Deposition of SnS Thin Films from Sn(II) Thioamidate Precursors

Abstract

Sunlight is the ultimate source of energy on Earth. Although the photovoltaic (PV) effect remains the most promising means to convert sunlight directly to useable electricity, current devices are dominated by crystalline silicon solar cells. While a mature technology, the indirect band gap of silicon means that the light absorption coefficient is low (102 cm-1) and thick layers (100-500 μm) are needed to absorb the requisite number of photons to produce the necessary efficiencies. The cost associated with the production of the thick layers of silicon needed to meet the increasing supply demand is, thus, very high. To date, the supply of sufficient high purity silicon has been economical due to the electronics industry, which produces large amounts of high purity silicon waste. This economic benefit will start to be eroded, however, should the silicon solar sector to continue to grow. To this end, materials containing post-transition metals with an ns 2 electronic configuration (e.g. Pb 2+ , Sn 2+ , Ge 2+ , Sb 3+ and Bi 3+) are attracting significant recent attention for their solar absorber ability. 1 A case in point is provided by tin(II) monosulfide (SnS), which possesses a high absorption coefficient (>104 cm-1) and a near ideal direct band gap of ca. 1.3 eV. 2 Although these properties, in conjunction with the high relative abundance of both tin and sulfur, denote SnS as an attractive candidate photovoltaic absorber, the efficiency of SnS-based devices has yet to reach 5% indicating that significant further advances in materials processing and thin film deposition methods will be required if this promise is to be fulfilled. 3-5 SnS thin films have been deposited by ALD, 6, 7 spray pyrolysis, 8-13 sputtering, 14-16 chemical bath deposition, 17-29 vacuum evaporation, 30-35 and CVD. 11, 36-49 In this latter regard, an initial report by Price et al. described the use of atmospheric pressure (AP) CVD to deposit a variety of tin sulfide stoichiometries from SnCl4 and H2S in the temperature range 300-545 °C, 36, 38 albeit SnS was only obtained at 545 °C. Similarly, (fluoroalkylthiolato)tin(IV) and organotin(IV) dithiocarbamates have been reported to provided SnS under APCVD conditions with the addition of H2S. 39, 44, 46 The tin thiolate precursor, (PhS)4Sn, has also allowed SnS deposition in the temperature range 350-500 ºC both with and without the presence of H2S by aerosol-assisted (AA) CVD. 40, 41 All the deposited films were amorphous, although Raman spectroscopy and EDX analysis confirmed the presence of tin sulfide. O'Brien and co-workers subsequently described a range of organo tin(IV) and Sn(II) dithiocarbamates, for example the diethyldithiocarbamate derivatives 1 and 2, which were suitable

Tin guanidinato complexes: oxidative control of Sn, SnS, SnSe and SnTe thin film deposition

“…In the case of compounds 6 and 9, microcrystalline powders were isolated. In all cases the products were characterised by solution state NMR ( 1 H, 13 C and 119 Sn) spectroscopy and elemental analysis.…”

“…6 As with previous complexes in this series, the nitrogen atoms within the guanidinate ligands experience some degree of pyramidalisation: in the case of complex 16 it is the axial nitrogen atoms N (2) Reaction of both 6 and 7, separately, with elemental Te in THF over 3 days at 70°C, followed by filtration and recrystallisation, results in the formation of large cubic red crystals in yields of 72% and 86% respectively. Analysis of the isolated materials by 1 H, 13 C, 119 Sn NMR spectroscopy and elemental analysis indicate the formation of mono-telluro complexes, 38,46 Single crystal X-ray diffraction studies confirm the exclusive formation of the five coordinate systems 17 and 18, which crystallise from toluene in the monoclinic space groups P2 1 /a and P2 1 /n respectively. The molecular structures of 17 and 18, shown in Fig.…”

“…Reaction of complex 6 with propylene sulfide results in the formation, and isolation, of yellow crystal of S 8 (identified by single crystal X-ray diffraction). The precise mechanism by which S 8 is formed is unclear, however, 1 H, 13 C and 119 Sn NMR spectroscopy of the resulting reaction mixture all suggest decomposition of the complex.…”

“…for C 34 3 J HH 7.93 Hz). 13 Synthesis of 12. 2.10 g (3.39 mmol) of compound 7 was dissolved in 50 ml of THF and 1.07 g (13.6 mmol) of elemental selenium powder was added to the solution and sonicated for 6 hours at 40°C.…”

“…We have recently reported the development of single source precursors, for the exclusive production of phase pure SnO 11,12 and SnS 10,13 materials respectively, which display unprecedented oxidation state control. These compounds provide the necessary kinetic control over the tin oxidation state through the design of ligand systems which decompose under only mild thermal stimulus.…”