Photovoltaic Effect in Ideal Carbon Nanotube DiodesOutlineSWNTPhotovoltaicFabrication of Ideal DiodeResults – ideal diodeResults – photovoltaicsSummary and ConclusionPhotovoltaic Effect in Ideal Carbon Nanotube DiodesSung Hwan KimUC Berkeley, Sung Hwan Kim 2OutlineMotivationFormation of ideal p-n junction diodes utilizing the structural purity of CNTExamination of photovoltaic effectSingle-walled carbon nanotubes(SWNTs)PhotovoltaicFabrication of Ideal DiodeResults – ideal diodeResults – photovoltaicsSummary and ConclusionUC Berkeley, Sung Hwan Kim 3SWNTStructural purity – free from impurities and defects Reduced carrier scatteringDirect bandgapWide range of bandgap to accommodate solar spectrum EG ≈ 0.8eV/d(nm)Enhanced optical absorptionUC Berkeley, Sung Hwan Kim 4PhotovoltaicFigure of MeritIsc – light-generated currentVoc = kT/q*ln(IL/Io+1)FF – measures the “squareness” of the I-V curveη – photon energy to electric power conversion efficiencyDiode 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 VocUC Berkeley, Sung Hwan Kim 5Fabrication of Ideal DiodeStandard 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 CVDA large number of devices(~400devices/cm2) need to be fabricated to find a single semiconducting SWNT between S and DUC Berkeley, Sung Hwan Kim 6Results – ideal diodeSince physical doping is not possible in SWNTs, split gate is used to create ambipolar deviceElectrostatic 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!UC Berkeley, Sung Hwan Kim 7Results – photovoltaics1.5μm(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 greatestResponsivity = Jsc/Pin = 30mA/Wη (max) = 0.2%Small absorption due to small diameter of SWNTsUC Berkeley, Sung Hwan Kim 8Summary and ConclusionSome thoughts:VG too highReproducibilityPrecise control of diameter of SWNTsNetwork of SWNTs + Si solar cell(substrate)Formation of ideal p-n junction diodes using SWNTsUnder illumination, photovoltaic effect and significant power conversion efficiency
View Full Document
Unlocking...