Kyotaro Nakamura

Influence of CdS window layer on 2-μm thick CdS/CdTe thin film solar cells

Influence of the CdS window layer on the PV performances of 2-μm thick CdS/CdTe solar cells has been studied as a function of the CdS thickness, d C d S . With a reduction of d C d S from 114 to 95 nm, J S C increases due to an increase in blue response. While, at d c d S <85 nm, the conversion efficiency largely decreases due to a decrease in V O C and FF. The deterioration of the crystallinity of CdTe due to a decrease in the sulfur composition x of the CdTe 1 - x S x mixed-crystal layer is concluded to be the most possible mechanism for the large decreases in V O C and FF.

Synthesis of silicon nitride films by ion-beam-enhanced deposition and their protective properties against oxidation☆

Abstract Silicon nitride films were synthesized on cold-rolled nickel coupons by concurrent electron beam evaporation of silicon and bombardment with nitrogen ions (ion-beam-enhanced deposition). IR, Rutherford backscattering, Auger electron spectroscopy, transmission electron spectroscopy, scanning electron microscopy, energy dispersive ellipsometry and spreading resistance measurements were performed for investigating the properties, composition, and structure of the films. Isothermal and cyclic oxidation tests were carried out on the uncoated nickel coupons and those coated with films of thicknesses up to 10 000 A at 1300 K in a flow of pure oxygen at atmospheric pressure. The films decrease the isothermal oxidation rates, the thicker films having a larger effect. However, films of thicknesses up to 3000 A are not effective for cyclic oxidation. The scanning electron microscopy observation and energy dispersive X-ray analysis revealed that thin films rich in silicon remained in the NiO scale near the scale-substrate interface after both isothermal and cyclic oxidation. Further, much thinner films containing silicon were present on the outer surface of the NiO scales formed on the specimens coated with nitride films of thickness 10 000 A. The oxidation mode initially follows an approximately linear law and approaches a parabolic law later.

Reduction of infrared response of CdS/CdTe thin-film solar cell with decreased thickness of photovoltaic active layer

Reduction of the infrared response is found in the CdS/CdTe thin-film solar cells with a decrease in the thicknesses of the photovoltaic active layers from 16 to 3 μm. The reduction is concluded to be mainly due to an increase in recombinations of photogenerated electrons in the p-layer caused by the unintentionally increased Cu acceptor concentration. A simple model for the depth profile of the donor and acceptor densities is proposed to explain that the acceptor concentration at the n-p junction can be unintentionally increased with a decrease in the thicknesses of the photovoltaic active layers, even if the process parameters for the Cu-doping are unchanged.

Influence of CdCl2 Treatment on Structural and Electrical Properties of Highly Efficient 2-µm-Thick CdS/CdTe Thin Film Solar Cells

The structural and electrical properties of the CdS/CdTe(S) interface region of 2-µm-thick CdS/CdTe solar cells have been studied in conjunction with photovoltaic (PV) performances with a focus on the influence of the annealing temperature (TCdCl2) and oxygen concentration of the CdCl2 treatment. When the CdCl2 treatment with the same conditions for a 5-µm-thick solar cell is applied to the 2-µm-thick solar cells, the PV performances largely decrease due to the structural deterioration at the CdS/CdTe(S) interface; however, they are improved by the rapid treatment with high TCdCl2 under a higher oxygen concentration. Similar behaviors are observed between the open-circuit voltage (VOC) and the signal intensity of the electromodulated photoluminescence against TCdCl2 or oxygen concentrations. These results suggest that the n+-CdTe1-xSx layer adjacent to the CdS/CdTe(S) interface plays a crucial role in the increase of VOC because of a decrease in the nonradiative recombination rate at and near the CdS/CdTe(S) interface and/or an increase in the built-in electric field.

Properties and structure of silicon nitride films synthesized by ion-beam-enhanced deposition

Abstract Silicon nitride films with a stoichiometric ratio of Si 3 N 4 were synthesized by ion-beam- enhanced deposition (IBED). It was observed that a silicon nitride film formed by IBED is composed of a thin silicon-enriched top layer, a stoichiometric Si 3 N 4 layer and a smooth transition layer at the interface next to the substrate. By detailed theoretical analysis and computer simulation of IR reflection interference spectra, refractive index profiles of the films were obtained. Composition profiles and refractive index profiles were correlated using the Lorentz-Lorenz equation. The fatigue and high temperature oxidation behaviour of metals can be improved using Si 3 N 4 coatings made by means of IBED, and the mechanisms of these are discussed.

Electromodulated Photoluminescence Study of CdS/CdTe Thin-Film Solar Cell

The n+-CdTe1-xSx layer, in which the most of the photocarriers are generated in the CdS/CdTe thin-film solar cell, has been characterized using a novel spectroscopic technique called electromodulated photoluminescence (EMPL), which is the first derivative spectroscopy monitoring electric-field-induced changes in the PL signal. In conjunction with the results of PL measurements, the ultraviolet-light-excited (UVE-) EMPL measurements reveal that the depletion layer is extended to a quite shallow region in the n+-CdTe1-xSx layer from the CdS/CdTe(S) metallurgical interface by <15 nm and not extended into the n-CdS layer. Also demonstrated here is that the strong UVE-EMPL signal is closely connected with the high photovoltaic (PV) performances because it is obtained from the shallow depleted n+ -CdTe1-xSx region with low density of nonradiative recombination centers.

CdS Window Layer for Large Open Circuit Voltages of Low Environmental-Load CdS/CdTe Solar Cells

The low ‘environmental-load’ CdS/CdTe solar cells for reducing consumption of Cd compounds have been investigated employing the CdS layers fabricated at various substrate temperatures, T CdS , and a conversion efficiency of 14.1% has been achieved. The nanostructure of CdS crystallites made at different T CdS are compared to the crystallinity of CdS, and CdTe deposited on CdS as well as sulfur fraction in CdTe 1-x S x mixed crystal layer unintentionally formed at CdS/CdTe(S) interface. The photovoltaic performances, especially obtained relative high open circuit voltages, are discussed in conjunction with the structural properties as well as electrical properties of the solar cells. The solar cells show a relative high V oc due to the large CdTe grains as well as the narrow depletion layer width. Besides, preventing deterioration of the CdS/CdTe(S) interface is found to be quite effective for achieving high open circuit voltages and fill factors.

Enhanced CdTe solar cell performance through surface engineering

Surface of indium-tin oxide (ITO) substrate were irradiated with accelerated Ar/sup +/, N/sub 2//sup +/, O/sub 2//sup +/ and H/sub 2//sup +/ ions (1 keV) at different doses. The change in sheet resistance (R/sub sh/), the water-contact angle (WCA) and the composition of ITO were measured. R/sub sh/ slightly increased with the ion dose. WCA dropped to less than 15/spl deg/ and the composition shifted to oxygen-rich. CdS films grown on ITO by chemical bath deposition showed an improvement in terms of the microstructure, the surface morphology, the optical transmittance, and the stoichiometry. X-ray photoluminescence was performed on CdS/ITO glass for the first time. The performance of CdTe/CdS solar cells was improved as a result of ITO surface treatments, this being due to improvements in the physical structure of the CdS interfaces.

Approaching high efficiencies over 13% with 2-/spl mu/m thick CdS/CdTe thin film solar cells

Highly efficient CdS/CdTe thin film solar cells with thinner PV active layers are required from a view point of reduction of production cost as well as toxicity of Cd. We have achieved the conversion efficiency of 13.5% (V/sub oc/: 0.799 V, J/sub sc/: 24.8 mA/cm/sup 2/, F.F.: 0.679, area: 0.42 cm/sup 2/) using 2.0-/spl mu/m thick PV active layers. High efficiencies were obtained with annealing temperatures at CdCl/sub 2/ treatment lower than those for conventional thick (5-10 /spl mu/m) solar cells. Furthermore, higher oxygen concentration of 7% at the CdCl/sub 2/ treatment was found to be effective for improvement of the efficiencies.