Correlation between the Photoluminescence Properties and the Crystal size in MAPbBr3 Perovskite Thin Films

Nikolaos Droseros^a^,^b, Giulia Longo^c, Jan C. Brauer^b, Michele Sessolo^c, Henk J. Bolink^c, Natalie Banerji^a^,^b

^aDepartment of Chemistry and Biochemistry, University of Bern - Switzerland, Freiestrasse, 3, Bern, Switzerland

^bUniversity of Fribourg, CH, Chemin des Verdiers, 4, Fribourg, Switzerland

^cUniversidad de Valencia - ICMol (Institute of Molecular Science), Catedrático José Beltrán Martinez 2, Paterna, Spain

Proceedings of Interfaces in Organic and Hybrid Thin-Film Optoelectronics (INFORM)

València, Spain, 2019 March 5th - 7th

Organizers: Natalie Stingelin, Hendrik Bolink and Michele Sessolo

Poster, Nikolaos Droseros, 062
Publication date: 8th January 2019

Perovskites with high absorption coefficients, low exciton binding energies and high carrier mobilities are proving themselves as efficient light-harvesters for solar cells applications. In addition, Perovskites with higher exciton binding energies, tunable band gap and narrow emission bandwidth are promising candidates for light-emitting diodes (LEDs), with the Methylammonium Lead Bromide (MAPbBr₃) being the major representative.Besides having been extensively investigated, MAPbBr₃, suffers by a low photoluminescence quantum yield (PLQY) preventing its use in visible perovskite LEDs. Fine-tuning the morphology and reducing the grain size of the polycrystalline perovskite to the micro- and nano- scale have proved to be efficient methods to increase the PLQY. However, the origins of the increased PLQY remain unclear. In this context the present study elucidates the physical processes underlying the light emission of MAPbBr₃ thin films using steady state photoluminescence and absorption, ns-time-resolved photoluminescence (TRPL), ns-transient absorption (ns-TA) and fs-transient absorption (fs-TA) spectroscopy techniques. After reducing the crystal size by a solution-processed, additive-assisted method, a systematic correlation between the enhanced PL properties and their size was obtained. The detection of a double-Gaussian emission band in as-cast polycrystalline MAPbBr₃ (crystal size of the order of few μm), further analyzed with the Burstein-Moss and Saha models, allowed for the interplay between the population of free-carriers and excitons to be monitored at different excitation densities. The co-existence of free carriers and excitons at low excitation densities was detected, while only excitons were found to be present at high excitation densities (TOC graphics). Furthermore, important quantities such as the exciton binding energy, the reduced exciton effective mass and the trap density were estimated. The increased PLQY upon crystal size reduction was attributed to the presence of a bright exclusively exitonic population even at low excitation densities, together with reduced surface trapping thanks to passivation by the additives.

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Acknowledgements:

This work has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program INFORM (grant agreement n° 675867), the Spanish Ministry of Economy and Competitiveness (MINECO) via the Unidad de Excelencia María de Maeztu MDM-2015-0538 and MAT2014-55200, PCIN-2015-255 and the Generalitat Valenciana (Prometeo/2012/053). M.S. thanks the MINECO for his RyC contract. N.D., J.B. and N.B. would like to acknowledge the Universities of Fribourg and Bern, Switzerland, for providing funding and infrastructure.

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