Prof. Eli Yablonovitch, Electrical Engineering & Computer Sciences Dept.

1. "Photovoltaics, High Efficiency Together with Low Cost" European Academies Science Advisory CouncilStockholm, SwedenThursday, Sept. 19, 2013 Prof. Eli Yablonovitch, Electrical Engineering & Computer Sciences Dept. & Lawrence Berkeley Laboratory University of California, Berkeley, CA 94720

2. 30 29 Alta Devices record, 28.8% 28 1-sun, single junction, solar cell efficiency record: 27 Efficiency (%) 26 25 24 1990 1995 2000 2005 2010 Year

3. 1.15 Alta Devices record cell, 1.122 volts 1.10 Open Circuit voltage for record efficiency cells: 1.05 Open Circuit voltage VOC (Volts) 1.00 0.95 1990 1995 2000 2005 2010 Year

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

5. h h h 25.1% efficiency 1990-2007 e- h+ h h hg hg h 28.8% efficiency 2011-2012 e- h+

6. Efficiency vs. Rear Reflectivity, GaAs 3m 90% Rear Reflectivity Is Not Enough! 33.5 33.2% 32.5 32.2% 31.9% Cell Efficiency (%) 31.5 30.5 0 0.2 0.4 0.6 0.8 1 Reflectivity 1.16 32.6 32.50 1.145 32.46 1.14 32.43 32.4 1.115 1.12 Jsc (mA/cm2) Voc (Volts) 1.104 1.10 32.2 1.08 32 1.06 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Reflectivity Reflectivity

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

8. h h h 25.1% efficiency 1990-2007 e- h+ h h hg hg h 28.8% efficiency 2011-2012 e- h+

9. What if the material is not ideal, and the electrons and holes are lost to heat before they can luminesce? qVoc = qVoc-ideal – kT|ln{ext}| The external fluorescence yield ext is what matters! Only external Luminescence can balance the incoming radiation.

10. 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, quality-control-metric, in solar cell manufacturing plants. ) This viewpoint is tautological: Good external luminescence actually is good voltage, and therefore good efficiency.

11. Objections to: “Good luminescence IS good voltage” • My solar cell doesn’t luminesce at all! • answer: Undoubtedly the voltage is very low, but there is always some small luminescence. • I need to separate the electron and hole as quickly as possible. There is little time for radiative recombination. • answer: That built-in electric field is costing voltage, which means less luminescence. • I need to suppress fluorescence occurring before the electron and hole have separated, which would cost current. • answer: The suppressed fluorescence is an indicator that voltage was sacrificed for current. The carrier extraction needs improvement. e- V h+

12. 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. Miller et al, IEEE J. Photovoltaics, vol. 2, pp. 303-311 (2012)

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

14. The Epitaxial Liftoff Process:

19. GaAs Courtesy of Alta Devices, Inc.

20. Dual Junction Series-Connected Tandem Solar Cell h h n-Al0.5In0.5P Eg~2.35eV Ga0.5In0.5P VOC=1.5V Solar Cell Ga0.5In0.5P VOC=1.5V Solar Cell n-Ga0.5In0.5P Eg~1.9eV p-Ga0.5In0.5P Eg~1.9eV Tunnel Contact p+-Al0.5In0.5P Eg~2.35eV Tunnel Contact n+-Al0.5In0.5P Eg~2.35eV n-Al0.5In0.5P n-GaAs Eg=1.4eV GaAs VOC=1.1V Solar Cell GaAs VOC=1.1V Solar Cell p-GaAs Eg=1.4eV p-Al0.2Ga0.8As All Lattice-Matched ~34% efficiency should be possible.

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

23. What is happening in the solar economy? c-Si  ~ 15%-23% in production 90% market share 60GW/year annual production capacity in China World-wide demand ~30GW/year ~28GW/year idle-capacity in China (moth-balled) Result is a Price war! The current world price has settled at $0.61/Watt!! This is very important information. It’s the variable cost of producing c-Si panels, does not cover fixed investment costs. New technologies have been shut down, including poly-CuInGaSe2, poly-CdTe, concentrators, etc. Companies are being kept alive by old fixed price contracts.

24. Common Fallacies, that have been over-turned: • Thin film implies poly or amorphous. • There is no such option as a single crystal thin-film. • Crystalline is inherently more expensive than poly or amorphous. • It is a competition between either low efficiency or low cost. • Conventional Wisdom that is true: • Thin-film is cheaper than bulk. • Both high efficiency AND low cost are needed to succeed. • The Shockley-Queisser limit is achievable, and the new battlegrounds are 30%-50% efficiency & storage.