Abstract
In polymer light-emitting diode (PLED), host-guest doping systems as the emitting layers have been extensively investigated in literature because they exhibit many advantages such as high device luminance and efficiency. But, it is easy to observe the occurrence of phase separation after the film was formed on a substrate, and this is attributed to the poor chemical compatibility between host and guest materials. However, there is no report focused on the effect of phase separation on the stability of this type of PLED to date. Therefore, the aim of this thesis is to study the relationship between phase separation and device stability (as well as performance) by using a well-known phosphorescent doping system which consists of poly(N-vinylcarbazole) (PVK, used as host), fac-tris(2-phenypyridine) iridium (Ir(ppy)3, used as guest), and 2-(4-tert-Butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD, used as electron-transporting material). The phase separation of PVK/Ir(ppy)3 is revealed by optical images of a doping film formed by spin-coating from its polymer solution. In these optical images, the aggregation of Ir(ppy)3 is apparent and results in the non-uniform emission as observed in photoluminescence (PL) and electroluminescence (EL) measurements. In addition, the PL spectra of polymer films of PVK doped with Ir(ppy)3 or PBD are collected at different time periods after polymer films are formed from its polymer solutions. For the case of PVK/Ir(ppy)3, the intensity of PVK emission at 410 nm (excimer emission) increases with time; for PVK/PBD system, the host emission is blue-shifted from 420 nm (exciplex of PVK and PBD) to 410 nm and two additional emission peaks at 375 and 395 nm are generated. These observations again demonstrate the occurrence of phase separation between PVK and Ir(ppy)3 (even to PBD). The phase separation between host and small-molecule materials implies the happening of device instability while operating with a longer time period. The results of life-time tests of devices based on PVK-PBD:Ir(ppy)3 with various doping ratios (weight ratio of PVK:PBD:Ir(ppy)3 = 100:40:1、3.5 and 8%) and cathode structures (Ca/Al, LiF/Ca/Al, and CsF/Al) show the rapid decays of device luminescence (the initial luminance was set at about 1000 cd/m2 for life-time tests) and life-times are calculated to range from 3 to 33 min. As compared to CBP/Ir(ppy)3-based emitting layer used in organic light-emitting diodes (OLEDs), the observed life-times of PVK-PBD:Ir(ppy)3-based devices are shorter than that of CBP:Ir(ppy)3-based device by a factor larger than one hundred. This is resulted from that Ir(ppy)3 fast aggregates and forms needle crystals upon electric field operation as observed by optic images taken from the device (weight ratio of PVK:PBD:Ir(ppy)3 = 100:40:8) operated under constant voltage at 7 V for 6 days. The aggregation of Ir(ppy)¬3 is attributed to that the heat generated by the pass of electric current in the device not only promotes the diffusion of Ir(ppy)3 but also makes the carbazole moieties of PVK locally move, forming channels which the guest can be gathered. As a result, the aggregates reduce the contact area between PVK and Ir(ppy)3 and, therefore, lower the device efficiency and life-time. Consequently, although device based on PVK-PBD:Ir(ppy)3 can exhibit high performance (25.9 cd/A and 7400 cd/m2), the life-time of the device is too short to be used in the application of PLED display. The reason for the weakness is the phase separation of PVK and Ir(ppy)3 (even to PBD) due to their bad chemical compatibility, and the aggregation of Ir(ppy)3 will be facilitated by the heat generated by the passing of current in device operation.