The fundamental (detailed balance) limit of the performance of a tandem structure is presented. The model takes into account the fact that a particular cell is not only illuminated by part of the solar irradiance but also by the electroluminescence of other cells of the set. Whereas, under 1 sun irradiance, a single solar cell only converts 30% of the solar energy, a tandem structure of two cells can convert 42%, a tandem structure of three cells can convert 49 %, etc. Under the highest possible light concentration, these efficiencies are 40% (one cell), 55 % (two cells), 63 % (three cells), etc. The model also allows us to predict the ideal efficiency of a stack with an infinite number of solar cells. Such a tandem system can convert 68% of the unconcentrated sunlight, and 86% of the concentrated sunlight.

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In the present paper a simple model is presented for a heat engine, where the power output is limited by the rate of heat supply (and/or heat release). The model leads to a variety of results. Some of them are established laws such as the Carnot law, the Curzon–Ahlborn efficiency, and the Castañs efficiency. Other results are new, and are related to phenomena as different as geothermal energy conversion and the Penfield paradox of electric circuits.

Recent calculations of the electron affinity difference between CdS and CuInSe2 indicate that the conduction band (CB) minimum of CuInSe2 is below the CB minimum of CdS. As a consequence, a spike occurs in the CB at the CdS/CuInSe2 interface. Such a spike is commonly considered as in conflict with good photovoltaic performance of heterojunction solar cells. It is outlined here that the simple assumption of thermionic emission across the junction can explain an unimpeded electron transport in the case of an n+p structure (n-type window, p-type absorber), even when a spike in the CB occurs.

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