Study On Blue Phosphorescent Organic Light-emitting Device With Periodic Emission Layer

Since the electroluminescence of organic materials was discovered, which has attracted much attention. It becomes a strong competitor for next display and lighting source. In which blue organic light-emitting device is an important component of full color display and white lighting. Although blue phosphorescent organic materials has much great efficiency than fluorescent organic materials, but blue phosphorescent organic materials have a poor stability and the electroluminescent spectra has an acromion, the intensity of this acromion has a obvious change with the voltage varing, which leads to a poor spectral stability. Researchers at home and abroad has a long-term work on efficiency of blue organic light-emitting and the synthesis of novel blue materials. Little attention has been paid to the spectral stability. We prepared a blue OLED with a periodic emission layer to improve performance in this paper.1. Compared the spectra of blue phosphorescent light with the spectra of red phosphorescent light and yellow phosphorescent light. We find blue phosphorescent light has a strong acromion and the intensity of this acromion has a obvious change with the voltage varing, which leads to a poor spectral stability. Through the analysis and research, we found that as the wavelength becomes shorter, the exciton lifetime of metal-ligand charge transfer state（MLCTs） which produces the main sources becomes longer, which leads to MLCTs can’t timely consumption exciton, and enhances the radiative transition probability of vibronic emission band as the wavelength becomes shorter.2. Through periodically inserting FIrpic doped Ir（ppz）₃ into emission layer, we find the efficiency becomes higher. And study on the number of inserted thin layers, in which the maximal current efficiency is improved from 16.55cd/A to 20.70cd/A, the maximal power efficiency is improved from 14.85lm/W to 16.95lm/W. The roll-off of current efficiency and power efficiency of device without Ir（ppz）₃ are 39.88% and 62.09% and the smallest current efficiency and the smallest power efficiency of device with Ir（ppz）₃ are 8.88% and 26.26% at 5000cd/m2, respectively. At the same time, with the number of Ir（ppz）₃ layer increasing, the spectra becomes more stable. The smallest change of chromaticity coordinate is（±0.0011,±0.0024）.3. Though the analysis of single carriers devices, we foud Ir（ppz）₃ inserted into m CP can block electrons and trap holes through the different LUMO and HOMO energy, It can slow down the accumulation of carriers at a certain position in emission layer, which reduces quenching and slow down the roll-off of efficiency. At the same time, the concentration of remainder excitons which can’t be used up by metal-to-ligand charge transfer becomes lower caused by reduction of excitons accumulation, this leads to lower radiative transition probability of vibronic emission band and solves the problem of acromion changes with voltage varing.4. The devices with Ir（ppz）₃ as host and m CP as host were analysized, respectively. We found the efficiency of device with Ir（ppz）₃ as host is lower than device with m CP as host. It demonstrates the efficiency improvement of device with Ir（ppz）₃:FIrpic layer isn’t because Ir（ppz）₃ is more suitable host for FIrpic than m CP.5. We analysized the performance of single carriers devices based on m CP or Ir（ppz）₃, respectively. We found the electron mobility of Ir（ppz）₃ is lower than m CP and the hole mobility of Ir（ppz）₃ is slightly larger than m CP. It also demonstrates Ir（ppz）₃ isn’t a suitable host material.6. Study on the distribution of carriers and excitions in emission layer of device with Ir（ppz）₃. We found the efficiency improvement of device with Ir（ppz）₃:FIrpic layer is caused by Ir（ppz）₃/m CP interface which block electrons and holes, and it also reveals the carriers distribution in emission layer.