Discrepancy between integral and local composition in off-stoichiometric Cu2ZnSnSe4 kesterites: a pitfall for classification

High-efficiency kesterite-based thin film solar cells typically feature Cu-poor, Zn-rich absorbers although secondary phases occur easily in non-stoichiometric CuZnSnSe. We therefore applied high-resolution X-ray fluorescence analysis using a synchrotron nanobeam to study the local composition of a CZTSe cross section lamella cut from a sample with an integral composition of Zn/Sn = 1.37 and Cu/(Zn+Sn) = 0.55. We find submicrometer-sized ZnSe-, SnSe/SnSe-, and even CuSe/CuSe-like secondary phases, while the local compositions of the kesterite are highly Zn-rich yet barely Cu-poor with 1.5 ≤ Zn/Sn ≤ 2.2 and Cu/(Zn+Sn) ∼ 1.0. Consequently, great care must be taken when relating the integral composition to other material properties including the device performance.This work was funded by ESRF, DFG (Deutsche Forschungsgemeinschaft) under Grant No. SCHN 1283/2-1, BMBF (Bundesministerium für Bildung und Forschung) under the “nano@work” project (Grant No. 05 K16SJ1), KESTCELLS 316488, FP7-PEOPLE-2012 ITN, Multi-ITN project, and MINECO (Ministerio de Economía y Competitividad de España) under the NASCENT Project (ENE2014-56237-C4-1-R). S.G. thanks the Government of Spain for the FPI fellowship (BES-2014-068533) and E.S. for the “Ramón y Cajal” fellowship (RYC-2011-09212)