Strain-Balanced Quantum Well Solar Cells From Multi-Wafer Production Jessica Adams 33 rd IEEE Photovoltaic Specialists Conference 12 th 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 EaEa EgEg V

Motivation for SB-QWSC [1] J. Ward et al., Photovoltaic Specialists Conference,1993., Conference Record of the Twenty Third IEEE, pages , Cells designed to work under concentrator conditions Need smaller band-gap than GaAs to operate at efficiency peak GaAs In 0.1 GaAs 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 J SC Photons from radiative recombination loss reflected back through wells => photon recycling => increased V OC Efficiency increased ~1 % absolute Photon Recycling Quantum wells Distributed Bragg reflector (mirror) n p Contact AR coat i

Distributed Bragg Reflector (DBR)

Device Structures Growth: MOVPE 50 quantum wells Control + DBR substrates p-i-n diodes p,n GaAs In 0.11 Ga 0.89 As wells GaAs 0.9 P 0.1 barriers Devices taken from 2 positions on 2 wafers Run-1Run-2 Stepped p-region emitter Heavy window doping Devices taken from 5 positions across 3 wafers X1X1 Y1Y1 Ctrl DBR X2X2 Y 2B Ctrl DBR Y 2A

DBR Spatial Reflectivity Wavelength of maximum reflectivity varies from 924 nm to 904 nm

X1X1 X1X1 Y1Y1 Y1Y1 Experimental QE - Run-1 X1X1 X1X1 Y1Y1 Y1Y1

Y 2A Y 2B X2X2 X2X2 X2X2 X2X2 Y 2A 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 X 1 -edgeRun-1 Y 1 -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 X1X1 Y1Y1 AM1.5D x500 5% shading Run-1 X2X2 Y 2A Y 2B Run-2 Efficiency (%)

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