InGaN-Based Solar Cells for Ultrahigh Efficiency ...ucsolar.org/files/public/documents/12-9-11...

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).