Excellent blue/pure blue iridium(iii) phosphors with high ΦPL, narrow FWHMs and robust chemical structures are designed for partially solution-processed OLEDs.
Organic light-emitting diodes (OLEDs) possess a number of advantages such as low power consumption, light weight, wide color gamut, high response speed, and high contrast ratio. They have received widespread attention due to their tremendous commercial applications in the fields of full-color flat panel display and solid-state lighting. Although nearly 100% internal quantum efficiency of OLED has been achieved through adopting phosphorescence or thermally activated delayed fluorescence emitters. However, the majority of light generated in an emitting layer is confined within the whole device but does not escape into air due to the induced surface plasmons at the interface between metal and dielectric layers as well as the differences in refractive index between layers of OLED structures including air, glass substrate, transparent electrode as well as organic or inorganic layers. The external quantum efficiency for an OLED with a flat glass substrate is limited to~20%. A low light out-coupling efficiency severely restricts the development and application of OLED. Therefore, enhancing the light out-coupling efficiency of OLED via light extraction technology offers the greatest potential for achieving a substantial increase in the external quantum efficiency of OLED and has been one of the most attractive projects. Up to now, lots of light out-coupling technologies such as micro-lens arrays, photonic crystal, Bragg mirrors and periodic grating have been suggested to enhance the out-coupling efficiency of OLEDs. However, the periodic light out-coupling structures have a limitation that the electroluminescence intensity and spectrum of OLED usually depend on the viewing angle. The angular dependence of the emission characteristic does not hold true for actual display applications due to its deviation from the Lambertian intensity distribution. In this review, we present recent research progress of using non-period micro/nanostructures to improve the light out-coupling efficiency of OLED. In contrast to the emission directionality for OLED using periodic light out-coupling structures, the luminance distribution and spectral stability of OLED based on non-period micro/nanostructures are insensitive to viewing angle. Various light out-coupling techniques such as random micro/nano lens structure, light scattering medium layer, polymer porous scattering films, random concave-convex corrugated structure, and random buckled structure are summarized and discussed. These techniques have the potential applications in displays and solid-state lighting. Finally, summary and prospects regarding to light-coupling techniques of OLEDs are presented.
Metal doping plays an important role in the resistance switching mechanism of the resistive random access memory (RRAM). The oxygen vacancy (V O ) formation energy under different oxygen partial pressures (P Od 2 ) and temperatures (T), the interaction of Mo dopant with V O , and the electronic structures of Mo-doped defect systems have been investigated by first-principles calculations. It is found that Mo doping significantly reduces the formation energy of neutral V O and decreases the stability of the +2 charged V O , especially in the Mo-doped HfO 2 system. Lower P Od 2 and higher T values are more energetically favorable to the formation of V O . The strong attraction between Mo and V O with charge states of 0 and +1q allows more oxygen vacancies to generate around the dopant, indicating that the Mo dopant can effectively modulate the distribution of V O , thus improving the uniformity of HfO 2 -and ZrO 2 -based RRAMs. The results of the electronic structures show that Mo doping can significantly enhance the conductivity of the devices; more electrons will be localized around the dopant, V O , and their surrounding atoms, thus promoting the formation of conductive filaments (CFs). It can be concluded that Mo doping can reduce the forming voltage and improve the resistance switching performance of HfO 2 -and ZrO 2 -based RRAMs, which provides theoretical guidance for the applications of metal-doped HfO 2 -and ZrO 2 -based RRAM devices.
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