Toward Ultra‐Efficient OLEDs: Approaches Based on Low Refractive Index Materials

Abstract

Organic light‐emitting diodes (OLEDs) have successfully established themselves as light sources vital in high‐end mobile displays and television due to various advances made for a wide range of technical aspects from materials, devices, and processes to air‐tight packaging and systems. However, a significant portion of the generated excitons in OLEDs are still lost to various channels. Among them, excitation of surface plasmon polaritons (SPPs) is a critical source of exciton loss, the recovery of which often involves structuring in nanoscale. Herein, a nano‐pattern‐free approach is reviewed to reducing SPP loss that makes use of materials with low refractive index. Because of the limited availability of such low refractive index materials, a method to use a low extraordinary refractive index of birefringent organic semiconductors is explored. With the proposed method combined with common lens‐based outcoupling schemes and high refractive index substrates, it is shown that an ultrahigh external quantum efficiency of ≈72% in a green, single‐junction phosphorescent OLED is within the reach. Transient photoluminescence measurement is used to monitor the change in Purcell factor incurred by the use of the birefringent low refractive index layer and to further verify the validity of the proposed concept.

Spirobifluorene modified electron transport materials for high efficiency in phosphorescent organic light-emitting diodes

“…It has been previously reported that the extent of the loss of the SPP mode is affected largely by the dielectric constant of the cathode and the electron transport layer. 16,17,30,31 Therefore, the refractive index of the electron transport layer can be partially responsible for the outcoupling efficiency of the PhOLEDs. The EQE roll-off of the devices was slightly large in the SBFTrz device possibly due to the large change of the electron mobility according to the electric field.…”

Understanding the charge dynamics in organic light-emitting diodes using convolutional neural network

“…After the first report of bilayer organic light-emitting diodes (OLEDs), tremendous improvement of device performance has been realized by comprehensive research toward fundamental organic photonics based on quantum chemistry and applied physics. [1][2][3][4][5][6] Internal quantum efficiency was enhanced from 25% to 100% because of the triplet harvesting mechanisms through spin-orbit coupling (phosphorescence) and thermally activated reverse intersystem crossing (thermally activated delayed fluorescence). 7,8 The radiative decay of triplets by heavy metal complex and thermal transition of triplets to singlets by minimizing the thermodynamic barrier between both states successfully realized utilization of the entire singlets and triplets formed by polaron recombination.…”

“…Controlling the refractive index of organic layers and transient dipole orientation was introduced as an effective approach to overcome the challenges to increase the portion of outcoupled air mode. 2,[9][10][11] As the first process modulating the factors involving the efficiency, optimization of polaron transport is an essential requirement to ensure the high-performance of devices. [12][13][14][15][16][17][18][19] First, driving voltage and power efficiency intrinsically depend on the charge carrier mobility of the charge transport layer since high mobility of the charge transport layer facilitates the transportation of a high number of polarons toward EML with a low electric field.…”

“…Controlling the refractive index of organic layers and transient dipole orientation was introduced as an effective approach to overcome the challenges to increase the portion of outcoupled air mode. 2,9–11…”

2,3‐Disubstituted Fluorene Scaffold for Efficient Green Phosphorescent Organic Light‐Emitting Diodes

“…[44][45][46][47] In order to accelerate the commercialization of OLED, the development of new functional materials and the optimization of device configuration are always studied to improve the performance of OLEDs. [48][49][50][51][52][53][54][55][56][57][58] Nowadays, fluorene derivatives have been widely used in OLED materials, such as hole transport materials, [59] host materials, [60][61][62] as well as emission materials. [63][64][65][66][67] In addition, polyfluorenes were employed as blue emission materials due to their high fluorescence quantum yields and wide band gaps.…”