Abstract
Abstract Electrophosphorescent device based on small molecules have been extensively reported in the application of full color devices and the maximum external quantum efficiency can be afforded larger than 5 % which is the up-limit value for the purely fluorescent device based on small molecule. This can be explained by that the triplet energy of organic molecules without perturbation by heavy-metal effect will be dissipated by energy transfer to ground state oxygen molecules or by non-radiative relaxation causing by collision of long-live triplet excited molecules. As a result, the triplet exciton can not relax through phosphorescence at room temperature. By doping with heavy-metal complexes both singlet and triplet excitons can be harvested owing to their efficient intersystem crossing from singlet to triplet excited states followed by relaxing through phosphorescence. Hence, Doping heavy metal complexes into host materials in organic light emitting diode for obtaining high external quantum efficiency has attracted a great attention. For such system, it includes a polymer host and an Ir-complex guest. For an efficient device, some requirements for such Ir-complexes have been suggested to be short emission lifetime and small exciton diffusion length as well as steric hindrance and suitable energy levels (HOMO and LUMO levels) lay in between those of the main chain. In addition, the requirements of host materials are also suggested be high triplet energy (ET) for confining triplet excitons on guest and suitable energy levels (electron affinity EA or ionization potential IP) for efficient charge injection from neighboring layers or electrodes and ambipolar characteristic. For this thesis, the design of polymer host is concerned. While the common blue-emitting polymers, polyfluorenes (PFs) (ET=2.18 eV) and poly(p-phenylene)s (PPPs) (ET=2.27 eV) are usually low in ET and expected to be not suitable for use as hosts for high ET guests, e.g. Ir-G or Ir-B, since a significant quench of triplet excitons by a low ET host for a high ET guests can occur. As a result, to exploit high triplet energy conjugated polymers as host, poly(3,6-carbazole) derivatives, P(3,6-Cz)s, can potentially obtain high efficient electrophosphorescent devices in green or blue emission. This thesis includes two parts: 1. The conjugated polymer P(tBu-CBP) for use as a host which possesses high ET (2.53 eV) and suitable HOMO (5.3 eV) and LUMO (2.04 eV) energy levels. Upon doping with green and red emission Ir-complexes, it gives devices with high luminous and external quantum efficiencies for green emission (ηLmax = 23.7 cd/A, Qext = 6.57 %) and for red emission (ηLmax = 5.1 cd/A, Qext = 4.23 %), respectively and low turn-on voltage (3 V). For both devices, the efficiencies are higher than those of the corresponding devices with P(3,6-Cz) as host by a factor of 4, even though the latter has ET (2.6 eV) slightly higher than the former. Evidently, the device efficiency of the phosphorescent emission is not solely dependent on the difference in ET between the host and guest, and the side group in P(tBu-CBP) also plays an important role. Incorporations of the co-monomers with higher ET value, di-alkoxy substituted phenylene and di-alkyl substituted fluorene, on the main chain to give the alternating copolymers, P(tBu-CBPP) and P(tBu-CBPF), do not promote, in fact decrease, the levels of ET and therefore lower the device efficiency due to back energy transfer of ET. 2. The design of high ET polymer as efficient host can be simplified by an alternative method that is to graft desired functional groups (electron-rich or/and electron-deficient) on backbone with non-conjugated alkyl chain as spacer. From time of flight (TOF) measurement, both polymers with pendant electron-deficient oxadiazole (OXD) and electron-rich carbazole Cz (25-75% content for both) were found not to improve the charge transport and electron mobility as compared with that of Poly(N-(2-ethylhexyl)carbazol-3,6-diyl) PCzB and significantly decrease the hole mobility by one order of magnitude at 75 % OXD content. From single carrier devices, hole current is still larger than electron current in most cases except for PCzB25Oxd75 and PCzOXD100 at electrical field smaller than 8 × 105 V/cm. This indicates that hole fluxes are dominant for such systems and polymers pendant with 75-100% OXD exhibit charge balance. By analyzing the results of TOF and single carrier (hole and electron) currents, pendant OXD significantly increases electron injection number by two orders of magnitude, but hole injection number is unexpectedly suppressed by one order of magnitude. In contrary, pendant Cz increase hole injection number by about one order, but electron injection number remains almost unaffected probably due to its LUMO of 2.0 eV is close to that of the backbone. This indicates that the nature of electron-deficient pendant OXD can improve electron injection and in the meantime significantly impedes hole injection. The nature of electron-rich pendant Cz can improve hole injection but does not affect electron injection. Upon doping with green emission Ir-complex, polymers pendant with OXD (25 – 100% content) and Cz (75 and 100% content) give device efficiencies better than polymer bound with 2-ethylhexyl chain, (PCzB maximum luminous efficiencies ηLmax of 10.5 cd/A), by a factor of about 2-3 and 1.5, respectively. Accordingly, the promoted efficiencies in EP devices with polymers pendant with OXD (25 – 100%) and Cz (75 and 100%) as hosts can be attributed to that pendant OXD can significantly hinder the hole transport and injection but pendant Cz can only impede the hole transport slightly. The device performances with PCzB25OXD75 as host provides the best performance: ηLmax of about 31 cd/A (corresponding to external quantum Qext = 8.5 %), maximum brightness (Bmax) of 28,000 cd/m2. Thus, the present work provides insight information of electrical properties influenced by pendant groups (electron-rich and electron-deficient), which is useful for molecular design of electroluminescent polymers as host for phosphor dopant or as emitting materials.