Fabrication Of CuPc/TiO₂ Ordered Nanocomposites And Their Photoconductivity

Tailoring the performance of materials via changing the morphologies and structures of materials has emerged as a new and important activity in optoelectronic functional materials and devices. The investigation of the relationship between structure and performance of optoelectronic composites would help to apprehend the optoelectronic mechanism, discover novel phenomenon, and raise new ideas. Photo-induced charge transfer in optoelectronic materials can be enhanced in one dimensional ordered nanocomposites and thus greatly improve the separation and transportation of charge carriers which are the key factors influencing optoelectronic device performance. The efficiency of photo-induced charge transfer in organic/inorganic nanocomposites can be effectively improved due to the effective contact in molecular level, larger contact area, and ordered directional donor/accepter nanocomposites construction which can be achieved through the ordered composite of organic and inorganic semiconductors and controlling the morphology of organic/inorganic ordered nanocomposites. In this dissertation, several approaches, anodic oxidation, sol-gel, vacuum thermal evaporation, and electrophoretic deposition, were carried out to fabricate high photosensitive CuPc/TiO₂ ordered nanocomposites. The optoelectronic property of the nanocomposites was investigated in details as well.The recent progress on the application of organic/inorganic nanocomposites in the area of photoconductivity, photovoltaic characteristic, sensors, etc. was reviewed firstly. A detailed description was covered on optoelectronic field based on copper phthalocyanine (CuPc), phthalocyanine dyes and titania, including the preparation methods of nanocomposites used in this dissertation.Size-controllable and high ordered vertically-oriented TiO₂ nanotube arrays were fabricated using anodic oxidation of pure titanium sheets in electrolyte solutions, and the crystallization of TiO₂ nanotubes could be tuned by altering annealing temperature. CuPc was filled into TiO₂ nanotubes by vacuum deposition at various deposition rates, a series of CuPc/TiO₂ ordered nanocomposites with various morphologies were achieved by tailoring the thickness of CuPc and the diameter of the TiO₂ nanotubes. A double-layered photoconductive device (Au/CuPc/TiO₂ nanotubes/titanium) was designed and fabricated with the CuPc/TiO₂ nanocomposites as photoactive layer. The photoconductive property of devices was studied via current-voltage(I-V) curves, and the current value increased by 2 or 3 orders of magnitude when exposed to light from dark, depending on the thickness of CuPc layer (100 nm for the best) and the diameter of TiO₂ nanotubes (70 nm for the best). For a control experiment, the I_l/I_d ratio of the device with disordered TiO₂ nanoparticles (TNPs) is obviously lower than that with ordered nanotube arrays. Several factors contribute to the improvement of photoconductive property of devices: the nanoarrays of TiO₂ nanotubes can enormously increase the contact area between CuPc and TiO₂, which offers much more sites for the exciton dissociation, and the one-dimensional and well-ordered structure of TiO₂ nanotube arrays can act as efficient transport channels. With the help of the aligned TiO₂ nanotubes, the collection and transfer efficiency of electrons is greatly enhanced, leading to an evident increase of photocurrent. The capturing and releasing majority carriers by traps or recombination centers at the donor/acceptor interface could effectively delay the process of collection or annihilation of charged carriers, which results in the hysteretic phenomenon of current change as device switches from light to dark and the reverse.Phthalocyanine, a p-type organic semiconductor, contains bridging nitrogen atoms and can be easily protonated in organic acid solution. The protonated phthalocyanine molecules can orientationally migrate and nucleate on the negative electrode under external electric field. Basing on these considerations, CuPc/TiO₂ nanocomposites were fabricated by the electrophoretic deposition method (EPD) from the mixed solution of chloroform and trifluoroacetic acid containing the protonated CuPc. The morphology of the CuPc nanofilms can be manipulated by deposition time, concentration of CuPc, and applied voltage. The aggregation structure of the CuPc nanofilms was investigated by UV-vis spectroscopy and X-ray diffraction (XRD). The shift of Q-band absorption of CuPc nanofilms was resulted from the transition of aggregated structure of CuPc molecules. The XRD patterns demonstrated that the stack style of the deposited CuPc molecules is influenced by the property of the underlayer molecules. The evolvement of CuPc molecular aggregated structure was investigated by molecular exciton model and plasmon coupling, and the electrochromatic property of CuPc nanofilms and solution is derived from the transition of H- and J-aggregates. A dual-layered photoreceptors containing CuPc/TiO₂ nanocomposites as the charge generation layer (CGL) were designed and fabricated. The photoconductive property of photoreceptors was tailored via changing the morphology of the CuPc nanofilms. It was found that, to guarantee the adequate utilization of light energy, the thickness of the CuPc layer must be appropriate to efficiently absorb incident photons as well as to favor the migration of the excitons, which improved the photoconductivities of photoreceptors greatly.Using highly-ordered porous anodic aluminum oxide (AAO) with nanopores perpendicular to the conductive substrate as the template, highly-ordered and well-aligned TiO₂ and CuPc nanowire arrays were prepared by assembly of TiO₂ precursors and CuPc in the AAO templates via sol-gel and EPD method, respectively. Coupling these two methods, a series of CuPc-coated TiO₂ ordered nanowire arrays were achieved via this secondary deposition technique. The photoluminescence spectra and the time-resolved fluorescence spectra of CuPc/TiO₂ nanowires confirmed the photoinduced charge transfer occurs between CuPc and TiO₂ nanowires, which contributes to the photoconductivity enhancement of CuPc/TiO₂ nanowires compared to pure CuPc or TiO₂ nanowires. The photoconductivity of the CuPc/TiO₂ nanowires can be tailored by changing the morphologies of CuPc/TiO₂ nanowires. It was found that the CuPc/TiO₂ nanowires arrays exhibited one or two orders of magnitude higher photoconductivity than that of pristine TiO₂ or CuPc nanowire arrays due to the large donor-acceptor (CuPc-TiO₂) contact area and the directional alignment of TiO₂ nanowires.