Spectral shifts upon halide segregation in perovskite nanocrystals observed via transient absorption spectroscopy
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.307
Spectral shifts upon halide segregation in perovskite nanocrystals observed via transient absorption spectroscopy Michael L. Crawford1, James C. Sadighian1, Yasser Hassan2, Henry J. Snaith2, Cathy Y. Wong1,3,4,* 1
Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97403, USA
2
Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, UK
3
Oregon Center for Optical, Molecular, and Quantum Science, University of Oregon, Eugene, Oregon 97403, USA
4
Materials Science Institute, University of Oregon, Eugene, Oregon 97403, USA
ABSTRACT
Lead halide perovskite nanocrystals (NCs) are promising for applications in light emitting devices owing to a strong emission spectrum that is tunable throughout the visible region by altering halide composition. However, in mixed-halide perovskite systems photoinduced migration drives formation of halide-segregated domains, altering the emission spectrum. The mechanism by which this segregation occurs is currently the subject of intense investigation. Processes involving the perovskite surface are expected to be of enhanced prevalence in NCs due to their large surface area to volume ratio. In this work, we use transient absorption spectroscopy to probe the excited-state dynamics of NCs before and after halide segregation. Comparison of global fit spectra of the measured signals suggests the accumulation of iodide at the surface, resulting in a redshifted emission spectrum.
1
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INTRODUCTION Research interest in hybrid organic-inorganic lead halide perovskites has been growing in recent years due to their potential for use in light absorbing and emitting applications. Hybrid organic-inorganic lead halide perovskite nanocrystals (NCs), first synthesized by Schmidt et al. in 2014 [1], exhibit photoluminescence spectra that can be tuned across the entire visible range by modulating NC size and halide composition [2]– [5]. The tunability of emission and ease of synthesis make these NCs attractive for use in light emitting devices [6], but their practicality is limited by the photo-instability of the bandgap in mixed-halide perovskite materials, e.g. MAPbIxBr3-x [7]–[17] (MA = methylammonium). Upon illumination, mixed iodide-bromide perovskite films have been widely reported to experience either a redshift of the photoluminescence (PL) spectrum [8]–[12] or the development of a second feature that is redder than the initial PL [7]. These developments, accompanied by a redshift in the band edge absorbance [8], are attributed to segregation of the halides into iodide-rich and bromide-rich regions. Because this halide segregation alters the optical characteristics of the material, a mechanistic understanding of this process is
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