Blue-Emitting Cs(Pb,Cd)Br3 Nanocrystals Resistant to Electric Field-Induced Ion Segregation.

High-performance solution-processed perovskite light-emitting diodes (PeLEDs) have emerged as a good alternative to the well-established technology of epitaxially grown AIIIBV semiconductor alloys. Colloidal cesium lead halide perovskite nanocrystals (CsPbX3 NCs) exhibit room-temperature excitonic emission that can be spectrally tuned across the entire visible range by varying the content of different halogens at the X-site. Therefore, they present a promising platform for full color display manufacturing. Engineering of highly efficient PeLEDs based on bromide and iodide perovskite NCs emitting green and red light, respectively, does not face major challenges except low operational stability of the devices. Meanwhile, mixed-halide counterparts demonstrating blue luminescence suffer from the electric field-induced phase separation (ion segregation) phenomenon described by the rearrangement (demixing) of mobile halide ions in the crystal lattice. This phenomenon results in an undesirable temporal redshift of the electroluminescence spectrum. However, to realize spectral tuning and, at the same time, address the issue of ion segregation less mobile Cd2+ ion could be introduced in the lattice at Pb2+-site that leads to the band gap opening. Herein, we report an original synthesis of CsPb0.88Cd0.12Br3 perovskite NCs and study their structural and optical properties, in particular electroluminescence. Multilayer PeLEDs based on the obtained NCs exhibit single-peak emission centered at 485 nm along with no noticeable change in the spectral line shape for 30 min which is a significant improvement of the device performance.

A quantum model of charge capture and release onto/from deep traps.

The rapid development of optical technologies and applications revealed the critical role of point defects affecting device performance. One of the powerful tools to study the influence of defects on charge capture and recombination processes is thermoluminescence. The popular models behind thermoluminescence and carrier capture processes are semi-classic though. They offer a good qualitative description, but implicitly exclude the quantum nature of the accompanying parameters, such as frequency factors and capture cross sections. As a consequence, results obtained for a specific host material cannot be successfully extrapolated to other materials. Thus, the main purpose of our work is to introduce a reliable analytical model that describes non-radiative capture and release of electrons from/to the conduction band (CB). The proposed model is governed by Bose-Einstein statistics (for phonon occupation) and Fermi's golden rule (for resonant charge transfer between the trap and the CB). The constructed model offers a physical interpretation of the capture coefficients and frequency factors, and seamlessly includes the Coulomb neutral/attractive nature of traps. It connects the frequency factor to the overlap of wavefunctions of the delocalized CB and trap states, and suggests a strong dependence on the density of charge distribution, i.e. the ionicity/covalency of the chemical bonds within the host. Separation of the resonance conditions from the accumulation/dissipation of phonons on the site leads to the conclusion that the capture cross-section does not necessarily depend on the trap depth. The model is verified by comparison to the reported experimental data, showing good agreement. As such, the model generates reliable information about trap states whose exact nature is not completely understood and allows performing materials research in a more systematic way.

Tuning the Optical Properties and Conductivity of Bundles in Networks of Single-Walled Carbon Nanotubes.

The films of single-walled carbon nanotubes (SWCNTs) are a promising material for flexible transparent electrodes, which performance depends not only on the properties of individual nanotubes but also, foremost, on bundling of individual nanotubes. This work investigates the impact of densification on optical and electronic properties of SWCNT bundles and fabricated films. Our ab initio analysis shows that the optimally densified bundles, consisting of a mixture of quasi-metallic and semiconducting SWCNTs, demonstrate quasi-metallic behavior and can be considered as an effective conducting medium. Our density functional theory calculations indicate the band curving and bandgap narrowing with the reduction of the distance between nanotubes inside bundles. Simulation results are consistent with the observed conductivity improvement and shift of the absorption peaks in SWCNT films densified in isopropyl alcohol. Therefore, not only individual nanotubes but also the bundles should be considered as building blocks for high-performance transparent conductive SWCNT-based films.

Electronic structure of CsPbBr3-xClx perovskites: synthesis, experimental characterization, and DFT simulations.

All-inorganic lead halide perovskites of various compositions have emerged as a prospective family of materials for light-emitting devices and photonic applications. However, a comprehensive study of their structural and electronic properties is still missing. Moreover, thin film fabrication of these perovskites comprising heterohalide anions by wet chemistry approaches also remains challenging. Here we fabricate high-quality CsPbBr3-xClx perovskite thin films using a wet chemical method accompanied by a chemical vapor anion exchange procedure, which allows rigorously studying their optical and structural properties at different compositions. Namely, we present both the numerical and experimental studies of the electronic properties of all-inorganic mixed-halide perovskites, showing their optical absorption, excitonic photoluminescence and exciton binding energy, phase, chemical composition, and band structure and the band gap evolution with a gradual change in x in CsPbBr3-xClx. The results reveal that as Cl ions substitute for Br ones in the perovskite crystal lattice the room-temperature phase does not change its orthorhombic symmetry, whereas the energy of the direct electronic transition from the valence to conduction band at the Γ-point increases nonlinearly. By using the experimentally derived nonlinear dependence it is easy to accurately predict the band gap for any CsPbBr3-xClx perovskite thin film consisting of grains with sizes beyond the quantum confinement regime.

Modeling and Assessment of Afterglow Decay Curves from Thermally Stimulated Luminescence of Complex Garnets.

Afterglow is an important phenomenon in luminescent materials and can be desired (e.g., persistent phosphors) or undesired (e.g., scintillators). Understanding and predicting afterglow is often based on analysis of thermally stimulated luminescence (TSL) glow curves, assuming the presence of one or more discrete trap states. Here we present a new approach for the description of the time-dependent afterglow from TSL glow curves using a model with a distribution of trap depths. The method is based on the deconvolution of the energy dependent density of occupied traps derived from TSL glow curves using Tikhonov regularization. To test the validity of this new approach, the procedure is applied to experimental TSL and afterglow data for Lu1Gd2Ga3Al2O12:Ce ceramics codoped with 40 ppm of Yb3+ or Eu3+ traps. The experimentally measured afterglow curves are compared with simulations based on models with and without the continuous trap depth distribution. The analysis clearly demonstrates the presence of a distribution of trap depths and shows that the new approach gives a more accurate description of the experimentally observed afterglow. The new method will be especially useful in understanding and reducing undesired afterglow in scintillators.