Nobuo Nakayama

CdS–CdTe Solar Cell Prepared by Vapor Phase Epitaxy

CdS–CdTe solar cells were prepared by epitaxial growth of CdS on pCdTe wafers, and properties of the cells were extensively studied. The cells have a junction structure of nCdS–(n or i)CdTe–pCdTe. Formation of the (n or i) layer is due to diffusion of In into CdTe; the thickness of the layer was 0.5 µm in the most efficient cell. The best cell exhibited a solar conversion efficiency of 10.5% under illumination by sunlight (AM1.3, 68 mW/cm2), and 6.0% under illumination by a simulated sunlight (AMO). The high efficiency is attributed to the reduction in both the series resistance and surface recombination, which results from the presence of the heavily doped CdS layer on the wafer. The cell is not humidity sensitive and is stable in the forward bias test, These results suggest strongly that practically useful solar cells can be manufactured from this type of junction.

Screen printed thin film CdS/CdTe solar cell.

Thin film CdS/CdTe solar cells with an efficiency of 6.3 % have been prepared on a borosilicate glass substrate of 4s×4 cm2 by successively repeating screen printing and heating (sintering) of each paste of CdS, CdTe and C. The CdS paste consists of CdS, CdCl2, GaCl2 and propylene glycol (PG). The CdTe paste contains CdCl2 and PG, and the C paste contains PG and a small amount of acceptor impurity. During the heating of C paste, an nCdS/pCdTe heterojunction is formed. In the most efficient cell, the peak of electron voltaic effect exists within 1 µm of the CdTe side from the CdS/CdTe metallurgical boundary. The C electrode cell is more stable than the previous Cu2Te electrode cell for an accelerated life test. From 25 elemental cells with 4×4 cm2 substrate, a 1 watt module has tentatively been constructed with a module efficiency of 2.9 %.

Ceramic CdS Solar Cell

This paper describes the properties and the model of ceramic CdS photovoltaic cell made by treating CdS ceramic plate electrochemically in copper ion solution. The compositions and structures were analysed by means of X-ray Ginuie Camera and X-ray microanalyser. It was found that Cu2-xS (0x0.2) is formed along grain boundaries over a range of a several ten microns of the surface layer, and that the photovoltaic junctions exist at grain boundaries in this surface layer. From examinations of the voltage dependence of the barrier capacitance, the junctions could be classified into step junction, graded junction and Mott barrier, according to the prepared method. The spectral response measurement indicates that the origin of the photo-emf of the Mott barrier is mostly impurity photovoltaic effect.

Preparation and Photovoltaic Properties of Screen Printed CdS/CuxS Solar Cells

CdS/CuxS solar cells were prepared by using a screen printing method. A low resistivity CdS film was obtained by screen printing CdS pasts on a glass substrate and then sintering it in N2 gas including a small amount of Cd vapor. A CuxS layer was formed on the surface of a CdS film by treating the surface electrochemically in an aqueous solution of CuSO4. An Ag paint electrode or Ni paint electrode was applied to the CuxS layer, which was then heat treated in N2 gas. The spectral responses of the cell suggest that the composition of the CuxS layer was changed to the Cu2S-rich side by this heat treatment. Fabrication of a low resistivity CdS film and heat treatment in N2 gas after applying electrodes were very important factors in obtaining a high efficiency cell. A maximum efficiency of 8.9% was attained for a cell with an active area of 0.90 cm2. The stability of the cell was found to be influenced by the electrode material on the CuxS layer.

Electrochemical Synthesis and Photovoltaic Effect of Copper Sulfides on CdS Single Crystals

Copper-sulfide layer is electrochemically produced on CdS single crystals in copper ion solution with various bias current densities. The compositions and structures of the copper sulfides are determined by means of the X-ray powder and single crystal methods. The phases of the copper sulfides formed on CdS crystals change not only by the bias electric current density but also by the temperature of solution. On examining the photovoltaic effects for all combinations of the copper sulfides and CdS, it is shown that the best combination for the solar cell is the Cu2S-CdS pair.

Screen-Printable Electrode Material for CdS and Its Application to Solar Cell

A screen-printable electrode material which makes an ohmic contact to an n-type CdS film was developed. The material consists of 20 wt% of indium fine powder and 80 wt% of a commercially available electroconductive silver paint. The electrodes formed on the CdS film under the optimum condition showed linear current-voltage characteristics, and gave a low contact resistance of 5.0×10-2 Ωcm2. The material was applied to fabrication of a CdS/CdTe solar cell as electrodes of CdS. The solar cell showed nearly the same intrinsic conversion efficiency, open circuit voltage and short circuit current as those with indium metal electrodes formed by the vacuum evaporation method.

Low Temperature Synthesis of CdTe and Its Application to CdS/CdTe Solar Cell

It has been found that CdTe can be synthesized from a paste consisting of Cd powder, Te powder and propylene glycol (binder) in N2 atmosphere at a temperature lower than 300°C. The reaction synthesizing CdTe from the paste begins at about 180°C. This method is also effective for lowering the sintering temperature of CdTe film. We can lower the sintering temperature of the CdTe film down 600°C. The new method of the CdTe synthesis is applied to the preparation of thin film CdS/CdTe solar cells with the screen-printing technique. The cell has an intrinsic conversion efficiency of 4.7%.