Departures from the principle of superposition in silicon solar cells

The principle of superposition forms the theoretical basis on which the comparison of illuminated and dark current-voltage (I-V) characteristics of solar cells depends. Two cases predicted from computer simulations where the superposition principle does not apply in silicon p-n junction solar cells are reported. These predictions are confirmed experimentally with measurements taken on existing high-efficiency devices, and cannot be accurately described by previous explanations for departures from this principle. The first case, which is the more important in terms of operation under 1 sun illumination, occurs in cells where recombination via defect levels (Shockley–Read–Hall recombination) with unequal electron and hole capture rates dominates the I-V characteristics. The second case is evident at small forward voltages in almost all silicon solar cells. It is shown that the former is due to a saturation in the recombination rate, while the latter is the result of a bias-dependent modification of the carrier concentrations across the p-n junction depletion region which is different under illumination from that in the dark.