Coupling problem: Nernst-Planck equation with Poisson not converging

Hi,

I am unsuccessfully trying to implement to Poisson equation through the Electrostatic Module into the Module "Transport of Diluted species" for a stationary case.
Beside those modules I use the laminar flow module to account for the fluidics.
The model is supposed to describe a biochemical cell, with a fixed ion concentration streaming into a reaction chamber onto a sensor. The variable of interest would be the resulting ion concentrations for both species. On that sensor species of one kind are transported out of the model, accounted for with the "flux" boundary condition.
I activated both migration through the Electrical field and convection through the fluid in the ion transport module.
As I understand the Electrical field created by the ion species are not automatically accounted for by these modules, so I created a space charge density as with z the respective ion charges and c the respective ion concentrations.
Without the space-charge density everything converges fine, with the space charge density implemented however, the solution does not converge. I tried different solver combinations, first solving for the fluidics, finer grid resolution and no steep gradients, but without any success.
Does anybody have any experience in coupling Poisson with NP? I'm starting to think some of the boundary conditions are not set up correctly...
It would be awesome if anybody could have a look at my model.

Thanks a lot,

Raphael