The function for calculating the current should never be used as a black-box. You need to inspect the transmission spectrum to understand the system, and then it is easy to see if the obtained current is sensible or not.
Indeed, as Serhan writes, 0.5 V is a respectable bias and the effect on the current should be noticeable. But if there is no change from 0 V bias, where the current by definition is zero, then it simply means that the system (at least as it has been calculated) is insulating.