Hironori Katagiri

Enhanced Conversion Efficiencies of Cu2ZnSnS4-Based Thin Film Solar Cells by Using Preferential Etching Technique

Cu2ZnSnS4 (CZTS) thin film solar cells have been fabricated by co-sputtering technique using three targets of Cu, SnS, and ZnS. CZTS-based thin film solar cells over 6.7% efficiency were obtained for the first time by soaking the CZTS layer on the Mo coated soda-lime glass substrate in deionized water (DIW) after forming the CZTS layer. It was found that DIW-soaking had the effect of preferential etching, which eliminated selectively metal oxide particles in the CZTS layer, by electron probe X-ray micro analysis.

Characterization of Cu2ZnSnS4 Thin Films Prepared by Vapor Phase Sulfurization

Cu2ZnSnS4 (CZTS) thin films could be successfully formed by vapor phase sulfurization of electron-beam-evaporated precursors on a soda-lime glass substrate. This film is an interesting material for absorber layer in a solar cell because all the constituents are readily available in the earths crust. In this study, using a new type of precursors containing ZnS, we could achieve the strong adhesion of CZTS films to a glass substrate. From the result of scanning electron microscope (SEM) observation, it was confirmed that the surface morphology of CZTS films is much improved by using this new type of precursor. The X-ray diffraction pattern revealed that CZTS thin films have kesterite structures. From the measurement of transmittance and reflectance, the optical band-gap energy was estimated as 1.40–1.45 eV, which is very close to the optimum value for a solar-cell absorber. The highest open-circuit voltage of our cells based on CZTS films is 735 mV, which is a higher value than that reported in numerous other studies on CZTS.

Investigation of Cu2ZnSnS4-Based Thin Film Solar Cells Using Abundant Materials

Aiming to develop the solar cells free from both environmental contaminants and rare metals, thin film solar cells were produced by using Cu2ZnSnS4 (CZTS) as an absorber. CZTS thin films could be successfully formed by the vapor phase sulfurization of precursors prepared by the electron-beam evaporation on the soda-lime glass substrates. The atomic ratio of Cu/(Zn+Sn) was varied from nearly 0.5 to 1.2 to investigate the influence of the composition on the surface morphology and the optical properties. In order to optimize the sulfurization condition, we varied the sulfurization temperature from 510°C to 550°C. As the results, the value of the conversion efficiency of 4.53% was obtained for the cell of which CZTS absorber sulfurized at 520°C.

6% Efficiency Cu2ZnSnS4-based thin film solar cells using oxide precursors by open atmosphere type CVD

An open atmosphere type chemical vapor deposition (OA-CVD) method is one of the most effective methods for producing functional thin films. Especially, the OA-CVD method is a unique technique which is able to deposit metal oxide thin films by decomposition of vaporized raw materials released through a nozzle onto substrates in the air. Cu2ZnSnS4 (CZTS)-based thin films as absorber layers of thin film solar cells were fabricated by sulfurizing oxide precursor thin films synthesized by the OA-CVD method. Cu(C5H7O2)2, Zn(C5H7O2)2 and Sn(C5H7O2)2 were used as raw materials. The oxide precursor thin films were sulfurized at 520–560 °C in 5 vol% H2S balanced with N2. The formed CZTS-based thin films included oxygen with the composition ratio of O/(S + O) = 0.17–0.27 according to energy dispersive X-ray spectroscopy. The thin film solar cells using the CZTS-based thin films including oxygen [CZT(S,O) films] as the absorber layers were fabricated. The CZT(S,O) thin film solar cell had a stack structure of Al/Al-doped-ZnO/CdS/CZT(S,O)/Mo/soda-lime glass substrate. The efficiency of the CZT(S,O) thin film solar cells was 6.03%, which was the high efficiency in the reported value for CZTS-based thin film solar cells using oxide thin film precursors. It was found that the OA-CVD method is suited to fabricate the absorber layers of thin film solar cells.

Cu2ZnSnS4 thin film deposited by sputtering with Cu2ZnSnS4 compound target

Cu2ZnSnS4 (CZTS) thin films were prepared by a single sputtering process with a CZTS compound target or a co-sputtering process with Cu, ZnS, and SnS targets followed by annealing in a H2S-containing atmosphere at several temperatures. Between the CZTS thin films prepared by both single and co-sputtering processes annealed at 500 °C, there are no major differences in the X-ray diffraction patterns, optical properties, and chemical composition. However, there is a difference in the scanning electron microscopy images. Although the CZTS thin films prepared by the co-sputtering process have some voids, the CZTS thin films prepared by the single sputtering process have no voids. The solar cells prepared with the CZTS thin films deposited by the single sputtering process followed by annealing at 500 °C show a higher efficiency of 4.40% than those prepared with the CZTS thin films prepared by the co-sputtering process (2.06%).

Development of rare metal-free CZTS-based thin film solar cells

Cu2ZnSnS4 (CZTS) is one of the promising materials for low cost thin film solar cells, because of a suitable band gap energy around 1.5 eV, and the large absorption coefficient over 104 cm−1 In addition, all constituents of this film are abundant on the crust of the earth, and they are not toxic. In 1996, we reported a fabrication process of CZTS films by sulfurization of E-B evaporated precursors. We formed a new type of thin film solar cells constructed with SLG/Mo/CZTS/CdS/ZnO:Al/Al and achieved the conversion efficiency of 0.66 % for the first time. Conversion efficiency was gradually increased by the modification of the fabrication process. This consists of the optimization of the back contact and the window layer as well as the development of the high quality CZTS absorber. In order to produce the high quality CZTS absorber, the stacking order of precursor was investigated. Then, using the multi-period precursor, we fabricated the CZTS absorber with preferable morphology. From theses results, we confirmed that the film morphology had large influence to the conversion efficiency as well as the composition ratio. After that, we introduced the three-phased co-sputtering system with annealing chamber. Nowadays, the conversion efficiency increased to over 6.7 % by optimizing many and many parameters in this system. Quite recently, using CZTS compound target, we achieved over 6.4 % efficiency with CZTS films prepared by the single sputtering followed by sulfurization.

In situ process monitoring during multistage coevaporation of Cu2ZnSnS4 thin films

A multistage coevaporation process for the direct growth of Cu2ZnSnS4 (CZTS) thin films without additional atmospheric sulfurization was investigated. To obtain reproducible CZTS films, in situ process monitoring of the film growth was developed by measuring the apparent substrate temperature (Tpyro) using a pyrometer. After CZTS depositions terminated at various endpoints, ex situ characterization of the film properties was performed to clarify the growth mechanism of the films. The results provided clear evidence that CZTS phase formation was significantly delayed via re-evaporation of Sn–S-based compounds in the early part of the first stage, leading to the initial formation of a dominant (CuS + ZnS) structure that coexisted with a small amount of CZTS. CZTS phase formation was then facilitated by the (CuS + ZnS) precursor via a Cu-rich to Cu-poor sequence with an apparent variation in Tpyro during the second stage, and the slightly segregated CuS phase was nearly consumed under (Zn + Sn + S) fluxes. C...

Effect of N2O-doped buffer layer on the optical properties of ZnO films grown on glass substrates using high-energy H2O generated by catalytic reaction

Improvement in the optical properties of ZnO films grown on glass substrates was investigated using a N2O-doped buffer layer inserted between the glass substrate and a ZnO film deposited by chemical vapor deposition (CVD). ZnO films were grown at 773 K using dimethylzinc (DMZn) and high-temperature H2O generated by catalytic reaction as zinc and oxygen sources, respectively. Crystal growth was enhanced for the ZnO film grown on the N2O-doped buffer layer, and the fluctuation in crystal orientation along the c-axis became less than that for a film grown directly on the glass substrate by CVD. The optical transmittance of the film in the wavelength range of 375–700 nm also increased with insertion of the buffer layer.

Training Courses and Utilization for Embedded System Development Engineers in Nagaoka National College of Technology

From 2006, Nagaoka National College of Technology (NNCT) , Niigata Industrial Creation Organization (NICO) and Society of Embedded Software Skill Acquisition for Managers and Engineers (SESSAME) began a joint training program for embedded systems development engineers. The courses consist of a basic course, a practical course, an intermediate course and PBL course according to ability of student. The training curriculum of the courses is planed corresponding to ETSS in cooperation with IPA-SEC. In this paper, the content of the training program for embedded system development engineers is described and the results are discussed. In addition, the application of teaching materials which were developed in this project can now be used to benefit the students and instructors of NNCT.