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Hyperspectral Imaging as a Tool to Study Optical Anisotropy in Lanthanide-Based Molecular Single Crystals

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Is Photoluminescence Spectroscopy a Suitable Probe of Halide Segregation?

Joshua R S Lilly¹, Vincent J-Y Lim¹, Jay B Patel²

¹Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom.

ACS Energy Letters

|May 14, 2026

Summary

Photoluminescence tracking inaccurately measures halide segregation in mixed-halide perovskites. X-ray diffraction reveals segregation rates are unchanged, contrary to photoluminescence data, necessitating new monitoring methods for photovoltaic stability.

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Area of Science:

Materials Science
Photovoltaics
Solid-State Chemistry

Background:

Mixed-halide perovskites offer tunable band gaps for efficient multijunction solar cells.
Photoinduced halide segregation in perovskites leads to performance degradation and instability.
Photoluminescence (PL) is a common technique for monitoring perovskite photoinstability.

Purpose of the Study:

To investigate the accuracy of photoluminescence (PL) for quantifying halide segregation in mixed-halide perovskites.
To compare PL measurements with X-ray diffraction (XRD) for assessing halide segregation.
To identify reliable methods for evaluating the stability of perovskite photovoltaics.

Main Methods:

Simultaneous recording of photoluminescence (PL) and X-ray diffraction (XRD) measurements.
Analysis of CH3NH3Pb-(I1-xBrx)3 films with 18 different halide compositions.
Correlation of PL intensity and spectral changes with XRD-derived structural information.

Main Results:

PL data suggested an exponential increase in segregation rates with bromide fraction, which was contradicted by XRD.
XRD patterns indicated that halide segregation rates remained largely unchanged across varying bromide fractions.
PL measurements were found to be influenced by charge funneling effects rather than solely reflecting halide segregation.

Conclusions:

Photoluminescence is an unreliable indicator of halide segregation in mixed-halide perovskites.
X-ray diffraction and absorption spectroscopy are more suitable for probing bulk material changes and assessing stability.
Accurate monitoring of perovskite photoinstability requires methods that probe the entire material volume, not just surface luminescence.