Strain-Balanced Quantum Well Solar Cells From Multi-Wafer Production

Jessica Adams

33rd IEEE Photovoltaic Specialists Conference

12th May 2008

Can we manufacture the strain-balanced quantum well solar cell on a multi-wafer

production run?

Research wafers Industrial wafers

2”

4”

1. Introduction

– Quantum well solar cell– Strain-balancing– Photon recycling

2. Details of devices

3. Experimental results– Spatial reflectivity– Quantum efficiency

4. Modeling results– Dark current suppression– Predicted efficiencies

5. Summary

Overview

Strain-Balanced Quantum Well Solar Cell (I)

• Wells inserted in i-region of p-i-n

• Extends absorption energy range to below that of bulk

p i n

Ea

Eg

V

Motivation for SB-QWSC

[1] J. Ward et al., Photovoltaic Specialists Conference,1993., Conference Record of the Twenty Third IEEE, pages 650-654, 1993.

•Cells designed to work under concentrator conditions

•Need smaller band-gap than GaAs to operate at efficiency peak

GaAs

In0.1GaAs

1000 Suns

GaAsP(barriers)

InGaAs

(wells)

GaAs(bulk)

> 65 wells without misfit dislocations

Strain-Balanced Quantum Well Solar Cell (II)

• Photons not absorbed on first pass reflected => increased JSC

• Photons from radiative recombination loss reflected back through wells => photon recycling => increased VOC

• Efficiency increased ~1 % absolute

Photon Recycling

Quantum wells

Distributed Bragg reflector (mirror)

n

p

ContactAR coat

i

Distributed Bragg Reflector (DBR)

Device Structures

Growth: MOVPE

50 quantum wells

Control + DBR substrates

p-i-n diodes• p,n GaAs• In0.11Ga0.89As wells• GaAs0.9P0.1 barriers

• Devices taken from 2 positions on 2 wafers

Run-1 Run-2• Stepped p-region emitter• Heavy window doping• Devices taken from 5

positions across 3 wafersX1

Y1

Ctrl

DBR

X2

Y2B

Ctrl

DBR

DBR

Y2A

DBR Spatial Reflectivity

Wavelength of maximum reflectivity varies from 924 nm to 904 nm

X1

X1

Y1

Y1

Experimental QE - Run-1

X1

X1

Y1

Y1

Y2A

Y2A

Y2B

X2

X2

X2

X2

Y2B

Y2A

Y2A

Experimental QE - Run-2

Carrier transport

Quantum well absorption

Carrier distributions

Modeling - SOL (I)

Fit QE to experimental data using parameters from literature

1 parameter fit to dark current!

Shockley injection current

Radiative current

SRH current in terms of single non-radiative carrier lifetime

[2] J. Connolly, et al., Proc. 19th European Photovoltaic SolarEnergy Conference, Paris, 2004.

Modeling - SOL (II)

Run-1 X1 -edge Run-1 Y1 -edge

Reduced radiative dark current in all of the DBR devices investigated Evidence of photon recycling

Reduced Shockley injection current in stepped emitter devicesEvidence of reduced surface recombination current

Predicted Efficiencies

X1

Y1

AM1.5D x500

5% shading

Run-1

X2

Y2A

Y2B

Run-2

Effi

cie

ncy

(%

)

25.5

28.0

27.5

26.0

27.0

26.5

• Investigated SB-QWSCs from 2 multi-wafer production runs

• Found suppressed radiative recombination in devices with DBRs– Photon recycling– Improved efficiency

• Investigated impact of stepped emitter– Reduced surface recombination– Improved efficiency

• Found that similar efficiencies can be produced from across the wafers– Results hold for both control and DBR substrates– Multi-wafer manufacture potentially viable

Summary

Keith Barnham, James Connolly and the QPV group at Imperial College London

J.S. Roberts and G. Hill at the EPSRC National Centre for III-V Technologies

T. Tibbits of QuantaSol Ltd.

M. Geen of IQE Europe

M. Pate of the Centre for Integrated Photonics

Acknowledgments

http://www.sc.ic.ac.uk/~q_pv