Organic light emitting diodes Research Papers (original) (raw)

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 emis-sive 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 photolu-minescence 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. In this work, it has been discovered that tris-[3-(3-pyridyl)mesityl]borane (3TPYMB) has a low refractive index of 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 (EQE). Optical simulation results of this study show that 59% of the enhancement comes from the low refractive index 3TPYMB, and the remaining 17% from the change in charge balance due to the 3TPYMB ETL in the OLED devices.