Tin-Substituted Chalcopyrite: An n-Type Sulfide with Enhanced Thermoelectric Performance

摘要

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-sulnde devices.Chalcopyrite(CuFeS2)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)SnAFeS2(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)FeS2.