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Mapping free carrier diffusion in GaAs with radiative and heat- generating recombination Tim Gfroerer and Ryan Crum Davidson College, Davidson, NC with.

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Presentation on theme: "Mapping free carrier diffusion in GaAs with radiative and heat- generating recombination Tim Gfroerer and Ryan Crum Davidson College, Davidson, NC with."— Presentation transcript:

1 Mapping free carrier diffusion in GaAs with radiative and heat- generating recombination Tim Gfroerer and Ryan Crum Davidson College, Davidson, NC with Mark Wanlass National Renewable Energy Lab, Golden, CO ~ Supported by the American Chemical Society – Petroleum Research Fund ~

2 Solar Cell Operation Conduction Band Valence Band PHOTON ENERGY ELECTRON E-Field E- HOLE E-Field E- ++ + + - - - - - CURRENT ABSORPTION When a photon is absorbed, an electron is excited into the conduction band, leaving a hole behind in the valence band. Some heat is lost, reducing efficiency. Then an internal electric field sweeps the electrons and holes away, creating electricity. HEAT

3 Light- and Heat-Generating Recombination Electrons can recombine with holes by releasing light or heat. This loss mechanism also reduces the efficiency of a solar cell. Conduction Band Valence Band Defect Level - + HEAT Conduction Band Valence Band ENERGY Photon - + Rate ≈ A x n (n = carrier density)Rate ≈ B x n 2 (n = carrier density)

4 Experimental Setup + - + - + + + + - + - + - - + - - - - - + + Laser spot ~ 4  m diameter GaAs sample (plan view) Thermal Camera Luminescence Camera

5 Time evolution of thermal profile Laser on! Heat loss Thermal diffusion

6 Luminescence and Thermal Profiles

7 Square-root of the Luminescence Rate ≈ B x n 2 Rate ≈ A x n

8 Free-Carrier or Thermal Diffusion?!

9 Conclusions We use optical and thermal imaging to map the free-carrier density near a localized photo-excitation source. The density profiles agree when we account for the bimolecular nature of radiative recombination. BUT: a thermal diffusion calculation also mimics the temperature profile … So what have we measured?! We’ll figure it out!

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