Improved GaAsP Solar Cells with Back Reflector for Space Applications

ECE443 Final ProjectBrian Li

Introduction/motivation Technical Background Simulation Results Conclusions

Outline

2

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

3

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)

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]

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

5

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.

Intro/motivation Technical Background Simulation Results Conclusions

Outline

6

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)

7

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

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

8

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.

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

9

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

Intro/motivation Technical Background Simulation Results

– 1J GaAsP cell design – DBR design– Improved Jsc of GaAsP cell with DBR

Conclusions

Outline

10

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

11

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.

Al0.1Ga0.9As/Al0.9Ga0.1As for the high/low index pairs

Design of DBR structures

12

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

DBR A and DBR B similarly improve EQE DBR C improves over wider region

Effect of DBR on EQE

13

300 400 500 600 7000

20

40

60

80

100

DBR C DBR B DBR A no reflection

EQE

(%)

Wavelength (nm)

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

14

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

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

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.

Supplemental: specs of DBR

17

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

Supplemental: Tabulated Jsc

18

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)

Voc surprisingly worsens after thinning the cell

Due to excess surface recombination at base/BSF?

Possibly non-physical artifact of simulation setup

Supplemental: LIV

19

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