ingan-based solar cells for ultrahigh efficiency ... · am1.5d, 1000x, 300k am1.5d, 1000x, 300k...
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InGaN-Based Solar Cells for Ultrahigh Efficiency Multijunction Solar Cell Applications
Umesh K. Mishra, Robert M. Farrell, Jordan R. Lang, Yan-Ling Hu, Carl J.
Neufeld, Nathan G. Young, Michael Iza, Samantha C. Cruz, Emmett E. Perl,
Dobri Simeonov, Nihal Singh, Stacia Keller, Daniel J. Friedman, John E.
Bowers, Shuji Nakamura, Steven P. DenBaars, and James S. Speck
2
Celebrating Shuji Nakamura’s Nobel Prize in Physics for the invention of the Blue
and White InGaN LEDs using InGaN
3
InGaN Advantages for Solar
• High absorption coefficient – ~105 cm-1 , 1000 X silicon
• Drift-based collection
• Radiation Resistant
• Tunable band gap – Spans solar spectrum in theory
– Only wide band gap material for high efficiency CPV
Figure 1: The band gap energies of various materials relative to the AM1.5 solar spectrum [5]
G.F. Brown and J. Wu, Laser & Photonics Review 3, 394-405 (2009)
4
Multijunction Solar Cells
• Junctions connected optically and electrically in series
• Current matching = optimal bandgaps
• Used in space or in concentrating systems up to 1000 suns
• Efficiencies exceeding 43% with 4 junctions
• Lattice matched or lattice mismatched approaches
J. Photon. Energy. 2012;2(1):021805-1-021805-8. doi:10.1117/1.JPE.2.021805
Eg1
Eg2
Eg3
>
>
5
Calculate Efficiencies for Hybrid Devices Inverted Metamorphic Multijunction (IMM) Concept
Ga0.45In0.55As 0.7 eV [3.9%]
Ga0.50In0.50P 1.82 eV [0%]
GaAs 1.40 eV [0%]
Ga0.70In0.30As 1.0 eV [2.1%]
Grade #1
Grade #2
Sunlight
Lattice-matched GaAs subcell
Lattice-mismatched Ga0.45In0.55As subcell
Lattice-matched Ga0.50In0.50P subcell
Lattice-mismatched Ga0.70In0.30As subcell
Grade #1
Grade #2
1.82 eV
1.40 eV
1.0 eV
0.7 eV
No grade needed
• Top junction(s) are lattice-matched and bottom junction(s) are lattice-mismatched
• GaAs substrate is removed to expose “top” Ga0.50In0.50P junction to incident sunlight
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Multijunction Integration
E.F. Schubert, Light Emitting Diodes, 2nd ed. (Cambridge University Press, 2006)
4-junction IMM design
Feasible band gap range for InxGa1-xN, down to 2.5eV today; 2.0 eV tomorrow
Courtesy of R.M. Farrell
• GaN’s wide band gap needed for more than 4 junctions
• Lattice mismatch requires bonded interface
7
Proposed Hybrid InGaN-GaAs Device Design
*3 terminal designs are also possible*
• 1J electrically-isolated InGaN-based cell (2.65 eV)
• 4J inverted metamorphic multijunction (IMM) GaAs-based cell
• Broadband top side antireflection (AR) coating
• Low-loss, low-dispersion bonding interlayer
• Aligned grid contacts
• Optical coatings on top and bottom side of bonding interlayer
Theoretical efficiency > 60% for technologically feasible 1J +4J design
Courtesy of R.M. Farrell
8
Calculated Efficiencies for Hybrid InGaN-GaAs Devices
InGaN/GaN MQW (2.65 eV)
Ga0.50In0.50P
(1.83 eV)
Little difference between ideal hybrid 1J + 4J cell design and
technologically feasible hybrid 1J + 4J cell design
1J InGaN-based + 3J GaAs-based 1J InGaN-based + 4J GaAs-based
AM1.5D, 1000X, 300K AM1.5D, 1000X, 300K
Green shaded region – Technologically feasible bandgaps Orange circles – Efficiency maxima for ideal bandgaps Blue lines – Bandgaps for proposed hybrid InGaN-GaAs design
Courtesy of R.M. Farrell
9
InGaN Challenges
• On (0001), 23% In needed for 2.65 eV
• Lattice mismatch, strain – Critical thickness << 10 nm
– Piezoelectric polarization field
• V-defect formation – Diameter increases with growth thickness
– Traps carriers and acts as recombination centers
– Leads to well disorder
X. Wu, C. Elsass, and A. Abare, Applied Physics Letters 72, 692-694 (1998).
10
Evolution of Active Region Design
i-InGaN
p-GaN
n-GaN
Substrate
p-GaN
n-GaN
InGaN/GaN
MQW
Substrate
Bulk InGaN PIN
Solar Cell
InGaN/GaN MQW
Solar Cell
Single thick
InGaN/GaN DH Thinner wells with
tInGaN < 10 nm
MQW design improves stability of InGaN material and prevents relaxation-related defect formation
• Must increase XIn to increase available optical power
• While keeping total InGaN thickness high enough to absorb most incident photons
Challenges
11
Proposed Hybrid InGaN-GaAs Device Design
*3 terminal designs are also possible*
• 1J electrically-isolated InGaN-based cell (2.65 eV)
• 4J inverted metamorphic multijunction (IMM) GaAs-based cell
• Broadband top side antireflection (AR) coating
• Low-loss, low-dispersion bonding interlayer
• Aligned grid contacts
• Optical coatings on top and bottom side of bonding interlayer
Theoretical efficiency > 60% for technologically feasible 1J +4J design
Courtesy of R.M. Farrell
12
2-Step GaN Barrier Growth (on Sapphire)
• Low temperature cap layer to
protect well integrity
– Optimized at 2 nm, 800 °C (well
temp)
• High temperature barrier grown
in H2 to mitigate V-defect
propagation
– Optimized at 2 nm, 900 °C
Growth Optimization
Filled V-pits, Ordered Wells
InGaN QW (2.5nm)
GaN Cap (2nm)
GaN Barrier (2nm)
N2 carrier gas, same temperature as QWs
H2 carrier gas, higher temperature than QWs
Y.-L. Hu, et al. Applied Physics Letters 100, 161101 (2012).
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0 1 2-1.5
-1.0
-0.5
0.0
Curr
ent
density (
mA
/cm
2)
Voltage (V)
Optimizing Cap Thickness
• Goal: find thinnest cap without QW damage
• 1.5 nm cap results in degraded performance
– Severe thickness fluctuations
– Wells exposed to indium desorption
• > 1.88 nm caps look good within normal variation
• Decided that 2 nm cap should be safe
300 350 400 4500
5
10
15
20
25
30
35 1.5nm cap
1.88nm cap
2.25nm cap
3.0nm cap
EQ
E (
%)
Wavelength (nm)
Y.-L. Hu, et al. Applied Physics Letters 100, 161101 (2012).
14
Optimizing Barrier Thickness
• Fixed cap thickness at optimal 2 nm
• Goal: thinnest possible barrier without device degradation
• Degrading Jsc below 4 nm barriers
• Probable cause: V-defect formation not suppressed
0 1 2
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0 2.5nm
3.2nm
3.5nm
3.9nm
4.4nm
Cu
rre
nt
de
nsity (
mA
/cm
2)
Voltage (V)
2.0 2.5 3.0 3.5 4.0 4.5 5.00.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
Jsc (
mA
/cm
2)
LBarrier
(nm)
V-defects
15
Active Region Trade-offs
• Active region must be fully depleted to maintain field
• Less total active region thickness with thinner barriers
• Total InGaN thickness of 100 nm needed for 1 absorption length with second pass (αInGaN = 5e4 cm-1)
• Wells must be < ~2.5 nm
• At least 50 periods needed for full absorption
10X
50X
N
P i
N
P
i
depleted undepleted
400 410 420 430 440 4500
2x104
4x104
6x104
8x104
Alp
ha
(cm
-1)
Wavelength (nm)
0
200
400
600
800
1000
Abso
rption L
ength
(nm
)
UID of 5e16 cm-3
16
Carrier Collection Sequence
Escape
Transit
Recapture
Recombination
hν
Generation
hν
17
Electric Field Effect on Transport
J.R. Lang, N.G. Young, R.M. Farrell, Y.-R. Wu, and J.S. Speck, Applied Physics Letters 101, 181105 (2012).
18
Temperature Dependent J-V
LB decreases TE to T escape FB increases FF increases
T
LB = 3.8nm
T
TE
T
LB = 9.7nm
TE
Tunneling vs. Thermionic Emission
• Thick Barriers: TE dominates
– Inefficient except at high temperatures
• Thin Barriers: Tunneling dominates efficiently
• Tunneling escape preferred
• Field-assisted tunneling
– Thinning of effective barrier
– Thinner barriers lead to higher field
J.R. Lang, N.G. Young, R.M. Farrell, Y.-R. Wu, and J.S. Speck, Applied Physics Letters 101, 181105 (2012).
19
Optimizing Barrier Temperature
• Hotter GaN barrier growth in H2 will suppress V-defect formation
• Too high temperature will degrade well quality
• Second hump seen in PL spectra
Tb (C) Vocavg
(V) FFavg
(%) Jscavg
(mA/cm2)
875 2.24 69.0 0.91
900 2.25 70.0 0.97
925 2.20 69.0 0.95
950 2.16 68.9 0.95
0 1 2-1.5
-1.0
-0.5
0.0
Cu
rren
t de
nsity (
mA
/cm
2)
Bias (V)
450 5000
1x104
2x104
3x104
4x104
5x104
6x104
7x104
BI: Tb = 875C
DI: Tb = 900C
FI: Tb = 925C
CI: Tb = 950C
EI: Tb = 975C
Inte
nsity (
coun
ts)
Wavelength (nm)
20
The GaN eco-system for energy conversion
• Output power 4.5kw (Single Phase
200V)
• Input voltage 60-400V
• Maximum Power Efficiency > 98% (vs.
>96.5% with Silicon)
• Volume about 10L <18L (existing
Silicon based)
>40% loss reduction
GaN modules allowed for kW class PV power conditioner with 40% smaller size and loss
40% volume reduction Courtesy: Testing done and published by Yaskawa Electric.
21
22
Penetration into price-sensitive markets
Transphorm Partners With Tata Power Solar to Introduce India’s Most Efficient Solar Inverter
Partnership provides next generation solar conversion solution, enabled by high performance
Gallium Nitride (GaN) power conversion products
Goleta, CA – August 26, 2014– Transphorm Inc. announced today that it is partnering with Tata Power Solar,
to introduce India’s most efficient solar inverter using Transphorm’s patented EZ-GaNTM technology. This
collaboration enhances Tata Power Solar’s position as a solar solutions and technology leader, by launching
inverters based on Transphorm’s unique GaN-based power switching platform.
Currently widely used in LEDs, GaN is the next generation in power electronics. The GaN transistor combines
low switching and conduction losses, offering reduced energy loss of more than 50 percent compared to
conventional silicon-based power conversion designs. Transphorm has established the industry’s first and
only qualified 600V GaN device platform with its TPH Series portfolio of GaN products, backed by its world-
leading GaN power IP portfolio.
Under the partnership, Transphorm will supply GaN transistors, while Tata Power Solar will locally
manufacture and market the GaN-powered solar inverters. The first PV Inverter product is scheduled to be
released in early 2015.
“We are pleased to partner with Transphorm to develop indigenous world-leading solar conversion,” said
Mr. Ajay Goel, CEO, Tata Power Solar. “Our intent is to lead in green energy. The inverter technology being
developed has broad applications beyond solar conversion and we anticipate these energy efficient
applications will find usage across various Tata companies.”
“By designing our solar inverter product family with Transphorm’s industry-leading and qualified EZ-GaNTM
platform, Tata Power Solar will provide the Indian energy sector with a compact and higher efficiency PV
Inverter as well as a roadmap of higher performance and smaller form factor solar PV power,” said Dr. Arul
Shanmugasundram, EVP Projects and CTO for Tata Power Solar. “This world-leading product family will
accelerate India’s adoption of solar energy, enabling the goal of using renewables to power 20 percent of
India’s energy needs by 2020.”
“Tata’s partnership with Transphorm is a testament to continued validation of Transphorm’s undisputed
leadership in addressing global power conversion needs,” said Dr. Umesh Mishra, Chairman, Transphorm
Inc. “We look forward to working with Tata to bring energy savings directly to enterprises and consumers.”
Voltage Accelerated Tests for 600V Rated GaN HEMTs
• Stressed at 1150V and 1100V to obtain
fails
• Obtain acceleration factor based on 2
groups of stresses
• Plotted at 600V
• inverse power law model used
• Standard voltage acceleration models give
range of projected mean lifetimes
• Most common models 1e8 ~1e10hr
• Most conservative model 1e7hrs
(Reciprocal Voltage TDDB model)
• In progress with 3rd stress level for model
determination
Even the most conservative model predicts
600V operation lifetime > 107hrs
Possible Failure Model Projected Lifetime
50 Mean failure time line
90% confidence limit
IPL model @ 600V
23
Step stress to 1800Volts !
SPC charts for over 90 device variables in place (additional for fab processes, epi and packaged tests) E.g. GaN/Silicon Production Dynamic Ron (Final Test)
Measure of the increase on Ron between static and dynamic measurements.
Increase in Ron generally referred to as current collapse At one time this was considered a difficult spec to achieve. The last 300 wafers have a Cpk =1
24
600 lots over more than two year time span Continuous improvement programs has put current collapse under control
GaN can dominate the new 600V power market which is here today and can also capture 1200V slots in the near future
25
Source: LUX Research market report for WBG for power and Transphorm’s own estimates for GaN portion and SiC portion of the WBG market
0
200,000,000
400,000,000
600,000,000
800,000,000
1,000,000,000
1,200,000,000
1,400,000,000
1,600,000,000
2015 2016 2017 2018 2019
GaN Market Estimate
GaN Market Estimate
GaN
sal
es (
60
0V
/12
00V
), $
Must Haves
GaN
Silicon
Form Factor
Volume Expansion
Efficiency
Proliferation in new
Markets
Industrial systems impact
Complete Adoption
Cost and Maturity
26
Challenges for InGaN Solar cells
27
Advantages of Bulk (0001) Substrates
• Two orders of magnitude lower threading dislocation density (TDD)
– < 3e6 cm-2 for Furukawa bulk GaN
– > 1e8 cm-2 for GaN on sapphire
• TDs are recombination centers and leakage paths
– Increase dark current and decrease Voc
• V-defects nucleate on TDs
– Lower TDD = fewer V-defects
50X: sapphire 50X: bulk
V-defects Smooth Surface
CL image of Furukawa bulk (0001) GaN substrate Courtesy of Erin Young
10 μm
28
Structural Investigation of 50X MQW Samples
a) On bulk c-plane; good integrity
b) On sapphire; well width fluctuation
c) On bulk c-plane; New TD generated partway through the stack – could explain decline in device performance
d) Close up on TD nucleation shows thinning and bending wells
N. G. Young, et al. Appl. Phys. Lett. 103, 173903 (2013).
29
Sapphire vs. Bulk Solar Cell Performance
Bulk substrates:
Improvement in Voc, Jsc, FF, EQE, Power
Increased improvement for more wells
300 350 400 4500
10
20
30
40
50
60 10X Bulk
10X Sapphire
30X Bulk
30X Sapphire
50X Bulk
50X Sapphire
EQ
E (
%)
Wavelength (nm)
0 1 2-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Cu
rre
nt
de
nsity (
mA
/cm
2)
Bias (V)
Co-loaded bulk and sapphire samples
Record performance for InGaN solar cells: For > 450 nm abs. edge Voc > 2.25V (<0.47V effective bandgap offset) Fill factors up to 80%
30
“Effective” Absorption Edge
Absorption tail due to
polarization fields
effabs
1240 nm eVeff
eff
g
abs
E
: "Effective" band gapeffgE
: "Effective" absorption edgeeffabs
Need to move absorption corner to 500 nm