The defect-related levels of n-type CdS/p-type SnS layers with thermal annealing were investigated through photoluminescence (PL) measurement. The SnS layer exhibited PL band at 1.13 eV. Based on the PL spectra observed at various excitation intensities and temperatures, PL peaks were attributed to donor to valence band transitions. Activation energy of donors in SnS was estimated to be 46 meV. The donor corresponding to the activation energy was due to interstitial Cadmium (Cd i ). Based on these data, a schematic diagram of several defects was created for post-annealed CdS/SnS.

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The effects of Ag in (Cu1–x Ag x )2SnS3 (CATS) were investigated in terms of photoluminescence (PL) lifetime, carrier concentration, and solar cell performance with varying Ag/(Cu + Ag) ratios, x. The PL lifetime of CATS solar cells had a maximum value when x was within 7%. A similar tendency was observed for other parameters of the CATS thin films and solar cells. One of the reasons for this is that the number of non-radiative recombination centers in the CATS layer is suppressed by the sulfurization process with Ag. Therefore, even if the Ag content changes slightly in the 0%–11% range, the crystal quality is changed in CATS, resulting in a significant effect on the properties of solar cells. These investigations on the characterization and device physics can be applied to improve the efficiency of CATS solar cells.

The synthesis of cubic‐phase SnS thin films using a sulfurization technique, in particular, the relationship between sulfurization conditions and extra phase contamination such as orthorhombic SnS, Sn2S3, or SnS2 is investigated. In case of SnS films sulfurized for 70–90 min, both cubic and orthorhombic SnS‐related diffractions are observed. Sulfurization time longer than 120 min, orthorhombic SnS and SnS2‐related diffractions are observed because the S vapor reacts with the SnS layer and forms a stable extra phase as SnS2. A single phase of cubic SnS is obtained via 18%‐HCl‐solution etching for 90 min without heating. Since the formation energy of cubic SnS is the same as that of orthorhombic SnS, obtaining single‐phase cubic SnS is difficult by optimizing a sulfurization and/or post‐thermal process. Moreover, because the band discontinuity of CdS/cubic SnS speculates a TYPE‐II band diagram, highly efficient SnS solar cells cannot be realized using CdS as an n‐type layer.