Record-High Thermoelectric Performance in Al-Doped ZnO via Anderson Localization of Band Edge States

Abstract

Oxides are of interest for thermoelectrics due to their high thermal stability, chemical inertness, low cost, and eco-friendly constituting elements. Here, adopting a unique synthesis route via chemical co-precipitation at strongly alkaline conditions, one of the highest thermoelectric performances for ZnO ceramics ( $P{F}_{\max }=$ 21.5 µW cm^-1 K^-2 and $z{T}_{\max }=$ 0.5 at 1100 K in ${\mathrm{Zn}}_{0.96}{\mathrm{Al}}_{0.04}O$ ) is achieved. These results are linked to a distinct modification of the electronic structure: charge carriers become trapped at the edge of the conduction band due to Anderson localization, evidenced by an anomalously low carrier mobility, and characteristic temperature and doping dependencies of charge transport. The bi-dimensional optimization of doping and carrier localization enable a simultaneous improvement of the Seebeck coefficient and electrical conductivity, opening a novel pathway to advance ZnO thermoelectrics.

Keywords: Anderson localization; ZnO; chemical co‐precipitation; oxides; thermoelectric materials; wet chemistry.

Figure 1

Schematic illustration of the semi‐automated laboratory‐made reactor used for chemical co‐precipitation.

Figure 2

a) PXRD patterns of ${\mathrm{Zn}}_{1-x}{\mathrm{Al}}_x\mathrm{O}$ ($x$ = 0, 0.02, 0.04, 0.06) samples after SPS and annealing with an enlarged section (on the right) of a 2$\theta$ range from ${42}^{\circ }$ to ${43}^{\circ }$ where ${\mathrm{ZnAl}}_2{\mathrm{O}}_4$ spinel phase has the most intensive reflection, which is indicated by a black solid triangle ($▾$). Bragg's reflections for the ZnO phase are indicated by black ticks on the top part of the figure. b) Calculated unit cell volumes for the ${\mathrm{Zn}}_{1-x}{\mathrm{Al}}_x\mathrm{O}$ samples (empty symbols) compared to literature data (filled gray symbols).^{[ 29} , ⁴² , ^{44 ]} c) Illustration of the crystal structure of ZnO.

Figure 3

a) Mixed backscattered electron and secondary electron HRSEM micrograph, b) linear EDS analysis results obtained in HRSEM with low‐acceleration beam mode, c) EBSD mapping, d) combined STEM and energy‐dispersive spectroscopy (EDS) mapping images showing the spatial distribution of Zn, O, and Al elements (in green, red, and orange, respectively), e) high‐magnification transmission electron microscopy micrograph, revealing the lattice image of two crystalline phases, and f) region with crystal orientations, determined using Fourier transform spectra, for ${\mathrm{Zn}}_{0.96}{\mathrm{Al}}_{0.04}\mathrm{O}$ sample.

Figure 4

Temperature dependence of (a) electrical conductivity $\sigma$, b) Seebeck coefficient $\alpha$, c) thermoelectric power factor $PF={\alpha }^2\sigma$, and d) weighted mobility ${\mu }_{\mathrm{w}}$ of ${\mathrm{Zn}}_{1-x}{\mathrm{Al}}_x\mathrm{O}$ ($x$ = 0, 0.02, 0.04, 0.06) prepared by chemical co‐precipitation.

Figure 5

Seebeck coefficient of ${\mathrm{Zn}}_{1-x}{\mathrm{Al}}_x\mathrm{O}$ (x = 0, 0.02, 0.04, 0.06) samples a) versus Hall carrier concentration (Pisarenko plot) at room temperature, b) versus logarithm of electrical conductivity (Jonker plot) at 1000 K, and c) versus temperature squared revealing variable range hopping conduction behavior $\alpha (T)\propto T^{1/2}$ below $\approx 600$ K. The reference values are taken from: $+$ ‐ Gayner et. al.,^{[ 53 ]} $\square$ ‐ Berardan et. al.,^{[ 54 ]} $\otimes$ ‐ Tsubota et. al.,^{[ 27 ]} $▵$ ‐ Jood et. al.,^{[ 29 ]} $\oplus$ ‐ Acharya et. al.,^{[ 32 ]} $★$ ‐ Han et. al.,^{[ 44 ]} $\nabla$ ‐ Nam et. al.,^{[ 55 ]} $◇$ ‐ Mayandi et. al.,^{[ 56 ]} $⊠$ ‐ Guan et. al.^{[ 57 ]}

Figure 6

a) Sketch of the density of states (DOS) for a disordered material (solid line) with localized states in the band tails and an ordered material with parabolic band dispersion (dashed line). The localized states are separated from the delocalized (extended) states by a mobility edge $E_{\mathrm{c}}$. When the Fermi level $E_{\mathrm{F}}$ lies within the region of localized states, the system behaves insulator‐like, whereas metallic‐liked behavior is observed for $E_{\mathrm{F}}>E_{\mathrm{c}}$. b) Energy‐dependent transport distribution function near the parabolic band edge of a semiconductor with dominant acoustic phonon scattering (dashed line) and near the mobility edge of a disordered material showing a different energy dependence. Variable‐range‐hopping‐like dependence of $\rho (T)$ at low temperatures for c) undoped ZnO and d) Al‐doped ${\mathrm{Zn}}_{0.98}{\mathrm{Al}}_{0.02}\mathrm{O}$. e) Hall carrier concentration and f) Hall mobility of ${\mathrm{Zn}}_{1-x}{\mathrm{Al}}_x\mathrm{O}$ (x = 0, 0.02, 0.04, 0.06). Anomalously low mobility is observed for undoped ZnO, which increases by an order of magnitude for ${\mathrm{Zn}}_{0.98}{\mathrm{Al}}_{0.02}\mathrm{O}$, despite the simultaneous increase of the carrier concentration in agreement with a disorder‐modified DOS sketched in (a). Unconventional temperature dependencies of the Hall mobility, which increases with temperature, confirm this scenario.

Figure 7

Temperature dependence of (a) total and (b) lattice thermal conductivity, and (c) dimensionless figure of merit ($zT$) of ${\mathrm{Zn}}_{1-x}{\mathrm{Al}}_x\mathrm{O}$ ($x$ = 0, 0.02, 0.04, 0.06) samples. In (b) solid lines are calculated by the Debye–Callaway model modified by Glassbrenner and Slack for high‐temperature regions, where $T\gg {\theta }_{\mathrm{D}}$.^{[ 70 ]} Dashed line in (b) is the minimum lattice thermal conductivity (glass limit) calculated from Cahill's model.^{[ 71 ]} d) Comparison of maximum $zT$ achieved for doped ZnO‐based ceramics.