Abstract
The ‘phonon-glass electron-crystal’ concept has triggered most of the progress that has been achieved in inorganic thermoelectrics in the past two decades. Organic thermoelectric materials, unlike their inorganic counterparts, exhibit molecular diversity, flexible mechanical properties and easy fabrication, and are mostly ‘phonon glasses’. However, the thermoelectric performances of these organic materials are largely limited by low molecular order and they are therefore far from being ‘electron crystals’. Here, we report a molecularly n-doped fullerene derivative with meticulous design of the side chain that approaches an organic ‘PGEC’ thermoelectric material. This thermoelectric material exhibits an excellent electrical conductivity of >10 S cm−1 and an ultralow thermal conductivity of <0.1 Wm−1K−1, leading to the best figure of merit ZT = 0.34 (at 120 °C) among all reported single-host n-type organic thermoelectric materials. The key factor to achieving the record performance is to use ‘arm-shaped’ double-triethylene-glycol-type side chains, which not only offer excellent doping efficiency (~60%) but also induce a disorder-to-order transition upon thermal annealing. This study illustrates the vast potential of organic semiconductors as thermoelectric materials.
Highlights
The ‘phonon-glass electron-crystal’ concept has triggered most of the progress that has been achieved in inorganic thermoelectrics in the past two decades
The thermoelectric performance is defined by the figure of merit ZT = S2σT/κ, where S, σ, T, and κ represent the Seebeck coefficient, the electrical conductivity, the absolute temperature, and the thermal conductivity, respectively[1,3]
Glen Slack proposed that an ideal TE material should be a “phonon-glass electron-crystal” (PGEC), which “means a material in which the phonon mean free paths are as short as possible and in which the electron mean free paths are as long as possible”[4,5]
Summary
The ‘phonon-glass electron-crystal’ concept has triggered most of the progress that has been achieved in inorganic thermoelectrics in the past two decades. The key to transforming this fullerene derivative close to an “electron crystal” is the use of “armshaped” double-triethylene-glycol-type side chains, which enable efficient and thermally stable n-doping and excellent molecular packing. This TE material exhibits an excellent electrical conductivity of >10 S cm−1 but an ultralow thermal conductivity of
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