Photovoltaic Effect in Ideal Carbon Nanotube Diodes

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Photovoltaic Effect in Ideal Carbon Nanotube
Diodes

• Sung Hwan Kim

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Outline

• Motivation
• Formation of ideal p-n junction diodes utilizing
  the structural purity of CNT
• Examination of photovoltaic effect
• Single-walled carbon nanotubes(SWNTs)
• Photovoltaic
• Fabrication of Ideal Diode
• Results ideal diode
• Results photovoltaics
• Summary and Conclusion

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SWNT

• Structural purity free from impurities and
  defects
• Reduced carrier scattering
• Direct bandgap
• Wide range of bandgap to accommodate solar
  spectrum
• EG 0.8eV/d(nm)
• Enhanced optical absorption

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Photovoltaic

• Figure of Merit
• Isc light-generated current
• Voc kT/qln(IL/Io1)
• FF measures the squareness of the I-V curve
• ? photon energy to electric power conversion
  efficiency
• Diode equation
• ID Ioexp(qVD/nKT - 1)
• Ideality factor(n) is 1 for ideal diode and
  approached 2 for materials with defects gt larger
  the n, the lower the power conversion efficiency
  through reduced Voc

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Fabrication of Ideal Diode

• Standard lithography and deposition used to form
  Mo split gates (gate spacing 0.5 µm to 1µm)
• Lift-off(Ti, Mo, Pd) to define S/D
• SWNTs grown on top of the S/D using Fe catalytic
  CVD
• A large number of devices(400devices/cm2) need
  to be fabricated to find a single semiconducting
  SWNT between S and D

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Results ideal diode

• Since physical doping is not possible in SWNTs,
  split gate is used to create ambipolar device
• Electrostatic doping - different bias polarities
  on the split gate electrostatically couple to
  form separate regions of electron and hole doping
  along single SWNT. This is possible via e-h
  tunneling through Schottky barriers from metal
  contacts to SWNT.
• I-V shown below(VG1 -VG210V) exhibits a fit
  with n1.0 gt ideal diode!

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Results photovoltaics

• 1.5µm(0.8eV) cw laser diode coupled to a
  multimode fiber
• Photogenerated e-h pairs become separated in the
  middle of the device where the electric field is
  the greatest
• Responsivity Jsc/Pin 30mA/W
• ? (max) 0.2
• Small absorption due to small diameter of SWNTs

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Summary and Conclusion

• Some thoughts
• VG too high
• Reproducibility
• Precise control of diameter of SWNTs
• Network of SWNTs Si solar cell(substrate)
• Formation of ideal p-n junction diodes using
  SWNTs
• Under illumination, photovoltaic effect and
  significant power conversion efficiency observed