Some Methods of Analyzing Solar Cell Efficiencies
A survey is given of various theoretical approaches to estimating solar cell efficiencies. We start (Section 4.2) with a development of the usual solar cell equation which is widely used and assume the so-called shift theorem. It is itself an approximation as is shown again here. A theory of the heterojunction solar cell is then developed (Section 4.4), following a brief survey of properties of efficiencies in general (Section 4.3). In this section we also give an introduction to the problem of estimating the effects of impact ionization. This is done by introducing a probability that a current carrier which has enough energy to impact ionize will actually do so. Following simpler special cases (Section 4.5), a more detailed theory of heterojunction cells with impact ionization is then presented (Section 4.6).
As well known, conversion efficiencies can be increased by connecting two or more cells in series, i.e., proceeding from a heterojunction ortandem cell to several cells, or even many cells. This problem is discussed in Section 4.7. It involves radiation theory, based on some elementary quantum mechanics and statistical mechanics. Thermophotovoltaic conversion (Section 4.8) has the benefit of yielding relatively high conversion efficiencies because the energy loss due to the thermalization of the current carries which occurs in a normal solar cell is here reduced. this is due to the fact that the solar energy is first absorbed by a material that reemits radiation at a lower temperature.
KeywordsEntropy Recombination Shrinkage GaAs Refraction
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