Nanoscale Energy Conversion in the Quantum Well Solar Cell

1
Nanoscale Energy Conversion in the Quantum Well
Solar Cell

• Keith Barnham, Ian Ballard, Amanda Chatten, Dan
  Farrell,
• Markus Fuhrer, Andreas Ioannides, David Johnson,
  
• Marianne Lynch, Massimo Mazzer, Tom Tibbits
• Experimental Solid State Physics, Imperial
  College London, London SW7 2BW, UK
• k.barnham_at_ic.ac.uk http//www.sc.ic.ac.uk
  /q_pv
• Rob Airey, Geoff Hill, John Roberts, Cath
  Calder,
• EPSRC National Centre for III-V Technology,
  Sheffield S1 3JD, UK
• Solarstructure , Permasteelisa, FULLSPECTRUM EU
  Framework VI,

2
Outline

• First practical nanoscale photovoltaic cell
• Enhanced spectral range of the strain-balanced
  quantum well solar cell (SB-QWSC)
• Efficiency enhancement by photon recycling
• Evidence for hot electron effects in the QW

3
Cell efficiency cell versus l or Eg

• GaAs cells - highest effic. single junction
  cells, Eg too high
• lower Eg gt higher efficiency
• Can grow InyGa1-yAs bulk cells on virtual
  substrates but never dislocation free
• Maximum at 1.1 mm 1.1 eV
• Multi-junction cells need 4th band-gap 1.1 mm
  1.1 eV

4
Enhancing GaAs Cell Efficiency

• From 30x 1000x AM1.5 optimum single junction
  efficiency band-gap 1.1 eV
• Multi-junction approaches going for GaInNAs cell
  
• No ternary alloy with lower Eg than GaAs lattice
  matched to GaAs/Ge

GaAs1-yPy (y 0.1) InxGa1-xAs, (x 0.1
0.2) strain-balanced to GaAs/Ge gt novel PV
material
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GaAsP/InGaAs Strain-Balanced QWSC
Balance stress between layers to match lattice
parameter of the substrate

• Advantages
• Can vary absorption band- edge and absorb
  wider spectral range without strain-relaxation
• no dislocations gt 65 wells
• single junction with wide spectral range
• ability to vary Eg gives higher tandem effic.

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SB-QWSC Ideal Dark-Currents at High
Concentration

• Dark current of 50 well QWSC
• Low current fits one parameter Shockley-Read-Hall
  model
• High (concentrator) current slope changes
• ideal Shockley current
• radiative recombination in QW
• Minimum recombination radiative at concentrator
  current levels

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Investigation of Photon Cavity Effects

• 50 well SB- QWSC
• In0.1Ga0.9As wells
• GaAs0.91P0.09 barriers
• Control and distributed
• Bragg reflector (DBR)
• devices grown
• side-by-side

• Processed as concentrator, fully metalised, and
  photodiode devices
• 11 finger concentrator mask, 3.6 shading

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Distributed Bragg Reflectors

• Increase photon
• absorption
• Increase photocurrent
• No series resistance
• In-situ growth

3 D.C. Johnson et al. Solar Energy Materials
and Solar Cells, 2005
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Concentrator Measurements

• 27 efficiency at 328x low-AOD spectrum
• Single junction record is (27.6 /-1) at 255x

• D.Johnson et al. WCPEC4, Hawaii May 06

• Efficiency increase higher than expect from
  double pass in QWs
• Enhanced Voc

3 Vernon S.M., et al. High-efficiency
concentrator cells from GaAs on Si, 22nd IEEE
PVSC 1991 pp5335
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Why the Efficiency Enhancement?

• Aim of DBR was to absorb
• photons on second pass
• Some photons from radiative
• recombination at high bias
• trapped in the device

MQW
DBR

• Photons reabsorbed in the QWs reduce dark current
• Generalised Plank model for EL shows reduction
  consistent with dark current suppression
• Photon recycling could take cell to 30 efficiency

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Single QW Electroluminescence low bias
Bulk
Well
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Single QW EL at high bias
Well
Bulk
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10 QW Electroluminescence low bias
Well
Bulk
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10 QW EL at high bias
Well
Bulk
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Model EL (radiative recombination)

• Detailed Balance leads to generalised Planck1
• a(E) (use measured QE) and T determine shape
• DEF requires absolute calibration

J.Nelson et al., J.Appl.Phys., 82, 6240,
(1997) M.Fuhrer et at Proc. EU PVSEC Dresden,Sept
06
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EL - model and experiment
data
model
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EL - Bulk Peak
Fits T 299 K
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Conclusions

• SB-QWSC concentrator cells (near) highest
  efficiency and widest spectral range of single
  junction cells
• Radiative recombination dominates at high
  current levels and photon recycling observed with
  DBR
• EL reduction with DBR consistent with
  dark-current
• Evidence for hot carrier effects at high current
  levels in EL shape consistent with generalised
  Planck
• These nanoscale properties occur at the high
  current levels to be expected in terrestrial
  concentrator systems

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Advantages of the SB-QWSC

• Approximately double the efficiency of current
  cells
• Widest spectral range in a single junction cell
  so keeps high efficiency as sunlight spectrum
  varies
• Nano-scale effectss photon cavity, hot
  electrons
• Small size mm optoelectronic fabrication.
• Need high concentration to bring price down
• What application?
• Building integrated concentrator photovoltaics
  (BICPV)

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Novel Application - Building Integrated
Concentrators
SB-QWSC - highest efficiency single junction
cell, 1mm size
UK over 60 electricity used in buildings over
7 x as much solar energy falls on those
buildings

• SMART WINDOWS
• No transmission of direct sunlight
• Reduce glare and a/c requirement
• Max diffuse sunlight - for illumination
• No need for lights when blinds working
  
• (2 3) x power from Silicon BIPV
• Electricity at time of peak demand
• Cell cooling in frame - hot water
• Barnham, Mazzer, Clive, Nature Materials, 5,
  161 (2006).

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Calculated output San Francisco
Average electricity generated by 1 m2 of façade
over 1 year
Savings
Consumption 145 kWh/m2
Fraction of electricity consumption provided by
photovoltaic cells
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Luminescent Concentrators for Diffuse Component
of Sunlight

• Dye-doped luminescent concentrators (1977)
• Advantages
• no tracking required
• accept diffuse sunlight
• stacks absorb different l
• Eg Eg, gives max. effic.
• thermalisation in sheet
• Disadvantages
• dyes degrade in sunlight
• loss from overlap of
  absorption/luminescence
• narrow absorption band

A Goetzberger and W Greubel, Appl. Phys. 14,
1977, p123.
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Quantum Dot Concentrator

• QDs replace dyes in luminescent concentrators
• QDs degrade less in sunlight
• core/shell dots high QE
• absorption edge tuned by dot size
• absorption continuous to short l
• red-shift tuned by spread in dot size
• spread fixed by growth conditions

(K.Barnham et al. App. Phys.Lett.,75,4195,(2000))
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• Thermodynamic Model for QDC
• The brightness, B(n), of a radiation field that
  is in equilibrium with electronic degrees of
  freedom of the absorbing species
• Applying the principle of detailed balance
  within the slab
• IC concentrated radiation field, Qe
  quantum efficiency,
• se absorption cross section
• Extend to 3-D fluxes boundary conditions

n refractive index b 1/kT m chemical
potential
A.J.Chatten et al, 3rd WCPEC, Osaka, 2003 E
Yablonovitch, J. Opt. Soc. Am. 70, 1362, 1980.
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Characterisation of ZnS/CdSe QDs in Acrylic with
Thermodynamic Model
SD387 Red
SD396 yellow

• Thermodynamic model fits PL shape and red-shift
  
• of Nanoco QDs assuming only absorption cross
  section
• Fitting current measured at cell on edge gives
• Qe(SD387) 0.56 (c.f. Nanoco 0.4 0.6)

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Thermodynamic Model confirms unexpected
luminescent stack result
Incident light
Total output 45.3 (mA/m2)
Incident light
Total output 52.3 (mA/m2)
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EL Modeling Confirms Recycling

• 50 QW dark current show 33 reduction of J01
• Model EL by detailed balance 30 reduction
• Supports efficiency increase results from
• photon recycling

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Compare SB-QWSC with Tandem in Smart Windows
London Vertical South - East Facing Wall A
tandem cell 13 more efficient than a SB-QWSC
harvests only 3 more electrical energy
Series current constraint means tandem optimised
for one spectral condition (and one temperature)

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Single Molecule Precursor ZnS/CdSe Core-Shell QDs

• Core shell ZnS/CdSe dots by thermolysis at 270 C
  of single-molecule precursors
• in PLMA using with TOPO cap
• Luminescence fit
• is two-flux thermodynamic model.
• Currently part of FULLSPECTRUM
• Framework VI Integrated Project

(T.Trindade et al. Chemistry of Materials, 9,
523, (1997)) (A.J.Chatten et al, Proc. 3rd
WCPEC, Osaka, 2003)
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BICPV Smart Windows

• Transparent Fresnel Lenses
• (300 500)x concentration
• 1.5 or 2-axis tracking
• Novel secondaries
• 1 mm solar cells
• Cell efficiency 30
• Adds 20 to façade cost