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Nanoscale Surface Potential Variation Correlates with Local S/Se Ratio in Solution-Processed CZTSSe Solar Cells Michael Salvador, Sarah M. Vorpahl, Hao Xin, Wesley Williamson, Guozheng Shao, Durmus U. Karatay, Hugh W. Hillhouse, and David S Ginger Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl503068h • Publication Date (Web): 05 Nov 2014 Downloaded from http://pubs.acs.org on November 9, 2014
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Nano Letters
Nanoscale Surface Potential Variation Correlates with Local S/Se Ratio in SolutionProcessed CZTSSe Solar Cells Michael Salvadora†, Sarah M. Vorpahla†, Hao Xinb, Wesley Williamsonb, Guozheng Shaoa, Durmus U. Karataya, Hugh W. Hillhouseb, David S. Gingera,* a
Department of Chemistry, University of Washington, Seattle, Washington 98195-1700.
b
Molecular Engineering & Science Institute, University of Washington, Seattle,
Washington, 98195-1652.
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ABSTRACT Thin film solar cells made from Cu, Zn, Sn, S/Se can be processed from solution to yield high-performing
kesterite
(CZTS
or
CZTSSe)
photovoltaics.
We
present
a
microstructural study of solution-deposited CZTSSe films prepared by nanocrystal-based ink approaches using scanning probe microscopy (SPM) and scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). We correlate scanning Kelvin probe microscopy (SKPM) maps of local surface potential with SEM/EDS images of the exact same regions of the film, allowing us to relate observed variations in surface potential to local variations in stoichiometry. Specifically, we find a correlation between surface potential and the S/(S+Se) composition ratio. In particular, we find that regions with high S/(S+Se) ratios are often associated with regions of more negative surface potential and thus higher work function. The change in work function is larger than the expected change in the valence band position with these small changes in sulfur, and thus the data suggest an increase in acceptor-like defects with increasing sulfur. These findings provide new experimental insight into the microscopic relationships between composition, structure and electronic properties in these promising photovoltaic materials. KEYWORDS: (CZTSSe, kesterite solar cells, surface potential, defects, scanning probe microscopy, scanning electron microscopy)
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Nano Letters
Cu2ZnSn(Sx,Se1 − x)4 (CZTSSe) is a kesterite-type pentenary compound semiconductor with a direct band gap suitable for solar cell applications. One key attraction is that the component elements are all considered Earth-abundant, non-toxic materials. In addition, the band gap can be tuned from ≈1.45 to 0.96 eV by continuously replacing sulfur with selenium, which allows for matching the band gap to the solar spectrum.1-3 Although kesterite-related photovoltaics have been known since the late 1980’s,4 this material has regained attention through the development of solution-based methods for fabrication of the active CZTSSe layer.5-8 This technological achievement may eventually form the basis for commercially viable low-cost, large-area solar cell devices that could be printed and offer versatility in form factors.9,10 One of the main limitations of CZTSSe solar cells is the widely observed deficiency in open-circuit voltage (VOC ) compared to the band gap.11 While the shortcircuit current for CZTSSe can reach values of 37 mA/cm2, close to the theoretical limit and comparable to what is typically observed in the case of CIGSe, both the fill factor (FF), typically
