Hongcai Wu

Enhancement of third-order optical nonlinearities by conjugated polymer-bonded carbon nanotubes

Third-order nonlinear susceptibility χ(3) and second-order hyperpolarizability γ of poly{[3-octylthiophene-2,5-diyl]-[p-aminobenzylidenequinomethane]-bonded multiwalled carbon nanotubes (POTABQ-MWNTs) were measured in chloroform solution using degenerate four-wave mixing at 532nm under nanosecond pulse excitation. Effective nonlinear absorption coefficient βeff and nonlinear refraction index n2 were obtained using open aperture and close aperture Z-scan technique at the same wavelength, respectively. Optical limiting property of the sample was also investigated. The resulting POTABQ-MWNTs exhibited large third-order nonlinear optical responses mainly due to the formation of intramolecular photoinduced charge-transfer system of polymers and carbon nanotubes.

A Fine Model for Evaluating Actual Output Performance of Crystalline Silicon Solar Modules

Textured crystalline silicon solar cells, planar crystalline silicon solar cells, textured glass, planar glass are commonly fabricated for crystalline silicon solar modules, thus, there are four kinds of crystalline silicon solar modules. The photovoltaic electricity outputs of the four kinds of modules with the same peak power measured under standard test conditions are different under true sunshine conditions; it is believed that the reflection losses of the four kinds of modules are different due to the different materials used in modules. Using the actual optical performance of a four-layer encapsulation, a simulation was made on the reflection losses and energy outputs of the four kinds of solar modules. The computer simulation shows that the module encapsulated by textured crystalline silicon solar cells and textured glass has more energy output than others in actual uses, even if they have equal electrical characteristics under standard test conditions, the result of simulation agrees approximately with the measured values. Performing energy production analysis for the four kinds of modules with the same peak power as a function of angle of incidence provides insight into economic analysis of photovoltaic systems and performance estimation of a photovoltaic roof within a fixed flat plate array

The third-order optical nonlinearities of carbon nanotube modified conjugated polymer in the femtosecond and nanosecond regimes

Multiwalled carbon nanotubes (MWCNTs) have been noncovalently modified with poly[2-methoxy 5-octyoxy]-p-phenylene vinylene (MO-PPV) to form a series of solution of MWCNTs and the conjugated polymers. The third-order nonlinearities of the solutions were investigated by the optical Kerr effect technique with femtosecond pulses, and degenerate four-wave mixing, open aperture, and closed aperture Z-scan techniques with nanosecond pulses. We attempted to quantify the contributions from various channels including the real and imaginary parts, the dynamic carriers, two-photon absorption, and thermal effect. The MO-PPV/MWCNTs composites showed much larger third-order nonlinearities than the pristine polymers. The enhanced optical nonlinearities were attributed to the strong interaction between the pi-pi electrons of MO-PPV and MWCNTs in their photoexcited state. (c) 2006 American Institute of Physics.

Sixth-order correlation on Raman-enhanced polarization beats with phase-conjugation geometry

The correlation effects of sixth-order on the Raman-enhanced polarization beats (REPB) with phase-conjugation geometry in carbon disulfide are investigated using chaotic and phase-diffusion models. Based on two types of models, the cases that the pump beams have either narrow band or broadband linewidth are considered and it is found that the beat signal oscillates not only temporally with a 50.5 fs period but also spatially. The overall accuracy of using REPB to measure the resonant frequency of Raman-active vibrational mode is determined by the relaxation rates of the Raman mode and the molecular-reorientational grating. Moreover, the temporal behavior of the beat signal depends on the stochastic properties of the lasers and the decay rates of the Raman mode and the molecular-reorientational grating. Different stochastic models of the laser field only affect the sixth- and fourth-order coherence function. Furthermore, the different roles of the phase fluctuation and amplitude fluctuation have been pointed out in the time domain. The difference between the REPB and the fourth-order coherence on ultrafast modulation spectroscopy is discussed as well from a physical viewpoint. Therefore, the sixth-order coherence function theory of chaotic field is of vital importance in REPB.

Analysis of Potential Distribution and Current-Voltage Characteristic in Polyimide Langmuir-Blodgett Films

The current–voltage (I–V) characteristics of Au electrode (M)–polyimide (PI) Langmuir–Blodgett (LB) film (I)–Al electrode (M)-MIM element are analyzed, taking into account the interfacial electrostatic phenomena and the presence of the interfacial electronic states. On the basis of the Richardson–Schottky model, the attractive force and potential energy created by a series of mirror image charges against both electrodes are calculated. A positive bias (Vex>0) applied to an Au–PI LB film–Al element gives rise to the injection of electrons from the Au electrode into the lowest unoccupied molecular orbitals (LUMO) of PI LB film, whereas a negative bias (Vex<0) causes the injection from Al electrode. The potential distributions across the PI LB film and the potential barrier height under the condition of various externally applied voltages (Vex) are calculated, assuming the presence of surface states at the metal/film interface. It is determined that the calculation results of the I–V characteristics can predict the experimental results.

Properties of Conducting Polymer-Dye Composite and Photovoltaic Characteristics of Junction Devices

We have prepared new soluble conducting polymer/dye composite and photovoltaic cells with three different architectures using soluble conducting polymer (CP) as hole transport material and perylene derivatives (PV) as electron transport material. These architectures comprise single-layer (ITO/CP-PV/Al) and double-layer (ITO/CP-PV/PV/Al) as well as three-layer (ITO/CP-C60/CP-PV/PV/Al) structures. Electrical and optical properties of soluble conducting polymer composite film and current-voltage characteristics and photocurrent spectra of photovoltaic cells are discussed. The results show that the soluble conducting polymer-dye composite represents a new class of organic semiconducting material that can be used to manufacture photovoltaic cells.

Design of Microstrip Ferrite-Coupled Line Devices Using Perturbation Theory

Ferrite perturbation theory is applied to analyze a ferrite-coupled microstrip line (FCL) with longitudinal magnetization. The properties of an FCL structure with a given finite length are discussed. Optimal conditions for the waveguide cross section and the length are derived. Using the results of the perturbation analysis, a circulator and an isolator based on the microstrip line FCL are designed, fabricated, and measured. Agreement between measured and calculated S parameters is obtained. The devices show an insertion loss of 2?3 dB within the frequency range of 6?10 GHz.

A modified model for large grain multicrystalline silicon used for solar cells

The large grain multicrystalline silicon for large scale terrestrial solar cells application has been well recognized, it shares with single crystal silicon numerous desirable physical and chemical properties such as abundance, low toxicity and stability, but the electrical behavior of multicrystalline silicon is different slightly than single crystal silicon. The electrical behavior of multicrystalline silicon is shown to be influenced by the properties of the grain boundaries, the study of the grain boundaries is very important for improving the efficiency of multicrystalline silicon solar cells. Hence, modeling the electrical properties of large grain multicrystalline silicon and their dependence on dopant concentration becomes very important for the proper design of solar cells and good understanding of multicrystalline silicon solar cells under illumination. In this paper, a new phenomenological model based on a combination of dopant segregation, carrier trapping and carrier reflection at grain boundaries for large grain multicrystalline silicon used for solar cells is developed, the grain boundaries are modeled by high-order cosine barriers. The model of the high-order cosine barriers is found to more accurately model the practical silicon grains in the body. The result shows that, as the grain size is increased, the resistivity approaches that of single crystalline silicon, although it always remains slightly higher at the same dopant concentration; and as the dopant concentration is increased, the resistivity approaches that of single crystalline silicon, although it always remains higher at the same grain size.

Electrostatic interfacial phenomena and I-V characteristics of Au/polyimide Langmuir-Blodgett film/Al element

In this paper, we have analyzed the I-V characteristic of metal (Au)-PI LB film-metal (Al) elements, taking into account the interfacial electrostatic phenomena and the existence of the interfacial electric states at the metal/PI LB film interfaces. A modification of the potential barrier height in the Richardson-Schottky model has been made. It is demonstrated that the interfacial electronic states play an important role in determining the I-V characteristic of the Au-PI LB film-Al structure.