exss quantum photovoltaics hot carriers in strain balanced quantum well solar cells m. f. führer,...
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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: [email protected]
EXSSQuantumPhotovoltaics
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.