Single-Exciton Amplified Spontaneous Emission in Thin Films of CsPbX₃ (X = Br, I) Perovskite Nanocrystals

Abstract

During the last four years, CsPbX 3 perovskite nanocrystals (PNC) have emerged as an excellent material for stimulated emission purposes, with even more prospective properties than conventional colloidal quantum dots. However, although the results under femtosecond excitation undoubtedly demonstrate potential performances, the achievement of more ambitious targets, such as stimulated emission under cw optical or electrical excitation, requires a better understanding of the physical and optical mechanisms responsible for the generation of the optical gain. In this publication, we establish intrinsic mechanisms underlaying the generation of amplified spontaneous emission (ASE) in PNCs with three different bandgaps (CsPbBr 3 , CsPbI 3 , and CsPbBr 1.5 I 1.5). For this purpose, optical gain is characterized at cryogenic temperatures in order to avoid the influence of non-radiative channels. In particular, our experimental results provide physical evidences that single excitons are the dominant species in the ASE generation. Moreover, the particular shape of ASE spectra is well reproduced by the model that takes into account the reabsorption effects and the inhomogeneous broadening of the PL band. These results are a new milestone towards the full understanding of light generation in PNCs, and with the development of competitive active optoelectronic devices.

Purcell Enhancement and Wavelength Shift of Emitted Light by CsPbI3 Perovskite Nanocrystals Coupled to Hyperbolic Metamaterials

“…It is worth mentioning that the above concentration and dipping time were optimized to obtain a partially covered monolayer of CsPbI 3 nanocubes (with 14 nm of average cube edge), 57 as illustrated in Figure 2 a. This single monolayer of PNCs (meaning a layer made of single nanocrystals dispersed throughout the sample surface without agglomeration, nominally) would ensure an efficient coupling of the emitters to the HMM for a given spacer, because the average PNC-HMM coupling strength would decrease significantly for too thick PNC films, as it would occur if increasing the spacer thickness.…”

“…The PL peak wavelength for the reference sample is observed at around 705 nm, in good agreement with what is reported by other authors for films of CsPbI 3 PNCs at cryogenic temperatures. 57 , 63 We also tested CsPbBr 3 PNCs deposited on top of the same HMM, whose spontaneous emission is peaked at around 520 nm. For these emitters, we did not observed any decay time reduction from 300 down to 15 K. This effect is ascribed to the wavelength dependence of the Purcell factor as a function of the metal fill factor in the HMM, which is equal/smaller than 0.5 in our case and consequently exhibiting a negligible Purcell factor enhancement at around 500 nm (see refs ( 39 ) and ( 64 ) for example, and section 4 of the Supporting Information ).…”

“…In order to explain the experimental results, we propose to consider the model schematically represented in Figure 3 b, assuming a monolayer of CsPbI 3 PNCs on top of the spacer. In the numerical calculations, the thickness of the CsPbI 3 layer and its refractive index are extracted from ref ( 57 ) (we are using the same PNCs) and ref ( 71 ) respectively. The permittivity of Lithium fluoride (LiF) is taken to be ϵ LiF = 1.95, whereas those of silver (Ag) and silicon (Si) are taken out from refs ( 72 ) and ( 73 ) respectively.…”

Ligand & band gap engineering: tailoring the protocol synthesis for achieving high-quality CsPbI3 quantum dots

“…Halide perovskite colloidal quantum dots (QDs), with tunable band gap under quantum confinement regime, are considered very promising materials for the next generation of a broad range of optoelectronic devices, such as high-quality displays, 1 lasers, [2][3][4] light-emitting diodes, 5 photodetectors, 6 solar cells. [7][8][9][10] This interest is based on their high photoluminescence quantum yield (PLQY), bandgap tunability in the entire visible region, and narrow emission line widths.…”

CsPbI3 nanocrystal films: towards higher stability and efficiency

“…While the double degeneracy of the ground state and the redshift of the ASE peak suggest that ASE is dominated by the biexciton-to-exciton transition, other interpretations have suggested the involvement of single excitons only, 97 as concluded from PL spectroscopy measurements. For the excitation fluency range <N>, between 1 and 2, they observe a decrease in the ASE QY, which cannot be understood if it is caused by biexciton-to-exciton transitions; rather, the decrease in QY is probably the result of Auger recombination.…”