Controlling Structural and Photophysical Properties of Triple Cation Perovskites by Light Soaking in Various Environmental Conditions
Miguel Anaya a, Krzysztof Galkowski a, Edoardo Ruggeri a, Tiarnan Doherty a, Stuart Macpherson a, Sam Stranks a
a Cavendish Laboratory, University of Cambridge, JJ Thomson Ave, Cambridge CB3 0HE, UK
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Poster, Miguel Anaya, 184
Publication date: 11th February 2019

Organic-inorganic metal halide perovskites have emerged over the past decade as a remarkably versatile class of semiconductor for high-performance optoelectronic devices.[1] Despite their unique properties, this family of perovskites has however been observed to display photo-instability and sensitivity to O2/H2O exposure, which hampers their long term stability and thus their definitive integration in real-world devices.[2,3]

In this talk, we will present a detailed structural and photophysical characterisation of state-of-the-art triple cation (Cs,FA,MA)Pb(I,Br)3 perovskite films under a series of atmospheric conditions (dry He, dry air and humid air), with the aim of assessing their stability and improving their properties. In this regard, we have performed a systematic experiment in which structural changes are tracked by means of Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS, synchrotron Diamond Light Source) while the perovskite films are illuminated. These measurements are correlated in-situ with macroscopic photoluminescence (PL) in order to discern whether different atmospheres during light soaking can instigate or mitigate defect annihilation and halide segregation in the systems. Interestingly, the different environmental conditions dictate the intrinsic levels of doping, i.e. the concentration of electrons and holes, in the material as observed by Kelvin Probe. Finally, we employ both a hyperspectral wide-field microscope imager and a fluorescence lifetime imaging microscope to better understand the implications of the described effects at the nanoscale. In particular, we confirm strong compositional re-organisation in the range of hundreds of nanometres when illuminating the films, whose impact in the optoelectronic properties of the material is governed by the surrounding atmosphere.

Our observations demonstrate that photobrightening in perovskite films is catalysed by the presence of oxygen and originates structural changes and halide redistribution that highly depend on the starting composition and have a remarkable impact in the electonic properties of the materials. As a conclusion, we propose a mechanism with which the intrinsic electrical properties and photophysical behaviour of triple cation perovskite films can be tuned by post-treating the material with light and a controlled atmosphere. These results constitute a rigorous and versatile method towards the development of metal halide perovskites approaching their full performance potential.

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