ABSTRACT
Quantum dots (QDs) are promising candidates for producing bright, color-pure, cost-efficient, and long-lasting QD-based light-emitting diodes (QDLEDs). However, one of the significant problems in achieving high efficiency of QDLEDs is the imbalance between the rates of charge-carrier injection into the emissive QD layer and their transport through the device components. Here we investigated the effect of the parameters of the deposition of a poly (methyl methacrylate) (PMMA) electron-blocking layer (EBL), such as PMMA solution concentration, on the characteristics of EBL-enhanced QDLEDs. A series of devices was fabricated with the PMMA layer formed from acetone solutions with concentrations ranging from 0.05 to 1.2 mg/mL. The addition of the PMMA layer allowed for an increase of the maximum luminance of QDLED by a factor of four compared to the control device without EBL, that is, to 18,671 cd/m2, with the current efficiency increased by an order of magnitude and the turn-on voltage decreased by ~1 V. At the same time, we have demonstrated that each particular QDLED characteristic has a maximum at a specific PMMA layer thickness; therefore, variation of the EBL deposition conditions could serve as an additional parameter space when other QDLED optimization approaches are being developed or implied in future solid-state lighting and display devices.
ABSTRACT
A new accessible sonochemical assembly method is developed for the preparation of photoluminescent oil-filled silica@CuS/Cu2O/CuO-graphene oxide (GO) microspheres that emit light of green, yellow, and red colors. This method is based on the ultrasonic emulsification of a biphasic mixture consisting of CuS/Cu2O/CuO-graphene oxide (GO) nanocomposites with poly(vinyl alcohol) (PVA) (aqueous phase) and tetraethyl orthosilicate with sunflower oil (organic phase). CuS/Cu2O/CuO-GO nanocomposites are composed of sonochemically formed three phases of copper: covellite CuS (p-type semiconductor), cuprite Cu2O (Bloch p-type semiconductor), and CuO (charge-transfer insulator). The photoluminescence properties of microspheres result from H-bridging between PVA and CuS/Cu2O/CuO-GO nanostructures, light absorption ability of Cu2O, and charge-transfer insulation by CuO. Substitution of PVA by S-containing methylene blue quenches fluorescence by enhanced dye adsorption on CuS/Cu2O/CuO-GO because of CuS and induced charge transfer. Non-S-containing malachite green is in a nonionized form and tends to be in the oil phase, prohibiting the charge transfer on CuS/Cu2O/CuO-GO.
