Tatsuo Fukano

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.

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.

Analysis of lattice site occupancy in kesterite structure of Cu2ZnSnS4 films using synchrotron radiation x-ray diffraction

The relationship between the composition and the kesterite crystal structure of Cu2ZnSnS4 (CZTS) thin films was investigated. CZTS thin films with three different compositions were produced by sulfurizing the precursors in 5 vol. % H2S balanced with N2. We measured the x-ray diffraction patterns of the CZTS powders scraped away from the CZTS thin films using the synchrotron radiation. The site occupancy of the CZTS thin films that had respectively different compositions were determined by the Rietveld analysis using x-ray powder diffraction patterns. Zn substitutes for Cu at the 2a site with decreasing the value of composition ratio Cu/(Zn+Sn). Meanwhile, Cu substitutes for Sn at the 2b site with increasing the value of Cu/(Zn+Sn). The CZTS thin film with Cu/(Zn+Sn) = 0.8 especially shows a marked tendency that Cu substitutes for Zn at the 2d site and/or Zn substitutes for Cu at the 2c site compared to the CZTS thin films with Cu/(Zn+Sn) = 1.0 and 1.2. We suppose that it is very important to realize the h...

Ionization potentials of transparent conductive indium tin oxide films covered with a single layer of fluorine-doped tin oxide nanoparticles grown by spray pyrolysis deposition

Indium tin oxide (ITO) films deposited with single layers of monodispersive fluorine-doped tin oxide (FTO) nanoparticles of several nanometers in size were grown on glass substrates by intermittent spray pyrolysis deposition using conventional atomizers. These films have significantly higher ionization potentials than the bare ITO and FTO films grown using the same technique. The ITO films covered with FTO particles of 7nm in average size show an ionization potential of 5.01eV, as compared with ∼4.76 and ∼4.64eV in ITO and FTO films, respectively, which decreases as the FTO particle size increases. The ionization potentials are practically invariant against oxidation and reduction treatments, promising a wide application of the films to transparent conducting oxide electrodes in organic electroluminescent devices and light-emitting devices of high efficiencies.

Enhancement of Conversion Efficiency of Cu2ZnSnS4 Thin Film Solar Cells by Improvement of Sulfurization Conditions

To enhance the conversion efficiency of Cu2ZnSnS4 (CZTS) thin film solar cells prepared by the sulfurization method, we investigated the formation process of the CZTS thin film. The holding temperature of the sulfurization was 580 °C. This study showed that the spreading resistance (SR) of the CZTS layer strongly depends on the holding time of the sulfurization. At the intermediate holding time (~30 min), the SR of the CZTS layer came to a minimum, and the efficiency of the CZTS solar cell came to a maximum. A 7.6% efficiency CZTS solar cell without a high-resistance buffer layer and an antireflection coating was fabricated.

Direct measurement of band offset at the interface between CdS and Cu2ZnSnS4 using hard X-ray photoelectron spectroscopy

We directly and non-destructively measured the valence band offset at the interface between CdS and Cu2ZnSnS4 (CZTS) using hard X-ray photoelectron spectroscopy (HAXPES), which can measure the electron state of the buried interface because of its large analysis depth. These measurements were made using the following real devices; CZTS(t = 700 nm), CdS(t = 100 nm)/CZTS(t = 700 nm), and CdS(t = 5 nm)/CZTS(t = 700 nm) films formed on Mo coated glass. The valence band spectra were measured by HAXPES using an X-ray photon energy of 8 keV. The value of the valence band offset at the interface between CdS and CZTS was estimated as 1.0 eV by fitting the spectra. The conduction band offset could be deduced as 0.0 eV from the obtained valence band offset and the band gap energies of CdS and CZTS.

Temperature Dependence of Cu2ZnSnS4 Photovoltaic Cell Properties

We studied the temperature dependence of current–voltage characteristics of Cu2ZnSnS4 (CZTS) photovoltaic cells to determine their fundamental properties. The open circuit voltage (Voc) in CZTS cells increased from 0.61 to 0.85 V and showed a linear relationship with decreasing temperature in the range from 150 to 350 K. The Voc at 0 K was extrapolated to about 1.2 V, which is lower than the bandgap energy of CZTS. These results suggest that the reason for the lower Voc in CZTS cells is recombination at the interface between the CZTS and buffer layers.

WO3/metal thin-film bilayered structures as optical recording materials

Interface reactions in WO3∕metal thin-film bilayered structures were examined for applications to optical recording materials. Decreases in the reflectance of the structures caused by temperature elevation were observed and were attributed to (i) coloration of the WO3 layers, and (ii) decreases in the reflectance of the metal layers resulting from redox reactions between the WO3 and the metals. The reflectance spectra of the structures before temperature elevation showed moderate wavelength dependence over a wide range, from the visible to the near infrared. Decreases in the reflectance due to temperature elevation occurred over a wide wavelength range. Examination of the WO3∕Al–Ti bilayered structures as potential optical disk memories revealed that the redox reactions occurred within submicrosecond time periods during irradiation with the recording laser. The results indicate that these structures possess promising properties for applications to optical recording materials that can be used over a wide w...

Compositional Dependence of Optical Constants and Microstructures of GeSbTe Thin Films for Compact-Disc-Rewritable (CD-RW) Readable with Conventional CD-ROM Drives

We studied the dependence of optical constants and microstructures of (GeTe)x(Sb2Te3)1-x films on composition to determine the optimal composition of phase change recording materials. The maximal differences in optical constants between crystalline and amorphous phases and the minimal crystalline grain sizes of the films were obtained at Ge39Sb10Te51. Using Ge39Sb10Te51 as a recording layer and TiO2 and SiO2 as interference layers, a compact disc rewritable (CD-RW) with 65% reflectivity and 60% modulation was obtained.

GeS2/metal thin film bilayered structures as write-once-type optical recording materials

GeS2/metals thin film bilayered structures were examined for write-once-type optical recording materials. Decreases in the reflectance of GeS2∕Ag and GeS2∕Cu structures at a certain elevated temperature were observed. The phenomena were attributed to the formation of metal sulfides with high optical absorption coefficients by the interface reaction between transparent GeS2 and highly reflective metals. In contrast to GeS2∕Ag or GeS2∕Cu, an increase in the reflectance was observed in GeS2∕Au, which was presumably due to the change of optical interference conditions caused by the decrease in the GeS2 film thickness and the improvements of the crystallinity of Au caused by the decrease of the lattice defects along the grain boundaries of Au. Examination of the GeS2/metals bilayered structures as optical disc memories revealed that the reactions occurred within submicrosecond during the recording laser irradiations. The recording sensitivities of GeS2∕Ag or GeS2∕Cu which corresponded to the reaction temperatu...