Metal halide perovskites (MHPs) have emerged as promising materials for optoelectronics and photonics, mostly due to their large absorption coefficient and excellent photoluminescence quantum yield at room temperature, being a drawback their stability under ambient conditions. Particularly, MHPs in the form of nanocrystals are useful for the fabrication of optoelectronic and photonic devices by using inkjet printing and similar solution processing techniques. For these applications it is very important to know the limitations of nanocrystal MHPs for such applications that are imposed by the exciton recombination dynamics. In this way, our “delayed photoluminescence model” will be explained in this talk as the basis to understand the role of shallow non-quenching traps on the observed exciton recombination dynamics as a function of temperature [1,2]. Another key point for the development of active photonic devices (e.g., optical amplifiers and lasers) is the observation of stimulated emission at room temperature, which was possible at very low thresholds in MHP films integrated on polymer waveguides both on rigid and flexible substrates [3,4]. In the case of films prepared by CsPbX₃ (X₃ = Br₃, X₃ = Br_1.5I_1.5, X₃ = I₃) nanocrystals with different thicknesses we have demonstrated Amplified Spontaneous Emission (ASE) in backscattering geometry for a wide range of temperatures [5]. In these films we demonstrated ASE thresholds lower than 5 μJ/cm2 at cryogenic temperatures under nanosecond laser excitation and the physical origin of ASE was elucidated (single exciton origin in contrast to biexcitonic, as claimed in literature). At single nanocrystal level, the emission properties of nanocrystals can be also profited for other applications, as near-field imaging and single photon emitters. In the first case, the application related to single nanocrystals coupled to Mie resonators formed by TiO2 nanoparticles will be presented [6], whereas in the second case the coupling of perovskite nanocrystals to the optical modes of hyperbolic metal-dielectric metamaterials (HMMs) will be examined [7]. For TiO2-perovskite resonators a notable absorption enhancement is observed at the level of an optical probe whose extent is around 100 nm [6] and for the HMM-perovskite a Purcell enhancement of its radiative rate very close to a factor 4 was recently demonstrated (paper in press).

Synthesis of lead halide nanocrystals within porous matrices provides a means to attain highly efficient light emitters in a protective and versatile environment. Furthermore, it allows spectral control over the luminescence, which can be finely tuned by means of quantum confinement effects, which arise when the nanocrystal size is of the order of the semiconductor exciton Bohr radius. Remarkably, this is mastered without the need of capping ligands, which ease both their integration in optoelectronic devices and the analysis of fundamental properties that depend on the direct exposure of the nanocrystal surface to its surroundings. In this talk, I will give an overview of the main features of these systems, focusing on aspects such as: the effect of the matrix on their radiative and non-radiative decay rates; their stability when subjected to different environments; their behavior as photoconductors, involving dot-to-dot charge transport, of great relevance if applications in electroluminescent devices are sought after; and the post-processing opportunities they offer to create novel color converting layers for LEDs.

In this talk, I will show results of using femtosecond (fs) time-resolved transient absorption and terahertz techniques to interrogate the photobehavior of a perovskite/QDs (Ps/QDs) film. Armed with these techniques, we were able to extract the real time dynamics (fs to ps regime) of excitons, and charge carriers happening in this hybrid material, and the interface between both materials. While the decays are dominated by e and h transition from PS to QDs, the increase in the QDs size results in an acceleration of the charge carriers’ transitions represented by the total transfer rate constants of electrons and holes. We extract the diffusion times and transfer rate constants to the interfaces. Furthermore, pumping with different fs-laser fluences indicates photoformation of excitonic states, which acceleration decreases the contribution of undesirable charge carriers trapping and non-radiative recombination within PS. Our results elucidate the importance of the QDs size for improving the efficiency of LEDs based on these nanocomposites.

Recently, all-inorganic perovskite light-emitting diodes (PeLEDs) have attracted both academic and industrial interest thanks to their outstanding properties, such as high efficiency, bright luminance, and excellent color purity, low cost and potentially good operational stability. Both the design and treatment of all-inorganic emitters and the device engineering are two key strategies to guarantee the high performance. The major practical bottleneck relates to the labile surface chemistry of the perovskite materials, and in turn their stability. [1] Here, ultra-high stable CsPbI₃ QDs for more than 15 months by controlling two main parameters, ligand concentration and temperature, have been synthetized. By increasing the capping ligand concentration during the QD synthesis, we were able to grow CsPbI₃ in a broad range of temperatures. Improved photophysical properties of QDs are obtained by increasing the synthesis temperature. We achieved the maximum photoluminescence quantum yield (PLQY) of 93% for a synthesis conducted at 185 °C, establishing an efficient surface passivation, which decreases the density of non-radiative recombination sites. [2] Moreover, we show that it is possible to produce stable CsPbI₃ QDs with high PLQY and red emission beyond the requirement of the Rec. 2020 standards for red color (0.708, 0.292). [3] By optimizing the hole transport layers and the electron transport layers (ETLs), in PeLEDs employing CsPbI₃ quantum dots as an emitter layer, we achieved electron/hole current balance. The devises based on the architecture (ITO/PEDOT:PSS 8000-p-TPD/CsPbI₃ QDs/ PO-T2T (40 nm)/Liq (2 nm)/Ag) show the highest external quantum efficiency (EQE) and are the most stable. [4] This result demonstrates how the device engineering is another significant factor to guarantee the high performance of LEDs, where a negligible charge injection barrier between charge injecting layers (CILs) and an optimized thickness of these CILs play a critical role for a controlled flow of charge carriers through the device and in turn for the performances.

Unraveling the structure-property relationships in hybrid lead halide perovskites is instrumental for the fundamental understanding not only of their exceptional optoelectronic performance but also their in-operando stability. Surprisingly, hybrid halide perovskites exhibit a great defect tolerance in their performance, despite their mechanical softness and the consequent low energetic barriers for point defect formation and ion migration. In a recent publication [1], we have constructed the complete phase diagram of organic-cation solid-solutions of lead iodide perovskites (FA_xMA_1-xPbI₃, where MA stands for methylammonium and FA for formamidinium) with compositions x ranging from 0 to 1 in steps of 0.1 and in the temperature range from 10 to 365 K by combining Raman scattering and photoluminescence (PL) measurements. In this talk, I will present an interesting byproduct of this work, which concerns the study of the evolution of shallow-defect signatures observed in the PL spectra at low temperatures. The strong free-exciton PL of the perovskites, apart from the discontinuous changes in the energy of its maximum at the different structural transitions, also exhibits a strong decrease in linewidth at low temperatures. This allows for the observation of the emission related to radiative recombination of bound exciton complexes (BECs) associated with shallow defects (donor and/or acceptor). I will report a tentative assignment of all PL features to the different shallow-defects typically present in hybrid perovskites, attained with the aid of state of the art ab-initio calculations [2]. The defect-related signatures exhibit a clear trend regarding the composition of the mixed crystals, indicating that the material becomes less prone to defect formation with increasing FA content.