2, National Tsing Hua University, Hsinchu, , Taiwan
3, Global Science Instruments Co. Ltd., Hsinchu, , Taiwan
Organic light-emitting diodes (OLEDs) have progressively attracted generous attention because of their versatile applications in solid-state lighting and displays. High-efficiency is crucial for OLEDs being energy-saving and to have a long lifetime. Numerous approaches have been attempted to attain high-efficiency OLEDs through the synthesis of novel organic materials, the design of light extraction structures, and the design of efficiency-effective device architectures. The organic materials used in optoelectronic devices have inherently low dielectric constant.
In this work, we demonstrate a comprehensive model to quantitatively investigate the role of dielectric constant of the electron transporting materials on the electric field distribution, charge drift, and exciton recombination probability in the emissive zone (EML) and electron transporting layer (ETL) of the organic light-emitting diode via commercialized electrical simulation package SETFOS. The simulation outcomes show the electric field distribution in the EML and ETL is heavily influenced by the dielectric constant of the ETL’s material. The electric field in the ETL dramatically decreases from 2.1 to 1.46 MVcm-1, a decrement of 30%, as its dielectric increases from 1 to 5 respectively, whilst electric field across the EML increases from 0.78 to 2.21 MVcm-1, that jeopardize the device reliability at elevated applied voltages. The organic layer with low dielectric constant material reduces the electron mobilities in the EML region, as the consequence electron and holes cannot overcome to their binding energy. It exceeds the internal thermal energy of the device at room temperature and finally leads to the deleterious effects on OLEDs performance.
Moreover, the recombination rate in EML increases from 11.12 to 28.21 cm-2s-1 as dielectric constant increases from 1 to 5 respectively, while in the ETL it decreases from 3.08x10-2 to 4.01x10-5 cm-2s-1. The simulation results show that the ETL with high dielectric constant material not only increases the number of electrons but also reduces the number of holes entering into the EML that leads the charge-balance into the emissive-recombination zone. In short, the ETL with a high dielectric constant enables low-operation voltage, balance-charge-distribution, and enhanced light-emissive exciton, hence better device performance.