Synthesis and Characterization of SbSe Thin Films for Photovoltaic Applications

Abstract

In this thesis study, synthesis and characterization of an alternative absorber, antimony selenide (Sb2Se3), layer for photovoltaic applications have been investigated. Synthesis of the Sb2Se3 thin films has been carried out in a two-step process where deposition of Sb metallic precursor film by RF magnetron sputtering on soda-lime glass (SLG) and molybdenum (Mo) coated SLG (SLG/Mo) substrates and a subsequent selenization of the deposited precursor films in a single zone tubular furnace are involved. Upon optimization of the sputtering conditions for the Sb growth, the effects of precursor thickness, selenization temperature, selenization time on the morphology and the structure of the synthesized Sb2Se3 thin films have been investigated by various techniques. Based on XRD and Raman analyzes, it has been understood that for a phase-pure complete selenization, the optimum selenization time is 150 min and the optimum selenization temperature is 250℃ for ~500 nm thick Sb precursor on SLG. However, for the same thickness Sb film on SLG/Mo, selenization temperature is kept constant but, optimum selenization time is needed to increase to 180 min. Raman spectra and XRD patterns of these samples have completely matched with orthorhombic Sb2Se3 structure. ii Although surface morphologies of all the thick samples investigated by SEM have revealed in plane arrangement of rod-like Sb2Se3 grains, cross-sectional image of the phase pure thick sample by FIB-SEM unveiled the fact that the rod-like morphology is only at the very surface and does not extend into the depth of the sample. Moreover, it has been observed that the thickness of the sample has increased excessively from 500 nm of the precursor to 790 nm. Since a thick absorber layer reduces the transportation of charge carriers to counter electrodes, the precursor thickness is decided to be reduced to 250 nm and the selenization conditions have been optimized, accordingly. The optimum selenization time for 250 nm Sb on SLG is determined to be 150 min and the optimum selenization temperature is 250℃ by investigating the XRD patterns. According to XRD and Raman analysis, selenization time and temperature have been optimized as 150 min and 250 ℃ for 250 nm thick Sb thin film deposited on SLG/Mo. Surface morphologies of thin Sb2Se3 samples via SEM have shown vertical arrangement of rod-like grains as opposed to those of thick Sb2Se3 samples. As expected, the variation of the surface arrangement has reflected itself on the XRD patterns as the inverse changes of I(211)/I(002) and I(221)/I(002) peak ratios. It has been determined that there is no impurity phase in fully selenized samples on SLG substrate and the increasing atomic concentration of Mo also matches with the decreasing atomic concentration of the Sb and Se elements at depth profiles of fully selenized samples on SLG/Mo substrate by XPS. Sb:Se atomic ratio obtained by EDS for the completely selenized 250 nm Sb thin film on SLG/Mo is in close agreement with that of Sb2Se3. Optical behavior of the fully selenized thin and thick samples have been determined by photoluminescence (PL) spectroscopy and PL spectra of these samples have commonly acquired a sharp interband transition peak at 1.53 eV. Therefore, we have thoroughly determined the proper Sb precursor film thickness and the selenization conditions in a two-step synthesis process for obtaining the desired Sb2Se3 structure on SLG/Mo substrates to be utilized in solar cell architecture.