The electroluminescence mechanism of non-doping PhOLEDs based on CBP/Ir(ppy)3 investigated by delayed EL measurements

Abstract

In this paper, the non-doped PhOLED based on 4,4′-bis(9-carbazolyl)-1,1′-biphenyl(CBP) and fac-tris(2-phenylpyridine) iridium(Ir(ppy)3) with the emitting layer of [CBP/Ir(ppy)3]n/CBP in which n is equal to 2,3,4 and 5, respectively, are prepared. The electroluminescence and the luminance-voltage-current density characteristics of devices are detected. The wavelength range of these devices is varied from 517 nm to 540 nm. And the delay EL measurements are used to elucidate the carrier recombination and light emission mechanism in non-doped PhOLEDs. In this technique, the devices are driven using a square pulse driving scheme, with a forward bias pulse width of 1 ms, which is sufficiently long for prompt EL to reach its steady-state intensity. The delay EL results show that changing the number of the non-doped EML leads to marked changes in the charge-trapping and host–host TTA patterns, which suggests that the carrier transport and recombination processes depends on the number of non-doped EML. When the number of non-doped EML is less than 3, only the trapped carrier recombination signal are detected in the delay EL measurement. For the devices with more non-doped EML than 3, both the trapped carrier recombination and host–host TTA signal are detected. All these results are discussed and give the evidence for the electroluminescence mechanism of prepared devices.