Injection and charge transport effects on electroluminescence characteristics of molecularly-doped polymer light-emitting diodes

Electrical and optical characteristics of single-layer light-emitting diodes (LEDs) comprised of a solid solution of electron transport and emitter molecules of Alq3, and hole transport molecules of TPD dispersed in polycarbonate (PC) have been studied using varying proportions of solutes and thickness of samples. The results show that the LEDs operate in the injection-controlled electroluminescence (EL) (ICEL) mode, the current being dominated by holes injected from the ITO anode. Electrons injected from the Mg/Ag cathode are trapped on Alq3 molecules serving as recombination centers for mobile holes. In particular, the current–voltage characteristics and their evolution with sample thickness allowed us to conclude that the hole injection follows the one-dimensional Onsager model. The EL efficiency, expected to be a function of the driving current due to the ICEL operation mode of the LEDs, appears to be quasi-constant for each given cell showing up in the quasi-linear brightness–current relationship. This behaviour can be explained by the field-increasing mobility of holes as shown in analytical considerations relating the EL efficiency to the driving current and hole mobility. The EL yield and its current dependence appeared to be only slightly affected by the concentration ratio TPD:Alq3. Its increase lowers the EL yield but strongly increases the absolute EL intensity. The conditions are examined to fabricate the polymer LEDs for different applications: for these where power consumption is of priority importance (high efficiency LEDs) and those where the high brightness of the EL display is required (high brightness LEDs). The results allow us to show a connection of polymer LED characteristics with their structure and technological parameters. This is a necessary step in fabrication of improved performance organic LEDs.