Eli Yablonovitch UC Berkeley Electrical Engineering & Computer Sciences Dept. & Lawrence Berkeley Nat’l Lab. Miller et al, IEEE J. Photovoltaics, vol. 2, pp. 303-311 (2012) Green Photonics Symposium at Technion Haifa, Israel, April 23, 2014 The Opto-Electronic Physics Which Just Broke the Efficiency Record in Solar Cells
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Eli Yablonovitch
UC Berkeley
Electrical Engineering & Computer Sciences Dept.
& Lawrence Berkeley Nat’l Lab.
Miller et al, IEEE J. Photovoltaics, vol. 2, pp. 303-311 (2012)
Green Photonics Symposium at Technion
Haifa, Israel,
April 23, 2014
The Opto-Electronic Physics Which Just Broke
the Efficiency Record in Solar Cells
GaAs solar cells are the preferred technology,
where cost is no objection: Space
The Epitaxial Liftoff Process:
GaAs Courtesy of
Alta Devices,
Inc.
Year
Eff
icie
ncy
(%
)
1990 1995 2000 2005 2010 24
25
26
27
28
29
30
1-sun, single junction,
solar cell efficiency record:
Alta Devices record, 28.8%
Open
Cir
cuit
vo
ltag
e V
OC (
Vo
lts)
0.95
1.00
1.05
1.10
1.15
Year 1990 1995 2000 2005 2010
Open Circuit voltage for
record efficiency cells:
Alta Devices record cell, 1.122 volts
What is the ideal voltage Voc to expect, i.e. the Quasi-Fermi
Level separation, chemical potential, or Free Energy?
In molecules and quantum dots:
qVoc=Free energy = kT ln
In semiconductors with mobile electrons & holes:
Free energy = EFc-EFv=2kT ln
darktheinpopulationstateexcited
lighttheinpopulationstateexcited
darktheindensityelectron
lighttheindensityelectron
darktheinpopulationstateexcited
lighttheinpopulationstateexcited
kT
EnergyFreeexp
Boltzmann Factor
What is the voltage to expect, i.e. the Quasi-Fermi
Level separation, chemical potential, or Free Energy?
Shockley-Queisser Limit (1961):
qVoc = kT ln
But in quasi-equilibrium:
qVoc = kT ln
darktheinemissionbandtoband
sunlightincoming
darktheinemissionbandtoband
emissionnt Luminesceexternal
What is the operating voltage?
e-
e-
e- e
-
e-
e-
h+
h+
h+
h+ h
+ h+ h
+
+ -
To extract current, voltage at contacts must be slightly lower than Voc
But, operating voltage linked directly to Voc
kT
qV
q
kTVV OC
OCOP ln
We only need to understand the open-circuit voltage