Jin-Wan Jeon

Towards a high efficiency amorphous silicon solar cell using molybdenum oxide as a window layer instead of conventional p-type amorphous silicon carbide

A thermally evaporated molybdenum oxide (MoO3) film was used as a window layer of a hydrogenated amorphous silicon (a-Si:H) solar cell instead of the conventional p-type hydrogenated amorphous silicon carbide (p-a-SiC:H) film. The short circuit current density (JSC) and fill factor were increased due to the wide optical band gap and high conductivity of the MoO3 film. As a result, the conversion efficiency of the fabricated MoO3 solar cell was increased to 6.21% compared to the typical a-Si:H solar cell (5.97%).

Tunable work function of a WOx buffer layer for enhanced photocarrier collection of pin-type amorphous silicon solar cells

An in situ postdeposition ultraviolet treatment was proposed to improve the electrical properties of a tungsten oxide (WOx) buffer layer of pin-type amorphous silicon-based solar cell. Based on the x-ray and ultraviolet photoelectron spectroscopy and the activation energy measurements, it was found that the work function of WOx is tunable by ultraviolet light treatment, and the collection performance of solar cells incorporating WOx with the lower work function is further improved. Moreover, the optimal band alignment scheme for a window layer is discussed in terms of obtaining enhanced carrier collection without open circuit voltage degradation.

Modeling, Design, Fabrication, and Demonstration of a Digital Micromirror With Interdigitated Cantilevers

We present the modeling, design, fabrication, and measurement results of a novel digital micromirror based on a new actuator called interdigitated cantilevers. In contrast to conventional micromirrors that rotate through the twisting actuation of a hinge, this micromirror has a symmetric bidirectional rotation through a bending actuation of interdigitated cantilevers hidden under a mirror plate. For the static and dynamic characteristics of the proposed micromirror, analytical models were developed first on the basis of the Euler-Bernoulli beam equation, as well as both distributed and lumped-parameter models. The results of the developed analytical models are in good agreement with those of a finite-element-method (FEM) simulation, having just a 10% deviation. On the basis of these analytical models, we successfully designed, fabricated, and evaluated a micromirror with a mirror size of 16 mum times 16 mum. The fabricated micromirror has a mechanical rotation angle of plusmn10deg, a pull-in voltage of 54 V, a resonant frequency of 350 kHz, and a switching response time of 17 mus. The measurement results compare favorably with those of analytical models and FEM simulations, with deviations of less than 15% and 10%, respectively.

Mechanical Reliability of a Digital Micromirror With Interdigitated Cantilevers

This paper is the first to report on analytical studies and experimental results pertaining to the mechanical reliability of a micromirror with a new spring structure called as interdigitated cantilevers. The bending actuation of the interdigitated cantilevers means that it is capable of a symmetrical bidirectional rotation. Through analytical calculations, finite element method (FEM) simulations, and experiments, we show that the mechanical reliability of the micromirror can simply be improved by changing the spring structure from a conventional twisting hinge type to the new bending interdigitated cantilever type. To quantitatively demonstrate the improvement, we compare the maximum Von Mises stress exerted in two types of micromirrors. The analytical calculations and FEM simulations confirm that the maximum Von Mises stress for cyclic rotations with regard to the micromirror with interdigitated cantilevers is only half that of a conventional micromirror with a hinge when the two micromirrors have the same performance and the same rotation angle. Also, we experimentally evaluate the mechanical reliability of the micromirror with interdigitated cantilevers. The micromirror that is made of pure aluminum was successfully operated without any failures after more than 500 h of operation and 2 × 1010 cycles under a laboratory air condition (23°C and 50 %).

A New Three-Dimensional Lithography Using Polymer Dispersed Liquid Crystal (PDLC) Films

In this paper, we have proposed a new three-dimensional (3-D) lithography using polymer dispersed liquid crystal (PDLC) films. The scattering or transmission rate of ultraviolet (UV) rays through the PDLC film can be continuously controlled by varying the applied voltage across its electrodes. Various slopes and profiles of 3-D photoresist microstructures are easily and effectively fabricated by controlling applied voltages and biasing times of the PDLC film during one UV exposure step of the lithography process.

Improvement of amorphous silicon solar cell performance by inserting a tungsten oxide layer between zinc oxide and p-type amorphous silicon carbide

Amorphous silicon (a-Si:H) solar cell fabricated on zinc oxide (ZnO) has poor fill factor (FF) resulting from a high resistive contact between ZnO and p-type amorphous silicon carbide (p-a-SiC:H) films. This is due to the existence of a wide depletion region in the p-a-SiC:H adjacent to the ZnO/p-a-SiC:H interface. To overcome this contact problem, an amorphous tungsten oxide (WO3) layer was inserted between ZnO and p-a-SiC:H. It was found that the insertion of the WO3 layer improves the cell performance by reducing series resistance. Since this layer has higher optical band-gap (3.35 eV) than a-SiC:H (2.1 eV), there was no change of current density in short wavelength regions. A radio-frequency magnetron sputtering apparatus was used to deposit ZnO:Al on glass. The structure of the cells was ZnO:Al (800 nm)/n-a-WO3 (4 nm)/p-a-SiC:H (10 nm)/i-a-Si:H (270 nm)/n-a-Si:H (30 nm)/Al (100 nm). As a result of the WO3 insertion, the efficiency of the solar cell enhanced from 6.89% to 7.45%.

Drastic performance enhancement by using a WOx buffer before ZnO back reflector in amorphous silicon solar cells fabricated at 121 °C

We report on drastic performance improvement by inserting amorphous tungsten oxide (WOx) with a wide optical band gap at the n-type amorphous silicon (n-a-Si)/zinc oxide (ZnO) back reflector interface in a-Si-based solar cells fabricated at 121 °C. We found that a 3-nm-thick WOx film could remarkably reduce the defect density at the n-a-Si/ZnO interface, resulting in decreased series and increased shunt resistances. Consequently, the fill factor and conversion efficiency could be markedly enhanced by 8.6% and 9.2%, respectively. A maximum efficiency of 8.05% was obtained. This technique may be applied to all kinds of thin-film solar cells.

Tungsten oxide as a p-type window material in amorphous silicon solar cells

A p-type amorphous tungsten oxide (p-a-WO3) film was prepared using a vacuum thermal evaporator with a WO3 source. By replacing a 10 nm-thick p-type amorphous silicon carbide window layer of a pin-type amorphous silicon based solar cell with a 10 nm-thick p-a-WO3 film, the short circuit current density increased from 12.75 to 13.83 mA/cm2. Although the open circuit voltage was limited to 0.65 V due to the smaller work function of the p-a-WO3, the a-Si based solar cell with the novel p-a-WO3 window layer has shown a conversion efficiency of 6.05 %. Our research opens a new application field for thin film solar cells.

High fill-factor micromirror array and its fabrication process

The micromirror array with high fill-factor of 91% is fabricated. For high fill-factor, the posts supporting the mirror plate are filled up with copper by the uniform electroplating process

Self-Aligned Series Connection of an Amorphous Silicon p-i-n Type Solar Cell Using Aluminum Oblique Deposition for Minimum Power Loss

We fabricated a self-aligned series connection of an amorphous silicon p-i-n type solar cell with a small dead area of 10 μm using oblique deposition (OD). An OD on a trench can readily pattern an evaporated aluminum (Al) layer. The fabrication process consists of forming trenches on a glass substrate, depositing and texturing ZnO:Al, Al OD for auxiliary electrode, depositing p-i-n layers, Al OD, dry etching the p-i-n layers, and Al OD to connect the front electrode to the back electrode in series. The predicted fractional power loss of our series connected solar module was under 1%.