The highest efficiency solar cells based on copper zinc tin sulfide (CZTS), a promising photovoltaic material comprised of earth abundant elements, are built on soda lime glass (SLG), a substrate which contains many impurities, including Na and K. These impurities may diffuse into CZTS films during processing and affect film structure and properties. We have investigated the effects of these impurities on the microstructure of CZTS films synthesized by ex situ sulfidation of Cu-Zn-Sn alloy films co-sputtered on SLG, Pyrex, and quartz.CZTS films synthesized on SLG were found to have significantly larger grains than films grown on the other substrates. Furthermore, we show that by including a bare additional piece of SLG in the sulfidation ampoule, the grain size of films grown on nominally impurity-free quartz increases from 100's of nm to greater than 1 mm. This demonstrates conclusively that impurities in SLG volatilize in S-containing atmospheres and incorporate into nearby CZTS films synthesized on other substrates. Impurity concentrations in these CZTS films were examined using depth profiling with time-of-flight secondary ion mass spectrometry (TOF-SIMS). Of all the impurities present in SLG, the TOF-SIMS experiments implicated Na, K, and Ca as possible elements responsible for the enhanced grain growth. To investigate the effects of these impurities individually, we introduced very small and controllable amounts of Na, K, or Ca into the sulfidation ampoule during CZTS synthesis. Impurity amounts as low as 10 À6 moles of Na or 10 À7 moles of K resulted in a dramatic increase in grain size, from 100's of nm to several microns, for films deposited on quartz, while Ca loading had no visible effect on the final microstructure. Based on this vapor transport mechanism, we thus demonstrate an approach for delivering precisely controlled amounts of specific impurities into CZTS films on arbitrary substrates to facilitate large-grain growth. Broader contextState-of-the-art thin-lm solar cells use cadmium telluride or copper indium gallium selenide as the light absorbing material. However, the production of these solar cells may be limited to less than 1 terawatt by the scarcity of tellurium and indium, and possibly by the toxicity of cadmium. These sustainability concerns provide a strong motivation to search for new photovoltaic materials comprised of nontoxic and abundant elements. The rapid rise in power conversion efficiencies of solar cells based on copper zinc tin sulde and selenide (CZTS and CZTSe) attests to their potential as low-cost, earth-abundant solar absorber materials. One of the properties that affects the CZTS solar cell performance is the lm's microstructure. Ideally, a monolayer of single crystal grains with sizes on the order of the lm thickness (1-3 micrometers) is required. The present study shows the remarkable effects of the presence of very small amounts of alkali metal atom impurities in enhancing grain growth in CZTS thin lms. The article also presents an approach for delivering preci...
Atomic layer deposition (ALD) is a layer-by-layer synthesis method capable of depositing conformal thin films with thickness and compositional control on subnanometer length scales. While many materials have been synthesized by ALD, the technologically important metal sulfides are underexplored, and homogeneous quaternary metal sulfides are absent from the literature. We report an ALD process to synthesize Cu 2 ZnSnS 4 (CZTS), a potentially low cost semiconductor being explored for photovoltaic applications. Two strategies are reported: one in which a trilayer stack of binary metal sulfides (i.e., Cu 2 S, SnS 2 and ZnS) is deposited and mixed by thermal annealing, as well as a supercycle strategy that is similar to the conventional ALD procedure for forming nanolaminates. Both routes rely on the facile solid state diffusion of chalcogenides for mixing. For this ALD route to the CZTS system, the challenges are nucleation, ion-exchange between the film and the volatile chemical precursors, and phase-stability of binary SnS 2 . The thin films were made with no sulfurization step. The X-ray diffraction and Raman spectra were consistent with the formation of CZTS. X-ray fluorescence measurements revealed that the films contained the expected amount of sulfur based on the target oxidation states. Photoelectrochemical measurements under simulated AM1.5 illumination using Eu 3+ as an electron acceptor demonstrated that the films were photoactive and had an average internal quantum efficiency (IQE) of 12%.
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