SnS thin films were prepared using automated chemical spray pyrolysis (CSP) technique. Single-phase, p-type, stoichiometric, SnS films with direct band gap of 1.33 eV and having very high absorption coefficient (N 10 5 /cm) were deposited at substrate temperature of 375°C. The role of substrate temperature in determining the optoelectronic and structural properties of SnS films was established and concentration ratios of anionic and cationic precursor solutions were optimized. n-type SnS samples were also prepared using CSP technique at the same substrate temperature of 375°C, which facilitates sequential deposition of SnS homojunction. A comprehensive analysis of both types of films was done using x-ray diffraction, energy dispersive x-ray analysis, scanning electron microscopy, atomic force microscopy, optical absorption and electrical measurements. Deposition temperatures required for growth of other binary sulfide phases of tin such as SnS 2 , Sn 2 S 3 were also determined.
Exploiting the potential of the material for the photovoltaic applications requires an extensive defect level analysis, mandatory. Photoluminescence (PL) technique was employed to probe the defect levels in p-SnS thin films deposited using chemical spray pyrolysis (CSP) technique. Three PL emissions were recorded at 1.09, 0.76, and 0.75 eV. Systematic investigations performed, focusing the 1.09 eV emission, revealed that the shoulder at 1.093 eV gets completely quenched beyond 110 K. From this study, we could identify a bound exciton associated to a shallow donor level whose activation energy was calculated to be 20 meV from Arrhenius plot. By studying the variation of PL intensity with excitation power, we could zeroin that the emission at 1.09 eV was a donor-acceptor pair (DAP) transition. Knowing the band gap to be 1.33 eV, we could identify a deep acceptor at 0.22 eV above valence band. The band structure deduced from the present analysis is depicted in the abstract figure.
The origin of various defect levels in the SnS thin films deposited using chemical spray pyrolysis (CSP) technique has been explored in this manuscript, by employing low‐temperature photoluminescence (PL) technique. Concentration of Sn in the samples was varied purposefully by ex situ diffusion in order to alter the defect levels. The acceptor level obtained at 0.22 eV from the Arrhenius plot, has been assigned as the defect level caused by the Sn vacancies present in the lattice. Two shallow donor levels are conclusively identified and their activation energies have been estimated. The present study could also unearth a trap level in the forbidden energy gap which was due to the oxygen contaminant occupied by the vacancy of Sn. This trap level could be removed by annealing the sample in vacuum or through the ex situ diffusion of Sn. Employing Kelvin probe force microscopy (KPFM), the work‐function of SnS was obtained as 4.925 eV, from which the position of the Fermi level could be assigned. Based on the present work, an energy level scheme for SnS thin films is proposed outlying origin of various defect levels.
Sn atoms were thermally diffused into tin (II) sulfide (SnS) films prepared using chemical spray pyrolysis. This was achieved by depositing a layer of Sn metal over SnS films followed by annealing of the Sn/SnS bilayer films at 100 °C in high vacuum for 30 min. There was no contamination due to the formation of additional phases of Sn compounds up to a very high percentage of Sn diffusion. Contamination due to Sn–O–S phase was removed by Sn diffusion. The samples were optimized to achieve higher photosensitivity and low resistivity. All these enhanced properties were obtained without altering the optimum band gap of the SnS film which is suitable for maximum photovoltaic conversion efficiency.
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