Carrier Multiplication and Quantum Dot Solar Cells

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Carrier Multiplication and Quantum Dot Solar Cells

• Devesh Khanal
• NSE C203
• Spring 2007

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Outline

1. Why QD solar cells?
2. Carrier multiplication and impact ionization
   intro.
3. Detecting impact ionization.
4. Possible QD arrangements for solar cells.

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Introduction

• Why Quantum Dots for solar cells?
• Thermal relaxation of excited charge carriers can
  be significantly slowed down.
• Enhanced photovoltage collect charges while
  their hot.
• Enhanced photocurrent get more from the hot
  ones.

Nozik. Physica E 14 115 (2002)
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Introduction
Carrier Multiplication one photon creates more
than one exciton via impact ionization, the
inverse of Auger recombination.
Franceschetti. Nano Letters 6 2191 (2006)
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Detection

• Experiment one pulsed laser for excitation and
  another for absorption.
• Basic idea distinguish between the relaxation
  dynamics of single excitons and biexcitons.
• Number of generated electron-hole pairs (Ne-h) is
  linearly proportional to absorption change for
  Ne-hlt3.
• Time of relaxation indicates whether there are
  biexcitons recombining via Auger recombination or
  single excitons recombining radiatively.

Schaller. PRL 92 186601 (2004)
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Detection

• Higher photon energy/band gap ratios give higher
  carrier multiplication efficiencies.
• Onset at 3Eg.

Schaller. PRL 92 186601 (2004)
Schaller. Nano Letters 6 424 (2006)
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QD Solar Cells

• The Trick
• Have to be able to extract charge carriers
  produced in quantum dots.

Nozik. Physica E 14 115 (2002)