The principle of superposition forms the theoretical basis on which the comparison of illuminated and dark current‐voltage (IV) characteristics of solar cells depends. Two cases predicted from computer simulations where the superposition principle does not apply in silicon pn 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 IV 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 pn junction depletion region which is different under illumination from that in the dark.

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