Design/Synthesis And Optoelectronic Properties Of Silicon-containing Wide Bandgap Hosts

In passed decades, major breakthroughs have led to significant improvements in the performance of organic light-emitting devices due to the use of phosphorescent heavy metal complexes. The triplet-triplet annihilation and triplet-polaron annihilation is occurred at high concentration of triplet state exciton, which is due to long triplet exciton life of phosphorescent materials. Incorporating electrophosphorescent metal complexes with a low concentration into the host organic/polymer materials is an effective way to resolve this problem. The host material is the main body of the light-emitting layer, and therefore the choice and nature of host material directly affect the performance of the device. In this thesis, we design and synthesize new host material system to study the relationship between structure and properties of materials and device performance, then finally to refine the ideas of the host material design.In Chapter 2, by 8-Si interrupted fluorene backbone and attaching carbazole on the side chain, we designed and synthesized a series of polymers and oligomers SiPPF,SiPPFCz,SiPF and SiPFCz, which have different bandgap and different side groups. These polymers and oligomers all show good thermal stability and film formation. Because of the conjugation length being effectively confined byδ-Si interrupted main chain, its'emission peaks in solution are less than 400 nm, especially for SiPF and SiPFCz, which have shorter conjugation length, show more blue-shifted absorption and emission. Due to the carbazole side chain, the HOMO energy levels of SiPPFCz and SiPFCz are elevated to -5.57 eV and -5.45 eV, effectively reduce the hole injecting barrier. The blue, green and red devices with the polymers and oligomers doped iridium complexes as the light emitting layer demonstrate that the introduction of carbazole group could effectively reduce the turn-on voltage of the devices. Moreover, the host material, such as SiPFCz, which has a wider bandgap and better hole injection property, has a more superior device performance.In previous chapter, we find that the carbazole group with a better hole injection property is more suitable as building blocks for host material, so in the third chapter, we designed and synthesized a series of wide bandgap polymers with a same backbone of tetraphenylsilicane and carbazole connected by meta-linkage. In order to enhance the electron injection and transport properties of the polymer host material, we apply the high electron affinitive cyano group to attach on the side chain with different content. From the solution, thin film and doping spectrum, we find that the cyano group has not affected the bandgap of the polymers. Single-carrier devices and UPS characterization of the polymer prove the effect of cyano group in the improvement of electron injection and transporting. In addition, atomic force photo display that the introduction and different content of cyano have a great impact in the polymer film morphology. The non-doped single layer devices of the polymer show that the cyano group effectively reduce the turn-on voltage of the devices, and also improve the brightness and efficiency of the devices. The blue and green devices with the polymers doped iridium complexes as the light emitting layer demonstrate that PCNCzSi25 and PCNCzSi50 have more superior device performance; the reason is that both PCNCzSi25 and PCNCzSi50 have not only balanced carrier transport properties but also good film formation. By further device optimized using PCNCzSi as host, a maximum luminous efficiency of 15 cd/A and a maximum external quantum efficiency of 6.7% were obtained, which are the highest efficiencies for blue phosphorescent device using polymers as hosts as we know. This inspiring result gives us a new foreground of the host design.In the above chapters, we study the silane-containing polymer host materials, and realize high efficiency devices with the synthesized polymer, so we also want to introduce silane into the small moleculer host material. In Chapter 4, through tetraphenyl silicon coupling carbazole by the non-conjugated nine position, we designed and synthesized a series of wide bandgap small molecular host materials CzSiCz,DCzSiCz,DDCzSi and DTCzSi, which have high triplet energy, good thermal and morphology stability. Blue evaporation process device based on CzSiCz with a high efficiency of 16.8 cd/A (6.8V) is realized, moreover, the device displays low efficiency roll-off, with a roll-off value of 16.3 cd/A (7.0V) at a luminance of 100 cd/m2 and 16.0 cd/A (8.6V) at a luminance of 1000 cd/m2. Blue solution process device based on DDCzSi with a high efficiency of 12.7 cd/A (5.8V) is also realized, moreover, the device displays low efficiency roll-off, with a roll-off value of 12.0 cd/A (6.2V)at a luminance of 1000 cd/m2. even at the brightness of 100 cd/m2 and 1000 cd/m2,12.7 cd/A (5.8V).In previous chapter we have designed and synthesized a series of small molecule host materials based on hole-transporting carbazole unit, in order to enhance its' electronic injection and transporting properties, so in Chapter 5, we introduce good electron injection and transporting pyridine unit into carbazole-based silane host materials, design and synthesis a series of bipolar host material CzSiPy, DCzSiPy and TCzSiPy. Through spectral analysis, we find that there are no intramolecular charge transfer in this system, though donar and accept group exist in it, so they still maintain a high triplet energy. The results from electrochemistry and DFT data demonstrate the pyridine effectively reduce the LUMO energy level. More over, the HOMO and LUMO of the bipolar host materials are separation and localization in the respective hole- and electron-transporting moieties, which will be helpful for the control of the carrier transporting properties. In addition, DCzSiPy and TCzSiPy also shown good thermal and morphology stability. The blue and green devices with the bipolar host materials and its' relevant cabazole-based host materials doped iridium complexes as the light emitting layer are fabricated. The results show that the DCzSiPy- and TCzSiPy-based devices have more superior performance than it's relevant cabazole-based host materials, which attest to our idea about balanced carrier injection and transporting in host material design achieve the desired results.