I'm not sure whether the dopant cations have accumulated or depleted at the surface. I do have the impedance results, however. But I'm not entirely sure how it would help in answering my question. 

Dear Jason,

well, it seems that I have just got an answer to one of my questions in my answer to your earlier question. Your system is a solid(?) doped/undoped CuWO4 layer ? What is the area and the thickness ? And what electrodes do you use ?

Although empirically perhaps interesting, the flat band potential, contact potential and the offset potential are derived quantities and one has to be careful not to give too much significance to them.

When you capture the depletion region response in your EIS measurement, your depletion resistance is a direct (and analytic !) function of the difference in the electrochemical potential between the electrode and the sample and therefore leads directly to the value of contact potential and flat band potential.

How do you define the onset potential ?

With best regards

Petr

Jason,

Your voltammetries show changes in the accumulation region, not only in the depletion. This may indicate that the electrode surface changes with illumination beyond the pure generation of charge carriers, so you are testing different electrodes with MS and photocurrent. Try doing MS under illumination and compare with dark MS. Also it is better measuring the photocurrent onset potential with chopped light.

Antonio. 

Dear Antonio,

How do you make the conclusion where the "voltammetries show changes in the accumulation region and not only in the depletion?"

I am genuinely curious and would like to learn