Photovoltaic Effect Derived from the Carnot Cycle

An expression for the maximum power derivable from any photovoltaic device is developed in terms of the incident light intensity measured in units of black body radiation. The expression shows the linear dependence of photovoltage on light intensity at low light levels, the logarithmic dependence at intermediate light levels and the saturation at high light levels. The analysis is used to resolve a paradox proposed by W. Shockley.