We investigated the effects of pulsed current (PC) and direct current (DC) driving modes on the stability of organic light-emitting diodes with and without hole-injection layers (HILs).Two different HIL materials were used:copper phthalocyanine (CuPc) and 4,4',4″-tris(3-methylphenylphenylamino)triphenylamine (m-MDTATA).It was found that the half-lives of devices using PC driving modes were different from those of comparable devices using DC driving modes.For the devices without HILs,with CuPc HILs and with MTDATA HILs,the half-lives of the devices were changed by factors of 1.91,1.41 and 0.86,respectively,when operated in PC rather than DC driving modes.Our analysis of the electrical characteristics of the corresponding hole-only devices showed that the number of holes injected into devices was greatly reduced by inserting an m-MTDATA layer compared with other designs.The results indicate that different ratios of injected electrons and holes can be obtained in these devices.Moreover,these ratios play a dominant role in the dependence of the stability of the device on the driving mode.
