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[Display omitted] • Surface and interface engineering was performed on VTD-SnS TFSCs. • E-beam evaporation optimized Se thickness. • Na-assisted Se diffusion was introduced. • The PCE notably increased from 3.01% to 3.70%. • Localized junction formation during Se deposition and concurrent SnSSe formation during Na-assisted Se diffusion. Surface and interface engineering in thin-film solar cells (TFSCs) is vital for optimizing charge carrier dynamics, reducing recombination, enhancing material properties, and ultimately improving efficiency and overall performance. In this study, e-beam evaporation was used to deposit selenium (Se) onto tin sulfide (SnS) absorber layers with a thickness of 10 nm to 50 nm. The Se layer optimization was validated using various characterization techniques, which confirmed the formation of micro-sized openings via Se droplet deposition. This Se passivation-plus-local opening geometry plays a crucial role in forming point-junctions with the n-type CdS during cell fabrication, presenting a novel approach in state-of-the-art technology. The results revealed a significant enhancement in the conversion efficiency of the SLG/Mo/SnS/Se/CdS/i-ZnO/AZO/Al device to 3.01 % (V OC = 0.326 V, J SC = 20.21 mA cm−2, and FF = 0.45) with 30 nm Se layer deposition. Furthermore, a 30 nm-thick NaF-assisted Se layer was deposited to improve device performance. The diffusion of both Se and Na into the SnS substrate was validated using XRD, SEM, EDS, AFM, and XPS. The power conversion efficiency (PCE) of the SLG/Mo/SnSSe-NaF/CdS/i-ZnO/AZO/Al device was further enhanced to 3.70 % (V OC = 0.340 V, J SC = 22.56 mA cm−2, and FF = 0.48). This enhancement is attributed to the creation of localized junctions via Se surface passivation, and the concurrent SnSSe formation induces band gap tuning and increases the grain size, contributing to additional PCE improvement during Na-assisted Se diffusion. [ABSTRACT FROM AUTHOR]

SnS x Se 1−x films can be fabricated by adjusting the Se/(Se + S) ratio by using two different source materials (SnS + SnSe or S + Se). However, maintaining a uniform composition throughout the film can be challenging when using conventional double-source methods. Here, we have developed a single-source vapor-transport-deposition (VTD) method to deposit highly compact and uniform SnS x Se 1−x alloy films with a Se/(Se + S) compositional ratio of 0.3. Furthermore, we fabricate thin-film solar cell (TFSC) devices with varying evaporation durations ranging from 3 to 7 h. By increasing the evaporation duration from 3 to 7 h, the thickness of the SnS 0.7 Se 0.3 absorber layer nearly doubled from around 1.05 µm to approximately 2.0 µm. As a result, the solar cell device (SLG/Mo/SnS 0.7 Se 0.3 /CdS/i-ZnO/AZO/Al) fabricated with a 5-h evaporation duration, which had an absorber thickness of approximately 1.55 µm and a bandgap of 1.18 eV, achieved a highest efficiency of 3.59%. In addition, its V OC , J SC , and FF were 0.284 V, 24.50 mA cm−2, and 51.3%, respectively. Furthermore, the band alignment at the SnS 0.7 Se 0.3 /CdS interface was investigated to determine the conduction band offset (CBO) and valence band offset (VBO). The results confirmed a cliff-like CBO of −0.07 eV at the SnS 0.7 Se 0.3 /CdS interface. The optimized device retained almost 99.9% of its initial efficiency after 6 months of storage in the air. [Display omitted] • Single-source vapor transport deposition of SnS x Se 1−x thin films. • Structural and morphological study of SnS x Se 1−x via varied evaporation durations. • Optimization of SnS x Se 1−x /CdS TFSCs with a competitive efficiency of 3.59% • Revealing band alignment at the SnS 0.7 Se 0.3 /CdS heterojunction interface [ABSTRACT FROM AUTHOR]

[Display omitted] • Design of SnSe/SnS hybrid bilayer absorber using VTD. • Role of SnSe thickness in bilayer absorber solar cells. • Enhancement in charge collection property using bilayer absorber. • Improvement in the fill factor and open-circuit voltage by introducing a thin SnSe layer. The vapor-transport deposition of SnS and SnSe can help achieve low-cost and scalable photovoltaic devices. However, the performance of SnS- and SnSe-based solar cells is limited by shunt losses and low open-circuit voltage. Single SnS absorber based solar cells exhibit a high open-circuit voltage and low short-circuit current density, whereas single SnSe based solar cells demonstrate higher short-circuit current and low open-circuit voltage. In this study, we introduce a hybrid bilayer absorber configuration combining SnSe and SnS and fabricate a solar cell device using this structure (Mo/SnSe/SnS/CdS/ZnO/AZO/Al). Specifically, we establish this bilayer absorber configuration and a heterojunction device using CdS as the buffer layer. The bilayer-absorber-based device exhibits a higher open-circuit voltage than SnS based devices, along with a short-circuit current density comparable to those of SnSe based device. The optimal device featuring the bilayer absorber exhibits an efficiency of 3.65%, representing a considerable improvement over single SnS (2.87%) and SnSe (1.4%) absorber-based solar cells. [ABSTRACT FROM AUTHOR]