Quantum Well Solar Cells - Quantum Electronics Group
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Transcript Quantum Well Solar Cells - Quantum Electronics Group
Nanoscale Energy Conversion in the
Quantum Well Solar Cell
Keith Barnham, Ian Ballard, Amanda Chatten, Dan Farrell,
Markus Fuhrer, Andreas Ioannides, David Johnson,
Marianne Lynch, Massimo Mazzer, Tom Tibbits
Experimental Solid State Physics, Imperial College London, London
SW7 2BW, UK
[email protected]
http://www.sc.ic.ac.uk/~q_pv
Rob Airey, Geoff Hill, John Roberts, Cath Calder,
EPSRC National Centre for III-V Technology, Sheffield S1 3JD, UK
Solarstructure , Permasteelisa, FULLSPECTRUM EU Framework VI,
Outline
First practical nanoscale photovoltaic cell
Enhanced spectral range of the strain-balanced
quantum well solar cell (SB-QWSC)
Efficiency enhancement by photon recycling
Evidence for hot electron effects in the QW
Cell efficiency cell versus l or Eg
GaAs cells - highest effic. single
junction cells, Eg too high
lower Eg => higher efficiency
Can grow InyGa1-yAs bulk cells
on virtual substrates but never
dislocation free
Maximum at 1.1 mm ~ 1.1 eV
Multi-junction cells need 4th
band-gap ~ 1.1 mm ~ 1.1 eV
Enhancing GaAs Cell Efficiency
From 30x – 1000x AM1.5
optimum single junction
efficiency band-gap ~ 1.1 eV
Multi-junction approaches
going for GaInNAs cell
No ternary alloy with lower Eg
than GaAs lattice matched to
GaAs/Ge
GaAs1-yPy (y ~ 0.1) + InxGa1-xAs, (x~ 0.1 – 0.2)
strain-balanced to GaAs/Ge => novel PV material
GaAsP/InGaAs Strain-Balanced QWSC
Balance stress between layers to
match lattice parameter of the
substrate
Advantages:
Can vary absorption bandedge and absorb wider
spectral range without
strain-relaxation
no dislocations > 65 wells
single junction with wide
spectral range
ability to vary Eg gives
higher tandem effic.
SB-QWSC – Ideal Dark-Currents
at High Concentration
Dark current of 50 well QWSC
Low current fits one parameter
Shockley-Read-Hall model
High (concentrator) current slope
changes
ideal Shockley current
+ radiative recombination in QW
Minimum recombination radiative
at concentrator current levels
Investigation of Photon Cavity Effects
50 well SB- QWSC
In0.1Ga0.9As wells
GaAs0.91P0.09 barriers
Control and distributed
Bragg reflector (DBR)
devices grown
side-by-side
Processed as concentrator,
fully metalised, and photodiode
devices
11 finger concentrator mask,
3.6% shading
Ta2O5 / SiNX
Ta2O5 / SiNX
BSF
BSF
DBR
100
100
100
100
80
80
80
80
60
60
60
60
40
40
40
40
20
20
00
400
400
20
20
500
500
600
600
700
700
800
800
900
900
Increase photon
absorption
Increase photocurrent
No series resistance
In-situ growth
Wavelength
Wavelength (nm)
(nm)
JSC (mA/cm2)
Device
AM1.5d 1000W/m2 AOD 913W/m2
NonDBR
28.0
DBR
28.6
100
00
1000
1000
26.3
26.9
Reflectivity (%)
DBR IQE
Non-DBR IQE
DBR reflectivity
Reflectivity (%)
Internal quantum efficiency (%)
Distributed Bragg Reflectors
80
60
40
20
[3]
0 D.C. Johnson et al. Solar Energy
0
20
40
60
80
Materials and Solar Cells, 2005
Incident angle (°)
Concentrator Measurements
27% efficiency at 328x
low-AOD spectrum
Single junction record is
(27.6 +/-1)% at 255x
26
Non-DBR
DBR
Efficiency (%)
25
24
D.Johnson et al. WCPEC4, Hawaii May 06
23
AM1.5d 1000W/m
2
22
10
100
Concentration (suns)
Efficiency increase
higher than expect from
double pass in QWs
Enhanced Voc
[3] Vernon S.M., et al. “High-efficiency concentrator cells
from GaAs on Si”, 22nd IEEE PVSC 1991 pp53–35
Why the Efficiency Enhancement?
MQW
DBR
Aim of DBR was to absorb
photons on second pass
Some photons from radiative
recombination at high bias
trapped in the device
Photons reabsorbed in the
QWs reduce dark current
Generalised Plank model
for EL shows reduction
consistent with dark
current suppression
Photon recycling could
take cell to 30% efficiency
MQW
DBR
Single QW Electroluminescence low bias
1
Bulk
Luminescence (a.u.)
Well
0.1
0.92V < Vapp < 0.98V
820
840
860
880
900
920
Wavelength (nm)
940
960
980
Single QW EL at high bias
1
Well
Luminescence (a.u.)
Bulk
Vapp= 1.10V
Vapp= 1.00V
0.1
0.92V < Vapp < 0.98V
820
840
860
880
900
920
Wavelength (nm)
940
960
980
10 QW Electroluminescence low bias
1
Luminescence (a.u.)
Well
0.1
Bulk
0.84V < Vapp < 1.02
840 850 860 870 880 890 900 910 920 930 940 950 960 970 980 990
wavelength (nm)
10 QW EL at high bias
1
Luminescence (a.u.)
Well
Bulk
Vapp = 1.16V
Vapp= 1.04V
0.1
0.84V < Vapp < 1.02
840 850 860 870 880 890 900 910 920 930 940 950 960 970 980 990
wavelength (nm)
Model EL (radiative recombination)
Detailed Balance leads to generalised Planck:1
2n LW a (E)E
L(E,F)dE
dE
3 2
(E DE F ) kB T
hc e
1
2
where
2
a(E) = absorption coefficient
T = temperature of recombining carriers
DEF = quasi-Fermi level separation
a(E) (use measured QE) and T determine shape
DEF requires absolute calibration
J.Nelson et al., J.Appl.Phys., 82, 6240, (1997)
M.Fuhrer et at Proc. EU PVSEC Dresden,Sept 06
EL - model and experiment
data
model
1
T=300.0K
T=320.0K
T=340.0K
T=360.0K
T=380.0K
luminescence
Luminescence (a.u.)
(a.u.)
1
increasing V
Increasing T
0.1
920
0.1
930
940
950
960
wavelength (nm)
970
980
990
920
930
940
950
960
wavelength (nm)
970
980
EL - Bulk Peak
1.1
1
0.9
0.8
Fits T = 299 K
Luminescence (a.u.)
0.7
0.6
0.5
0.4
0.3
0.2
840
850
860
870
Wavelength (nm)
880
890
Conclusions
SB-QWSC concentrator cells (near) highest efficiency and
widest spectral range of single junction cells
Radiative recombination dominates at high current levels and
photon recycling observed with DBR
EL reduction with DBR consistent with dark-current
Evidence for hot carrier effects at high current levels in EL
shape consistent with generalised Planck
These nanoscale properties occur at the high current levels to
be expected in terrestrial concentrator systems
Advantages of the SB-QWSC
Approximately double the efficiency of current cells
Widest spectral range in a single junction cell so
keeps high efficiency as sunlight spectrum varies
Nano-scale effectss – photon cavity, hot electrons
Small size ~ mm – optoelectronic fabrication.
Need high concentration to bring price down
What application?
Building integrated concentrator photovoltaics (BICPV)
Novel Application - Building Integrated Concentrators
SB-QWSC - highest efficiency single junction cell, ~ 1mm size
UK – over 60% electricity used in buildings
over 7 x as much solar energy falls on those buildings
SMART WINDOWS
No transmission of direct sunlight
Reduce glare and a/c requirement
Max diffuse sunlight - for illumination
No need for lights when blinds working
(2 – 3) x power from Silicon BIPV
Electricity at time of peak demand
Cell cooling in frame - hot water
Barnham, Mazzer, Clive, Nature Materials, 5, 161 (2006).
Calculated output : San Francisco
9.3%
180
5.0% 1.2%
Electricity
2.1%
170
Space Heating
kWh/m 2
160
Water Heating
17.9%
150
Cooking
23.1%
Average electricity generated by
1 m2 of façade over 1 year
140
130
Lighting
Cooling
Ventilation
120
110
Refrigeration
8.6%
Office Equipment
100
1.0
10.0
100.0
Larger side/Smaller side
Other
7.3%
25.6%
Savings
120%
100%
80%
Consumption = 145 kWh/m2
60%
Fraction of electricity consumption
provided by photovoltaic cells
40%
20%
0%
0
30
60
90
120 150 180 210 240 270 300 330 360
Day
L
6L
3L
Luminescent Concentrators for
Diffuse Component of Sunlight
Dye-doped luminescent concentrators (1977):
Advantages
no tracking required
accept diffuse sunlight
stacks absorb different l
Eg ~ Eg, gives max. effic.
thermalisation in sheet
Disadvantages
dyes degrade in sunlight
loss from overlap of
absorption/luminescence
narrow absorption band
A Goetzberger and W Greubel, Appl. Phys. 14, 1977, p123.
Quantum Dot Concentrator
QDs replace dyes in luminescent concentrators:
QDs degrade less in sunlight
core/shell dots high QE
absorption edge tuned by dot size
absorption continuous to short l
red-shift tuned by spread in dot size
spread fixed by growth conditions
(K.Barnham et al. App. Phys.Lett.,75,4195,(2000))
Thermodynamic Model for QDC
The brightness, B(n), of a radiation field that is in equilibrium
with electronic degrees of freedom of the absorbing species:
n = refractive index
8n 2n 2
1
Bn )
= 1/kT
c 2 ehn m ) 1
m = chemical potential
Applying the principle of detailed balance within the slab:
Ns e n )
F m ) dnNs e n )I C n ) dn
Bn ) 0
Qe
IC = concentrated radiation field, Qe = quantum efficiency,
se = absorption cross section
Extend to 3-D fluxes + boundary conditions
I1(n)
x
Wc
y
A.J.Chatten et al, 3rd WCPEC, Osaka, 2003
E Yablonovitch, J. Opt. Soc. Am. 70, 1362, 1980.
z=0
Wc
z
W2
z=D
Characterisation of ZnS/CdSe QDs in
Acrylic with Thermodynamic Model
SD387 Red
SD396 yellow
Thermodynamic model fits PL shape and red-shift
of Nanoco QDs assuming only absorption cross section
Fitting current measured at cell on edge gives
Qe(SD387) = 0.56 (c.f. Nanoco 0.4 – 0.6)
Thermodynamic Model confirms
unexpected luminescent stack result
Layer
Experimental Jsc
(mA/m2)
Predicted
Jsc (mA/m2)
Top
10.2 ± 2.0
9.1 ± 2.1
Bottom
35.1 ± 2.0
37.9 ± 1.3
Incident light
Total output = 45.3 (mA/m2)
Incident light
Layer
Experimental Jsc
(mA/m2)
Predicted
Jsc (mA/m2)
top
47.5 ± 2.0
46.9 ± 2.1
Bottom
4.8 ± 2.0
3.8 ± 1.3
Total output = 52.3 (mA/m2)
EL Modeling Confirms Recycling
50 QW dark current show 33% reduction of J01
Model EL by detailed balance ~ 30% reduction
Supports efficiency increase results from
photon recycling
0.1
10
DBR
Non-DBR
Non-DBR Ideality n=1
DBR Ideality n=1
Current (A)
Normalised emission (a.u.)
Calculated
Measured
0.01
Ideality n = 1 reduction
1
1.32
1.34
1.36
1.38
1.40
Energy (eV)
1.42
1.44
1.04 1.06 1.08 1.10 1.12 1.14 1.16 1.18
Bias (V)
Compare SB-QWSC with Tandem in Smart Windows
A tandem cell 13%
more efficient than a
SB-QWSC harvests
only 3% more
electrical energy
P.Tandem
P.Single
1.2
Power/(kWh/m 2)
London –
Vertical South - East
Facing Wall
1.0
0.8
0.6
0.4
0.2
0.0
0
28 56 84 112 140 168 196 224 252 280 308 336 364
Day
Series current constraint means tandem optimised
for one spectral condition (and one temperature)
Single Molecule Precursor ZnS/CdSe Core-Shell QDs
Currently part of
“FULLSPECTRUM”
Framework VI Integrated
Project
300
absorption and luminescence of Nanoco OMN29 QDs
1.2
Experimental absorption with a linear background subtracted
Gaussian used to fit absorption threshold
250
Absorption fit used in predicting the luminescence
1
Normalised predicted luminescence
Normalised experimental luminescence
200
0.8
150
Absorption and emission
data from Sarah
Gallagher
0.6
100
0.06% by mass QDs in
chloroform
0.4
Pathlength 1cm
0.2
50
0
1.90
2.10
2.30
2.50
2.70
2.90
3.10
3.30
0
3.50
E/eV
(T.Trindade et al. Chemistry of Materials, 9, 523, (1997))
(A.J.Chatten et al, Proc. 3rd WCPEC, Osaka, 2003)
Luminescence/a.u.
Absorption/a.u.
Core shell ZnS/CdSe
dots by thermolysis at
270 °C of singlemolecule precursors
in PLMA using with
TOPO cap
Luminescence fit
is two-flux
thermodynamic model.
BICPV – Smart Windows
Transparent Fresnel Lenses
(300 – 500)x concentration
1.5 or 2-axis tracking
Novel secondaries
Lenses
~ 1 mm solar cells
Direct
Sunlight
Cell efficiency ~ 30%
Adds ~ 20% to façade cost
Diffuse
Daylight
Solar
Cells
Diffuse
Daylight
Front
Glass
Heat
Electricity