  We theoretically study the current-voltage relation, $I-V$ characteristic, of
the photovoltaics due to the shift current, i.e., the photocurrent generated
${\it without}$ the external dc electric field in noncentrosymmetric crystals
through the Berry connection of the Bloch wavefunctions. We find that the $I-V$
characteristic and shot noise are controlled by the difference of group
velocities between conduction and valence bands, i.e., $v_{11}-v_{22}$, and the
relaxation time $\tau$. Since the shift current itself is independent of these
quantities, there are wide possibilities to design it to maximize the energy
conversion rate and also to suppress the noise. We propose that the Landau
levels in noncentrosymmetric two-dimensional systems are the promising
candidate for energy conversion.
