1. Ben Browne © Imperial College LondonPage 1 B.C. Browne, A. Ioannides, J.P.Connolly, K.W.J.Barnham Imperial College London John Roberts, Geoff Hill, Rob Airey, Cath Calder EPSRC National Centre for III-V Technology G.Smekens, J. Van Begin Energies Nouvelles et Environnement, B-1150 Brussels, Belgium TANDEM QUANTUM WELL SOLAR CELLS

2. Ben Browne © Imperial College LondonPage 2 Introduction and motivation Description of our cells Modelling Characterisation of two generations of cells Predictions under a concentrator spectrum TANDEM QUANTUM WELL SOLAR CELLS

3. Ben Browne © Imperial College LondonPage 3 Single Junction Cells:  The GaAs band gap is below the theoretical optimum  There are no ternary alloys lattice matched to Ge or GaAs with a lower bandgap than GaAs  Efficiency peaks predicted for In 0.1 Ga 0.9 As and In 0.3 Ga 0.7 As Tandems:  The bandgap of both cells in a GaInP/GaAs tandem are too high  QWs can move the limiting efficiency at 500 suns from 40% to 50% BANDGAP ENGINEERING InGaP/GaAs SB-QWSC Dual SB-QWSC

4. Ben Browne © Imperial College LondonPage 4 GaAsP (barrier) InGaAs (well) GaAs (bulk) STRAIN BALANCING Energy EfEf We are able to grow up to 65 quantum wells with this technique Strain balanced quantum well solar cells are dislocation free

5. Ben Browne © Imperial College LondonPage 5 pi EaEa EgEg Ideal Shockley Recombination Quantum Well Recombination Barrier Recombination Δμ n Thermal escape Generation GENERATION AND RECOMBINATION Under concentration, recombination is radiatively dominated At short circuit current all generated carriers escape from the wells

6. Ben Browne © Imperial College LondonPage 6 OUR TANDEM SOLAR CELLS Grown by MOVPE: Bottom Cells: EPSRC National Centre for III-V Technologies, UK Top Cells: ENE, Belgium

7. Ben Browne © Imperial College LondonPage 7 QUANTUM EFFICIENCY Sample 1 AM1.5D Top Cell Bottom Cell Tandem QW solar cell grown with 50 InGaAs well in the bottom cell Absorbing out to 932nm

8. Ben Browne © Imperial College LondonPage 8 DARK CURRENT MODELLING

9. Ben Browne © Imperial College LondonPage 9 Using the measured Jsc, modelling dark current and assuming additivity: LIGHT CURRENT (1 SUN A0D) Sample 1 1 ST cell modelling prediction: 29.8±0.3% at 200 suns low AOD This cell had a low top cell emitter doping → poor performance at concentration

10. Ben Browne © Imperial College LondonPage 10 Sample 2 IMPROVED QW TANDEM CELL A 2 nd cell was grown with higher: top cell emitter doping—decreased Rs QW band gap—better current matching Fill FactorEfficiency (%) QW Cell81.430.6 Control81.931.7 Tested at ENE under a red-rich Xenon Lamp, 54  concentration:

11. Ben Browne © Imperial College LondonPage 11 2 ND CELL PERFORMANCE Sample 2  The Control Top Cell absorbs out to longer wavelengths  This explains the superior performance of the control in a red-rich spectrum

12. Ben Browne © Imperial College LondonPage 12 Sample 2 2 ND CELL SHORT CIRCUIT CURRENT A good quality Top Cell on a quantum well bottom cell would be current matched in a concentrator spectrum

13. Ben Browne © Imperial College LondonPage 13 Sample 2 2 ND CELL EFFICIENCY Under a low AOD spectrum (1000W/m²) & assuming additivity we expect: We are working to improve our top cell in order to realise 34% efficiencies

14. Ben Browne © Imperial College LondonPage 14 CONCLUSIONS Tandem quantum well solar cells offer a path to increased multi-junction cell efficiency by band gap engineering We have achieved 30.6% under a Xe lamp at 54 suns Two junction quantum well solar cells have the potential to reach efficiencies above 34% Tandem cells with quantum wells in both junctions could perform better still