Tin-Substituted Chalcopyrite: Ann-Type Sulfide with Enhanced Thermoelectric Performance

Abstract

The dearth of n -type sulfides with thermoelectric performance comparable to that of their p- type analogues presents a problem in the fabrication of all-sulfide devices. Chalcopyrite (CuFeS 2 ) offers a rare example of an n -type sulfide. Chemical substitution has been used to enhance the thermoelectric performance of chalcopyrite through preparation of Cu 1- x Sn x FeS 2 (0 ≤ x ≤ 0.1). Substitution induces a high level of mass and strain field fluctuation, leading to lattice softening and enhanced point-defect scattering. Together with dislocations and twinning identified by transmission electron microscopy, this provides a mechanism for scattering phonons with a wide range of mean free paths. Substituted materials retain a large density-of-states effective mass and, hence, a high Seebeck coefficient. Combined with a high charge-carrier mobility and, thus, high electrical conductivity, a 3-fold improvement in power factor is achieved. Density functional theory (DFT) calculations reveal that substitution leads to the creation of small polarons, involving localized Fe 2+ states, as confirmed by X-ray photoelectron spectroscopy. Small polaron formation limits the increase in carrier concentration to values that are lower than expected on electron-counting grounds. An improved power factor, coupled with substantial reductions (up to 40%) in lattice thermal conductivity, increases the maximum figure-of-merit by 300%, to zT ≈ 0.3 at 673 K for Cu 0.96 Sn 0.04 FeS 2 .

Local structural distortions and reduced thermal conductivity in Ge-substituted chalcopyrite

“…The slightly higher Cu contents determined by EDS may be due to the overlap of the K α and L α characteristic lines of Cu and Fe, as has been reported previously. 31 The Ge contents are in excellent agreement with the nominal values, indicating successful substitution of Ge in the chalcopyrite matrix. X-ray photoelectron spectroscopy data for CuFe 0.94 Ge 0.06 S 2 (Figure 4) reveals the presence of Cu + , Fe 3+ and S 2-, as has been observed in XPS data for unsubstituted chalcopyrites.…”

“…29,30 High oxidation state substituents offer the potential to achieve greater increases in the charge-carrier concentration than is possible with the more usual 2+ transition-metal cations. Although, as we have demonstrated recently, 31 in the case of substitution with Sn 4+ , the increase in charge-carrier concentration can deviate from expected values due to the presence of localised states, associated with the formation of a small polaron.…”

“…X-ray photoelectron spectroscopy data for CuFe 0.94 Ge 0.06 S 2 (Figure 4) reveals the presence of Cu + , Fe 3+ and S 2-, as has been observed in XPS data for unsubstituted chalcopyrites. 25,29,31,48,49 However, an additional weak peak (Figure 4b) corresponding to Fe 2+ is observed in the Fe 2p spectrum, suggesting that Ge substitution results in a reduction of a fraction of the Fe 3+ ions to Fe 2+ . A similar observation has been made for tin and indium substitution in chalcopyrite 30,31 Significantly, the 3d spectrum of Ge (Figure 4d) indicates that this cation is in a mixed oxidation state, with peaks assignable to both Ge 2+ and Ge 4+ present; the latter having the higher intensity.…”

“…25,29,31,48,49 However, an additional weak peak (Figure 4b) corresponding to Fe 2+ is observed in the Fe 2p spectrum, suggesting that Ge substitution results in a reduction of a fraction of the Fe 3+ ions to Fe 2+ . A similar observation has been made for tin and indium substitution in chalcopyrite 30,31 Significantly, the 3d spectrum of Ge (Figure 4d) indicates that this cation is in a mixed oxidation state, with peaks assignable to both Ge 2+ and Ge 4+ present; the latter having the higher intensity. While data were obtained for materials with lower germanium contents, the Ge signal was too weak to provide definitive information on the oxidation states.…”

“…The calculation details for these parameters are provided in the supplementary information (ESI). Previously, 31 it has been shown that scattering of acoustic phonons with mean free paths between 50 -500 nm, can reduce the lattice thermal conductivity of chalcopyrite-type phases significantly. High-resolution transmission electron microscopy (HRTEM) (x = 0.06 and 0. which aids in maintaining the local charge balance.…”