Tuning PEDOT:Tos thermoelectric properties through nanoparticle inclusion

Organic materials have recently gained attention as a valuable choice for electronic applications, such as organic field-effect transistors (OFETs)1, organic light-emitting diodes (OLEDs)2, memories3, photovoltaic cells4 and sensors5. Also thermoelectric applications have been proposed, and a variety of studies about this topic have been developed in the last few years6,7. Particularly, the use of the conductive polymer poly(3,4-ethylendioxithiophene), or PEDOT, appeared attractive due to its high electrical conductivity, its low thermal conductivity, its good environmental stability and its easy processability. Now a days the thermoelectric performance of this polymer is still poor, in comparison with inorganic benchmarks, though several attempts have been made to increase its efficiency8,9. In order to obtain a PEDOT-based material with improved features, we decided to embed inorganic nanoparticles in the polymeric matrix. This will generate an energy filtering effect in the hybrid material, which can lead to an increase of the Seebeck coefficient and, at the same time, an improvement of the carrier average mobility through a selection of the more energetic holes. The expected total result is the enhancement of the nanocomposite power factor10. For such purpose, we developed a chemical path to allow an interaction between the polymer and the inorganic nanoparticles, which involves the grafting of the monomer on the nanoparticle surface (Figure 1), and defined a protocol to obtain samples of hybrid material that can be thermoelectrically characterized. In order to understand if it is possible to exploit the energy filtering effect in such samples, several parameters have to be modified, such as the oxidation level of the polymer and the size and the concentration of the nanoparticles, and the resulting variations in the features of the material are currently being studied. In the present work we report the investigation of the variation of the nanoparticle concentration in the hybrid material. The results obtained for hybrid films made with nanoparticles of Mn3O4 (diameter 120±20 nm) are shown in Figure 2. The variation of the power factor due to the nanoparticles concentration implies the existence of an interaction between the polymer and the nanoparticles, but, as reported, there is a worsening of the performance as nanoparticle concentration increases. This is probably owed to the nanoparticle large size: since the Mn3O4 charge carrier mobility is low (5.3-8.1.10-3cm2/Vs)11, the hole free mean path inside this material is far shorter than the diameter of the nanoparticles used. An outlook of the possible modifications to overcome this problem and reach the desired result will be given. Figure 1: Schematic representation of the monomer (Succinyl-EDOT) grafted nanoparticle Figure 2: Power factor measured for hybrid films plotted versus nanoparticle concentration References: (1) Dodabalapur, A.; Torsi, L.; Katz, H. E. Sci. 1995, 268 (5208 ), 270–271. (2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347 (6293), 539–541. (3) Cavallini, M.; Biscarini, F.; Zerbetto, F.; Bottari, G.; Leigh, D. A. Science 2003, 299 (5606), 531. (4) Reddy, V. S.; Karak, S.; Ray, S. K.; Dhar, A. Org. Electr. 2009, 10, 138. (5) Street, R. A.; Mulato, M.; Lau, R.; Ho, J.; Graham, J.; Popovic, Z.; Hor, J. Appl. Phys. Lett. 2001, 78, 4193. (6) Bubnova, O.; Crispin, X. Energy Environ. Sci. 2012, 5, 9345–9362. (7) He, M.; Qiu, F.; Lin, Z. Energy Environ. Sci. 2013, 6 (5), 1352. (8) Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X. Nat. Mater. 2011, 10 (6), 429–433. (9) Wei, Q.; Mukaida, M.; Kirihara, K.; Naitoh, Y.; Ishida, T. Materials (Basel). 2015, 8 (2), 732–750. (10) Neophytou, N.; Zianni, X.; Kosina, H.; Frabboni, S.; Lorenzi, B.; Narducci, D. Nanotechnology 2013, 24, 205402. (11) Metselaar, R. Journal of Solid State Chemistry 1981, 341, 335–341.