1

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

6

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).

13

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