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Thermoelectrics of Cu2Se:Organic-Inorganic Hybrid Approaches to zT
Enhancement
David Brown, Tristan Day and Dr. G. Jeffrey Snyder
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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) SelenideMixed ion-electron conductor (MIEC)
Copper interstitials
Obviously there is a phase transition in between. (≈410K)The sub-lattice melts (1st order transition)The ions disorder (2nd order transition)
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Mixed Ion Electron Conductors
Materials that conduct both ions and electronsLow 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)
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Near the Phase Transition
80% increase in thermopower over 40 Kelvin
Tk
SZT
2
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1st Order Transition• i.e. melting• Sudden structural transition• Enthalpy of formation• 1st order discontinuity
2nd Order Transition• i.e. ferromagnetism• “gradual” transformation• Critical power law behavior
• 2nd order discontinuity
1st versus 2nd Order Transitions
Plot: Water enthalpy with temperature
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Critical Scattering
Follow critical power laws below the transitionGo rapidly to zeroPossible critical enhanced scattering
Critical Exponent: .80
Critical Exponent: .32
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Temperature Resolved pXRD
Continuous TransformationObserved
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Heat Capacity
DSC Heat Capacity With PPMS to 400K
Continuous “Lambda” Transition
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Rigid Band Thermopower
Degenerate rigid band model
Below 360K: Below 385K rigid band model fits
There is extra thermopower
Carrier concentration changes at 360K360K to 420K ion ordering range
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Resulting zT Enhancement
Why do Seebeck and zT increase?
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Thermopower and EntropyS* Entropy transported per carrier
Increase entropy transported Increased efficiency
Ji 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
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Degrees of FreedomDegree of Freedom
Entropy per Carrier
Scale
Configurational
≈ 86 uV/K kb
Configurational
Spin Entropy
W. Koshibae et al., Phys Rev B 62 6869 (2000)
Degree of Freedom
Entropy per Carrier
Scale
Configurational
≈ 86 uV/K kb
Spin state[1] ≈ 86 uV/K kb
Degree of Freedom
Entropy per Carrier
Scale
Configurational
≈ 86 uV/K kb
Spin state[1] ≈ 86 uV/K kb
2nd 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 Cu2Se.
-200
-150
-100
-50
0
50
100
150
200
0 0.2 0.4 0.6 0.8 1
10 spaces100 spacesHiekes formula
See
beck
Coe
ffici
ent
(µV
/K)
Fractional Concentration
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Entropy and Thermopower
6
Near a phase transition:
Tc is the critical temperature m is the order parameter
Expand the presence term
Tc depends on copper concentration[1]Copper ions thermally diffuse ordering component migratesHoles couple to Cu+ electrically
[1] Z. Vučić, O. Milat, V. Horvatić, and Z. Ogorelec, Phys Rev B 24, 5398 (1981)
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Organic/Inorganic Hybrids
heat
Charge transfer to NC film
S
S2Semiconductors Metals
Carrier concentrationDTA of various Cu2-xSe
Organic Ligands
Donate charge carriers Dope sampleStructure unchanged
Z. Vucic and Z. Ogorelec, Philos Mag B 42, 287 (1980)
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Gated Seebeck
Gate
T1 T2
Semiconductor
Gate dielectric
Source Drain
Degenerate Si with SiO2
Change carrier concentrationDon’t alter structure or chemistry
Perfect way to probe this effect
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Thin Film Cu2Se
film thickness: 270 nmroughness average: 2.5 nmroot mean square: 3.2 nm
1 inch diameter hot-pressed disk Made at CaltechPLD at 300°C and 10-6 mBar
Danish Technical University
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Initial Data on Cu2Se
250 300 350 400 450 500 550 6000
1
2
3
4
5
6
Temperature (K)
Con
duct
ivity
(10
3 S*c
m)
Cu2Se 1 micron Thin Film
Heating 1
Cooling 1Heating 2
Cooling 2
Initial measurement unstableBehavior atypical of the bulkElectromigration?
Soon we will have:PPMS running (DC Hall measurements 4K-400K)Lower current Hall chamber (80K – 450K)Position resolved Seebeck data
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Conclusions
The phase transition enhances the thermopower and zT
New method for zT enhancement
Study fundamentals of transport Thrust 3: Understand and engineer
Gate
T1 T2
Semiconductor
Gate dielectric
Source Drain
Degenerate Si with SiO2
heat
Charge transfer to NC film
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Acknowledgments
Yunzhong Chen & Nini PrydsKasper Borup & Bo B. IversenHuili Liu, Xun Shi & Lidong Chen
Alex Z. Williams & NASA JPL
Caltech Thermoelectrics Group
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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.
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More Transport Data
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More Transport Data
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Seebeck Methodology