Thermoelectric Properties of Ge-doped Cu 3 SbSe 4
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Thermoelectric Properties of Ge-doped Cu3SbSe4 Eric J. Skoug1, Jeffrey D. Cain1, and Donald T. Morelli1 1 Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824 U.S.A. ABSTRACT Ternary variations of the II-VI zincblende semiconductors have received little attention for thermoelectric applications. Here we present the first systematic doping study on Cu3SbSe4, a zincblende-like ternary semiconductor with a unit cell four times larger than the parent II-VI compounds. The large unit cell of Cu3SbSe4 results in a low room temperature thermal conductivity (~3.0 W/m*K) and its large hole effective mass produces a Seebeck coefficient approaching 500 μV/K in the undoped compound. Our results show that Ge is an effective p-type dopant in Cu3SbSe4, and the power factor reaches nearly 16 μW/cm*K2 at 630K when 3% Ge is added, rivaling that of state-of-the-art thermoelectric materials at this temperature. INTRODUCTION Thermoelectric materials convert directly between thermal and electrical energy, and thus find applications in waste heat recovery, remote power generation, and solid-state cooling. The widespread use of thermoelectric technology has been frustrated primarily by low conversion efficiency, which is defined by the thermoelectric figure of merit Z=S2σ*κ-1 (where S is the Seebeck coefficient, σ is electrical conductivity, and κ is thermal conductivity). A common approach to increasing Z is to decrease the lattice thermal conductivity by introducing phonon scattering centers into the material, which has proven successful in, for example, solid solutions12 , filled skutterudites3-4, and nanostructured materials5-6. A simplified approach is to find materials that have low intrinsic thermal conductivities, thus eliminating the need for additional scattering centers that can often adversely affect the electronic properties. Slack7 found that the lattice thermal conductivity tends to decrease with increasing unit cell size as n-2/3, where n is the number of atoms per primitive unit cell. This effect has been used recently8-9 to identify AgSbTe2 (rocksalt structure, twice the unit cell of PbTe) as a high-Z thermoelectric material. Work pioneered in the 1950s by Goodman10 showed that ternary variations of the II-VI zincblende semiconductors are also possible if charge balance is maintained, resulting in compounds of the form I-III-VI2 with twice the unit cell of the II-VI compounds. Shortly thereafter Wernick11 synthesized zincblende-like compounds of the form I3V-VI4, which have even larger unit cells (n=8). The I3-V-VI4 compounds, typified by Cu3SbSe4, are known to have relatively low thermal conductivities (< 3 W/m*K), but have remained largely unexplored for thermoelectric applications12. The compound Cu3SbSe4 is known to be a narrow bandgap (0.13 eV) semiconductor13, suggesting possible applications at low and intermediate temperatures. Here we aim to characterize the effect of Ge doping on the thermoelectric properties of Cu3SbSe4. EXPERIMENT Samples of Cu3Sb1-xGexSe4
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