1. The Role of Solar Electricity in Sustainable Building (Smart Windows) Quantum Photovoltaic Group Keith Barnham, Ian Ballard, Andreas Ioannides, David Johnson, Marianne Lynch, Massimo Mazzer, Tom Tibbits, (Cell design, cell characterisation and modelling) Experimental Solid State Physics, Imperial College London, London SW7 2BW, UK http://www.sc.ic.ac.uk/~q_pv John Roberts, Geoff Hill, Cath Calder (Sample growth and fabrication) EPSRC National Centre for III-V Technology, University of Sheffield, Sheffield S1 3JD, UK Tim Green (Systems, local power networks, storage) Electrical Engineering, Imperial College London, SW7 2AZ Optical Products Ltd, SolarStructure, Permasteelisa Centre for Integrated Photonics

2. Growth in World PV Capacity PV installations World-wide increased by 57% in 2004 UK PV one of lowest in EU but a 40%/yr increase would generate 23% by 2023 P.Maycock, Renewable Energy World, Aug. 2005.

3. The Three Generations of PV lFirst Generation èCrystalline and poly-crystalline Si è~15% efficiency, ~ $3/W p lSecond Generation èThin film cells CdTe, CuInSe 2 è(10-15)% effic., ~ $(1-2)/W p lThird Generation  expensive - III-V cells  (400-1000)x concentration for large scale power < $ 1/W p. Our Target (1000x) Our Present State (200x) Silicon - no concentration M.A.Green, “Photovoltaics for the 21st Century II”, Electrochemical Soc. Proc. Vol. 2001-10, 1, (2001).

4. http://www.pvsystem.net/ Shibuya, Japan The First BIPV Building in Japan First Generation cells in BIPV Cell Efficiency ~ 15%

5. Power Glass™ Power Glass™ represents a new breed of solar cell design that balances solar cell efficiencies and manufacturing costs with broad applications and uses. Power Glass™ solar cells operate at as much as 50%, or half, the efficiency of conventional opaque amorphous solar cells yet costing as little as 25%, or one fourth, to produce. http://www.xsunx.com/tech-power.htm XsunX, Inc. Aliso Viejo, CA (USA) Second Generation Thin Film Cells in BIPV Cell Efficiency ~ 7%

6. Efficiency versus band- gap lGaAs cells highest single junction efficiencies lLower E g => higher efficiency l No lower E g III-V alloys lattice matched to GaAs or Ge substrates lMulti-junction approaches also would like bulk-cell with band-gap ~ 1.1 mm ~ 1.1 eV lCan grow In y Ga 1-y As bulk cells on virtual substrates but never dislocation free l SB-QWSC lowers GaAs absorption edge without dislocations

7. GaAsP/InGaAs Strain-Balanced QWSC Advantages:  Can vary absorption band- edge and absorb wider spectral range without strain- relaxation  No dislocations  Higher barriers than GaAs => reduced recombination  single junction so can cope with varying spectra Balance stress between layers to match lattice parameter of the substrate

8. SB-QWSC Efficiency vs. Concentration 50 well SB-QWSC ~ 2% higher efficiency than p-n control l 65 well cell should achieve World record at 500x Single Junction Cell World Record

9. What is the Electricity DEMAND in Buildings? 63% of electricity in UK used in buildings Sunlight on buildings ~ 7x electricity consumption in the buildings DEMAND similar through year, peaks daily ~2x BASELOAD – Nuclear only provides baseload 3 rd Generation cells on 25% of S-facing walls replace all nuclear contribution At 500x a UK semiconductor facility could produce cells = 5 nuclear reactors in 10 years Imperial College London

10. Novel Application of 3 rd Generation Cells Solarstucture Ltd– new company for Building Integrated Concentrator PV Curtain walls (glass facades) => Use tracking blinds to cut direct sunlight to remove glare and reduce air conditioning demand Our cells are very small (~ 1mm) high efficiency (~ 30%) need 500 x concentrators to reduce costs The concentrators must track the sun

11. Third Generation Cells in Concentrators (Smart Windows) Unique advantages: lNo transmission of direct sunlight lReduce a/c requirements lMax diffuse sunlight - for illumination l(2 – 3) x power from Silicon BIPV lProvide electricity at peak times lCell cooling provides hot water l(500 – 1000)x concentration Transparent modules 1 mm solar cells Cell efficiency ~ 30% l innovative 2-axis tracking l adds ~ 20% to façade cost

12. 65% 21% 22% (cell cooling) 6% Direct Sunlight Diffuse Sunlight Available for Illumination Electricity 31% (IR rad.) Smart Windows Energy Budget Thin 2 nd Generation Cells BeneficialDetrimental 11% 6% Electricity 80% Usable Heat Heat Dispersion Light Reflection Direct Sunlight Diffuse Sunlight 10%

13. Annual electricity generation as a function of the curtain wall slope 90° = vertical wall The maxima correspond to the latitude angle, i.e.: 37.8° for S.Francisco 51.5° for London Comparison with 2nd Generation BIPV

14. SB-QWSC + Ge 32% vertical S - W facing wall in London World record 3- junction cell 35% harvests same electrical energy over year Compare SB-QWSC + Ge with multi-junction Cell MJ cell optimised for one spectrum, one temperature. Tunnel Junctions problematic at high conc. high current

15. Conclusions 3rd generation cells, higher efficiency-lower cost in concentrators can help maintain > 40% per year PV market expansion Smart-windows alone could replace the nuclear component quicker than new-build Smart windows – new active architectural component