The Opto-Electronic Physics That Just Broke the Efficiency Record in Solar Cells

Transcription

1 The Opto-Electronic Physics That Just Broke the Efficiency Record in Solar Cells Solar Energy Mini-Series Jen-Hsun Huang Engineering Center Stanford, California Sept. 26, 2011 Owen D. Miller & Eli Yablonovitch UC Berkeley Electrical Engineering & Computer Sciences Dept.

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3 2010

4 Cost per peak Watt for Solar Panels $10 X 40GW installed cumulative $1 $0.30 X 1TW installed cumulative Year subsidies will no longer be needed Date

5 Cost of Eletricity ( /kwh) Value of High Photovoltaic Efficiency Efficiency and Cost of Electricity (COE) inversely related: COE (Cost module Cost BOS) Insolation Efficiency Finance Operating Factors that impact COE: 40 Location: decides the available input energy % 8% 12% Efficiency: decides the portion that can be converted to electricity % 27% 40% From Allen Barnett Univ. of Delaware Cost of Photovoltaics ($/m 2 ) 9

6 CdTe 11% Efficiency $

7 c-si 23% Efficiency $

8 1. Why the pn junction is merely optional in solar cells? 2. What determines the voltage of a solar cell? 3. What is the statistical mechanical approach to optics that is needed in solar cells? 4. Are photonic crystals of any help toward solar cells? 5. What are the top competing technologies? 6. What is the status of the industry today?

9 What is a Solar Cell? E h - e - e - e - E Fn Wider Bandgap top, bottom and sides e - h + e - h + e - h + e - h + e - h + h + h + e - + h + h + h + V E Fp Selective hole contact double hetero-structure + - Selective electron contact k Diffusion potential to contacts is typically <1mVolt. A Solar Cell does not require a p-n junction!

10 What is a Selective Contact? It passes one type of carrier but not the other. Most Ohmic electrical contacts barely work on even one type of carrier. Therefore they are almost all selective. The ideal type of selective contact is a hetero-contact: electron contact: E hole contact: E E C E Fn E Fp E F E C E Fn E V E Fp E V E F semiconductor metal semiconductor metal

11 Solar Cell as a Bucket feeding a Water Wheel rainfall incident sunlight pressure head voltage sidewall leakage surface recombination bottom leakage bulk recombination sidewall leakage surface recombination water flow current power flow pressure water wheel output solar cell power

12 There are three things to know about Solar Cells: 1.Short Circuit Current 2.Open Circuit Voltage 3.Fill Factor

13 Three possible valve conditions Open valve Low-pressure, high-flow Closed valve High-pressure, low-flow Partially restricted valve Optimal pressureflow (Water wheel barely turns) (Water wheel doesn t turn) (Max water wheel output)

14 Fill-Factor = ratio of these two rectangles Fill-Factor is more a thermodynamic property, rather than a circuit property. Ideal Fill-Factor ~ 89%

15 What is the current to expect? Absorb all the incoming light, in as thin a layer as possible. A direct bandgap uses less material than an indirect bandgap

16 Solar Cell: h single pass only h semiconductor semiconductor light trapping: path length increased by 4n 2 =50 where n = refractive index

17 Solar Cell: Light Emitting Diode: h single pass only h semiconductor semiconductor h h only 1/4n 2 = 1/50 = 2% of the light escapes. h semiconductor light trapping: path length increased by 4n 2 =50 where n = refractive index semiconductor all the light eventually escapes.

18 Non-Ergodic Shapes: Ergodic Shapes: h h sphere h ellipse rectangle h h parallel ogram trapezoid Most of the internal light can never escape All the internal light eventually escapes

19 Density of Optical Modes per Unit Volume in air 8π c 3 2 Density of Optical Modes per Unit Volume in semiconductor 8π n c What is the ratio? n 2 There is a factor 2 from double pass and There is a factor 2 from cos averaging The net benefit is 4n 2

20 Light Trapping Requirement: 1. Either the high index material has to be textured, or the photon recycling should be very efficient. The Benefit: a. 4n 2 ~50 times thinner layer can be sufficient to absorb the sunlight saving semiconductor cost. b. In a thinner cell, there is less series resistance, and a higher Fill Factor c. The operating point voltage, (not just V oc ), increases by kt ln{4n 2 } ~ 0.1Volts. Total Internal Reflection is completely compatible with an Anti-Reflection Coating.

21 h h h h h semiconductor layer A randomly rough surface does approach the statistical mechanical limit, regardless what angle the light is coming in on. Can a photonic crystal patterned surface do better than a randomly rough surface?

22 What is the operating voltage? e - h + e - h + e - h + e - h + e - h + h + h + e To extract current, voltage at contacts must be slightly lower than Voc But, operating voltage linked directly to Voc V OP V OC kt q ln qv kt OC We only need to understand the open-circuit voltage

23 What is the ideal voltage V oc to expect, i.e. the Quasi-Fermi Level separation, chemical potential, or Free Energy? exp Free Energy kt Boltzmann Factor excited excited state state population in the light population in the dark In molecules and quantum dots: qv oc =Free energy = kt ln excited excited state state population population in in the light the dark In semiconductors with mobile electrons & holes: Free energy = E Fc -E Fv =2kT ln electron electron density in density in the light the dark

24 What is the voltage to expect, i.e. the Quasi-Fermi Level separation, chemical potential, or Free Energy? Shockley-Queisser Limit (1961): qv oc = kt ln external Luminescent emission band to band emission in the dark But in quasi-equilibrium: qv oc = kt ln band incoming sunlight to band emission in the dark

25 qv = operating point Yes photons have entropy, S Photon Free Energy = h - TS Photon Free Energy = h - kt lnw -0.28eV -0.1eV -0.1eV eV E kt +0.02eV E g kt{ln(/ s ) + ln(4n 2 g ) + ln(qv op /kt) ln()-ln } s 2 T T s Entropy due to loss of directivity information Entropy due to incomplete light trapping Free energy loss due to power-point optimization Free energy loss due to poor Quantum Efficiency optical mode density correction where s is the solid angle subtended by the sun nicest treatment: R.T.Ross "Some Thermodynamics of PhotoChemical Systems", J. Chem. Phys. 46, (1967)

26 Small bandgaps are particularly vulnerable to entropy: After you subtract off all the entropy terms, you don't have much Free Energy left. qv operating point =1.1eV 0.8eV = 0.3eV A lousy 0.3eV from all those big photons In general we cannot afford to compromise with regard to quantum efficiency.

27 normal solar cells new physics efficiency % 33.5% ~25% Shockley-Queisser limit (single-junction) high performing cells 0%

28 What did Shockley-Queisser say? Any absorber also has to be an emitter! At the open-circuit condition the electrons & holes have no place to go. They build up, and ideally they are lost only to luminescence. Outgoing luminescence Solar radiation In an ideal solar cell, at open-circuit, the outgoing luminescence will equal the incoming solar radiation

29 h semiconductor only 1/4n 2 = 1/50 = 2% of the light escapes. h

30 What if the material is not ideal, and the electrons and holes are lost to heat before they can luminesce? qv oc = qv oc-ideal kt ln{ ext } The external Only external Luminescence can balance the incoming radiation. fluorescence yield ext is what matters!

31 normal solar cells new physics efficiency % 33.5% ~25% Shockley-Queisser limit (single-junction) high performing cells 0%

32 (a) h h h e - h + (b) h h g h g h h h g e - h +

33 h semiconductor only 1/4n 2 = 1/50 = 2% of the light escapes. h You may need an internal efficiency of int =99% just to get an external efficiency of ext =50%

34 h semiconductor only 1/4n 2 = 1/50 = 2% of the light escapes. h Another way to look at this, 1. the recycled photons are not lost, 2. the carrier lifetime increases, 3. increasing carrier density 4. Increasing V oc

35 h semiconductor only 1/4n 2 = 1/50 = 2% of the light escapes. h But this is really hard to do: You may need an internal efficiency of int =99% just to get an external efficiency of ext =50%

36 Voc (V) Jsc ( ma/cm 2 ) Cell Efficiency (%) Efficiency vs. Internal Fluorescence Yield, GaAs 3m % 31.2% 30.6% Internal Fluorescence Yield 90% Internal Fluorescence Yield Is Not Enough! mA/cm Internal Fluorescence Yield Internal Fluorescence Yield

37 Voc (Volts) Jsc (ma/cm 2 ) Cell Efficiency (%) Efficiency vs. Rear Reflectivity, GaAs 3m % 32.2% 31.9% 90% Rear Reflectivity Is Not Enough! Reflectivity Reflectivity Reflectivity

38 Solar Cell Efficiency Solar Cell Efficiency Limits for int < % 31.0% % int =100% int =90% int =80% Bandgap (ev)

39 Open-circuit Voltage (V) mv 20 mv Bandgap (ev)

40 Textured front surface, perfectly reflecting mirror Planar front surface, perfectly reflecting mirror Planar front surface, absorbing mirror Textured good mirror Untextured good mirror Untextured bad mirror Thickness 500nm 1 m 10m 500nm 1m 10m 500nm 1m 10m Voc (volts) Jsc (ma) Fill Factor efficiency %

41 GaAs, theory & expt GaAs 33.5% Theory 30 ~30% % V oc =1.11V (2011) Alta Devices 26.4% Previous Record (2010) V oc =1.03V 25.1% record ( )

42 For 3m thickness: Normalized for 1 cm 2 area. Planar, good mirror efficiency = 33.2% Incoming light 32.5mA 77.3mA Reabsorption Auger ma Mirror Loss External emission 0.79mA 0mA Internal luminescence 78.1mA Extracted current 31.7mA Planar, bad mirror efficiency = 30.6% Incoming light 32.1mA 5.36mA Reabsorption Auger ma Mirror Loss External emission 0.04mA 0.79mA Internal luminescence 5.40mA Extracted current 31.3mA In both cases, you lose 0.76 ma. In the first case, that is luminescence out the top. In the second case, it is lost in the mirror. But the difference in external luminescence, and hence voltage, is significant.

43 For 3m thickness: Normalized for 1 cm 2 area. Planar, good mirror efficiency = 33.2% Incoming light 32.5mA 77.3mA Reabsorption Auger ma Mirror Loss External emission 0.79mA 0mA Internal luminescence 78.1mA Extracted current 31.7mA Planar, bad mirror efficiency = 30.6% Incoming light 32.1mA 5.36mA Reabsorption Auger ma Mirror Loss External emission 0.04mA 0.79mA Internal luminescence 5.40mA Extracted current 31.3mA In both cases, you lose 0.76 ma. In the first case, that is luminescence out the top. In the second case, it is lost in the mirror. But the difference in external luminescence, and hence voltage, is significant.

44 (a) h h h e - h + (b) h h g h g h h h g e - h +

45 Counter-Intuitively, to approach the Shockley-Queisser Limit, you need to have good external fluorescence yield ext!! Internal Fluorescence Yield int >> 90% Rear reflectivity >> 90% Both needed for good ext

46 What is the voltage to expect, i.e. the Quasi-Fermi Level separation, chemical potential, or Free Energy? Shockley-Queisser Limit (1961): qv oc = kt ln band incoming sunlight to band emission in the dark But in quasi-equilibrium: qv oc = kt ln external Luminescent emission band to band emission in the dark The external Luminescence intensity is a Volt-meter into the solar cell.

47 For solar cells at 25%, good electron-hole transport is already a given. Further improvements of efficiency above 25% are all about the photon management! A good solar cell has to be a good LED! Counter-intuitively, the solar cell performs best when there is maximum external fluorescence yield ext.

48 Dual Junction Series-Connected Tandem Solar Cell h h Ga 0.5 In 0.5 P V OC =1.5V Solar Cell Tunnel Contact n-al 0.5 In 0.5 P n-ga 0.5 In 0.5 P p-ga 0.5 In 0.5 P p + -Al 0.5 In 0.5 P n + -Al 0.5 In 0.5 P n-al 0.5 In 0.5 P E g ~2.35eV E g ~1.9eV E g ~1.9eV E g ~2.35eV E g ~2.35eV Ga 0.5 In 0.5 P V OC =1.5V Solar Cell Tunnel Contact GaAs V OC =1.1V Solar Cell n-gaas p-gaas p-al 0.2 Ga 0.8 As E g =1.4eV E g =1.4eV GaAs V OC =1.1V Solar Cell All Lattice-Matched ~34% efficiency should be possible.

49 There is much latitude for creativity in the higher bandgap nano-particles; nano-rods, etc. Wide Bandgap V OC >1.5V Tunnel Contact GaAs V OC =1.1V Solar Cell h n-al 0.5 In 0.5 P E g ~2.35eV p-al 0.5 In 0.5 P p + -Al 0.5 In 0.5 P n + -Al 0.5 In 0.5 P n-al 0.5 In 0.5 P n-gaas h p-gaas p-al 0.2 Ga 0.8 As E g ~2.35eV E g ~2.35eV E g ~2.35eV E g =1.4eV E g =1.4eV >35% efficiency should be possible. Wide Bandgap V OC >1.5V Tunnel Contact GaAs V OC =1.1V Solar Cell

50 GaAs solar cells are the preferred technology, where cost is no objection: Space

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52 The Epitaxial Liftoff Process:

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56 5. What are the top competing technologies? a. c-si ~ 15%-23% in production 90% market share b. poly-cuingase 2 ~ 13% promised ~100 Start-Ups c. poly-cdte ~ 11% in production First Solar Corp. d. flat-plate GaAs ~ 28.3% in R&D Alta Devices Inc. e. Concentrators ~ 43.5% in R&D triple-junction III-V Solar Junction Inc.