Nanoscale Energy Conversion in the Quantum Well Solar Cell: 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@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,


Outline: 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 l or Eg: Cell efficiency cell versus l or Eg GaAs cells - highest effic. single junction cells, Eg too high lower Eg => 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


Slide4: 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 => novel PV material


GaAsP/InGaAs Strain-Balanced QWSC : 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 Eg gives higher tandem effic. Balance stress between layers to match lattice parameter of the substrate


SB-QWSC – Ideal Dark-Currents at High Concentration: 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


Investigation of Photon Cavity Effects: 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


Distributed Bragg Reflectors: 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


Concentrator Measurements: Concentrator Measurements [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 Voc D.Johnson et al. WCPEC4, Hawaii May 06


Why the Efficiency Enhancement?: 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


Single QW Electroluminescence low bias: Single QW Electroluminescence low bias Bulk Well


Single QW EL at high bias: Single QW EL at high bias Bulk Well


10 QW Electroluminescence low bias : 10 QW Electroluminescence low bias Bulk Well


10 QW EL at high bias: 10 QW EL at high bias Bulk Well


Model EL (radiative recombination): Model EL (radiative recombination) Detailed Balance leads to generalised Planck:1 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


EL - model and experiment: EL - model and experiment data model


EL - Bulk Peak: EL - Bulk Peak Fits T = 299 K


Conclusions : 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: 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)


Slide20: Novel Application - Building Integrated Concentrators 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). 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


Slide21: Calculated output : San Francisco Fraction of electricity consumption provided by photovoltaic cells Consumption = 145 kWh/m2 Savings Average electricity generated by 1 m2 of façade over 1 year


Luminescent Concentrators for Diffuse Component of Sunlight: 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.


Quantum Dot Concentrator: 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))


Slide24: 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.


Characterisation of ZnS/CdSe QDs in Acrylic with Thermodynamic Model : 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)


Thermodynamic Model confirms unexpected luminescent stack result : Thermodynamic Model confirms unexpected luminescent stack result Incident light Incident light Total output = 45.3 (mA/m2) Total output = 52.3 (mA/m2)


EL Modeling Confirms Recycling : 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


Slide28: 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 : 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)


BICPV – Smart Windows: 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