Improvement of Interface Quality Between Chalcogenide-Based Absorption Layer and Buffer Layer for Low-Cost Thin Film Solar Cells

Orthorhombic tin sulfide (SnS) is a promising absorber material for thin-film solar cells (TFSCs) due to its ideal optical band gap of ~1.3 eV, non-toxic and relatively earth-abundant constituents, high absorption coefficient (≥ 10^-5), and the high theoretical limit of ~32%. To date, the cell efficiency has mostly remained below 4% for SnS TFSCs, which is fairly low compared to its theoretical limit. The improvement in the efficiency of SnS is mostly hindered because of its low heterojunction quality with the CdS buffer layer.<br/>This work reports the study on improving the heterojunction quality at vapor-transport-deposited (VTD) SnS absorber/CdS interface via an annealing process. The highest efficiency of ~4.3% with good long-term stability was achieved by optimizing the annealing conditions for the heterojunction interface. This improved performance was primarily attributed to the reduction in interface defects of the SnS/CdS heterojunction, which can occur either during the deposition of transparent electrodes (Al-doped ZnO) or direct annealing of SnS/CdS heterojunction at 300 °C. The enhanced heterojunction interface quality is well supported by the reduced reverse saturation current density and shunt conductance of the fabricated devices measured under dark conditions. Although the SnS/CdS device exhibits an efficiency of over 4%, significant short-circuit current loss, mainly due to recombination, was revealed by quantum efficiency and optical analysis. Admittance spectroscopy analysis shows the presence of numerous defect densities of Sn and S vacancies (〉10¹⁷ cm^-3) in the VTD-grown SnS absorber. Here, a detailed analysis of the device’s performance will be presented.