## 3 Results and Discussion
### Crossover Effect in the J-V Characteristic  
From Figure 1b, it is evident that the J-V characteristic under light is not a simple translation of the dark J-V characteristic, as in an ideal junction. The two characteristics show a crossover, indicating a potential-dependent photocurrent [39; 40; 41]. It is typically assumed that the photogenerated current primarily depends on the intensity of incident light, with only a minor dependence on the voltage V. However, in this case, the dependence on V is not sufficiently weak to be disregarded. Consequently, we can describe the current flowing in the solar cell as follows (superposition principle) [33]:  
\[J_{L}(V,g)=J_{Dsc}(V,g)+J_{ph}(V,g) \tag{6}\]  
where J\({}_{L}\)(V,g) is the total flowing current, J\({}_{DSC}\)(V,g) is the current flowing in the diode (p-n junction), and J\({}_{ph}\)(V,g) is the current generated by light. Here, V is the applied bias and g is the photocarrier generation rate. The net current of the diode without the contribution of light-induced generation can be expressed, considering the dark current, as follows:  
\[J_{Dsc}(V,g)=J_{j}(V,g)-J_{dark} \tag{7}\]  
Here, J\({}_{j}\)(V,g) represents the injection current of the diode, and J\({}_{dark}\) is the current of the unilluminated diode, which, being minimally dependent on the potential, can be considered as constant. Consequently, the difference between J\({}_{L}\)(V,g) and J\({}_{dark}\) can give some information about the crossover:  
\[\Delta=J_{L}(V,g)-J_{dark}=\left[J_{j}(V,g)-J_{dark}\right]+\left\{J_{ph}(V,g )-J_{dark}\right\} \tag{8}\]  
From the first term (in square brackets) on the right-hand side of Equation (8), we can infer that, if the current injected by the junction also depends on the illumination, then, in \(\Delta\), this term predominates, and the crossover essentially depends on this. On the contrary, if photogeneration depends on the potential, then the second term (curly brackets) predominates. The difference \(\Delta\) has a trade-off at the point V\({}_{x}\). In our Sb\({}_{2}\)Se\({}_{3}\)-based cell, the p-n junction can be considered as a hole-blocking heterojunction, attributed to the electron affinity of the n-type layer ((Cd,Zn)S) and the doping level of the p-type layer (Sb\({}_{2}\)Se\({}_{3}\)). As mentioned earlier, this implies an accumulation of positive charge in the p-type region of the junction and, consequently, \(\left(\text{J}_{\text{j}}(V,g)-\text{J}_{dark}\right)>0\). This remains valid even when considering the intermixing layer, which reduces the lattice mismatch but does not fully adapt the offset in the valence bands between (Cd,Zn)S and Sb\({}_{2}\)Se\({}_{3}\). Concerning the presence of the crossover, there are, however, two more questions to be discussed. First, it should be considered that the potential we observe at the contacts is not the built-in potential V\({}_{\text{bi}}\) but rather the potential that drops on the resistances of materials and contacts. We can denote this potential as V\({}_{\text{bi},c}\) to indicate that it is the potential observed at the contacts. Secondly, as mentioned earlier, we need to consider that, in the (Cd,Zn)S/Sb\({}_{2}\)Se\({}_{3}\) cell under moderate forward bias, only electrons can flow through the p-type contact, while holes are partially blocked by the barrier between the valence band of (Cd,Zn)S and Sb\({}_{2}\)Se\({}_{3}\). The photocurrent is, therefore, voltage-dependent but cannot become zero until all electrons and holes have reached their respective contacts. Nevertheless, the presence of positive charge accumulation in the junction region under light implies a change in the internal electric fields. This, in turn, means that the diode current becomes dependent on the light-induced generation of charge carriers. This influences the point where the photocurrent and the dark current intersect.