Carnot factor in solar cell efficiencies

We study the dependence of solar cell efficiency on the illumination factor G, which governs the solid angle subtended by the radiation source at the cell. While the 2π-geometry (or, equivalently, maximum illumination factor G = 1) yields a Carnot factor, this disappears in more general cases when G lies between zero and one and the cell receives both solar and ambient radiation. The Carnot factor also applies to cells with particular angularly selective absorption properties and this can be made obvious even in the more general case, for which a new efficiency formula is found here. This is achieved by a simple algebraic transformation. In this way, the connection between the thermodynamic and statistical mechanical approach to solar cell efficiencies emerges very clearly. The illumination factor is subsequently defined for an arbitrary radiation source and the theory is applied in the case of moonlight.