Junsin Yi

Growth and properties of the Cd1−xZnxS thin films for solar cell applications

Cd1−xZnxS (0≤x≤1) thin films have been prepared by the co-evaporation of CdS and ZnS. When the ZnS mole ratio was less than 0.85, the crystal structure of Cd1−xZnxS films was hexagonal with the c axis aligned perpendicular to the substrate. For x>0.85, however, the Cd1−xZnxS films were grown with cubic zincblende structure. As the ZnS mole ratio increased, the lattice constant of Cd1−xZnxS films decreased. The optical band gap of Cd1−xZnxS films varies from 2.41 eV for CdS to 3.48 eV for ZnS with x. The open circuit voltage of Cd1−xZnxS/CdTe solar cells increased with x due to reducing of the electron affinity difference between Cd1−xZnxS and CdTe films, having approximately 830 mV of the maximum value at x=0.35.

ZnO nanowire-embedded Schottky diode for effective UV detection by the barrier reduction effect

A zinc oxide nanowire (ZnO NW)-embedded Schottky diode was fabricated for UV detection. Two types of devices were prepared. The ZnO NW was positioned onto asymmetric metal electrodes (Al and Pt) for a Schottky device or symmetric metal electrodes (Al and Al) for an ohmic device, respectively. The Schottky device provided a rectifying current flow and was more sensitive to UV illumination than the ohmic device. The Schottky barrier plays an important role for UV detection by a UV-induced barrier reduction effect. The fabrication of the ZnO NW-embedded Schottky diode and the UV reaction mechanism are discussed in light of the UV light-induced Schottky barrier reduction effect.

RIE texturing optimization for thin c-Si solar cells in SF6/O2 plasma

Dry etching plasma parameters were optimized for texturing single crystalline thin silicon solar cells and hence for high efficiency. In reactive ion etching (RIE) texturing, a low etch depth (~2 µm) was obtained compared with the etch depth that occurred in the wet chemical texturing process. For the flow ratios (SF6/O2) of 2 and 3, needle-like and cylindrical type structured surfaces were obtained. In the RIE process, the effects of working pressure, flow ratio, and etching time on reflectance and electrical properties of single crystalline silicon solar cells were investigated. The textured c-Si wafer with needle-like structure has good antireflectance behaviour. The interface defect density (Dit) in these textured silicon wafers increased with etching time. But an improvement in the reduction of interface trap density was observed through the annealing effect. Single crystalline solar cells with cylindrical type texture have higher values for open circuit voltage, short circuit current and efficiency in spite of a higher reflectance as compared with those needle-like structures. It is observed that the optimized SF6/O2 based chemistry in the RIE method is more suitable for thin crystalline silicon solar cells instead of the conventional wet texturization processes.

Novel low cost chemical texturing for very large area industrial multi-crystalline silicon solar cells

Multi-crystalline silicon surface etching without grain-boundary delineation is a challenging task for the fabrication of high efficiency solar cells. The use of sodium hydroxide–sodium hypochlorite (NaOH–NaOCl) solution for texturing a multi-crystalline silicon wafer surface in a solar cell fabrication line is reported in this paper. The optimized etching solution of NaOH–NaOCl does not have any effect on multi-crystalline silicon grain boundaries and it also has excellent isotropic etch characteristics, which ultimately helps to achieve higher values of performance parameters, especially the open circuit voltage (Voc) and fill factor (FF), than those in the case of conventional silicon texturing. Easy control over the reaction of the NaOH–NaOCl solution is also one of the major advantages due to which sophistication in controlling the temperature of the etching bath is not required for the industrial batch process. The FTIR analysis of the silicon surface after etching with the current approach shows the formation of Si–Cl bonds, which improves the quality of the diffused junction due to chlorine gettering during diffusion. We are the first to report 14–14.5% efficiency of very large area (150 mm × 150 mm) multi-crystalline silicon solar cells using a NaOH–NaOCl texturing approach in an industrial production line with a yield greater than 95%.

Negative gate-bias temperature stability of N-doped InGaZnO active-layer thin-film transistors

Stability of negative bias temperature stress (NBTS) of nitrogen doped amorphous InGaZnO (a-IGZO) thin-film transistor (TFT) is investigated. Undoped a-IGZO TFT stressed at 333 K exhibit a larger negative ΔVTH (−3.21 V) with an unpredictable sub-threshold swing (SS) of hump shaped transfer curve due to the creation of meta-stable traps. Defects related hump formation has disappeared with small ΔVTH (−1.13 V) and ΔSS (0.018 V/dec) in nitrogen doped a-IGZO TFT. It is observed that nitrogen doping enhances device stability by well controlled oxygen vacancy and trap sites in channel and channel/dielectric interface.

Improvement in the performance of an InGaZnO thin-film transistor by controlling interface trap densities between the insulator and active layer

An amorphous InGaZnO film fabricated by radio frequency magnetron sputtering in only an Ar-reactive gas shows high conductivity, and a thin-film transistors (TFTs)-based IGZO active layer expresses a poor on/off current ratio with a high off current and high subthreshold swing (SS). This paper presents the post-annealing effects on IGZO thin films to compensate the oxygen deficiencies in films as well as on TFT devices to reduce the densities of the interface trap between the active layer and insulator. The ratio of oxygen vacancies over total of oxygen (O2/Otot) in IGZO estimated by the XPS measurement shows that they significantly diminish from 24.75 to 17.68% when increasing the temperature treatment to 350 °C, which is related to the enhancement in resistivity of IGZO. The TFT characteristics of IGZO treated in air at 350 °C show a high ION/IOFF ratio of ~1.1 × 107, a high field-effect mobility of 7.48 cm2 V−1 s−1, and a low SS of 0.41 V dec−1. The objective of this paper is to achieve a successful reduction in the interface trap density, ΔDit, which has been reduced about 3.1 × 1012 cm−2 eV−1 and 2.0 × 1012 cm−2 eV−1 for the 350 and 200 °C treatment samples compared with the as-deposited one. The resistivity of the IGZO films can be adjusted to the appropriate value that can be used for TFT applications by controlling the treatment temperature.

Incident light adjustable solar cell by periodic nanolens architecture.

Could nanostructures act as lenses to focus incident light for efficient utilization of photovoltaics? Is it possible, in order to avoid serious recombination loss, to realize periodic nanostructures in solar cells without direct etching in a light absorbing semiconductor? Here we propose and demonstrate a promising architecture to shape nanolenses on a planar semiconductor. Optically transparent and electrically conductive nanolenses simultaneously provide the optical benefit of modulating the incident light and the electrical advantage of supporting carrier transportation. A transparent indium-tin-oxide (ITO) nanolens was designed to focus the incident light-spectrum in focal lengths overlapping to a strong electric field region for high carrier collection efficiency. The ITO nanolens effectively broadens near-zero reflection and provides high tolerance to the incident light angles. We present a record high light-conversion efficiency of 16.0% for a periodic nanostructured Si solar cell.

Transparent conductor-embedding nanocones for selective emitters: optical and electrical improvements of Si solar cells

Periodical nanocone-arrays were employed in an emitter region for high efficient Si solar cells. Conventional wet-etching process was performed to form the nanocone-arrays for a large area, which spontaneously provides the graded doping features for a selective emitter. This enables to lower the electrical contact resistance and enhances the carrier collection due to the high electric field distribution through a nanocone. Optically, the convex-shaped nanocones efficiently reduce light-reflection and the incident light is effectively focused into Si via nanocone structure, resulting in an extremely improved the carrier collection performances. This nanocone-arrayed selective emitter simultaneously satisfies optical and electrical improvement. We report the record high efficiency of 16.3% for the periodically nanoscale patterned emitter Si solar cell.

Silicon nitride films prepared by high-density plasma chemical vapor deposition for solar cell applications

Abstract Silicon nitride films were deposited by means of high-density inductively coupled plasma chemical vapor deposition in a planar coil reactor. The process gases used were pure nitrogen and a mixture of silane and helium. Buried contact solar cells, passivated by the silicon nitride layer, show efficiency above 17%. Strong H-atom release from the growing SiN film and Si–N bond healing are responsible for the improved electrical and passivation properties of the SiN film. This paper presents the optimal refractive index of SiN for a single layer antireflection (SLAR) coating in solar cell applications.

Effect of the short collection length in silicon microscale wire solar cells

Electrical and optical properties of silicon microscale wire (SiMW) solar cells were investigated. Diverse designs were applied for SiMW geometries as light absorbers. Finite-difference time-domain simulation shows a focused optical field in the wires inducing an optical absorption enhancement in SiMW solar cells. SiMW solar cells provided remarkably higher Voc values (0.597-0.61 V) than that of the planar solar cell (0.587 V). As for the electrical aspects, the position of the space charge region in a SiMW directly affects the carrier collection efficiency according to the SiMW diameter and significantly modulates the photogenerated-currents and voltages in solar cells.