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Photovoltaic Effect in Ideal Carbon Nanotube Diodes

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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  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/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

  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= -VG2=+10V) exhibits a fit with n=1.0 => ideal diode!

  7. 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

  8. 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

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