 Solution

Above-gap photon absorption generates electrons and electron holes. A p-n junction's built-in field and carrier diffusion direct them toward selective contacts; their nonequilibrium populations can maintain separated quasi-Fermi levels and deliver electrical power. The equilibrium built-in voltage alone is not a battery supplying continuous power.

Choose diode current positive into the cell. An idealized characteristic isIllumination shifts the curve downward. At short circuit $I(0)=-I_{\mathrm{sc}}$; at open circuit $I(V_{\mathrm{oc}})=0$. Between those points, $V>0$ and $I<0$, so the delivered power $P=-VI$ is positive.

The solar-cell fill factor compares maximum delivered power with the open/short-circuit rectangle:Series resistance, leakage, recombination and temperature change the shape and hence the fill factor. Neither the short-circuit nor the open-circuit point itself delivers power.