  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.
