Transcript Document

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?
2”
Research wafers
Industrial wafers
4”
Overview
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
Strain-Balanced Quantum Well Solar Cell (I)
Eg
V
Ea
p
i
• Wells inserted in i-region
of p-i-n
• Extends absorption
energy range to below
that of bulk
n
Motivation for SB-QWSC
In0.1GaAs
GaAs
1000 Suns
•Cells designed to work under concentrator conditions
•Need smaller band-gap than GaAs to operate at efficiency peak
[1] J. Ward et al., Photovoltaic Specialists Conference,1993., Conference Record of the Twenty Third IEEE, pages 650-654, 1993.
Strain-Balanced Quantum Well Solar Cell (II)
GaAsP
(barriers)
InGaAs
(wells)
GaAs
(bulk)
> 65 wells without misfit dislocations
Photon Recycling
AR coat
Contact
p
i
n
Quantum wells
Distributed Bragg
reflector (mirror)
• 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
Distributed Bragg Reflector (DBR)
Device Structures
Growth: MOVPE
50 quantum wells
Control + DBR substrates
Run-1
• Devices taken from 2
positions on 2 wafers
X1
Ctrl
p-i-n diodes
•
•
•
p,n GaAs
In0.11Ga0.89As wells
GaAs0.9P0.1 barriers
Run-2
• Stepped p-region emitter
• Heavy window doping
• Devices taken from 5
positions across 3 wafers
X2
Ctrl
Y1
DBR
Y2B
DBR
Y2A
DBR
DBR Spatial Reflectivity
Wavelength of maximum reflectivity varies from 924 nm to 904 nm
Experimental QE - Run-1
X1
X1
X1
X1
Y1
Y1
Y1
Y1
Experimental QE - Run-2
Y2A
Y2A
Y2A
Y2B
Y2A
X2
Y2B
X2
X2
X2
Modeling - SOL (I)
Fit QE to experimental data using parameters from literature
Carrier transport
Shockley injection current
Quantum well absorption
Radiative current
Carrier distributions
SRH current in terms of single non-radiative carrier lifetime
1 parameter fit to dark current!
[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 devices
Evidence of reduced surface recombination current
Predicted Efficiencies
28.0
Run-2
Run-1
X2
Efficiency (%)
X1
27.5
Y2A
27.0
26.5
26.0
Y1
Y2B
25.5
AM1.5D
x500
5% shading
Summary
• 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
Acknowledgments
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
http://www.sc.ic.ac.uk/~q_pv