Abstract
Abstract Compared to the high brightness and high efficiency of inorganic LED, late developing organic LED (OLED) is difficult to compete with it. However, in solid-state lighting industry, the point lighting source of the former cannot be applied to large area, and requires utilizing a diffuser to improve the problem of harsh in one point and uneven in full plane. On the other hand, OLED possesses natural plane lighting, and is no need to be fabricated by complicated epitaxy process like inorganic LEDs. Because of flexibility of organic molecules, it can be fabricated on flexible substrates and provides suitability for carrying. Furthermore, the use of vacuum deposition for depositing various layers allows flexibility in device structure design for exciton and charge blocking layers to enhance device performance. Normally, an efficient OLED device contains more than five layers. However, the device with large number of layers could lead to lower production yields and is able to be slow fabrication speed. On the contrary, due to the advantages of wet process, PLED is able to be fabricated at higher speed and thus at lower costs. The most important characteristic of PLED is that permits an integration of various functional groups like charge transport moieties and/or emitting species into one polymer chain for simplifying the fabrication process. In the meantime, it not only simplifies the device structure but also excludes possibility of phase separation host-guest systems. Nowadays, for OLED system, some research groups have attempted to fabricate device using wet process. However, as compared to vacuum deposition process, the efficiency was dropped by 36% resulting from poor film quality. The current state of art for green emission efficiency is in the level of 20-22% in external quantum efficiency (EQE); while green emission of PLED with conjugated polymers is in the level 11%, only half of the above level. The use of non-conjugated polymer (polyvinyl carbazole, PVK) as host with green phosphor as dopant and large amount (about 30%) of electron transport material (PBD) to assist charge balance able to reach EQE at the level of 16-18%. However it suffers from rapid phase separation observable within hours or even minutes and is not suitable for practical use. So far, the most challenging problem in PLED is inadequate efficiency, which is in needs of being promoted immediately. In this thesis, the contents are divided into three parts giving in chapters 4, 5 and 6. In the first part, we design a highly efficient and voltage independent white light device. In the second part, we design a new series of polymers by introducing hole transport moiety (TPA) and electron transport moiety (OXD) as side arms on the silicone of silylene-diphenylene backbone to enhance its charge transport abilities. Since each unit in the polymer has triplet energy higher than 2.9 eV, we may expect that the polymer has triplet energy of 2.9 eV and is suitable to act as hosts for phosphor dopant. In the third part, we center on the physical properties and device performance affected by end groups at side arms of the transport moieties. In chapter 4, we utilize our developed material, PFCn6, as an interlayer and introduce β-PFO for blue emission layer and PFO: rubrene for yellow emission layer to obtain stable white light with fully wet process. Due to the whole device host materials are consisted of PFO main chain structure, there are no energy barriers between each layer. Based on the hole mobility of β-PFO, the electron mobility of PFO from literatures and both mobilities of PFCn6 determined using SCLC measurement, we calculate the ratio of travel time (R) for hole to pass through β-PFO and PFCn6 layers to that for electron to pass through PFO: rubrene and PFCn6; for the device R60/20 (60 nm β-PFO and 20 nm PFO: rubrene layers) and R40/40, they are 1.09 and 0.58, respectively. These results show that the times for holes and electrons traveling before recombination in the devices with 60/20 nm (blue/yellow layers) are more balanced than that with 40/40 nm system. Thus, whatever the external electric field varies, the electroluminescences are almost the same, and the maximum brightness of 15695 cd/m2, the maximum luminance efficiency of 5.43 cd/A are achieved. Besides, the CIE coordinates of (0.32, 0.36) are very close to the standard white light of (0.33, 0.33). In chapter 5, we introduce the phosphor Ir(ppy)2(acac) as green dopant to the proposed bipolar polymers with silylene-diphenylene as backbone, Si(OXD)(TPA), because its HOMO/LUMO levels are 5.2/2.5 eV just lying in between HOMO of TPA (5.3 eV) and LUMO of OXD (2.4 eV). From the photoluminescence (PL) measurement, although the bipolar polymers contain exciplex emission, the films only emit green light after doped 8 wt% Ir(ppy)2(acac), indicating that the exciplex can effectively transfer its energy to green emitter without significant loss. By introducing Cl-ITO as anode (allowing an elimination of PEDOT:PSS layer which is usually used as HTL) and adjusting the thicknesses of ETL and EML, we achieve the high device performance 80.1 cd/A (EQE 21.2%) of Si(tOXD)(oTPA) and 73.5 cd/A (EQE 19.5%) of Si(tOXD)(tTPA), which are extremely high compared to the current reported value of PLED 11% in EQE. This molecular design strategy opens a broad avenue leading toward industrialization of PLED for its two-layer-only device and high performance. In chapter 6, we focus on the effects of using tert-butyl instead of hexyloxy at the side arms of transport moieties, and we found that even the steric hindrance by the tert-butyl group does not affect much on the charge transport. From single carrier examination, the electron current density is 2-3 magnitude higher than hole current density in the bipolar materials. Therefore, we introduce a PVK layer as HTL/EBL, replace CsF by Ca to reduce electron current, and use high conductivity PEDOT:PSS, the resulting device reaches the efficiency 41.6 cd/A in green emission.