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:

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

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

EXSS Quantum Photovoltaics Electroluminescence Setup

EXSS Quantum Photovoltaics Results – Single Medium Well

EXSS Quantum Photovoltaics Results – Ten Medium Well

EXSS Quantum Photovoltaics Results – 50 Deep Wells

EXSS Quantum Photovoltaics Bias Dependence

EXSS Quantum Photovoltaics Results - Depth dependence?

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

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.

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

EXSS Quantum Photovoltaics Interpretations – Hot Carriers

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

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

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

EXSS Quantum Photovoltaics Interpretations – Exciton Effects

EXSS Quantum Photovoltaics Interpretations – Exciton Effects

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

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.