Flexible fiber-type CdS quantum dot-sensitized solar cells (CdS FF-QDSSCs), in which all the components are assembled into a flexible plastic capillary tube, are designed and fabricated. In each FF-QDSSC, a Pt wire along the capillary tube axis acts as the counter electrode. A number of surrounding Ti wires coated with CdS-sensitized ZnO nanorods act as the working electrodes. This design provides the cells with good flexibility and all-direction light harvesting capability. The photovoltaic performances of the FF-QDSSCs can be effectively improved by increasing the number of the working electrodes up to six. The conversion efficiency of the cell with six working electrodes (FF-QDSSC-6) is more than five times that of the cell with only one working electrode. Furthermore, for simultaneously illuminating the cell from all the directions perpendicular to the capillary tube, an Al reflector is placed behind the cell. This measure raised the maximum power output of an FF-QDSSC-6 by 55%. Index Terms-Flexible, multiple working electrodes, quantum dot-sensitized solar cell, simultaneous light harvesting from all directions, ZnO nanorod.

No abstract

The generation of a field emission (FE) cathode electron source has attracted wide attention for its miniaturization, high-frequency operability, and low energy consumption. However, the FE performance of cold cathodes is limited by poor current stabilities of carbon nanotube (CNT) emitters. Coating CNTs with sp 3 -bonded carbon coatings is considered as a successful approach to stabilize the FE currents. High-quality ultrathin diamond-like carbon (DLC) films, which serve as sp 3 carbon coatings, are deposited uniformly on CNTs by filtered cathodic vacuum arc evaporation in this research. The thicknesses of DLC coatings and the field enhancement factors of pristine CNTs affect to a large extent the FE properties. An optimum coating thickness of DLC layers corresponding to the lowest threshold field exists due to space-charge-induced band bending, at which the depletion region of the DLC layer in equilibrium is maximized. The optimum coating thickness increases with the geometric field enhancement factor of pristine CNTs bundles, the relationship of which illustrates an obvious difference between planar cold cathodes and nanostructured field emitters, and could be extended to optimize double-layer or multi-layered nanostructures serving as FE emitters with high reliability.

The electric field on a hemisphere-on-post nanowire is numerically calculated using the finite element method (FEM). The FEM calculation results show that the field is sufficiently strong for extracting a significant field emission current only in a small area at the top of the hemisphere, while the contribution to the field emission from the other part of the hemisphere and the flank side of the cylinder is negligible owing to the rapid drop of the electric field. Both the local current density at the top of the hemisphere ([Formula: see text]) and the average current density across the nanowire cross-section ([Formula: see text]) are calculated and the [Formula: see text]-to-[Formula: see text] ratio ([Formula: see text]) is introduced to reflect the nonuniformity of the field emission. An empirical formula with proper parameters that can best fit the simulation results is derived for describing the dependence of [Formula: see text] on the macroscopic electric field ([Formula: see text]). As a result, the [Formula: see text]–[Formula: see text] relationship is attained and the revision to the traditional Fowler–Nordheim (FN) formula caused by the nonuniformity of field distribution is found in both the pre-exponent part and the exponent part, so that the deviation of the FN plots from linearity often observed in experiments is partly accounted for. Moreover, the resistance at the emitter-substrate interface is shown to cause saturation in the field emission current and a downward bending of the FN plot in the high-field region.

In field emission under a non-dc voltage, a displacement current is inevitable due to charging the cathode–anode condenser. Under an often-used square voltage pulse, in which the voltage rises from zero to a certain value abruptly, the charging current in the circuit is very large at the rising and falling edges. This large charging current makes measurement of the actual emissive current from the cathode difficult, constitutes a threat to the components in the circuit and causes attenuation of the emissive current within the pulse. To alleviate these drawbacks, trapezoid voltage pulses, whose rising edges are extended dramatically in comparison with square voltage pulses, are employed to extract the field emission. Under a trapezoid voltage pulse, the charging current is clearly lowered as expected. Furthermore, the heat generated by the charging current under the trapezoid voltage pulse is much smaller than that under the square voltage pulse. Hence the emissive current does not show any attenuation within the pulse. Finally, the average emissive currents are found to decrease with the repetition frequency of the pulses.