Copper zinc tin sulfide (Cu2ZnSnS4 or CZTS) is a potential candidate for next generation thin film solar cells because it contains abundant and nontoxic elements and exhibits high light absorption. Thin films of CZTS are typically synthesized by sulfidizing a stack of zinc, copper, and tin films. In addition to CZTS, a variety of binary and ternary metal sulfides can form and distinguishing among phases with similar crystal structure can be difficult. Herein, the authors show that confocal Raman spectroscopy and imaging can distinguish between CZTS and the other binary and ternary sulfides. Specifically, Raman spectroscopy was used to detect and distinguish between CZTS (338 cm-1), Cu2SnS3 (298 cm-1), and Cu4SnS4 (318 cm-1) phases through their characteristic scattering peaks. Confocal Raman spectroscopy was then used to image the distribution of coexisting phases and is demonstrated to be a useful tool for examining the heterogeneity of CZTS films. The authors show that, during sulfidation of a zinc/copper/tin film stack, ternary sulfides of copper and tin, such as Cu2SnS3 form first and are then converted to CZTS. The reason for formation of Cu2SnS3 as an intermediary to CZTS is the strong tendency of copper and tin to form intermetallic alloys upon evaporation. These alloys sulfidize and form copper tin sulfides first, and then eventually convert to CZTS in the presence of zinc. As a consequence, films sulfidized for 8 h at 400C contain both CZTS and Cu2SnS3, whereas films sulfidized at 500C contain nearly phase-pure CZTS. In addition, using Cu K radiation, the authors identify three CZTS X-ray diffraction peaks at 37.1 [(202)], 38 [(211)], and 44.9 [(105) and (213)], which are absent in ZnS and very weak in Cu2SnS3.
|Original language||English (US)|
|Journal||Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films|
|State||Published - Sep 2011|
Bibliographical noteFunding Information:
This work was supported partially by the National Science Foundation (NSF) under Award No. CBET-0931145, partially by the Initiative for Renewable Energy & the Environment, IREE (RL-0004-11) and the MRSEC Program of the National Science Foundation under Award No. DMR-0819885. Parts of this work were carried out in the Nanofabrication Center, which receives partial support from NSF through the NNIN program. Part of this work was also carried out in the University of Minnesota Characterization Facility, a member of the NSF-supported Material Research Facilities Network.