DE3241982A1 - A solar cell - Google Patents

Description

37 789

MITSUBISHI DENKI KABUSHIKI KAISHA
Tokyo / JAPAN

Solar cell

The invention relates to solar cells in which GaAs or other semiconductor compounds are used.

Solar cells are used and most widely used as energy sources in satellites and spacecraft are silicon solar cells. The properties of such Solar cells are made by emitting high energy cosmic rays such as electron beams and proton beams exposed to severe deterioration, which is one of the reasons for the limited useful life of satellites is. Attempts have therefore been made to use solar batteries in which the silicon is replaced by GaAs is replaced in order to obtain a design in which the properties of the cell by the influence of the cosmic rays are not deteriorated.

Fig. 1 shows a conventional solar cell in which a GaAs substrate is used, wherein a p-type semiconductor layer 2 is formed on one surface of the n-GaAs substrate 1, whereby an n-type semiconductor layer 3 is formed as a result of the formation of the p-type semiconductor layer 2 on the substrate. A plurality of parallel first electrodes 4 are arranged on the p-semiconductor layer 1, while a second electrode 5 is placed on the n-semiconductor layer 3.

The embodiment of FIG. 1 has a p / n structure, in where a thin p-layer is formed on a thick η-layer is, but the same result can be obtained with an n / p structure in which a thin η layer is applied a thick p-layer is formed.

The theoretical functioning of solar cells is known and therefore does not need to be discussed. if Light hits the p-n-overgano, generate photons,

whose energy is greater than the band jump, an excess of defects and free electrons, which are pumped away by the voltage applied to the p-n junction and become move to p-layer 2 or n-layer 3. This creates a photo voltage, and by closing an external one A photocurrent flows in the circle. So the light has been converted into electrical energy by the solar cell.

The properties and the performance of a solar cell depend on the yield with which an excess of holes and free ones Electrons caused by light absorption move in opposite directions through the p-n junction move. With a given special semiconductor material, the research attempts concentrate on a to create optimal cell design in order to yield this increase. However, the properties of a solar cell deteriorate significantly when high energy radiation such as cosmic rays strike. The main reason for this is that this radiation increases the mobility and the lifetime of the carriers in the p-semiconductor layer 2 and the n-layer 3 reduce, resulting in a substantial decrease in the ratio of the diffusion of excess charge carriers to the pn junction leads. This drop is unaffected by whether the semiconductor material not exposed to the radiation is good has crystalline properties or not and this establishes the Suggests that the worsening effect of the higher

getic radiation on the cell properties is very large is.

It is therefore the object of the invention to create a solar cell that uses GaAs or others as the substrate Semiconductor materials are used whose properties are not impaired by high-energy radiation. This goal can be achieved with an element which has a first semiconductor layer of a first conductivity type, a second semiconductor layer of the same conductivity type, but with a lower concentration of impurities than that first semiconductor layer, a third semiconductor layer of the opposite conductivity type, which has a Forms p-n junction with the second semiconductor layer, and has a layer that has a junction with the third Forms semiconductor layer, the combination of the third semiconductor layer and the layer forming the junction is thinner than the depletion layer formed between the second and third semiconductor layers.

To explain the invention, reference is now made to the drawing Referenced. This shows in detail in:

Fig. 1 is a section through a conventional one

Solar cell;

■ Fig. 2 'a section through an inventive Solar cell in a first embodiment; and 30

3 shows a section through a solar cell

with a Schottky barrier layer according to a second embodiment the invention. 35

The solar cell according to FIG. 2 also contains a p-half

• ■ ·· «" α

► *% α β * ο «α β -

Conductor layer 12, which with one surface of a p ~ half ~ conductor layer 2 forms a transition, with the first electrodes on the other surface of the p-type semiconductor layer 2 4 are arranged and the impurity concentration of the p »semiconductor layer is higher than that of the p-semiconductor layer 12 is. An η semiconductor layer 13 forms with one side an n-semiconductor layer 3 has a junction, while on the other surface of this n-semiconductor layer 3 the second Electrode 5 is formed and the impurity concentration of the n-semiconductor layer 3 is higher than that of the n-semiconductor layer 13 is. Between the p-type semiconductor layer 12 with a low impurity concentration and the n-type semiconductor layer13 a depletion layer 6 is formed with a low concentration of impurities,

The element according to FIG. 2 is characterized by the presence of a thicker depletion layer. Their purpose is to create a gradual transition and reduce cell resistance to increase compared to high-energy radiation in that more carriers are generated in the depletion layer than in the other areas, namely the p- and n-semiconductor layers. The basic concept of this design is as follows: the depletion layer is exposed to a strong electric field and stands out due to the high mobility of the carrier and the reluctance of the carrier to do so for recombination, so that even if the carrier mobility and service life are reduced by high-energy radiation, the charge carriers in the depletion layer are only slightly influenced and the properties of the entire cell are practically essentially constant stay.

A structure with a thicker depletion layer can be achieved by reducing the concentration of impurities in the p- and achieve n-type semiconductor layers. On the other hand, it is

Open circuit voltage / the one parameter of a solar cell i & t, high when the p- and n-semiconductor layers have high concentrations of impurities and if the Fermi limit has been shifted accordingly from the zero position. These two opposing demands can be made by the Gestal. 2 can be reconciled with the device according to FIG p- and n-semiconductor layers each consist of two sub-layers with different concentrations of impurities.

The relationship between the p-type semiconductor layer and the depletion layer will be described below with reference to a GaAs solar cell. A single GaAs crystal has one Such a high absorption factor that a strength of a few Micron is sufficient to absorb almost all visible light (wavelengths below 0.9 μ, ΐη). Cosmic rays like Electron beams and proton beams, on the other hand, have high energy and can even pass through a GaAs crystal evenly several hundred microns thick. Thus, a solar cell with high resistance can be achieved against high energy radiation using a GaAs crystal with a narrow operating range. In a conventional p-to-n solar cell, this is Thickness of the p-semiconductor layer determined so that the carrier generated in this layer when illuminated for the larger Part of the photocurrent are responsible. With such a structure, however, the cell performance decreases once the crystalline properties (e.g. lifespan and mobility minority carriers) have deteriorated. To avoid this, the thickness of the p-type semiconductor layer is increased reduced to a minimum by the fact that between p- and n-semiconductor layers a depletion layer is formed as close as possible to the crystal surface, and at the same time becomes the thickness of the depletion layer is increased to such an extent that more carriers are generated in the depletion layer than in the p-type semiconductor layer. If with such a structure,

_ Q -

that high-energy radiation strikes the p-semiconductor layer becomes worse, the internal stress on the depletion layer facilitates a concentration of charge carriers in it, and the deterioration in cell properties can be kept low. This is the main reason why the depletion layer is stronger than the p-type semiconductor layer is made. The thickness of the depletion layer can be determined from the wavelength dependence of the absorption factor Semiconductor material and the spectral distribution of sunlight can be calculated.

Another requirement, which is why the cell resistance to high-energy radiation should be improved, is that the total thickness of the p-type semiconductor layer and the depletion layer are to be determined so that the Part of the incident sunlight whose energy is higher than the band jump of the special crystal (z. B. light with a wavelength below 0.9 μπι for GaAs), of is absorbed by these layers. The required thickness of the p-type semiconductor and the depletion layer formed together Layer can be calculated from the absorption factor of the special semiconductor material. The majority of the load carriers, which is to be generated by irradiation in the solar cell must be in the p-semiconductor layer and in the depletion layer are generated because the diffusion of holes in the n-type semiconductor layer is less than that of the is free electrons in the p-type semiconductor layer, and the former is more sensitive to high-energy radiation. A cell structure in which the greater part of the incident light is not in the p-type semiconductor layer or the depletion layer is absorbed, has inherently poor properties and is very sensitive to high energy Radiation.

A good cell structure that prevents the build-up of more charge

slower made possible by the irradiation in the depletion layer than in any other layer and its properties essentially depend on this ability, can be determined by determining the absorption factor of the special Obtain crystal and the distribution of the impurity concentration in the crystal. The concept of the invention shows the greatest benefit in a device that uses a direct band hop semiconductor how GaAs is used, which has a large absorption factor in the range of the visible Spectrum and makes it possible to absorb the greater part of the incident light in a flat area. However, other semiconductor materials can also be used by reducing the impurity concentrations that are present in the p- and n-semiconductor layers, accordingly be reduced; this increases the strength of the depletion layer, and it can be exposed to light more charge carriers are generated in the depletion layer than in any other area.

As stated above, it is important for a cell according to the invention that the majority of the incident Light is absorbed by the upper semiconductor layer and the depletion layer. For this purpose a High resistivity layer can be formed by applying the technique of doping compensation to the p-type semiconductor layer located on the surface (or the n-semiconductor layer in an n-to-p structure). Another option is using a solar cell Schottky barrier layer can be formed by dividing the top semiconductor layer through a metallic Schottky barrier layer 7 is replaced as shown in FIG.

Claims (8)

37 789 MITSUBISHI DENKI KABUSHIKI KAISHA
Tokyo / JAPAN Solar cell Pat tan claims for "e nJ solar cell, characterized by a first semiconductor layer (3) of a first conductivity type, a second semiconductor layer (13) of the same conductivity type with a lower concentration of impurities With respect to the first semiconductor layer (3), a third semiconductor layer (12) of the opposite conductivity type which has a p-n junction with the second semiconductor layer (13) (6) forms, and one with the third semiconductor layer (12) a transition-forming layer (2; 7), the combination of the third semiconductor layer (12) and the layer (2; 7) forming a transition with it thinner than the depletion layer (6) between the second and the third semiconductor layer (13, 12). 2. Solar cell according to claim 1, characterized, that the layer connected to the third semiconductor layer (12) forms a transition, the third semiconductor layer (12) and the depletion layer (6) absorb most of the incident sunlight. 3. Solar cell according to claim 1,
characterized, that with the third semiconductor layer (12) an over * - Gang-forming layer (2) is of the same conductivity type as the third semiconductor layer (12), but higher Has impurity concentration. 4. Solar cell according to claim 1,
characterized, that with the third semiconductor layer (12) a transition forming layer (7) consists of metal. 5. solar cell, marked by first and second semiconductor layers (13, 12) of opposite conductivity types and relatively low concentration of impurities, a third and a fourth semiconductor layer (3, 2), which are connected to the first or second semiconductor layer (13, 12) and higher Have impurity concentrations than this and on the third or fourth semiconductor layer (3, 2) Electrodes (5, 4). 6. Solar cell according to claim 5,
characterized, that the semiconductor layers consist of GaAs. 30th 7. Solar cell according to claim 5,
characterized, that there is one between the first and the second semiconductor layer (13, 12) located depletion layer (6) over a layer thickness that is greater than the common * - same thickness of the second and fourth semiconductor layers (12, 2) is. . 8. Solar cell according to claim 7, characterized in that the predominant charge carrier part, which is caused by lighting the solar cell arises, has its origin in the depletion layer (6).