Guilin Sun
Ansys Employee

Most likely the substrate setting is wrong: it should be large enough to penertate out PML, instead of a finite sized slab. I have seen this before:

the above setting is wrong. Below is the correct setting:

When it is a slab, it forms strong cavity which needs long simulation time. 

In addition, please visualize the np density you imported and see if the charge density changes with voltage.