Abstract
This study is for divided for four parts. The first two parts discuss the interaction between hole-transport layer poly(3,4-ethylenedioxythiophene): poly (styrenesulfonic acid) (PEDOT:PSS) and both emitting layer poly(9,9-dioctylfluorene) (PFO) and anode indium tin oxide (ITO). The third part is the improvement of the performance of bipolar devices in which the PEDOT:PSS is mixed with nanoparticles. The last part discuss the species and energy level (hole injection barrier) of the modified ITO anode by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). In order to understand the barrier heights of PFO spin-coated onto PEDOT:PSS and the interface formation between them, we use UPS and the current profiles of hole-dominated and bipolar devices to investigate. The hole-injection barrier of PFO cast on top of PEDOT:PSS electrode is found to vary with thickness of both films. There are three thicknesses of PEDOT:PSS, which are 15, 25 and 50 nm. In hole-dominated devices, the current density of the device with 15 nm PFEDOT:PSS is one order larger than that with 50 nm PFEDOT:PSS. For the bipolar devices, the maximum brightness and efficiency are 8,374 cd/m2 and 0.75 cd/A (9V), respectively, also for the devices with 15 nm PFEDOT:PSS. The variation in hole-injection barrier can be attributed to a migration of PSS chain as a dopant into PFO layer, as indicated by the broadening and shifting (towards higher binding energy) of HOMO edge peak of PFO film. The barrier at the thinnest PEDOT:PSS film (15nm) is found to be the smallest due to the least extent of doping by PSS chains. It is known that there is reaction after PEDOT:PSS spins coated onto ITO. We used ultraviolet photoelectron spectroscopy to investigate ionization potential (IP) changes of PEDOT:PSS spin-coated on indium tin oxide (ITO), and found that increasing delay time to baking after spin-coating decreases its IP and increases hole injection barrier to emitting polymer. The IP change is attributed to dedoping of PEDOT in PEDOT:PSS due to reaction of ITO with protons in PSS and those in doped PEDOT in a presence of water. To get good performance of bipolar device, PEDOT:PSS film should be baked right after spin-coating. The bipolar device with no delay time has the highest brightness and efficiency, 15,625 cd/m2 (9.5 V) and 0.88 cd/A (9.5 V), respectively. The performance of PFO devices with PEDOT:PSS added nanoparticles improves and the nanoparticles are Ni、Pd、Pt、ZrO2及TiO2. High and low conductivity of PEDOT:PSS with different weight percent of nanoparticles spins onto ITO to from one to three layers. The results show that the better device performance is for the devices with high conductivity of PEDOT:PSS mixed with metal nanoparticles or the single and double layer with Pt nanoparticles. The device with maximum brightness is that with Pt (5 wt%) as bottom layer and TiO2 (2 wt%) as top layer, which is 34,424 cd/m2 (11 V) and the efficiency is 2.18 cd/A. The device with maximum efficiency is that with Ni (2 wt%) as bottom layer and TiO2 (2 wt%) as top layer, which is 2.43 cd/A (9.5 V), and the brightness is (10 V). PEDOT:PSS mixed with nanoparticles can improve its conductivity and also the performance of the devices can be improved with some certain PEDOT:PSS structures and the work function of nanoparticles. We also use XPS and UPS to investigate the modified ITO by HMDS and CFX to understand the composition at the surface and the changes of hole injection barriers by the band diagramed derived by UPS. These understandings will help to improve the device performance.