The second law of thermodynamics imposes a limit on the maximum efficiency achievable with any rectifier in converting radiation into dc power. By using the second law and Bose photon statistics, a general expression is derived showing that this efficiency depends upon the bandwidth and effective temperature of the incoming radiation. Only for completely coherent radiation can the conversion efficiency be unity. The theoretical implications are illustrated by examining specific thermodynamic cycles involving idealized recombination diodes and thermodynamically ideal rectifiers. The relationship between minimum bandwidth of radiation and bias voltage in a recombination diode is derived. The connection between conversion efficiency and Carnot efficiency is made clear.