Commercially produced, unencapsulated, Czochralski (CZ) silicon solar cells can lose 3%–4% (relative) of their initial efficiency after exposure to light, after minority-carrier injection during dark forward bias, or after thermal treatment at 100–400 °C. All three degradation methods reduce the minority-carrier diffusion length in the cell substrate. Under light, the decrease in efficiency is rapid (<30 min at 1 sun), but the cell power remains stable thereafter. The effect is completely reversible and the cell performance recovers in <12 h in the dark at room temperature. The various conditions under which CZ silicon cells degrade, and the reverse process, annealing, are characterized by spectral response and current–voltage (I–V) measurements. Iron impurities are a possible cause of this effect.