3 years ago

Exciton Recombination in Formamidinium Lead Triiodide: Nanocrystals versus Thin Films

Exciton Recombination in Formamidinium Lead Triiodide: Nanocrystals versus Thin Films
Maria Antonietta Loi, Daniel M. Balazs, Loredana Protesescu, Maksym V. Kovalenko, Sampson Adjokatse, Hong-Hua Fang
The optical properties of the newly developed near-infrared emitting formamidinium lead triiodide (FAPbI3) nanocrystals (NCs) and their polycrystalline thin film counterpart are comparatively investigated by means of steady-state and time-resolved photoluminescence. The excitonic emission is dominant in NC ensemble because of the localization of electron–hole pairs. A promisingly high quantum yield above 70%, and a large absorption cross-section (5.2 × 10−13 cm−2) are measured. At high pump fluence, biexcitonic recombination is observed, featuring a slow recombination lifetime of 0.4 ns. In polycrystalline thin films, the quantum efficiency is limited by nonradiative trap-assisted recombination that turns to bimolecular at high pump fluences. From the temperature-dependent photoluminescence (PL) spectra, a phase transition is clearly observed in both NC ensemble and polycrystalline thin film. It is interesting to note that NC ensemble shows PL temperature antiquenching, in contrast to the strong PL quenching displayed by polycrystalline thin films. This difference is explained in terms of thermal activation of trapped carriers at the nanocrystal's surface, as opposed to the exciton thermal dissociation and trap-mediated recombination, which occur in thin films at higher temperatures. The newly developed formamidinium lead triiodide (FAPbI3) nanocrystals (NCs) show quantum yield above 70% with a large absorption cross-section (5.2 × 10−13 cm−2). Biexcitonic recombination is observed, featuring a slow recombination lifetime of 0.4 ns. The optical properties of the NC ensemble are further compared with their polycrystalline thin film counterpart, showing two very distinct recombination mechanisms.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/smll.201700673

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