Optical Transmittance Maximization in Superior Performance Tunnel Junctions for Very High Concentration Applications.

The light transmission through a tunnel junction in a multijunction solar cell depends on the optical properties and thickness of the whole solar cell layers stack, which configure the light absorption, reflection and interference processes taking place inside the semiconductor structure. In this paper the focus is put on the AlGaAs barrier layers of p++AlGaAs/n++GaAs and p++AlGaAs/n++GaInP tunnel junctions inserted into a GaInP/GaAs dualjunction solar cell. The aim is to analyze the effect of the thickness and Al-composition of these barrier layers on the light transmittance of the tunnel junction, using the bottom cell Jsc as the merit figure to appraise it. An intricate relation between this Jsc and the barrier layers parameters, caused by interferential reflectance, was observed. The importance of an appropriate optical design of the semiconductor structure was corroborated by a non-negligible gain in the bottom cell Jsc when choosing the appropriate barrier layers Al-compositions and thicknesses from a range of practical values for which the optical absorption is not the main contributor to the optical losses