Investigating the effect of steric hindrance within CdS single-source precursors on the material properties of AACVD and spin coat-deposited CdS thin films

Cadmium sulfide (CdS) is an important semiconductor for electronic and photovoltaic applications, particularly when utilised as a thin film for window layers in CdTe solar cells. Deposition of thin film CdS through the decomposition of single source precursors is an attractive approach due to the facile, low-temperature and rapid nature of this approach. Tailoring the precursor to affect the decomposition properties is commonly employed to tune desirable temperatures of decomposition. However, altering the precursor structure and the effect this has on the nature of the deposited material is an area far less commonly investigated. Here we seek to investigate this by altering the ligands around the Cd metal centre to increase the steric hinderance of the precursor and investigate the effect this has on the decomposition properties and the properties of deposited thin film CdS from these precursors. For this, we report the synthesis of four CdS precursors with xanthate and pyridyl ligands ([Cd(n-ethylxanthate)2(3-methylpyridine)2] [1], [Cd(n-ethylxanthate)2(3,5-lutidine)2] [2], [(Cd2(isopropylxanthate)4(3-methylpyridine)2)n] [3], [Cd(isopropylxanthate)2(3,5-lutidine)2] [4]). These single source precursors for CdS were fully characterised by elemental analysis, NMR spectroscopy, single crystal XRD and thermogravimetric analysis. It was found that even with subtle alterations in the xanthate (n-ethyl to isopropyl) and pyridine (3-methyl and 3,5-dimethyl) ligands, a range of hexa-coordinate precursors were formed (two with cis configuration, one with trans configuration and one as a 1D polymer). These four precursors were then used in AACVD and spin coating experiments to deposit eight thin films of CdS, which were characterised by Raman spectroscopy, powder x-ray diffraction and scanning electron microscopy. Comparative quantitative information concerning film thickness and surface roughness was also determined by atomic force microscopy. Finally, the optical properties of all thin films were characterised by UV-Vis absorption spectroscopy, from which the band gap of each deposited film was determined to be commensurate with that for bulk CdS (ca. 2.4 eV).