prof. eli yablonovitch, electrical engineering & computer sciences dept

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

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"Photovoltaics, High Efficiency Together with Low Cost" European Academies Science Advisory Council Stockholm, Sweden Thursday, Sept. 19, 2013. Prof. Eli Yablonovitch, Electrical Engineering & Computer Sciences Dept. & Lawrence Berkeley Laboratory - PowerPoint PPT Presentation

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Page 1: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

"Photovoltaics, High Efficiency Together with Low Cost"

European Academies Science Advisory CouncilStockholm, Sweden

Thursday, Sept. 19, 2013

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

& Lawrence Berkeley LaboratoryUniversity of California, Berkeley, CA 94720

Page 2: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

Year

Eff

icie

ncy

(%)

1990 1995 2000 2005 201024

25

26

27

28

29

30

1-sun, single junction, solar cell efficiency record:

Alta Devices record, 28.8%

Page 3: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

Ope

n C

ircu

it v

olta

ge V

OC (

Vol

ts)

0.95

1.00

1.05

1.10

1.15

Year1990 1995 2000 2005 2010

Open Circuit voltage for record efficiency cells:

Alta Devices record cell, 1.122 volts

Page 4: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

norm

al s

olar

cel

lsne

wph

ysic

s

efficiency %

high performing cells

~25%

33.5% Shockley-Queisser limit (single-junction)

0%

Page 5: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

e-

h+

hh hh hh

25.1%efficiency

1990-2007

hhhhhhhg hg

e-

h+

28.8%efficiency

2011-2012

Page 6: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

0 0.2 0.4 0.6 0.8 130.5

Reflectivity

Cel

l Eff

icie

ncy

(%)

0 0.2 0.4 0.6 0.8 1

1.16

Reflectivity

Voc

(V

olts

) 1.14

1.12

1.10

1.08

1.06

Reflectivity0 0.2 0.4 0.6 0.8 1

Jsc

(mA

/cm

2)

32

32.2

32.2%

33.2%

31.9%

1.1041.115

1.145 32.43

32.4632.50

31.5

32.5

33.5

32.4

32.6

Efficiency vs. Rear Reflectivity, GaAs 3m

90% Rear

ReflectivityIs Not

Enough!

Page 7: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

Latest 1 sun single-junction

results fromAlta Devices, Inc.

Expected to reach34% dual junction,

eventually.

Page 8: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

e-

h+

hh hh hh

25.1%efficiency

1990-2007

hhhhhhhg hg

e-

h+

28.8%efficiency

2011-2012

Page 9: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

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.

Page 10: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

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.

Page 11: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

Objections to: “Good luminescence IS good voltage”

1.My solar cell doesn’t luminesce at all!

answer: Undoubtedly the voltage is very low, but there is always some small luminescence.

2.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 costingvoltage, which means less luminescence.

3.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-

h+

V

Page 12: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

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)

Page 13: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

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

Page 14: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

The Epitaxial Liftoff Process:

Page 15: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept
Page 16: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept
Page 17: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept
Page 18: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept
Page 19: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

GaAsCourtesy of Alta Devices, Inc.

Page 20: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

p-Al0.2Ga0.8As

n-GaAs Eg=1.4eV

n-Al0.5In0.5Pn+-Al0.5In0.5P Eg~2.35eVp+-Al0.5In0.5P Eg~2.35eVp-Ga0.5In0.5P Eg~1.9eV

n-Al0.5In0.5P Eg~2.35eV

n-Ga0.5In0.5P Eg~1.9eV

Tunnel Contact

GaAsVOC=1.1V Solar Cell

p-GaAs Eg=1.4eV

Ga0.5In0.5PVOC=1.5V Solar Cell

Tunnel Contact

GaAs VOC=1.1V Solar Cell

Ga0.5In0.5PVOC=1.5V Solar Cell

Dual Junction Series-Connected Tandem Solar Cell

hh

All Lattice-Matched ~34% efficiency should be possible.

Page 21: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

Latest 1 sun dual-junction

results fromAlta Devices, Inc.

Expected to reach34% dual junction,

eventually.

Page 22: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept
Page 23: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

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.

Page 24: Prof. Eli Yablonovitch,  Electrical Engineering & Computer Sciences Dept

Common Fallacies, that have been over-turned:1.Thin film implies poly or amorphous.

2.There is no such option as a single crystal thin-film.

3.Crystalline is inherently more expensive than poly or amorphous.

4.It is a competition between either low efficiency or low cost.

Conventional Wisdom that is true:1.Thin-film is cheaper than bulk.

2.Both high efficiency AND low cost are needed to succeed.

3.The Shockley-Queisser limit is achievable, and the new battlegrounds are 30%-50% efficiency & storage.