## 4 Discussion  
Taking into the results presented, one may evaluate the complexity of the layer-formation process on the TiO\({}_{2}\):ZnO/CuO solar cell and the influence of the process flow on Cu diffusion and the final structure behavior. To study the electronic properties of the TiO\({}_{2}\)/ZnO film doped with Cu and CuO atoms, we obtained distributions of valence electrons by energy bands for the cubic structure of the space group Fm\(\overline{3}\)m.  
It was established that doping the film with Cu/CuO atoms of the TiO\({}_{2}\)/ZnO film system led to an improvement in the internal properties of the binary oxides. At the same time, the absorption of an oxygen molecule on the TiO\({}_{2}\)/ZnO/Cu surface was more likely in the interatomic void of Cu (100) atoms. The results obtained allow for a deeper understanding and control of the optimization of TiO\({}_{2}\)/ZnO thin-film systems. Based on the structural analyses of Samples #18 and #15, shown in Figure 5c,d, the columnar structure of the CuO layer can still be recognized but is less obvious in the cross-sectional images. This diffusion can be quantitatively described using the surface diffusion coefficient D = D\({}_{0}\)exp[\(-\)E\({}_{A}\)/kT] (D and D\({}_{0}\) measured in m\({}^{2}\)/s, and E\({}_{A}\) in J (or eV) so as kT), where D\({}_{0}\) (the preexponential factor) is related to the frequency of attempts by the adatoms to jump from one surface site to another, E\({}_{A}\) is the activation energy of the jump phenomenon, k is the Boltzmann constant, and T is the absolute temperature at which the adatom jump process occurs [27; 29]. Key factors in establishing the activation energy are, for example, the interatomic energy between adatoms and substrate atoms and between the adatoms themselves. Surface diffusion is one of the most important processes in crystal growth. On a crystallographically singular surface, arriving atoms are not directly incorporated into the crystal. Rather, they migrate on the surface until they are incorporated into the crystal or evaporate to the vapor phase [30; 31]. Owing to a proper crystallographic structure created by column grain growth, Samples #26 and #27 achieved a detectable photovoltaic effect. Both of these samples presented low electric parameters; however, the characteristic flow of these samples was very in  
Figure 7: Reflection spectra of samples #15, #18#, #26, and 27 in the wavelength range of 200–2500 nm. teresting according to the possible construction issues, which may be addressed in the forthcoming research. Sample #27 presented uniform, flat characteristics with an FF value close to 25%. This was due to the high value of series resistance caused by the poor electrical contact of the inner layers and some problems with the effective ohmic contact with the electrodes. This resulted in very limited Voc (11 mV) and Isc (6.5 uA) values. More interesting are the results presented by Sample #26. Observing the I-V characteristics apart from the visible photovoltaic effect with the functioning p-n junction, one may distinguish two sectors of the curve. For the lower voltage part (Part 1) the FF may be calculated at the value of 50%, whereas in the higher voltage range (Part2), the FF was equal to 45%. Part 1 additionally presents a lower value of series resistance and a higher value of shunt resistance, whereas the Rsh in total was 20% higher than in Sample 27 and the Rs was more than 30% higher as well. This phenomenon is similar to the overshadowing of a single cell in a series string (sub-graph). This observation leads to a conclusion of non-homogeneity within the structure which must be further explored. Providing the homogenous characteristic flow of Sample #26, one may expect at least a 20% higher Rsh and a 30-50% lower Rs value. Under these conditions, the expected FF may achieve a value of 50-55%. All observed phenomena originated from the deposition process conditions and also influenced the specific carrier concentrations and mobilities, which will be further investigated.