The Opto-Electronic Physics Which Just Broke the Efficiency Record in Solar Cells. Green Photonics Symposium at Technion Haifa, Israel, April 23, 2014

Transcription

1 The Opto-Electronic Physics Which Just Broke the Efficiency Record in Solar Cells Green Photonics Symposium at Technion Haifa, Israel, April 23, 2014 Eli Yablonovitch UC Berkeley Electrical Engineering & Computer Sciences Dept. & Lawrence Berkeley Nat l Lab. Miller et al, IEEE J. Photovoltaics, vol. 2, pp (2012)

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

4 The Epitaxial Liftoff Process:

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6 GaAs Courtesy of Alta Devices, Inc.

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8 Efficiency (%) Alta Devices record, 28.8% 1-sun, single junction, solar cell efficiency record: Year

9 Open Circuit voltage V OC (Volts) Alta Devices record cell, volts Open Circuit voltage for record efficiency cells: Year

10 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 populationin state populationin the light the dark In semiconductors with mobile electrons & holes: Free energy = E Fc -E Fv =2kT ln electron electron density in the light density in the dark

11 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 But in quasi-equilibrium: external Luminescent emission band to band emission in the dark qv oc = kt ln band incoming sunlight to band emission in the dark

12 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

13 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 +0.02eV E 1. 4T E g kt{ln(/ s ) + ln(4n 2 s kts ) + ln(qv op /kt) ln()-ln( e )} T g 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 correction for Planck emissionbandwidth 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)

14 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.

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

16 h h h 25.1% efficiency h + e - h h g h h h g 28.8% efficiency e - h +

17 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!

18 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%

19 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%

20 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

21 Latest 1 sun single-junction results from Alta Devices, Inc. Expected to reach 34% dual junction, eventually.

22 h h h 25.1% efficiency h + e - h h g h h h g 28.8% efficiency e - h +

23 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

24 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: 1. Thin-film cells are more efficient than the best wafer cells. 2. Solar cells perform best when there is maximum external fluorescence yield ext. Miller et al, IEEE J. Photovoltaics, vol. 2, pp (2012)

25 Why the record-setting Voltage? semiconductor h h only 1/4n 2 = 1/50 = 2% of the light escapes. 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 This Photon-Recycling explanation is incomplete! Good external luminescence can be achieved with texturing and no-photon-recycling.

26 Paradox: Why is external luminescence is good for solar cell efficiency? Reason #4; Luminescence IS Voltage: External luminescence is sometimes used as a type of contactless voltmeter, indicating the separation of quasi-fermi levels in the solar material. At quasi-equilibrium: Luminescence = (Black Body) exp{qv/kt} (This is sometimes employed as a contactless, qualitycontrol-metric, in solar cell manufacturing plants. ) This viewpoint is tautological: Good external luminescence actually is good voltage, and therefore good efficiency.

27 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!

28 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.

29 Latest 1 sun dual-junction results from Alta Devices, Inc. Expected to reach 34% dual junction, eventually.

30 Conclusions: 1. Thin-Film GaAs of ~28% efficiency is in the process of scale-up for solar panels. 2. Opto-electronics can provide the motive power for automobiles. 3. The automotive power market is 10 bigger than the panel electricity market.