Progress in halide-perovskite nanocrystals with near-unity photoluminescence quantum yield

Colloidal halide perovskite nanocrystals (PNCs) are an outstanding case study due to their remarkable optical features, such as a high photoluminescence (PL) quantum yield (PLQY), tunable band gap, and narrow emission. Despite the impressive first reports of PLQYs beyond 70%, it has been observed that PLQY is limited by structural defects arising from labile interactions between the organic capping ligand and the inorganic core. Structural defects acting as trap states are key factors limiting both PNC PLQY and stability. In this review, we present the most studied, common, and alternative protocols to fully compensate for surface defects (e.g., halide vacancies, loss of protective capping ligands) as well as how to increase their stability and PLQY to unity (i.e., 100% when PLQY is expressed as a percentage)

Stabilization of Black Perovskite Phase in FAPbI3 and CsPbI3

Although halide perovskites allow a great versatility, the application on single-absorber solar cells restricts significantly the number of available materials. In this context, CsPbI3 and FAPbI3 (FA, formamidinium) present a huge potential for the inorganic approach with enhanced stability and narrow bandgap, respectively. However, for these materials, Cs+ and FA+ are relatively too small and too big to stabilize the perovskite black phase at room temperature, both presenting a nonphotoactive yellow phase as the most stable phase. This fact limits dramatically their application and also helps in the understanding of the main research lines in the halide perovskite photovoltaic field in the quest for the stabilization of FAPbI3. In this Perspective, we present an overview of different strategies for the stabilization of the perovskite black phase of these two materials. We evaluate the stability approaches envisioning efficient and stable materials, with a particular focus on the positive and limiting aspects of the exploitation of low dimensionality and chemi-structural mechanisms

Recent insights for achieving mixed halide perovskites without halide segregation

The incorporation of halide perovskites in optoelectronics has provided a fast advance in the fabrication of new sensitizers with a balanced light-harvesting, free carrier transportation, and a progressive overcoming of the low tolerance to the moisture. Within these emerging materials, mixed halide perovskites as APbX3−xYx, (A = MA+, Cs+, FA+; X, Y=Cl−, Br−, I−) have been highlighted due to their facile band gap tunability in the entire visible region by varying the halide composition, which making these systems enormously appealing for the design of optoelectronic devices. Nonetheless, their performance in real devices is strongly limited as mixed halide perovskites exhibit photoinduced and current-induced phase segregation, losing their original photophysical properties and effective band gap tunability to generate halide-rich domains. The phase segregation has been the key factor to decrease the photovoltaic parameters in solar cells as open-circuit voltage and photoconversion efficiency, also limiting the performance of tandem devices and the potentiality of color design in perovskite LEDs. This review summarizes recent trends to hinder the phase segregation

Application of Halide Perovskite Nanocrystals in Solar-Driven Photo(electro)Catalysis

Photo(electro)catalysis (PEC) is a promising strategy to conduct attractive solar-driven reactions such as CO2 reduction to form added-value products, H2 production, and organic substrates oxidation, taking advantage of the spatial separation of photocarriers generated in semiconductor-based electrodes. Halide perovskite nanocrystals (PNCs) are attracting increasing attention due to their tunable optical features and band structure, which is pivotal to modulate their oxidizing/reducing power and perform chemical reactions efficiently. However, the defective structure and ionic nature of PNCs make them prone to be unstable in polar media, where most of the relevant solar-driven chemical reactions of interest take place. In this review, an overview of recent strategies to stabilize PNCs in polar solvents is presented, some of them with promising application or already introduced in PEC systems. Perovskite encapsulation, including the formation of heterostructures, surface passivation engineering, and the synthesis of PNCs with intrinsic stability in polar media, is focused upon. Furthermore, perspectives for using stable lead-free PNCs in solar-driven PEC reactions are presented, showing the benefits of this future technology for energy production/transformation.Funding for open access charge: CRUE-Universitat Jaume

High Quality Inkjet Printed-Emissive Nanocrystalline Perovskite CsPbBr3 Layers for Color Conversion Layer and LEDs Applications

Metal halide perovskites (MHPs) have shown outstanding optical emissive properties and can be employed in several optoelectronics devices. In contrast with materials of well-established technologies, which are prone to degradation or require expensive processes, MHPs can be obtained by solution processing methods and increase stability. Inkjet printing is proposed as an industrial friendly technique to deposit MHPs. The inks have been developed from colloidal CsPbBr3 nanocrystals and printing procedures that allow the deposition of thin layers with intense green emission. High emissive printed layers are assured by carrying out thermal annealing in vacuum oven, which is demonstrated to promote compact layers with low roughness, corroborated by SEM and AFM. XRD measurements show CsPbBr3 crystalline layers with cubic symmetry and XPS provides insight into the stoichiometric composition and local bonding. Optical properties of inkjet-printed CsPbBr3 films have been analyzed by UV–vis absorbance and photoluminescence (PL), to extract the bandgap energy and photoluminescence quantum yield (PLQY). CsPbBr3 printed layers emit at 524 nm with a narrow emission (FWHM ≈ 15 nm), exhibiting a PLQY up to 20%. These results enabled the large-scale fabrication by inkjet printing of CsPbBr3 color conversion layers (CCLs) and pave the way for flexible LEDs.The authors wish to thank the financial support from the European Commission via FET Open Grant (862656 – DROP-IT), MINECO (Spain) for grant PID2019-105658RB-I00 (PRITES project), Ministry of Science and Innovation of Spain under Project STABLE (PID2019-107314RB-I00), and Generalitat Valenciana via Prometeo Grant Q-Devices (Prometeo/2018/098). J.L.F. acknowledges the financial support from the Spanish Ministry of Education, Culture and Sports (FPU16/06257)

Photo-Induced Black Phase Stabilization of CsPbI3 QDs Films

α-CsPbI3 quantum dots (QDs) show outstanding photoelectrical properties that had been harnessed in the fabrication of perovskite QDs solar cells. Nevertheless, the stabilization of the CsPbI3 perovskite cubic phase remains a challenge due to its own thermodynamic and the presence of surface defects. Herein, we report the optimization of the CsPbI3 QDs solar cells, by monitoring the structure, the morphology and the optoelectronic properties after a precise treatment, consisting of the conventional solvent washing with a time limited ultraviolet (UV) exposure combination, during the layer-by-layer deposition. The UV treatment compensates the defects coming from the essential but deleterious washing treatment. The material is stable for 200 h and the PCE improved by the 25% compared with that of the device without UV treatment. The photo-enhanced ion mobility mechanism is discussed as the main process for the CsPbI3 QDs and solar cell stability

Recycled Photons Traveling Several Millimeters in Waveguides Based on CsPbBr3 Perovskite Nanocrystals

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 CsPbBr3 nanocrystals into a planar waveguide. An experimental set-up based on a frequency modulation spectroscopy allows to characterize the PR effect and the determination of the effective decay time of outcoupled photons. A correlation between the observed photoluminescence redshift and the increase of the effective decay time is demonstrated, which grows from 3.5 to near 9 ns in the best device. A stochastic Monte Carlo model reproduces these experimental results and allows the extraction of the physical mechanisms involved. In the waveguide under study recycled photons follow a drift (directional enhancement) velocity ≈5.7 × 105 m s−1, dominating over the diffusive regime observed in a standard thin film (D ≈ 420 m2 s−1). This means that recycled photons propagate mm-distances in shorter traveling times in the waveguide (≈5 ns) as compared to the film (>20 ns). These results are expected to pave the road for exploiting the PR effect in future optoelectronic and photonic devices

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

CsPbX3 perovskite nanocrystals (PNCs) have emerged as an excellent material for stimulated emission purposes, with even more prospective applications than conventional colloidal quantum dots. However, a better understanding of the physical mechanisms responsible for amplified spontaneous emission (ASE) is required to achieve more ambitious targets (lasing under continuous wave optical or electrical excitation). Here, we establish the intrinsic mechanisms underlying ASE in PNCs of three different band gaps (CsPbBr3, CsPbBr1.5I1.5, and CsPbI3). Our characterization at cryogenic temperatures does not reveal any evidence of the biexciton mechanism in the formation of ASE. Instead, the measured shift toward long wavelengths of the ASE band is easily explained by the reabsorption in the PNC layer, which becomes stronger for thicker layers. In this way, the threshold of ASE is determined only by optical losses at a given geometry, which is the single-exciton mechanism responsible for ASE. Experimental results are properly reproduced by a physical model

Fotoánodos modificados con óxido de grafeno reducido para mejorar el rendimiento fotoelectrocatalítico de B-TiO2 bajo luz visible

"The effect of reduced graphene oxide (rGO) content in boron-modified TiO2 nanocrystalline films on their photocatalytic activity in phenol oxidation is investigated. Visible-light-active TiO2 modified photoanodes were prepared by incorporating graphene sheets into the sol-gel reaction of B-TiO2, followed by depositing the reaction products on 304 stainless steel plates by dip-coating technique. Thin films obtained by in situ sol-gel synthesis were characterized by FESEM, GIXRD and UV–vis diffuse reflectance spectroscopy. FESEM examination showed cracked films due to the tensile stress generated by solvent evaporation. GIXRD results showed that boron in the films inhibits the growth of crystallites. Comparing to unmodified TiO2, B-TiO2/rGO showed a red shift in the band gap. The potentiodynamic anodic polarization measurements showed that graphene incorporation improved the photogenerated electron transport within the film, hence increasing the photocurrent. These enhancements are explained on the basis of the ability of graphene in promoting the charge carrier separation by transferring the photogenerated electrons from the illuminated photoanode to the substrate. The film B-TiO2/rGO obtained from the sol solution containing 0.03 wt/v% boron and 3 wt/v% graphene exhibited the highest photocurrent, which was 30 times larger compared with the photocurrent of TiO2 film.""Se investiga el efecto del contenido de óxido de grafeno reducido (rGO) en películas de TiO2 modificadas con boro sobre su actividad fotocatalítica en la oxidación de fenol. Fotoánodos modificados de TiO2 activos a la luz visible fueron preparados incorporando hojas de grafeno en la reacción sol-gel de B-TiO2, seguido por el depósito de los productos de la reacción sobre láminas de acero inoxidable 304 por la técnica dip-coating. Las películas delgadas obtenidas por síntesis sol-gel in-situ fueron caracterizadas por FESEM, GIXRD y espectroscopia de reflectancia difusa UV-vis. La observación por FESEM mostró películas agrietadas debido al estrés mecánico generado por la evaporación del solvente. Los resultados de GIXRD mostraron que el boro en las películas inhibe el tamaño de los cristalitos. Comparando con el TiO2, el dióxido de titanio modificado presentó un desplazamiento de la banda de energía prohibida hacia el rojo. Las mediciones de polarización anódica potenciodinámica mostraron que la incorporación de grafeno mejora el transporte de electrones fotogenerados dentro de las películas compuestas incrementando así la fotocorriente. Estas mejoras se explican en base a la habilidad del grafeno para facilitar la separación de portadores de carga, transfiriendo los electrones fotogenerados desde la película iluminada de B-TiO2 hasta el sustrato. La película compuesta B-TiO2/rGO obtenida a partir de la solución con 0.03 % p/v de boro y 3 % p/v de grafeno presentó la fotocorriente más alta, la cual fue 30 veces mayor comparada con la fotocorriente de la película de TiO2.

Enhanced spontaneous emission of CsPbI3 perovskite nanocrystals using a hyperbolic metamaterial modified by dielectric nanoantenna

In this work, we demonstrate, theoretically and experimentally, a hybrid dielectric-plasmonic multifunctional structure able to provide full control of the emission properties of CsPbI3 perovskite nanocrystals (PNCs). The device consists of a hyperbolic metamaterial (HMM) composed of alternating thin metal (Ag) and dielectric (LiF) layers, covered by TiO2 spherical MIE nanoresonators (i.e., the nanoantenna). An optimum HMM leads to a certain Purcell effect, i.e., an increase in the exciton radiative rate, but the emission intensity is reduced due to the presence of metal in the HMM. The incorporation of TiO2 nanoresonators deposited on the top of the HMM is able to counteract such an undesirable intensity reduction by the coupling between the exciton and the MIE modes of the dielectric nanoantenna. More importantly, MIE nanoresonators result in a preferential light emission towards the normal direction to the HMM plane, increasing the collected signal by more than one order of magnitude together with a further increase in the Purcell factor. These results will be useful in quantum information applications involving single emitters based on PNCs together with a high exciton emission rate and intensity