Band Alignment and Charge Transfer in CsPbBr3-CdSe Nanoplatelet Hybrids Coupled by Molecular Linkers

Controllable interface characteristics are demonstrated at the nanoscale, using two types of semiconducting nanoparticles with nearly identical optical band gaps, CsPbBr3 nanocrystals and CdSe nanoplatelets, capped with molecular linkers. By exploiting chemical recognition of the capping molecules, the two types of nanoparticles are brought into mutual contact, thus initiating spontaneous charge transfer and the formation of a strong junction field. Depending on the choice of capping molecules, the magnitude of the latter field is shown to vary in a broad range, corresponding to an interface potential step as large as circa 1 eV. The band diagram of the system, as well as the emergence of photo-induced charge transfer processes across the interface are studied here by means of optical and photoelectron based spectroscopies. Our results propose an interesting template for generating and harnessing internal built-in fields in heterogeneous nanocrystal solids.</p

PbS quantum dots as additives in methylammonium halide perovskite solar cells: the effect of quantum dot capping

Colloidal PbS quantum dots (QDs) have been successfully employed as additives in halide perovskite solar cells (PSCs) acting as nucleation centers in the perovskite crystallization process. For this strategy, the surface functionalization of the QDs, controlled via the use of different capping ligands, is likely of key importance. In this work, we examine the influence of the PbS QD capping on the photovoltaic performance of methylammonium lead iodide PSCs. We test PSCs fabricated with PbS QD additives with different capping ligands including methylammonium lead iodide (MAPI), cesium lead iodide (CsPI) and 4-aminobenzoic acid (ABA). Both the presence of PbS QDs and the specific capping used have a significant effect on the properties of the deposited perovskite layer, which affects, in turn, the photovoltaic performance. For all capping ligands used, the inclusion of PbS QDs leads to the formation of perovskite films with larger grain size, improving, in addition, the crystalline preferential orientation and the crystallinity. Yet, differences between the capping agents were observed. The use of QDs with ABA capping had a higher impact on the morphological properties while the employment of the CsPI ligand was more effective in improving the optical properties of the perovskite films. Taking advantage of the improved properties, PSCs based on the perovskite films with embedded PbS QDs exhibit an enhanced photovoltaic performance, showing the highest increase with ABA capping. Moreover, bulk recombination via trap states is reduced when the ABA ligand is used for capping of the PbS QD additives in the perovskite film. We demonstrate how surface chemistry engineering of PbS QD additives in solution-processed perovskite films opens a new approach towards the design of high quality materials, paving the way to improved optoelectronic properties and more efficient photovoltaic devices

From dilute isovalent substitution to alloying in CdSeTe nanoplatelets

International audienceCadmium chalcogenide nanoplatelet (NPL) synthesis has recently witnessed a significant advance in the production of more elaborate structures such as core/shell and core/crown NPLs. However, controlled doping in these structures has proved difficult because of the restrictive synthetic conditions required for 2D anisotropic growth. Here, we explore the incorporation of tellurium (Te) within CdSe NPLs with Te concentrations ranging from doping to alloying. For Te concentrations higher than ∼30%, the CdSexTe(1−x) NPLs show emission properties characteristic of an alloyed material with a bowing of the band gap for increased concentrations of Te. This behavior is in line with observations in bulk samples and can be put in the context of the transition from a pure material to an alloy. In the dilute doping regime, CdSe:Te NPLs, in comparison to CdSe NPLs, show a distinct photoluminescence (PL) red shift and prolonged emission lifetimes (LTs) associated with Te hole traps which are much deeper than in bulk samples. Furthermore, single particle spectroscopy reveals dramatic modifications in PL properties. In particular, doped NPLs exhibit photon antibunching and emission dynamics significantly modified compared to undoped or alloyed NPLs

Nucleation, Growth, and Structural Transformations of Perovskite Nanocrystals

Despite the recent surge of interest in lead halide perovskite nanocrystals, there are still significant gaps in the understanding of nucleation and growth processes involved in their formation. Using CsPbX₃ as a model system, we systematically study the formation mechanism of cubic CsPbX₃ nanocrystals, their growth via oriented attachment into larger nanostructures, and the associated phase transformations. We found evidence to support that the formation of CsPbX₃ NCs occurs through the seed-mediated nucleation method, where Pb° NPs formed during the course of reaction act as seeds. Further growth occurs through self-assembly and oriented attachment. The polar environment is a major factor in determining the structure and shape of the resulting nanoparticles, as confirmed by experiments with aged seed reaction mixtures, and by addition of polar additives. These results provide a fundamental understanding of the influence of the environment polarity on self-assembly, self-healing, and the ability to control the morphology and structure over the perovskite structures. As a result of this understanding, we were able to extend the synthesis to produce other materials such as CsPbBr₃ nanowires and orthorhombic CsPbI₃ nanowires with tunable length ranging from 200 nm to several microns

Resolving the Controversy in Biexciton Binding Energy of Cesium Lead Halide Perovskite Nanocrystals through Heralded Single-Particle Spectroscopy

Understanding exciton-exciton interaction in multiply excited nanocrystals is crucial to their utilization as functional materials. Yet, for lead halide perovskite nanocryst-als, which are promising candidates for nanocrystal-based technologies, numerous contradicting values have been reported for the strength and sign of their exciton-exciton interaction. In this work, we unambiguously determine the biexciton binding energy in single cesium lead halide perovskite nanocrystals at room temperature. This is enabled by the recently introduced single-photon avalanche diode array spectrometer, capable of temporally isolating biexciton-exciton emission cascades while retaining spectral resolution. We demonstrate that CsPbBr3 nanocrystals feature an attractive exciton-exciton interaction, with a mean biexciton binding energy of 10 meV. For CsPbI3 nanocrystals, we observe a mean biexciton binding energy that is close to zero, and individual nanocrystals show either weakly attractive or weakly repulsive exciton-exciton interaction. We further show that, within ensembles of both materials, single-nanocrystal biexciton binding energies are correlated with the degree of charge-carrier confinement.AQU