Koeng Su Lim

Highly textured and conductive undoped ZnO film using hydrogen post-treatment

We proposed a method to enhance the characteristics of undoped ZnO films by H2 post-treatment using photochemical vapor deposition. The resistivity of a H2-treated film decreased from 1×10−2 to 2×10−3 Ω cm, the haze ratio increased from 37% to 48%, and no degradation of total transmittance was observed. There are two possible explanations for these phenomena. First, hydrogen atoms assist the desorption of oxygen atoms from the film, resulting in decreased resistivity. Second, hydrogen atoms etch small grains growing among large ones on the surface of the film, resulting in a rough surface.

Highly stable and textured hydrogenated ZnO thin films

We investigated intentionally hydrogenated zinc oxide (ZnO:H) fabricated by combining photoassisted metalorganic chemical vapor deposition and mercury-sensitized hydrogen addition methods. We found that intentionally incorporated hydrogen plays an important role in n-type conduction as a donor, improving free carrier concentration and electrical stability. We simultaneously obtained improved surface roughness of the ZnO:H film due to an enhancement of (1120) orientation. The high-quality ZnO:H film is promising as a back reflector material for thin-film solar cells.

Extremely Transparent and Conductive ZnO:Al Thin Films Prepared by Photo-Assisted Metalorganic Chemical Vapor Deposition (photo-MOCVD) Using AlCl3(6H2O) as New Doping Material.

Extremely transparent and conductive ZnO:Al thin films were successfully prepared by a photo-assisted metalorganic chemical vapor deposition (photo-MOCVD) technique at a temperature of 140° C using diethylzinc and H2O as source materials. The vapor from an aqueous solution of aluminum chloride hydrate ( AlCl3(6H2O)) was used as a doping gas. ZnO:Al thin films with a minimum resistivity of 6.22×10-4 Ω cm were obtained. Their total transmittance at 550 nm was 91%. Moreover, the average transmittance in the wavelength region of 400 nm to 1200 nm was over 91%. The new Al-doping method using AlCl3(6H2O) by the photo-MOCVD, proposed for the first time in this study, is economical as well as safe, and high-quality ZnO:Al can be successfully applied to a transparent conductive electrode for large area thin-film solar cells.

Improvement of pin-type amorphous silicon solar cell performance by employing double silicon-carbide p -layer structure

We investigated a double silicon-carbide p-layer structure consisting of a undiluted p-type amorphous silicon-carbide (p-a-SiC:H) window layer and a hydrogen diluted p-a-SiC:H buffer layer to improve a pin-type amorphous silicon based solar cell. Solar cells using a lightly boron-doped (1000 ppm) buffer layer with a high conductivity, low absorption, well-ordered film structure, and slow deposition rate improves the open-circuit voltage (Voc), short-circuit current density, and fill factor by reducing recombination in the buffer layer and at p/buffer and buffer/i interfaces. It is found that a natural hydrogen treatment generated throughout the buffer layer deposition onto the p-a-SiC:H window layer is an advantage of this double p-layer structure. We achieved a considerable initial conversion efficiency of 11.2% without any back reflector.

A ZnO cross-bar array resistive random access memory stacked with heterostructure diodes for eliminating the sneak current effect

We report cross-bar array resistive random access memory (RRAM) devices based on a ZnO thin film fabricated at room temperature. To prevent the sneak current path in a conventional cross-bar array device, two types of heterostructure diodes, a NiO/ZnO p-n junction and a WO3/ZnO tunnel barrier both stacked on cross-bar array RRAM were employed. With rectifying characteristics and high forward current density, the sneak current path was effectively eliminated. We believe that the proposed structures are promising for cross-bar type RRAM applications.

Double amorphous silicon-carbide p-layer structures producing highly stabilized pin-type protocrystalline silicon multilayer solar cells

We have applied double p-type amorphous silicon-carbide (p-a-SiC:H) layer structures to pin-type protocrystalline silicon (pc-Si:H) multilayer solar cells. The less-pronounced initial short-wavelength quantum efficiency variation against the biased voltage and the wide overlap of dark current—voltage (JD-V) and short-circuit current—open-circuit voltage (Jsc-Voc) characteristics prove that the double p-a-SiC:H layer structure successfully reduces recombination at the p∕i interface. Therefore, we achieved highly stabilized efficiency of 9.0% without any backreflector.

Boron-doped amorphous diamondlike carbon as a new p-type window material in amorphous silicon p-i-n solar cells

A boron-doped hydrogenated amorphous diamondlike carbon (a-DLC:H) was prepared using a mercury-sensitized photochemical vapor deposition (photo-CVD) method. The source gases were B2H6 and C2H4. By increasing the boron doping ratio (B2H6/C2H4) from 0 to 12 000 ppm, the dark conductivity increased from ∼10−9 to ∼10−7 S/cm. A boron-doped a-DLC:H with an energy band gap of 3.8 eV and a dark conductivity of 1.3×10−8 S/cm was obtained at a doping ratio of 3600 ppm. By using this film, amorphous silicon (a-Si) solar cells with a novel p-a-DLC:H/p-a-SiC double p-layer structure were fabricated using the photo-CVD method and the cell photovoltaic characteristics were investigated as a function of a-DLC:H layer thickness. The open circuit voltage increased from 0.766 V for the conventional cell with a 40-A-thick p-a-SiC to 0.865 V for the cell with a p-a-DLC:H (15 A)/p-a-SiC (40 A) double p-layer structure. The thin (<15 A) p-a-DLC:H layer proved to be an excellent hole emitter as a wide band gap window layer.

Inclusion of nanosized silicon grains in hydrogenated protocrystalline silicon multilayers and its relation to stability

Photoluminescence and Fourier transform infrared spectroscopy measured at room temperature produce strong evidence that nanosized silicon (nc-Si) grains embedded in hydrogenated protocrystalline silicon (i-pc-Si:H) multilayers. Thus, we propose the structure of the i-pc-Si:H multilayer possessing isolated nc-Si grains and their wrapping layers with a high hydrogen concentration embedded in highly hydrogen-diluted sublayers. The isolated nc-Si grains may act as radiative recombination centers of photoexcited carriers, and hence suppress the photocreation of dangling bonds caused by the nonradiative recombination in amorphous silicon matrix. Because of the repeatedly layered structure, the i-pc-Si:H multilayers have a fast light-induced metastability with a low degradation.

In situ ultraviolet treatment in an Ar ambient upon p-type hydrogenated amorphous silicon–carbide windows of hydrogenated amorphous silicon based solar cells

We proposed an in situ postdeposition ultraviolet treatment in an Ar ambient (UTA) to improve the p∕i interface of amorphous silicon based solar cell. We have increased the conversion efficiency by ∼16% by improving the built-in potential and reducing recombination at the p∕i interface. Through spectroscopic ellipsometry and Fourier-transform infrared measurements, it is concluded that the UTA process induces structural modification of the p-type hydrogenated amorphous silicon–carbide (p-a-SiC:H) window layer. An ultrathin p-a-SiC:H contamination layer formed during the UTA process acts as a buffer layer at the interface.

Stable protocrystalline silicon and unstable microcrystalline silicon at the onset of a microcrystalline regime

We investigated the light-soaking behaviors and the thermal annealing kinetics of amorphous silicon-based solar cells incorporating hydrogen-diluted films as i-layers deposited at several hydrogen dilution ratios. From the investigation, we confirmed that the protocrystalline silicon was most stable against light soaking, and also that the film deposited at the onset of the microcrystalline regime, which were known to have the most competent device quality and stability, was less stable. Charged dangling bonds defects caused by inhomogeneous microstructure of the onset of the microcrystalline regime, was suggested as one of reasons for the instability at the onset regime.