RADIATIVE RECOMBINATION AND PHOTON RECYCLING IN PHOTOVOLTAIC SOLAR-CELLS

The generalized van Roosbroeck-Shockley formula has long provided the standard method for calculating radiative recombination rates. Recently, however, it has been realised that reabsorption of the photons emitted in such recombination events can significantly diminish their effectiveness; this phenomenon is refered to as ''photon recycling''. Starting from the photon Boltzmann equation, a correct expression for the recombination rate has been derived, in the form of an integral. Its evaluation, in the case of InP, suggests a forty-fold increase in the lifetime and explains some otherwise anomalous measurements in this material. This integral has been incorporated in a proven one dimensional computer code for solving the semiconductor transport equations and then applied to InP solar cells in two significant cases. The first is when only radiative recombination is included; here neglect of photon recycling leads to a predicted efficiency of 25.7% whilst its inclusion gives results in agreement with the Shockley-Queisser figure of close to 30%. The second relates to the performance to be expected of cells produced from the highest quality semiconductor. Here, taking account of photon recycling raises the limiting efficiency from about 22% to well over 23%.