Using transfer coefficients to model series-connected multi-junction solar cells with radiative coupling

When the quality of multi-junction solar cells becomes sufficiently high, radiative exchange of photons between cells must be included to properly model such devices. In this work, it is shown how constants called transfer coefficients can account for radiative coupling in series-connected multi-junction solar cells consisting of several sub-cells, which obey the single diode equation. The introduction of the transfer coefficients allows the relation between the voltage and current of the device to be expressed by a convenient closed-form expression that captures the general physics of radiatively coupled multi-junction cells analogous to how the single diode equation describes single-junction solar cells. Another advantage of this model is that it allows the short circuit current and open circuit voltage of radiatively coupled multi-junction cells to be easily calculated. The possibility of extending the model by including some non-idealities is briefly discussed.