Photovoltaic effect in ideal carbon nanotube diodes

Ji Ung Lee
2005 Applied Physics Letters  
We demonstrate that individual single-walled carbon nanotubes ͑SWNTs͒ can form ideal p-n junction diodes. An ideal behavior is the theoretical limit of performance for any diode, a highly sought after goal in all electronic materials development. We further elaborate on their properties by examining photovoltaic effects, an application where its performance is intimately related to the quality of the diode. Under illumination, SWNT diodes show significant power conversion efficiencies owing to
more » ... iciencies owing to enhanced properties of an ideal diode. The p-n junction diode forms the basis for nearly allmodern semiconductor electronics. 1 As such, the quality of the diode is often a good predictor of the performance of a semiconductor device. A diode is degraded by impurities and structural defects in the semiconductor, forming defect states in the energy band gap. Commensurate with defect states are deviations from the theoretical ideal behavior of a diode. 1 Nowhere is the quality and performance relation more intimate than in the photovoltaic ͑PV͒ application. 1-4 This is because two charge carriers, electrons and holes, are created when photons of sufficient energy are absorbed in a semiconductor. Together, they probe a wider range of defect energy levels through recombination than a unipolar device involving only one of the carriers. Here, we demonstrate that individual single-walled carbon nanotubes ͑SWNTs͒ can form ideal p-n junction diodes, providing direct evidence for their structural purity. We further elaborate on the ideal behavior by examining PV effects, an application where the conversion efficiency of photon energy to electrical power is directly related to the quality of the diode. [1] [2] [3] [4] Additionally, SWNTs may provide unique advantages for PV. They offer a wide range of band gaps 5 to match the solar spectrum, enhanced optical absorption, 6,7 and reduced carrier scattering for hot carrier transport. 8, 9 A necessary component in a PV device is a built-in electric field that separates the photogenerated electron-hole ͑e-h͒ pairs. Early bulk semiconductor devices utilized metal/ semiconductor Schottky diodes where the difference in the work function between these two materials resulted in the built-in electric field. 3 However, these make inefficient PV devices because the full band-gap of the semiconductor is never utilized, resulting in poor device performance from reduced photovoltage and high leakage currents. The p-n junction diode overcomes these issues and serves as the model system with the highest-power conversion efficiencies. 3,4 Previous results dealing with photocurrents in SWNTs were either Schottky or ohmic contacted devices, 10-12 or possibly thermoelectric devices 13 with no details on the relation between photogenerated current and voltage. PV devices utilizing small amounts of SWNTs in a conjugated polymer blend have been reported with poor efficiencies. 14 Here, we examine the fundamental properties of nanotubes by examining individual ideal SWNT diodes.
doi:10.1063/1.2010598 fatcat:oeiqupyn55bfpdwplbjztpbqjm