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Photovoltaic Effect in Ideal Carbon Nanotube Diodes. Sung Hwan Kim. 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

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Photovoltaic Effect in Ideal Carbon Nanotube Diodes


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Presentation Transcript
outline
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
slide3
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
photovoltaic
Photovoltaic
  • Figure of Merit
    • Isc – light-generated current
    • Voc = kT/q*ln(IL/Io+1)
    • FF – measures the “squareness” of the I-V curve
    • η – photon energy to electric power conversion efficiency
  • Diode equation
    • ID = Io[exp(qVD/nKT - 1)]
    • Ideality factor(n) is 1 for ideal diode and approached 2 for materials with defects => larger the n, the lower the power conversion efficiency through reduced Voc
fabrication of ideal diode
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
results ideal diode
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= -VG2=+10V) exhibits a fit with n=1.0 => ideal diode!
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

η (max) = 0.2%

Small absorption due to small diameter of SWNTs

Results – photovoltaics
summary and conclusion
Summary and Conclusion
  • Some thoughts:
    • VGtoo 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