Kavli Affiliate: Eli Yablonovitch
| First 5 Authors: Eli Yablonovitch, Zunaid Omair, , ,
| Summary:
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.
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