Origin of Leakage Currents and Nanowire-to-Nanowire Inhomogeneity in Radial p-i-n Junction GaAs Nanowire Array Solar Cells on Si

Abstract

To realize the promising potential of radial junction nanowire (NW) array-based solar cells, it is crucial to get physical insight into how the overall photoconversion efficiency (PCE) is impacted by the hole mask properties, NW growth, and post-growth device processing. In this work, we have fabricated and analyzed a radial p–i–n junction GaAs NW array solar cell grown by molecular beam epitaxy (MBE) on a Si substrate. Multiple electrical measurements are correlated through a range of characterization techniques such as nanoprobing of as-grown individual NWs, multicontact single-NW studies, and structural characterization of the fabricated NW array solar cell. A relatively high leakage current density (∼120 mA/cm2 at −1 V) was measured from the solar cell, resulting in a PCE of only ∼2.1%. The origin of this high leakage current was further investigated by measuring the electrical transport properties of individual as-grown NWs in the array through nanoprobing, revealing a high variation in electrical properties from NW to NW. In contrast to this, planar single-NW solar cells have shown rectifying characteristics with high on/off ratios and an average PCE of ∼5.2%, indicating a leakage path in the vertical configuration of the NWs. Furthermore, structural analysis of the NW array reveals a regular occurrence of NWs with off-centered nucleation for the p-GaAs NW core in the holes in the hole mask, leading to a partial or full electrical shortening of the n-GaAs NW shell to the p-Si substrate. This is shown to be predominantly responsible for the high leakage current density and poor PCE from the NW array solar cell. These findings will help to improve the structural design of radial junction NW array solar cells in order to further improve the PCE.