  We demonstrate electrical control of the A-exciton interband transition in
mono- and few-layer $MoS_{2}$ crystals embedded into photocapacitor devices via
the DC Stark effect. Electric field dependent low-temperature photoluminescence
spectroscopy reveals a significant tuneability of the A-exciton transition
energy, up to $\sim 16 meV$, from which we extract the mean DC exciton
polarisability $<\bar{\beta}>_{N} = (0.58 \pm 0.25) \cdot 10^{-8} DmV^{-1}$.
The exciton polarisability is shown to be layer-independent, indicating a
strong localisation of both electron and hole wave functions in each individual
layer.
