  Creating a single bandgap solar cell that approaches the Shockley-Queisser
limit requires a highly reflective rear mirror. This mirror enhances the
voltage of the solar cell by providing photons with multiple opportunities for
escaping out the front surface. Efficient external luminescence is a
pre-requisite for high voltage. Intermediate mirrors in a multijunction solar
cell can enhance the voltage for each cell in the stack. These intermediate
mirrors need to have the added function of transmitting the below bandgap
photons to the next cell in the stack. In this work, we quantitatively
establish the efficiency increase possible with the use of intermediate
selective reflectors between cells in a tandem stack. The absolute efficiency
increase can be up to ~6% in dual bandgap cells with optimal intermediate and
rear mirrors. A practical implementation of an intermediate selective mirror is
an air gap sandwiched by antireflection coatings. The air gap provides perfect
reflection for angles outside the escape cone, and the antireflection coating
transmits angles inside the escape cone. As the incoming sunlight is within the
escape cone, it is transmitted on to the next cell, while most of the
internally trapped luminescence is reflected.
