Radiative coupling the easy way: Using transfer coefficients to model series-connected multi-junction solar cells

Abstract: When the quality of multijunction solar cells becomes sufficiently high, radiative exchange of photons between cells has to be taken into account to properly model these devices. In this work it is shown how this radiative coupling can be accounted for in series connected multi-junction solar cells by constants called transfer coefficients. Under the assumption that the exchanged radiation only travels one way, from higher to lower band gaps, the transfer coefficients allows the relation between the voltage and current of the device to be expressed by a convenient mathematical expression. Another advantage of this model is that it allows the short circuit current of radiatively coupled multijunction cells to be calculated in a straightforward way. Non-ideality may be included by means of the external radiative efficiency. A number of examples are discussed to show how the transfer coefficients describes the radiative coupling. In addition, the model is used to find the efficiency peaks of radiatively coupled multi-junction cells when illuminated by the AM1.5 spectrum.