Raffaella Ferraiuolo,1 Rodolfo Esposito,2 Maria Grazia Maglione,3 Paolo Tassini,3 Giuseppe Vitiello,4 Gerardino d'Errico,3 Alessandro Pezzella1,5,6

1 Department of Physics “Ettore Pancini”, University of Naples “Federico II”, 80126 Naples, Italy;
2 Dept. of Chemical Science, University of Naples Federico II, Naples (IT)
3 Laboratory of Nanomaterials and Devices (SSPT-PROMAS-NANO) ENEA – C. R. Portici P.le E. Fermi 1 Loc. Granatello I-80055 Portici (NA), Italy
4 Dept. of Chemical, Materials and Production Engineering, University of Naples Federico II, Naples (IT)
5 National Interuniversity Consortium of Materials Science and Technology (INSTM), Piazza S. Marco, 4,
50121 Florence, Italy
6 Institute for Polymers Composites and Biomaterials (IPCB) CNR Via Campi Flegrei 34, I-80078 Pozzuoli (NA), Italy

Organic thermoelectric materials have potential for a wide range of applications from wearable heating, cooling, and energy generation at room temperature to continuously harvest waste heat from the human body to eventually satisfy the energy requirements of personalized healthcare devices.1 For this to see a full technological exploitation, high-conductance and high-Seebeck-coefficient materials are needed and there is a growing interest in developing organic thermoelectric materials which are flexible, cost-effective, eco-friendly and potentially energy-efficient.
Organic materials can exhibit electrical conductivity comparable to leading inorganic thermoelectric materials, but the Seebeck coefficients of many of the highest electrically conductive polymers significantly under perform those their inorganic counterparts.2
A recently explored approach to improve thermoelectric properties of organic material relies on the use of molecules bearing stable radical groups to increase the Seebeck coefficient.3
Eumelanins do exhibit a persistent EPR signal due to the presence of exceptionally stable free radicals located on the pigment backbone,4 and their integration with PEDOT:PSS provided a valuable tool to access innovative conducting material.5 6
In this perspective, here combining EPR spectroscopy and electrical measurements we address the impact of eumelanin-PEDOT:PSS blending on the Seebeck coefficient.
EPR analysis indicates a different paramagnetic behavior exerted by PEDOT:PSS polymeric mixture as function of the presence and content of in-situ prepared DHI eumelanin. Indeed, pure PEDOT:PSS blend, which is prepared by an exposure to ammonia vapors and thermally treated, presents a bi-component EPR peak indicating the presence of carbon-centered radicals with different relaxation times. The formation of DHI eumelanin leads to the progressive disappearance of the second component. This effect is quantitatively described by the changes in the spectral parameters, such as signal linewidth (AB) and area (A), determined through the direct analysis of the experimental spectra as a function of the incident microwave power. The correlation between these parameters and the Seebeck coefficients obtained by electrical analysis is investigated with the aim to define a relationship between the paramagnetic behavior and the electric conductivity of the composite material.

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1.    Massetti, M.; Jiao, F.; Ferguson, A. J.; Zhao, D.; Wijeratne, K.; Wurger, A.; Blackburn, J. L.; Crispin, X.; Fabiano, S., Unconventional Thermoelectric Materials for Energy Harvesting and Sensing Applications. Chem. Rev. 2021, 121 (20), 12465-12547.
2.    Shakouri, A., Recent Developments in Semiconductor Thermoelectric Physics and Materials. Annual Review of Materials Research 2011, 41 (1), 399-431.
3.    Tomlinson, E. P.; Willmore, M. J.; Zhu, X.; Hilsmier, S. W. A.; Boudouris, B. W., Tuning the Thermoelectric Properties of a Conducting Polymer through Blending with Open-Shell Molecular Dopants. ACS Applied Materials & Interfaces 2015, 7 (33), 18195-18200.
4.    d'Ischia, M.; Wakamatsu, K.; Napolitano, A.; Briganti, S.; Garcia-Borron, J.-C.; Kovacs, D.; Meredith, P.; Pezzella, A.; Picardo, M.; Sarna, T.; Simon, J. D.; Ito, S., Melanins and melanogenesis: methods, standards, protocols. Pigment Cell & Melanoma Research 2013, 26 (5), 616-633.
5.    Migliaccio, L.; Aprano, S.; Iannuzzi, L.; Maglione, M. G.; Tassini, P.; Minarini, C.; Manini, P.; Pezzella, A., Eumelanin-PEDOT:PSS Complementing En Route to Mammalian-Pigment-Based Electrodes: Design and Fabrication of an ITO-Free Organic Light-Emitting Device. Advanced Electronic Materials 2017, 3 (5).
6.    Migliaccio, L.; Altamura, D.; Scattarella, F.; Giannini, C.; Manini, P.; Gesuele, F.; Maglione, M. G.; Tassini, P.; Pezzella, A., Impact of Eumelanin–PEDOT Blending: Increased PEDOT Crystalline Order and Packing–Conductivity Relationship in Ternary PEDOT:PSS:Eumelanin Thin Films. Advanced Electronic Materials 2019, 5 (3).