pNB possessed a low LUMO energy level of-4.2 eV.However,the backbone of pNB had a large wavy configuration,which led to weak crystallinity in thinfilm(Fig.6a)and was harmful for charge transport.The authors then introduced a thiazole unit into the backbone,and further changed the alkyl side chains from 2-branching points to 3-branching points,to synthesize pNB-TzDP(Fig.2).The modifications maintained the deep LUMO energy level but endowed the resulting polymer with a(pseudo)straight-line configuration.This led to greatly enhanced crystallinity with much closerπ-πand lamellar stacking of polymer backbones(Fig.6b),which was beneficial for charge transport.The modifications also changed the stacking orientation from face-on to a bimodal orientation,which was speculated to facilitate better accommodation of dopants.After n-doping,theπ-πand lamellar stacking of the pNB-TzDP backbones are nearly undisturbed(Fig.6c).As a result,pNB-TzDP achieved a conductivity of up to 11.6 S cm-1and aPFof up to 53.4μW m-1K-2,which were much higher than those of pNB(σ=0.01 S cm-1,PF=0.3μW m-1K-2).

Fig.6.Backbone configuration and GIWAXS linecuts for(a)pristine pNB and(b)pristine pNB-TzDP.(c)Cartoon representations of molecular packing of pNB-TzDPfilms before and after doping.Reproduced from ref.19 with permission from WILEY-VCH.

3.5.DTzTI-based homopolymer

Guo et al.reported a homopolymer,PDTzTI,with all acceptor units based on DTzTI(Fig.2),showing unipolar electron mobility of as high as 1.6 cm2V-1s-1[36].Recently,cooperating with Liu et al.,they investigated the TE performance of PDTzTI[17].Compared with N2200,PDTzTI had a nearly coplanar backbone(Fig.7a and b),leading to a much enhanced crystallinity.PDTzTI showed much closerπ-πstacking and larger coherence length in both(100)and(010)directions than those of N2200(Fig.7c and d).The greatly enhanced crystallinity with compact stacking and the A-A featured backbone of PDTzTI all contributed to the improvement of the two-dimensional charge delocalization,thereby facilitating free charge generation by overcoming the Coulomb interaction in the doped state.The high crystallinity of PDTzTI was also beneficial for charge transport.As a result,PDTzTI achieved a maximum conductivity of 4.6 S cm-1,which was 500 times higher than that of N2200.After optimizing the doping process,thePFof PDTzTI reached 7.6μW m-1K-2.

Fig.7.DFT-optimized configuration for the repeating unit of(a)N2200 and(b)PDTzTI.GIWAXS linecuts of(c)N2200 and(d)PDTzTI in pristinefilms.Reproduced from ref.17 with permission from American Chemical Society,copyright 2019.

3.6.DPP-based copolymers

DPP-based copolymers could achieve high crystallinity and high electron mobility of over 5 cm2V-1s-1,which are very attractive for TE applications[9,37].Pei et al.enhanced the conductivity and TE performance of DPP-based D-A copolymer by modifying the donor moiety[22].They synthesized two polymers,PDPH and PDPF,alternating 2-pyridinyl substituted DPP and bi(2-thienyl)ethylene as the backbone without and with four fluorine atoms on the donor moiety,respectively(Fig.2).PDPF showed a lower LUMO energy level than PDPH because of the fluorine substitution.In addition,the introduction of fluorine atoms changed the stacking orientation of the polymer backbones from preferential edge-on to mixed face-on/edge-on(Fig.8a,c).The bimodal packing orientation of PDPF provided more transition regions and thus facilitated the accommodation of dopants,leading to the relatively uniform morphology of the dopedfilms(Fig.8e and f).While the highly doped PDPHfilm exhibited diffractions from the aggregated dopants(Fig.8b),there were no obvious diffraction bands of dopants for the highly doped PDPFfilm(Fig.8d),implying improved miscibility between the polymer and the dopants.Moreover,the modification of the donor moiety had almost no negative in fluence on the charge carrier mobility but reduced the hopping barrier in the conductive PDPF.As a result,PDPF exhibited a TE performance 1000 times higher than that of PDPH,with a maximum conductivity of 1.30 S cm-1and aPFof 4.65μW m-1K-2.

Recently,Lei et al.developed a strongly electron-deficient building block by means of flanking a pyrazine unit to a DPP core.They further copolymerized this building block with 2,2′-dicyanobithiophene to obtain P(PzDPP-CT2)(Fig.2)[12].P(PzDPP-CT2)exhibited the lowest LUMO energy level among the DPP-based copolymers.Additionally,due to the planar backbone and the far-branching-point alkyl side chains,P(PzDPP-CT2)exhibited very high crystallinity with an extremely small π-πstacking distance of 3.38?,which would facilitate the charge transport in conductivefilms.After doping withN-DMBI,P(PzDPP-CT2)achieved excellent TE performance with a conductivity of up to 8.4 S cm-1and aPFof up to 57.3μW m-1K-2,which were among the highest values reported for n-type polymer TE materials.

3.7.Organoboron polymers

Organoboron chemistry provides new methods to tune the optoelectronic properties of conjugated polymers for various applications,e.g.,OSCs and OFETs[38–42].Liu et al.developed a series of n-type polymer semiconductors based on the boron nitrogen coordination bond(B←N)using the strong electron-withdrawing character of B←N[43,44].These organoboron polymers have already achieved high performance in OSCs and OFETs[45–47].

Fig.8.GIWAXS patterns of PDPH in pristinefilm(a)and in highly dopedfilm(b);GIWAXS patterns of PDPF in pristinefilm(c)and in highly dopedfilm(d).Schematic illustration of phase separation for doped PDPH(e)and PDPF f).Reproduced from ref.22 with permission from WILEY-VCH,copyright 2018.

Recently,Liu and coworkersfirst proved that B←N is beneficial for facilitating the n-doping of resultant polymers and could be used to design n-type polymers for TE applications[33].Based on a typical D-A type polymer with an alternating isoindigo(IID)unit and bithiophene unit,PI-BT,they embedded B←N into the donor moiety to synthesize a novel organoboron polymer,PI-BN(Fig.9a).The introduction of B←N into the polymer backbone not only reduced the LUMO energy level by about 0.3 eV,but it also weakened the D-A character of the polymer backbone,which were considered to be helpful for the n-doping of the resultant polymer.As a result,PI-BN with B←N could be readily n-doped byN-DMBI,while PI-BT without B←N nearly could not be n-doped.The conductivity of PI-BN was enhanced by more than 5 orders of magnitude compared to PI-BT(Fig.9b).PI-BN gave a preliminary conductivity of 0.001 S cm-1and aPFof 0.02μW m-1K-2,which was comparable to that of N2200 under the same condition.In addition,PI-BN exhibited good miscibility with the n-dopant.The TE performance of organoboron polymers needs to be substantially improved to catch up with the other kinds of n-type polymer TE materials.

4.Summary and outlook

In this review,we summarized the recent progress on n-type polymer TE materials and discussed the structure-performance relationship of several types of representative material systems.Several strategies to improve the n-doping efficiency and TE performance have been demonstrated,e.g.,lowering the LUMOenergy levels of n-type polymers,weakening the D-A character of polymer backbones,improving the miscibility between polymer and dopants,planarizing the backbone and enhancing the crystallinity of polymers.Application of these strategies led to the greatly enhanced TE performance of n-type polymers,which is expected to catch up with that of the p-type soon.In addition,some single-component conductive polymers appeared,such as open-shell conjugated polymers[48]and“self-doping”conjugated polymers[49],which might also be used to develop high-performance and stable n-type polymer TE materials.Towards practical applications of organic TE technology,great attention should be paid to the Seebeck coefficient and the issue of the stability of n-type polymer TE materials in the future.

Fig.9.(a)Schematic illustration of the molecule design of PI-BN.(b)Conductivities of doped PI-BT and PI-BN at different doping concentration.Reproduced from ref.33 with permission from American Chemical Society,copyright 2020.

Declaration of competing interest

The authors declare no con flict of interest.

Acknowledgement

The authors are grateful for thefinancial support of the National Natural Science Foundation of China(No.21625403,21875244,21875241,22075271).