Kesterite Cu 2 ZnSn(S,Se) 4 Absorbers Converted from Metastable, Wurtzite-Derived Cu 2 ZnSnS 4 Nanoparticles Wei-Chang Yang, †,§ Caleb K. Miskin, † Charles J. Hages, † Evan C. Hanley, † Carol Handwerker, § Eric A. Stach,* ,‡,§ and Rakesh Agrawal* ,† † School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, United States § School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States ‡ Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States * S Supporting Information ABSTRACT: Wurtzite-derived copper-zinc-tin sulfide nanoparticle films were observed to undergo a phase transformation to a kesterite phase when exposed to Se vapor at 500 °C. The resulting dense and selenized Cu 2 ZnSn(S,Se) 4 (CZTSSe) films were found to have the same bilayer kesterite structure as absorber layers derived directly from kesterite Cu 2 ZnSnS 4 (CZTS) nanoparticles (Guo, Q.; Ford, G. M.; Yang, W.-C.; Walker, B. C.; Stach, E. A.; Hillhouse, H. W.; Agrawal, R. J. Am. Chem. Soc. 2010, 132, 17384-17386). The top layer was fully sintered into micrometer size grains, while the bottom unsintered layer consisted of small, nanometer size kesterite grains. When compared to films formed from kesterite CZTS nanoparticles, solar cells fabricated from the wurtzite-derived CZTS nanoparticles were found to have lower power conversion efficiencies (PCE). Surprisingly, for those CZTSSe films that were formed from wurtzite-derived nanoparticles, it was found that extensive selenization leads to the disappearance of the bottom unsintered layer and the formation of a thin film composed of only micrometer-sized grains. These results have significant importance for the improvement of the performance of CZTSSe solar materials. Solar cells fabricated from kesterite nanoparticles have delivered a PCE of 9%despite the presence of an unsintered layer. These results indicate that the use of wurtzite-derived CZTS nanoparticles has the potential to remove the unsintered layer in kesterite CZTSSe solar cells (Miskin, C. K.; Yang, W.-C.; Hages, C. J.; Carter, N. J.; Joglekar, C. S.; Stach, E. A.; Agrawal, R. Prog. Photovoltaics: Res. Appl. 2014, DOI: 10.1002/pip.2472). ■ INTRODUCTION Copper-zinc-tin sulfide (CZTS) and its selenide version (CZTSe) are important materials for potentially low-cost thin film solar cells. Furthermore, Cu, Zn, and Sn are earth abundant elements that ensure a supply of materials for any foreseeable harvesting of solar energy. CZTSe and CZTS devices have shown a power conversion efficiency (PCE) of 9.15% and 8.4% using coevaporation. 3,4 Kesterite-structure CZTSSe thin films prepared using hydrazine-based solution processing have achieved an 12.6% PCE. 5 The kesterite CZTSSe devices made by selenizing kesterite CZTS nanoparticle inks have attained a PCE of 9%, and this has been improved to 9.4% by partially doping Sn with Ge. 2,6,7 The commonality shared by the above-mentioned high-PCE CZTSSe solar cells is they all possess a kesterite-structure absorber. First-principles calculations suggest that the kesterite structurealong with cation-disorder within the Cu-Zn layeris the most stable crystallographic structure within the CZTS phases derived from a binary II-VI zinc blende structure having ABCABC stacking. 8-10 A wurtzite-derived CZTS phase from the binary II-VI wurtzite structure, however, has been reported among the quaternary I 2 -II-IV-VI 4 semiconductors featuring ABABAB stacking. 11-13 Another study based on first- principles calculations has investigated the structural stability of wurtzite-derived CZTS phases in which the Cu, Zn, and Sn cations are ordered and have specific positions in the unit cells. 14,15 Although first-principles total-energy calculations of wurtzite-derived CZTS have indicated that the zinc-blende- derived kesterite and stannite are relatively more stable than the wurtzite-kesterite and wurtzite-stannite structures, the small difference in the total energy between kesterite and wurtizte- kesterite cannot rule out the possible formation of wurtzite- derived CZTS. 14 Both wurtzite-derived CZTS and CZTSe have been demonstrated experimentally in the forms of nanocrystals and nanorods. 16-20 In addition to the wurtzite-kesterite and wurtzite-stannite structures, another crystallographic structure in which the metal cations are randomly distributed at the Zn sites of a wurtzite ZnS unit cell has been proposed. 16 The presence of organic solvent or capping ligands has been suggested to influence the crystallographic structure of nanoparticles. 21 With a careful choice of organic solvents, CZTS nanoparticles can be synthesized as a metastable Received: March 28, 2014 Revised: May 6, 2014 Article pubs.acs.org/cm © XXXX American Chemical Society A dx.doi.org/10.1021/cm501111z | Chem. Mater. XXXX, XXX, XXX-XXX