Alignment And Optoelectronic Properties Of Discotic Molecules And Their Preliminary Application In Devices

Macrocyclic discotic liquid crystal(DLC)molecules are conposed of large aromatic core and have a tendency to self-assemble and align.Aligned disco tic liquid crystals often have excellent photoelectric physical properties and highly anisotropic structure can optimize their performance in a certain direction,resulting in high carrier mobility and long exciton diffusion length.Especially the ability of self-healing can make their excellent properties to maintain a high degree of stability.Discotic liquid crystals are considered to be a novel generation of organic optoelectronic materials with great potential.Current work focused on the application of a series of novel discotic liquid crystals in optoelectronic devices,especially in solar cells,and carried out a series of fundamental research.The specific research contents include the following aspects:First of all,the fabrication of homeotropic or face-on alignment film of discotic liquid crystals was investigated.Generally,conventional homeotropic alignment of discotic liquid crystal is obtained by the use of standard liquid crystal cell,into which discotic liquid crystal is injected,but this method is clearly not suitable for the production of optoelectronic devices using layer-by-layer method.Current work would explore the technique to fabricate the homeotropic alignment of DLC molecules with a large scale on a single substrate surface.Although there are a lot of reports on the homeotropic alignment in liquid cell,to date only a limited number of studies related to the homeotropic alignment of DLC on a single substrate surface have been reported in the literature,which making challenges for the applications of DLC in optoelectronic devices.The alignments of a series of discotic liquid crystalline materials were investigated by a combination of techniques such as polarized optical microscope and X-ray diffraction.The molecular alignment characteristics of two kinds of novel discotic liquid crystalline materials,such as triphenylene derivative SK-75 with electronic ambipolar property and anthraquinone derivative AQ6 with electron transport ability,were investigated on emphases.Current work fabricated the stable,uniform,hexagonal symmetric and highly ordered homeotropic alignment of SK-75 and AQ6 on a single substrate surface using a thermally controlled method,which were firstly achieved in internationally.By comparing the molecular alignment of H4TP,H5TP and TPT-BAE,the molecular structure,especially the structure of the flexible side chains,was found to be a key factor affecting the alignment of the triphenylene,and the appropriate thermal control conditions need to be selected in order to obtain better molecular alignment.Doping induction-assisted alignment technique was introduced to investigate the molecular alignment based on the above thermally controlled homeotropic alignment.The control of induced alignment of do nor-acceptor composite system oo-CuPc:AQ6 and homogeneous composite system H4TP:SK-75 were achieved.Secondly,the influence of molecular orientation on photophysical properties and carrier mobility was studied by using discotic liquid crystal materials triphenylene derivative TPT-BAE and anthraquinone derivative AQ6 as the representative.Because of the difficulty to obtain homeotropic alignment on a single substrate,the investigation of optoelectronic characteristics of most DLCs are carried out by using sandwiched cells,which can not fully reflect the real situation in optoelectronic devices.In current work,the controllable growth and molecular orientation of ordered structure of TPT-BAE and AQ6 discotic material were realized on the surface of the substrate by thermally controlled method,which was consistent with the actual organic photoelectric device fabrication in a layer-by-layer way.The devices with a structure similar to the actual device were fabricated,and the optoelectronic properties of aligned films of those two discotic materials were investigated.Finally,an attempt was made to fabricate the preliminary photovoltaic device.It was demonstrated that the absorption spectra of short wavelengths were closely related to the alignment of discotic molecules.The UV absorption of TPT-BAE and AQ6 in the short wavelength region was significantly enhanced after the alignment.TPT-BAE film exhibited the characteristics of electronic ambipolar carrier(hole and electron)migration.The homeotropic alignment increased the hole and electron mobility by nearly three orders of magnitude to 6.25 × 10-2 and 4.28×10-2 cm2V-1s-1,respectively,close to the reported maximum ambipolar carrier mobility of triphenylene discotic liquid crystal so far.An electron-only device was firstly fabricated using anthraquinone derivative AQ6 and the steady-state space-charge limited current(SCLC)technique was adopted to investigated the charge carrier transport properties of AQ6 thin film before and after homeotropic alignment.It was demonstrated that the homeotropic alignment could increase the electron mobility by nearly two orders of magnitude to 1.2 ×10-2 cm2V-1s-1.The third,for the first time,n-type anthraquinone derivative AQ6 was used as acceptor to study the feasibility of preparing the bulk heterojunction solar cells with donor polymers.AQ6 was blended with p-type donor materials MEH-PPV or P3HT,and the corresponding bulk heterojunction solar cell devices were prepared.The feasibility of applying AQ6 as an acceptor material to polymer solar cell was explored.It was found that there was intermolecular interaction between AQ6 and MEH-PPV,which could not produce the PV response effectively.It was presumed that intermolecular charge transfer occurred,so that the combination could not be effective for the do nor-acceptor composite system.While the interaction between AQ6 and P3HT was weak,and this combination could produce photovoltaics output,which indicated that anthraquinone discoid liquid crystal material can be used as acceptor in organic polymer bulk heterojunction solar cells.Finally,discotic macro molecular metal phthalocyanines were used as the organic layer to fabricate the hybrid solar cells with amorphous silicon film for the first time.The research on the composite or hybrid structure of amorphous silicon and organic materials is extremely rare.In current work,a simple composite structure of amorphous silicon and discotic macromolecule metal phthalocyanines were used for the first time,the corresponding solar cells were fabricated and investigated.It was found that amorphous silicon/metal phthalocyanine hybrid heterojunction could produce photovoltaic output,although the efficiency is not high.It was demonstrated that the photovoltaic performance of the device was closely related to the thickness of the metal phthalocyanine layer.However,it was also found that the contribution of the photocurrent mainly come from the amorphous silicon layer.The unbalanced charge carrier transport capacity between the metal phthalocyanine layer and the amorphous silicon layer limits the carrier transport and collection efficiency,which provides a new direction for the subsequent optimization of the structure and performance of hybrid photovoltaic devices.