Recent advances in diketopyrrolopyrrole (DPP) based next-generation thermoelectric materials: an overview
0 Abstract
The imminent global energy crisis and the growing global demand for electricity, which require the development of alternative energy conversion technologies such as organic thermoelectrics, have attracted much attention from the scientific community due to their capability to convert low-grade waste heat into electrical energy. In the last decades, p-type and n-type thermoelectric polymers have been studied extensively and have achieved significant progress in thermoelectrics. In particular, diketopyrrolopyrrole (DPP)-based thermoelectric materials have gained much attention from researchers due to their unique structural properties. This review discusses potential of
Keywords
INTRODUCTION
The combustion of fossil fuels provides almost 90% of the power generated by heat engines globally. However, the world’s fast-growing demand for electricity cannot be met by fossil fuel technologies. Therefore, an urgent and essential need to develop alternative energy conversion technologies to meet the energy requirement challenge[1]. However, most renewable energy conversion devices have fundamental limitations on their maximum conversion efficiency, which is at the best less than 40%, with a significant part of that energy dissipated into the environment as heat[2]. Therefore, to address global environmental challenges and provide new opportunities for the usage of renewable energy resources, it is imperative to develop novel technologies with higher energy conversion efficiency and less waste heat. Recently, thermoelectric (TE) and photovoltaic (PV) technologies have emerged as two viable clean, sustainable energy conversion methods to address the energy crisis from an environmental and sustainable standpoint.
Significant progress has been achieved in the development of PV materials over the past ten years, but the research in the field of TE is still behind. Unlike photovoltaic techniques, TE technology has shown remarkable performance, as TE devices rely not only on solar energy but also on various heat sources, including body heat, which is a promising and alternative energy source for the future[3]. TE materials hold immense promise for powering a wide range of low-power technologies due to their ability to convert waste heat into electricity. One of the most attractive features of TE generators (TEGs) is their scalability and maintenance-free operation, making them ideal for small-scale and remote applications. This makes them excellent candidates for portable and wearable electronics. Additionally, TEGs are increasingly used to power low-energy devices such as radiator valves (Micropelt), wireless sensor nodes[4], and industrial monitoring systems[5]. In the biomedical field, TE materials are particularly valuable for powering biosensors and implants by harvesting body heat[6,7], thus eliminating the need for external power sources. Beyond healthcare, TEGs are beneficial in environments where solar energy harvesting is not viable, such as in mines, pipelines, and aircraft. In such critical or hazardous zones, TE-based sensors offer reliable solutions for fire detection, homeland security, and environmental monitoring[8,9].
Traditional TE devices are generally based on inorganic materials due to their superior TE performances and stability over organic materials. The inorganic materials such as lead telluride (PbTe)[10], bismuth telluride (Bi2Te3)[11], silicon-germanium (SiGe)[12] and tin selenide (SnSe)[13] are extensively studied, where the ZT value (dimensionless merit figure) reaches to 2.6[14-17]. On the downside, many in organic semiconductor (OSC) materials have certain disadvantages inherent to the material, such as scarcity, toxic nature, inadequate processability, high thermal conductivity, and high manufacturing cost. In the recent years, research on carbon nanotube (CNT)-based are promising TE materials. Single-walled CNTs (SWCNTs) are highly promising for thermoelectrics due to their high conductivity, ease of doping, and surface functionalization. Their charge carrier type and density can be tuned through chemical doping, though stable n-type doping remains a challenge. Forming CNT-based composites with organic or inorganic materials enhances TE performance by combining high conductivity, improved Seebeck coefficient (S), and low thermal conductivity (κ)[18,19]. On the other hand, OSCs have been neglected for decades because of their low energy conversion efficiency and potential bad thermal stability, but they are potential candidates for the conversion of low-grade thermal energy into useful electricity, as they possess low-cost, lightweight, and mechanically flexible properties compared to classical in OSCs[20]. Over the past several decades, extensive research has been dedicated to the development and modification of organic polymers and molecules with promising performance capabilities for use as TE materials[21]. Figure 1 demonstrates an increase in the number of research publications conducted in TE (keyword as thermoelectric polymer) which shows the significant efforts that have been made in the field of TE. Despite the current challenge presented by the low κ value of polymers, which remains an ongoing issue that researchers are striving to overcome, there have been many reports of organic TE materials exhibiting ZT values greater than 1, attributed to their advantageous traits of high electrical conductivities along with impressive power factor (PF)[14,22].
Figure 1. Trend in the number of publications related to “thermoelectric polymers” over the past decades, based on data from the Web of Science.
The most common organic materials showing TE properties are conducting polymer (CP) such as
Recently, diketopyrrolopyrrole (DPP)-based polymers attracted much attention in the field of OSCs such as OSCs[27], organic field-effect transistor (OFETs)[28-31], organic light emitting diodes[32] Organic memory devices[33-35], chemical sensors[36-38], photodetectors[39,40], and light-emitting electrochemical cells[41]. The keen interest of DPP derivatives is due to their attractive features such as low production cost, wide optical absorption, exceptionally high photochemical stability, high thermal stability, functionalized groups, and tunable semiconductivity, etc.[42]. Numerous DPP derivatives that have been reported in studies are either small molecules or polymers that exhibit exceptional power conversion efficiency (PCE) or high
DPP-based TE materials offer several advantages over conventional TE polymeric materials. DPP units possess a rigid, planar structure that facilitates strong π-π stacking interaction between polymer chains, leading to high charge carrier mobility and, consequently, enhanced σ[55]. Additionally, the electronic properties of DPP polymers can be readily tuned through chemical modifications, such as varying the side chains, incorporating electron-donating/withdrawing groups, and copolymerization with other monomers[56] as shown in Figure 2. Furthermore, many DPP polymers exhibit excellent thermal stability and can be processed using solution-based techniques, enabling the fabrication of flexible and low-cost TE devices.
Figure 2. Schematic representation of the DPP central unit, flanked by structural moieties tailored for n-type (left) and p-type (right) thermoelectric materials. DPP: Diketopyrrolopyrrole.
The main objective of this review is to give a comprehensive overview of recent advancements in the TE materials, mainly focused on DPP-based donor-acceptor p-type and n-type copolymers. More specifically, the structure-property relationship, along with the strategies we have explored to enhance performance. Furthermore, we will introduce results and comparisons of different strategies aimed at improving TE performance, providing a comprehensive overview of achievements and ongoing challenges in this exciting field.
THEORY OF THERMOELECTRIC
The intrinsic property of materials to generate power from heat truly opens up the potential to harvest
Figure 3. Schematic illustration of fundamental thermoelectric effects: (A) Seebeck effect, where a thermal gradient induces an electric potential, and (B) Peltier effect, showing heat exchange at junctions upon current flow.
The energy conversion efficiency of TE materials is generally determined by using the dimensionless
Where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the working temperature. From Equation (1), a higher S, higher σ, and lower k can therefore result in better TE performance. Materials with intrinsically high ZT are rare, nevertheless, because these three factors (S, σ, and k) are highly interdependent in most of the materials[66]. However, the intrinsic problems arise from the determination of κ for organic-based TE materials compared to that of inorganic materials; the performance of organic thermoelectric (OTE) materials is generally assessed based on the PF given by
Therefore, ZT is frequently substituted with PF for assessing the TE performance of organic polymer materials and their corresponding composites, owing to their low thermal conductivity.
STATUS OF THERMOELECTRIC MATERIALS
P-type thermoelectric polymers
Conventional p-type TE polymers
The TE performance of various p-type conventional CP including PANI[67], PPy[68], polythiophene (PTH)[69], poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)[70-73], polyacetylene (PA)[74,75], polycarbazoles (PC)[76], and their derivatives have been investigated, with their performance metrics summarized in Table 1[20,68,70,76-87]. Generally, the intrinsic conductivities of these CPs range from 10-16 to
Thermoelectric performance of conventional polymers
| Sr. no. | Polymer | k W m-1K-1 | [S cm-1] | S [μV K-1] | ZT max | Ref. |
| 1 | PEDOT:PSS | 0.34 | 0.06-945 | 8-888 | 1 × 10-2 at 300 K | [70,77,78] |
| 2 | PEDOT:Tos | 0.37 | 6×10-2-300 | 40-700 | 0.25 at RT | [20] |
| 3 | PANI | 0.02-0.542 | 10-7-320 | -16-225 | 1.1 × 10-2 at 423K | [79-82] |
| 4 | PPy | 0.2 | 6×10-2-300 | -1-40 | 3 × 10-2 at 423K | [68,83] |
| 5 | PTH | 0.028-0.17 | 10-2-10-3 | 10-100 | 2.9 × 10-2 at 250 K | [84-86] |
| 6 | PC | 0.34 | 4×10-5-5×10-2 | 4.9-600 | [76,87] |
Research efforts have led to significant improvements in TE performance of PEDOT:PSS. However, PEDOT derivatives remain promising; challenges such as hygroscopicity and insulating counterions have driven researchers to explore new p-type TE polymers that may exhibit superior charge-transport characteristics.
Diketopyrrolopyrrole based p-type TE polymers.
DPP has gained considerable attention due to its unique electronic properties and versatility in molecular design. Its strong electron-accepting features, combined with its high planarity and robust thermal stability, make it an ideal candidate for enhancing the performance of organic TE devices. Recent advancements in DPP-based materials have led to significant improvements in charge carrier mobility and thermoelectric efficiency, paving the way for innovative applications in organic electronics. Figure 4 displays the chemical structures of several DPP-based p-type D-A polymers and Table 2 summarizes their optimal doped TE performance.
Figure 4. Chemical structure of a DPP-based p-type donor-acceptor polymer used in organic thermoelectrics. DPP:
Summary of thermoelectric performances on DPP-based p-type thermoelectric D-A polymers at the optimized doping condition
| Sr. no. | Polymer | Dopants | σ [S cm-1] | S [μV K-1] | PF [μW m-1 K-2] | Ref. |
| 1 | PDPP-3T | FeCl3 | 55 | 226 | 276 | [95] |
| 2 | PDPP-4T | FeCl3 | 10 | 150 | 23.5 | [96] |
| Mo(tfd-CO2Me)3 | 0.3 | 220 | 15 | |||
| 3 | PDPP-5T | FeCl3 | 120 | 42.2 | 11.1 | [97] |
| 4 | PDPP-4T-EDOT | FeCl3 | 272 | 174.2 | 298.2 | |
| 5 | PDPPS-12T | FeCl3 | 318 | 80 | 178 | [101] |
| 6 | PDPPSe-12T | FeCl3 | 997 | 62 | 364 | |
| 7 | EHT6-20DPP | FeCl3 | 93.3 | 78 | 56.7 | [102] |
| 8 | PDPP-TT | FeCl3 | 45 | 92 | 38 | [103] |
| 9 | PDPP-g32T0.3 | FeCl3 | 360 | 56 | 110 | |
| 10 | PDPPS-5 | FeCl3 | 96.0 | 52.4 | 26.4 | [104] |
| 11 | PDTP-DPP | FeCl3 | 58 | 39.8 | 10.8 | [105] |
| 12 | PCZ-DPP | FeCl3 | 2.5 | 75 | 1.8 | |
| 13 | C6-ICPDPP | FeCl3 | 0.204 | 254 | 1.32 | [106] |
| 14 | DPP-MeDTP | FeCl3 | 92.8 | 80.9 | 60.7 | [107] |
| F4TCNQ | 14.6 | 242 | 85.2 | |||
| 15 | PCD-1 | F4TCNQ | 0.64(Mixing) 0.80(Spin coating) | 307(Mixing) 334(Spin coating) | 6.1(Mixing) 9.0(Spin coating) | [108] |
| 16 | PCD-2 | F4TCNQ | 3.1(Mixing) 1.3(Spin coating) | 266(Mixing) 321(Spin coating) | 21.8(Mixing) 13.6(Spin coating) | |
| 17 | PCD-3 | F4TCNQ | 0.02(Mixing) 0.84(Spin coating) | 464(Mixing) 431(Spin coating) | 0.47(Mixing) 15.6(Spin coating) | |
| 18 | P(BDTTT-DPP) | FeCl3 | 1.8 | 193.7 | 6.5 | [110] |
| 19 | P(BDT-DPP) | FeCl3 | 0.10 | 161 | 0.26 | |
| 20 | P10 | FeCl3 | 0.2 | 360 | 3.1 | [111] |
| 21 | P12 | FeCl3 | 8 | 124 | 12.3 | |
| 22 | P01 | FeCl3 | 8.8 | 103 | 9.5 | |
| 23 | P29DPP-BT | FeCl3 | 298 | 72 | 158 | [112] |
| 24 | P29DPP-BTOM | FeCl3 | 242.4 | 111 | 195 | |
| F4TCNQ | 89 | 122 | 132 | |||
| 25 | CN1 | FeCl3 | 56 | 122 | 79 | [113] |
| 26 | CN3 | FeCl3 | 78 | 95 | 65 | |
| 27 | CN5 | FeCl3 | 66 | 96 | 59 | |
| 28 | CN7 | FeCl3 | 15 | 155 | 34 | |
| 29 | CN9 | FeCl3 | 13 | 58 | 10 | |
| 30 | PDPP | FeCl3 | 152 | 33 | 17 | [119] |
| 31 | DITT30 | FeCl3 | 38 | 57 | 12.5 | |
| 32 | DITT80 | FeCl3 | 2.6 | 87.5 | 2.01 | |
| 33 | DITT100 | FeCl3 | 1.6 | 138 | 3.0 |
In 2017, Jung et al. synthesized poly(diketopyrrolopyrrole-terthiophene) (PDPP3T) due to its excellent performance in field of OFET. The study compares the thermoelectric efficiency of the DPP semiconductor PDPP3T against conventional thermoelectric polymer poly(3-hexylthiophene) (P3HT). By finely doping with FeCl3, PDPP-3T has higher than 200 μW m-1 K-2 PFs and peaks at 276 μW m-1 K-2, which is far more than the 56 μW m-1 K-2 PF of P3HT. High mobility PDPP3T increases σ without having a high dopant volume. It can be highly transparent due to a low band gap in electronic devices. In general, PDPP3T has all the attributes of high mobility, low band gap and well-controlled doping, to be a new gold standard for thermoelectric semiconducting polymers[95]. Besides, Liang et al. investigated the doping effects on
In 2020, Liu et al. investigated incorporation thiophene unit in the structure of PDPP-4T and synthesized PDPPT-5T and further modified the structure by replacing one thiophene unit with EDOT yielding PDPPT-4T-EDOT. From the study, it has been ascertained that PDPP-4T-EDOT can attain comparatively higher values of the Highest Occupied Molecular Orbital (HOMO), which facilitates efficient p-doping. Theoretical calculations indicate that PDPP-4T-EDOT exhibits slightly improved coplanarity compared to PDPP-5T to enhance charge carrier transport. Both polymers exhibit an increase in σ with an initial increase in dopant concentration, but the σ decreases at higher concentrations and with it, the S also reduces. The PF that the best results obtained by PDPP-4T-EDOT at a doping concentration of 0.5 mM was
Similarly earlier on, selenophene substitution was demonstrated to be an effective approach for improving intermolecular interactions and achieving high charge carrier mobility[98-100]. Ding et al. designed and synthesized selenium-substituted DPP-based polymer for TE devices. They modified the PDPP-3T structure by introducing a branched side chain on one side and a linear alkyl chain on the other end of each DPP unit, resulting in PDPPS-12. The structure was further modified by replacing one sulfur atom with selenium to obtain PDPPSe-12. The incorporation of selenium leads to strong intermolecular interactions and ordered molecular packing. PDPPSe-12, a DPP-selenophene copolymer, displayed impressive hole mobility, approaching 7 cm2 V-1 s-1. Its maximal σ, when doped with FeCl3, is nearly 997 S cm-1, over three times that of the doped PDPPS-12. Stronger intermolecular interactions occur within PDPPSe-12 because of larger atomic radius of selenium compared to that of sulfur. As a result, PDPPSe-12’s molecular arrangement remains mainly unaltered by doping, although PDPPS-12’s structure becomes less organized under similar doping conditions. Atomic force microscopy (AFM) analysis revealed that FeCl3 doping profoundly altered the morphology of PDPPS-12 by demolishing its fiber-like polycrystalline microstructure and reducing the surface roughness. On the other hand, PDPPSe-12 retained its fiber-like intercalating network and exhibited negligible morphological alteration upon doping. This suggests better dopant accommodation in
Notably, at low doping levels, PDPPSe-12 exhibits a remarkable hole mobility of around 1.9 cm2 V-1 s-1, leading to a maximum PF of 364 μW m-1 K-2, which contributes to a ZT value of 0.25-marking a record for CP. This work provides insights into the structure-property relationship of doped OSc and indicates a pathway for the rational design of high-performance OTE materials[101].
In 2021 Kim et al. synthesized a novel DPP-based conjugated polymer, EHT6-20DPP, containing eight thiophene units in its repeat unit structure. The incorporation of multiple thiophene groups leads to increased doping efficiency with the p-type dopant FeCl3 and thus improves thermoelectric properties. EHT6-20DPP achieves an σ of 93.28 S cm-1 along with a PF of 56.73 μW m-1 K-2, performing considerably better than the reference DPP polymer, PDPP3T, with three thiophene units. There is a close similarity in saturation field-effect mobilities in their undoped states for both polymers, but the conductivity of
Side-chain engineering has become a key method for developing DPP-based D-A TE polymers, enabling the optimization of dopant miscibility and tuning of physical parameters. One example is the copolymerization and combination of thienothiophene (TT) with oligo ethylene glycol (OEG) bithiophene functionalized with side-chain on donating framework. In this study, the design of new random copolymers incorporating planar DPP as an acceptor unit alongside TT and OEG functionalized bithiophene as donor units. Key design principles include promoting strong interchain donor-acceptor interactions through the inclusion of DPP-TT, which facilitates efficient charge transport via close packing and crystalline microstructures. The introduction of g32T-TT raises the HOMO level, improving charge transfer upon doping. Additionally, the dual incorporation of long alkyl chains and OEG side chains improves solubility and stabilizes dopants while fine-tuning the morphology of the films. Notably, the copolymer PDPP-g32T0.3 achieves an impressive σ of 360 S cm-1, S of 56 µV K-1, and PF of 110 μW m-1 K-2, significantly outperforming its counterparts
Zhong et al. come up with a strategic approach to enhance the TE performance of D-A polymers through donor engineering, specifically by substituting the dithieno[3,2-b:2’,3’-d]pyrrole (DTP) moiety
Lee et al. synthesized a new D-A conjugated polymer, C6-ICPDPP, to enhance TE performance by integrating electron-donating fused benzene, thiophene, and cyclopentadithiophene (CDT) units, with the electron-accepting DPP core in the polymer backbone. Through extended π-conjugation, this novel structural design promoted efficient charge transfer by achieving outstanding planarity. Using FeCl3 only
In 2021, Li et al. chose a benzodithiophene (BDT) donor building block whose fused-ring structure causes high planarity and a narrow band gap, and it possesses excellent electron-donating properties[109]. Furthermore, to enhance backbone planarity, the TT unit can also be incorporated into the side
At the same time, another innovative approach was made by Cao et al. to enhance the TE performance by using D-A copolymers with 1D EDOT framework with 2D BDTTT units, being randomly copolymerized to combine the high-Seebeck-coefficient of BDTTT with the strong-doping tendency of EDOT[111]. The copolymer P12 demonstrates a superior PF of 12.3 μW m-1 K-2, which is significantly higher than P10
Significant efforts have been made by Lee et al. to enhance the TE performance. Recently, they have designed and synthesized methoxy-functionalization donor-acceptor conjugated polymer based on DPP and bithiophene units and compared the performance between the methoxy-functionalized polymer, P29DPP-BTOM and its unfunctionalized counterpart, P29DPP-BT. The introduction of methoxy group increased lamellar stacking distance which enhanced the dopant’s diffusion and consequently its doping efficiency, notably for the bigger dopants. Hence, the methoxy-functionalized polymer, P29DPP-BTOM, had increased carrier concentration and mobility when compared with P29DPP-BT. In the case of the FeCl3 dopant, the resulting TE performance based on P29DPP-BTOM was impressive, with a maximum σ reaching 242.4 S cm-1 and a charge carrier mobility of 0.18 cm2 V-1 s-1 when compared to 298 S cm-1 and
Diketopyrrolopyrrole-based n-type thermoelectric polymers
Even though significant amount of research has been carried out on p-type TE materials and dopants in recent decades, studies of their n-type counterparts have significantly lagged behind those of p-type materials due to a variety of intrinsic and extrinsic limitations. One of the fundamental issues is the relatively low electron affinity (EA) of most conjugated polymers, which makes efficient and stable n-type doping difficult[120]. Additionally, n-type dopants tend to be more air-sensitive, chemically labile, or have harsh doping conditions such as high temperature or vacuum treatment that prevent practical use and scalability[121-123]. Furthermore, poor miscibility and limited dopant-polymer backbone compatibility often result in inhomogeneous doping, phase segregation, or inefficient charge transfer, resulting in reduced conductivity[124]. Some p-type materials have reached σ of over 1000 S cm-1[20]. With the increasing demand for high-performance OTE materials, the development of n-type CPs has been one of the major focuses of research; most n-type materials have low σ, and only a few of them have reached σ values above 1 S cm-1 after doping[22]. As a result, there has been growing interest in n-type materials and dopants, leading to significant advancements in the development of new n-type TE materials. The designed strategy for good
Currently, various kinds of n-type building blocks are used for the development of n-type OTE materials, such as naphthalene diimide (NDI)[135,136], perylene diimide (PDI)[130], benzodifurandione polyphenylenevinylene (BDOPV)[137,138], and bithiophene diimide (BTI)[139,140]. Among these, the most used electron-deficient building block for n-type OTE is NDI, and its representative compound would be N2200[141]. The compound has an σ of about 10-3 S cm-1 and a PF of about 10-2 μW m-1K-2[135]. In other words, it is of the utmost importance for the production of n-type OTE materials in order to develop new
Figure 5. Chemical structure of a DPP-based n-type donor-acceptor polymer used in organic thermoelectrics. DPP:
Summary of thermoelectric performances of DPP-based n-type thermoelectric polymers at the optimized doping condition
| Sr. no. | Polymer | Dopants | σ [S cm-1] | S [μV K-1] | PF [μW m-1 K-2] | Ref. |
| 1 | PDPH | N-DMBI | 1.01×10-3 | -80 | 5.11×10-4 | [133] |
| 2 | PDPF | N-DMBI | 1.3 | -235 | 4.65 | |
| 3 | P(TDPP-CT2) | N-DMBI | 0.39 | -580 | 9.3 | [123] |
| 4 | P(PzDPP-CT2) | N-DMBI | 8.4 | -370 | 57.3 | |
| 5 | P(PzDPP-2FT) | CoCp2 | 129 | - | - | [145] |
| 6 | PDCNBT-DPP | N-DMBI-H | 11.78 | -146 | 7.96 | [146] |
| 7 | PDCNBSe-DPP | N-DMBI-H | 12.36 | -102 | 9.22 | |
| 8 | PTz-5-DPP | N-DMBI | 8.39 | -338 | 106 | [147] |
| 9 | P(DPP-CNPz) | N-DMBI | 25.30 | -131.9 | 41.4 | [148] |
| 10 | P(DPP-DCNPz) | N-DMBI | 33.94 | -95.25 | 30.4 | |
| 11 | ThDPP-CNBTz | N-DMBI | 50.6 | -145 | 126 | [149] |
| 12 | pDSe | N-DMBI | 5.9 | -217.9 | 27.8 | [150]. |
| 13 | pDFSe | N-DMBI | 62.6 | -154.8 | 133.1 | |
| 14 | PTh-DPP | N-DMBI | 0.03 | -403.7 | 0.5 | [151] |
| 15 | PThTz-DPP | N-DMBI | 0.68 | -277.0 | 5.2 | |
| 16 | PTz-DPP | N-DMBI | 63.2 | -133.0 | 111.8 |
Recently, Wang et al. designed and synthesized new n-type TE polymers using novel approach that is development of A-A polymers designed strategy. This A-A strategy will help reduce intramolecular charge transfer (ICT) properties that influence the performance of semiconductors achieved with low FMOs. Therefore, they synthesized two A-A polymers based on this strategy namely, PDCNBT-DPP and PDCNBSe-DPP by integrating DPP with coplanar building block and cyano-functionalized benzothiadiazole (DCNBT) and benzoselenadiazole (DCNBSe) possessing high electron deficiency[146]. The synthesized polymers demonstrated very narrow bandgaps (~1.0 eV) and deep-lying LUMO energy levels of ~-3.90 eV, thus leading to n-type behavior within the OTE devices. The highest electrical conductivities, 11.78 and 12.36 S cm-1, are displayed by PDCNBT-DPP and PDCNBSe-DPP following N-DMBI-H doping, respectively, contributing to high PFs of 7.96 and 9.22 μW m-1 K-2. Relatively higher PF value obtained for the PDCNBSe-DPP than PDCNBT-DPP attributed to decrease in aromaticity and increase in quinodal nature of the acceptor unit when S was replaced with Se. Moreover, the PDCNBSe-DPP has low LUMO energy levels and high electron mobility than PDCNBT-DPP. These results highlight the fact that cyano-substituted benzoselenadiazole insertion into A-A-type polymers is a successful strategy for building high-performance n-type OTEs.
Shi et al. synthesized a new high-performance n-type homopolymer PTz-5-DPP, using firstly adopted C-H/C-H oxidative direct arylation polycondensation method using newly developed monomer 3,6-di(thiazol-5-yl)-diketopyrrolopyrrole (Tz-5-DPP). The polymer has demonstrated significant potential as an n-type OTE application. The PTz-5-DPP polymer displays both edge-on and face-on orientations in thin films with stable lamellar (22 Å) and π-π stacking (3.62 Å) distances prior to and after doping. After doping with
For thermoelectric applications, Tu et al. developed a novel class of structurally easy, low-cost, and easily available electron-deficient structural moiety for n-type polymers. They have come up with 3,6-dibromopyrazine-2-carbonitrile (CNPz) and 3,6-dibromopyrazine-2,5-dicarbonitrile (DCNPz), two cyano-functionalized pyrazine electron-deficient building blocks. Their great planarity is a major advantage for their use in n-type systems since the steric hindrance effect on the cyano-functionalized pyrazine building blocks has been theoretically studied. Two A-A type polymers, P(DPP-CNPz) and P(DPP-DCNPz), were synthesized from CNPz and DCNPz through incorporating them with the DPP unit[148]. The planar backbones and deep-lying LUMO levels of these polymers aid in the achievement of high n-type performance. The polymers exhibited unipolar electron mobilities of up to 1.85 cm2 V-1 s-1 for P(DPP-DCNPz) and 0.85 cm2 V-1 s-1 for P(DPP-CNPz). When doped with the molecular dopant N-DMBI, they revealed high PFs of 41.4 μWm-1 K-2 and 30.4 μWm-1 K-2, along with outstanding electrical conductivities of 25.30 S cm-1 and 33.93 S cm-1, respectively. The results point to CNPz and DCNPz as promising and
Low crystallinity conjugated polymers have been considered to be potential candidates for OTEs, especially for flexible devices, since their disordered structure provides an efficient means of introducing dopants and retains high flexibility innately. Therefore, Gao et al. designed and synthesized two n-type conjugated polymers, ThDPP-BTz and ThDPP-CNBTz, low crystallinity polymers with dual acceptor backbone featuring a thiophene-flanked DPP and cyano-substituted benzothiadiazole[149]. Due to its low LUMO energy level of below -4.20 eV and low crystallinity, ThDPP-CNBTz achieved high doping efficiency and better polaron delocalization. After doping with N-DMBI, ThDPP-CNBTz realized high σ of 50.6 S cm-1 and a PF of 126.8 μW m-1 K-2 at the best, which is among the highest values reported for solution-processed n-doped polymers. A flexible OTE device also fabricated based on 20 mol% doped polymer; exhibits a maximum PF of 70 μW m-1 K-2 and excellent bending stability with almost no change in conductivity after 600 cycles. This finding provided a pathway for the development of high-performance n-type materials suitable for flexible OTE devices.
Recently, Shen et al. made a significant approach to enhance the performance of TE polymer by incorporation of noncovalently fused-ring strategy. They synthesized pDFSe conjugated polymer using unique acceptor-triad structure containing DPP and difluorobenzoselenadiazole with noncovalently
MOLECULAR DOPING OF THERMOELECTRIC POLYMERS
Doping is an effective approach to optimize the performance of CP thin films, mainly for thermoelectric application. With the presence of dopants, there is considerable enhancement in charge carrier concentration within the polymer matrix, leading to an improvement in the σ, a key component in realizing high thermoelectric efficiency. Chemical doping and electrochemical doping are the two primary techniques employed to dope CP thin films. Chemical doping is typically accomplished by introducing small molecular oxidizing or reducing agents into the CP matrix. The dopants interact with the polymer chains through charge transfer mechanisms. Good alignment in the energy levels enables the energy exchange to add or extract electrons, thus altering the electrical properties of the CP. Conversely, electrochemical doping is performed by casting the CP film on a conductive substrate, which is then immersed in an electrolyte solution. These two methods represent controllable and effective ways to modulate the electrical properties of CP thin films, which is important for their applications in high-performance TE devices.
Doping mechanism and method
The enhancement of σ in CPs is highly dependent on the doping strategy employed. Redox doping and acid/base doping are the two main doping mechanisms that have been commonly recognized as shown in Figure 6A. Electron transfer between the CP and a dopant species governs the redox doping process and in case of acid/base doping the CP interacts with acidic or basic species that can donate or accept protons (H+) or hydride ions (H-). Depending on the direction of charge movement, this electron exchange may result in either n-type or p-type doping. In particular, n-type doping takes place when electrons transport from the dopant’s HOMO to the polymer’s LUMO, whereas p-type doping happens when electrons move from HOMO of CP to LUMO of the dopant. The efficiency of this doping process is critically influenced by the relative alignment of the energy levels between the dopant and the polymer.
Figure 6. (A) Schematic illustration of redox and acid-base doping mechanisms in organic semiconductors. Redox doping involves charge transfer, while acid-base doping occurs via hydride ion or proton transfer; (B) Schematic illustration of mixed-solution doping and sequential doping approaches.
The selection of an appropriate dopant is one of the most important factors in optimizing the TE properties of CPs because it directly influences critical parameters such as σ, S, and overall PF. For effective p-type doping, the EA of the dopant must be greater than the ionization energy (IE) of the polymer (EAdopant > IECP) for efficient charge transfer through the integer charge transfer (ICT) mechanism. Similarly, n-type doping requires dopants with ionization energies lower than the EA of the polymer (EACP > IEdopant). This energy level compatibility not only enables efficient doping but also prevents the formation of trap states that would hinder charge mobility.
The method by which dopants are introduced into TE polymers significantly influences doping efficiency, morphology, and the resulting σ[152,153]. Among the commonly employed methods, the mixing doping and sequential doping methods are common techniques as shown in Figure 6B, in the mixing doping the dopant and CP are individually dissolved in suitable solvents, then mixed together in specific ratios to produce a homogeneous mixture. This one-step, simple process enables easy control of the doping level by simply adjusting the concentration of the dopant. At higher doping concentration of the dopant, dopant molecules form aggregates, inducing phase separation and disturbing the polymer microstructure. Such aggregation can disturb the packing of polymer chains, reduce film crystallinity, and form trap states that eventually hinder the mobility of charge carriers[154]. Apart from that, miscibility of the dopant with the polymer and dopant solubility in the chosen solvent system are major concerns to ensure a homogeneous doping effect. For instance, the low solubility of high-electron-affinity dopants such as F4TCNQ in the majority of organic solvents can restrict their applicability for the mixing approach. To address the limitations associated with mixing doping, sequential doping methods have been developed. The sequential doping method has several advantages over the mixing doping method[155,156]. In this method, the CP film is first cast and dried, then a solution of dopant is deposited on top of it. In spin-coating, a minute quantity of the dopant solution dissolved in a semi-orthogonal solvent that swells but doesn’t dissolve the polymer is dispensed on the polymer surface and left to interact for a measured period of time. It is then spun rapidly to remove excess solution, giving a controlled and homogeneous doping layer. This method minimizes disruption of the native polymer morphology and helps preserve the chain packing and order, even at higher doping levels. The spin-coating process enables accurate control over the doping level by adjusting key parameters such as dopant concentration and contact time, resulting in enhanced electrical properties without compromising film integrity. Overall, mixing and spin-coating-based SQD both have advantages and challenges, and their selection should be guided by factors that include polymer-dopant compatibility, doping precision needed, and process scalability.
p-type dopants
To improve the TE performance of CPs, a variety of p-type dopants have been used; each one has unique benefits depending on its electronic characteristics and interactions with the polymer as shown in Figure 7A. The EA of p-type dopants determines their efficiency in CP, which further governs charge transfer efficiency during the doping process. Higher EA dopants are more suitable to accept electrons from the HOMO of the polymer, so oxidizing the polymer and raising hole concentration. The compound 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is a well-known p-type dopant due to its high EA, which plays a critical role in facilitating efficient p-type doping. Measuring at almost 5.25 eV, the improved EA of F4TCNQ is mostly attributed to the strong electron-withdrawing effects of the four fluorine atoms and four cyano groups symmetrically arranged around the TCNQ core[157]. F4TCNQ is a highly effective electron acceptor in redox doping systems since these substituents stabilize the LUMO. Moreover, changing the degree of fluorination on the molecule helps one to precisely adjust the EA of TCNQ derivatives. For instance, F1TCNQ and F2TCNQ, which include one and two fluorine atoms respectively, show lower EAs of roughly 5.01 eV and 5.10 eV, so reflecting a lower capacity to accept electrons than F4TCNQ. This tuning allows exact control over the charge transfer interactions with CP, so affecting the doping level and thermoelectric characteristics. Conversely, F6TCNNQ, a more highly fluorinated derivative, shows even more EA of roughly 5.37 eV, implying better doping potential because of its stronger electron-accepting properties[158].
Figure 7. Chemical structures of common (A) p-type dopants and (B) n-type dopants.
Besides F4TCNQ, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is also extensively employed as a
Another foremost interesting dopant is FeCl3, a popular inorganic oxidant that has proven to be an efficient and useful p-type dopant for numerous CP beyond typical systems such as P3HT and PEDOT. Recent studies have revealed it to be an exceptional doping efficiency in a family of DPP-based polymers such as PDPP-4T-EDOT, PDPSS-12T, PDPPSe-12T, and P29DPP-BTOM, all of which exhibit remarkable thermoelectric enhancements upon doping with FeCl3. In such systems, FeCl3 efficiently increases carrier concentration without disrupting the semi-crystalline arrangement of the polymer backbone necessary for sustaining effective charge transport and high σ. For instance, FeCl3-doped PDPP-4T-EDOT was found to possess a PF of 298.2 μW m-1 K-2 and a conductivity of 272 S cm-1, indicative of extremely high doping efficiency. Similarly, FeCl3-doped PDPSS-12T and PDPPSe-12T showed PF of 178 μW m-1 K-2 and
In contrast to molecular redox dopants, acid dopants such as (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (FTS), PTSA (p-toluenesulfonic acid), 4-ethylbenzenesulfonic acid (EBSA), and dodecylbenzenesulfonic acid (DBSA) operate via protonic (acid/base) doping methods and do not rely on EA-driven redox process. Successful p-type doping of the high-mobility polymer poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) with FTS and EBSA has been demonstrated, providing an insight into non-redox doping mechanisms[162]. FTS, specifically, functions via a protonation mechanism facilitated by acidic silanol groups produced upon hydrolysis, which form a self-assembled monolayer at the polymer surface and donate protons to the polymer backbone. This process enhances doping efficiency without compromising the structural integrity and stability of the film. Among, FeCl3 has emerged as the most useful and powerful p-type dopant for DPP-based TE materials, due to its strong oxidative ability and the ability to enhance charge carrier concentration without disturbing the polymer’s semi-crystalline order. In contrast to other molecular dopants such as F4TCNQ or CN6-CP, which are prone to solubility or miscibility limitations in certain backbones, FeCl3 boasts remarkable compatibility in a broad variety of DPP structures. For instance, FeCl3-doped PDPP-4T-EDOT, PDPSS-12T, PDPPSe-12T, and P29DPP-BTOM exhibit excellent TE performance. The improved performance is attributed to the ability of the dopant to penetrate deep-lying HOMO levels of the DPP units and introduce holes effectively, with the order of packing of the backbone for effective charge transfer.
n-type dopants
Effective n-type doping of OTE is essential to enhance σ, S, and thus the PF of CP. The chemical structure of common n-type dopants is shown in Figure 7B. Among the extensively studied dopants, N-DMBI is the most widely used n-type dopant. The EHOMO of N-DMBI is -4.44 eV, which is lower in energy than the ELUMO of typical n-type conjugated polymers, making direct electron transfer unfavorable thermodynamically. Instead, it follows other mechanisms of doping, mainly the hydrogen radical/electron transfer and hydride transfer mechanisms[120]. Thermal activation in the radical transfer mechanism enables cleavage of the C-H bond of N-DMBI to form a hydrogen radical and a neutral N-DMBI radical. The electron from the energy of singly occupied molecular orbitals ESOMO (-2.23 eV) of the N-DMBI radical is subsequently transferred to the ELUMO of the polymer, leading to the formation of a stable N-DMBI+ cation. For the hydride transfer mechanism, a hydride ion (H-) is heterolytically cleaved and transferred to the polymer. The two mechanisms are energetically demanding, with enthalpy changes of 80.2 and 74.6 kcal mol-1 for the formation of neutral hydrogen and hydride, respectively. Thus, thermal treatment (typically 80-180 °C) is required to initiate doping and the efficiency is highly sensitive to the chemical structure of the polymer[121-123]. In an attempt to overcome the solubility and miscibility problem of N-DMBI with polymers, scientists have synthesized many molecular derivatives. One of these is N-DPBI (4-(1,3-dimethyl-2,3-dihydro-1H-benzimidazol-2-yl)-N,N-diphenylaniline), wherein the dimethylamino group in N-DMBI is replaced with a diphenylamino substituent[163]. This modification was performed to stabilize the radical species via resonance by phenyl rings. However, despite structural modification, N-DPBI showed similar conductivity (σ ~4 × 10-3 S cm-1) to that of N-DMBI (~8 × 10-3 S cm-1) when used to dope N2200, largely due to poor miscibility of dopants in the polymer matrix. TP-DMBI is another novelty where a 3,4,5-trimethoxyphenyl unit is introduced into the N-DMBI backbone to enhance miscibility as well as electron-donating capability[124]. Further, a dimeric species of N-DMBI, (N-DMBI)2, which had the advantages of strong reducing ability and small cation size, was synthesized to deliver two electrons directly to the polymer and form two N DMBI+ cations. This pathway significantly enhances doping efficiency with σ
The air stability of n-type CP remains a challenge for its practical application in OTEs. Unlike their p-type counterparts, the majority of n-type polymers are stable under inert atmospheres only since organic anions, particularly carbanions, are susceptible to oxidative degradation. The reaction rapidly quenches mobile electrons and decreases σ[128,168]. One of the potential approaches for preventing this problem is the synthesis of polymers with deep levels of LUMO (typically lower than -4.7 eV), reducing energy offset to oxidative degradation[169,170]. In order to get over this, strategies such as locating electron-deficient groups on the polymer backbone and employing dopants that form stable charge-transfer complexes have proven useful. For instance, recently, PTz-DPP-based TE polymers have shown deepened LUMO levels with higher nitrogen content which directly affect the DOS, making it easier for polaron formation and leading to improved charge transport properties[151]. In addition, designing a thicker dopant or film layer enriched on the surface might utilize a self-encapsulation effect, preventing physical oxygen and moisture entry. For example, 3-8 μm thick ClBDPPV films doped with 25 mol% TBAF retained over 0.1 S cm-1 conductivity even after exposure to air for one week, showing the potential of combinations of low-LUMO polymers with stable dopants[167]. In combination, synergistic polymer chemistry design, dopant selection, and film morphology are required to achieve both high performance and stable durability for n-type OTE materials.
COMPARATIVE STUDY ON DPP-BASED THERMOELECTRIC POLYMERS
Comparative study in p-type DPP-based thermoelectric polymers
In a comparative study of p-type DPP-based thermoelectric polymers, PDPP-4T-EDOT, PDPPSe-12 and P29DPP-BTOM emerge as potential candidates, each excelling in different performance metrics. PDPP-4T-EDOT realizes a remarkably high PF of 298.2 μW m-1 K-2 at an optimal doping concentration of 0.5 mM. This work highlights that the incorporation of electron-rich EDOT moiety into a DPP framework results in significant enhancement of the electronic characteristics of the polymer, significantly outpacing other contenders such as PDPP-3T and P29DPP-BTOM. The EDOT incorporation increases the HOMO levels, allowing efficient p-doping without sacrificing high coplanarity and charge carrier transport characteristics. PDPPSe-12 is a selenium-substituted DPP-selenophene copolymer with strong intermolecular forces and stable morphology due to the larger atomic radius of selenium. PDPPSe-12 exhibited high hole mobility
Similarly, the methoxy-functionalized P29DPP-BTOM exhibits a remarkable PF of 195 μW m-1 K-2, which is substantiated by the judicious introduction of methoxy groups in the polymer backbone. These methoxy substituents increase the lamellar stacking distance of the polymer matrix and thus allow the dopant diffusion in a more effective way, directly increasing the doping efficiency, especially for larger dopants, including FeCl3. P29DPP-BTOM exhibits excellent σ up to 242.4 S cm-1 with a high S. Optimization of TE performance requires such a balance between conductivity and S, and thus P29DPP-BTOM becomes a strong competitor in this field. In comparison, although some other polymers, such as PDPP3T, also show excellent properties in regard to hole mobility and σ, the overall thermoelectric efficiency is less than
However, P29DPP-BTOM’s design illustrates the effectiveness of structural optimization to enhance doping efficiency and mobility and also underlines the importance of tailored strategies in polymer design for optimizing TE materials. The comparative study of σ and PF of DPP-based p-type thermoelectric polymers over time is shown in Figure 8A and chemical structures of the high performance p-type thermoelectric polymer are shown in Figure 8B.
Figure 8. (A) Comparative analysis of the progress in electrical conductivity and PF of DPP-based p-type thermoelectric polymers reported over time and (B) Chemical structures of representative high-performance p-type thermoelectric polymers, showcasing key structural features contributing to enhanced TE properties. PF: Power factor; TE: thermoelectric; DPP:
Comparative study in n-type DPP-based thermoelectric polymers
In the search for advanced n-type thermoelectric polymers, several novel materials have emerged, each demonstrating unique structural features and electron-transport properties. Among these, the significant examples are P(PzDPP-CT2), PTz-5-DPP, ThDPP-CNBTz, PTz-DPP and pDFSe. The compound P(PzDPP-CT2), which incorporates pyrazine groups and cyano-functionalized bithiophene, has shown an excellent σ of 8.4 S cm-1 and a PF of 57.3 μW m-1 K-2. Such performance indicates a remarkable EA and effective charge transport, which is supported by intramolecular hydrogen bonding interactions.
Figure 9. (A) Comparative analysis of the progress in electrical conductivity and PF of DPP-based n-type thermoelectric polymers reported over time and (B) Chemical structures of representative high-performance n-type thermoelectric polymers. PF: Power factor; DPP:
From the literature discussed, several strategies have been identified to overcome low TE performance. Firstly, the synthesis of electron-deficient backbones such as DPP-based polymers lowers LUMO energy levels and facilitates better n-doping thermodynamics. For instance, the DPP-based copolymer P(PzDPP-CT2) showed high σ of 8.4 S cm-1 and PF of 57.3 μW m-1 K-2, which indicates improved n-type doping efficiency due to its well-structured molecular architecture. Second, dopant design advancements, particularly with N-DMBI derivatives such as TP-DMBI and (N-DMBI)2, have improved doping efficiency through improved dopant-polymer miscibility, enhanced radical stability, and promotion of multielectron transfer. Moreover, the unprecedented enhancement of n-type polymer TE performance by the design of pDFSe with a noncovalently fused-ring structure with lower CCL 26.4 Å, higher paracrystalline disorder (g = 21%), and more planar surface morphology. These features enhance doping efficiency so that pDFSe can show a high electron mobility of 6.15 cm2 V-1 s-1 and, upon n-doping, an exceptional σ of 62.6 S cm-1 and a PF of 133.1 μW m-1 K-2. These findings in aggregate indicate a clear direction forward in the development of high-performance n-type TE materials and illustrate the necessity of dopant polymer synergy.
CONCLUSION AND OUTLOOK
In this review, we provided an overview of recent advances in organic TE materials, more precisely based on the DPP p-type and n-type thermoelectric conjugated polymers as DPP has proven to be an extremely promising building block in the preparation of state-of-the-art OTE materials. Its unique electronic features, including a strong electron-accepting tendency, high planarity, and good thermal stability, have enabled the advancement in p-type and n-type materials with a significantly enhanced TE performance. Molecular design strategies for OTE materials are evolving from simply optimizing σ, S, and κ, toward a deeper understanding of the structure-property-performance relationships at the molecular level. For DPP-based systems, particularly, fine-tuning these interdependent parameters through rational backbone and
Overall, DPP-based materials are a promising candidate for efficient organic TE materials. The insights gained from the comparative studies of p-type and n-type materials pave the way for optimizing these promising TE materials. Although significant progress has been made in enhancing the thermoelectric properties of DPP-based polymers, their implementation in practical devices such as flexible thermoelectric modules or wearable energy harvesters remains limited. Future research should focus on scalable processing techniques and device fabrication strategies to translate these promising materials into real-world applications. Continued research and development in this field is expected to lead to significant advances in the field.
DECLARATIONS
Authors’ contributions
Proposed the topic of this review: Kim, Y. H.
Manuscript writing: Girase, J. D.; Kim, Y. H.
Data curation: Girase, J. D.; Kim, I. C.
Availability of data and materials
Not applicable.
Financial support and sponsorship
This research was supported by the National Research Foundation (NRF) of Korea (Grant numbers RS-2023-00301974, RS-2024-00336766 and RS-2024-00406548).
Conflicts of interest
Kim Y. H. is an Editorial board member of the journal Energy Materials and was not involved in any part of the editorial process, including reviewer selection, manuscript handling, or decision-making. The other authors declare that they have no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
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