Silicon Tunnel Junctions Produced by Ion Implantation and Diffusion Processes for Tandem Solar Cells

Abstract

This work reports on the fabrication of silicon tunnel junctions that can be used in silicon-based bottom cells for tandem solar cells. The tunnel junction was formed on n-type monocrystalline silicon wafers using the following two different processes: 1) a thermal diffusion of boron followed by an arsenic ion implantation; and 2) successive implantation steps of boron and arsenic ions. For both the processes, a rapid thermal annealing step was used to activate the dopants and cure the defects generated by the ion implantation. The activated dopant distribution across the tunnel junction was determined by electrochemical capacitancevoltage (ECV) measurements. The quality of the device was assessed from the peak-to-valley ratio and peak current density of its currentvoltage characteristics, as well as the resistivity. As a significant result, a peak current density value and a peak-to-valley ratio of approximately 50 A/cm2 and 1.7, respectively, were obtained for two consecutive implantations of boron and arsenic, followed by thermal annealing at 925C for 30 min. A very narrow depletion zone of 3.9nm was calculated from the ECV dopant concentrations measurements under full depletion approximation. Finally, solar cells fabricated using this optimized process resulted in a conversion efficiency of 10.1 against 12 obtained from the reference monocrystalline solar cell.