Using impedance spectroscopy in the 103–107 Hz range, researchers at the National University of Tucumán, Argentina, have shown that the degree of spatial overlap of the n-core and the p-shell along the NW axis is critical to obtain good p-n junction electrical characteristics in GaAs NW-based core-shell PV devices.

The tested NW devices were fabricated by the LaPierre research group at McMaster University, Canada, and consisted of n-cores and coaxial p-shells aligned perpendicular to a GaAs substrate. Tellurium was used as the n-type dopant instead of the more common silicon due to the amphoteric behaviour that silicon may have in GaAs NWs. The two kinds of studied devices had very different degrees of core-shell spatial overlap along the NW axes.

When the DC voltage was changed from forward to reverse bias at low frequencies in devices having large core-shell overlap ("sample D" in the figure), the measured capacitance qualitatively followed the behaviour expected for a cylindrical space-charge region characterizing the core-shell p-n interface. In contrast, samples with little overlap ("sample A" in the figure) did not follow this behaviour.

Furthermore, when the excitation frequency was increased above 104 Hz, the device responses decayed abruptly, an effect that the team attributed to carrier trapping at and release from deep bandgap levels. By comparing the estimated trap energies with those reported in the literature for planar GaAs surfaces, the deep levels could be attributed to NW surface states. This was supported by electrostatic calculations of the NW core-shell geometry, which showed that NW surfaces lay within or very close to the depletion region.

The scientists believe that these findings will help improve future NW-based PV devices for efficient solar-energy conversion.

The group presented its work in the journal Nanotechnology.