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Carrier Multiplication and Quantum Dot Solar Cells

Carrier Multiplication and Quantum Dot Solar Cells. Devesh Khanal NSE C203 Spring 2007. Outline. Why QD solar cells? Carrier multiplication and impact ionization intro. Detecting impact ionization. Possible QD arrangements for solar cells. Schaller. Nano Letters 6 424 (2006).

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Carrier Multiplication and Quantum Dot Solar Cells

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  1. Carrier Multiplication and Quantum Dot Solar Cells Devesh Khanal NSE C203 Spring 2007

  2. Outline • Why QD solar cells? • Carrier multiplication and impact ionization intro. • Detecting impact ionization. • Possible QD arrangements for solar cells.

  3. Schaller. Nano Letters 6 424 (2006) 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)

  4. Introduction Carrier Multiplication = one photon creates more than one exciton via impact ionization, the inverse of Auger recombination. Franceschetti. Nano Letters 6 2191 (2006)

  5. 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-h<3. • Time of relaxation indicates whether there are biexcitons recombining via Auger recombination or single excitons recombining radiatively. Schaller. PRL 92 186601 (2004)

  6. 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)

  7. QD Solar Cells The Trick: • Have to be able to extract charge carriers produced in quantum dots. Nozik. Physica E 14 115 (2002)

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