## 3 Results and Discussion  
### The Sb\({}_{2}\)Se\({}_{3}\) Solar Cell  
Figure 1a illustrates a non-scaled diagram of the structures of the Cd\({}_{0.85}\)Zn\({}_{0.15}\)S/Sb\({}_{2}\)Se\({}_{3}\)-based solar cell. The key photovoltaic parameters, extracted from the J-V characteristic presented in Figure 1b, are reported in Table 1. \begin{table}
\begin{tabular}{c c c c} \hline \(V_{\rm{oc}}\) [mV] & 365 \(\pm\) 6.0 & _PCE_ [\%] & 4.36 \(\pm\) 0.3 \\ \hline \(V_{x}\) [mV] & 375 \(\pm\) 6.0 & \(R_{\rm{sh}}\) [\(\Omega\)cm\({}^{2}\)] & 378 \(\pm\) 16 \\ \hline \(J_{\rm{sc}}\) [mA/cm\({}^{2}\)] & 17.1 \(\pm\) 0.3 & \(R_{\rm{S}}\) [\(\Omega\)cm\({}^{2}\)] & 3.6 \(\pm\) 0.1 \\ \hline _FF_ [\%] & 49 \(\pm\) 3.0 & \(n\) & 2.1 \(\pm\) 0.1 \\ \hline \end{tabular}
\end{table} 
Table 1: Main photovoltaic parameters of the investigated solar cell. V\({}_{\rm{oc}}\) is the open circuit voltage, V\({}_{\rm{X}}\) is the crossover voltage, J\({}_{\rm{sc}}\) is the current density, FF is the fill factor, PCE is the power conversion efficiency, R\({}_{\rm{sh}}\) is the shunt resistance, R\({}_{\rm{S}}\) is the series resistance, n is the ideality factor. Figure 1: **(a)** Non-scaled sketch of the Sb\({}_{2}\)Se\({}_{3}\)-based solar cell and cross-sectional bright field (BF) TEM image of the solar cell; **(b)** current density–voltage characteristics in dark (black) and under solar simulator illumination with 700 W/m\({}^{2}\) AM1.5G filtered light (red); the magnification highlights the crossover point V\({}_{\rm{X}}\). To evaluate the series resistance (\(R_{s}\)) and the ideality factor (\(n\)) of the solar cell, a manipulation of the J-V expression, neglecting the saturation current and supposing \(R_{s}<<R_{slt}\), yields the following [14]:  
\[\frac{dV}{dI}=\frac{nKT}{q}\left(J+J_{ph}\right)^{-1}+R_{S} \tag{1}\]  
Here, \(n\) represents the diode ideality factor and \(J_{ph}\) is the current attributed to the photogenerated carriers, which can be approximated by the short-circuit current \(J_{sc}\). From the plot in Figure 2a, it is possible to extract the series resistance \(R_{S}\) while, from the plot of \(dI/dV\) in Figure 2b, one can obtain the shunt resistance \(R_{sl}\) (both values reported in Table 1) once the value of \(R_{S}\) is known from the fit of Figure 2a [21]. \[\frac{dJ}{dV}=\frac{qJ_{0}}{nKT}e^{[\frac{(V-R_{S})}{nKT}]}+\frac{1}{R_{sh}} \tag{2}\]  
For the subsequent analysis of the cell properties, it is important to note that the interface between the p-type Sb\({}_{2}\)Se\({}_{3}\) and the n-type (Cd,Zn)S region is not sharp but rather graded due to layer intermixing, which helps minimize the lattice mismatch between the two materials. According to relation (3), the lattice mismatch (\(\delta\)) between Sb\({}_{2}\)Se\({}_{3}\) and Zn\({}_{0.15}\)Cd\({}_{0.85}\)S is indeed large [22]:  
\[\delta(\%)=\frac{2\cdot\left|a_{(Cd,Zn)S}-b_{Sb_{2}Se_{3}}\right|}{a_{(Cd,Zn )S}+b_{Sb_{2}Se_{3}}}\cdot 100=2.63\% \tag{3}\]  
The lattice parameters of (Cd,Zn)S and Sb\({}_{2}\)Se\({}_{3}\) are denoted as \(a_{(Cd,Zn)S}\) and \(b_{Sb_{2}Se_{3}}\), respectively. The lattice parameter of (Cd,Zn)S (with 15% atomic concentration of Zn) is estimated using Vegard law [23; 24] with \(a_{CdS}=4.14\) A and \(a_{ZnS}=3.82\) A [25; 26]. \[a_{Zn_{0.15}Cd_{0.85}S}=0.15a_{ZnS}+0.85a_{CdS}=4.092\text{ \AA} \tag{4}\]  
The lattice parameter of Sb\({}_{2}\)Se\({}_{3}\) to be considered is \(b_{Sb_{2}Se_{3}}=3.9858\) A, as for our growth conditions the Sb\({}_{2}\)Se\({}_{3}\) preferential orientation is [001] direction, i.e., parallel to the growth axis [9]. Figure 2: (a) Linear fit (in red) of dV/dJ data for the determination of R\({}_{S}\). (b) Linear fit (in red) of dJ/dV data for the determination of R\({}_{sh}\).