Dipolar-modulated charge-doped trilayer organic semiconductor n-n heterojunction

A novel dipolar-modulated charge-doped trilayer n-n organic heterojunction with a bidirectional tunable energy band discontinuity is constructed. The rectifying mechanism of the trilayer is similar to the rectifying and inverse-rectifying characteristics from n-p and p-n junctions, respectively. Zero-bias optoelectronic behavior and persistent photoconductivity are discovered. These results show that what are viewed as technological hurdles in the development of an organic n-n heterojunction should, in fact, lead to a better approach in organic optoelectronics.     

Hybrid organic/inorganic molecular heterojunctions based on strained nanomembranes

In this work, we combine self-assembly and top-down methods to create hybrid junctions consisting of single organic molecular monolayers sandwiched between metal and/or single-crystalline semiconductor nanomembrane based electrodes. The fabrication process is fully integrative and produces a yield loss of less than 5% on-chip. The nanomembrane-based electrodes guarantee a soft yet robust contact to the molecules where the presence of pinholes and other defects becomes almost irrelevant. We also pioneer the fabrication and characterization of semiconductor/molecule/semiconductor tunneling heterojunctions which exhibit a double transition from direct tunneling to field emission and back to direct tunneling, a phenomenon which has not been reported previously.     

Hybrid organic/inorganic composites based on silica and weak synthetic polyelectrolytes

The preparation and characterization of new organic/inorganic composites by the consecutive adsorption of weak polyelectrolytes on silica particles were studied in the article. Two polycations containing primary amine groups in the side chains, poly(vinylamine) or poly[N(β-aminoethylene) acrylamide], and poly(acrylic acid) as polyanion were used for the hybrid materials construction. The stability of the organic/inorganic composites has been increased by a heat-induced reaction at 150 °C. The organic/silica hybrids properties were monitored by potentiometric titration, laser light scattering, infrared spectroscopy, and thermogravimetric analysis. The adsorption of methylene blue by the composite materials has been tested. The dye adsorption capacity was strongly influenced by the dye concentration, the nature of the last adsorbed layer, the polyions concentration, and the composite thermal treatment.     

Hybrid Nanocomposite Materials—between inorganic glasses and organic polymers

The sol–gel process, with its associated mild conditions, offers a new approach to the synthesis of composite materials with domain sizes approaching the molecular level. Transparent organic–inorganic composites can be prepared by dissolving preformed polymers into sot–gel precursor solutions, and then allowing the tetraalkyl orthosilicates to hydrolyze and condense to form glassy SiO₂ phases of different morphological structures. Alternatively, both the organic and inorganic phases can be simultaneously formed through the synchronous polymerization of the organic monomer and the sol–gel precursors. Depending upon such factors as the structures of the organic and inorganic components, the phase morphology, the degree of interpenetration, and the presence of covalent bonds between the phases, the properties of these composites can vary greatly and range from elastomeric rubbers to high–modulus materials.     

Highly efficient resonant coupling of optical excitations in hybrid organic/inorganic semiconductor nanostructures

The integration of organic and inorganic semiconductors on the nanoscale offers the possibility of developing new photonic devices that combine the best features of these two distinct classes of material. Such devices could, for example, benefit from the large oscillator strengths found in organic materials and the nonlinear optical properties of inorganic species. Here we describe a novel hybrid organic/inorganic nanocomposite in which alternating monolayers of J-aggregates of cyanine dye and crystalline semiconductor quantum dots are grown by a layer-by-layer self-assembly technique. We demonstrate near-field photon-mediated coupling of vastly dissimilar optical excitations in the two materials that can reach efficiencies of up to 98% at room temperature. By varying the size of the quantum dots and thus tuning their optical resonance for absorption and emission, we also show how the ability of J-aggregates to harvest light can be harnessed to increase the effective absorption cross section of the quantum dots by up to a factor of ten. Combining organic and inorganic semiconductors in this way could lead to novel nanoscale designs for light-emitting, photovoltaic and sensor applications.     

Hybrid epitaxial-colloidal semiconductor nanostructures

We present a growth technique which combines wet-chemical growth and molecular beam epitaxy (MBE) to create complex semiconductor nanostructures with nanocrystals as active optical material. The obtained results show that wet-chemically prepared semiconductor nanocrystals can be incorporated in an epitaxally grown crystalline cap layer. As an exemplary system we chose CdSe nanorods and CdSe(ZnS) core-shell nanocrystals in ZnSe and discuss the two limits of thin (d approximately 2R) and thick (d>2R) ZnSe cap layers of thickness d for CdSe nanorods and nanodots of radii R between 2 and 4 nm. In contrast to the strain-induced CdSe/ZnSe Stranski-Krastanow growth of a quantum dot layer in a semiconductor heterostructure, the technique proposed here does not rely on strain and thus results in additional degrees of freedom for choosing composition, concentration, shape, and size of the nanocrystals. Transmission electron microscopy and X-ray diffractometry show that the ZnSe cap layer is of high crystalline quality and provides all parameters for a consecutive growth of Bragg structures, waveguides, or diode structures for electrical injection.     

Determining the optical absorption edge in organic semiconductor composites with a bulk heterojunction by the constant photocurrent method

Spectral dependences of photoconductivity in thin layers of polyconjugated polymers (PCDTBT, PTB7) and their composites with a fullerene derivative (PC₇₀BM), which are promising for the development of organic solar cells, have been studied. It was found that the photoconductivity in the polymeric composite exceeds that in the polymer in the whole spectral range under study and the edge of the photoconductivity spectrum is shifted to the long-wavelength part of the spectrum. Use of the constant photocurrent method made it possible to obtain spectral dependences of the absorption coefficients and determine the optical gap width of the materials studied.     

Half wave rectification of inorganic/organic heterojunction diode at the frequency of 1 kHz

An inorganic/organic vertical heterojunction diode has been demonstrated with p-type Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) deposited by spin coating on n-type Ga-doped ZnO (GZO) thin films. Transparent conducting GZO thin films are deposited on glass substrate by rf-magnetron sputtering. Electrical properties of GZO thin films are investigated depending on the processing temperatures. The resistivity, mobility and carrier concentration of the GZO thin films deposited at processing temperatures of 500 °C are measured to be about 3.6 × 10⁻⁴ Ω cm, 23.8 cm²/Vs and 7.1 × 10²⁰ cm³, respectively. The root mean square surface roughness of the GZO thin films is calculated to be ~ 0.9 nm using atomic force microscopy. Current-voltage characteristics of the n-GZO/p-PEDOT:PSS heterojunction diode present rectifying operation. Half wave rectification is observed with the maximum output voltage of 1.85 V at 1 kHz. Low turn-on voltage of about 1.3 V is obtained and the ideality factor of the n-GZO/p-PEDOT:PSS diode is derived to be about 1.8.     

CuO-ZnO异质结半导体陶瓷气敏机理的研究

ＣｕＯ－ＺｎＯ异质结气敏传感器是一种新型的气敏传感器,它有成本低,工艺简单、检测方便等众多优点,本工作主要研究了ＣｕＯ和ＺｎＯ不同比例情况下,该传感器的气敏性能。测试了它的阻温特性及在不同温度,不同气氛条件下器件的灵敏度,并从理论上对测试结果及敏感机理进行了分析和讨论。     

Influence of organic salt concentration on the performance of bulk heterojunction organic solar cell

The effect of organic salt on the performance of bulk heterojunction organic solar cell was investigated by varying the concentration of tetrabutylammonium hexafluorophosphate (TBAPF₆). Organic solar cells based on TBAPF₆-blended poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV): (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) thin films with aluminium (Al) as cathode have been fabricated on ITO substrates. The MEHPPV:PCBM films with different concentrations of TBAPF₆ (10, 20, 30 and 40 wt% with respect to MEHPPV) were deposited onto the ITO by spin coating technique, followed by deposition of Al using electron gun evaporation technique to build the devices. Experimental results showed that the short circuit current density and open circuit voltage improved with increasing of TBAPF₆ concentration up to 20 wt% since more dissociated ions accumulated at the photoactive layer-electrode interfaces resulted in higher built in electric field. However, the short circuit current density and open circuit voltage started to decrease at TBAPF₆ concentration of 30 wt%, indicating higher charge recombination as a result of agglomeration of TBAPF₆.     

The influences of substrate temperature on ambipolar organic heterojunction transistors

Organic heterojunction thin-film transistors are fabricated based on copper phthalocyanine (CuPc) and hexadecafluorophtholocyaninatocopper (F₁₆CuPc) as double active layers, which exhibit typical ambipolar conduction. Several substrate temperatures are utilized to tune film morphology, which results in a remarkable change on the electric characteristics of organic transistors. The highest balanced mobility value of 2.91 × 10⁻² cm²/V s for hole and 1.04 × 10⁻² cm²/V s for electron are obtained by depositing F₁₆CuPc at 150 °C and CuPc at 200 °C, respectively, which are comparable to those conventional single-layer devices. This result demonstrates that the growth conditions of organic heterojunctions play a crucial role in ambipolar devices.     

Ambipolar organic heterojunction transistors with various p-type semiconductors

Ambipolar transport has been realized in organic heterojunction transistors with metal phthalocyanines, phenanthrene-based conjugated oligomers as the first semiconductors and copper-hexadecafluoro-phthalocyanine as the second semiconductor. The electron and hole mobilities of ambipolar devices with rod-like molecules were comparable to the corresponding single component devices, while the carrier mobility of ambipolar devices with disk-like molecules was much lower than the corresponding single component devices. The much difference of their device performance was attributed to the roughness of the first semiconductor films, which was original from their distinct growth habits. The flat and continuous films for the first semiconductors layer can lead to a smooth heterojunction interface, and obtained a high device performance for ambipolar organic heterojunction transistors.     

Localized photovoltaic investigations on organic semiconductors and bulk heterojunction solar cells

Newly synthesized organic electronics materials are often available in submicrogram amounts only. Photoelectrochemical scanning droplet cell microscopy is a powerful method that allows a comprehensive characterisation of such small amounts including oxidation, reduction potentials, doping, determination of charge carriers, band gap, charge capacity, over-oxidation sensitivity and many more. Localized photoelectrochemical characterization of the poly[4,8-bis-substituted-benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl-alt-4-substituted-thieno [3,4-b] thiophene-2,6-diyl] (PBDTTT-c) and PBDTTT-c:PCBM bulk heterojunction was performed using photoelectrochemical scanning droplet cell microscopy (PE-SDCM). The optical properties and the real and imaginary part of the dielectric function, of the polymer were determined using spectroscopic ellipsometry. The photoelectrochemical characterizations were performed in a three and two electrode configuration of PE-SDCM under laser and white light illumination. The effect of illumination was characterized using dark/illumination sequences. The stability of the photocurrent was studied using longer term (600 s) illumination. Finally the effect of cell configuration and illumination conditions on the photovoltage was studied.     

Controlling heterojunction abruptness in VLS-grown semiconductor nanowires via in situ catalyst alloying

For advanced device applications, increasing the compositional abruptness of axial heterostructured and modulation doped nanowires is critical for optimizing performance. For nanowires grown from metal catalysts, the transition region width is dictated by the solute solubility within the catalyst. For example, as a result of the relatively high solubility of Si and Ge in liquid Au for vapor-liquid-solid (VLS) grown nanowires, the transition region width between an axial Si-Ge heterojunction is typically on the order of the nanowire diameter. When the solute solubility in the catalyst is lowered, the heterojunction width can be made sharper. Here we show for the first time the systematic increase in interface sharpness between axial Ge-Si heterojunction nanowires grown by the VLS growth method using a Au-Ga alloy catalyst. Through in situ tailoring of the catalyst composition using trimethylgallium, the Ge-Si heterojunction width is systematically controlled by tuning the semiconductor solubility within a metal Au-Ga alloy catalyst. The present approach of alloying to control solute solubilities in the liquid catalyst may be extended to increasing the sharpness of axial dopant profiles, for example, in Si-Ge pn-heterojunction nanowires which is important for such applications as nanowire tunnel field effect transistors or in Si pn-junction nanowires.     

Quantitative nanoscale surface voltage measurement on organic semiconductor blends

We report on the validation of a method based on Kelvin probe force microscopy (KPFM) able to measure the different phases and the relative work function of polymer blend heterojunctions at the nanoscale. The method does not necessitate complex ultra-high vacuum setup. The quantitative information that can be extracted from the topography and the Kelvin probe measurements is critically analysed. Surface voltage difference can be observed at the nanoscale on poly(3-hexyl-thiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends and dependence on the annealing condition and the regio-regularity of P3HT is observed.     

Ambipolar field effect transistors with heterojunction of organic semiconductors

Ambipolar operations in organic field effect transistors (OFETs) with heterojunction structures have been demonstrated. We have selected a biphenyl capped thiophene oligomer (BP2T) as p-type and fullerene (C₆₀) as n-type materials in the active layer of the OFETs. To investigate their intrinsic behaviors we measured the OFET characteristics in vacuum without breaking vacuum after device fabrication. Their electric characteristics depended on the heterostructure configurations. The OFET prepared with a co-deposited thin film of BP2T/C₆₀ showed high carrier transport performance and both carriers were efficiently injected into the channel of the active layer. In the bi-layered device, ambipolar characteristics were only observed when the n-type C₆₀ molecules penetrated deeply into the BP2T layer.     

Tailoring organic heterojunction interfaces in bilayer polymer photovoltaic devices

In an ideal model, a p-n junction is formed by two stacked slabs of semiconductors. Although the construction of actual devices is generally more complex, we show that such a simple method can in fact be applied to the formation of organic heterojunctions. Two films of the organic semiconductors poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) can be connected by a simple film-transfer method without disturbing their flat surfaces. Each film can further be modified with a surface-segregated monolayer to tune the strength and direction of the surface dipole moment. Using this method, we fabricated bilayer organic photovoltaic devices with interfacial dipole moments that were selected to align the energy levels at the heterojunction. The open-circuit voltages of the P3HT/PCBM devices could be tuned over a wide range between 0.3 and 0.95 V, indicating that, even if the same combination of bulk materials is used, the interfacial properties drastically alter the performance of organic photovoltaic devices.     

Organic photovoltaic cell employing organic heterojunction as buffer layer

Hexadecafluorophthalocyaninatocopper (F₁₆CuPc)/zinc phthalocyanine (ZnPc) heterojunction layer has been used as buffer layer in organic photovoltaic (OPV) cells based on ZnPc and C₆₀. The F₁₆CuPc/ZnPc heterojunction with highly conductive property decreased the contact resistance between the indium-tin-oxide anode and the organic layer. As a result, the short-circuit current density and fill factor were increased, and the power-conversion efficiency was improved by over 60%. Therefore, the method provides an effective path to improve the performance of OPV cells.