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Chen Li1 Ekin Simsek Sanli1 Daniel Barragan-Yani2 Helena Stange3 Marc-Daniel Heinemann4 Dieter Greiner4 Wilfried Sigle1 Roland Mainz4 Karsten Albe2 Peter A. van Aken1 Daniel Abou-Ras4

1, Stuttgart Center for Electron Microscopy, Max Planck Institute for Solid State Research, Stuttgart, , Germany
2, Institute of Materials Science, Technical University Darmstadt, Darmstadt, , Germany
3, Institute of Materials Science and Technology, Technical University Berlin, Berlin, , Germany
4, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, , Germany

High-efficiency Cu(In,Ga)Se2 (CIGS) thin-film solar cells are achieved by depositing CIGS absorber layers using a three-stage co-evaporation process. During the second stage of this process, Cu-Se deposition on a (In,Ga)2Se3 precursor during heating leads to the formation of a CIGS layer that passes from a Cu-poor to a Cu-rich composition. It has been reported that the interdiffusion of Cu and In/Ga between CIGS and Cu-Se layers is essential for the grain growth of CIGS during this Cu-poor to Cu-rich transition1-5. However, little has been known about how the atomic structures evolve during this process.

In the present study, we use the CuInSe2 (CIS) system, as it is simpler than CIGS for an initial study. Cu-Se was deposited on top of CIS, and the grain growth was directly monitored during in-situ heating in a scanning electron transmission microscope (STEM). The compositional changes were revealed simultaneously with electron energy-loss spectroscopy (EELS). The corresponding time-series STEM images and elemental maps will be presented in as videos in the conference. The results show that planar defects play an important role in grain growth. The grains with high-density planar defects tend to be consumed by the grains without planar defects, and this does not occur when there is no extra Cu-Se layer deposited on CIS. Simultaneously acquired STEM and EELS spectrum images reveal that Cu-rich grain boundaries (GBs) between highly defected and non-defected CIS grains migrate towards the highly defected CIS grains during heating. Further EELS elemental mapping reveals Cu enrichment and In depletion at most GBs within regions having widths of more than 10 nm. Atomically resolved STEM images show a gradual phase change from a chalcopyrite CIS structure at CIS grain interiors towards anantifluorite Cu2-xSe structure at GBs, which is also evidenced by atomic-resolution EELS. In this gradual phase change, the Se sublattice stays the same, while In and Cu interdiffuse gradually without any dislocations between the Cu2-xSe and CIS phases. Moreover, In atoms are found at interstitial sites in Cu2-xSe near GBs. Density functional theory (DFT) calculations show that the diffusion barrier of In atoms from interstitial to interstitial sites in Cu2-xSe is very low. These results not only show that Cu diffusion at GBs assists the recrystallization of CIS, but also reveal the mechanism beneath it, which will be discussed at the conference.

Acknowledgment: Research funded in part by the Helmholtz Virtual Institute “Microstructure Control for Thin- Film Solar Cells”, VH-VI–520 project. CL acknowledges the support from Max Planck Society.

References:

1. R. Klenk et al., A model for the successful growth of polycrystalline films of CuInSe2 by multisource physical vacuum evaporation, Adv. Mater. 5, 114-119 (1993)
2. N. Barreau et al., Recrystallization of CIGSe layers grown by three-step processes: a model based on grain boundary migration, Acta Mater. 58, 5572-5577 (2010)
3. R. Mainz et al., Sudden stress relaxation in compound semiconductor thin films triggered by secondary phase segregation, Phys. Rev. B 92, 155310 (2015)
4. H. Rodriguez-Alvarez et al., Recrystallization of Cu(In,Ga)Se2 thin films studied by X-ray diffraction, Acta Mater. 61, 4347-4353 (2013)
5. H. Stange et al., Diffusion-induced grain boundary migration as mechanism for grain growth and defect annihilation in chalcopyrite thin films, Acta Mater. 111, 377–384 (2016)

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