Organic Donor-acceptor Heterojunction Optoelectronic Composites With Ordered Nanostructures

The performance of optoelectronic functional devices mainly exhibits as the generation, separation of excited charge carriers and the followed transportation of free charge carriers. The aggregation state of organic composite materials plays a vital role in determining the device performance. Photo-induced charge transfer can get enhanced in one dimensional ordered nanocomposites and thus can greatly improve the separation and transportation of charge carriers which are the key factors influencing optoelectronic device performance. The transportation capability of charge carriers in organic semiconductors can be effectively improved through controlling the morphology of organic nano-films and the assembly of highly-ordered organic nanoarrays. The present dissertation focuses on organic semiconductor composite materials at nanometer scale, especially the ordered nanocomposites with the low dimensional inorganic nanostructures as the template, so as to enhance the separation and orientational transportation of charge carriers, and to achieve the co-enhancement of optoelectronic properties. In this work, ordered nanostructures and nanoarrays made from phthalocyanines, perylene-diimide derivative and C₆₀ semiconductors were prepared and their optoelectronic properties were studied, which presents fundamental experimental results for the fabrication of optoelectronic devices based on the phthalocyanines, perylene-diimide derivatives and C₆₀ semiconductors.The recent progress on the application of organic nano-films with optoelectronic functionalities in the area of organic solar cells (OSCs) was reviewed first. A detailed description was covered on organic photovoltaic devices based on phthalocyanine dyes, perylene-diimide derivatives and fullerenes.In this work, metal phthalocyanines, rare earth bisphthalocyanine (REPC2) with sandwich molecular structure and pyridine-perylenediimide (PPD) were synthesized and purified. The chemical structure and purity were characterized by fourier transform infrared spectroscopy, ultra violet-visible (UV-vis) spectroscopy, and elemental analysis. The DSC-TGA analysis indicated good thermal stability for these compounds.The p-type organic semiconductor of phthalocyanines 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. Based on this phenomenon, erbium bisphthalocyanine (HErPc₂) films on indium-tin-oxide (ITO) glass were prepared by electrophoretic deposition (EPD) method. The aggregation structure of the HErPc₂ films could be controlled either by changing the EPD time or EPD voltage or by annealing treatment. Through prolonging EPD time or increasing EPD voltage, the aggregation morphologies of HErPc₂ films on the ITO glass could change from nanowires to microwires. V-type, Y-type or flower-shape HErPc₂ microstructures were formed after annealing treatment of the HErPc₂ films prepared by EPD. The photoresponse of the HErPc₂ films was investigated by UV-vis spectroscopy and near infrared (NIR) spectroscopy. The high photoresponse of the HErPc₂ films was observed in the UV-vis and NIR region, which indicates the potential application of rare-earth biphthalocyanines as OSCs devices and NIR photodetectors. Similarly, PPD films with nanostructures were also prepared by electrophoretic deposition method because the N-atom-bearing PPD molecules can also be protonated in solution.HErPc₂/PPD nanocomposite films were prepared by two-step EPD and electrophoretic co-deposition method. The morphologies and the UV-vis absorption spectra of the composite films indicated that, due to more easily protonized nitrogen, the rare earth bisphthalocyanine molecules exhibited a much faster electrophoretic velocity than the PPD did at the same applied electric field. I-V characteristic curves demonstrated p/n heterojunction rectification for the HErPc2/PPD nanocomposite films.Using highly-ordered porous anodic aluminum oxide (AAO) with nanopores perpendicular to the conductive substrate as the template, highly-ordered and well-aligned organic nanowire arrays were prepared by assembly of CuPc, HErPc₂ and PPD in the AAO templates via EPD and vacuum deposition technique. Further studies revealed that the growth process of the nanowires in the AAO pores followed a "bottom-up" growth model, which enables us to readily control the length of the organic nanowire arrays by varying the deposition time. The highly-ordered organic nanoarrays were free-standing after removing the AAO supports. The length and diameter of the as-prepared organic nanowires could be easily controlled by selecting different pore sizes and film thicknesses of the AAO templates. The photoreceptor with the organic nanowire arrays as charge generation layer (CGL) showed one order of magnitude higher photosensitivity than the photoreceptor with the casting bulk film as CGL. The improved photosensitivity was attributed to the large interfacial area for exciton splitting and orientational nanochannels for charge carrier transportation.HErPc₂/PPD composite with p/n heterojunction nanostructure was prepared by EPD HErPc₂ nanowire arrays in AAO template and followed by the vacuum deposition of PPD on them. The optoelectronic devices based on the HErPc₂/PPD p/n heterojunction nanostructures exhibited photovoltaic properties. Therefore, this mentioned template-based synthesis of organic p/n heterojunction opens up a new approach for the preparation of ordered organic nanostructured composites with optoelectronic functions.C₆₀ is an n-type semiconductor with excellent electron transportation property and thus has been extensively used in OSC devices as the electron-acceptor. By electrophoretic deposition of C₆₀ aggregates from mixed solvent of toluene and acetonitrile into the nanopores of the AAO template, C₆₀ nanoarrays have been successfully prepared. Under the same EPD conditions, C₆₀ nanowire arrays and C₆₀ nanotube arrays were formed by using two different AAO templates with Al conductive substrate and Au conductive substrate, respectively. It is worth to mention that C₆₀ nanowire arrays were prepared by template-surface-wetting method for the first time. The photosensitivity of the photoreceptors with the highly-ordered nanowire arrays as CGL was enhanced by one order of magnitude compared with that of their bulk films. The study on the photosensitivity of photoreceptors made from different C₆₀ nanowire arrays with 40, 60 and 80 nm in diameter and 4, 8, 10 and 15μm in length revealed the high dependence of the photoconductivity on the diameter and the length of the C₆₀ nanowires. This effect of the nanowire size on the photosensitivity should be related to the transportation of the photo-generated carriers in the nanowire arrays. The faster carrier transportation was found in the nanowires with a smaller diameter and a shorter length.