Hi. As we know certain threshold voltage is needed to drive a minimum current through semiconductor. Depending on the type of contacts, ohmic or non-ohmic, the increase in current with applied voltage varies. As always, increase in voltage drives the more number of charge carriers in to the metal electrode from semiconductor. The situations sustains for the photogenerated carriers to, but the concentration gets saturated after certain voltage. So sensitivity looks to be incremented with applied bias and after particular value of voltage saturates or declines.