Clean Energy: Thermoelectrics and Photovoltaics. Akram Boukai Ph.D.

Transcription

1 Clean Energy: Thermoelectrics and Photovoltaics Akram Boukai Ph.D.

2 Solar Energy Use

3 Hydrocarbons vs. Photons Arabian Oil: 600 years Sun: 1.5 billion years

4 The Sun can Power both Solar Cells and Thermoelectrics TE PV

5 Voyager Powered by Thermoelectrics Us

6 Thermoelectrics 101 Seebeck Effect E dx = 0 L. Onsager, Physical Review 37, 405 (1931)

7 Thermoelectrics 101 FOR A METAL Carriers within kt are excited ~ 1 µv/k At 300K for a typical metal FOR A SEMICONDUCTOR A semiconductor is like a classical gas ~ 100 µv/k

8 Off the Shelf Thermoelectrics COLD Thermally Conductive/Electrically Insulating n p n p n p n p Thermally Conductive/Electrically Insulating HOT V OC = N(SΔT)

9 DC and AC Power-Generating Systems DC Power AC Power

10 What Governs Particle Flow? du = TdS + pdv + µdn + φde η = µ + eφ Particles move from high electrochemical potential to low electrochemical potential

11 Requirements for Electric Power 1. An Electrochemical Potential Difference Must be Present 2. A Selective Barrier Must be Present

12 The Contact Potential µ µ E η

13 Batteries V Δµ anode Δµ cathode redox chemistry E Anode Electrolyte Cathode

14 Batteries Continued V V OC E Anode Electrolyte Cathode V OC = Δµ anode + Δµ cathode

15 Solar Cells Light e - µ electrons µ holes h +

16 Solar Cells Light η electrons V OC η holes V OC = Δµ electrons + Δµ holes

17 Thermoelectrics as Heat Engines W is the work output Q is the heat input Work extracted is: V TE R TE Heat input consists of 3 terms: Plugging into η and maximizing:

18 Heat Engines and Efficiency Vining, C. Nature Materials 8, 83 (2009)

19 Figure of Merit for Thermoelectrics is ZT Dimensionless number. Larger the better S σ κ Thermopower Electrical conductivity Thermal conductivity

20 Is There a Ceiling to ZT? Standard Compression Based Refrigeration Bi 2 Te 3 /Sb 2 Te 3 superlattice PbSeTe/PbTe superlattice A. Majumdar, Science 303,

21 Is Bismuth a Good Thermoelectric? L Δ = 38meV L Te states E F L T Bulk Bismuth T Bismuth wire with diameter < 50nm T Tellurium doped Bismuth nanowires m * =.001m e µ = 2.59X10 5 cm 2 V -1 s -1 S = 100µV/K κ = 8 W m -1 K -1 Electron mean free path is ~30 to 50nm at room temperature

22 Density of States S T N(E) E EF DOS DOS Bulk Metal 1-D Systems E F E E F E

23 ZT for Bismuth Nanowires M.S. Dresselhaus, Phys. Rev. B 62,

24 Bismuth is Not an Easy Material to Work With State of the art: Alumina assisted electrodeposition M.S. Dresselhaus et. al., Int. Mater. Rev. 48, Bismuth is sensitive to acids and bases and oxidizes readily S.B. Cronin et. al., Nanotechnology 13, Measurement limited to 2-point and large thermocouples Y.M. Lin et. al., Mat. Res. Soc. Symp. Proc. 691,

25 Bismuth Nanowire Thermoelectric Devices A. Boukai, K. Xu, J.R. Heath, Advanced Materials 18, (2006)

26 Bi Nanowire Electrical Conductivity Results A. Boukai, K. Xu, J.R. Heath, Advanced Materials 18, Heremans et. al., Phys. Rev. B 61,

27 Measuring the Thermopower Left Thermometer Heater Right Thermometer

28 Measuring the Thermoelectric Voltage (TEV) This gives us: V/W

29 Measuring ΔT 17Hz I Lock-In ΔV I ΔV Lock-In 13Hz

30 Measuring ΔT This gives us: Ω/W

31 Measuring ΔT This gives us: Ω/K

32 Multiply: Measuring ΔT 72nm Wide Bi Wire

33 Bi Nanowire Thermopower Results A. Boukai, K. Xu, J.R. Heath, Advanced Materials 18,

34 Surface States Dominate Carrier Transport 40nm wide Bi wire at 20K Results Our results indicate that surface states dominate the carrier transport Thermopower is well correlated to Mott diffusion formula S DOS E F 1-D Systems E

35 And God Said, Let there be Silicon and it was good. Chemistry of Si is well understood +50 years of Silicon R&D D. Li, et al. APL 83, κ for bulk Si is ~150 With SNAP, we have control over wire width, doping, crystal orientation, etc.

36 Superlattice Nanowire Pattern Transfer (SNAP) GaAs/AlxGa1-xAs Nanowire transfer Selective etching AlxGa1-xAs Pt nanowire formation Pt deposition Nanowire contact N.A. Melosh, A. Boukai, F. Diana, B. Gerardot, A. Badolato, P.M. Petroff, J.R. Heath, Science 300, (2003)

37 Array of Si Nanowires Made With SNAP N.A. Melosh, A. Boukai, F. Diana, B. Gerardot, A. Badolato, P.M. Petroff, J.R. Heath, Science 300, (2003)

38 SNAP s Versatility 20nm 7.5nm 400 NWs 1400 NWs

39 Si Nanowire Thermoelectrics Akram Boukai, Yuri Bunimovich, Jamil Tahir-Kheli, Jen-Kan Yu, Bill Goddard and Jim Heath, Nature, 461, (2008)

40 Suspended Platform Allows Measurement of ZT K = Q/ΔT Akram Boukai, Yuri Bunimovich, Jamil Tahir-Kheli, Jen-Kan Yu, Bill Goddard and Jim Heath, Nature, 461, (2008)

41 Measurements are Taken on an Array of Si NWs Akram Boukai, Yuri Bunimovich, Jamil Tahir-Kheli, Jen-Kan Yu, Bill Goddard and Jim Heath, Nature, 461, (2008)

42 Si Nanowire Electrical Conductivity

43 Minimum Thermal Conductivity D.G. Cahill, et al. Phys. Rev. B 46, 6131 (1992) κ min for Si ~ 1 This occurs when Si is amorphous

44 Si Nanowire Thermal Conductivity κ min Si κ for bulk Si is ~150

45 Diffuse vs Specular Scattering

46 Lots of Data to Minimize Error Bars Our error in the temperature measurement is ~.01%!!!

47 Si Nanowire Thermopower

48 Phonon Drag Bulk Silicon L. Weber, E. Gmelin, Applied Physics A 53, (1991) Phonons are not in equilibrium Longitudinal modes push the electrons down the temperature gradient

49 Phonon Drag in Our Si NWs

50 Phonon Drag is Supposed to Disappear at the Nanoscale Thank you Jamil and Bill! L. Weber, et al. Phys. Rev. B 46, 9511 (1992)

51 Phonon Drag in a 1-D System S = S diffusion + S phonon drag

52 Efficient Si Nanowires