Solution processed organic thermoelectric generators as energy harvesters for the Internet of Things

B. n-type thermoelectric materials for TEGs

Achieving a stable, highly conductive n-doped organic semiconductor has considerably been more difficult than for p-type systems, although very recently Tang et al. have demonstrated a high performing n-type polymer, poly(benzodifurandione) (PBFDO), with σmax > 2000 S cm−1 that could potentially close the gap between zT values of n- and p-type materials.5454. H. Tang, Y. Liang, C. Liu, Z. Hu, Y. Deng, H. Guo, Z. Yu, A. Song, H. Zhao, D. Zhao, Y. Zhang, X Guo, J. Pei, Y. Ma, Y. Cao, and F. Huang, Nature 611, 271 (2022). https://doi.org/10.1038/s41586-022-05295-8 One of the challenges for n-type systems is the small number of available electron-withdrawing building blocks that also exhibit stable electron transport characteristics.5555. J. T. E. Quinn, J. Zhu, X. Li, J. Wang, and Y. Li, J. Mater. Chem. C 5, 8654 (2017). https://doi.org/10.1039/C7TC01680H For stable transport in the doped state, the acceptor moiety in the organic semiconductor should have a sufficiently high electron affinity (EA), which translates to a low-lying energy level of the lowest unoccupied molecular orbital (LUMO), to avoid de-doping oxidation reactions in ambient air.5555. J. T. E. Quinn, J. Zhu, X. Li, J. Wang, and Y. Li, J. Mater. Chem. C 5, 8654 (2017). https://doi.org/10.1039/C7TC01680H Theoretical calculations and empirical data show that to obtain a stable n-doped state, an organic semiconductor should ideally have a LUMO energy, ELUMO, less than –4.0 eV.56–5956. H. Yan, Z. Chen, Y. Zheng, C. Newman, J. R. Quinn, F. Dötz, M. Kastler, and A. Facchetti, Nature 457, 679 (2009). https://doi.org/10.1038/nature0772757. D. M. De Leeuw, M. M. J. Simenon, A. R. Brown, and R. E. F. Einerhand, Synth. Met. 87, 53 (1997). https://doi.org/10.1016/S0379-6779(97)80097-558. J. Zaumseil and H. Sirringhaus, Chem. Rev. 107, 1296 (2007). https://doi.org/10.1021/cr050154359. B. A. Jones, A. Facchetti, M. R. Wasielewski, and T. J. Marks, J. Am. Chem. Soc. 129, 15259 (2007). https://doi.org/10.1021/ja075242e A low ELUMO would also energetically favor charge transfer from the highest occupied molecular orbital (HOMO) of the electron-donating dopant molecule or from the singly occupied molecular orbital (SOMO) in case a multi-step doping process.60,6160. J. E. Anthony, A. Facchetti, M. Heeney, S. R. Marder, and X. Zhan, Adv. Mater. 22, 3876 (2010). https://doi.org/10.1002/adma.20090362861. J. Han, A. Chiu, C. Ganley, P. McGuiggan, S. M. Thon, P. Clancy, and H. E. Katz, Angew. Chem., Int. Ed. 60, 27212 (2021). https://doi.org/10.1002/anie.202110505 In addition to the ionization potential of the dopant and the EA of the host, the molecular size and shape of n-type dopants and vicinity to the conjugated backbone will affect the doping efficiency, charge carrier concentration, and, thus, the electrical conductivity.6262. Y. Lu, J. Y. Wang, and J. Pei, Chem. Mater. 31, 6412 (2019). https://doi.org/10.1021/acs.chemmater.9b01422

1. n-type dopants for TEGs

When considering TEG fabrication, n-doping processes, such as mixed-solution doping (the dopant is dissolved in the same solution as the organic semiconductor before deposition) or sequential-solution doping (the dopant is dissolved in an orthogonal solvent with respect to the film and the film is soaked or coated in the dopant solution), are easier to implement than a method such as sequential-vapor doping (the dopant is volatilized in a closed chamber containing the cast film).6363. W. Zhao, J. Ding, Y. Zou, C. A. Di, and D. Zhu, Chem. Soc. Rev. 49, 7210 (2020). https://doi.org/10.1039/D0CS00204F There are a number of different classes of soluble n-dopants including benzimidazoles,64–6764. B. Saglio, M. Mura, M. Massetti, F. Scuratti, D. Beretta, X. Jiao, C. R. McNeill, M. Sommer, A. Famulari, G. Lanzani, M. Caironi, and C. Bertarelli, J. Mater. Chem. A 6, 15294 (2018). https://doi.org/10.1039/C8TA04901G65. M. Cassinelli, S. Cimò, T. Biskup, X. Jiao, A. Luzio, C. R. McNeill, Y. Y. 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2. n-type semiconductors for TEGs

Organic n-type semiconductors are often seen as the limiting factor for application-oriented solution-processed TEGs, because there is not a widely available material which exhibits high electrical conductivity, a reasonable negative thermovoltage, solution processability, and ambient stability, although there are claims that PBFDO could potentially be that breakthrough material.5454. H. Tang, Y. Liang, C. Liu, Z. Hu, Y. Deng, H. Guo, Z. Yu, A. Song, H. Zhao, D. Zhao, Y. Zhang, X Guo, J. Pei, Y. Ma, Y. Cao, and F. Huang, Nature 611, 271 (2022). https://doi.org/10.1038/s41586-022-05295-8 Despite the fact there is no clearly superior n-type thermoelectric material, the concerted effort to find such a material has resulted in several examples of solution-processable n-type organic semiconductors which can, and should, be put to use in application-oriented TEGs. 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Mater. 30, 1704630 (2018). https://doi.org/10.1002/adma.201704630104. D. Nava, Y. Shin, M. Massetti, X. Jiao, T. Biskup, M. S. Jagadeesh, A. Calloni, L. Duo, G. Lanzani, C. R. Mcneill, M. Sommer, and M. Caironi, ACS Appl. Energy Mater. 1, 4626 (2018). https://doi.org/10.1021/acsaem.8b00777 meaning that the reduced state of the organic semiconductors is still susceptible to interaction with oxygen and water. De-doping occurs rapidly for thin films, and the electrical conductivity of these NDI polymers typically decreases by several orders of magnitude within an hour after taking a sample out of the glovebox.104104. D. Nava, Y. Shin, M. Massetti, X. Jiao, T. Biskup, M. S. Jagadeesh, A. Calloni, L. Duo, G. Lanzani, C. R. Mcneill, M. Sommer, and M. Caironi, ACS Appl. Energy Mater. 1, 4626 (2018). https://doi.org/10.1021/acsaem.8b00777 Thionation of PNDI-T2 has been shown to lower the ELUMO to −3.96 eV, resulting in a relatively stable doped state over 16 h, but with a limited σmax just above 10−3 S cm−1.104104. D. Nava, Y. Shin, M. Massetti, X. Jiao, T. Biskup, M. S. Jagadeesh, A. Calloni, L. Duo, G. Lanzani, C. R. Mcneill, M. Sommer, and M. Caironi, ACS Appl. Energy Mater. 1, 4626 (2018). https://doi.org/10.1021/acsaem.8b00777Lactam-lactone-based polymers, like benzodifurandione-based polyphenylenevinylene (BDPPV), are another class of promising n-type organic semiconductors for TEGs. Pei and co-workers have published several studies on BDPPV-based polymers doped with well-known n-type dopants including N-DMBI and TAM that have demonstrated outstanding thermoelectric properties with σmax ranging from 14 to 90 S cm−1.68,73,106–10968. C. Y. Yang, Y. F. Ding, D. Huang, J. Wang, Z. F. Yao, C. X. Huang, Y. Lu, H. I. Un, F. D. Zhuang, J. H. Dou, C. A. Di, D. Zhu, J. Y. Wang, T. Lei, and J. Pei, Nat. Commun. 11(1), 3292 (2020). https://doi.org/10.1038/s41467-020-17063-173. H. I. Un, S. A. Gregory, S. K. Mohapatra, M. Xiong, E. Longhi, Y. Lu, S. Rigin, S. Jhulki, C.. Y. Yang, T. V. Timofeeva, J. Y. Wang, S. K. Yee, S. Barlow, S. R. Marder, and J. Pei, Adv. Energy Mater. 9, 1900817 (2019). https://doi.org/10.1002/aenm.201900817106. Y. Lu, Z. D. Yu, H. I. Un, Z. F. Yao, H. Y. You, W. Jin, L. Li, Z. Y. Wang, B. W. Dong, S. Barlow, E. Longhi, C. Di, D. Zhu, J. Y. Wang, C. Silva, S. R. Marder, and J. Pei, Adv. Mater. 33, 2005946 (2021). https://doi.org/10.1002/adma.202005946107. K. Shi, F. Zhang, C. A. Di, T. W. Yan, Y. Zou, X. Zhou, D. Zhu, J. Y. Wang, and J. Pei, J. Am. Chem. Soc. 137, 6979 (2015). https://doi.org/10.1021/jacs.5b00945108. W. Ma, K. Shi, Y. Wu, Z. Y. Lu, H. Y. Liu, J. Y. Wang, and J. Pei, ACS Appl. Mater. Interfaces 8, 24737 (2016). https://doi.org/10.1021/acsami.6b06899109. X. Y. Wang, Y. Liu, Z. Y. Wang, Y. Lu, Z. F. Yao, Y. F. Ding, Z. D. Yu, J. Y. Wang, and J. Pei, J. Polym. Sci. 60, 538 (2022). https://doi.org/10.1002/pol.20210493 In addition, BDPPV and its derivatives like ClBDPPV and FBDPPV have an ELUMO below −4.0 eV due to carbonyl groups with strong electron-withdrawing characteristics,107,110,111107. K. Shi, F. Zhang, C. A. Di, T. W. Yan, Y. Zou, X. Zhou, D. Zhu, J. Y. Wang, and J. Pei, J. Am. Chem. Soc. 137, 6979 (2015). https://doi.org/10.1021/jacs.5b00945110. K. Shi, Z. Y. Lu, Z. D. Yu, H. Y. Liu, Y. Zou, C. Y. Yang, Y. Z. Dai, Y. Lu, J. Y. Wang, and J. Pei, Adv. Electron. Mater. 3, 1700164 (2017). https://doi.org/10.1002/aelm.201700164111. A. Tripathi, Y. Lee, S. Lee, and H. Y. Woo, J. Mater. Chem. C 10, 6114 (2022). https://doi.org/10.1039/D1TC06175E although long-term stability testing in a TEG has yet to be confirmed.Poly(benzimidazobenzophenanthroline) (BBL) is another good electron transporting, highly planar, semiconducting polymer that has recently gained attention after Yang et al. developed a stable n-type ink in combination with the amine-based n-dopant, polyethylenimine (PEI).112112. C. Y. Yang, M. A. Stoeckel, T. P. Ruoko, H. Y. Wu, X Liu, N. B. Kolhe, Z. Wu, Y. Puttisong, C. Musumeci, M. Massetti, H. Sun, K. Xu, D. Tu, W. M. Chen, H. Y. Woo, M. Fahlman, S. A. Jenekhe, M. Berggren, and S. Fabiano, Nat. Commun. 12, 2354 (2021). https://doi.org/10.1038/s41467-021-22528-y The films spray-coated from the ethanol-based BBL:PEI ink demonstrated σmax up to 8 S cm−1 and a PFmax = 11 μW m−1 K−2.112112. C. Y. Yang, M. A. Stoeckel, T. P. Ruoko, H. Y. Wu, X Liu, N. B. Kolhe, Z. Wu, Y. Puttisong, C. Musumeci, M. Massetti, H. Sun, K. Xu, D. Tu, W. M. Chen, H. Y. Woo, M. Fahlman, S. A. Jenekhe, M. Berggren, and S. Fabiano, Nat. Commun. 12, 2354 (2021). https://doi.org/10.1038/s41467-021-22528-y The BBL:PEI ink was also shown to be air-stable over the course of 24 h, which is promising for TEG applications, although the method of deposition seems to be limited to spray-coating for now.112112. C. Y. Yang, M. A. Stoeckel, T. P. Ruoko, H. Y. Wu, X Liu, N. B. Kolhe, Z. Wu, Y. Puttisong, C. Musumeci, M. Massetti, H. Sun, K. Xu, D. Tu, W. M. Chen, H. Y. Woo, M. Fahlman, S. A. Jenekhe, M. Berggren, and S. Fabiano, Nat. Commun. 12, 2354 (2021). https://doi.org/10.1038/s41467-021-22528-ySmall molecule fullerene derivatives, such as [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), have extensively been studied for organic photovoltaic applications,113,114113. G. Paternò, A. J. Warren, J. Spencer, G. Evans, V. G. Sakai, J. Blumberger, and F. Cacialli, J. Mater. Chem. C 1, 5619 (2013). https://doi.org/10.1039/c3tc31075b114. J. A. Carr and S. Chaudhary, Energy Environ. Sci. 6, 3414 (2013). https://doi.org/10.1039/c3ee41860j but modifications of fulleropyrrolidine with oligoethylene glycol (OEG) side chains have demonstrated record high zT values, above 0.3, for n-type organic thermoelectric materials.115115. J. Liu, B. van der Zee, R. Alessandri, S. Sami, J. Dong, M. I. Nugraha, A. J. Barker, S. Rousseva, L. Qiu, X. Qiu, N. Klasen, R. C. Chiechi, D. Baran, M. Caironi, T. D. Anthopoulos, G. Portale, R. W. A. Havenith, S. J. Marrink, J. C. Hummelen, and L. J. A. Koster, Nat. Commun. 11(1), 5694 (2020). https://doi.org/10.1038/s41467-020-19537-8 The excellent thermoelectric performance of the fullerene derivative, PTEG-2, is due in part to the ethylene glycol side chains, which have been shown to improve dopant miscibility resulting in σmax > 10 S cm−1.115115. J. Liu, B. van der Zee, R. Alessandri, S. Sami, J. Dong, M. I. Nugraha, A. J. Barker, S. Rousseva, L. Qiu, X. Qiu, N. Klasen, R. C. Chiechi, D. Baran, M. Caironi, T. D. Anthopoulos, G. Portale, R. W. A. Havenith, S. J. Marrink, J. C. Hummelen, and L. J. A. Koster, Nat. Commun. 11(1), 5694 (2020). https://doi.org/10.1038/s41467-020-19537-8 The other huge advantage of the fullerene derivatives synthesized by Koster and co-workers (PTEG-2, PTEG-1, PDEG-1, PPEG-1, etc.) are their exceptional Seebeck coefficients, in the framework of doped organic materials, ranging from ∼−200 μV K−1 when overdoped to an outstanding ∼−600 μV K−1 at low doping concentrations.81,116,11781. L. Qiu, J. Liu, R. Alessandri, X. Qiu, M. Koopmans, R. W. A. Havenith, S. J. Marrink, R. C. Chiechi, L. J. Anton Koster, and J. C. Hummelen, J. Mater. Chem. A 5, 21234 (2017). https://doi.org/10.1039/C7TA06609K116. J. Liu, L. Qiu, G. Portale, M. Koopmans, G. ten Brink, J. C. Hummelen, and L. J. A. Koster, Adv. Mater. 29, 1701641 (2017). https://doi.org/10.1002/adma.201701641117. J. Liu, L. Qiu, G. Portale, S. Torabi, M. C. A. Stuart, X. Qiu, M. Koopmans, R. C. Chiechi, J. C. Hummelen, and L. J. A. Koster, Nano Energy 52, 183 (2018). https://doi.org/10.1016/j.nanoen.2018.07.056 The PFmax of PTEG-2 surpasses 40 μW m−1 K−2 making it one of the best n-type candidates for a TEG.115115. J. Liu, B. van der Zee, R. Alessandri, S. Sami, J. Dong, M. I. Nugraha, A. J. Barker, S. Rousseva, L. Qiu, X. Qiu, N. Klasen, R. C. Chiechi, D. Baran, M. Caironi, T. D. Anthopoulos, G. Portale, R. W. A. Havenith, S. J. Marrink, J. C. Hummelen, and L. J. A. Koster, Nat. Commun. 11(1), 5694 (2020). https://doi.org/10.1038/s41467-020-19537-8The dopants and semiconductors listed above are those which we believe to have the highest potential to be incorporated into an application-oriented TEG, but they make up a small subsection of organic thermoelectric materials as a whole. There are several comprehensive reviews giving a more complete account of organic thermoelectrics.14,15,25,7814. B. Russ, A. Glaudell, J. J. Urban, M. L. Chabinyc, and R. A. Segalman, Nat. Rev. Mater. 1, 16050 (2016). https://doi.org/10.1038/natrevmats.2016.5015. M. Massetti, F. Jiao, A. J. Ferguson, D. Zhao, K. Wijeratne, A. Würger, J. L. Blackburn, X. Crispin, and S. Fabiano, Chem. Rev. 121, 12465 (2021). https://doi.org/10.1021/acs.chemrev.1c0021825. L. M. Cowen, J. Atoyo, M. J. Carnie, D. Baran, and B. C. Schroeder, ECS J. Solid State Sci. Technol. 6, N3080 (2017). https://doi.org/10.1149/2.0121703jss78. Y. Lu, J. Y. Wang, and J. Pei, Acc Chem. Res. 54, 2871 (2021). https://doi.org/10.1021/acs.accounts.1c00223 Furthermore, the section above primarily focuses on electrical conductivity as the distinguishing property between materials, but the sections below will present a number of different strategies to influence thermal conductivity, thermovoltage, and morphology of a thermoelectric material that can be employed to enhance the overall performance of a TEG.

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