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InGaN-Based Solar Cells for Ultrahigh Efficiency Multijunction Solar Cell
Applications
Robert M. Farrell, Carl J. Neufeld, Samantha C. Cruz, N. G. Young, Michael Iza, Jordan R. Lang, Yan-Ling Hu, Dobri Simeonov, N. Singh, Emmett E. Perl, Tony Lin,
Nikholas G. Toledo, Stacia Keller, Daniel J. Friedman, John E. Bowers, Shuji Nakamura, Steven P. DenBaars, James S. Speck, and Umesh Mishra
• Higher efficiency multijunction cells will require higher bandgap top junctions than current GaAs-based designs
• InxGa1-xN spans the entire solar spectrum
• Integrate InGaN-based cells with GaAs-based multijunction cells to enable efficient collection of high energy photons
Goal: Achieve >50% conversion efficiency with a hybrid InGaN-GaAs
multijunction cell design
Motivation
Bulk InGaN Solar Cells
0
20
40
60
80
100
350 375 400 425 450
EQ
E, A
bsor
ptio
n (%
)
Wavelength (nm)
Absorption
EQE rough
0
20
40
60
80
100
345 365 385 405
IQE
(%)
Wavelength (nm)
Recombination in p-GaN
Absorption in InGaN
#carriers collectedIQE# photons absorbed
=
Internal Quantum Efficiency 350 nm p-GaN
3 μm n-GaN
Sapphire
60 nm InGaN
>90% IQE for InGaN active region!
E. Matioli et al., Appl. Phys. Lett. 98, 021102 (2011).
0.0 0.2 0.4 0.6 0.8 1.00.00
0.04
0.08
0.12
0.16
0.20
855 oC 850 oC 845 oC
Curre
nt d
ensit
y (m
A/cm
2 )
Voltage (V)
TInGaN Voc (V) FF (%)
855 .98 63
850 .61 42
845 .38 32
320 340 360 380 400 420 440 4600
20
40
60
80
EQE
(%)
Wavelength (nm)
855 oC 850 oC 845 oC
LInGaN = 90 nm
High Indium Content Bulk InGaN Cells
EQE, Voc and FF degrade with high Xin due to strain, defect formation, and polarization
Polarization and Carrier Collection
320 340 360 380 400 420 4400
20
40
60
80
100
EQE
(%)
Wavelength (nm)
Total
Front
DepletionRegion
Band Diagram Structure Spectral response
Silic
on S
olar
Cel
l In
GaN
-bas
ed S
olar
Cel
l Drift-Based vs. Diffusion-Based Devices
S. M. Sze and K. K. Ng, Physics of Semiconductor Devices, 3rd ed. (Wiley, Hoboken, NJ, 2006).
Polarization in InGaN-Based Emitters
J.S. Speck et al., MRS Bull. 34, 304 (2009). E. F. Schubert, Light-Emitting Diodes, 2nd ed. (Cambridge University Press, Cambridge, 2006).
Unstrained Compressive
Strain Tensile Strain
• Polarization sheet charges “tilt” the energy bands in InGaN/GaN MQWs
• Reduction in radiative recombination efficiency
• Redshift in emission wavelength
Ec
Ev
c-plane (polar)
Ebi EP
- - -
++ ++
- - -
+ +
p-GaN InGaN n-GaN
ρ(x)
ε(x)
m-plane (nonpolar)
p-GaN InGaN n-GaN
Nd+
Na-
σp+
σp-
E(x)
Growth Direction [0001] Growth Direction [1010]
Net polarization charges opposite sign of depletion region fixed charges
Results in reduced or negative field in i-region
Junction voltage is dropped across p-GaN & n-GaN instead of i-region. Carrier collection is hindered!
No polarization charges
Junction voltage is dropped across i-region.
Field in i-region is in correct direction for carrier collection
Polarization sheet charges exist at heterointerfaces for polar orientations
Polarization in InGaN-Based Solar Cells
Ebi EP = 0
0 50 100 150 200-4
-2
0
2
4 Nd (cm-3) 1.0x1018
Ener
gy (e
V)
Distance from Surface (nm)
p-GaN Na=5x1019 cm-3 In0.2Ga0.8N n-GaN
Nd=0.1-2x1019 cm-3
ε(x)
Energy Band Diagram
Device Structure
Schematic Electric Field Profile
• Increasing doping in n-GaN: • Screens polarization charge • Reduces voltage drop on n side • Reduces electric field in InGaN
Light doping Field is reversed
Doping and Electric Fields
0 50 100 150 200-4
-2
0
2
4 Nd (cm-3)
1.4x1018
Ener
gy (e
V)
Distance from Surface (nm)
p-GaN Na=5x1019 cm-3 In0.2Ga0.8N n-GaN
Nd=0.1-2x1019 cm-3
ε(x)
Energy Band Diagram
Device Structure
Schematic Electric Field Profile
• Increasing doping in n-GaN: • Screens polarization charge • Reduces voltage drop on n side • Reduces electric field in InGaN
“Flat Band” no field in InGaN
Doping and Electric Fields
0 50 100 150 200-4
-2
0
2
4 Nd (cm-3)
2.0x1018
Ener
gy (e
V)
Distance from Surface (nm)
p-GaN Na=5x1019 cm-3 In0.2Ga0.8N n-GaN
Nd=0.1-2x1019 cm-3
ε(x)
Energy Band Diagram
Device Structure
Schematic Electric Field Profile
• Increasing doping in n-GaN: • Screens polarization charge • Reduces voltage drop on n side • Reduces electric field in InGaN
Field in InGaN in negative (correct) direction
Doping and Electric Fields
0 50 100 150 200-4
-2
0
2
4 Nd (cm-3)
4.0x1018
Ener
gy (e
V)
Distance from Surface (nm)
p-GaN Na=5x1019 cm-3 In0.2Ga0.8N n-GaN
Nd=0.1-2x1019 cm-3
ε(x)
Energy Band Diagram
Device Structure
Schematic Electric Field Profile
• Increasing doping in n-GaN: • Screens polarization charge • Reduces voltage drop on n side • Reduces electric field in InGaN
Increasing field
Doping and Electric Fields
0 50 100 150 200-4
-2
0
2
4 Nd (cm-3)
2.0x1019
Ener
gy (e
V)
Distance from Surface (nm)
p-GaN Na=5x1019 cm-3 In0.2Ga0.8N n-GaN
Nd=0.1-2x1019 cm-3
ε(x)
Energy Band Diagram
Device Structure
Schematic Electric Field Profile
• Increasing doping in n-GaN: • Screens polarization charge • Reduces voltage drop on n side • Reduces electric field in InGaN
Increasing field
Doping and Electric Fields
-6 -5 -4 -3 -2 -1 0 1 2 3-2.0
-1.5
-1.0
-0.5
0.0
0.5
Cur
rent
Den
sity
(mA/
cm2 )
Voltage (V)
Vk= -3.4 V
Bias-Dependent Carrier Collection
Dark
Illuminated
75 nm p-GaN
3 μm n-GaN
Sapphire
InGaN/GaN MQW 12 nm InGaN QWs 9 nm GaN barriers 10X
Reverse biasing the device recovers the photoresponse
C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).
-6 -5 -4 -3 -2 -1 0 1 2 3-2.0
-1.5
-1.0
-0.5
0.0
0.5
Cur
rent
Den
sity
(mA/
cm2 )
Voltage (V)
Vk= -3.4 V
300 320 340 360 380 400 420 4400
10
20
30
40
50
60
EQE
(%)
Wavelength (nm)
Bias-Dependent Carrier Collection
Dark
Illuminated
75 nm p-GaN
3 μm n-GaN
Sapphire
InGaN/GaN MQW 12 nm InGaN QWs 9 nm GaN barriers 10X
Reverse biasing the device recovers the photoresponse
C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).
-6 -5 -4 -3 -2 -1 0 1 2 3-2.0
-1.5
-1.0
-0.5
0.0
0.5
Cur
rent
Den
sity
(mA/
cm2 )
Voltage (V)
Dark
Illuminated
300 320 340 360 380 400 420 4400
10
20
30
40
50
60
EQE
(%)
Wavelength (nm)
300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V
EQE
(%)
Wavelength (nm)
Bias-Dependent Carrier Collection
75 nm p-GaN
3 μm n-GaN
Sapphire
InGaN/GaN MQW 12 nm InGaN QWs 9 nm GaN barriers 10X
Reverse biasing the device recovers the photoresponse
C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).
-6 -5 -4 -3 -2 -1 0 1 2 3-2.0
-1.5
-1.0
-0.5
0.0
0.5
Cur
rent
Den
sity
(mA/
cm2 )
Voltage (V)
Dark
Illuminated
300 320 340 360 380 400 420 4400
10
20
30
40
50
60
EQE
(%)
Wavelength (nm)
300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V
EQE
(%)
Wavelength (nm)
Bias-Dependent Carrier Collection
75 nm p-GaN
3 μm n-GaN
Sapphire
InGaN/GaN MQW 12 nm InGaN QWs 9 nm GaN barriers 10X
Reverse biasing the device recovers the photoresponse
C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).
-6 -5 -4 -3 -2 -1 0 1 2 3-2.0
-1.5
-1.0
-0.5
0.0
0.5
Cur
rent
Den
sity
(mA/
cm2 )
Voltage (V)
Dark
Illuminated
300 320 340 360 380 400 420 4400
10
20
30
40
50
60
EQE
(%)
Wavelength (nm)
300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V
EQE
(%)
Wavelength (nm)
Bias-Dependent Carrier Collection
75 nm p-GaN
3 μm n-GaN
Sapphire
InGaN/GaN MQW 12 nm InGaN QWs 9 nm GaN barriers 10X
Reverse biasing the device recovers the photoresponse
C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).
-6 -5 -4 -3 -2 -1 0 1 2 3-2.0
-1.5
-1.0
-0.5
0.0
0.5
Cur
rent
Den
sity
(mA/
cm2 )
Voltage (V)
Dark
Illuminated
300 320 340 360 380 400 420 4400
10
20
30
40
50
60
EQE
(%)
Wavelength (nm)
300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V -2.0 V
EQE
(%)
Wavelength (nm)
Bias-Dependent Carrier Collection
75 nm p-GaN
3 μm n-GaN
Sapphire
InGaN/GaN MQW 12 nm InGaN QWs 9 nm GaN barriers 10X
Reverse biasing the device recovers the photoresponse
C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).
-6 -5 -4 -3 -2 -1 0 1 2 3-2.0
-1.5
-1.0
-0.5
0.0
0.5
Cur
rent
Den
sity
(mA/
cm2 )
Voltage (V)
Dark
Illuminated
300 320 340 360 380 400 420 4400
10
20
30
40
50
60
EQE
(%)
Wavelength (nm)
300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V -2.0 V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V -2.0 V -2.5 V
EQE
(%)
Wavelength (nm)
Bias-Dependent Carrier Collection
75 nm p-GaN
3 μm n-GaN
Sapphire
InGaN/GaN MQW 12 nm InGaN QWs 9 nm GaN barriers 10X
Reverse biasing the device recovers the photoresponse
C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).
-6 -5 -4 -3 -2 -1 0 1 2 3-2.0
-1.5
-1.0
-0.5
0.0
0.5
Cur
rent
Den
sity
(mA/
cm2 )
Voltage (V)
Dark
Illuminated
300 320 340 360 380 400 420 4400
10
20
30
40
50
60
EQE
(%)
Wavelength (nm)
300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V -2.0 V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V -2.0 V -2.5 V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V -2.0 V -2.5 V -3.0 V
EQE
(%)
Wavelength (nm)
Bias-Dependent Carrier Collection
75 nm p-GaN
3 μm n-GaN
Sapphire
InGaN/GaN MQW 12 nm InGaN QWs 9 nm GaN barriers 10X
Reverse biasing the device recovers the photoresponse
C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).
-6 -5 -4 -3 -2 -1 0 1 2 3-2.0
-1.5
-1.0
-0.5
0.0
0.5
Cur
rent
Den
sity
(mA/
cm2 )
Voltage (V)
Dark
Illuminated
300 320 340 360 380 400 420 4400
10
20
30
40
50
60
EQE
(%)
Wavelength (nm)
300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V -2.0 V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V -2.0 V -2.5 V
EQE
(%)
Wavelength (nm)300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V -2.0 V -2.5 V -3.0 V
EQE
(%)
Wavelength (nm)
300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V -2.0 V -2.5 V -3.0 V -4.0 V
EQE
(%)
Wavelength (nm)
Bias-Dependent Carrier Collection
75 nm p-GaN
3 μm n-GaN
Sapphire
InGaN/GaN MQW 12 nm InGaN QWs 9 nm GaN barriers 10X
Reverse biasing the device recovers the photoresponse
C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).
300 320 340 360 380 400 420 4400
10
20
30
40
50
60 Bias Voltage -4.0 V -3.0 V -2.5 V -2.0 V -1.5 V -1.0 V -0.5V 0.0 V +0.5 V
EQE
(%)
Wavelength (nm)
Increasing Reverse Bias
40 80 120 160 200 240 280
-6
-4
-2
0
2
4
0 V -3V
Ener
gy (e
V)
Distance From Surface(nm)
-3 V
Bias-Dependent Carrier Collection
75 nm p-GaN
3 μm n-GaN
Sapphire
InGaN/GaN MQW 12 nm InGaN QWs 9 nm GaN barriers 10X
Reverse biasing the device recovers the photoresponse
C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).
• Increasing Si doping: • Reduces voltage
dropped on n-side • Shifts knee voltage to
positive voltages • Results in good device
performance: Voc = 1.9 V and FF = 74%
-5 -4 -3 -2 -1 0 1 2 3
-1.5
-1.0
-0.5
0.0Si Doping(1018 cm-3)
0.6 1.1 2.3 6.8
Curre
nt D
ensit
y (m
A/cm
2 )Voltage (V)
Effect of Doping on J-V Characteristics
High doping on both sides of the i-region is essential for screening polarization fields
Na = 5 x 1019 cm-3
C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).
InGaN/GaN MQW Solar Cells
i-InGaN
p-GaN
n-GaN
Substrate
p-GaN
n-GaN
i-InGaN i-GaN
i-GaN
Substrate
i-InGaN
i-InGaN p-GaN
n-GaN
InGaN/GaN MQW
Substrate
Bulk InGaN PIN Solar Cell
InGaN/GaN MQW Solar Cell
InGaN/GaN MDH Solar Cell
Thicker InGaN layers Thinner InGaN layers
Single thick InGaN/GaN DH
Break absorbing region Into discrete sections
tInGaN > 10 nm
Thinner wells for better stability at high XIn
tInGaN < 10 nm
Evolution of Active Region Design
• 3 key elements – High doping to screen
polarization sheet charges – Thin QWs to avoid InGaN
degradation (XIn ~ 0.28) – Roughened p-GaN to increase
optical path length
30X Smooth 30X Rough
RMS = 0.5 nm RMS = 75 nm
2.2 nm In0.28GaN QWs / 8 nm GaN barriers
Device Structure and Surface Morpholgy
R. M. Farrell et al., Appl. Phys. Lett. 98, 201107 (2011).
Structural Data
• Dotted vertical lines indicate that all samples have similar MQW period and average composition
• RSM from sample D shows that 30X In0.28GaN/GaN MQW is coherently strained
All samples exhibit excellent structural quality
R. M. Farrell et al., Appl. Phys. Lett. 98, 201107 (2011).
*Solid lines: EQE *Dotted lines: Absorption
Device Performance
No decrease in IQE with more QWs; Roughening increases EQE substantially
R. M. Farrell et al., Appl. Phys. Lett. 98, 201107 (2011).
InGaN-based cells should reduce operating temperature of underlying lower bandgap cells at all temperatures by simply absorbing high energy photons
Thermal Performance
Increase in efficiency of InGaN-based cells at elevated temperatures should help offset decrease in efficiency of underlying lower bandgap cells with temperature
30X In0.28Ga0.72N/GaN
C. J. Neufeld et al., Appl. Phys. Lett. 99, 071104 (2011).
Typical Si Solar Cell Temp Response
Radziemska et al., Renew. Energy 43, 1889 (2002).
InGaN-based cells should reduce operating temperature of underlying lower bandgap cells at all temperatures by simply absorbing high energy photons
Thermal Performance
Increase in efficiency of InGaN-based cells at elevated temperatures should help offset decrease in efficiency of underlying lower bandgap cells with temperature
C. J. Neufeld et al., Appl. Phys. Lett. 99, 071104 (2011).
