Optical properties of reactively sputtered Cu 2 ZnSnS 4 solar absorbers determined by spectroscopic ellipsometry and spectrophotometry Shu-Yi Li n , Carl Hägglund, Yi Ren, Jonathan J.S. Scragg, Jes K. Larsen, Christopher Frisk, Katharina Rudisch, Sven Englund, Charlotte Platzer-Björkman Ångström Solar Center, Solid State Electronics, Uppsala University, Sweden article info Article history: Received 27 October 2015 Received in revised form 11 January 2016 Accepted 12 January 2016 Keywords: Ellipsometry Cu 2 ZnSnS 4 Optical properties Bandgap Absorption coefficient Thin film solar cell abstract We have determined for the first time the device-relevant optical constants of 500 nm and 800 nm-thick Cu 2 ZnSnS 4 absorbers, grown on bare and Mo-coated soda-lime glass (SLG), using spectroscopic ellip- sometry (SE). The composition, structure, phase purity and morphology were characterized by X-ray fluorescence, X-ray photoelectron spectroscopy depth profiling, X-ray diffraction, Raman spectroscopy, scanning-electron microscopy and atomic force microscopy. For the SE analysis, carefully determined sample characteristics were utilized to build a multilayer stack optical model, in order to derive the dielectric functions and refractive indices. The SE-derived absorption coefficients from CZTS/SLG samples were compared with those derived from complementary spectrophotometry measurements and found to be in good agreement. The bandgap determined from Tauc plots was E g ¼1.57 70.02 eV. The absorption coefficients just above the bandgap were found to be a few 10 4 cm 1 and to exceed 10 5 cm 1 at energies above 2.5 eV, which is much higher than previously found. The sub-bandgap k-value was found to be k 0.05 or less, suggesting that a moderate band tail is present. Separate device character- ization performed on identical samples allowed us to assign device efficiencies of, respectively, 2.8% and 5.3% to the 500 nm and 800 nm-thick samples featured in this study. & 2016 Elsevier B.V. All rights reserved. 1. Introduction The quaternary compound kesterite Cu 2 ZnSnS 4 (CZTS) is a pro- mising earth-abundant choice for thin film solar cell applications [1], owing to its direct bandgap of optimal size  1.5 eV [2–5], its high absorption coefficient 410 4 cm 1 in the visible region [2,3], as well as crystallographic and electronic structures similar to that of high-efficiency Cu(In,Ga)Se 2 [6] based absorbers. Since the first 0.66%-efficient CZTS solar cell device made by Katagiri et al. in 1996, great progress was seen over the years [1,7]. The current efficiency records are  9.2% for CZTS [8] and  12.6% for its close relative CZTSSe [9]. However, more improvements are needed for CZTS to reach above  15% in order to satisfy the requirements for practical applications. To understand the loss mechanisms and further improve the efficiency, electrical and optical device modeling [10,11] are needed. High-quality wavelength-dependent optical constants that accurately represent the device-relevant CZTS material are in demand as a crucial and a fundamental input if correct conclusions are to be drawn from such analyses. In general, optical constants, being fundamental material properties, are worth being repeatedly determined for an important emerging material like CZTS. So far, the existing literature ellipsometry data for the pure sulfide CZTS are limited [12–14] and have been deficient in one way or another: sample composition and description of surface preparation methods were sometimes lacking; the importance of careful material characterization was frequently overlooked; pos- sible influences from secondary phases were often neglected; the analyzed absorbers were rarely grown in the same way or on the same substrates as those for devices; none have compared ellip- sometry data with that from transmittance and reflectance mea- surements in the same study even though their transparent sub- strates permit such comparison. Furthermore, there is a general lack of comparable device performance values provided in the same study despite the generally observed spread in performance from different CZTS processing routes or even between nominally identical processes. To reach accurate conclusions from the com- plex optical analysis, careful complementary characterization of the studied samples is key – as demonstrated by Ahn et al. in their detective work that settled the controversy over the bandgap value of CZTSe [15]. In this work, we attempt to rectify these oversights, providing much sought-after ellipsometry data deter- mined from device-relevant material. We analyzed  500 nm and  800 nm thick Cu-poor and Zn-rich CZTS absorbers grown on Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2016.01.014 0927-0248/& 2016 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: Shuyi.Li@angstrom.uu.se (S.-Y. Li). Solar Energy Materials & Solar Cells 149 (2016) 170–178