JPH02201973A - Thin film solar cell - Google Patents

Abstract

PURPOSE:To provide a solar cell with increased photovoltaic current and efficiency by combining multilayered films comprising two type of semiconductors different in refractive indexes. CONSTITUTION:A solar cell active layer 4 comprises an n-GaAs layer (base) 41 and a p<+>-GaAs (emitter) 42, and includes a light reflecting layer 8 formed of semiconductor multilayered films of two types different in refractive indexes and disposed at the rear end of the active layer. The reflecting layer is constructed so as to reflect light available having wavelengths transmitted through the thin film solar cell active layer, and is designed such that respective reflecting layers BR1, BR2, and BR3 satisfy the formula I. In the formula I, denotes a central wavelength, film thickness, and ni a refractive index. Hereby, long wavelength light component with reduced light absorbance can effectively be taken out as an electrical output.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a compound semiconductor thin film solar cell formed on a Si substrate.

(Prior art) Generally, when a compound semiconductor thin film solar cell is formed on a Si substrate, high density dislocations occur in the compound semiconductor thin film due to mismatch in lattice constant between the Si substrate and the compound semiconductor, resulting in activation of the solar cell. reach the territory. Dislocations tend to serve as recombination centers for electrons and holes. Electrons and holes generated by sunlight incident on the solar cell active layer recombine at dislocations, making it impossible to efficiently convert light to electrical energy. It is known that for this reason, the performance of thin film solar cells is significantly reduced. For this reason, efforts are being made to reduce dislocations in compound semiconductors in order to improve performance. For example, FIG. A buffer layer 3 is introduced between the solar cell active layer 4 and the Si substrate 2 to reduce dislocations contained in the solar cell active layer. In the figure, 31 is a thermal cycle buffer layer made of GaAs with a thickness of 1 to 2 pm and subjected to thermal cycle treatment, and 32 is Ino, +5G.
33 is a strained superlattice buffer layer grown repeatedly for 5 cycles of ao, asAs (thickness Ions)/GaAs (thickness 10n+1);
m) / GaAs (alternating N buffer layers with a thickness of 10100 nm grown three times repeatedly, and the structure of each is determined from the viewpoint of dislocation and low acidity. Note that 1 is the lower electrode, 5
6 shows the wind layer, 6 shows the antireflection layer, and 7 shows the upper part.

(Problems to be Solved by the Invention) Even in the thin film solar cell using the above buffer layer, 10'c
It has a dislocation density of m''z, and electrons and holes, that is, carriers, are easy to recombine, so the diffusion length of minority carriers is short, and it is smaller than the absorption depth of light in the wavelength range that thin-film solar cells can use. For this reason, carriers generated in a deep region away from the pn junction recombine before reaching the pn junction and cannot be used as the electrical output of the solar cell.

The present invention was proposed to improve the above-mentioned drawbacks, and makes it possible to achieve higher performance than conventional structures in thin-film solar cells that utilize compound semiconductor thin films epitaxially grown on Si substrates. .

(Means for Solving the Problems) In order to achieve the above object, the present invention newly arranges a light reflecting layer formed of two types of semiconductor multilayer films with different refractive indexes at the rear end of the active layer. This is a thin film solar cell with special characteristics. As a result, it has a feature that long wavelength light components with small optical absorption coefficients can be efficiently extracted as electrical output.

(Function) In the thin film solar cell of the present invention, a multilayer reflective layer is provided, which doubles the effective absorption area and brings the absorption area closer to the pn junction, so that the photovoltaic current is large and the efficiency is increased. You can wander around.

(Example) Next, an example of the present invention will be described. Note that the embodiments are merely illustrative, and it goes without saying that various changes and improvements can be made without departing from the spirit of the invention.

FIG. 1 shows an embodiment of a thin film solar cell according to the invention. In the figure, 1 is a lower electrode, 2 is a Si substrate, and 3 is a buffer layer for improving crystallinity. 4 is the solar cell active layer n −
GaAsH (base) 41, p-GaAsN (Emi-
) 42.5 is p” -GaAIAs (wind)
It is a layer. 6 indicates an antireflection layer, and 7 indicates an upper electrode.

8 is a reflective layer introduced according to the present invention, and in this example, three types of reflective layers with different center wavelengths (BJ71.BR
2, BR3).

The configuration of this reflective layer is determined by the optimal thickness of the thin film solar cell active layer relative to the diffusion length of minority carriers in the y4 film solar cell active layer. That is, the structure is such that out of the light having a wavelength that passes through the thin-film solar cell active layer, light that can be used as cell output is reflected.

The configurations of the reflective layers BRI, BR2, and BR3 are shown below. λr is the center wavelength, formed by Ali.

Wl, thickness, and refractive index are shown in this order.

BRI: λr = 0.72n (AIAs (film thickness 59
2 people, refractive index 3.04) / Ale, zsGao,
? 5AS (486 people, 3.70)) BR2:λr
=0.18n (AIAs (645 people, 3.02)
/Ale, +5Gao, 5sAs (542 people,
3.60)) BR3: λr = 0.871 (AIAs
(730 people, 2.98) /^L, +5Gao,
5sAs (621 people, 3.50)) Therefore, for each reflective layer BRI, BR2, BR3,
λr n+L+book nzLz=λ.

n, t 3 = It is designed so that it holds true.

Figure 3 shows the effect of the reflective layer of the present invention on photovoltaic current when the diffusion length of minority carriers is 1.5n.From this, the optimal active layer thickness is 2μ for the conventional structure, and for the present invention. It turned out to be 1.2n. In this case, an effective active layer thickness of about 0.6 nm is effective.

FIG. 4 shows the photon energy and spectral sensitivity of thin film solar cells of the present invention and conventional structures. From this, it can be seen that the present invention has a sufficient effect when the reflection wavelength is from 650 na+ to 870 nm.

FIG. 5 shows the relationship between incident wavelength and reflectance, and this shows three types of reflective layers: BR1°BF? 2. B
By combining R3, from 700na+ to 900na+
It can be confirmed that light in the nm wavelength range is effectively reflected.

That is, in the thin film solar cell of the present invention, GaAs, AI, G
a+-xA3, A as a compound semiconductor constituting a multilayer film
I, Ga+-, As+ AIJa+-gAs (x <
y < z).

In addition, since the forbidden band width of the semiconductor constituting the reflective layer is larger than that of the semiconductor constituting the solar cell active layer (i.e., X<y<z), light is not absorbed by the reflective layer and has no effect. reflected.

That is, the thin film solar cell of the present invention is characterized in that the active layer is composed of InP as a compound semiconductor, and the multilayer film is composed of InAlAs and InAIAsP as compound semiconductors.

(Effects of the Invention) As explained above, according to the present invention, two kinds of materials having a wide forbidden band width and different refractive indexes, which have a lattice constant close to that of the material of the active region and have different refractive indexes, are prepared at the rear end of the thin film solar cell active region on a Si substrate. By combining multilayer films made of semiconductors, it is possible to not only improve efficiency while maintaining crystallinity compared to conventional thin-film solar cells, but also create a structure that efficiently obtains carriers with a small diffusion length. Therefore, a thin film solar cell with high radiation resistance can be obtained while maintaining efficiency compared to the conventional structure. Using this technology, thin-film solar cells with performance approaching that of bulk-type GaAs solar cells can be fabricated using lightweight and inexpensive Si.
It becomes possible to manufacture on a substrate, and the economic effect of the system can be enhanced for use in satellites and various civilian applications.

[Brief explanation of the drawing]

FIG. 1 shows a schematic cross-sectional structure of an embodiment of the thin film solar cell of the present invention. FIG. 2 shows a schematic cross-sectional structure of a conventional thin-film solar cell, and FIG. 3 shows the dependence of photovoltaic current on the solar cell active layer thickness depending on the presence or absence of a reflective layer in the thin-film solar cell. Figure 4 illustrates the dependence of photovoltaic current conversion efficiency on incident light energy in thin film solar cells with different thicknesses of the solar cell active layer, and shows the effect of the reflection wavelength of the solar cell activity 1iL of the reflective layer. . FIG. 5 shows the reflection characteristics of an actual reflection layer. 1... Lower electrode 2... Si substrate 3 . . . 4 . . . . 5 . . . 6 . . . 7 . . . 8 .・ ・ ・ 41・ ・ ・ ・ 42・ ・ ・ ・ 8f, 82.83 Battery active layer p''-GaAs solar cell active layer reflective layer (BR) Fig. 1 ■---Chobe katakoku 5---Wind diagram fig.

Claims (1)

[Claims] In a thin film solar cell having a structure in which a compound semiconductor thin film is epitaxially grown on a Si substrate, a combination of refractive index n and thickness t is formed below the active layer of the semiconductor thin film solar cell.
At least one multilayer film of two types of semiconductors composed of n_1, t_1) and (n_2, t_2) is arranged, and each multilayer film has n_1t_1 with respect to the wavelength λ_r of the light to be absorbed by the active layer. =n_2t_2=λ_r
1. A thin film solar cell characterized by being a multilayer film with a relationship of /4.