Showing papers by "Keisuke Ohdaira published in 2015"

Comparison of crystalline-silicon/amorphous-silicon interface prepared by plasma enhanced chemical vapor deposition and catalytic chemical vapor deposition

Abstract: The properties of the interface between crystalline silicon (c-Si) and amorphous silicon (a-Si) were studied for two deposition methods of a-Si. One is the conventional plasma enhanced chemical vapor deposition (PECVD) and the other is catalytic chemical vapor deposition (Cat-CVD), often called hot-wire CVD. Surface recombination velocity (SRV) and atomic scale fine structure at the a-Si/c-Si interface were particularly studied. It is found that the a-Si/c-Si interface is very sharp for Cat-CVD a-Si and the width of transition-layer from c-Si to a-Si is less than 0.6 nm, which is three times smaller than for PECVD a-Si. It is also found that the SRV for Cat-CVD a-Si is about 1/3 of that for PECVD, and that the SRV appears to depend on the effective area of the interface which increases due to increase of surface roughness caused by plasma damage.

Improvement in passivation quality and open-circuit voltage in silicon heterojunction solar cells by the catalytic doping of phosphorus atoms

Abstract: We apply phosphorus (P) doping to amorphous silicon (a-Si)/crystalline silicon (c-Si) heterojunction solar cells realized by exposing c-Si to P-related radicals generated by the catalytic cracking of PH3 molecules (Cat-doping). An ultrathin n+-layer formed by P Cat-doping acts to improve the effective minority carrier lifetime (?eff) and implied open-circuit voltage (implied Voc) owing to its field effect by which minority holes are sent back from an a-Si/c-Si interface. An a-Si/c-Si heterojunction solar cell with a P Cat-doped layer shows better solar cell performance, particularly in Voc, than the cell without P Cat-doping. This result demonstrates the feasibility of applying Cat-doping to a-Si/c-Si heterojunction solar cells, owing to the advantage of the low-temperature (<200 ?C) process of Cat-doping.

Enhancement of Photoresist Removal Rate by Using Atomic Hydrogen Generated under Low-pressure Conditions

Abstract: Department of Electrical and Computer Engineering, National Institute of Technology, Kagawa College, 355 Chokushi-cho, Takamatsu, Kagawa 761-8058, Japan Applied Chemistry and Biochemical Engineering Course, Graduate School of Integrated Science and Technology, Shizuoka University, Johoku, Naka, Hamamatsu, Shizuoka 432-8561, Japan Green Devices Research Center, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan Department of Electronic Systems Engineering, National Institute of Technology, Kagawa College, 551 Kohda, Takuma-cho, Mitoyo, Kagawa 769-1192, Japan Department of Applied Chemistry and Bioengineering, Graduate School of Engineering, Osaka City University, 3-3-138, Sugimoto-cho, Sumiyoshi, Osaka 558-8585, Japan

Application of heterojunction to Si-based solar cells using photonic nanostructures coupled with vertically aligned Ge quantum dots

Abstract: We have recently developed a new structure for solar cells that consists of photonic nanostructures coupled with vertically aligned Ge quantum dots on a crystalline Si substrate For the fabrication of a solar cell device, a heterojunction with a-Si:H was chosen because its processing temperature is less than 200 °C, at which point Ge atoms cannot diffuse into Si layers In this study, we developed a guideline for the most appropriate cell structures to take advantage of the Ge layer by fabricating heterojunction solar cells with various structures As a result, we confirmed that the carriers absorbed in Ge quantum dots contributed output current when Ge quantum dots are fabricated on the pn-junction side Hence, the presence of built-in potential in a Ge dot layer was found to be necessary to extract the carriers generated in the Ge layer In addition, carrier transport in Ge quantum dots is improved under conditions of reverse bias Therefore, the electrical field in the Ge layer is a key parameter to improve solar cell performance in our proposed structure

Thermal Conductivity Measurements of Aggregated (Bi 1−x Sb x ) 2 Te 3 Nanoparticles Using 3 ω Method

Abstract: Using the 3ω method based on the two heat flow model, the thermal conductivity of as-dried and thermally treated samples of (Bi1−xSbx)2Te3 inks in the form of aggregated nanoparticles was measured. For the as-dried sample, κsample was 0.2 W K−1 m−1 to 0.3 W K−1 m−1. That of the thermally treated sample was 0.4 W K−1 m−1 to 0.7 W K−1 m−1. Organic solvent remained in the as-dried sample, covering the (Bi1−xSbx)2Te3 nanoparticles. In the region where organic solvent remained between the nanoparticles, heat flow was impeded because the thermal conductivity of the organic solvent is lower than that of the (Bi1−xSbx)2Te3 nanoparticles. However, in the thermally treated sample, the organic solvent evaporated and the nanoparticles grew due to the effects of the thermal treatment process. In this case, factors impeding heat flow, such as the interfacial thermal resistance between the particles, were limited. Therefore, this difference in κsample, which is consistent with the grain growth as ascertained from powder x-ray diffraction measurements, can be attributed to the evaporation of the organic solvent.

Efficient Organic Devices Based on π-Electron Systems: Comparative Study of Fullerene Derivatives Blended with a High Efficiency Naphthobisthiadiazole-Based Polymer for Organic Photovoltaic Applications

Abstract: This study focuses on a polymer based on quaterthiophene and naphthobisthiadiazole units (PNTz4T). Three fullerene derivatives, namely, PC61BM, PC71BM, and IC60BA, were selected as potential electron acceptors in regular device architectures. The resulting average PCE are 7.52, 8.52, and 2.58 % for PC61BM, PC71BM, and IC60BA, respectively. Through a careful and systematic study, we investigated the origins of the differences observed in devices’ performances. In particular, the higher Jsc and consequently higher PCE of the PC71BM devices (as compared to PC61BM devices) can be easily explained by the better light-harvesting properties in the visible range of the larger fullerene derivative. Furthermore, we demonstrate that the limiting factor in these devices is the electron collection which is closely related to the crystallinity of the fullerene derivative. The low crystallinity and resulting low electron-transporting properties of IC60BA is at the origins of the low performances of the IC60BA-based devices. Through this comparative study, we confirm that developing new materials is the key to remarkably increase the PCE of polymer solar cells. However, in order to obtain PCE over 10 %, a particular attention should be given to material combination, process, and charge balance in the devices.

Heterojunction solar cell and process for producing same

Abstract: [Problem] To provide a heterojunction solar cell of a structure in which carrier recombination on the upper surface of the crystalline silicon can be dramatically inhibited and which attains a heightened efficiency. [Solution] This heterojunction solar cell is a solar cell in which a thin film of intrinsic amorphous silicon has been bonded by heterojunction to a crystalline silicon substrate, wherein the crystalline silicon substrate has a doped layer formed by doping an extreme surface layer having a depth of 10 nanometers or less with phosphorus or boron. According to an electron photomicrograph obtained when a sample (900) was examined with a transmission electron microscope, the sample (900) having been produced by depositing a film of intrinsic amorphous silicon (902) right on an n-type crystal silicon substrate (901) by a catalytic-CVD method, the thickness of an interface transition layer (903) between the n-type crystal silicon (901) and the amorphous silicon (902) is 0.6 nanometers or less.

Photo-carrier generation at a-Si layer in SiNx/a-Si stacked passivation with extremely low surface recombination velocity

Abstract: This is to study on the feasibility of photo-carrier generation at amorphous-silicon (a-Si) insertion layer in silicon-nitride (SiNx)/a-Si stacked passivation system, which realizes extremely low surface recombination velocity (SRV). When the thickness of crystalline silicon (c-Si) becomes thinner than 100 ixm for reducing cell cost, the method collecting 100% sun light is to be very important. If a-Si or a-Si-compounds on c-Si in hetero-structure can absorb sun light enough and if the photo-carriers generated inside such a-Si or a-Si compounds are effectively transferred to c-Si and used as currents of c-Si solar cells, the formation of hetero-structure has an advantage for thinning c-Si substrates. In the present research, the ratio of photo-generated carriers in a-Si to the carriers transferred to c-Si is experimentally studied in SiNx/a-Si stacked passivation system, which is expected to be used as surface passivation of back-contact c-Si solar cells. We compared the SiNx/a-Si/c-Si system prepared by the plasma-enhanced chemical vapor deposition (PECVD) with that by catalytic chemical vapor deposition (Cat-CVD), often called “Hot-Wire CVD”. It is found that the a-Si/c-Si interface prepared by Cat-CVD is much smoother than that by PECVD, indicating better quality of Cat-CVD a-Si/c-Si interface and that almost 100% photo-carriers generated in Cat-CVD a-Si can be transferred into c-Si through the interface when the a-Si thickness is less than 15 nm. This demonstrates the feasibility as power generator for Cat-CVD a-Si in a-Si/c-Si hetero-structure.