Claas J. Reckmeier

Enhancing the Brightness of Cesium Lead Halide Perovskite Nanocrystal Based Green Light-Emitting Devices through the Interface Engineering with Perfluorinated Ionomer

High photoluminescence quantum yield, easily tuned emission colors, and high color purity of perovskite nanocrystals make this class of material attractive for light source or display applications. Here, green light-emitting devices (LEDs) were fabricated using inorganic cesium lead halide perovskite nanocrystals as emitters. By introducing a thin film of perfluorinated ionomer (PFI) sandwiched between the hole transporting layer and perovskite emissive layer, the device hole injection efficiency has been significantly enhanced. At the same time, PFI layer suppressed charging of the perovskite nanocrystal emitters thus preserving their superior emissive properties, which led to the three-fold increase in peak brightness reaching 1377 cd m(-2). The full width at half-maximum of the symmetric emission peak with color coordinates of (0.09, 0.76) was 18 nm, the narrowest value among perovskite based green LEDs.

Fabrication of efficient planar perovskite solar cells using a one-step chemical vapor deposition method

Organometallic trihalide perovskites are promising materials for photovoltaic applications, which have demonstrated a rapid rise in photovoltaic performance in a short period of time. We report a facile one-step method to fabricate planar heterojunction perovskite solar cells by chemical vapor deposition (CVD), with a solar power conversion efficiency of up to 11.1%. We performed a systematic optimization of CVD parameters such as temperature and growth time to obtain high quality films of CH3NH3PbI3 and CH3NH3PbI3-xClx perovskite. Scanning electron microscopy and time resolved photoluminescence data showed that the perovskite films have a large grain size of more than 1 micrometer, and carrier life-times of 10 ns and 120 ns for CH3NH3PbI3 and CH3NH3PbI3-xClx, respectively. This is the first demonstration of a highly efficient perovskite solar cell using one step CVD and there is likely room for significant improvement of device efficiency.

Luminescent colloidal carbon dots: optical properties and effects of doping [Invited].

We review the effect of doping on the optical properties of luminescent colloidal carbon dots. They are considered as a hybrid material featuring both molecular and semiconductor-like characteristics, where doping plays an important role. Starting from the short overview of synthetic strategies, we consider the evolution of carbon dots from molecular precursors to fluorescent nanoparticles, and the relevant structural properties of carbon dots. Choice of the reactant materials, dopant atoms and reaction parameters provide carbon dots with varying optical properties. High chemical stability, bright luminescence and customizable surface functionalization of carbon dots open their use in a broad range of applications, which are exemplary presented at the end of this review.

Room-Temperature Solution-Processed NiOx: PbI2 Nanocomposite Structures for Realizing High-Performance Perovskite Photodetectors

While methylammonium lead iodide (MAPbI3) with interesting properties, such as a direct band gap, high and well-balanced electron/hole mobilities, as well as long electron/hole diffusion length, is a potential candidate to become the light absorbers in photodetectors, the challenges for realizing efficient perovskite photodetectors are to suppress dark current, to increase linear dynamic range, and to achieve high specific detectivity and fast response speed. Here, we demonstrate NiOx:PbI2 nanocomposite structures, which can offer dual roles of functioning as an efficient hole extraction layer and favoring the formation of high-quality MAPbI3 to address these challenges. We introduce a room-temperature solution process to form the NiOx:PbI2 nanocomposite structures. The nanocomposite structures facilitate the growth of the compact and ordered MAPbI3 crystalline films, which is essential for efficient photodetectors. Furthermore, the nanocomposite structures work as an effective hole extraction layer, which provides a large electron injection barrier and favorable hole extraction as well as passivates the surface of the perovskite, leading to suppressed dark current and enhanced photocurrent. By optimizing the NiOx:PbI2 nanocomposite structures, a low dark current density of 2 × 10(-10) A/cm(2) at -200 mV and a large linear dynamic range of 112 dB are achieved. Meanwhile, a high responsivity in the visible spectral range of 450-750 nm, a large measured specific detectivity approaching 10(13) Jones, and a fast fall time of 168 ns are demonstrated. The high-performance perovskite photodetectors demonstrated here offer a promising candidate for low-cost and high-performance near-ultraviolet-visible photodetection.

Mesoporous Aluminum Hydroxide Synthesized by a Single-Source Precursor-Decomposition Approach as a High-Quantum-Yield Blue Phosphor for UV-Pumped White-Light-Emitting Diodes

Strongly emissive (photoluminescence quantum yield up to 65%), thermally stable aluminum hydroxide blue phosphors are synthesized by a single-source precursor-decomposition approach. Blue-emitting UV-pumped light-emitting diodes (LEDs) based on the aluminum hydroxide phosphor reach luminous efficiency of 27.5 lm W-1 , while UV-white-LEDs integrating blue-emitting aluminum hydroxide and red-emitting CuInS2 nanocrystals achieve high color-rendering-index values of 94.3 and luminous efficiency of 23.5 lm W-1 .

Efficient near-infrared light-emitting diodes based on organometallic halide perovskite–poly(2-ethyl-2-oxazoline) nanocomposite thin films

Organometallic halide perovskites have recently drawn considerable attention for applications in light emission diodes (LEDs). However, the small exciton binding energy of the CH3NH3PbI3 perovskite has the concerns of large exciton dissociation and low radiative recombination on its use in near-infrared LEDs (NIR-LEDs). Herein, we propose and demonstrate that the introduction of poly(2-ethyl-2-oxazoline) (PEtOz) into the perovskite can simultaneously improve the recombination rate and radiative decay rate for improving perovskite LED performances. Additionally, our approach results in smooth perovskite films with increased thickness, reduced roughness, and pin-hole free, which facilitates other film deposition on top for practical device fabrication, and reduces current leakage. After optimizing the perovskite-PEtOz nanocomposite emission layer in NIR-LEDs (emission peak at 760 nm), a high radiance of 12.3 W sr-1 m-2 and 70-fold enhancement of the external quantum efficiency (EQE) compared to that of the pristine perovskite case are achieved. The maximum EQE reaches 0.76%, which is the highest EQE reported so far for the CH3NH3PbI3 based NIR-LEDs. The simplicity of our fabrication approach combined with the outstanding device performances further highlights the enormous potential of perovskite-based LEDs.

Impact of D2O/H2O Solvent Exchange on the Emission of HgTe and CdTe Quantum Dots: Polaron and Energy Transfer Effects.

We have studied light emission kinetics and analyzed carrier recombination channels in HgTe quantum dots that were initially grown in H2O. When the solvent is replaced by D2O, the nonradiative recombination rate changes highlight the role of the vibrational degrees of freedom in the medium surrounding the dots, including both solvent and ligands. The contributing energy loss mechanisms have been evaluated by developing quantitative models for the nonradiative recombination via (i) polaron states formed by strong coupling of ligand vibration modes to a surface trap state (nonresonant channel) and (ii) resonant energy transfer to vibration modes in the solvent. We conclude that channel (i) is more important than (ii) for HgTe dots in either solution. When some of these modes are removed from the relevant spectral range by the H2O to D2O replacement, the polaron effect becomes weaker and the nonradiative lifetime increases. Comparisons with CdTe quantum dots (QDs) served as a reference where the resonant energy loss (ii) a priori was not a factor, also confirmed by our experiments. The solvent exchange (H2O to D2O), however, is found to slightly increase the overall quantum yield of CdTe samples, probably by increasing the fraction of bright dots in the ensemble. The fundamental study reported here can serve as the foundation for the design and optimization principles of narrow bandgap quantum dots aimed at applications in long wavelength colloidal materials for infrared light emitting diodes and photodetectors.

Gamma ray shifted and enhanced photoluminescence of graphene quantum dots

We demonstrate how the emission of graphene quantum dots with a surface consisting of hydroxyl and quinone groups is tuned from green to blue by gamma-ray irradiation ranging from 0 to 500 kGy and in the presence of radical scavengers. The photoluminescence quantum yield increases from 13 to 52% under optimal irradiation conditions. Gamma-ray irradiation triggered radicals either oxidize or reduce the surface groups of graphene quantum dots altering their chemical structure, reducing surface traps and enhancing the emission. The resulting chemical composition of surface groups derived from XPS analysis influences emission pathways and trap formation. Furthermore, the irradiated nanoparticles show a better performance and cell viability in bioimaging applications as compared to the pristine graphene quantum dots.