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 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,

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

Cell efficiency cell versus or E g lGaAs cells - highest effic. single junction cells, E g too high llower E g => higher efficiency lCan grow In y Ga 1-y As bulk cells on virtual substrates but never dislocation free Maximum at 1.1  m ~ 1.1 eV Multi-junction cells need 4th band-gap ~ 1.1  m ~ 1.1 eV

Enhancing GaAs Cell Efficiency l From 30x – 1000x AM1.5 optimum single junction efficiency band-gap ~ 1.1 eV l Multi-junction approaches going for GaInNAs cell l No ternary alloy with lower E g than GaAs lattice matched to GaAs/Ge GaAs 1-y P y (y ~ 0.1) + In x Ga 1-x As, (x~ 0.1 – 0.2) strain-balanced to GaAs/Ge => novel PV material

GaAsP/InGaAs Strain-Balanced QWSC Advantages: Can vary absorption band- edge and absorb wider spectral range without strain-relaxation no dislocations > 65 wells single junction with wide spectral range ability to vary E g gives higher tandem effic. Balance stress between layers to match lattice parameter of the substrate

SB-QWSC – Ideal Dark-Currents at High Concentration lDark current of 50 well QWSC lLow current fits one parameter Shockley-Read-Hall model lHigh (concentrator) current slope changes ideal Shockley current + radiative recombination in QW l Minimum recombination radiative at concentrator current levels

Investigation of Photon Cavity Effects 50 well SB- QWSC In 0.1 Ga 0.9 As wells GaAs 0.91 P 0.09 barriers Control and distributed Bragg reflector (DBR) devices grown side-by-side Ta 2 O 5 / SiN X Processed as concentrator, fully metalised, and photodiode devices 11 finger concentrator mask, 3.6% shading Ta 2 O 5 / SiN X

Increase photon absorption Increase photocurrent No series resistance In-situ growth Distributed Bragg Reflectors [3] D.C. Johnson et al. Solar Energy Materials and Solar Cells, 2005 J SC (mA/cm 2 ) DeviceAM1.5d 1000W/m 2 AOD 913W/m 2 Non- DBR DBR

Concentrator Measurements 27% efficiency at 328x low-AOD spectrum Single junction record is (27.6 +/-1)% at 255x [3] Vernon S.M., et al. “High-efficiency concentrator cells from GaAs on Si”, 22nd IEEE PVSC 1991 pp53–35 Efficiency increase higher than expect from double pass in QWs Enhanced V oc D.Johnson et al. WCPEC4, Hawaii May 06

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 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

Single QW Electroluminescence low bias Bulk Well

Single QW EL at high bias Bulk Well

10 QW Electroluminescence low bias Bulk Well

10 QW EL at high bias Bulk Well

Model EL (radiative recombination) Detailed Balance leads to generalised Planck: 1  (E) (use measured QE) and T determine shape  E F requires absolute calibration  (E) = absorption coefficient T = temperature of recombining carriers  E F = quasi-Fermi level separation where J.Nelson et al., J.Appl.Phys., 82, 6240, (1997) M.Fuhrer et at Proc. EU PVSEC Dresden,Sept 06

EL - model and experiment datamodel (nm) (a.u.)

EL - Bulk Peak Fits T = 299 K

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

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)

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

Calculated output : San Francisco Fraction of electricity consumption provided by photovoltaic cells Consumption = 145 kWh/m 2 Savings Average electricity generated by 1 m 2 of façade over 1 year

Luminescent Concentrators for Diffuse Component of Sunlight Dye-doped luminescent concentrators (1977): lAdvantages èno tracking required èaccept diffuse sunlight  stacks absorb different  E g ~ E , gives max. effic. èthermalisation in sheet lDisadvantages èdyes degrade in sunlight èloss from overlap of absorption/luminescence  narrow absorption band A Goetzberger and W Greubel, Appl. Phys. 14, 1977, p123.

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 red-shift tuned by spread in dot size spread fixed by growth conditions (K.Barnham et al. App. Phys.Lett.,75,4195,(2000))

Thermodynamic Model for QDC The brightness, B( , 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: I C = concentrated radiation field, Q e = quantum efficiency,  e = absorption cross section Extend to 3-D fluxes + boundary conditions n = refractive index  = 1/kT  = chemical potential I 1 ( ) z = 0 z = D x y z cc 22 cc A.J.Chatten et al, 3rd WCPEC, Osaka, 2003 E Yablonovitch, J. Opt. Soc. Am. 70, 1362, 1980.

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 Q e (SD387) = 0.56 (c.f. Nanoco 0.4 – 0.6)

Thermodynamic Model confirms unexpected luminescent stack result Incident light LayerExperimental Jsc (mA/m 2 ) Predicted Jsc (mA/m 2 ) Top10.2 ± ± 2.1 Bottom35.1 ± ± 1.3 Incident light LayerExperimental Jsc (mA/m 2 ) Predicted Jsc (mA/m 2 ) top47.5 ± ± 2.1 Bottom4.8 ± ± 1.3 Total output = 45.3 (mA/m 2 ) Total output = 52.3 (mA/m 2 )

EL Modeling Confirms Recycling 50 QW dark current show 33% reduction of J 01 Model EL by detailed balance ~ 30% reduction Supports efficiency increase results from photon recycling

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

Single Molecule Precursor ZnS/CdSe Core-Shell QDs lCore shell ZnS/CdSe dots by thermolysis at 270 °C of single- molecule precursors in PLMA using with TOPO cap lLuminescence 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. 3 rd WCPEC, Osaka, 2003)

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