The asked questions is very interesting and put into a broad sense. As we know that, both optical properties and electronic transport properties are mainly determined by the electronic structure of the materials. Here, for optical properties, bandgap, and type of bandgap, i.e indirect or direct bandgap, is important to discuss the optical property of the materials. In the latter case, i.e. electronic transport properties, the bandgap is crucial, and density of state near chemical potential is also very important. Now, it is well known that ZnO is wide bandgap material i.e. the bandgap is about 3.37 eV. Now, if we substitute the nickel, which is a transition metal element with partially filled d orbital, at the zinc site. The electronic structure is expected to be modified. Nickel has less number of an electron than zinc (2 electrons less), and by substitution, it becomes hole type doping. Now, it will be interesting to see how it modify the electronic structure. Here, I mean the change in the bandgap, nature of band (flatten of the bands near chemical potential, band shifting, or formation of impurity states). If the nickel doping effect in the band flattening, band convergence at the valance band edge, then the effective mass of charge carriers will be modified. this will tune the carrier mobility. I would recommend making some DFT calculations and seeing the band structure result. If you have pure experimental research facility and interest, then just dope the nickel within the solubility limit and take the optical measurement for estimation of bandgap, and the properties related to the optical and electronic. You can easily make a good analysis based on the results. Good luck.
The characterization of ZnO film doped with Ni showed that the material under focus is a good candidate for optoelectronic applications such as the manufacture of solar cells with transparent electrodes.