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Photovoltaic Effect in Ideal Carbon Nanotube Diodes

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Title: Photovoltaic Effect in Ideal Carbon Nanotube Diodes


1
Photovoltaic Effect in Ideal Carbon Nanotube
Diodes
  • Sung Hwan Kim

2
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

3
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

4
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

5
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

6
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!

7
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

8
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
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