N-type organic thermoelectrics: demonstration of ZT > 0.3

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.

Fig. 1. Side-chain variations and thermal annealing effect.

a Chemical structures of different fullerene derivatives (PTEG-1, PTEG-2, PPEG-1, and F2A), and dopant (n-DMBI); b Plots of electrical conductivity at room temperature as a function of the annealing temperature for different fullerene derivatives doped at a concentration of 8 wt% n-DMBI. Error bars indicate the standard errors of the mean values of electrical conductivity obtained by the measurement of six different samples.

Fig. 2. Thermal response and stability of thin-film samples.

a Plots of variable temperature ellipsometry scans for pristine PTEG-1, PTEG-2, and F2A, and doped PTEG-2 films; b evolution of normalized electrical conductivity for various doped fullerene derivatives maintained at a temperature of 150 °C.

Fig. 3. Molecular packing of PTEG-2 films.

a, b 2D-GIWAXS patterns of pristine PTEG-2 films annealed at (a) 120 °C and (b) 150 °C and (c, d) the corresponding linecuts together with the simulated scattering linecuts (the simulated linecuts are plotted on a linear scale in both cases); e representative snapshot of PTEG-2 molecular packing, as atomistically resolved by molecular dynamics simulations; the unit cell is highlighted in blue.

Fig. 4. Optimization of thermoelectric parameters by controlling the doping.

a electrical conductivity, b Seebeck coefficient, and c power factor as a function of doping concentration for doped PTEG-2 film at room temperature. Error bars indicate the standard errors of the mean values of electrical conductivity, Seebeck coefficient and power factor obtained by the measurement of six different samples.

Fig. 5. Temperature dependent thermoelectric parameters.

a electrical conductivity (the red star represents the conductivity after cooling down to 25 °C), b Seebeck coefficient (blue) and power factor (red), c in-plane thermal conductivity and d figure of merit, ZT at various operating temperatures for the doped PTEG-2 film at a 5 wt% doping concentration. Error bars (b, c) represent the standard errors of Seebeck coefficient and thermal conductivity obtained by best-fits; error bars (d) represent the corresponding calculated deviations of ZT.