Hindrance-Functionalized π-Stacked Polymer Semiconductor Materials For Organic Electronics:Their Design, Synthesis And Properties Investigation

Compared with complex and tedious coevaporation techniques of small-molecule based devices, polymer diode devices employ the simple, low cost solution process methods, such as as spin-coating and inkjet printing, which is favorable for the development and commercialization of organic diode devices.Thus, the demands on novel polymeric semiconducting materials are increased accordingly. Vinyl-type π-stacked polymer, such as poly(N-vinylcarbazole)(PVK), with the characteristics of unique stacked conformation, excellent hole-transporting ability along with relative high triplet energy state(ET), could be applied as host material or hole-transporting material in optoelectronic devices. In order to expand the applications of π-stacked polymers in optoelectronic devices, our group proposed the strategy of hindrance functionalization that incorporate of functional groups with steric hindrance effect into π-stacked polymers to finely tune their electronic structures, stacked conformations, morphological and themal stability, succeeded in getting polymer hosts for polymer phosphorescent light-emitting devices (PPLEDs) and new-generation electrically bistable materials for flash memory devices. The completed works and the results are mainly summarized as follows:1. There are no reports on π-stacked polymer host materials except PVK. Our group first proposed the strategy of hindrance-functionalization design of novel π-stacked polymer host materials for PPLEDs. A model hindrance-functionalized π-stacked polymers,, poly(N-vinyl-3-(9-phenyl-fluoren-9-y1)-carbazole)(PVPKF), has been designed and synthesized serving as the blue electrophosphorescent polymer host materials. The introduction of bulky9-phenylfluoren-yl moieties (PFMs) with3D Cardo structure obviously alters the photophysical, thermal and electrochemical properties of the precursor poly(N-vinyl-carbazole)(PVK) according to various characterizations of UV-vis, PL, TGA, DSC and CV. PVPFK shows high triplet energy level of2.80eV and gives an efficient Forster energy transfer to blue phosphorescent material, bis[(4,6-difluorophenyl)pyridinato-N,C2]picolinatoiridium(III)(FIrpic), making it as a potential polymer host for blue PPLEDs. A series of prototype PPLEDs have been fabricated using the PVPFK as the blue phosphorescent host and FIrpic as the guest. Compared with the controlled experiments of PVK, PVPFK-based devices exhibited overall high performance with lower turn-on voltage of6.1V, higher maximum current efficiency of14.2cd/A and higher maximum brightness of13287cd/m2as well as better CIE chromaticity coordinate. The results indicated hindrance-functionalized PVK will be more suitable polymer host materials for blue electrophosphorescent PLEDs than PVK.2. In order to achieve novel hindrance-functionalized π-stacked polymers, tetraphenylsilane moieties as the star group among the small molecular hosts was introduced as hindrance groups into the carbazole units of PVK to achieve novel polymer host materials with high ET of2.98eV for PPLEDs application. To the best our knowledge, this is the highest ET for the reported polymer hosts up to date. Compared with PVK and PVPFK, the electron-transporting ability of PVKSi improved. PVPFK shows an efficient Forster energy transfer to blue phosphorescent material, FIrpic, making it as a potential polymer host for blue PPLEDs. Although the unsatisfactory film-forming ability of PVKSi limited it as host material for PPLEDs, the PVKSi mixed with PVK as host materials will take advantage of the potential of PVKSi, being as more suitable polymer host materials for blue electrophosphorescent PLEDs than PVK。3. The photophyscial properities of terfluorene-functionalized π-stacked polymer, PVK-TF, was investigated by us. PVK-TF shows higher triplet energy level than Bis(2-benzo[b]thiophen-2-yl-pyridine)(acetylacetonate)iridium(III)(Ir(btp)2(acac)) and gives an efficient Forster energy transfer to Ir(btp)2(acac), making it as a potential polymer host for white PPLEDs. Thus, PVK-TF acting as not only blue light-emitting material and but also host material doped with Ir(btp)2(acac) was applied as emitting layer in PPLEDs. Through the control of the doping ratio, the PPLEDs realized white light emission. Furthermore, the different electron-transporting materials were tried in order to optimize the device, resulted in the improvement of device efficiency. PVK-TF and Ir(btp)2(acac) based devices exhibited overall high performance with higher maximum current efficiency of10.69cd/A, higher maximum brightness of15723cd/m2and CIE chromaticity coordinate of (0.378,0.387).4. Among the electroresistance mechanisms, conformation induced electrical switching mechanism is vital for the design of polymer memory materials. Based on our previous research works, a novel vinyl-type π-stacked polymer based on PVK with pendent cardo group, poly(N-vinyl-3-(9-pyridine--fluoren-9-yl)-carbazole)(PVKPy), for flash (rewritable) memory devices has been designed and synthesized. PVKPy shows excellent thermal, morphological stabilities. Compared with the performance of PVKPy and PVPFK based memory devices, PVKPy and PVPFK devices all exhibited flash memory performance with threshold voltage of0.55eV and0.34eV. The lower threshold voltage of PVPFK device was attributed to the smaller hole injection barrier. On the other hand, the ON/OFF current ratio of104for PVKPy device was higher than ON/OFF current ratio of103for PVPFK device, indicated the practical applied value of PVKPy device. PVKPy device shows high stability in retention time up to104s and number of read cycles up to104under a read voltage of1.5V in both ON and OFF states. According to the operating mechanism of PVKPy based memory device, our previous suggested conformation induced electrical switching mechanism could explain the electrical switching behavior of PVKPy appropriately.5. There is no report on the functionalized PS with excellent thermal stability to achieve flash memory performance. A novel vinyl-type π-stacked polymer based on polystyrene with pendent cardo group, poly(4-(9-phenyl-fluoren-9-yl)styrene)(PPFS), for flash (rewritable) memory devices has been designed and synthesized. Bulky9-phenylfluoren-yl moieties (PFMs) with steric hindrance effect were introduced into all the phenyl units of polystyrene (PS) through the sp3hybridized C-9carbon atom to obtain PPFS with improved thermal, morphological stabilities and conductivity. Memory devices using PPFS as active layers exhibited flash memory performance with ON/OFF current ratio of up to104, high stability in retention time up to104s and number of read cycles up to105under a read voltage of1.5V in both ON and OFF states, while PS based device exhibited no memory performance.