Thermoelectrics of Cu 2 Se: Organic-Inorganic Hybrid Approaches to zT Enhancement David Brown, Tristan Day and Dr. G. Jeffrey Snyder

High Temperature Phase Phonon Liquid (low κ) High ion conductivity Anti-fluorite structure Room Temperature Phase Lower ion conductivity Ion-ordered stucture – Crystallography unresolved Copper(I) Selenide Mixed ion-electron conductor (MIEC) Copper interstitials Obviously there is a phase transition in between. (≈410K) The sub-lattice melts (1 st order transition) The ions disorder (2 nd order transition)

Mixed Ion Electron Conductors Materials that conduct both ions and electrons Low thermal conductivities due to unstable structure Separate out ion and electron contributions  Gated Seebeck and hybrid thermoelectrics H. Liu, et al., Nat Mater 11, 422 (2012)

Near the Phase Transition 80% increase in thermopower over 40 Kelvin

1 st Order Transition i.e. melting Sudden structural transition Enthalpy of formation 1 st order discontinuity 2 nd Order Transition i.e. ferromagnetism “gradual” transformation Critical power law behavior 2 nd order discontinuity 1 st versus 2 nd Order Transitions Plot: Water enthalpy with temperature

Critical Scattering Follow critical power laws below the transition Go rapidly to zero Possible critical enhanced scattering Critical Exponent:.80 Critical Exponent:.32

Temperature Resolved pXRD Continuous Transformation Observed

Heat Capacity DSC Heat CapacityWith PPMS to 400K Continuous “Lambda” Transition

Rigid Band Thermopower There is extra thermopower Carrier concentration changes at 360K 360K to 420K  ion ordering range

Resulting zT Enhancement Why do Seebeck and zT increase?

Thermopower and Entropy S *  Entropy transported per carrier Increase entropy transported  Increased efficiency J i are transport integrals  i.e. Kubo or Boltzmann integral How much does entropy change when a carrier is added? Can we increase it? Quasi-equilibrium term: The “presence” thermopower term

Degrees of Freedom Degree of Freedom Entropy per Carrier Scale Configurational≈ 86 uV/Kkbkb Configurational Spin Entropy W. Koshibae et al., Phys Rev B (2000) Degree of Freedom Entropy per Carrier Scale Configurational≈ 86 uV/Kkbkb Spin state[1]≈ 86 uV/Kkbkb Degree of Freedom Entropy per Carrier Scale Configurational ≈ 86 uV/K kbkb Spin state[1] ≈ 86 uV/K kbkb 2 nd order transition 50,000 uV/K 10 J/(mol∙K) Analogously, we suggest that structural entropy of a phase transformation may be coupled to transport in Cu 2 Se.

Entropy and Thermopower 6 Near a phase transition: Tc is the critical temperature m is the order parameter Expand the presence term T c depends on copper concentration[1] Copper ions thermally diffuse  ordering component migrates Holes couple to Cu + electrically [1] Z. Vučić, O. Milat, V. Horvatić, and Z. Ogorelec, Phys Rev B 24, 5398 (1981)

Organic/Inorganic Hybrids heat Charge transfer to NC film S S2S2 SemiconductorsMetals Carrier concentration DTA of various Cu 2-x Se Organic Ligands Donate charge carriers Dope sample Structure unchanged Z. Vucic and Z. Ogorelec, Philos Mag B 42, 287 (1980)

Gated Seebeck Gate T1T1 T2T2 Semiconductor Gate dielectric SourceDrain Degenerate Si with SiO 2 Change carrier concentration Don’t alter structure or chemistry Perfect way to probe this effect

Thin Film Cu2Se film thickness: 270 nm roughness average: 2.5 nm root mean square: 3.2 nm 1 inch diameter hot-pressed disk Made at Caltech PLD at 300°C and mBar Danish Technical University

Initial Data on Cu 2 Se Initial measurement unstable Behavior atypical of the bulk Electromigration? Soon we will have: PPMS running (DC Hall measurements 4K-400K) Lower current Hall chamber (80K – 450K) Position resolved Seebeck data

Conclusions The phase transition enhances the thermopower and zT New method for zT enhancement Study fundamentals of transport Thrust 3: Understand and engineer Gate T1T1 T2T2 Semiconductor Gate dielectric SourceDrain Degenerate Si with SiO 2 heat Charge transfer to NC film

Acknowledgments Yunzhong Chen & Nini Pryds Kasper Borup & Bo B. Iversen Huili Liu, Xun Shi & Lidong Chen Alex Z. Williams & NASA JPL Caltech Thermoelectrics Group

Seebeck Stability Figure S5. The sample was held at an average temperature of 390 K and a temperature difference of 16 K for 13 hours. The measured thermopower, 152 µV/K, varied by less than 1% during this time period.

More Transport Data

Seebeck Methodology