## 3 Results and Discussion
### The Sb\({}_{2}\)Se\({}_{3}\) Solar Cell
The curves were derived from J-V curves measured under solar simulator illumination of 700 W/m\({}^{2}\) AM1.5G at a temperature of 298 K.  
The nominal concentration of 15 at% Zn inside the CdS crystal structure was confirmed by taking the E\({}_{\text{g}}\) of (Cd,Zn)S (2.67 \(\pm\) 0.01 eV) and CdS (2.48 \(\pm\) 0.01 eV) from a Tauc plot (see Figure 3). With the assumption that the energy gap varies linearly with the Zn concentration between the E\({}_{\text{g}}\) of CdS (0%) and that of ZnS (100%), and taking the E\({}_{\text{g}}\) of ZnS = 3.6 eV [27], we estimated the at% of Zn in CdS to be (17 \(\pm\) 2)%, which is consistent with the present 15 at% value. A lattice mismatch exceeding 1% normally introduces a significant density of crystallographic defects at the interface, which provides energy levels acting as recombination centers. This well-known fact limits the number of photocarriers that actually reach the device electrodes. Such a phenomenon was previously observed in CdS/CdTe-based solar cells and pre-vented the formation of an adapting layer [28], which gradually reduced the effective mismatch. Similarly, in antimony selenide-based solar cells, the introduction of such an adapting layer proved to be essential to circumvent the lattice mismatch issue. The introduction of (Cd,Zn)S at the junction, besides decreasing the lattice mismatch, has two further advantages. First, as previously shown [9], the introduction of a Zn\({}_{0.15}\)Cd\({}_{0.85}\)S layer allows the growth of antimony selenide with preferential grain growth orientation along the [001] direction. Second, as demonstrated in ref. [14], a zinc concentration ranging from 15 at% to 20 at% within CdS enables the achievement of a spike-like type conduction band offset (CBO) at the interface with Sb\({}_{2}\)Se\({}_{3}\), which is sufficiently small so as to not induce significant effects on the photovoltaic parameters of the cell. The hypothesis regarding the existence of the intermixing layer between (Cd,Zn)S and Sb\({}_{2}\)Se\({}_{3}\), based on the experience with the CdS/CdTe system, has been experimentally verified using HRTEM and EDS measurements in cross-section on the complete Sb\({}_{2}\)Se\({}_{3}\)-based cell. Figure 4b shows the EDS spectrum related to the cross-section highlighted in Figure 4a. Figure 3: Tauc plot of (**a**) (Cd,Zn)S and (**b**) CdS thin films. Red lines represent the linear fitting with the dotted part being the extrapolation to the energy axis. In Figure 4c-e,h, an interdiffusion of selenium and antimony atoms into the sulfide layer is observed, which forms an adaptive layer with an estimated thickness of about 20 nm in accordance with the EDS profile spectrum depicted in Figure 4b. The interdiffusion of Se and Sb into the sulfide layer occurs during the CSS deposition of Sb\({}_{2}\)Se\({}_{3}\) at temperatures between 350 and 400 \({}^{\circ}\)C. This temperature range is sufficient to promote diffusion at the (Cd, Zn)S/Sb\({}_{2}\)Se\({}_{3}\) interface, leading to the formation of an intermixing layer [29]. The observed interdiffusion is uniquely produced during the CSS growth as no additional heating is applied after deposition. Additionally, a non-negligible amount of oxygen (10 to 20 at%) seems present outside the ZnO layer, including in the Sb\({}_{2}\)Se\({}_{3}\) layer. This amount cannot be explained by assuming the presence of oxygen during deposition since the starting vacuum of all the different deposition processes is less than 10\({}^{-6}\) mbar, and the oxygen presence has been shown to be undetectable with an oxygen vapor pressure that is more than one order of magnitude greater [30]. A hypothesis regarding this anomalous experimental result is that the cross-sectional ca. 30 nm thin TEM lamella with a large free surface tends to oxidize during sample transfer, and, thus, the oxygen is not actually present in the subsequent layers, especially not in the Sb\({}_{2}\)Se\({}_{3}\).