1. EXSS Quantum Photovoltaics Hot Carriers in Strain Balanced Quantum Well Solar Cells M. F. Führer, J.P. Connolly, M. Mazzer, I. M. Ballard, D.C. Johnson, K. W. J. Barnham, A.Bessière, J. S. Roberts, R. Airey, C. Calder, G. Hill Contact: markus.fuhrer01@imperial.ac.uk

2. EXSS Quantum Photovoltaics Quantum Well Solar Cells Extended Absorption Strain Balancing Increased Efficiency

3. EXSS Quantum Photovoltaics Our Samples various depth InGaAs Wells GaAsP Barriers Concentrator Masks: Homogeneous at high bias

4. EXSS Quantum Photovoltaics Electroluminescence Setup

5. EXSS Quantum Photovoltaics Results – Single Medium Well

6. EXSS Quantum Photovoltaics Results – Ten Medium Well

7. EXSS Quantum Photovoltaics Results – 50 Deep Wells

8. EXSS Quantum Photovoltaics Bias Dependence

9. EXSS Quantum Photovoltaics Results - Depth dependence?

10. EXSS Quantum Photovoltaics Interpretations Sample Heating Non-Equilibrium Effects (Hot Carriers) High Exciton Density Effects

11. EXSS Quantum Photovoltaics Interpretations – Sample Heating Yes, some samples are heating up… identify with: –  E G – Bulk slope …and discard. However, effect persists even in samples without changing bulk T. Not just sample heating.

12. EXSS Quantum Photovoltaics Interpretations Model for Recombination: Generalised Planck Non-equilibrium interpretations focus on T and  E F Other interpretations focus on  (E) Measuring these quantities is difficult

13. EXSS Quantum Photovoltaics Interpretations – Hot Carriers

14. EXSS Quantum Photovoltaics Interpretations – Hot Carriers assume constant absorption Vary T and  E F for non- equilibrium effects

15. EXSS Quantum Photovoltaics Interpretations – Hot Carriers Raises possibilities of hot carrier solar cells – (which bypass Shockley-Queisser limit) Most attractive Interpretation Fits data reasonably well

16. EXSS Quantum Photovoltaics Interpretations – Exciton Effects Most of the recombination stems from recombining excitons Excitons are bosons only at long distance Interactions amplified by confinement in the quantum well

17. EXSS Quantum Photovoltaics Interpretations – Exciton Effects

18. EXSS Quantum Photovoltaics Interpretations – Exciton Effects

19. EXSS Quantum Photovoltaics Interpretations – Exciton Effects Kink in the I SC vs Power curve suggests decreased absorption beyond a certain optical bias Some evidence against heating Needs further study

20. EXSS Quantum Photovoltaics Conclusions Significant evidence of a high-bias effect in GaAs based quantum well solar cells – relative reduction in peak exciton emission Several interpretations – Hot Carriers – Exciton Interactions More work needed.