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.

In this work, doctor blading is proposed for the fabrication of strongly-coupled QD solids from a PbS nanoink for photodetection at telecom wavelengths.

Such multiple (re)absorption/(re)emission cycles result in a certain population of "recycled photons" concentrated inside the semiconductor. Consequently, an efficient PR effect provides another degree of freedom to control the photon and carrier densities in a semiconductor [2] and hence a way to tailor-made its optoelectronic properties. [3,4] Demonstrated applications include solar cells, light-emitting diodes, or optical modulators. [2] In this context, PR has been recently claimed in halide perovskites (HP), as an effect that could contribute to a certain extent to the excellent conversion efficiencies and emission rates reported for this family of semiconductors. [5,6] Indeed, the strong absorption coefficient above the bandgap and sharp (excitonic) band edge, [7,8] the small Stokes shift (SS) between photoluminescence (PL) and absorption, [9,10] and the high PL quantum yield (PLQY) [11,12] are outstanding characteristics of HPs and needed ingredients for leading to important PR effect. In this way, over the last 3-4 years, different experimental and theoretical reports have analyzed the efficiency of PR in HPs and the potential benefits in solar cells [13][14][15] or light-emitting diodes, [15,16] among other devices. 15 In particular, experimental studies carried out in CH 3 NH 3 PbX 3 (X = Cl, Br, I) polycrystalline thin films, [17,18] CH 3 NH 3 PbX 3 single crystals, [10,19,20] CsPbBr 3 nano/microwires [21][22][23] or CsPbBr 3 nanocrystals [9] always show that PL spectra experience an important redshift and an elongation of the decay time after traversing some microns of the HP material. Although there has been a controversy about the impact of PR in the total PL spectra, [19,24] or if PR dominates or not over carrier diffusion on the effective decay time, [23,25] recent studies on perovskite single crystals [6] and MAPI polycrystalline thin films [18] confirm that PR is the dominant transport mechanism for propagation lengths longer than the diffusion of carriers. Besides, the theoretical analysis predicts that multiple absorption and emission processes produce a certain "diffusive regime of traveling photons" that increases the effective lifetime of photons outcoupled from the sample (thin film, microwire, single crystal …). [26] Indeed, recent theoretical and experimental works demonstrated an enhancement of the open-circuit voltage in solar cells [14,18,27,28] or radiation efficiency in light-emitting diodes [29] when PR is optimized. The standard optical configuration that has been chosen to demonstrate the PR effect in most works consists of a semiconductor thin film deposited on a specific Reabsorption and reemission of photons, or photon recycling (PR) effect, represents an outstanding mechanism to enhance the carrier and photon densities in semiconductor thin films. This work demonstrates the propagation of recycled photons over several mm by integrating a thin film of CsPbBr 3 nanocrystals into a planar waveguide. An experimental set-up based on a frequency modulation spectroscopy allows to ch...

Control of quantum-dot (QD) surface chemistry offers a direct approach for the tuning of charge-carrier dynamics in photoconductors based on strongly coupled QD solids. We investigate the effects of altering the surface chemistry of PbS QDs in such QD solids via ligand exchange using 3-mercaptopropionic acid (MPA) and tetrabutylammonium iodide (TBAI). The roll-to-roll compatible doctor-blade technique was used for the fabrication of the QD solid films as the photoactive component in photoconductors and field-effect phototransistors. The ligand exchange of the QD solid film with MPA yields superior device performance with higher photosensitivity and detectivity, which is due to less dark current and lower noise level as compared to ligand exchange with TBAI. In both cases, the mechanism responsible for photoconductivity is related to trap sensitization of the QD solid, in which traps are responsible of high photoconductive gain values, but slow response times under very low incident optical power (<1 pW). At medium–high incident optical powers (>100 pW), where traps are filled, both MPA- and TBAI-treated photodevices exhibit similar behavior, characterized by lower responsivity and faster response time, as limited by the mobility in the QD solid.