## 4 Applications of Bulk SnS  
Tin(II) sulfide is a promising material for absorber layers in thin film solar cells. [PERSON] et al. developed one of the first solar cells with a significant power conversion efficiency based on evaporation of SnS into an indium doped tin oxide (ITO)/n-CdS/p-SnS/Ag heterostructure. The cell had a short circuit current of 7 mA cm\({}^{-2}\), an open circuit voltage of 0.12 V, a fill factor of 0.35, and a conversion efficiency of 0.29% [6]. [PERSON] et al. have reported the record-high efficiency 4.36% of p-type SnS-based solar cells, where an n-type zinc oxysulfide, Zn(O,S) layer has been combined with SnS, which acts as a hole blocking layer and favours electron transport. The major losses present in a typical SnS/ZnO cell are: losses from recombination at SnS grain boundaries and recombination at the SnS/ZnO(S) interface where both act as defects. A low open circuit voltage is the main limitation to obtaining a high conversion efficiency, with open-circuit voltage being dependant on the quantity of recombination pathways with a low open-circuit voltage suggesting a greater quantity of defects [8]. Solution-processed SnS cells made from quantum dots may have far lower efficiencies, with values of 0.008-0.066% due to the increased number of grain boundaries and recombination losses [20]. To reduce recombination and obtain highly efficient (4.36%) devices, multiple strategies have been applied, for example: (i) annealing of the SnS layer under a H\({}_{2}\)S environment to form largergrains with fewer grain boundaries; (ii) SnO\({}_{2}\) of several monolayer thickness has been inserted between the p-SnS/n-Zn(O,S) junction; and finally, (iii) adjusting the conduction band offset by tuning the composition of the Zn(O,S) [8]. In Table 4, [PERSON] et al. used commercially available SnS powder to produce phase-pure SnS films using a congruent thermal evaporation unit under high vacuum. Before film deposition, the commercially available SnS powder was pre-annealed in a separate furnace at 500 \({}^{\circ}\)C for 1 h. After this deposition method, thermally evaporated SnS film absorber layers in SnS-based solar cells achieved a National Renewable Energy Laboratory (NREL) certified efficiency of 3.88% [69]. [PERSON] et al. reported devices with efficiencies of up to 2.9% through an improvement in the morphology via annealing of the SnS layers deposited using pulsed-chemical vapour deposition using N\({}^{2}\),N\({}^{3}\)-di-tert-butyl-butane-2,3-diamido-tin(II) precursors. Optimising the oxygen and sulfur content of Zn(O,S) helps in tuning the conduction band offset (CBO) at the p-n junction interface [70]. SnS has been used as a counter electrode (CE) in iodide/triiodide (I\({}^{-}\)/I\({}_{3}\)\({}^{-}\))-based liquid junction dye-sensitised solar cells (DSSCs), as an alternative to expensive platinum (Pt). [PERSON] et al. synthesised SnS nanosheets via the thermal decomposition of tin diethyldiditocarbamate and Na-DEDTC molecular precursors, with SnS applied as a CE in DSSCs. Excellent catalytic activity for triiodide reduction, resulting in a power conversion efficiency of 6.56%, is reported. [PERSON] et al. [9] reported stoichiometric SnS nanostructured films deposited using a continuous spray pyrolysis (CoSP) technique with an aqueous spray solution of tin chloride and thiourea, with the film used as a CE in DSSCs as a replacement for Pt [10]. DSSCs have attracted a significant degree of interest, however they suffer from encapsulation issues due to solvent leakage of the liquid-based electrolyte. Another third-generation solar cell technology, i.e., inorganic-organic hybrid solar cells, are adopted from DSSCs; in these systems, inorganic semiconductor materials are used as sensitizers and low bandgap polymers act as hole transport materials. [PERSON] et al.