Dependence of the stability of organic light-emitting diodes on driving mode

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.