Substrate free GaAs photovoltaic cells on Pd-coated silicon with a 20% AM1.5 efficiency

Abstract

Thin-film epitaxial 2/spl times/2.5 mm/sup 2/ GaAs crack free solar cells grown with metal organic vapor phase epitaxy (MOVPE) have been successfully lifted-off from their original GaAs substrate using the epitaxial lift-off (ELO) technique and Van der Waals bonded on Pd-coated silicon substrates without degrading the electrical, optical and structural properties of the devices. A 20% AM1.5 energy conversion efficiency was measured for an anti-reflection coated device, with a fill factor of 0.79, a short-circuit current density of 21.84 mA/cm/sup 2/ with respect to the total area and a V/sub oc/ value of 957 mV at 25/spl deg/C. Electron beam induced current (EBIC) and cathodoluminescence (CL) measurements at room temperature were performed on the devices to study their structural properties. The EBIC signal image evidenced a very good homogeneous material with an EBIC contrast (C=(I/sub max/-I)/I/sub max/) below 1%. The variation of the penetration depth of the electron beam for a detection of the different layers of the cell gave nearly the same EBIC contrast. Only a low density of point like defects, that we assume to be structural defects, was evidenced. The CL image signal in comparison to the EBIC image didn't give any additional results for the existence of strong nonradiative recombination centers. Cross-sectional transmission electron measurements (TEM) of the GaAs-Pd interface showed a very intimate and homogeneous bonding between the grafted layer and its host substrate.