EXSSQuantumPhotovoltaics

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

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Quantum Well Solar Cells

Extended AbsorptionStrain Balancing

Increased Efficiency

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Our Samples

various depth InGaAs WellsGaAsP Barriers

Concentrator Masks:Homogeneous at high bias

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Electroluminescence Setup

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Results – Single Medium Well

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Results – Ten Medium Well

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Results – 50 Deep Wells

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Bias Dependence

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Results - Depth dependence?

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Interpretations• Sample Heating• Non-Equilibrium Effects (Hot Carriers)• High Exciton Density Effects

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Interpretations – Sample Heating• Yes, some samples are heating

up… identify with:– EG

– Bulk slope…and

discard.• However, effect persists even in

samples without changing bulk T.

• Not just sample heating.

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Interpretations• Model for Recombination: Generalised Planck

• Non-equilibrium interpretations focus on T and EF

• Other interpretations focus on (E)

• Measuring these quantities is difficult

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Interpretations – Hot Carriers

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Interpretations – Hot Carriers• assume

constant absorption

• Vary T and EF for non-equilibrium effects

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Interpretations – Hot Carriers• Raises possibilities of hot carrier solar cells

– (which bypass Shockley-Queisser limit)• Most attractive Interpretation• Fits data reasonably well

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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

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Interpretations – Exciton Effects

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Interpretations – Exciton Effects

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Interpretations – Exciton Effects• Kink in the ISC vs Power curve suggests

decreased absorption beyond a certain optical bias

• Some evidence against heating

• Needs further study

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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.