We present highly efficient electroluminescent devices using size-separated silicon nanocrystals (ncSi) as light emitting material. The emission color can be tuned from the deep red down to the yellow-orange spectral region by using very monodisperse size-separated nanoparticles. High external quantum efficiencies up to 1.1% as well as low turn-on voltages are obtained for red emitters. In addition, we demonstrate that size-separation of ncSi leads to drastically improved lifetimes of the devices and much less sensitivity of the emission wavelength to the applied drive voltage.

Organic light-emitting diodes (OLEDs) are currently being commercialized for lighting and display applications, but more work has to be done. In addition to the ongoing optimization of materials and devices in terms of efficiency and lifetime, the substitution of processing steps involving vacuum deposition for solution processing techniques is favorable. To reach this aim, good soluble materials are required. A modular family of highly emissive PyrPHOS-copper iodide complexes featuring various ancillary phosphine ligands has been synthesized. Photoluminescence spectroscopy, TCSPC (time-correlated single photon counting), cyclic voltammetry, X-ray diffraction, and DFT calculations were performed to gain a broad understanding of the complexes. While the photophysical properties are consistent within the family, thermal stability and solubility depend on the ligands. The materials showed very high photoluminescence quantum efficiencies up to 99% in powders and 85% in thin films. Selected examples were tested in devices, confirming the suitability of heteroleptic PyrPHOS-complexes for OLEDs.

Bragg gratings incorporated into organic light-emitting diodes (OLEDs) establish a coupling between waveguide modes and useful light (leaky modes). Here we demonstrate that the net coupling direction depends on the OLED stack design. We fabricated two different device structures with gold Bragg gratings. Angle resolved electroluminescence spectra were recorded. For the first device peaks of enhanced emission due to the Bragg grating are observed corresponding to a net energy transfer in direction of the leaky modes. The second device, on the other hand, exhibits dips in the emission spectrum. This reversed direction of energy transfer from the leaky modes to the waveguide modes is explained considering transfer matrix simulations of modal intensity distributions and device emission simulations. An OLED efficiency enhancement is only achieved, if the waveguide mode extraction is dominant.

White organic light emitting diodes (WOLEDs) suffer from poor outcoupling efficiencies. The use of Bragg-gratings to enhance the outcoupling efficiency is very promising for light extraction in OLEDs, but such periodic structures can lead to angular or spectral dependencies in the devices. Here we present a method which combines highly efficient outcoupling by a TiO 2 -Bragg-grating leading to a 104% efficiency enhancement and an additional high quality microlens diffusor at the substrate/air interface. With the addition of this diffusor, we achieved not only a uniform white emission, but also further increased the already improved device efficiency by another 94% leading to an overall enhancement factor of about 4.

In this study, we investigate for the first time morphological and compositional changes of silicon quantum dot (SiQD) light-emitting diodes (SiLEDs) upon device operation. By means of advanced transmission electron microscopy (TEM) analysis including energy filtered TEM (EFTEM) and energy dispersive X-ray (EDX) spectroscopy, we observe drastic morphological changes and degradation for SiLEDs operated under high applied voltage ultimately leading to device failure. However, SiLEDs built from size-separated SiQDs operating under normal conditions show no morphological and compositional changes and the biexponential loss in electroluminescence seems to be correlated to chemical and physical degradation of the SiQDs. By contrast, we found that, for SiLEDs fabricated from polydisperse SiQDs, device degradation is more pronounced with three main modes of failure contributing to the reduced overall lifetime compared to those prepared from size-separated SiQDs. With this newfound knowledge, it is possible to devise ways to increase the lifetimes of SiLEDs.