Effect of triplet energy and transporting property of electron transporting material on iridium complex yellow phosphorescent organic light-emitting devices

摘要

Balanced charge carrier and appropriate exciton confinement are considered as the key factors for the realization of highly efficient and stable organic light-emitting devices (OLEDs). An effective way to reach a loss free hole-electron recombination and exciton leakage is to use suitable electron transporting layers (ETLs). To investigate the influence of triplet energy and electron transporting properties of ETLs on the performance of iridium complex yellow phosphorescent OLEDs, we fabricated three devices based on bis[2-(4-tert-butylphenyl)benzothiazolato-N,C2']iridium (acetylacetonate) [(t-bt)_2Ir(acac)] doped 4,4'-bis(carbazol-9-yl) biphenyl (CBP) host by using 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), l,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi), and 4,7-diphyenyl-l,10-phenanthroline (BPhen) as ETLs, respectively. It was found that there was no apparent correlation between the device efficiency and the triplet energy of ETLs. Instead, device efficiency was determined by the electron mobility of ETLs only. With an optimized device using BPhen as ETL, a power efficiency of 23.1 lm/W and a current efficiency of 28.0 cd/A at 0.08 mA/cm2 were achieved, which was much higher than that of the control devices (7.5 lm/W for BCP-device and 8.5 lm/W for TPBi-device). The improved efficiency was attributed to that BPhen had the highest electron mobility and provided better charge balance in the hole-dominant devices. Moreover, the EL spectra of three devices showed no obvious difference with a light emission from iridium complex peaked at 562 nm and a shoulder peak at 600 nm. This indicated that no matter BCP, TPBi or BPhen acted as ETLs, the (t-bt)_2Ir(acac) triplet exciton can be confined within the emissive layer effectively.