Intermediate Mirrors to Reach Theoretical Efficiency Limits of Multi-Bandgap Solar Cells

TL;DR

This paper demonstrates that using intermediate selective mirrors, such as air gaps with antireflection coatings, can significantly increase the efficiency of multi-junction solar cells by enhancing voltage and photon management.

Contribution

It quantitatively analyzes the efficiency gains from intermediate mirrors in tandem solar cells and proposes a practical implementation using air gaps and antireflection coatings.

Findings

Efficiency increase up to ~6% with optimal mirrors

Air gap with antireflection coatings effectively transmits sunlight

Reflects internally trapped luminescence to boost voltage

Abstract

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.