(809e) Enhanced Grain Growth in Cu2ZnSnS4 Thin Films Via Vapor Transport of Alkali Metal Impurities

While power conversion efficiencies of Cu₂ZnSnS₄ (CZTS) based solar cells have increased relatively rapidly, very little research has been focused on the effects of the substrate on CZTS films. To date, the best performing CZTS solar cells have been deposited on Mo-coated soda lime glass (SLG), a carryover from Cu(In_xGa_1-x)Se₂ (CIGS) solar cells where Na diffusion from the SLG into the CIGS layer enhances the power conversion efficiency. Impurity diffusion is also expected when CZTS is deposited on Mo-coated SLG, but SLG hosts many other impurities including K, Ca, Mg, and Al and a systematic investigation of whether these impurities diffuse into CZTS and what effect they may have on the film properties has not been conducted.  To this end, we have investigated the effects of these impurities on the microstructure of CZTS films. Thin CZTS films were synthesized via ex situ sulfidation of Cu-Zn-Sn films co-sputtered on a variety of substrates, including, SLG, quartz, and Pyrex. The metallic precursor films were loaded into a quartz ampoule with 1 mg of S, evacuated to 10^-6 Torr, sealed and sulfidized at 600 ^oC for 8 hours. The sulfidized films were then characterized using X-ray diffraction, Raman spectroscopy and scanning electron microscopy. Concentration depth profiles were examined using time-of-flight secondary ion mass spectrometry (TOF-SIMS). CZTS films synthesized on SLG have significantly larger grains than films grown on other substrates. We have conducted experiments where identical Cu-Zn-Sn precursor films on quartz were sulfidized both in the absence and in the presence of a bare additional piece of SLG in the sulfidation tube. Remarkably, the grain sizes (>1 μm) in CZTS films sulfidized with SLG present are dramatically larger than the grain sizes (100’s of nm) in CZTS films sulfidized without SLG. This demonstrates conclusively that impurities in SLG volatilize in a S atmosphere and incorporate into nearby CZTS films synthesized on a quartz substrate. Of all the impurities present in SLG, the TOF-SIMS experiments implicated Na, K and Ca as possible elements responsible for enhanced grain growth. To investigate the effects of these impurities individually, we then introduced very small and controllable amounts of Na, K, or Ca into the ampoule during sulfidation of the CZTS. Impurity amounts as low as 10^-6 moles of Na or 10^-7 moles of K loaded into the sulfidation tube resulted in a dramatic increase in grain size for films deposited on quartz, while Ca loading had little effect on the final microstructure. The results presented will demonstrate that subtle changes in impurity types and compositions lead to drastic changes in CZTS films.