Improved GaAsP Solar Cells with Back Reflector for Space … › files › 2020 › 05 ›...
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Improved GaAsP Solar Cells with Back Reflector for Space Applications
ECE443 Final ProjectBrian Li
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Introduction/motivation Technical Background Simulation Results Conclusions
Outline
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Terrestrial PV dominated by low-cost Si solar cells
Space PV mainly uses III-V multijunction cells
Space PV prioritizes high-efficiency over cell cost– High specific power (W/kg)
reduces launch cost
Space Solar Power
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Module type Module cost ($/W)III-V 150Si 0.3-0.5
Estimated cost for cell modules [1]
Cell type efficiency (%) Ref.Si 18.3 [2]
InGaP/GaAs/InGaAs 31.5 [3]
Efficiency of cells under AM0 spectrum
[1] Horowitz, K. A. et al. Technical Report: National Renewable Energy Laboratory (2018)[2] Crotty, G. T. et al. Conf. Rec. IEEE Photovolt. Spec. Conf. 1035–1038 (1997)[3] Takamoto, T. et al. 2014 IEEE 40th Photovolt. Spec. Conf. PVSC 2014 1–5 (2014)
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Recent trend: low-earth orbit (LEO) satellite constellations [1]– Cheaper, shorter duration than
geostationary (GEO) satellites
Cell cost may be more important for LEO satellites
III-V on Si could achieve high efficiency at low cost
Low-earth orbit satellites
4
Satellite Type
Altitude (km)
Averageduration
(yrs)GEO 35000 15-20
LEO 500-2000 7
GEO vs. LEO satellites [3]
[1] G. Ritchie, “Why Low-Earth Orbit Satellites are the New Space Race,” Washington Post. [Online]. Available: https://www.washingtonpost.com/business/why-low-earth-orbit-satellites-are-the-new-space-race/2019/08/15/6b224bd2-bf72-11e9-a8b0-7ed8a0d5dc5d_story.html.[2] M Williams, “Starlink’s Satellites Will be Orbiting at a Much Lower Altitude, Reducing the Risks of Space Junk” [Online]. Available: https://www.universetoday.com/142134/starlinks-satellites-will-be-orbiting-at-a-much-lower-altitude-reducing-the-risks-of-space-junk/[3] J. Pelton, S. Madry, and S. Camacho-Lara, Handbook of Satellite Applications. New York: Springer US, 2013.
Sketch of LEO constellation [2]
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GaAsP/Si has high theoretical efficiency 34% (AM0), above record Si cells
Real cells suffer from lattice mismatch defects, and need improved growth and design
This work: Improve GaAsPcells with back reflector
GaAsP/Si tandem cells
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tunnel junction
1.7eV GaAsP top cell
1.1eV Si bottom cell
GaAsyP1-y graded buffer
Structure of GaAsP/Si cell
Experiment (AM1.5G) 20.1 [1]Theoretical (AM1.5G) 37.0 [2]Theoretical (AM0) 34.0 [2]
Efficiency (%) of GaAsP/Si cells
[1] M. A. Green, E. D. Dunlop, J. Hohl-Ebinger, M. Yoshita, N. Kopidakis, and A. W. Y. Ho-Baillie, “Solar cell efficiency tables (Version 55),” Prog. Photovoltaics Res. Appl., vol. 28, no. 1, pp. 3–15, 2020.
[2] J. Geisz and D. Friedman, “III–N–V semiconductors for solar photovoltaic applications,” Semicond. Sci. Technol., vol. 769, 2002.
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Intro/motivation Technical Background Simulation Results Conclusions
Outline
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Achieving high EQE:– Low reflectance– Long carrier lifetime – Low surface recomb.– High absorption
Short-circuit current density (Jsc) is dependent on EQE
External Quantum Efficiency (EQE)
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Region 1 carrier losses1. Reflectance2. Emitter recomb.3. Front surface recomb.
WavelengthEQ
E (%
)𝜆𝜆𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏
100Perfect EQE
0
Region 1
Region 2
Region 2 carrier losses1. Base recomb.2. Back surface recomb.3. Transmission of light
p-typebase
n-type emitter
junction
𝐸𝐸𝐸𝐸𝐸𝐸(𝜆𝜆) =𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑐𝑐𝑐𝑐 𝑗𝑗𝑗𝑗𝑗𝑗𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑗𝑗
𝑐𝑐𝑗𝑗𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑗𝑗𝑐𝑐 𝑝𝑝𝑝𝑐𝑐𝑐𝑐𝑐𝑐𝑗𝑗𝑐𝑐
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500 1000 15000.0
0.5
1.0
1.5
2.0
2.5
Spec
tral i
rradi
ance
(W*m
-2*n
m-1
)
Wavelength (nm)
Tandem Jsc is limited by the worse of the 2 sub-cells
Jsc should be equal to minimize loss (current matching)
GaAsP cell is current-limiting due to defects harming carrier collection [1]
Irradiation in space will further harm cell [2]
Tandem cells and current-matching
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tunnel junction
1.7eV GaAsP top cell
1.1eV Si bottom cell
GaAsyP1-y graded buffer
AM0spectrum
[1] S. Fan et al., “20%-efficient epitaxial GaAsP/Si tandem solar cells,” Sol. Energy Mater. Sol. Cells, vol. 202, no. March, pp. 1–8, 2019.[2] N. Gruginskie et al., “Electron radiation – induced degradation of GaAs solar cells with different architectures,” Prog. Photovoltaics Res. Appl., vol. 28, no. 4, pp. 266–278, 2020.
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Distributed Bragg reflector (DBR)– Alternating high/low index layers – Creates reflectance “stop-band” at
central wavelength 𝜆𝜆𝑐𝑐 Thin cell with reflector can
improve long-wavelength EQE
Improving EQE with backside reflector
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nH = high index
nL = low index
DBR(N pairs)
reflected light
Δλ
𝜆𝜆𝑐𝑐
Base (p-type)
Emitter (n-type)
e-
x e- recombines beforereaching junction
Base (p-type)
Emitter (n-type)
e-e- collects at junction
DBR
Thick cell w/out reflector Thin cell with reflector
Reflectance stop-band of DBR
𝑐𝑐𝐻𝐻 =𝜆𝜆𝑐𝑐
4𝑗𝑗𝐻𝐻
𝑐𝑐𝐿𝐿 =𝜆𝜆𝑐𝑐
4𝑗𝑗𝐿𝐿
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Intro/motivation Technical Background Simulation Results
– 1J GaAsP cell design – DBR design– Improved Jsc of GaAsP cell with DBR
Conclusions
Outline
10
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1J GaAsP cell modeled after literature [1]
Thin emitter, thick base to generate carriers near the junction
Window and back surface field (BSF) to block minority carriers
Design of 1J GaAsP cell
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Window n-Al0.65In0.35P 20 nm 1×1018 cm-3
Emitter n-GaAs0.77P0.23 50 nm 1×1018 cm-3
Base p-GaAs0.77P0.23 1150 nm 1×1017 cm-3
BSF p-In0.37Ga0.63P 25 nm 1×1018 cm-3
Contact Layer p-GaAs0.77P0.23 50 nm 1×1019 cm-3
contact
contact
0 200 400 600 800 1000 1200
-2
-1
0
1
2
Ene
rgy
(eV
)
Depth from surface (nm)
AlInP window InGaP BSFEc
EF
Ev
2% front reflection
[1] S. Fan et al., “20%-efficient epitaxial GaAsP/Si tandem solar cells,” Sol. Energy Mater. Sol. Cells, vol. 202, no. March, pp. 1–8, 2019.
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Al0.1Ga0.9As/Al0.9Ga0.1As for the high/low index pairs
Design of DBR structures
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400 500 600 700 800 9000
20
40
60
80
100
Sim
ulat
ed E
QE
(%)
Wavelength (nm)
0
20
40
60
80
100 DBR C DBR B DBR A
DBR
refle
ctan
ce (%
)
Description of three DBR designs
DBR label No. of layer pairs 𝝀𝝀𝒄𝒄 (nm)
A 10 650
B 20 650
C 20 600 and 680 (10 pairs each)
Window n-Al0.65In0.35P 20 nm 1×1018 cm-3
Emitter n-GaAs0.77P0.23 50 nm 1×1018 cm-3
Base p-GaAs0.77P0.23 1150 nm 1×1017 cm-3
BSF p-In0.37Ga0.63P 25 nm 1×1018 cm-3
Contact Layer p-GaAs0.77P0.23 50 nm 1×1019 cm-3
contact
contact
2% front reflection
DBR reflection profile
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DBR A and DBR B similarly improve EQE DBR C improves over wider region
Effect of DBR on EQE
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300 400 500 600 7000
20
40
60
80
100
DBR C DBR B DBR A no reflection
EQE
(%)
Wavelength (nm)
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1 2 3 421.5
22.0
22.5
23.0
23.5
24.0
24.5
J sc (m
A/cm
2 )
no backreflection
DBR A DBR B DBR C
1200nm, 1ns
800nm, 1ns
800nm, 0.1ns
1200nm, 0.1ns
baseline
Jsc for current-matching to Si cell: ~24mA/cm2 [1]
Vary thickness and lifetime 𝜏𝜏:– 1ns = nominal carrier lifetime– 0.1ns = “irradiated” carrier lifetime
800nm + DBR has better Jsc at 0.1ns carrier lifetime– Carriers generated closer to junction
leads to improved collection
Effect of DBR on Jsc
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Window n-AlInP 20 nm
Emitter/base n- and p- GaAsP
1200nm or 800nm
𝝉𝝉 = 1ns or 0.1ns
BSF p-InGaP 25 nmContact Layer p-GaAsP 50 nm
contact
contact
2% front reflection
AM0 spectrum
Thickness (nm)
Reflector 𝝉𝝉 (ns) Jsc(mA/cm2)
1200 none 1 23.42
800 DBR C 1 23.87
1200 none 0.1 21.99
800 DBR C 0.1 22.83
Jsc for baseline vs. optimized cells
[1] G. T. Crotty, P. J. Verlinden, M. Cudzinovic, R. M. Swanson, and R. A. Crane, “18.3% Efficient Silicon Solar Cells for Space Applications,” Conf. Rec. IEEE Photovolt. Spec. Conf., pp. 1035–1038, 1997.
DBR reflection profile
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The Jsc of GaAsP cells was improved with thin 800nm cell and a high-performance DBR– Jsc is 1.9% higher under nominal 1ns lifetime and
3.8% higher under degraded 0.1ns lifetime– Jsc with 1ns lifetime was near current-matching
condition of 24 mA/cm2
Overall, new cell design would improve performance over long-term use in space
Conclusion
15
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Adjusted GaAsP minority carrier lifetime 𝜏𝜏 and interface recomb. velocities (IRV) to fit EQE from ref. [1]
Obtained similar long-wavelength EQE to ref.
Same Jsc of 17.8mA/cm2
under AM1.5G
Supplemental: Fitting for EQE
16
Window n-Al0.65In0.35P 20 nm 1×1018 cm-3
Emitter n-GaAs0.77P0.23 50 nm 1×1018 cm-3
Base p-GaAs0.77P0.23 1150 nm 1×1017 cm-3
BSF p-In0.37Ga0.63P 25 nm 1×1018 cm-3
Contact Layer p-GaAs0.77P0.23 50 nm 1×1019 cm-3
contact
contact
300 400 500 600 7000
20
40
60
80
100
Reference Simulation
EQE
(%)
Wavelength (nm)
Important region for study
2% front reflection
Carrier lifetime 𝝉𝝉 (ns) 1Emitter/window IRV (m/s) 1x103
Base/BSF IRV (m/s) 1x105
Fitted lifetime and velocity parameters
[1] S. Fan et al., “20%-efficient epitaxial GaAsP/Si tandem solar cells,” Sol. Energy Mater. Sol. Cells, vol. 202, no. March, pp. 1–8, 2019.
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Supplemental: specs of DBR
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DBR label No. of layer pairs 𝝀𝝀𝒄𝒄 (nm) High/low indices Al0.1Ga0.9As/Al0.9Ga0.1As thicknesses (nm)
Thickness(nm)
A 10 650 3.58/2.99 45.43/54.29 997
B 20 650 3.58/2.99 45.43/54.29 1994
C 10 + 10 600 and 680 3.58/2.99 41.93/50.12 and47.53/56.80 1963
400 500 600 700 800 9000
20
40
60
80
100
Sim
ulat
ed E
QE
(%)
Wavelength (nm)
0
20
40
60
80
100 DBR C DBR B DBR A
DBR
refle
ctan
ce (%
) Al0.1Ga0.9As/Al0.9Ga0.1As for the high/low index pairs– Index values of 3.58 and 2.99– Set absorption = 0
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Supplemental: Tabulated Jsc
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Cell conditions Jsc (mA/cm2) for different reflectance casesGaAsP
Thickness (nm)Bulk
lifetime (ns)No back
reflectionDBR
10 pairDBR
20 pairsDBR
10+10 pairsTotal back reflection
1200 1 23.42 23.80 23.80 23.98 24.101200 0.1 21.99 22.29 22.29 22.43 22.52800 1 22.74 23.55 23.58 23.87 24.06800 0.1 21.82 22.55 22.58 22.83 23.01
1 2 3 4 521.5
22.0
22.5
23.0
23.5
24.0
24.5
J sc (m
A/cm
2 )
no backreflection
DBR A DBR B DBR C 100% backreflection
1200nm, 1ns
800nm, 1ns
800nm, 0.1ns
1200nm, 0.1ns
500 550 600 650 700 75040
50
60
70
80
90
100% reflection DBR C DBR B DBR A no reflection
EQE
(%)
Wavelength (nm)
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Voc surprisingly worsens after thinning the cell
Due to excess surface recombination at base/BSF?
Possibly non-physical artifact of simulation setup
Supplemental: LIV
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0.00 0.25 0.50 0.75 1.00 1.250
5
10
15
20
25
800nm - DBR C - 1ns 1200nm - no reflection - 1ns
800nm - DBR C - 0.1ns 1200nm - no reflection - 0.1ns
Cur
rent
Den
sity
(mA/
cm2 )
Voltage (V)
AM0
Voc(V)
Jsc(mA/cm2)
FF (%)
𝜂𝜂(%)
800nm 1ns 1.172 23.87 86.1 17.62
1200nm 1ns 1.209 23.42 88.9 18.40
800nm 0.1ns 1.069 22.83 85.2 15.21
1200nm 0.1ns 1.152 21.99 82.4 15.27