Organic Light - Emitting Diodes and their Applications

Abstract

Organic light emitting diodes (OLEDs) have been the focus of intense study since the late 1980s, when the low voltage organic electroluminescence in small organic molecules such as Alq3, and large organic molecules such as polymers (PPV), was reported. Since that time, research has continued to demonstrate the potential of OLEDs as viable systems for displays and eco-friendly lighting applications. OLEDs offer full colour display, reduced manufacturing cost, larger viewing angle, more flexible, lower power consumption, better contrast, slimmer, etc. which help in replacing the other technologies such as LCD. The operation of OLEDs involves injection of charge carriers into organic semiconducting layers, recombination of charge carriers, formation of singlet and triplet excitons, and emission of light during decay of excitons. The maximum internal quantum efficiency of fluorescent OLEDs consisting of the emissive layer of fluorescent organic material is 25% because in this case only the 25% singlet excitons can emit light. The maximum internal quantum efficiency of phosphorescent OLEDs consisting of the emissive layer of fluorescent organic material mixed with phosphorescent material of heavy metal complexes such as platinum complexes, iridium complexes, etc. is nearly 100% because in this case both the 25% singlet excitons and 75% triplet excitons emit light. Recently, a new class of OLEDs based on thermally activated delayed fluorescence (TADF) has been reported, in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates of more than 106decays per second. These molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels and provides an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.The OLED technology can be used to make screens large enough for laptop, cell phones, desktop computers, televisions, etc. OLED materials could someday be applied to plastic and other materials to create wall-size video panels, roll-up screens for laptops, automotive displays, and even head wearable displays. Presently, the OLEDs are opening up completely new design possibilities for lighting in the world of tomorrow whereby the offices and living rooms could be illuminated by lighting panels on the ceiling. The present paper describes the salient features of OLEDs and discusses the applications of OLEDs in displays and solid state lighting devices. Finally, the challenges in the field of OLEDs are explored. Contents of Paper

Combining an Ionic Transition Metal Complex with a Conjugated Polymer for Wide-Range Voltage-Controlled Light-Emission Color

“…For a manifold of emissive applications, including high- and low-information content displays and signage as well as mood-controlled illumination, it is fundamental to be able to shift or tune the emission color from a pixel or an entire device on-demand during operation. One notable success story in this aspect is the organic light-emitting diode (OLED), which recently was commercially introduced as the high-information content display in various high-end applications such as cellular phones and digital cameras. , However, the fabrication of OLEDs and similar display-fit technologies (e.g., the inorganic LED and the liquid crystal display) is technically challenging and expensive, and the interest for alternative low-cost technologies capable of achieving an on-demand control of the light-emission color is significant. , …”

Recent advances in efficient emissive materials-based OLED applications: a review

“…The formation of excitons depends on the dielectric constant (e) of the emitting material. The lower dielectric constant has better exciton formation than the higher dielectric constant [34,64,65]. The photon out-coupling efficiency of the OLED device mainly depends on the substrate refractive index and the electrodes emitting sides.…”

Recent Advances in the Optimization of Organic Light‐Emitting Diodes with Metal‐Containing Nanomaterials

“…h f is the PL quantum yield of emitting molecules, and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 h out is the light out-coupling efficiency. [51] In conventional OLEDs, the light generated in the EML usually escapes into free space after passing through various organic layers, ITO and glass substrates. [52] Figure 4 shows the light propagation path in an OLED device.…”

“…Optical channels in a conventional OLED. [51] Figure 5. Schematic diagram of LSPR process of metal nanoparticles.…”

“…where γ is the charge balance factor of injected holes and electrons, n r is probability of radiative exciton (i. e. the ratio of singlet excitons for fluorescent emitting materials and the ratio of triplet excitons for phosphorescent emitting materials). f is the PL quantum yield of emitting molecules, and out is the light out‐coupling efficiency . In conventional OLEDs, the light generated in the EML usually escapes into free space after passing through various organic layers, ITO and glass substrates .…”