Exciton Recombination in Formamidinium Lead Triiodide: Nanocrystals versus Thin Films

Abstract

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.

Properties of Excitons and Photogenerated Charge Carriers in Metal Halide Perovskites

“…Many other studies also revealed the ultrafast behaviors of localized excitons, biexcitons, trions and multiple exciton generation in perovskites by transient techniques. He et al reported that the radiative recombination of the photogenerated species in solution‐processed methylammonium–lead–halide films was dominated by excitons weakly localized in band tail states, under the excitation level close to the working regime of solar cells, and such exciton localization effect was found to be general in several solution‐process perovskite films .…”

“…In contrast, the photoluminescence in CsSnX 3 perovskites has been reported to be extrinsic and assigned to the acceptor‐bound excitons at Sn vacancies . Meanwhile, besides the typically observed free‐charge‐carrier or free‐exciton PL peak, additional redshifted PL peaks with larger Stokes shifts and broadened peak widths appeared in certain perovskites, which have been ascribed to other types of recombination mechanisms such as the emission of bound exciton, self‐trapped exciton, trapped carrier, biexciton, or impurity, respectively.…”

“…One is to concentrate carriers in a small size, thus significantly increasing the charge‐carrier densities to produce high PLQYs even at low excitation intensity . The other one is to enable the formation of free exciton due to the enhanced exciton binding energy, which makes the radiative recombination in LD perovskites dominated by free exciton emission and thus further ensures the high PLQYs at low excitation fluence. Nevertheless, it is worth mentioning that the photoluminescence in LD perovskites originating from free‐carrier rather than exciton emission has also been reported in certain cases …”

“…For instance, Aneesh et al found that biexciton formation was detected in colloidal CsPbBr 3 nanocrystals within 1 ps . Fang et al revealed that biexcitonic recombination existed in FAPbI 3 nanocrystals at high pump fluence, with a slow lifetime of 0.4 ns . Makarov et al demonstrated that, similar to other types of nanocrystals, cesium–lead–halide perovskite QDs were prone to photocharging, leading to the formation of trions that decayed primarily via the Auger process on the time scale of ≈20–100 ps .…”

Efficient and Tunable Electroluminescence from In Situ Synthesized Perovskite Quantum Dots

“…Carrier dynamics of nanoscale crystals are different from those of their bulk counterparts . Due to the small exciton binding energy in the bulk perovskite crystals, emissions of these materials are from radiative bimolecular recombination; the other two decay pathways, trap‐assisted monomolecular recombination and Auger recombination, are nonradiative .…”