JPS6333308B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an amorphous silicon solar cell, particularly to an amorphous silicon solar cell in which two or more amorphous silicon layers are stacked in series.

Photovoltaic devices, or solar cells, can convert sunlight into useful electrical energy. This energy conversion is performed by the photovoltaic effect, which is well known in the field of solar cells. Sunlight incident on a solar cell and absorbed by its active region generates electrons and holes. The generated electrons and holes are separated by the internal electric field of the solar cell, such as a junction, and the separation of the electrons and holes generates a current as explained below. For example, this internal electric field is created in the solar cell by a semiconductor layer with an active region of P-type, intrinsic, and N-type hydrogenated amorphous silicon. In the intrinsic region, sunlight with a suitable band gap is absorbed and electron-hole pairs are generated. Electrons flow toward the N-type region and, at the same time, holes flow toward the P-type region, and when these electron-hole pairs are separated, a photovoltage and a photocurrent are generated.

The photocurrent output of a solar cell can be increased by increasing the total number of photons of different energies and wavelengths absorbed by the semiconductor material.
The spectrum of sunlight is distributed in the wavelength range from approximately 30 Å to approximately 220 Å, and these are approximately equivalent to energy.
This corresponds to approximately 0.59eV from 4.2eV. The spectral range of sunlight absorbed by a solar cell is determined by the bandgap energy of the semiconductor material. Traditionally, solar cells have been fabricated from single crystal materials such as gallium arsenide, which has a band gap energy of about 1.45 eV, or silicon, which has a band gap energy of about 1.1 eV. Sunlight with energy below the bandgap energy is not absorbed by the semiconductor material and does not contribute to the photocurrent output of the solar cell.

Semiconductor materials such as gallium arsenide and single-crystal silicon have been used together in solar cells to improve the total conversion efficiency of solar energy into electrical energy, but it is difficult to use different semiconductor materials in the same solar cell. There was a problem. The first solution to the problem of manufacturing solar cells using different semiconductor materials is
reflect light of the appropriate wavelength onto the solar cell made of material,
The solution was to use a filter that transmits the unabsorbed light to the solar cell of a second semiconductor material. Second
The solution was to use different bandgap semiconductor materials that could be epitaxially grown together, such as aluminum gallium arsenide, gallium arsenide, and gallium phosphide. These two structures have been loosely referred to as series junction solar cells. The third option was to create individual solar cells with different bandgap energies, connect them in series, and stack them. All three methods are complicated,
This method has drawbacks such as being expensive and requiring large-sized equipment.

Therefore, in the case of optical filters and epitaxial growth of mutual active regions, it is strongly desirable to develop series junction solar cells with separate active regions in a stacked structure without the need for serial connection of separate solar cells. It was rare.

The solar cell of this invention has a configuration in which a plurality of hydrogenated amorphous silicon layers are separated by a tunnel junction and arranged in a series stacked structure. The thickness of the amorphous silicon layer is adjusted to maximize conversion efficiency and equalize the current generated in each layer. Series-stacked hydrogenated amorphous silicon solar cells can reduce their bandgap energy by adjusting the hydrogen concentration.
It can also be varied over a range of 1.5eV to about 1.8eV. The present invention will be described in detail below with reference to the attached drawings.

FIG. 1 shows a series junction type hydrogenated amorphous silicon solar cell 10. As shown in FIG. The sunlight 100 incident on the solar cell 10 serves as a reference point for the incident plane of each layer of the solar cell. Solar cell 10 is formed on a transparent substrate 12 of glass or other suitable material that is transparent to sunlight.
Transparent conductive oxide film (hereinafter abbreviated as TCO) 14
is deposited on substrate 12 to form ohmic contact with a cermet layer 16 of PtSiO ₂ containing about 10-40 volume percent platinum and having a thickness of about 0.2-1 Å. This cermet layer 16 is as described in U.S. Pat. No. 4,167,015.
It can also be made from dielectric materials such as _TiO2 .

An active substrate 20 is formed on the cermet layer 16, consisting of a plurality of hydrogenated amorphous silicon layers, each pair of layers separated by a light-transmissive tunnel junction. In FIG. 1, active substrate 20 has two hydrogenated amorphous silicon layers 22, 26 separated by a tunnel junction layer 24. In FIG. The active substrate 20 can be made from two to five or more layers of hydrogenated amorphous silicon.

Regions 22 of different conductivity types on the cermet layer 16
A first active layer 22 of hydrogenated amorphous silicon is deposited, consisting of a, 22b and 22c. Region 22a is a hydrogenated amorphous silicon layer doped with boron or other suitable P-type conductivity converter. This region has a thickness of about 1-4 Å, preferably about 3.75 Å. Region 22b is an intrinsic hydrogenated amorphous silicon layer having a thickness of approximately 5-50 Å. It is known that intrinsic type hydrogenated amorphous silicon exhibits a slight N type as described in US Pat. No. 4,064,521. Region 22c of the N ⁺ type hydrogenated amorphous silicon layer has a thickness of approximately 1-4 Å and is in contact with intrinsic region 22b.

A tunnel junction layer 24 is provided between the active layers 22 and 26.
are formed, forming a current path toward the back substrate 28 through these layers. This bonding layer 24 has a thickness of about 0.2 to 1.5 Å and is made of PtSiO ₂ cermet,
Consists of a thin metal layer and PtSiO ₂ cermet, or a thin metal layer. The metal layer is platinum, which allows sunlight to pass through.
It can be made of metal such as titanium or nickel. If the insulator is made sufficiently thin, electrons can pass through it due to the tunnel effect, but if the tunnel junction layer 24 becomes an insulator, the performance of the series junction solar cell 10 will deteriorate.

Theoretically, this tunnel junction layer 24 is a conductivity converter of sufficient N type and P type, with a region 22c of the N ⁺ type hydrogenated amorphous silicon layer and a region 26a of the P ⁺ type hydrogenated amorphous silicon layer. If the active layers 22 and 26 can be removed, a tunnel junction will occur between the active layers 22 and 26, so that they can be removed. solar cell 1
0 can be manufactured without the tunnel junction layer 24, but
Doping levels high enough to create tunnel junction conditions with properties comparable to PtSiO ₂ cermets and similar materials or thin metal layers have not been obtained in hydrogenated amorphous silicon.

The second active layer 26 consists of regions 26a, 26b, and 26c of hydrogenated amorphous silicon of different conductivity types. Area 26a is similar to area 22a,
It is doped with a suitable P ⁺ type conductivity converter.
The area 26b is the area 22b, and the area 26c is the area 2.
They are made in the same way as 2c. area 26
a, 26b, and 26c may be deposited at elevated temperatures to form a layer with a lower hydrogen concentration and lower bandgap energy than active layer 22. The thickness of this second active layer 26 is such that the current generated in the first and second active layers 22, 26 is equal, since the total current of the solar cell is limited by the lower current of the active layer 22 or 26. You have to adjust accordingly.

The absorption efficiency of hydrogenated amorphous silicon solar cells approaches a constant value when the thickness of the intrinsic region becomes approximately 50 Å. Increasing the thickness of this region further in a series junction solar cell will increase the absorption of sunlight but will not improve the performance of the solar cell, it will only eliminate the need for the next layer to absorb sunlight. . Therefore, the thickness of each intrinsic region must be reduced as the number of hydrogenated amorphous silicon layers in the stack increases.

Finally, N ⁺ type hydrogenated amorphous silicon region 26
Titanium, which adheres closely to c and forms a good ohmic contact.
Metal layer 28 made of material such as molybdenum, niobium, etc.
is deposited on region 26c. Wirings 15 and 29 are connected to the metal layers 14 and 28, and the current generated by irradiating the solar cell 10 is extracted.

FIG. 2 shows another embodiment of the invention. Solar cell 50 in FIG. 2 has substrate 12 on the opposite side from solar cell 10 in FIG. Each layer and region of the solar cells 50, 10 is designated by the same reference numeral in FIG. Since the sunlight 100 does not need to pass through the substrate 12, the substrate can be made of a metal plate such as stainless steel, molybdenum, titanium, etc. Furthermore, the metal layer 14 for making the substrate of the solar cell conductive is not necessary in the solar cell 50.

When the solar cell 50 has an inverted structure, the cermet layer 52 of the thick film ballast resistor, the grid electrode 56,
A TCO antireflection layer 58 may be provided. Cermet layer 52 is effective in reducing electrical shorts as described in U.S. Pat. No. 4,162,505.

The solar cells of FIGS. 1 and 2 can be fabricated in several ways. The metal layer 14 is applied to the substrate 1 by vapor deposition or other known methods such as electrochemical deposition.
2. The cermet layer is described in the above U.S. patent no.
Manufactured based on the method described in No. 4167015.
The hydrogenated amorphous silicon layers 22 and 26 are deposited by glow discharging silane or other suitable silicon-hydrogen compound as described in the above-mentioned U.S. Pat. do. Layers 22, 26 can also be formed with a high frequency discharge device. Examples of suitable conditions for high-frequency discharge include: high-frequency power of approximately 0.5 W/cm ² or less, target area of approximately 160 cm ² , gas pressure of approximately 2 to 5 x 10 ^-2 mmHg, silane flow rate of approximately 30 secm, and device temperature of approximately 200 to 350°C. There is.
The P-type region of layer 22 or 26 is formed by mixing diborane as a P-type dopant in the coating gas at a concentration of about 10 ^-4 to ^{10 -5} %. The N ⁺ type region is N
Phosphine as a type dopant at a concentration of approximately 2×10 ^-3 %
Mix and form. Finally, a metal layer is applied by vapor deposition, radio frequency sputtering or other suitable method to complete the cell.

The overall voltage of the solar cell produced by the method described above is slightly lower than the sum of the voltages from the individual active layers. This difference in voltage is considered to be due to light absorption of sunlight passing through the hydrogenated amorphous silicon layer.

Next, this invention will be explained in more detail by giving examples, but this invention is not limited to these examples, and modifications that can be easily inferred by those skilled in the art belong to the technical scope of this invention.

Example 1 Approximately 5.8 Å transparent conductive oxide (TCO) indium oxide is placed on a low soda glass plate approximately 1.6 mm thick.
After being deposited by high frequency sputtering at a pressure of ×10 ^-2 mmHg, it was annealed at about 400°C for about 24 hours. this
The TCO layer has a sheet resistance of approximately 10Ω/□ or less. Next, platinum cermet (PtSiO ₂ ) was deposited on this TCO layer to a thickness of about 1.1 Å by high-frequency sputtering. Cermet contains about 10-15% platinum by volume, has a transmittance of about 95% in the visible light range, and has a resistance of about 10 ⁶ Ω-cm at room temperature.
It has the following specific resistance. on this platinum cermet
A first hydrogenated amorphous silicon layer with P ⁺ , intrinsic, and N ⁺ regions was deposited as described in U.S. Pat. Nos. 4,167,015 and 4,162,505. The P ⁺ type region is in contact with the cermet, and other regions are adjacent to the P ⁺ type region as described above. P ⁺
The thickness of the type and N ⁺ type regions is approximately 4.1 Å, approximately
4.5 Å, and the thickness of the intrinsic region is about 24 Å. A PtSiO ₂ cermet layer was again deposited as a tunnel junction layer, and a second hydrogenated amorphous silicon layer with three regions was formed thereon. The second cermet layer is approximately 0.9 Å thick. The thickness of the P ⁺ type, intrinsic and N ⁺ type regions is approximately 3.2 Å and approximately 50 Å, respectively.
and approximately 4.5 Å. Finally, an aluminum electrode approximately 20 Å thick was deposited on the N ⁺ type region of this second layer.

When this series-configured stacked solar cell is irradiated with a light source with a light intensity of 1 solar constant, the open circuit voltage (abbreviated as V _pc ) is approximately 1.21 V, and the short circuit current (abbreviated as J _sc ) is approximately 1.71 V.
Values in mA/cm ² were obtained.

Example 2 A series junction solar cell was fabricated using the method described in Example 1. In this example, a titanium layer approximately 0.5 Å thick is deposited on the N ⁺ type region of the first hydrogenated amorphous silicon layer before depositing the PtSiO ₂ cermet and the second hydrogenated amorphous silicon layer. It was covered. PtSiO ₂ layer for both amorphous silicon layers, P ⁺ type region and
The N ⁺ type regions were the same and had thicknesses of about 1.8 Å, about 3.6 Å, and about 3.6 Å, respectively. The thicknesses of the first and second intrinsic regions were about 9.1 Å and about 50 Å, respectively. The TCO layer is approximately 2.5 Å thick, and the titanium layer in contact with the second amorphous silicon layer is approximately 2.6 Å thick.
It was hot. When this solar cell was irradiated with a light source with a solar constant of 1, a V _pc of about 1.34 V and a J _sc of about 2.5 mA/cm ² were obtained.

Comparative Example 1 A series junction solar cell was fabricated using the method described in Example 1. In this example, there was no cermet or metal layer separating the first and second hydrogenated amorphous silicon layers. The thickness of the cermet layer on the front side is about 1.5 Å, both P ⁺ type and N ⁺ type of amorphous silicon layer.
The thickness of the mold region was approximately 4.5 Å. 1st and 2nd
The thickness of the intrinsic region is about 7.6 Å and about 5.9 Å, respectively.
It was set as Å. This solar cell had a V _pc of about 0.96 V and a J _sc of about 1.98 mA/cm ² . A low V _pc value indicates the presence of a reverse diode between the N ⁺ type region of the first amorphous silicon layer and the P ⁺ type region of the second amorphous silicon layer.

Example 3 A series junction solar cell with five hydrogenated amorphous silicon layers was fabricated using the method described in Example 1.
The thickness of the intrinsic region is sequentially thicker in each layer, i.e. 4.5 Å, respectively, to equalize the cell current in each layer.
They were 4.5 Å, 5.3 Å, 6.8 Å, and 36.3 Å. The obtained solar cell had a V _pc of about 2.4 V and a J _sc of about 0.58 mA/cm ² .

Comparative example 2 A series junction solar cell was fabricated using the method of Example 1.

In this example, three hydrogenated amorphous silicon layers were separated from each other by two silicon nitride insulating layers approximately 0.2 Å thick. Amorphous silicon layer N ⁺ type, P ⁺
The thickness of the type and intrinsic region is 3.8 Å and 14.5 Å, respectively.
Å, 3.8 Å. The insulating layer was deposited by glow discharge, and the cell's V _pc was approximately 0.2V. Although the insulating layer is thin enough to allow electrons to tunnel through it, the performance of the solar cell is incomparable to the V _pc of the previous example.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view showing a series junction type solar cell in which hydrogenated amorphous silicon layers separated by a tunnel junction are stacked, and Figure 2 is a series junction type amorphous silicon solar cell equipped with a grid electrode and a thick film cermet. FIG. DESCRIPTION OF SYMBOLS 10... Amorphous silicon solar cell, 12... Substrate, 20... Active substrate, 22, 26... Amorphous silicon layer, 24... Tunnel junction layer, 28... Cermet layer, 29... Contact means.

Claims (1)

[Claims] 1 an active substrate having a conductive substrate, a first major surface in ohmic contact with the substrate and a second major surface opposite the first major surface, and a high-work substrate in contact with the first major surface; a functional metal cermet layer and means for electrically contacting the metal cermet layer, the active substrate comprising a plurality of hydrogenated amorphous silicon layers having regions of different conductivity types and arranged in a stacked structure. An amorphous silicon solar cell comprising a transparent tunnel junction layer of high work function metal cermet or metal disposed between the amorphous silicon layers.