The Synthesis, Characterization And Optoelectronic Properties Of The Cruciform Conjugated Molecules

The organic semiconductor materials have attracted great attentions of researchers in electroluminescent light-emitting diodes, field effect transistors, organic thin film photovoltaic cells, optically pumped lasers, and other areas for the wide range of source, can be simply control the photoelectric properties, morphology by chemical modification. Polymer has strong workability due to good solubility, become the focus of the study. Later studies of small organic molecules widely concerned about the introduction of aliphatic chain by researchers to solve the solubility of small organic molecules, crystallization of the problem, but reduced thermal stability. The Joule heat may induce the change of chemical structure and morphology of active layer materials and makes the destroying of device, so the thermal stability will determine the life of the device. Amorphous films could avoid the intermolecular aggregation and non-radioactive process in optoelectronic devices. But the suitable intermolecular forces could be benefit for the effective carrier transfer in organic thin films. So fabricate materials which have high thermal stability, easy to dissolve, could form a uniform dense amorphous thin film is very important for researchers. Shirota.etc reported that the non-planar nature of the molecule can form amorphous materials that can suppress the crystallization and the strong intermolecular stacking, promoting the improvement of material dissolved. The biphenyl with big substituted groups at the meta- and ortho-position of the biphenyl was constructed so distorted configuration, which endows the molecular better solubility for the spin-coated devices fabricated. The relatively flexible biphenyl center allows the molecule form amorphous films with variety of configurations. Because of the proper rigid and flexible of the molecule makes it has a high thermal stability, so the amorphous film used for device could turn around the Joule heat. The relatively free rotate of the biphenyl center could depress the fluorescence quenching induced by molecular aggregation. Accordingly, the biphenyl center with partial flexibility may be superior to the molecules with fully rigidity, because partial flexibility may help the molecules assemble into dense film through solution casting or vacuum depositing processes may be suitable to improve the stability and carrier transport ability of opto-electronic devices. Based on the above advantages, biphenyl central compounds possess unique and excellent optical performance.4,4,-Bis(2,2-diphenylvinyl)-1,1,-biphenyl (DPVBi) is a well-known blue-emitting material for electroluminescent devices, but its vacuum evaporated films exhibit a phase transition trend from amorphous to crystalline state, which impacts morphological stability and device lifetime. We made a cruciform DPVBi derivate, named as 2,5,2',5'-tetrakis (2,2-diphenylvinyl)biphenyl (TDPVBi), in which two 1,4-bis(2,2-diphenylvinyl)benzene segments are linked through a central biphenyl linkage. TDPVBi exhibits very high photoluminescence efficiency in the solid state (80%), and fully amorphous characteristics with high glass transition temperature (Tg=110℃). High performance non-doped blue organic light-emitting device (OLED) with Commission Internationale de I'Eclairage (CIE) coordinates of (0.16,0.21), maximum luminance efficiency of 6.2 cd A-1 (corresponding external quantum efficiency of 3.8%), and maximum luminance exceeding 31170 cd m-2 is achieved based on TDPVBi. The operation lifetime of TDPVBi-based device exhibits a 1.7-fold enhancement relative to that of a similar device based on DPVBi. We demonstrated the first Aggregation-induced Enhanced Emission(AIEE) materials for white organic light-emitting diodes (WOLEDs). AIEE properties of materials provide a possibility to turnaround the bad thermal effects to positive effects for device performance as WOLEDs working at high brightness and high current density. The blue AIEE molecule TDPVBi showes high glass transition temperature (Tg) and special applicability for OLEDs, ca. enhanced PL intensity and smoothness as annealing its film. Combining both AIEE materials (TDPVBi and CN-DPASDB) to construct WOLED results in devices with a maximum current efficiency of 7.9 cd A-1, a maximum power efficiency of 6.2 lm W-1 and a maximum luminance of 25350 cd m-2, respectively. The AIE WOLED displays simplification in device fabrication and excellent color stability, which belongs the best fluorescent WOLED reported.Benzothiadiazole-based materials have drawn much attention in research and industry due to their excellent opto-electronic property. A series of benzothiadiazole-based linear molecules have been synthesized and display high performance in the opto-electronic field. But the linear small molecules benzothiadiazole-based has a tendency of crystalline could affect the performance of the device. Due to low cost solution processing and the purity, the solution process-able small molecules have attracted more attention as active materials fabricated in organic solar cells (OSCs), OFET and organic light-emitting diodes (OLEDs). We fabricate the bibenzothiadiazole-core compounds. Bibenzothiadiazole-core based molecules have many advantages, such as monodispersity, excellent solubility, and vast structures with different functional groups. The bibenzothiadiazole as the core, oligo triphenylamine as the arm, and its linear counterpart was designed and synthesized. The OLED based on bibenzothiadiazole-core displayed a turn-on voltage, maximum power efficiency and maximum luminance before damaged are 4.0 V,0.67% and 761 cdm-2, respectively. Relative to the linear molecule, the cruciform one made in the device display two-fold enhancement.Benzothiadiazole as the electron-deficient units primitives widely used in the preparation of organic photovoltaic material. The linear molecule with, oligothiophene as the arm, and triphenylamine as the end group and their linear counterparts were designed and synthesized. Cruciform molecule tTPATh-dBTz shows high thermal stability, completely amorphous, which is benefit for long life-time device fabricated. Because of its cross-configuration of the material extended conformation, more primitive makes the cruciform molecule has a greater absorbance value relative to the linear material, it is beneficial for the absorption of sunlight. The cruciform molecule with partial flexibility can be assembled into a dense film during spin-coating process to improve roughness of the film and more miscibilities with PCBM, which is especially advantageous for the transport of charge carrier. The performance of photovoltaic devices shows a more linear molecule with the photoelectric conversion efficiency of 11% of the increase.The cruciform molecule fabricated by single bond has greatly improved performance relative to linear molecule. Due to the intramolecular twist of the cruciform molecule makes the spectrum displays no apparent broadening compare with the linear one. We consider using the rigid phenylene unites to build cruciform compounds. The device based on the linear molecular dTPA=BTz has good performance, we constructed the dimer cruciform molecule tTPA=dBTz. Relative to the linear small molecule tTPA=dBTz has a red shift of absorb spectrum. The higherεmax and the broader absorption spectrum of the tTPA=dBTz increase the absorbability of the molecule, which is benefit for the short-circuit current improved. The cruciform molecule tTPA=dBTz with partial flexibility can be assembled into a dense film during spin-coating process to improve roughness of the film and more miscibilities with PCBM, which is especially advantageous for the transport of charge carrier. The complex tTPA=dBTz based device shows a typical photovoltaic response:the short circuit current (Jsc) was 3.49 mA cm-2, the open-circuit voltage (Voc) was 0.80 V and the fill factor (FF) was 0.39. These values corresponded to the power conversion efficiency (ηc) of 1.15%. Meanwhile, the device based on compound dTPA=BTz has also been inspected; the Jsc, Voc and FF were 2.66 mA cm-2,0.75V and 0.34, respectively. The value ofηc is 0.70%. The PCE of the tTPA=dBTz displays 2-fold enhancement relative to the linear molecule dTPA=BTz. The photovoltaic properties of tTPA=dBTz are deeply investigated by utilizing spin-cast blend films of tTPA=dBTz and PC70BM (1:4, w/w) as the active layer in single-layer OSCs. The device with the structure of ITO/PEDOT:PSS/ tTPA=dBTz PC70BM=1:4, w/w) /Ca showed a high performance with Voc of 0.85 V, a Jsc of 6.04 mA/cm2, a FF of 0.40, and a PCE of 2.17%, make it a potential donor material for OSCs.