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

Motivation • 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

Bulk InGaN Solar Cells

Internal Quantum Efficiency 350 nm p-GaN

IQE =

60 nm InGaN 3 μm n-GaN

>90% IQE for InGaN active region!

Sapphire Absorption

80

100 80

EQE rough

60

IQE (%)

EQE, Absorption (%)

100

40

60

20

0

0

375

400

425

Wavelength (nm)

450

E. Matioli et al., Appl. Phys. Lett. 98, 021102 (2011).

Recombination in p-GaN

40

20 350

# carriers collected # photons absorbed

345

365

Absorption in InGaN

385

Wavelength (nm)

405

EQE (%)

80

855 oC 850 oC 845 oC

60 40 20

0 320 340 360 380 400 420 440 460

Wavelength (nm)

TInGaN

Voc (V)

FF (%)

855

.98

63

850

.61

42

845

.38

32

Current density (mA/cm2)

High Indium Content Bulk InGaN Cells 0.20 0.16

LInGaN = 90 nm

855 oC 850 oC 845 oC

0.12 0.08 0.04 0.00 0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

EQE, Voc and FF degrade with high Xin due to strain, defect formation, and polarization

Polarization and Carrier Collection

Drift-Based vs. Diffusion-Based Devices Band Diagram

Spectral response

100

EQE (%)

InGaN-based Solar Cell

Silicon Solar Cell

Structure

80 60 40 20 0 320

Depletion Region

Total

Front

340

360

380

400

420

Wavelength (nm) S. M. Sze and K. K. Ng, Physics of Semiconductor Devices, 3rd ed. (Wiley, Hoboken, NJ, 2006).

440

Polarization in InGaN-Based Emitters 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 J.S. Speck et al., MRS Bull. 34, 304 (2009). E. F. Schubert, Light-Emitting Diodes, 2nd ed. (Cambridge University Press, Cambridge, 2006).

Polarization in InGaN-Based Solar Cells Polarization sheet charges exist at heterointerfaces for polar orientations c-plane (polar)

m-plane (nonpolar)

Growth Direction [0001] p-GaN

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!

InGaN

σp+

ρ(x)

Na- -

n-GaN ++ ++

σp-

Growth Direction [1010] p-GaN

InGaN

Nd+ -

n-GaN + +

No polarization charges

Field in i-region is in correct direction for carrier collection

ε(x) Ec

Junction voltage is dropped across i-region.

E(x)

Ev

Ebi EP

Ebi EP = 0

Doping and Electric Fields • Increasing doping in n-GaN: • Screens polarization charge • Reduces voltage drop on n side • Reduces electric field in InGaN

Device Structure p-GaN Na

Energy Band Diagram Nd (cm )

Energy (eV)

1.0x1018

2

ε

(x)

0 -2 -4

0

50

100

150

Distance from Surface (nm)

200

In0.2Ga0.8N

n-GaN

Nd=0.1-2x1019 cm-3

Schematic Electric Field Profile

-3

4

=5x1019 cm-3

Light doping Field is reversed

Doping and Electric Fields • Increasing doping in n-GaN: • Screens polarization charge • Reduces voltage drop on n side • Reduces electric field in InGaN

Device Structure p-GaN Na

Energy Band Diagram Nd (cm )

“Flat Band” no field in InGaN

18

Energy (eV)

1.4x10

2

ε

(x)

0 -2 -4

0

50

100

150

Distance from Surface (nm)

200

In0.2Ga0.8N

n-GaN

Nd=0.1-2x1019 cm-3

Schematic Electric Field Profile

-3

4

=5x1019 cm-3

Doping and Electric Fields • Increasing doping in n-GaN: • Screens polarization charge • Reduces voltage drop on n side • Reduces electric field in InGaN

Device Structure p-GaN Na

Energy Band Diagram

Energy (eV)

In0.2Ga0.8N

Nd (cm ) 2.0x1018

2

ε

(x)

0 -2 -4

0

50

100

150

Distance from Surface (nm)

200

n-GaN

Nd=0.1-2x1019 cm-3

Schematic Electric Field Profile

-3

4

=5x1019 cm-3

Field in InGaN in negative (correct) direction

Doping and Electric Fields • Increasing doping in n-GaN: • Screens polarization charge • Reduces voltage drop on n side • Reduces electric field in InGaN

Device Structure p-GaN Na

Energy Band Diagram Nd (cm )

Energy (eV)

2

4.0x1018

ε

(x)

0 -2 -4

0

50

100

150

Distance from Surface (nm)

200

In0.2Ga0.8N

n-GaN

Nd=0.1-2x1019 cm-3

Schematic Electric Field Profile

-3

4

=5x1019 cm-3

Increasing field

Doping and Electric Fields • Increasing doping in n-GaN: • Screens polarization charge • Reduces voltage drop on n side • Reduces electric field in InGaN

Device Structure p-GaN Na

Energy Band Diagram Nd (cm )

Energy (eV)

2 2.0x1019

0

ε

(x)

-2 -4

0

50

100

150

Distance from Surface (nm)

200

In0.2Ga0.8N

n-GaN

Nd=0.1-2x1019 cm-3

Schematic Electric Field Profile

-3

4

=5x1019 cm-3

Increasing field

Bias-Dependent Carrier Collection 75 nm p-GaN

12 nm InGaN QWs 9 nm GaN barriers

InGaN/GaN MQW

10X

Reverse biasing the device recovers the photoresponse

3 μm n-GaN

Current Density (mA/cm2)

Sapphire 0.5 0.0

Dark

-0.5

Illuminated

-1.0 -1.5

Vk= -3.4 V

-2.0 -6 -5 -4 -3 -2 -1 0 Voltage (V)

1

2

3

C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).

Bias-Dependent Carrier Collection 75 nm p-GaN

12 nm InGaN QWs 9 nm GaN barriers

InGaN/GaN MQW

10X

Reverse biasing the device recovers the photoresponse

3 μm n-GaN

60

0.5 0.0

Dark

50

-0.5

EQE (%)

Current Density (mA/cm2)

Sapphire

Illuminated

-1.0 -1.5

30 20 10

Vk= -3.4 V

-2.0 -6 -5 -4 -3 -2 -1 0 Voltage (V)

40

1

2

3

C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).

0 300

320

340

360

380

400

Wavelength (nm)

420

440

Bias-Dependent Carrier Collection 75 nm p-GaN

12 nm InGaN QWs 9 nm GaN barriers

InGaN/GaN MQW

10X

Reverse biasing the device recovers the photoresponse

3 μm n-GaN

60

0.5 0.0

Dark

-0.5

Illuminated

-1.0 -1.5 -2.0 -6 -5 -4 -3 -2 -1 0 Voltage (V)

Bias Voltage 0.0 V -0.5V

50

EQE (%)

Current Density (mA/cm2)

Sapphire

40 30 20 10

1

2

3

C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).

0 300

320

340

360

380

400

Wavelength (nm)

420

440

Bias-Dependent Carrier Collection 75 nm p-GaN

12 nm InGaN QWs 9 nm GaN barriers

InGaN/GaN MQW

10X

Reverse biasing the device recovers the photoresponse

3 μm n-GaN

60

0.5 0.0

Dark

-0.5

Illuminated

-1.0 -1.5 -2.0 -6 -5 -4 -3 -2 -1 0 Voltage (V)

Bias Voltage 0.0 V -0.5V -1.0 V

50

EQE (%)

Current Density (mA/cm2)

Sapphire

40 30 20 10

1

2

3

C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).

0 300

320

340

360

380

400

Wavelength (nm)

420

440

Bias-Dependent Carrier Collection 75 nm p-GaN

12 nm InGaN QWs 9 nm GaN barriers

InGaN/GaN MQW

10X

Reverse biasing the device recovers the photoresponse

3 μm n-GaN

60

0.5 0.0

Dark

-0.5

Illuminated

-1.0 -1.5 -2.0 -6 -5 -4 -3 -2 -1 0 Voltage (V)

Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V

50

EQE (%)

Current Density (mA/cm2)

Sapphire

40 30 20 10

1

2

3

C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).

0 300

320

340

360

380

400

Wavelength (nm)

420

440

Bias-Dependent Carrier Collection 75 nm p-GaN

12 nm InGaN QWs 9 nm GaN barriers

InGaN/GaN MQW

10X

Reverse biasing the device recovers the photoresponse

3 μm n-GaN

60

0.5 0.0

Dark

-0.5

Illuminated

-1.0 -1.5 -2.0 -6 -5 -4 -3 -2 -1 0 Voltage (V)

Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V -2.0 V

50

EQE (%)

Current Density (mA/cm2)

Sapphire

40 30 20 10

1

2

3

C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).

0 300

320

340

360

380

400

Wavelength (nm)

420

440

Bias-Dependent Carrier Collection 75 nm p-GaN

12 nm InGaN QWs 9 nm GaN barriers

InGaN/GaN MQW

10X

Reverse biasing the device recovers the photoresponse

3 μm n-GaN

60

0.5 0.0

Dark

-0.5

Illuminated

-1.0 -1.5 -2.0 -6 -5 -4 -3 -2 -1 0 Voltage (V)

Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V -2.0 V -2.5 V

50

EQE (%)

Current Density (mA/cm2)

Sapphire

40 30 20 10

1

2

3

C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).

0 300

320

340

360

380

400

Wavelength (nm)

420

440

Bias-Dependent Carrier Collection 75 nm p-GaN

12 nm InGaN QWs 9 nm GaN barriers

InGaN/GaN MQW

10X

Reverse biasing the device recovers the photoresponse

3 μm n-GaN

60

0.5 0.0

Dark

-0.5

Illuminated

-1.0 -1.5 -2.0 -6 -5 -4 -3 -2 -1 0 Voltage (V)

Bias Voltage 0.0 V -0.5V -1.0 V -1.5 V -2.0 V -2.5 V -3.0 V

50

EQE (%)

Current Density (mA/cm2)

Sapphire

40 30 20 10

1

2

3

C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).

0 300

320

340

360

380

400

Wavelength (nm)

420

440

Bias-Dependent Carrier Collection 75 nm p-GaN

12 nm InGaN QWs 9 nm GaN barriers

InGaN/GaN MQW

10X

Reverse biasing the device recovers the photoresponse

3 μm n-GaN

60

0.5 0.0

Dark

-0.5

Illuminated

-1.0 -1.5 -2.0 -6 -5 -4 -3 -2 -1 0 Voltage (V)

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

50

EQE (%)

Current Density (mA/cm2)

Sapphire

40 30 20 10

1

2

3

C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).

0 300

320

340

360

380

400

Wavelength (nm)

420

440

Bias-Dependent Carrier Collection 75 nm p-GaN

12 nm InGaN QWs 9 nm GaN barriers

InGaN/GaN MQW

10X

Reverse biasing the device recovers the photoresponse

3 μm n-GaN

4

60

2

50

0

40

-3 V

-2 -4

EQE (%)

Energy (eV)

Sapphire

0V -3V

-6 40

80

120 160 200 240 280

Distance From Surface(nm)

C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).

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

Increasing Reverse Bias

30 20 10 0 300

320

340

360

380

400

Wavelength (nm)

420

440

Effect of Doping on J-V Characteristics • 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%

Current Density (mA/cm2)

Na = 5 x 1019 cm-3

0.0 -0.5 -1.0

Si Doping (1018 cm-3) 0.6 1.1 2.3 6.8

-1.5 -5 -4 -3 -2 -1 0

Voltage (V) High doping on both sides of the i-region is essential for screening polarization fields

C. J. Neufeld et al., Appl. Phys. Lett. 98, 243507 (2011).

1

2

3

InGaN/GaN MQW Solar Cells

Evolution of Active Region Design Bulk InGaN PIN Solar Cell p-GaN i-InGaN

n-GaN

InGaN/GaN MDH Solar Cell p-GaN i-InGaN i-GaN i-InGaN i-GaN i-InGaN

InGaN/GaN MQW Solar Cell p-GaN InGaN/GaN MQW

n-GaN

n-GaN

Substrate

Substrate

Substrate

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

Thicker InGaN layers

Thinner InGaN layers

Device Structure and Surface Morpholgy • 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

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).

Device Performance

*Solid lines: EQE *Dotted lines: Absorption

No decrease in IQE with more QWs; Roughening increases EQE substantially

R. M. Farrell et al., Appl. Phys. Lett. 98, 201107 (2011).

Thermal Performance

30X In0.28Ga0.72N/GaN

InGaN-based cells should reduce operating temperature of underlying lower bandgap cells at all temperatures by simply absorbing high energy photons 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).

Thermal Performance

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 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).