A new methodology for calculating the efficiency of multi-junction solar cells

The future of high efficiency photovoltaics lies in better utilizing the full solar spectrum. Spectrally splitting the solar input either by using stacked multi-junctions or advanced photonic redistribution of the light into different cell segments can approach theoretical conversion limits. However, while the algorithm for calculating the efficiency for these systems is considered to be definitive, a careful examination of some of the fundamental parameters shows that there are crucial aspects to the calculation that have been overlooked. By recognizing a new set of parameters such as the limited bandwidth of light absorbed in each cell, the non-radiative losses of each segment, and the photon management of incoming and outgoing light, a new approach to the calculation can be obtained. Here, we provide a blueprint for this calculation stemming from the perspective of Information Theory, and its association with the efficiency of solar cells via the relationship between the Entropy of the photons and the Open-Circuit Voltage of a cell. We list these important parameters, and how they relate to the overall efficiency, in a simple, elegant manner. Using these concepts we show how to compensate for these inherent losses by modifying the effective area of each cell segment.