Lecture 7 Solar Cells

Transcription

1 Lecture 7 Solar Cells review solid-state thermionics solar cells: basic principle solar cells: maximum efficiency factors impacting efficiency different types of cells

2 Compare Schottky diode and pn diode p-type E c Δ E c C hole V diffusion A n-type ψ e ψ h B electron diffusion D E v x E f x C x A x B x D J ev ( = J s exp 1 k B T J = J s ev / k T 1) B e J s = AT 2 exp Δ k T B J s = en c N v 1 N A a h τ h + 1 N D a e τ e E G exp κ B T

3 C hole V diffusion A ψ e ψ h B p-type E c electron ectron diffusion D E v Current and Energy Distribution n-type x C x A x B x x x D Energy Source Distribution J Positive Positive x c x A x B x D x 0 J n Negative J p x x c x A x B x D

4 Thermionic Emission and Energy Filtering E S φ b Energy filtering Figure by MIT OpenCourseWare. D(E) Moyzhes and Nemchinsky, Appl. Phys. Lett., 73, (1998). Shakouri and Bowers, Appl. Phys. Lett., 71, 1234 (1997).

5 Potential-Step Amplified Thermal-Electrical Energy Converter Region 1 Potential Interface E c1 Region 2 E f Forward Structure E c2 E c1 Reverse Structure Ef ICT 02 Sharp Interface: Electron Mean Free Path > Space Charge Region Single Carrier Transport

6 Amplification of Temperature Discontinuity E c1 (T e1 T e2 ) f k e2 dt e2 J R f dx, E f Forward Structure 2 J R, f = AT e1 e (E c1 E f 1 )/ (κ B T e1 ) ( Δ /( κ e BT T e) e1 T e2 ) f ~ Λe2 dt e2 dx Amplification Factor E c1 E c2 Reverse Structure E f (T e1 T e2 ) r k e2 dt e2 J Rr, dx (T e1 T e2 ) f ~ Λ e2 dt e2 dx No Amplification

7 Two-Temperature Modeling Results TEMPERATURE (K) Hot E c1 PHONON ELECTRON Forward Structure Cold E f TEMPERATURE (K) Dashed Lines: Electron Solid Lines: Phonon 450 n 2 =1.3x10 18 cm -3 δ=10 G 2 = W/m 3 K Fig. REVERSE 1(b) μ=7700vcm/s Reverse THERMAL BIAS Kp =1W/mK 400 Structure 350 FORWARD THERMAL Fig. 1(a) BIAS Forward Structure COORDINATE (m) DISTANCE( m) Δ = 8.3 μ=20,000 cm 2 /Vs; m*=0.014 m e ; G=10 10 W/m 3 K κ B T k p =1W/mK; n 2 =3.18x10 17 cm -3; n 1 =5.8x10 16 cm -3

8 Power Generation Efficiency EFFICIENCY PANTEC Regular 0.02 Thermoelectric Device CURRENT DENSITY (A/m 2 ) VOLTAGE (V)

9 Photovoltaic Cells p-type E c hν p-type E c C hole V diffusion A ψ e ψ h B electron diffusion D E v ev hν E v n-type x x C x A x B x D Load R J = J s (e ev / k BT 1) J = J s (e ev / k BT 1) J L J = Aexp E G J L --- Excitation due to photon s κ B T Short Circuit Current

10 Solar Cell: Open Circuit Voltage V oc = κ B T ln J L +1 e J s J = A exp E G s κ T B V E κ BT ln oc eg e A JG

11 IV Characteristics of Solar Cell Images removed due to copyright restrictions. Please see Fig. 5 in Chapter 14, "Solar Cells." Sze, Simon M. Physics of Semiconductor Devices. 2nd ed. New York, NY: Wiley, From S.M. Sze, Physics of Semiconductor Devices, 2 nd Ed., p.795

12 Maximum Power Output ev / k B T W e = J e V = J s V (e 1) J L V Find maximum: dw e /dv=0 ev ev ev m m m J m = J s exp J L 1 κ B T κ B T κ B T V = m κ B T J L / J s +1 κ B T ev m ln oc e 1+ evm /(κ B T ) V ln e 1+ κ B T Fill Factor: J m V F = m J L V oc

13 Source Term Radiation J L = ef se (1 R bω ω ) I (T,ω) dω hω E G / h Electrode p-type film Fraction of solar radiation reaching earth One photon generates one electron-hole pair n-type substrate JV η = I s Incident solar radiation flux Question: what is the maximum possible efficiency?

14 Schokley-Quisser Limit C hole V diffusion A n-type x C x A ψ e ψ h B x B p-type B J = J s (e ev / k T 1) J L E c electron diffusion D E v x x D h e G + exp N A τ h N D τ s c v J = en N 1 a 1 a e κ E BT If we do not include nonradiative recombination at all, there is still radiative recombination in the pn junction. When a voltage develops across the pn junction, the average number of photons per mode is: f (T,ω) = 1 exp hω ev k T 1 B Shockley, W. and Queisser, H.J., Journal of Applied Physics, 32, 510 (1961). Henry, C.H., Journal of Applied Physics, 51, 4494 (1980).

15 Schokley-Quisser Limit Recombination Current J r n---refractive index J L = ef se (1 R ω ) I bω (T,ω) dω = efse hω E G / h I bω (T,ω) dω hω E G / h

16 Ideal Device ev E g J = Aexp J k B T Follow same efficiency analysis to maximize efficiency L Image removed due to copyright restrictions. Please see Fig. 3 in Henry, C. H. "Limiting Efficiencies of Ideal Single and Multiple Energy Gap Terrestrial Solar Cells." Journal of Applied Physics 51 (August 1980): Henry, C.H., Journal of Applied Physics, 51, 4494 (1980).

17 Multijunction Cells Image removed due to copyright restrictions. Please see the schematic of a tandem PV cell in Pentland, William. "Solar Energy's Bleeding Edge - Breakthrough PV Research Projects." CleanBeta Blog, June 22, magazin/images/imagen1_web.g if 520x420_tandempv.jpg Courtesy of Antonio Luque. Used with permission. g Image removed due to copyright restrictions. Please see Fig. 4 in Henry, C. H. "Limiting Efficiencies of Ideal Single and Multiple Energy Gap Terrestrial Solar Cells." Journal of Applied Physics 51 (August 1980): Henry, C.H., Journal of Applied Physics, 51, 4494 (1980). Courtesy of Claudio Pelosi, Matteo Bosi, and SPIE. Used with permission.

18 Courtesy of Elsevier, Inc., Used with permission.

19 Challenge: Recombination Conduction band Li + (0.033) P + (0.044) As + (0.049) Sb + (0.039) S + (0.18) E c Deep levels Dangling bonds Grain boundaries Ni - (0.35) S ++ (0.37) Zn - (0.55) Mn + (0.53) Cu - (0.49) Au - (0.54) Ni - (0.22) B - (0.045) Zn - (0.31) In - (0.16) Ga - (0.065) Valence band Au + (0.35) Cu + (0.24) Al - (0.057) E v Purify Single crystals Figure by MIT OpenCourseWare. Delhi/Semiconductor%20Devices/LMB2A/3b.htm

20 Photon Absorption Absorption Coefficient of Semiconductor Mate I s Reflection I(z) z I (z) = I s e αz α--- absorption coefficient Courtesy of Christiana Honsberg and Stuart Bowden. Used with permission. δ=1/α---penetration depth (H.J Moller, 1993)

21 Direct vs. Indirect Semiconductors ENERGY ENERGY E g k E g Phonon Emission or Absorption k

22 Thin and Thick Dilemma Solar Radiation Photon p n Electrons Holes For Light Capture Crystalline Si: >100 μm Amorphous Si: ~ 1 μm For Charge Transfer Thinner is better

23 Issue of Cost 1% 4% 2% 4% 1% 0% 27% 61% Multi-crystalline Si Single-crystalline Si Ribbon Si a-si/mono Si Thin-film Si a-si CIS Others Installed PV capacity by cell type, 2003 Figure by MIT OpenCourseWare. Courtesy of Thomas Surek. Used with permission.

24 Single Crystalline and Polycrystalline Si Cells gle-solar-cell.png tructure-sicrystal-eng_full.jpg az-001.jpg Solar_cell_structure_and_mechanism.svg/400px- Silicon_Solar_cell_structure_and_mechanism.svg.png Images from EERE, Wikimedia Commons, sbma44 on Flickr, and Cyferz on Wikipedia.

25 Thin Film Solar Cells Courtesy of Elsevier, Inc., Used with permission. Image removed due to copyright restrictions. Please see Fig. 2 in Compaan, Alvin. "Photovoltaics: Clean Electricity for the 21st Century." APS News 14 (April 2005). m/ampp/image?path=/ / /001f0003. png apsnews/200504/images/fig2_tri ple_junction_cell.jpg Courtesy of EERE.

26 Polymer Cells Photon electron transport _ + hole transport anode Electric field cathode Three_different_views_of_IMEC_s_spray_coated_organic_ solar_cell.jpg Courtesy of IMEC. Used with permission. Figure by MIT OpenCourseWare. n/userfiles/image/pics/opv6.j pg

27 Photon TiO 2 particle Dye Grätzel Cells Dye (Monolayer) e e - Load + "hole" (aq.) PV Efficiency: ~11% (~7% module) Hydrogen Generation: ~5% e quantum efficiency Nanoparticle Electrolyte 0.13% Flat 88% electrode Nanocrystal electrode Grätzel, Nature, nm wavelength

28 Trends in Solar PV Courtesy of Elsevier, Inc., Used with permission. (Source: Martin Green) Can we bring Si into Gen. III paradigm?

29 MIT OpenCourseWare Direct Solar/Thermal to Electrical Energy Conversion Technologies Fall 2009 For information about citing these materials or our Terms of Use, visit: