Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface

ARTICLE https://doi.org/10.1038/s41467-020-17434-8 OPEN 1234567890():,; Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface Diego Colombara 1,2,3,9 ✉, Hossam Elanzeery 1,4,9 ✉, Nicoleta Nicoara 2, Deepanjan Sharma2, Marcel Claro 2, Torsten Schwarz5, Anna Koprek5, Max Hilaire Wolter1, Michele Melchiorre 1, Mohit Sood 1, Nathalie Valle6, Oleksandr Bondarchuk 2, Finn Babbe 1,7, Conrad Spindler1, Oana Cojocaru-Miredin5,8, Dierk Raabe5, Phillip J. Dale 1, Sascha Sadewasser 2 & Susanne Siebentritt1 The electrical and optoelectronic properties of materials are determined by the chemical potentials of their constituents. The relative density of point defects is thus controlled, allowing to craft microstructure, trap densities and doping levels. Here, we show that the chemical potentials of chalcogenide materials near the edge of their existence region are not only determined during growth but also at room temperature by post-processing. In particular, we study the generation of anion vacancies, which are critical defects in chalcogenide semiconductors and topological insulators. The example of CuInSe2 photovoltaic semiconductor reveals that single phase material crosses the phase boundary and forms surface secondary phases upon oxidation, thereby creating anion vacancies. The arising metastable point defect population explains a common root cause of performance losses. This study shows how selective defect annihilation is attained with tailored chemical treatments that mitigate anion vacancy formation and improve the performance of CuInSe2 solar cells. 1 Physics and Materials Science Research Unit, University of Luxembourg, Belvaux L-4422, Luxembourg. 2 International Iberian Nanotechnology Laboratory, Av. Mestre Jose Veiga, Braga 4715-330, Portugal. 3 Università degli Studi di Genova, via Dodecaneso 31, Genova 16146, Italy. 4 Avancis, Otto-Hahn-Ring 6, 81739 München, Germany. 5 Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Strasse 1, 40237 Düsseldorf, Germany. 6 Luxembourg Institute of Science and Technology, Belvaux L-4422, Luxembourg. 7 Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, 94720 Berkeley, CA, USA. 8 Institute of Physics, RWTH Aachen University, Sommerfeldstrasse 14, 52062 Aachen, Germany. 9These authors contributed equally: Diego Colombara, Hossam Elanzeery. ✉email: ; NATURE COMMUNICATIONS | (2020)11:3634 | https://doi.org/10.1038/s41467-020-17434-8 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17434-8 A b Sphalerite CuInSe2 700 400 CuIn3Se5 600 β ODC m CIS-P CIS-S In2Se3 Cu-poor G.B. 400  25 30 22 Cu2Se 23 m,poor CIS-P Cooling CIS-R Cu2Se 200 100 °C Cu (at. %) InCu VCu 2 μm Cu2Se + CuInSe2 300 20 CIS-R 300 100  m,poor 15 c 500 800 500 °C 530 °C 24 Cu (at. %) m,rich 25 0 26 CuIn Cui m,rich ? d CIS-P DA1 Cu-rich  900 Temperature (°C) Liquid DA2 1000 Cu-poor CIS-R 0.80 0.90 EX a DA3 1110 Here, the case is made for CuInSe2 (CIS), a 3D chalcogenide belonging to the adamantine family of materials with a wide single-phase existence region (Fig. 1a)7,8. CIS is a suitable proxy for the successful Cu(In,Ga)(S,Se)2 (CIGS) PV technology and will become increasingly important for the third-generation concepts, e.g., future tandem cells9 in combination with widergap chalcopyrites or halide perovskites10,11. Realizing today’s highly efficient CIGS, solar cells require a carefully conceived fabrication process, one that derives from a decades-long research endeavour12. One key innovation in CIGS fabrication was enabled in the 1990s by crossing the phase homogeneity boundary during growth from ‘Cu-poor’ to ‘Curich’, then back again to Cu-poor compositions (known as the three-stage process)13. The strategy received widespread success, because it allows combining the superior microstructure of Cu-rich material (cf. cross-sections in Fig. 1b) and the superior performance typical for Cu-poor compositions. However, besides showing larger grains, Cu-rich CIS displays a more ideal luminescence signature than Cu-poor CIS, a prerequisite for high efficiency potential (Fig. 1d)14. This fact has puzzled the community for a long time, because it is at odds with the worse performance of Cu-rich CIS devices. It is well known that during CIGS solar cell manufacturing, exposure of the absorber surface to ambient air15, alkali metal fluorides16,17 or chemical etchants18,19 before buffer deposition influences heavily the device’s optoelectronic properties. It is likely, but has never been investigated, that CIGS grown under different conditions displays different resilience to point defect formation during and after growth. It is unknown how the altered defect populations influence the PV performance in commonly processed CIGS absorbers from this fundamental viewpoint. Here we identify the root causes of chemical instability of CIS when crossing the edge of its existence region and relate them to device’s performance losses. Three thin-film compositions with increasing Cu concentration are investigated as bare absorbers, after surface treatments and as PV cells. These are identified Offset normalised PL flux density (arb.u.) toms in a crystalline structure align in a regular lattice, but due to off-stoichiometry, thermal energy, reactions or phase changes, some of the atoms leave their lattice sites or fail to occupy them, generating point defects. The density of these defects (such as vacancies, antisites and interstitials) and their charge state (positive, negative and neutral) depend on the (electro)chemical potentials of the constituent atoms and electrons. These potentials are usually controlled by the elemental compositions during the growth of a material and are vital in shaping its properties. Indeed, significant property changes can be observed with slight modifications of the synthesis conditions, especially when crossing the boundaries of phase homogeneity regions, as epitomized by the assorted realm of steels1. Therefore, a deliberate positioning along the edges of single-phase existence regions during growth can be exploited to benefit from the characteristics of the different phases involved, such as a superior native doping or microstructure. However, these advantages may come at a cost. The growthdependent (electro)chemical potential of the constituent atoms is a source of interface instability. Different interface reactivity manifests itself as undesirable chemical reactions affecting the material obtained on either side of the phase boundary. Furthermore, the detrimental nature of the reactions may spring from the formed secondary phase or from the altered defect population at the interface or both. Understanding the nature of the defects involved, their concentration and mobility during the growth and after subsequent interface reactio (...truncated)