Owen D. Miller & Eli Yablonovitch UC Berkeley Electrical Engineering & Computer Sciences Dept. http://arxiv.org/abs/1106.1603 Solar Energy Mini-Series Jen-Hsun Huang Engineering Center Stanford, California Sept. 26, 2011 The Opto-Electronic Physics That Just Broke the Efficiency Record in Solar Cells
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Owen D. Miller & Eli Yablonovitch
UC Berkeley
Electrical Engineering & Computer Sciences Dept.
http://arxiv.org/abs/1106.1603
Solar Energy Mini-Series
Jen-Hsun Huang Engineering Center
Stanford, California
Sept. 26, 2011
The Opto-Electronic Physics That Just Broke
the Efficiency Record in Solar Cells
2010
Cost per peak Watt for Solar Panels
Date
$10
$1
$0.30
X
X
1981 2011 2020
40GW installed cumulative
1TW installed
cumulative
subsidies will no
longer be needed
Year
From Allen Barnett
Univ. of Delaware
Value of High Photovoltaic Efficiency
Efficiency and Cost of Electricity (COE) inversely related:
OperatingFinanceEfficiencyInsolation
)Cost(CostCOE BOSmodule
Factors that impact COE:
Location: decides the available
input energy
Efficiency: decides the portion
that can be converted to
electricity
5% 8% 12%
18%
27%
40%
0
5
10
15
20
25
30
35
40
0 100 200 300 400 500 600 700 800 900 1000
Cost of Photovoltaics ($/m2)
Co
st
of
Ele
tric
ity
(¢
/kW
h)
9
$
CdTe 11% Efficiency
$
c-Si 23% Efficiency
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?
What is a Solar Cell?
A Solar Cell does not require a p-n junction!
E
EFn
EFp
V
h
e-
e-
e- e
-
e-
e-
h+
h+
h+
h+ h
+ h+ h
+ Wider Bandgap
top, bottom
and sides
double hetero-structure Selective
electron
contact
Selective
hole
contact
k
+ -
-
+
e-
e- e-
h+
h+
h+
e-
Diffusion potential to contacts is typically <1mVolt.
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:
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.
Normalized for 1 cm2 area.
Internal luminescence 78.1mA
Reabsorption 77.3mA
For 3m thickness:
32.1mA
Incoming light
External emission 0.04mA
Auger 710-6mA
Internal luminescence 5.40mA
Reabsorption 5.36mA Extracted
current 31.3mA
Planar, good mirror efficiency = 33.2%
efficiency = 30.6%
Planar, bad mirror
Mirror Loss 0.79mA
32.5mA
Incoming light
External emission 0.79mA
Auger 410-4mA
Extracted current 31.7mA
Mirror Loss 0mA
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.
Normalized for 1 cm2 area.
Internal luminescence 78.1mA
Reabsorption 77.3mA
For 3m thickness:
(a)
e-
h+
h h h
(b) h h h hg hg
hg
e-
h+
Internal Fluorescence Yield int >> 90%
Rear reflectivity >> 90%
Both
needed for
good ext
Counter-Intuitively, to approach the
Shockley-Queisser Limit, you need to have
good external fluorescence yield ext !!
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
The external Luminescence intensity is a “Volt-meter”
into the solar cell.
darktheinemissionbandtoband
sunlightincoming
darktheinemissionbandtoband
emissionnt Luminesceexternal
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.
http://arxiv.org/abs/1106.1603
p-Al0.2Ga0.8As
n-GaAs Eg=1.4eV
n-Al0.5In0.5P
n+-Al0.5In0.5P Eg~2.35eV
p+-Al0.5In0.5P Eg~2.35eV
p-Ga0.5In0.5P Eg~1.9eV
n-Al0.5In0.5P Eg~2.35eV
n-Ga0.5In0.5P Eg~1.9eV
Tunnel
Contact
GaAs
VOC=1.1V
Solar Cell p-GaAs Eg=1.4eV
Ga0.5In0.5P
VOC=1.5V
Solar Cell
Tunnel
Contact
GaAs
VOC=1.1V
Solar Cell
Ga0.5In0.5P
VOC=1.5V
Solar Cell
Dual Junction Series-Connected Tandem Solar Cell
h h
All Lattice-Matched ~34% efficiency should be possible.
p-Al0.2Ga0.8As
n-GaAs Eg=1.4eV
n-Al0.5In0.5P
n+-Al0.5In0.5P Eg~2.35eV
p+-Al0.5In0.5P Eg~2.35eV
n-Al0.5In0.5P Eg~2.35eV
Tunnel
Contact
p-GaAs Eg=1.4eV
Wide
Bandgap
VOC>1.5V
Tunnel
Contact
Wide
Bandgap
VOC>1.5V
There is much latitude for creativity in the higher