Ultra-thin, high performance tunnel junctions for III-V multijunction cells

Tunnel junctions (TJ) made of p-Al0.1Ga0.9As/n-GaAs are used because of their high peak current and low series resistance, but are not fully transparent. The influence of reducing the thickness of these tunnel junctions on the characteristics of InGaP/GaAs tandem cells was investigated. It was found that ultra-thin TJs with excellent performance can be realized. Even for a 7.5/6-nm thick TJ, which is the thinnest possible in our growth reactor, the peak current density is at least 600A/cm(2). The series resistance of the TJs was found to be at a constant level of 0.6 +/- 0.2mcm(2) for all total thicknesses of the TJ in the 13.5-40nm range. Because of a lower absorption in the TJ, a tandem cell with a 7.5/6-nm thick TJ, compared with a cell with a 20/20-nm thick TJ, gained 0.53 +/- 0.05mA/cm(2) in short circuit current to a value of 14.8mA/cm(2). Copyright (c) 2012 John Wiley & Sons, Ltd.