FIG. 3 shows a cross section of a prior art solar cell by which a plurality of similar solar cell elements are able to be interconnected.
In FIG. 3, reference numeral 1 designates a solar cell device and reference numeral 2 designates a solar cell element. The solar cell element 2 has a light receiving surface 2a and a rear surface 2b. An electrode 3 is disposed on the light receiving surface 2a and an electrode 4 is disposed on the rear surface 2b. An interconnector 5 comprising Ag is disposed on the electrode 3. A p type GaAs layer 11 and an n type GaAs layer 12 constitute a pn junction. A p type AlGaAs contact layer 10 is disposed on the p type GaAs layer 11. A reflection preventing film 9 comprising, for example, Si.sub.3 N.sub.4 is disposed on the p type GaAs layer.
Light incident on the light receiving surface 2a generates charge carriers in the p type AlGaAs layer 10, p type GaAs layer 11, and n type GaAs layer 12, thereby generating an electromotive force in the solar cell element 2. Thus generated electromotive force is outputted to the outside of the solar cell element 1 via the electrodes 3 and 4. In order to obtain a desired output, it is required to interconnect the electrodes 3 of respective solar cell elements 1 by using interconnectors 5.
Conventionally, the interconnector 5 is connected to the external connection electrode 3 by soft soldering. In a solar cell device in which a plurality of solar cell elements are mutually connected by interconnectors 5 and mounted on an artificial satellite, the wide range of change in the temperatures places stresses on the solder layer between the interconnector 5 and the electrode 3. The solder may be decomposed, causing variation in the resistance of the junction and reduction in its mechanical strength.
In order to avoid these problems, a the electrode 3 and the interconnector 5 are connected by direct welding. A method called parallel gap welding is generally used. The welding preferably does not influence the element characteristics such as reducing power output due to leakage at the junction and the mechanical strength of the junction portion should be higher than the element strength.
In the prior art solar cell thus constructed, 45 degree pull strength distribution of the welding portion which is obtained after the welding of interconnector 5 is shown in FIG. 5. The 45 degree pull strength means the magnitude of force applied to separate the interconnector 5 from the electrode 3 when the interconnector 5 is pulled up in the direction F shown in FIG. 4. The the interconnector 5 is pulled in a direction of 45 degrees from the junction of the interconnector 5 and the electrode 3.
As a result of this experiment, it has been found that the welding strength distribution for a large number of samples is as shown in FIG. 5. There, one sample has quite a low strength. The main cause of such variations in welding strength is thought to be concentration of force. That is, since the p type GaAs layer 11 and the p type AlGaAs layer 10 are grown by liquid phase epitaxy, the surface of the solar cell 2 is likely to have corrugations and the electrode surface of the element is not always flat. The interface of the weld between the electrode 3 and the interconnector 5 is not a straight line.
Accordingly, when a pulling force is applied to the weld, the force is thought to be mainly applied to the edge of the weld of the interconnector 5 and the electrode 3. Then, various kinds of distribution of applied force may be conceived. In an extreme case, the force may be concentrated on quite a small area. In such a case, even when the junction strength between element and electrode is uniform, the force is concentrated on a narrow area and the welding strength between the interconnector 5 and the electrode 3 shows a low value. Accordingly, for solving the above described problems, the following three methods are conceived:
(1) enhance the junction strength between the solar cell device 1 and the electrode 3
(2) make the surface of electrode 3 planar, and
(3) enhance the strength of the electrode 3 itself.
However, all of these methods have problems, which result in difficulty in realization.
As the first method which attempts to enhance the junction strength between the element 1 and the external connection electrode 3, generally, mutual diffusion by sintering can be conceived. However, in solar cells the depth of the pn junction between p type GaAs layer 11 and n type GaAs layer 12 from the light receiving surface 2a is shallow. The shallow junction depth permits photocarriers to be collected even if the diffusion length of the photocarriers is reduced due to crystal defects generated by radiation. Therefore, when the temperature is raised by sintering, Ti or Pd from the electrode 3 may enter the light to electricity conversion section, causing leakage at the pn junction, and deterioration in the element characteristics.
The above-described second method which attempts to make the concave-convex of the surface of electrode 3 flat also has difficulty. That is, the p type AlGaAs layer 10 and the p type GaAs layer 11 below the electrode 3 are produced by liquid phase epitaxy, inevitably causing concave and convex features in the surface. This makes flattening the surface on which electrode 3 is subsequently disposed difficult.
In the above described third method which attempts to enhance the strength of the electrode itself, it is necessary to increase the thickness of electrode 3 using the same material for the electrode 3. The stress which is applied to the element is also increased with an increase in the electrode thickness, thereby causing unfavorable mechanical and electrical influences on the solar cell device.
USPTO Backgrounds
USPTO Backgrounds