## 3 Results and Discussion
### Kink Effect in the J-V Characteristic  
As clearly shown in Figure 1b, the current-density characteristic for the polycrystalline Sb\({}_{2}\)Se\({}_{3}\)-based solar cell exhibits different behavior in the dark and under illumination. Specifically, under illumination, a kink is observed at high voltages in the first quadrant of the forward J-V characteristic. This behavior is attributed to the presence of an energy barrier within the absorber layer, which can be explained by the two-diode model in series configuration (Figure 6a) [31; 32]. In the first quadrant of the J-V characteristic, the main junction barrier is completely flattened by the voltage applied to the contacts. In this situation, holes must exit from the negative pole and electrons from the positive pole,  
Figure 5: HRTEM image from the ZnO/CdS/(Cd,Zn)S/Sb\({}_{2}\)Se\({}_{3}\) interfaces. The FFT from the signed area is indexed as orthorhombic (s.g. Pnma) Sb\({}_{2}\)Se\({}_{3}\) from the [010] projection. In the right-upper corner, the HAADF image with the projected atomic structure of the Sb\({}_{2}\)Se\({}_{3}\) is shown. CdS and the (Cd,Zn)S layer are polycrystalline. contrary to what happens in the fourth quadrant, where the built-in electric field of the junction pushes the photogenerated carriers in the opposite direction. In the case of the (Cd,Zn)S/Sb\({}_{2}\)Se\({}_{3}\) junction, for a forward bias above V\({}_{\rm{oc}}\), this is certainly true for electrons, but holes tend to accumulate at the interface of the p-side of the junction. This accumulation is a result of the potential step caused by the offset between the valence bands of (Cd,Zn)S and Sb\({}_{2}\)Se\({}_{3}\) (see Figure 7). These materials possess quite different band gaps, and the low conduction band offset (CBO) leads to a big cliff-like discontinuity in the valence band, even under the conditions of the first quadrant of the J-V characteristic, as explained in [33]. Therefore, a fraction of the holes, partly photogenerated and partly injected, are trapped in the Sb\({}_{2}\)Se\({}_{3}\) layer near the interface with (Cd,Zn)S. The trapped holes, unable to move freely, promote recombination  
Figure 6: (**a**) The solar cell model highlights the second barrier diode positioned in anti-series with respect to the primary barrier diode. R\({}_{0}\) and D\({}_{\rm{B}}\) are the contact resistance and threshold barrier of the anti-series diode, respectively. (**b**) Non-scaled sketch for the proposed model of the (Cd,Zn)S/Sb\({}_{2}\)Se\({}_{3}\) junction highlighting the intermixing layer between (Cd,Zn)S and Sb2 Se3, the penetration depth in which 63% of the incoming light is absorbed, and the depletion layer. Figure 7: Schematic representation of the (Cd,Zn)S/Sb\({}_{2}\)Se\({}_{3}\) interface band profiles. with electrons, causing a noticeable decrease in current, which is seen as a kink in the J-V characteristic. In the dark, the phenomenon is much less evident, as the resistance offered by the materials only allows the flow of a weaker current and, accordingly, the number of trapped holes is also lower. To determine the barrier height, we performed temperature-dependent J-V characteristics at different temperatures, and we assumed that the barrier height as a function of the temperature follows a thermally activated behavior, as reported in ref. [34]:  
\[R_{S}T=C\,e^{\frac{\Phi_{B}}{kT}} \tag{5}\]  
where \(R_{S}\) is the total resistance comprising the resistance due to the materials constituting the solar cell, the resistances arising from contacts \(R_{0}\), and the resistance of the additional barrier \(D_{B}\) (see Figure 6a). \(C\) is a constant independent of the temperature \(T\), and \(\Phi_{B}\) is the barrier height. If \(R_{0}\) is sufficiently small compared to the barrier resistance, and, therefore, negligible, then the temperature dependence of \(R_{s}\) is primarily attributed to \(D_{B}\), which is the circuital representation of the barrier due to charge trapping.