Flexible Porous Bismuth Telluride Thin Films with Enhanced Figure of Merit using Micro‐Phase Separation of Block Copolymer

Abstract

Thermoelectric materials have attracted increasing attention because of their importance in applications such as energy harvesting and electronic cooling devices. [1][2][3][4] The performance of the thermoelectric devices is determined by the fi gure of merit, ZT , defi ned as ZT = S 2 σT / κ , where S is the Seebeck coeffi cient; σ , the electric conductivity; κ , the thermal conductivity; and T , the temperature. To date, thermoelectric devices have been too ineffi cient to be cost effective in most applications. Many attempts have been made to enhance ZT using nanostructured materials. It has been increasingly recognized that thermal conductivity in nanostructured materials is not an intrinsic parameter, unlike for bulk materials. [ 5 , 6 ] There have been several recent reports on the use of nanostructured thermoelectric materials to effi ciently increase the ZT value. [ 3 , 7-11 ] If the characteristic length of the microstructure is longer than the mean free path of the carriers, the transport of the carriers becomes diffusive. In contrast, if the characteristic length of the microstructures is smaller than the mean free path of the carriers, the transport of the carriers becomes ballistic. If the structure spacing or length is shorter than the mean free path of the phonons and is longer than the mean free path of electrons, the lattice thermal conductivity can be reduced while simultaneously maintaining high electrical conductivity. [ 12 , 13 ] It has been found that the reduction of thermal conductivity by the introduction of a microporous structure is an effective approach for increasing thermoelectric performance. However, the materials (Bi, Te, Pb, and Sb) and processes used can be diffi cult to scale to practically useful dimensions. Here, we report a selfassembled porous structure using a block copolymer (BCP) over a large area. [ 9 , 10 ] We attempted to fabricate microporous structures containing periodically occurring pores with critical dimensions/spacing smaller than the mean free paths of the phonons. The pores need to be isolated to avoid a signifi cant reduction of electrical conductivity as a result of percolation mechanism. [ 9 , 10 , 12-15 ] The transport of carriers in the random porous structure is drastically reduced with few increase of the porosity because the effective width of carrier paths will become narrow and their effective length will become long in the random porous materials. Bi 0.4 Te 3.0 Sb 1.6 thin fi lms with microporous structures were fabricated on alumina substrates such that the thin fi lms have extremely low thermal conductivity and high thermoelectric perfomance, but fl exibility of the fi lm was very low. [ 10 ] Microporous fi lms should thus be promising as substrates for the synthesis of scalable thermoelectric thin fi lms that exhibit both good electrical properties and low thermal conductivities. [ 16 ] The resulting fl exible porous thermoelectric thin fi lms using both organic and inorganic materials can be expected to effi ciently recover waste heat at...

Tailoring Nanoporous Structures in Bi2Te3 Thin Films for Improved Thermoelectric Performance

“…Aiming at practical application, the TE film should be thick enough to lower the internal resistance. However, such arranged nanopores would be buried during the thick-film deposition process. , Once the pores are filled by the TE film, the phonon-pore scattering is inhibited without the suppression effect on thermal conductivity. Moreover, the template method is not applicable for large-scale production because of its low cost efficiency.…”

Origin of N-Type Thermoelectric Properties in Single-Wall Carbon Nanotube Films With Anionic Surfactants Investigated by Experimental Analyses and First-Principles Calculations

“…For thin-lm thermoelectric materials, the power factor (PF), de ned as PF = σS 2 , is occasionally used as an alternative to ZT 13,14,15 . In addition to their good thermoelectric performance, SWCNTs are inherently exible; hence, they can be used to create exible thermoelectric generators with high thermoelectric performance 16,17,18,19,20 . Therefore, a exible thermoelectric generator using SWCNTs can be used as a power supply for wearable electronic devices and wireless sensor networks 21,22,23,24 .…”

Towards high refrigeration capability: the controllable structure of hierarchical Bi0.5Sb1.5Te3 flakes on a metal electrode

“…4,5 Bi 0.5 Sb 1.5 Te 3 is the best-known p-type material for room-temperature refrigeration. 6 Nano-structured Bi 2 Te 3based materials, such as thin films, 7 thick films, 8,9 and nanoscale inclusions in bulk materials, 10 can be fabricated by methods such as metal-organic chemical vapor deposition (MOCVD), 11 molecular beam epitaxy (MBE), 12 ion track lithography, 13 flash evaporation (FE), 14 spark plasma sintering (SPS) 15 and hot pressing. 16 Furthermore, the ZT value can be enhanced by controlling the preferential direction.…”