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

To realize the promising potential of radial junctionnanowire(NW) array-based solar cells, it is crucial to get physical insightinto how the overall photoconversion efficiency (PCE) is impactedby the hole mask properties, NW growth, and post-growth device processing.In this work, we have fabricated and analyzed a radial p-i-njunction GaAs NW array solar cell grown by molecular beam epitaxy(MBE) on a Si substrate. Multiple electrical measurements are correlatedthrough a range of characterization techniques such as nanoprobingof as-grown individual NWs, multicontact single-NW studies, and structuralcharacterization of the fabricated NW array solar cell. A relativelyhigh leakage current density (& SIM;120 mA/cm(2) at -1V) was measured from the solar cell, resulting in a PCE of only & SIM;2.1%.The origin of this high leakage current was further investigated bymeasuring the electrical transport properties of individual as-grownNWs in the array through nanoprobing, revealing a high variation inelectrical properties from NW to NW. In contrast to this, planar single-NWsolar cells have shown rectifying characteristics with high on/offratios and an average PCE of & SIM;5.2%, indicating a leakage pathin the vertical configuration of the NWs. Furthermore, structuralanalysis of the NW array reveals a regular occurrence of NWs withoff-centered nucleation for the p-GaAs NW core in the holes in thehole mask, leading to a partial or full electrical shortening of then-GaAs NW shell to the p-Si substrate. This is shown to be predominantlyresponsible for the high leakage current density and poor PCE fromthe NW array solar cell. These findings will help to improve the structuraldesign of radial junction NW array solar cells in order to furtherimprove the PCE.