Highly Efficient Organic Light‐Emitting Diode Using A Low Refractive Index Electron Transport Layer

Abstract

use a low refractive index electron transport layer (ETL).To design a high-efficiency OLED, it is important to have a multilayer structure to control the radiative recombination such that excitons are confined to the emissive layer (EML) using high triplet charge blocking layers. In addition to exciton confinement, the electron and hole transport layers should be chosen in such a way to maintain the charge balance to avoid triplet-polaron quenching. In a multilayer OLED, the ETL also plays a key role in determining the light extraction efficiency since it is the intermediate layer between the EML and the metallic cathode, and a large portion of the dipole radiation from the EML is lost to the evanescent region where the in-plane wave-vector is larger than the total wave-vector, resulting in radiation that is coupled to the surface plasmon polariton (SPP) and the "lossy surface waves" on the metallic cathode. [4] The magnitude of the loss to the SPP mode is determined by the dielectric constants of the metallic cathode and the ETL, therefore the refractive index of the ETL can significantly impact the out-coupling efficiency of an OLED. There have been some reports demonstrating the effect of the ETL's refractive index on light extraction efficiency in OLEDs by simulation. [5,6] However, experimental study of the effect of ETL refractive index on device performance is rather limited. [7] In this work, we demonstrate the effect of ETL's refractive index on device efficiency using a solution processed OLED with a copper-based thermally activated delayed fluorescent (TADF) emitter, [(2-(Diphenylphosphino)-4-isobutylpyridine) (PPh3)2Cu2I2] (Cu(I)-iBuPyrPHOS). The emitter has a photoluminescence quantum yield (PLQY) of 70%. [8] Based on the PLQY and assuming an out-coupling efficiency of 20%, a maximum external quantum efficiency (EQE) of 14% is expected. Using 2,9-di(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (NBPhen) and 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T) as ETLs, we showed that an optimized device has a maximum EQE of 12% which is close to the estimate of the maximum EQE of 14%. Surprisingly, using tris-[3-(3-pyridyl)mesityl] borane (3TPYMB) as the ETL we achieved a maximum EQE of 21%, showing nearly a 76% enhancement just by changing the ETL alone. Upon investigation of the origin of the efficiency enhancement, we found that the refractive index of 3TPYMB is 1.65, which is the lowest among all commonly used ETLs, resulting in a significant enhancement in light extraction efficiency. Our device data are also confirmed by the optical simulation results.A low refractive index electron transport layer (ETL) can be very effective in enhancing the out-coupling efficiency of an organic light-emitting diode (OLED). However, most organic films show a refractive index close to 1.8. 1.65 (at 550 nm), which is the lowest refractive index ETL among the commonly used ETLs up to date. Using 3TPYMB as an ETL, a solution processed OLED is demonstrated with nearly a 76% enhancement in external quantum efficiency (...

TADF Material Design: Photophysical Background and Case Studies Focusing on CuIand AgIComplexes

“…Basic research of photophysical and chemical properties of organo-transition-metal compounds was strongly activated by their potential commercial use. This became particularly apparent for classes of compounds that may be applied as emitters in organicl ight emitting diodes (OLEDs) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] or in light emitting electrochemical cells (LEEC). [7,[20][21][22][23][24][25][26][27][28] These scientific investigations led to am uch deeper understanding of photophysical principles and of the compound's properties resultingi nt he development of an enormousn umber of new materials in part with drastically improved properties for OLED applications.…”

Omnidirectional Reflector based on Transparent Oxide Materials for Light Extraction Enhancement of Organic and Inorganic Solid-state Light Emitting Devices

“…On the other hand, (b) OLED needs low refractive index organic conductive materials as electron transport layer (ETL) to introduce TI-ODR. It is usually hard to realize high conductivity and low refractive index simultaneously in organic materials, so the structure has currently been in study phase yet [25,26]. In this simulation model, the lowest refractive index already reported was adopted [25].…”

“…It is usually hard to realize high conductivity and low refractive index simultaneously in organic materials, so the structure has currently been in study phase yet [25,26]. In this simulation model, the lowest refractive index already reported was adopted [25]. The detail of reflector's structure was simulated by ThinFilmView version 2.2.0, which is a Fresnel calculation program for optical thin films manufactured by Nary Software.…”

Flexible top-emitting organic light emitting diodes with a functional dielectric reflector on a metal foil substrate

“…1,2 For display devices and general lighting, glass has been used as a substrate material of OLEDs because of its transparency, high-temperature processing capability, and smooth surface. [3][4][5] Current studies on OLEDs have been motivated by the potential for low-cost manufacture with high throughput using roll-to-roll (R2R) techniques, for which the glass substrate is unfavourable because of its non-exibility. 6,7 Thus, the exible substrate materials are an essential prerequisite for performance, reliability, and practical application of exible OLEDs.…”