The Shockley-Queisser detailed balance calculation of solar-cell behaviour is not quite exact, but if the correct expression is used in a straightforward way, then the dark current shows an unphysical infinity when the terminal voltage approaches the energy gap value from below. To eliminate this, it is necessary to allow for the reduction in optical absorption which occurs at the same time, turning to emission whenqV>Eg. If this is done, there is a reduction in photocurrent and an increase in dark current, however there is no discontinuity in behaviour whenqV=Eg. WhenqV>Eg, the lasing condition holds, so that the calculation shows clearly the transition of diode behaviour from solar cell to laser.