Synthesis and properties of ferrocene-functionalised polythiophene derivatives

The ferrocene-functionalised thiophene derivatives (TFn) with different length of oxyethylene chains were synthesized and polymerized chemically with iron (III) chloride as an oxidant. The resulting ferrocene-functionalised polythiophenes (PTFn) show good solubility in most solvents, such as chloroform (CHCl3) tetrahydrofuran (THF), acetone, etc. The structure and properties of the PTFn polymers were confirmed by IR, H-1 NMR, AFM and photoluminescence (PL). The polymers PTFn show good redox activity with no attenuation of the electroactivity after multiple potential cycling. (C) 2009 Elsevier B.V. All rights reserved.

Effects of fullerene solubility on the crystallization of poly(3-hexylthiophene) and performance of photovoltaic devices

The substantial crystallization suppression of poly(3-hexylthiophene) (P3HT) in the untreated P3HT:C60 composite film prepared from o-dichlorobenzene (ODCB) solution has been revealed. Besides, the effective conjugation length of P3HT in this composite has been nearly maintained to that in the solution. The different crystallization behaviors of P3HT in its composites with C60 and [6,6]-phenyl C-61 butyric acid methyl ester (PCBM) are mainly attributed to the relative solubility of C60 and PCBM with respect to P3HT in ODCB. The solution to overcome this disadvantage of chain conformation and crystallinity of P3HT in the composite with C60 is thus proposed and finalized by resorting to the addition of low volatile solvent with much higher solubility of C60 than P3HT into the main solvent used, so as P3HT can crystallize before C60 forms crystallites in the solution. The feasibility of this approach has been proven by the improved efficiency of devices based on composites of P3HT and the low cost C60 without resorting to post-treatments.

Constructing Thin Polythiophene Film Composed of Aligned Lamellae via Controlled Solvent Vapor Treatment

Thin poly(3-butylthiophene) (P3BT) film composed of aligned lamellae attached to the edge of the original film has been achieved via a controlled solvent vapor treatment (C-SVT) method. The polarized optical microscopy operated at both single-polarization and cross-polarization modes has been used to investigate the alignment of the fiber-like lamellae. A numerical simulation method is used to quantitatively calculate angle distributions of the lamellae deviated from the film growth direction. Prepatterned P3BT film edge acts as nuclei which densely initialize subsequent crystal growth by exhausting the materials transported from the partially dissolved film. The growth of new film upon crystallization is actually a self-healing process where the two-dimensional geometric confinement is mainly responsible for this parallel alignment of P3BT crystals. The solvent vapor pressure should be carefully chosen so as to induce crystal growth but avoid liquid instability which will destroy the continuity of the film.

Rational Design, Synthesis, and Optical Properties of Film-Forming, Near-Infrared Absorbing, and Fluorescent Chiromophores with Multidonors and Large Heterocyclic Acceptors

A new series of film-forming, low-bandgap chromophores (1a,b and 2a,b) were rationally designed with aid of a computational study., and then synthesized and characterized. To realize absorption and emission above the 1000 nm wavelength, the molecular design focuses on lowering the LUMO level by fusing common heterocyclic units into a large conjugated core that acts an electron acceptor and increasing the charge transfer by attaching the multiple electron-donating groups at the appropriate positions of the acceptor core. The chromophores have bandgap levels of 1.27-0.71 eV, and accordingly absorb at 746-1003 nm and emit at 1035-1290 nm in solution. By design, the relatively high molecular weight (up to 2400 g mol(-1)) and non-coplanar structure allow these near-infrared (NIR) chromophores to be readily spin-coated as uniform thin films and doped with other organic semiconductors for potential device applications. Doping with [6,6]-phenyl-C-61 butyric acid methyl ester leads to a red shift in the absorption on]), for la and 2a. An interesting NIR electrochromism was found for 2a, with absorption being turned on at 1034 nm when electrochemically switched (at 1000 mV) from its neutral state to a radical cation state. Furthermore, a large Stokes shift (256-318 nm) is also unique for this multidonor-acceptor type of chromophore.

Why [6,6]- and 1,2-Benzal-3-N-4-O-Cyclic Phenylimidate C-60 Undergo Electrochemically Induced Retro-Addition Reactions while 1,4-Dibenzyl-2,3-Cyclic Phenylimidate C-60 Does Not? C-H center dot center dot center dot X (X = N, O) Intramolecular Interactions in Organofullerenes

The electrochemical properties of a series of structurally related fullerooxazoles, [6,6] cyclic phenylimidate C-60 (1), 1,2-benzal-3-N-4-O-cyclic phenylimidate C-60 (2), and 1,4-dibenzyl-2,3-cyclic phenylimidate C-60 (3), are described, and the spectroscopic characterizations of their anionic species are reported. The results show that compounds I and 2 undergo retro-cycloaddition reactions that lead to the formation of C-60 and C61HPh, respectively, upon two-electron-transfer reduction. However, compound 3 demonstrates much more electrochemical stability as no retro-cycloaddition reaction occurs under similar conditions. Natural bond orbital (NBO) calculations on charge distribution show there is no significant difference among the dianions of 1, 2, and 3, indicating that the electrochemical stability of 3 is unlikely to be caused by the charge distribution difference of the dianions of three compounds. Examination on the crystal structure of compound 3 reveals close contacts of the C-H group with the heteroatoms (N and O) of cyclic phenylimidate, suggesting the existence of C-H center dot center dot center dot X (X = N, O) intramolecular hydrogen bonding among the addends, which is further confirmed by NBO analysis.

A simple method to prepare titania nanomaterials of core-shell structure, hollow nanospheres and mesoporous nanoparticles

A simple method to prepare titania nanomaterials of core-shell structure, hollow nanospheres and mesoporous nanoparticles has been developed. The core-shell nanostructures with NH4Cl as core and TiO2 center dot xH(2)O-NH4Cl as shell were prepared in nonaqueous system by the deposition on the surface of the aggregated NH4Cl crystals, which could be transformed into mesoporous anatase nanoparticles or hollow nanospheres by calcination at 500A degrees C or extraction with methanol, respectively. The hierarchical mesoporous nanostructures benefited the photocatalytic activities of the resultant titania nanomaterials, demonstrated by the UV light photodegradation of Methyl Orange.

A plasma sputtering decoration route to producing thickness-tunable ZnO/TiO2 core/shell nanorod arrays

We report a radio frequency magnetron sputtering method for producing TiO2 shell coatings directly on the surface of ZnO nanorod arrays. ZnO nanorod arrays were firstly fabricated on transparent conducting oxide substrates by a hydrothermal route, and subsequently decorated with TiO2 by a plasma sputtering deposition process. The core/shell nanorods have single-crystal ZnO cores and anatase TiO2 shells. The shells are homogeneously coated onto the whole ZnO nanorods without thickness change. This approach enables us to tailor the thickness of the TiO2 shell for desired photovoltaic applications on a one-nanometer scale. The function of the TiO2 shell as a blocking layer for increasing charge separation and suppression of the surface recombination was tested in dye-sensitized solar cells. The enhanced photocurrent and open-circuit voltage gave rise to increased photovoltaic efficiency and decreased dark current, indicating successful functioning of the TiO2 shell.

Study on the Single Crystals of Poly(3-octylthiophene) Induced by Solvent-Vapor Annealing

Needle-like single crystals of poly(3-octylthiophene) (P3OT) have been prepared by tetrahydrofuran-vapor annealing. The morphology and structure of the crystals were characterized with optical microscopy, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and wide-angle X-ray diffraction. It is observed that the P3OT molecules are packed with the backbones parallel to the length axis of the crystal and the alkyl side chains perpendicular to the substrate. The field effect transistor based on the P3OT single crystal exhibited a charge carrier mobility of 1.54 x 10(-4) cm(2)/(Vs) and on/off current ratio of 37, and the molecular orientation of the crystal is ascribed to account for the device performance. The time-dependent morphological evolution demonstrated that the crystals underwent Ostwald ripening when annealed.

Achieving perpendicular alignment of rigid polythiophene backbones to the substrate by using solvent-vapor treatment

The rigid backbone of the poly(3-butylthiophene) molecule adopts a perpendicular orientation with respect to the substrate by using a solvent-vapor treatment (see figure). Small and closely contacting spherulites instead of conventional whisker-like crystals are achieved. This could be utilized to improve charge-carrier mobility particularly in the direction normal to the film plane by designing and constructing thick crystalline domains in the functional layer.

Enhanced electrical conductivity of highly crystalline polythiophene/insulating-polymer composite

Poly(3-butylthiophene) (P3BT)/insulating-polymer composites with high electrical conductivity have been prepared directly from the solution. These composites exhibit much higher conductivity compared to pure P3BT with the same preparation method provided that P3BT content is higher than 10 wt %. Morphological studies on both the pure P3BT and the composites with insulating polymer show that P3BT highly crystallizes and develops into whisker-like crystals. These nanowires are homogeneously distributed within the insulating polymer matrix and form conductive networks, which provide both extremely large interface area between conjugated polymer and insulating polymer matrix and highly efficient conductive channels through out the whole composite. In contrast, the conductivity enhancement of P3HT/PS composite is not so obvious and drops down immediately with increased PS content due mainly to the absence of highly crystalline whisker-like crystals and much larger scale phase separation between the components. The results presented here could further illuminate the origin of conductivity formation in organic semiconducting composites and promote applications of these polymer semiconductor/insulator composites in the fields of organic (opto-)electronics, electromagnetic shielding, and antistatic materials.

Studies on microstructure bilayer film of ultrasonic dipped cadmium sulfide and d.c. sputtered indium tin oxide

A bilayer CdS/ITO film was obtained. The dipped CdS was grown by an ultrasonic colloid deposition (USCD) method. Microstructure of the CdS film made by USCD has a wider transmission range and a higher transmittance. Amorphous indium-tin-oxide (ITO) thin film was deposited using d.c. magnetron-sputtering at room temperature. The ITO films exhibited good conductivity and maximum transmittance of 94%. The CdS/ITO bilayer was investigated by means of GIXD (grazing incidence X-ray diffraction) at different incidence angles (alpha = 0.20-5.00degrees) and XRD. We discuss a model for the thin bilayer film. SEM and AFM show that homogeneous CdS films with a bar-shaped ultrafine particles and ITO film with nanometer structure. The mechanism of the bilayer CdS/ITO film is discussed.

Photosensitization of TiO2 nanoparticulate thin film electrodes by CdS nanoparticles

Surface photovoltage spectra (SPS) measurements of TiO2 show that a large surface state density is present on the TiO2 nanoparticles and these surface states can be efficiently decreased by sensitization using US nanoparticles as well as by suitable heat treatment. The photoelectrochemical behavior of the bare TiO2 thin film indicates that the mechanism of photoelectron transport is controlled by the trapping/detrapping properties of surface states within the thin films, The slow photocurrent response upon the illumination can be explained by the trap saturation effect. For a TiO2 nanoparticulate thin film sensitized using US nanoparticles, the slow photocurrent response disappears and the steady-state photocurrent increases drastically, which suggests that photosensitization can decrease the effect of surface states on photocurrent response.

Surface photovoltage spectra and photoelectrochemical properties of semiconductor-sensitized nanostructured TiO2 electrodes

Three kinds of TiO2 nanostructured thin films and their CdS-sensitized films, consisting of different sizes of TiO2 nanoparticles prepared with different methods, have been investigated. The surface photovoltage spectra (SPS) measurements indicate that the density of surface states on TiO2 is likely dependent upon the details of prepared methods. TiO2 particles prepared from basic sol have more surface states than that prepared from acidic sol. When the TiO2 thin films prepared using the TiO2 sols were sensitized by CdS particles, the SPS responses relative to the surface states on TiO2 from 350 to 800 nm were decreased. The photoelectrochemical properties of nanostructured TiO2 electrodes suggest that the fewer the surface states and the smaller the particle sizes of TiO2, the larger the photocurrent response. For CdS sensitized TiO2 thin film electrode, it is shown that the semiconductor sensitization is an efficient way to decrease the influence of surface states on the charge separation, and can improve the intensity of photocurrent response. (C) 2001 Elsevier Science B.V. All rights reserved.

Semiconductor nanowire fabrication via bottom-up & top-down paradigms

Semiconductor nanowires are pseudo 1-D structures where the magnitude of the semiconducting material is confined to a length of less than 100 nm in two dimensions. Semiconductor nanowires have a vast range of potential applications, including electronic (logic devices, diodes), photonic (laser, photodetector), biological (sensors, drug delivery), energy (batteries, solar cells, thermoelectric generators), and magnetic (spintronic, memory) devices. Semiconductor nanowires can be fabricated by a range of methods which can be categorised into one of two paradigms, bottom-up or top-down. Bottom-up processes can be defined as those where structures are assembled from their sub-components in an additive fashion. Top-down fabrication strategies use sculpting or etching to carve structures from a larger piece of material in a subtractive fashion. This seminar will detail a number of novel routes to fabricate semiconductor nanowires by both bottom-up and top-down paradigms. Firstly, a novel bottom-up route to fabricate Ge nanowires with controlled diameter distributions in the sub-20 nm regime will be described. This route details nanowire synthesis and diameter control in the absence of a foreign seed metal catalyst. Additionally a top-down route to nanowire array fabrication will be detailed outlining the importance of surface chemistry in high-resolution electron beam lithography (EBL) using hydrogen silsesquioxane (HSQ) on Ge and Bi2Se3 surfaces. Finally, a process will be described for the directed self-assembly of a diblock copolymer (PS-b-PDMS) using an EBL defined template. This section will also detail a route toward selective template sidewall wetting of either block in the PS-b-PDMS system, through tailored functionalisation of the template and substrate surfaces.

Orientation effects in copper phthalocyanine films studied by electron paramagnetic resonance spectroscopy

The metallo-phthalocyanines (MPcs) are an interesting group of organic semiconductor materials for applications such as large area solar cells due to their optoelectronic properties coupled with the possibility of easily and cheaply fabricating thin films of MPcs [1, 2]. As for organic semiconductors in general, many of the interesting properties of the MPcs such as magnetism, light absorption and charge transport, are highly anisotropic [2, 3]. To maximise the efficiency of a device based on these materials it is therefore important to study their molecular orientation in films and to assess the influence of different growth conditions and substrate treatments.
X-ray diffraction is a well established and powerful technique for studying texture (and hence molecular orientation) in crystalline materials, but it cannot provide any information about amorphous or nanocrystalline films. In electron paramagnetic resonance (EPR) spectroscopy the signal comes from the spin of unpaired electrons in the material. This technique therefore does not require the sample to be crystalline. It works for any sample with paramagnetic centres such as the MPcs where the unpaired electrons are contributed by the metal. In this paper we present a continuous-wave X-band EPR study using the anisotropy of the EPR spectrum of CuPc [4] to determine the orientation effects in different types of CuPc films. From these measurements we gain insight into the molecular arrangement of films with different spin concentrations, and apply our technique to the study of molecular orientation in photovoltaic cells.