There Are Two Distinct Photon Gases Present Inside Every Solar Cell

TL;DR

This paper reveals the existence of a second photon gas of infrared luminescence inside solar cells, which, along with the traditional light trapping, influences efficiency and requires optical modeling for optimal design.

Contribution

It introduces the concept of a second photon gas of luminescence inside solar cells and emphasizes its importance for achieving higher efficiencies.

Findings

Infrared luminescence photon gas is trapped inside solar cells.

Optical modeling is crucial alongside electron-hole modeling.

Proper design can approach the Shockley-Queisser limit.

Abstract

It has gradually been recognized that incoming sunlight can be trapped within a high refractive index semiconductor, n~3.5, owing to the narrow 16degree escape cone. The solar light inside a semiconductor is 4n^2 times brighter than incident sunlight. This is called light trapping and has increased the theoretical and practical efficiency of solar panels. But there is a second photon gas of equal importance that has been overlooked. Inside every forward-biased solar cell there is a gas of infrared luminescence photons, also trapped by total internal reflection. We introduce the idea of super-equilibrium, when the luminescence photon gas freely exchanges energy with the two quasi-Fermi levels. Nonetheless, the loss of a single photon from either gas is equivalent to the loss of a precious minority carrier. Therefore optical modeling & design becomes equally important as electron-hole modeling in high efficiency solar cells. It becomes possible to approach the idealistic Shockley-Queisser limit, by proper material selection and design of the solar cell optics.