Compositional Variation in FAPb1- xSnxI3and Its Impact on the Electronic Structure: A Combined Density Functional Theory and Experimental Study

Simon Kahmann, Zehua Chen, Oleh Hordiichuk, Olga Nazarenko, Shuyan Shao, Maksym V. Kovalenko, Graeme R. Blake, Shuxia Tao (Corresponding author), Maria A. Loi (Corresponding author)

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Abstract

Given their comparatively narrow band gap, mixed Pb-Sn iodide perovskites are interesting candidates for bottom cells in all-perovskite tandems or single junction solar cells, and their luminescence around 900 nm offers great potential for near-infrared optoelectronics. Here, we investigate mixed FAPb1-xSnxI3 offering the first accurate determination of the crystal structure over a temperature range from 293 to 100 K. We demonstrate that all compositions exhibit a cubic structure at room temperature and undergo at least two transitions to lower symmetry tetragonal phases upon cooling. Using density functional theory (DFT) calculations based on these structures, we subsequently reveal that the main impact on the band gap bowing is the different energy of the s and p orbital levels derived from Pb and Sn. In addition, this energy mismatch results in strongly composition-dependent luminescence characteristics. Whereas neat and Sn-rich compounds exhibit bright and narrow emission with a clean band gap, Sn-poor compounds intrinsically suffer from increased carrier recombination mediated by in-gap states, as evidenced by the appearance of pronounced low-energy photoluminescence upon cooling. This study is the first to link experimentally determined structures of FAPb1-xSnxI3 with the electronic properties, and we demonstrate that optoelectronic applications based on Pb-Sn iodide compounds should employ Sn-rich compositions.

Original languageEnglish
Pages (from-to)34253–34261
Number of pages9
JournalACS Applied Materials and Interfaces
Volume14
Issue number30
DOIs
Publication statusPublished - 3 Aug 2022

Bibliographical note

Funding Information:
Arjen Kamp and Teodor Zaharia are thanked for technical support. S.K. is grateful for a research fellowship (Grant No.: 408012143) awarded by the Deutsche Forschungsgemeinschaft (DFG). This work was financed through the Materials for Sustainability (Mat4Sus) programme (739.017.005) of The Netherlands Organisation for Scientific Research (NWO). Z.C. acknowledges funding from the Eindhoven University of Technology. This work made use of the Dutch national e-infrastructure with the support of the SURF Cooperative using Grant No. EINF-2988. S.T. acknowledges funding by the Computational Sciences for Energy Research (CSER) tenure track program of Shell and NWO (Project No. 15CST04-2) as well as the NWO START-UP from The Netherlands. The work at ETH Zurich (O.N., O.H., and M.V.K.) was financially supported by the Swiss National Science Foundation (grant agreement 186406, funded in conjunction with SPP2196 through the DFG-SNSF bilateral program) and by ETH Zurich through the ETH+ Project SynMatLab.

Keywords

  • band bending
  • crystallography
  • DFT calculations
  • lead-tin mixed perovskites
  • photoluminescence
  • single crystals

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