JP3472837B2 - Solar cell and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a highly efficient and low cost solar cell and a method for manufacturing the same.

[0002]

2. Description of the Related Art As a low-cost practical solar cell, S grown by a casting method, which is one of the solidification growth methods from a melt, is used.
i-bulk polycrystals are used. Since Si bulk polycrystals can be grown using the casting method,
It has the advantage of low manufacturing costs. However, since only the Si bulk polycrystal cannot absorb the long wavelength side of the sunlight spectrum, and since the casting method is the main growth technique, there are many defects in the bulk polycrystal. Therefore, the Si bulk polycrystal was used. It is difficult to make solar cells highly efficient.

On the other hand, Si is used as a highly efficient solar cell crystal.
A heterostructure tandem solar cell in which a compound semiconductor thin film such as GaAs is laminated on a substrate or a Ge substrate is used.
However, the tandem solar cell with a heterostructure has a complicated structure and requires advanced technology for growth. Therefore, it cannot be used as a versatile solar cell because of its high manufacturing cost, and as a result, it is used only for limited applications such as space applications.

[0004]

As described above,
Conventionally, there is no solar cell that can achieve both high efficiency and low cost, and the present situation is that the solar cell is used properly according to the application. However, in order to make extensive use of solar cells as clean energy, there has been a strong demand for a technology that can achieve both high efficiency and low cost.

The present invention provides a solar cell capable of achieving both high efficiency and low cost, and a method of manufacturing the same.

[0006]

Means for Solving the Problems The present invention comprises (1) a substrate made of a multi-element polycrystal having a non-uniform microscopic composition distribution, and a deposition growth layer formed by depositing and growing thin film crystals on the substrate. A solar cell having the two-layer heterostructure of. (2) The solar cell according to (1), wherein the polycrystalline substrate is a SiGe system and the thin film crystal deposition growth layer is Si. (3) The solar cell according to (1), wherein the polycrystalline substrate is an InGaAs system and the thin film crystal deposition growth layer is a GaInP system. (4) The polycrystalline substrate is SiC-S mainly composed of SiC.
The solar cell according to (1), wherein the i-based or Si-SiC based mainly on Si, and the deposition growth layer of the thin film crystal is Si or SiC based.

(5) The polycrystalline substrate is a GaAsSb type,
The solar cell according to (1), wherein the thin film crystal volume growth layer is a GaAs system.

(6) The solar cell according to any one of (1) to (5), wherein the polycrystalline substrate has a columnar crystal structure.

(7) When a substrate made of a multi-source polycrystal is produced by a melt growth method, a step of adjusting the growth conditions to obtain a desired micro-compositional distribution is obtained. And a step of depositing and growing a thin film crystal on the substrate by an epitaxial growth method to form a two-layer heterostructure. In the description of this specification, “heterogeneous microscopic composition distribution” means that the composition has a predetermined composition macroscopically, but has an arbitrary compositional distribution microscopically. For example, when the content of a predetermined composition is plotted on the horizontal axis and the content ratio (content) of the composition is plotted on the vertical axis, the shape of the composition distribution such as a normal distribution or the shape of the composition distribution deviating from the normal distribution, It can take the form of any composition distribution.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is intended to obtain a high-efficiency solar cell having a heterostructure based on a multi-element polycrystal such as SiGe. A multi-element polycrystal such as SiGe has a non-uniform composition distribution in a microscopic manner and a uniform composition distribution in a macroscopic manner. By using this multi-element polycrystal as a substrate and depositing a thin film crystal of Si or the like having a larger energy band gap than the multi-element polycrystal on the substrate to form a heterostructure, the spectrum of sunlight can be absorbed in a wide range. Solar cells with efficient heterostructure crystals can be manufactured.

Hereinafter, a SiGe binary polycrystal-Si thin film polycrystal solar cell will be specifically described as an example. First,
A SiGe binary binary polycrystal obtained by adding Ge to a Si polycrystal is used as a substrate. This SiGe polycrystal is produced by a melt growth method such as a casting method, and its composition distribution is freely adjusted by controlling the growth conditions (cooling rate etc.). This polycrystal is not a conventional crystal having a uniform micro composition, but a SiG having a novel arbitrary micro-dispersive composition distribution.
e Polycrystal. By controlling the microscopic composition distribution, the wavelength dependence of the physical properties such as the absorption coefficient of SiGe polycrystal can also be freely controlled, and the crystal structure / structure that can absorb sunlight most efficiently and generate electricity can be designed. can do. That is, by giving a distribution of absorption coefficient over a wide range of wavelengths,
Specifically, it is possible to manufacture a polycrystalline substrate such as SiGe having an absorption coefficient on the long wavelength side larger than that expected from the distribution of Ge composition.

As described above, in the SiGe polycrystal having the microdispersive composition distribution according to the present invention, the absorption coefficient on the long wavelength side is larger than the absorption coefficient expected from the distribution of the Ge composition. Just mix it in a small amount,
A sufficient effect can be given to the wavelength dependence of the absorption coefficient. Therefore, SiGe having a microdispersive composition distribution
Polycrystals are extremely effective as crystal substrates for high efficiency solar cells. In this case, the polycrystalline structure is preferably a columnar crystal because it prolongs the diffusion length of carriers.

Next, the obtained SiGe polycrystal is used as a substrate to form a two-layer heterostructure. That is, a thin film crystal of Si or the like having an energy bandgap larger than that of the multi-source polycrystal is deposited and grown to form a deposited growth layer. Here, in order to prevent a voltage drop, it is desirable that the pn junction be present in the Si thin film crystal. Further, the composition control of the SiGe polycrystal and the size and size distribution of the crystal grains are controlled so that misfit dislocations do not enter the Si / SiGe hetero interface. Further, it is preferable to grow the Si polycrystalline thin film to which the highest electric field is applied, by the epitaxial growth method capable of reducing the defects most in order to improve the efficiency. By this method, it is possible to obtain a thin film crystal having good quality, large grain size, and few defects.

Thus, the Si / SiGe heterostructure in which the SiGe polycrystal having the micro-dispersive composition distribution is used as the substrate and the Si thin film polycrystal is epitaxially grown thereon can be obtained the solar cell of the present invention.

In this heterostructure solar cell, sunlight passing through the Si layer can also be absorbed in the lower SiGe layer to contribute to the generation of carriers, and the carriers diffused from the SiGe layer can have pn in the Si layer. The driving force is given by the potential difference at the junction, and it can be effectively utilized as a current.

As described above, the present invention can realize a high-efficiency solar cell by using the combination of the composition distribution with the highest efficiency and the optimized heterostructure without using a complicated structure such as a tandem type. In addition to the Si / SiGe system, In
It is a highly practical technology that can be easily applied to multi-element polycrystals such as GaAs.

[0017]

EXAMPLES Examples of the present invention will be described below based on a Si / SiGe heterostructure solar cell, but the present invention is not limited to these examples, and various modifications can be made without changing the gist of the present invention. It goes without saying that changes and modifications can be made.

Using B-doped p-type Si crystal and Ge crystal as raw materials, 16.6 g and 37.1 g were mixed and melted to prepare a melt of 50% Si—Ge binary system having a uniform composition. This was unidirectionally solidified and grown at a cooling rate of 10 ° C./min using a Bridgman type cast growth furnace to produce a columnar SiGe polycrystal having a microdispersive composition distribution. At this time, by changing the cooling rate, it is possible to obtain crystals having various composition distributions microscopically. This crystal structure and composition distribution state can be prepared by growth conditions such as melt composition and cooling rate. FIG. 1 shows the wavelength dependence of the absorption coefficient of Si _0.5 Ge _0.5 polycrystals having the same average composition of about 0.5 but different microdispersive composition distributions. The SiGe polycrystal used for this measurement is not a columnar crystal, but has a Si-rich needle-like structure and Ge-r.
It is a polycrystal having a matrix portion of ich. As can be seen from FIG. 1, crystals (Si: G) having the same composition on average
Even with e = 1: 1), it is apparent that the wavelength dependence of the absorption coefficient differs due to the difference in the microscopic composition distribution, and the wavelength dependence of the absorption coefficient corresponding to the shape of the composition distribution is obtained.

Further, for high efficiency, these SiGe columnar polycrystals having a microscopic composition distribution are sliced in the direction perpendicular to the columnar crystals to prepare a plate crystal, which is used as a substrate. A two-layer heterostructure was produced by depositing and growing a B-doped p-type Si thin film crystal. Molecular beam epitaxial growth (MBE) was used for the growth of this Si thin film crystal. P was diffused from the surface of the Si thin film crystal to make the surface side of the Si thin film crystal n-type. This created a pn junction in the Si thin film crystal. FIG. 2 schematically shows the structure of the Si / SiGe heterostructure solar cell thus manufactured. In the figure, 10 is a p-SiGe polycrystalline substrate, 20
Is a p-SiGe polycrystalline substrate 1 which is a Si polycrystalline thin film.
There is a hetero interface 30 between 0 and the Si polycrystalline thin film 20, the Si polycrystalline thin film has a p-Si layer and an n-Si layer, and a pn junction surface is formed between these layers. In Figure 3,
The calculation result of the wavelength dependence of a short circuit current at the time of irradiating a Si / SiGe heterostructure solar cell with sunlight of AM1.5 is shown. By using the heterostructure, the value of the short circuit current is about double that in the case of only Si polycrystal. Although the use of SiGe having a small band gap causes a decrease in open circuit voltage, it can be expected that the conversion efficiency will be improved as compared with the conventional Si polycrystalline solar cell by optimizing the structure (see FIG. 4). A solar cell was produced from this Si / SiGe heterostructure crystal by using a normal process.

Although the above embodiment was carried out using a SiGe system,
It can also be applied to a crystal system having an all-solid-state phase diagram such as an As-GaAs system. Ga on InGaAs polycrystalline substrate
Higher efficiency can be achieved by using a heterostructure in which an InP thin film crystal is deposited. In addition, as a material combination, a III-V group ternary system such as GaSb-GaAs or I
It is also possible to use a quaternary system such as nAs-GaP-GaAs-GaSb. Further, the substrate is a SiC-Si system mainly composed of SiC or a Si-Si mainly composed of Si.
It is also possible to use C system and use Si or SiC for the thin film deposition layer.

According to this embodiment, as shown in FIG. 4, SiG having a microdispersive composition distribution according to the present invention is used.
The conversion efficiency of the Si / SiGe heterostructure solar cell using the e-polycrystal could be made higher than that using the SiGe crystal of uniform composition. Furthermore, this value is S under the same conditions.
Greater than the conversion efficiency of i-polycrystalline solar cells.

As is apparent from this example, by changing the microscopic composition distribution according to the present invention depending on the growth conditions, the sensitivity distribution for the spectrum of sunlight can be controlled as shown in FIG. Therefore, a SiGe polycrystal having an optimal composition distribution can be used as a substrate, and a heterostructure having a Si thin film crystal grown thereon can be used to manufacture a highly efficient solar cell. In addition, the heterostructure solar cell of the present invention has the following effects. Si and S
Two types of crystals of iGe can efficiently absorb the spectrum of sunlight. Since SiGe polycrystal has a columnar structure, S
The carriers generated in the iGe crystal can diffuse to the pn junction in the Si layer without being blocked by the grain boundaries. pn
Since the junction is in the Si layer, the voltage drop is small. Si
Since the layer is formed by the epitaxial growth method, it has few defects. Even in the Ge-rich portion, since it is polycrystalline, the grain size is finite, the stress applied to the interface is small, and misfit dislocations are hard to enter. Furthermore, at the Si / SiGe hetero interface, SiGe has a higher potential in the conduction band, and when strain is introduced into Si, this tendency becomes more prominent, and electrons that are minority carriers are not blocked by the potential barrier. Can be diffused to the Si side. Since the present invention has such features in the structure, it can effectively absorb the spectrum of sunlight, diffuse carriers without disappearing in the middle, suppress the drop of voltage as much as possible, have high current, and are highly efficient solar cells. Can be produced.

[0023]

As described above, according to the present invention,
A high-efficiency solar cell can be realized at low cost by combining the most efficient composition distribution and the optimized heterostructure without using a complicated structure such as a tandem type. Further, not only the Si / SiGe system, but also a remarkable effect that it can be easily applied to a multi-element polycrystal such as InGaAs system.

[Brief description of drawings]

FIG. 1 is a graph showing the wavelength dependence of the absorption coefficient of SiGe polycrystals having the same average composition but different microdispersive composition distributions.

FIG. 2 is a schematic sectional view of a Si / SiGe heterostructure solar cell according to the present invention.

FIG. 3 Si / SiGe under AM1.5 sunlight irradiation
The graph which shows the wavelength dependence of the short circuit current of a heterostructure solar cell.

FIG. 4 shows Si film thickness dependence of conversion efficiency of a Si / SiGe heterostructure solar cell using a SiGe polycrystal having a microdispersive composition distribution. For comparison, S with uniform composition
The graph which shows the conversion efficiency of the Si / SiGe heterostructure solar cell which used the iGe crystal.

[Explanation of symbols]

10. ． ． p-SiGe polycrystalline substrate 20. ． ． Si polycrystalline thin film 30. ． ． Hetero interface

Continuation of the front page (56) Reference JP-A-53-104157 (JP, A) JP-A-4-249374 (JP, A) JP-A-4-87326 (JP, A) JP-A-6-224455 (JP , A) JP 10-135494 (JP, A) JP 4-168769 (JP, A) JP 3-184324 (JP, A) JP 2002-9312 (JP, A) P. Geiger et al, Multicrystalline SiGe Solar Cells with Ge Content above 10 at%, Proc. 16th European Photovoltaic Solar Energy Conference, UK, 2000, p. 150-153 (58) Fields investigated (Int.Cl. ⁷ , DB name) H01L 31/04-31/078 C01B 33/00-33 / 193 C30B 1/00-35/00

Claims (7)

(57) [Claims] 1. A solar cell having a two-layer heterostructure comprising a substrate composed of a multi-element polycrystal having a non-uniform microscopic composition distribution and a deposition growth layer formed by depositing and growing a thin film crystal on the substrate. 2. The solar cell according to claim 1, wherein the polycrystalline substrate is SiGe-based, and the thin film crystal deposition growth layer is Si. 3. The solar cell according to claim 1, wherein the polycrystalline substrate is an InGaAs system and the thin film crystal deposition growth layer is a GaInP system. 4. The polycrystalline substrate is composed mainly of SiC.
The solar cell according to claim 1, wherein the C-Si system or the Si-SiC system mainly composed of Si, and the deposition growth layer of the thin film crystal is Si or the SiC system. 5. The solar cell according to claim 1, wherein the polycrystalline substrate is a GaAsSb system and the thin film crystal volume growth layer is a GaAs system. 6. The solar cell according to claim 1, wherein the polycrystalline substrate has a columnar crystal structure. 7. A step of adjusting a growth condition to obtain a desired micro-composition distribution when a substrate made of a multi-source polycrystal is manufactured by a melt growth method,
And a step of depositing and growing a thin film crystal on the obtained substrate by an epitaxial growth method to form a two-layer heterostructure.