Study Of The Influence Of The Interface And Hole Transport Layer On The Performance Of OLED

Organic light-emitting diodes(OLED) have attracted much attention for autoluminescence, flexibility, wide angle of view, light weight and so on. However their efficiency, stability and other aspects still need to improve,which are all related to interfacial modification of electrodes and charge transport ability. In this thesis, we mainly study the effect of interface and transport layer on the performance of OLEDs.1. Based on the device structure of ITO/HIL/Alq3: C545T/LiF/Al(HIL = MoO3, PEDOT: PSS, MoO3: NPB), we discuss the effect of hole injection layer on the device performance. Through comparison we found that the transport ability of these three HILs was as follows: MoO3:NPB > MoO3 > PEDOT: PSS. But when PEDOT: PSS was used as hole injection layer, the device with the structure of ITO/PEDOT: PSS(3500r/min, 1min)/Alq3:C545T(1%, 90 nm)/LiF(0.7 nm)/Al gave the best performance: turn-on voltage of 3.7V and maximum current efficiency of 2.91cd/A. Then we concluded that the hole injection layers(HILs) play an important role in the adjustment of the electron/hole injection to attain transport balance of charge carriers in the single emission layer of OLEDs with electron-transporting host in single-layer organic light-emitting diodes(OLEDs).2. Interfacial modification of cathodes. The polarity of ITO can be modified to efficiently collect either holes as anode or electrons as cathode by coating with functional interlayers of different work functions. In this paper, we study inverted bottom-emission organic light-emitting diodes(IBOLEDs), consisting of tris-(8–hyd- roxyquinoline) aluminum(Alq3) as the emissive layer and an ultrathin layer of sodium hydroxide(NaOH) on top of indium tin oxide(ITO) as the electron injection layer. The devices with NaOH treated by water vapor and CO2 and annealing show higher current efficiency than those with Na OH that are untreated. Furthermore, we also present the influence of different concentration of NaOH solution on device performance, the optimal device’s maximum current efficiency can reached 6.0cd/A.3. Investigate the influence of the materials, thickness of hole transport layer and device structure. Phosphorescent OLEDs were studied based on Ir(ppy)3:CBP as emitting layer(Ir(ppy)3 as guest material, CBP as light emitting host). Comparing the performances of the device without hole transport layer and the devices with CBP、NPB、NPB/TCTA as hole transport layer respectively, it was found that the device with CBP as hole transport layer achieved the best performance. And this can be ascribed to the fact that CBP also served as light-emitting layer and there is not barrier between transport layer and emitting layer, which avoids accumulation holes in the interface, depresses efficiency roll-off. Combined with the spectrum, we also concluded that when CBP was used as hole transport layer, triplet excitons can limited in the light-emitting layer and higher current efficiency could be achieved. The optimal device structure is as follows: MoO3(1nm)/CBP(45nm)/CBP:Ir(ppy)3(8%,15nm)/TPBi(55nm)/Li F(0.7nm) and the turn-on voltage of 3.1V and the maximum current efficiency over 70cd/A.