There Are Two Distinct Photon Gases Present Inside Every Solar Cell

It has gradually been recognized that incoming sunlight can be trapped within a high refractive index semiconductor, n≈3.5, owing to the narrow 16° escape cone. The solar light inside a semiconductor is 4n<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 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 photon confinement and luminescent photon density becomes equally important as minority carrier confinement in high efficiency solar cells. It becomes possible to approach the idealistic Shockley-Queisser limit, by using a double heterostructure for carrier confinement, and light trapping design for luminescence concentration.