Hiroshi Uda

Preparation of Low Resistance Contact Electrode in Screen Printed CdS/CdTe Solar Cell

Heating conditions of carbon electrodes for CdTe and the effect of impurities in the carbon paste on the characteristics of solar cells are investigated. The series resistance (Rs) and the conversion efficiency (ηi) of solar cells exhibit strong dependence on O2 concentration in the heating atmosphere. Addition of Cu to the carbon paste causes Rs and diode factor (n) to decrease, resulting in a remarkable improvement in ηi. As a result of preparation of low resistance contact electrode for CdTe, a cell with 0.78 cm2 active area which was made from 50 ppm Cu-added carbon paste showed Voc=0.754 V, Isc=0.022 A, FF=0.606 and ηi=12.8%.

Structural and Electrical Properties of Chemical-Solution-Deposited CdS Films for Solar Cells

The structural and electrical properties of CdS films prepared by chemical-solution deposition have been studied. As-deposited films have about 0.4 µm thickness, a grain size of about 0.3 µm and consist of the coalesence of columnar structure. The dark resistivity of more than 104 Ωcm of as-deposited films decreased by about three orders of magnitude to about 10 Ωcm upon annealing in N2 atmosphere at 300–500°C for 20 minutes. The decrease in resistivity was caused by an increase in electron concentration. The resistivity of annealed CdS films was low enough to use as a window layer of thin-film solar cells.

Effect of Resistivity of CdS Sintered Film on Photovoltaic Properties of Screen-Printed CdS/CdTe Solar Cell

The preparation conditions of low-resistance CdS sintered film and the effect of the resistance of CdS film on the photovoltaic properties of screen-printed CdS/CdTe solar cells were studied. CdS films were sintered in an alumina case with a perforated cover, and the rate of evaporation of CdCl2 in a printed CdS film was found to be influenced by the area of the holes in the cover. The resistivity of CdS film was less than 0.5 ?-cm for the samples sintered with the cover (cover area is 102 cm2) which had a hole area between 0.4 and 1.4 cm2. CdS/CdTe solar cells prepared from CdS films with resistivities of about 0.3, 0.7 and 1.4 ?-cm had conversion efficiencies of 8, 6 and 4% respectively. The decrease in efficiency was due to the increase in series resistance, which depended on the sheet resistance of the CdS film.

Effect of Substrate Temperature on the Photovoltaic Properties of a CdS/CdTe Solar Cell

Structural and electrical properties of CdTe thin films prepared by the conventional vacuum evaporation method were studied. Photovoltaic properties of thin-film CdS (chemical-solution-deposited)/CdTe (vacuum-evaporated) solar cells were also investigated. The crystal structure of CdTe films deposited on the CdS films at the substrate temperature of 300~520°C was of the zincblende type with a preferential orientation of the (111) plane parallel to the substrate. The dark resistivity of the CdTe films deposited on the glass substrates was about 107 Ωcm. Illumination caused the resistivity decrease of the films by about two orders of magnitude. The conversion efficiency of the CdS/CdTe solar cells increased with increasing substrate temperature. The present thin-film CdS/CdTe solar cell showed a conversion efficiency of greater than 5%.

Annealing effect of Cu2TeAu contact to evaporated CdTe film on photovoltaic properties of CdS/CdTe solar cell

Abstract The annealing effect of an evaporated Cu 2 TeAu contact to CdTe film on the photovoltaic properties of thin-film CdS/vacuum-evaporated CdTe solar cells has been investigated. V oc and J sc for the cells with Cu 2 TeAu contact increased greatly with increasing annealing temperature and showed a maximum value at around 250. Photovoltaic properties of the cell with Cu 2 TeAu contact were improved by annealing in a greater extent than those of the cell with TeAu or TeCu contacts. The cells with Cu 2 TeAu contact exhibit a higher conversion efficiency, comparing with the cells with TeAu or TeCu contacts. The cell with Cu 2 TeAu contact showed a conversion efficiency of 10%. Cu 2 TeAu contact acts as the best pseudoohmic contact on vacuum evaporated CdTe film.

Stability of Screen Printed CdS/CdTe Solar Cells

Some stability studies have been conducted on screen printed CdS/CdTe solar cells. Encapsulated CdS/CdTe modules exhibited excellent stabilities in output performance over 206 days when subjected to outdoor tests under various loading conditions. Accelerated tests were performed on unencapsulated cells. No degradation was observed after 90 days in continuous illumination test, A decrease in conversion efficiency due to an increase in series resistance of the unencapsulated cell took place under high temperature and high humidity tests. It is concluded that the increase in series resistance is caused by a decrease in adhesion between CdTe and C layers, and by swelling of the carbon layer with water absorption. Improved cells showed no degradation in efficiency over periods of 110 days at 100°C and over periods of 28 days at 65°C in 95% relative humidity.

8.5% Efficient Screen-Printed CdS/CdTe Solar Cell Produced on a 5×10 cm2 Glass Substrate

The preparation conditions of CdS sintered film for 5×10 cm2 screen-printed CdS/CdTe solar cells were investigated. Increasing the belt speed of the belt furnace increased the residual amount of Cl ions in the CdS sintered film and lowered the efficiency of the cell. The optimum belt speed was 2 cm/min, corresponding to a sintering time of 90 min. The thickness of the CdS film was changed by changing the screen thickness. Increasing the thickness of the CdS film lowered its surface resistivity and improved the fill factor of a cell. A solar cell of 8.5% intrinsic efficiency was obtained from CdS film printed by an 80 mesh screen and sintered at 690°C at a belt speed of 2 cm/min.

Screen printed CdS/CdTe cells for visible-light-radiation sensor

Thick-film CdS/CdTe photovoltaic cells have been prepared on borosilicate glass substrates by sequential screen printing and sintering steps. The residual amount of acts as a flux in a sintered CdS film and the sintering temperature of the CdTe film has a strong influence on the spectral response of the cell. The cells fabricated with the sintered CdS film having a larger amount of residual Cl ion and the cells sintered at a higher temperature show a poor response in the short wavelength. This is due to a formation of solid solution at the CdS/CdTe interface during the CdTe sintering. The screen printed CdS/CdTe cells also exhibit a higher spectral sensitivity in the longer wavelength region compared with typical CdS/CdTe cells. This may be due to the narrowing of the band gap resulting from layer. The cells prepared under suitable conditions show a spectral sensitivity nearly constant in the range of 530 to 850 nm. cell has shown an increase in the spectral response at 500 nm wavelength region. These cells may be used as a visible-light-radiation sensor with a constant sensitivity in a wider wavelength region.

Te-metal Contact on Evaporated CdTe Film for a CdS/CdTe Solar Cell

Abstract The photovoltaic properties of thin-film CdS/vacuum-evaporated CdTe solar cells with evaporated TeAu and TeCu contacts to the CdTe film have been investigated. The contact properties were improved by annealing at 350°C and 240°C, respectively, for the cell with pinhole-free CdTe film. The improvement of the cell performance was achieved for an about 1 μm thick CdTe cell with TeAu contact and for a 2–4 μm thick CdTe cell with TeCu contact. The Te film of TeAu contact may prevent the CdS film from touching Au or diffused Au through pinholes in thin CdTe film. The optimum thickness of the Te film for improving the cell performance depends on the thickness of the CdTe film for the cell with TeCu contact.

Properties of CdS/CdTe Solar Cells with Cu2Te–Au Contacts

Cu2Te-based contact layers are annealed onto CdTe films of CdS/CdTe solar cells. The photovoltaic properties of cells with Cu2Te–Au contacts are greatly improved when the annealing temperature was increased. The V oc and J sc of the cells reached a maximum value at around 250° C. An increase in the spectral response of the short wavelength region was observed for cells with thinner CdS films. CdS/CdTe solar cells with Cu2Te–Au contacts had a conversion efficiency of 10.5%.