Abstract
Although organic polymer thermoelectric (TE) materials have witnessed explosive advances in the recent decade, the molecular mechanism of crystallization engineering of TE performance, even for the most successful polymer of poly(3,4-ethylenedioxythiophene) : poly(styrene sulfonate) (PEDOT : PSS), is still far from clear. Here, we deepen the understanding of the role of annealing-induced crystalline microstructure evolution on TE performance of the PEDOT : PSS film with thickness of 10 μ m, which is usually more effective than thin ones in applications. Annealed at optimized temperature of 220°C, the film displays a power factor of 162.5 times of that of the pristine film before annealing. The enhanced TE performance is associated with the changes of crystallographic and morphologic microstructures, including increased crystallinity and crystal grain size, a domain morphology transformation from granular to crystalline nanofibril, and reduced insulating PSS in the skin layer. These variances facilitate the carrier transport by a transition from 3D to 1D hopping, reduce the activation energy, and improve the carrier mobility. The mechanism of crystallization engineering reported here can be conceptually extended to other TE polymers and guides the future rational design of preparation principles for organic and composite TE materials.
Highlights
Thermoelectric (TE) materials are irreplaceable in their unique capabilities of harvesting low-grade heat and wide applications in TE generators (TEGs), local cooling, eskin and wearable devices, etc. [1,2,3,4]
Organic and organic/inorganic composite TE materials have witnessed drastic advances due to their diverse advantages, including solution processability, lightweight, rich source in earth, super flexibility, and highly adjustable molecular structure or component [5,6,7,8]. Because of their low thermal conductivities (κ, typically between 0.1 and 0.4 W m-1 K-1), their TE performance is often evaluated by the power factor (PF = S2σ) rather than the figure of merit (ZT = S2σT/κ), where σ, S, and T stand for the electrical conductivity, the thermopower or Seebeck coefficient, and the absolute temperature, respectively
Thick films of the pristine PEDOT : PSS were obtained by vacuum-filtrating commercial Clevios PH1000 solution on the porous poly(vinylidene fluoride) (PVDF) membrane (Figure S1)
Summary
Thermoelectric (TE) materials are irreplaceable in their unique capabilities of harvesting low-grade heat and wide applications in TE generators (TEGs), local cooling, eskin and wearable devices, etc. [1,2,3,4]. Organic and organic/inorganic composite TE materials have witnessed drastic advances due to their diverse advantages, including solution processability, lightweight, rich source in earth, super flexibility, and highly adjustable molecular structure or component [5,6,7,8]. Because of their low thermal conductivities (κ, typically between 0.1 and 0.4 W m-1 K-1), their TE performance is often evaluated by the power factor (PF = S2σ) rather than the figure of merit (ZT = S2σT/κ), where σ, S, and T stand for the electrical conductivity, the thermopower or Seebeck coefficient, and the absolute temperature, respectively. The underlying mechanism of the role of crystalline microstructure evolution on TE performance of PEDOT : PSS films is discussed
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