Electrostatic Potential Continuous

You can try this:

solution name=DevPsi continuous

But if that doesn't work, try this:

set eqnsurf "DevPsi_Oxide - DevPsi_Silicon"
pdbSetBoolean Oxide_Silicon DevPsi FixedOxide 1
pdbSetBoolean Oxide_Silicon DevPsi FluxConstant 1
pdbSetString  Oxide_Silicon DevPsi Equation_Oxide $eqnsurf