Deposition of semiconductor thin films and nanoparticles by molecular precursors approach for solar cell applications

Abstract

Semiconductor materials in compound form have been extensively investigated in recent years as photoabsorber materials for thin film solar cells. The research work presented in this thesis reports the use of a number of dialkyldichalcogenophosphinatometal complexes as molecular precursors for deposition of binary, ternary and quaternary semiconductor thin films and nanoparticles. Several complexes belonging to diphenyldiselenophosphinatometal family with general formula [Mx(Ph2PSe2)y] (Where M = Cu, In, Ga, Zn, Sn Pb and Fe) have been synthesized in high yield by a new efficient and reproducible method. Similarly, a number of diisobutyldithiophosphinatometal complexes [Mx(iBu2PS2)y] (Where M = Cu, In, Ga, Zn, Sn, Pb, and Fe) have been synthesized by a facile and reproducible approach utilizing commercially available ligand. The assynthesized complexes have been characterized using mass spectrometry, NMR (1H and 13C) spectroscopy, FTIR spectroscopy and elemental analysis. Thermogravimetric analyses were carried out to study the degradation of these complexes under the influence of temperature. These complexes have been used as single source precursors (SSPs) for deposition of binary metal selenide (Cu2-xSe, In2Se3, ZnSe, and PbSe) and metal sulfide (Cu2-xS, In2S3, ZnS and PbS) thin films by aerosol-assisted chemical vapour deposition (AACVD). Similarly, thin films of ternary semiconductor materials (CuInSe2, CuGaSe2, Cu2SnSe3, CuFeSe2, CuInS2, CuGaS2, Cu2SnS3, CuFeS2) as well as quaternary materials (CuIn1- xGaxSe2, CuIn1-xGaxS2, Cu2ZnSnSe4 and Cu2ZnSnS4) have been deposited using suitable molar combinations of these molecular precursors in AACVD experiments. Depositions were carried out at four different temperatures (350, 400, 450 and 500 °C) to study the influence of deposition temperature on morphology, mean diameter, stoichiometry and crystallographic phase of the deposited material. Similarly, parametric studies were also undertaken to investigate the effect of solvent, precursor concentration and carrier gas flow rate on quality of the deposited thin films. Characterization of the thin films was carried out by powder X-ray diffraction (p-XRD) studies, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and atomic force microscopy (AFM). Significant variation of grain size, shape, stoichiometry and phase structure was observed by varying the deposition parameters, particularly the deposition temperature. In short, a range of materials has been deposited by AACVD, with good control over properties of the material like crystallographic phase, stoichiometry and morphology of the crystallites. Keeping in view the recent trend of using semiconductor nanocrystals as solar inks for deposition of photoabsorber layer in solar cells, diphenyldiselenophosphinatometal and diisobutyldithiophosphinatometal complexes have also been used for colloidal preparation of binary, ternary, and quaternary semiconductor nanoparticles. Effect of various nanoparticles growth parameters like growth temperature, reaction duration and precursors concentration was investigated in detail. Nanocrystals were characterized by p-XRD studies, transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Band gaps of the asgrown nanocrystals were determined by using Tauc plots drawn from UV-Vis-NIR absorption data. It was demonstrated that mean diameter, morphology and band gap of the nanocrystals could be controlled by judicious choice of the growth parameters. Furthermore, stoichiometry of the as-grown nanoparticles could also be controlled by suitably adjusting the molar ratios of molecular precursors used in the reaction. Good quality, phase pure and monodispersed nanoparticles have been prepared by thermolytic degradation of these dialkyldichalcogenophosphinatometal precursors. In conclusion, a comprehensive study on preparation of a wide range of semiconductor nanoparticles, by using molecular precursors approach, has been carried out. These nanoparticles may potentially be used as solar inks, thus providing an attractive alternative route for deposition of photoabsorber layer in thin film solar cells. The upshot of this novel study is that a facile and effective alternate route for deposition of wide ranging semiconductor thin films and nanoparticles, using dialkyldichalcogenophosphinato-metal precursors, has been developed. This molecular precursor route offers the possibility for judicious tuning of material properties for their optimal utilization in solar cell applications, thus providing a viable solution of energy shortage through a renewable route.