Semiconducting nanocrystals for large-area light-emitting devices
Ekaterina Nannen, Universität Duisburg-Essen, Duisburg, GermanyMiriam Di Marcantonio, OSRAM GmbH, Augsburg, GermanyJulia Frohleiks, University Duisburg-Essen, Duisburg, GermanySvenja Wepfer, University Duisburg-Essen, Duisburg, GermanyGerd Bacher, University Duisburg-Essen, Duisburg, Germany
Large-area, ultrathin light-emitting devices such as organic light emitting diodes (OLEDs) or light-emitting electrochemical cells (LECs) currently inspire lighting design in interior and automotive industry all over the world. Still, some fundamental challenges regarding the performance and long-term stability need to be solved to transfer these new lighting concepts into application. Semiconducting nanocrystals (NCs) for example offer promising potential as an active material with high long-term stability and adjustable emission color by variation of size and composition. Due to their excellent electrical properties, NCs can also be applied to optimize charge carrier injection and charge carrier balance resulting in increased luminance and efficiency values.
In hybrid devices based on ionic transition metal complexes (iTMCs), we demonstrate the usage of NCs for optimizing charge carrier injection and color tunability. Water-free synthesis of monocrystalline ZnO-NCs of 3.5 nm in size is established. The particles can be stabilized without any additives in different organic solvents, enabling spin-coating processes on various layers including the active LEC layer. In the LEC device geometry, the ZnO layer acts as electron injecting and hole blocking layer simultaneously. The resulting devices yield a luminance of about 490 cd/m² and efficacies of 3.5 cd/A and 2.4 lm/W at pulsed current long-term operation (20 mA/cm²), i.e., luminance and efficiency is doubled with respect to reference devices. By implementing colloidal quantum dots (QDs) of different size as additional emissive material in yellow light-emitting iTMC-LECs, the emission color can be expanded, even realizing white emission.
An alternative promising device concept for improving robustness and long-term stability of light-emitting devices is the implementation of QDs instead of organic molecules as light-emitting species. Hereby, Cd-based materials, which are prohibited in the European Union, have to be avoided. We develop solution-based QD-LEDs with CuInS2-QDs as a Cd-free alternative yielding luminance up to 200 cd/m² and external quantum efficiency and current efficacy of around 1 % and 2.6 cd/A, respectively. Due to the broad defect-related emission spectra of CuInS2-QDs, white light emitting devices with high CRI values (> 75) are demonstrated in combination with blue QDs.