Abstract
Hybrid inorganic–organic superlattice with an electron-transmitting but phonon-blocking structure has emerged as a promising flexible thin film thermoelectric material. However, the substantial challenge in optimizing carrier concentration without disrupting the superlattice structure prevents further improvement of the thermoelectric performance. Here we demonstrate a strategy for carrier optimization in a hybrid inorganic–organic superlattice of TiS2[tetrabutylammonium]x[hexylammonium]y, where the organic layers are composed of a random mixture of tetrabutylammonium and hexylammonium molecules. By vacuum heating the hybrid materials at an intermediate temperature, the hexylammonium molecules with a lower boiling point are selectively de-intercalated, which reduces the electron density due to the requirement of electroneutrality. The tetrabutylammonium molecules with a higher boiling point remain to support and stabilize the superlattice structure. The carrier concentration can thus be effectively reduced, resulting in a remarkably high power factor of 904 µW m−1 K−2 at 300 K for flexible thermoelectrics, approaching the values achieved in conventional inorganic semiconductors.
Highlights
Hybrid inorganic–organic superlattice with an electron-transmitting but phonon-blocking structure has emerged as a promising flexible thin film thermoelectric material
The power factor can be remarkably improved if the carrier concentration was reduced, as shown in Supplementary Fig. 1 and Supplementary Note 1
Completely different strategies than chemical dopant co-firing should be developed for tuning carrier concentration in an inorganic–organic material
Summary
Hybrid inorganic–organic superlattice with an electron-transmitting but phonon-blocking structure has emerged as a promising flexible thin film thermoelectric material. There is a rapidly growing interest in flexible thin film thermoelectrics[1] for ambient temperature cooling and power generation on the microwatt-to-watt scale, where temperature gradients are moderate, surfaces are irregular, conventional thermoelectric materials with toxic or rare elements are prohibited, and low cost is preferred[2,3,4,5,6] These flexible thin film thermoelectric devices are under active pursuit for wearable energy harvesting, as they can generate electricity to power sensors by the temperature differences between human body and the environmental atmosphere[3]. A low power factor of 0.6 μW m−1 K−2 was reported for the n-type polymer poly[N,N′-bis(2-octyl-dodecyl)−1,4,5,8-napthalenedicarboximide-2, 6-diyl]-alt-5,5′-(2,2′-bithiophene)] (P(NDIOD-T2)[10] and 1.4 μW m−1 K−2 was reported for the self-doped perylene diimides (PDI)[11] To overcome this challenge, inorganic components have been incorporated into polymers to make n-type flexible materials with enhanced thermoelectric performance[12,13,14,15,16]. Interfacial doping of bismuth into organic materials shows n-type thermoelectric power factor of 108 μW m−1 K−2 18
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