High thermoelectric power factor in two-dimensional crystals ofMoS2

Abstract

The quest for high-efficiency heat-to-electricity conversion has been one of the major driving forces towards renewable energy production for the future. Efficient thermoelectric devices require high voltage generation from a temperature gradient and a large electrical conductivity, while maintaining a low thermal conductivity. For a given thermal conductivity and temperature, the thermoelectric powerfactor is determined by the electronic structure of the material. Low dimensionality (1D and 2D) opens new routes to high powerfactor due to the unique density of states (DOS) of confined electrons and holes. 2D transition metal dichalcogenide (TMDC) semiconductors represent a new class of thermoelectric materials not only due to such confinement effects, but especially due to their large effective masses and valley degeneracies. Here we report a powerfactor of MoS 2 as large as 8.5 mWm −1 K −2 at room temperature, which is amongst the highest measured in traditional, gapped thermoelectric materials. To obtain these high powerfactors, we perform thermoelectric measurements on few-layer MoS 2 in the metallic regime, which allows us to access the 2D DOS near the conduction band edge and exploit the effect of 2D confinement on electron scattering rates, which result in a large Seebeck coefficient. The demonstrated high, electronically modulated powerfactor in 2D TMDCs holds promise for efficient thermoelectric energy conversion.

Thermal Transport in 2D Semiconductors—Considerations for Device Applications

“…With reducing thickness, the thermoelectric power factor (PF) keeps increasing until a peak, where a high ≈50 µW cm −1 K − 2 PF was shown for two‐layer MoS 2 sample in its ON state, before dropping greatly for the monolayer, which was theoretically attributed to a difference in the energy dependence of the electron MFP. A similar thickness‐dependent PF trend was reported in paper, where a relatively higher PF of two‐layer MoS 2 was demonstrated due to the better conductivity in the degenerate metallic regime and a large Seebeck coefficient due to the high valley degeneracies and large effective masses. Quite recently, Wu et al showed that the PF of layered MoS 2 can be even enhanced by a strong interaction of electrons at the Fermi level with a local magnetic impurity (i.e., sulfur vacancy) and this so‐called magnetic impurity‐induced Kondo effect leads to a new record PF around 50 mW m −1 K −2 , as shown in Figure d.…”

“…The bilayer MoS 2 shows a maximum PF with a larger electrical mobility. Reproduced with permission . Copyright 2017, American Physical Society.…”

2D Materials for Large‐Area Flexible Thermoelectric Devices

“…More recently, thickness‐dependent thermoelectric measurements were performed with cleaved MoS 2 flakes . Interestingly, the largest power factor of 8500 µW m −1 K −2 was realized in bilayer samples and is also the largest value among semiconducting thermoelectric materials (Figure f) . This large enhancement of the power factor in bilayer MoS 2 was not only because of 2D confinement effects but was also attributed to the large effective mass and valley degeneracies in the heavily doped conduction band.…”

“…More recently, thickness‐dependent thermoelectric measurements were performed with cleaved MoS 2 flakes . Interestingly, the largest power factor of 8500 µW m −1 K −2 was realized in bilayer samples and is also the largest value among semiconducting thermoelectric materials (Figure f) .…”

“…Green, red, and blue symbols indicate the results for mono‐, bi‐, and trilayer samples, respectively. Reproduced with permission . Copyright 2015, American Institute of Physics.…”