JPS5850034B2 - photovoltaic device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to improving the non-reflection conditions of photovoltaic devices, and in particular to fine tuning the non-reflection properties of photovoltaic devices having thin film active regions.

The total power generation efficiency in a photovoltaic device such as a solar cell is directly dependent on the efficiency with which light is incident on and absorbed by the active region of the solar cell.

The active region of a photovoltaic device is the region in which light or solar radiation is absorbed and thereby generates charge carriers.

Power in solar cells is obtained by collecting these light-generated charge carriers by the active region or by semiconductor junctions within it.

In the case of single-crystal silicon solar cells, the diffusion distance of the charge button is relatively long, at most 100μ.

Therefore, electron-hole pairs generated within about 100 microns from the semiconductor junction will contribute to the power generated by the solar cell.

In this case, it is necessary to provide a broadband antireflection layer on the surface of the solar cell in order to effectively allow light to enter the inside of the solar cell.

This anti-reflection layer usually consists of a thin film of dielectric material formed on the surface to suppress reflection of light and improve the transmittance of light into the solar cell.

In general, for many photovoltaic devices with thin active regions of a few microns or less in thickness, the diffusion distance of photogenerated charge carriers is significantly reduced for photovoltaic devices with thick active regions of good crystalline materials such as single-crystal silicon. This is significantly shorter than the diffusion distance of power equipment.

This short diffusion distance is due to the fact that the thin film material is polycrystalline or amorphous and thus has low charge carrier mobility.

The thin-film active region of a photovoltaic device can be formed from materials such as amorphous silicon created by glow discharge in silane, but is generally made of holes in amorphous silicon created by glow discharge in silane. The diffusion distance is very short (only 3μ) compared to 100μ in single crystal silicon.
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However, photovoltaic devices with thin film active regions as described above are cheaper to fabricate and are therefore economically advantageous compared to photovoltaic devices with thicker active regions made of higher quality crystals. be.

The solar spectrum of Air Mass-1 (AMl) contains a considerable portion of its energy in photons with a wavelength of 6000 Å or more, which is called the high wavelength part of the solar spectrum.
Solar radiation at wavelengths greater than oA, in many thin-film active region materials, including amorphous silicon created by glow discharge in silane, has optical absorption lengths that exceed the diffusion lengths of charge carriers occurring in these materials. show.

This means that if a solar cell with a thin active region is manufactured using the same design theory as a solar cell with a thick active region,
Since none of the high wavelengths of the solar spectrum are absorbed by the active region of this thin film, the efficiency of the solar cell drops to zero.

Therefore, it is desirable to design photovoltaic devices with thin film active regions that have high absorption in the high wavelength portion of the solar spectrum.

The photovoltaic device includes a main body having an active region and a light-transmitting layer having an incident surface for collecting solar radiation, and in the present invention, the thickness of the light-transmitting layer is adjusted to reflect solar radiation of a first wavelength. In addition, the total thickness of the light-transmitting layer and the active region is adjusted so that the incidence surface exhibits non-reflection property against solar radiation having a second wavelength longer than the first wavelength. choose the way to present it.

The present invention will now be described in more detail with reference to the accompanying drawings.

FIG. 1 shows a first embodiment of the photovoltaic device of the present invention.
Indicated by

The photovoltaic device 10 is illustrated as a Schottky barrier solar cell, but as will be apparent to those skilled in the art, the present invention may be a solar cell or a photodetector;
It may also have other semiconductor junctions such as an IN junction structure or a heterojunction structure.

Schottky barrier solar cell 10 has a body 11 that includes a thin film active region 12 .

The thickness of this thin film active region 12 is about 0.1~10μ, the active region means the part of the body 110 that can absorb sunlight and generate charge carriers, and the thin film has good light absorption properties. Made from semiconductor materials.

For convenience of explanation of this invention, the active region 12 is assumed to be made of amorphous silicon produced by glow discharge in silane, but the active region 12 may be made of other thin film semiconductor materials such as polycrystalline gallium arsenide or cadmium tellurium. It can also be made from.

One surface of active region 12 is in contact with substrate 14 .

This substrate 14 is made of a material, such as aluminum, which reflects incoming solar radiation and at the same time makes good electrical contact with the active region 12, serving as an electrical contact to the active region 12.

A thin metal film 18 is deposited on the surface 13 of the active region 120 opposite the substrate 14 .

The metal thin film 18 is a continuous metal thin film capable of forming a Schottky barrier or surface barrier type junction with the amorphous silicon active region 12 on the surface 13.

Since undoped amorphous silicon is normally only slightly N-type, the metal thin film 18 must be a metal with a high work function of 4.5 eV or higher to form a Schottky barrier.

Metals with high work functions suitable for this purpose include platinum, indium, rhodium, and palladium.

Furthermore, the metal film 18 must be so thin that it is at least semitransparent to solar radiation and must have a relatively low layer resistance.

It is clear to those skilled in the art that the light transmission properties and low thin film resistance required for the metal thin film 18, together with the fact that the metal thin film is a continuous thin film, are factors that determine the thickness of the thin film.

A light-transmitting layer 20 having an entrance surface 22 through which solar radiation 24 can enter the base body 11 is arranged on the side of the thin metal film 18 facing away from the active region 12 .

This light transmitting layer 20 is made of titanium dioxide, zirconium oxide,
It is comprised of a dielectric material that is substantially transparent to solar radiation, such as silicon nitride.

The active region 12 of amorphous silicon fabricated by glow discharge in silane has an absorption curve for the solar spectrum such that solar radiation in the range of about 3500 to 5000 is strongly absorbed (hereinafter this region will be referred to as the high absorption region). ), but the absorption by the active region 12 of amorphous silicon is 5000
It decreases for solar radiation with wavelengths longer than Å (hereinafter this region will be referred to as the low absorption region).

One function of the light transmitting layer 200 is to allow solar radiation in the high absorption region of the absorption curve of amorphous silicon to enter the entrance surface 22 with little reflection.

Therefore, the thickness of the light-transmitting layer 20 must be selected so that the sunlight having the first predetermined wavelength in the high absorption region is not reflected at the incident surface 22.

This first predetermined wavelength is selected so that the solar radiation in the high absorption region is absorbed to the maximum extent in the active deposit region 12 and the first non-reflection condition is satisfied.

The thickness of the metal thin film 18 slightly influences the thickness of the light transmitting layer 20 when this first non-reflection condition is met.

The most useful point of this invention is that the light transmitting layer 20 and the active region 1
2 is adjusted to the second predetermined wavelength of the low absorption region, and the absorption ability of the device 10 is improved even in this low absorption region.

This second predetermined wavelength is selected to maximize the amount of solar radiation absorbed by the active region 12 in the low absorption region.

This improvement in absorption ability is due to the fact that the second non-reflection condition is satisfied at the entrance surface 22 for solar radiation in the low absorption region.

If the active region 12 is made of amorphous silicon by glow discharge in silane, the thickness is such that the light-transmissive layer 20 is non-reflective to solar radiation at a wavelength of about 5oooA (first predetermined wavelength). It is desirable that the active region 12 and the light transmitting layer 20 have a combined thickness such that the active region 12 and the light transmitting layer 20 are non-reflective to solar radiation at a wavelength of about 6,500 holes (the second predetermined wavelength).

In this case, the second predetermined wavelength is longer than the first predetermined wavelength.

As is well known to those skilled in the art, when a particular wavelength of radiation is selected in the design of an anti-reflection layer or condition, the non-reflection condition operates such that the reflection at the input surface is minimized at that wavelength.

Near this selected wavelength (slightly longer wavelength side or shorter wavelength side)
The incident surface has a relatively low reflectance for solar radiation, but as the radiation wavelength deviates from the selected wavelength, the reflectance of the incident surface increases.

FIG. 2 shows the reflectance of the incident surface 22 for the first and second predetermined wavelengths of 5,000 A and 6,500 holes of 4,250 to 6.
It has been shown to be relatively low for solar radiation in the 750 wavelength range.

During operation of solar cell 10 , solar radiation near a second predetermined wavelength that is not absorbed during its initial passage through active region 12 is reflected by reflective substrate 14 back to active region 12 and then passes through active region 12 . It is absorbed during

However, solar radiation near the second predetermined wavelength may cause the active region 12 to
Even if it is not absorbed in the second pass of
does not reflect solar radiation in the vicinity of a predetermined wavelength, so this solar radiation cannot exit the device 10 through the entrance surface 22.

This solar radiation, which is only slightly absorbed by the active region 12, is therefore mostly trapped inside the light-transmissive layer 20 and the active region 12, thereby increasing the probability of absorption in the active region 12.

This trapping of solar radiation does not normally occur for the first predetermined wavelength of radiation that is more likely to be absorbed, since it is absorbed on the first pass through the active region 12.

FIG. 3 shows a comparison of the solar radiation absorption efficiency of a conventional solar cell having an amorphous silicon active region fabricated by glow discharge in silane and a solar cell of the present invention.

Curve A in the figure shows the absorption efficiency of a conventional amorphous silicon solar cell, and curve B shows the absorption efficiency of the solar cell 10 of the present invention.

The conventional solar cell selected here for comparison has an active region approximately 10 μm thick fabricated by glow discharge in silane, with the active region formed adjacent to a substrate of a material such as aluminum. ing.

A thin film of platinum is deposited on the surface of the active region opposite the back contact to form a Schottky barrier.

The thickness of this platinum thin film is approximately 75 mm.

An antireflection layer of titanium oxide (T i02 ) with a thickness of about 340 nm is formed on the platinum thin film surface opposite to the active region, so that it exhibits a non-reflective state against solar radiation with a wavelength of 500A. .

Light transmitting layer 20 and metal thin film 18 of solar cell 10 of this invention
is made of the same material and has the same thickness as the conventional solar cell described above, and also exhibits no reflection against solar radiation of 500A.

However, the active region 1 also has a thickness of about 3250 Å and is located on an aluminum reflective substrate 14, and this active region 12 has a second non-reflective state for solar radiation with a total thickness of about 6500 Å with the light-transmitting layer 20. Curve B showing the solar cell 10 of the present invention in FIG.
As shown by the diagonal line below, there is clearly an increase in the absorption of solar photon flux.

While the photon conversion efficiency of a conventional amorphous silicon solar cell is approximately 28%, the conversion efficiency of the solar cell 10 of the present invention is approximately 33%, which is 18% higher than the initial efficiency of the conventional solar cell. % has also increased.

When manufacturing the solar cell 10, a substrate 14 of, for example, aluminum is placed in a conventional glow discharge apparatus, and a glow discharge is performed in a substantially silane atmosphere to form an active region 12 of amorphous silicon on the substrate 14. After the deposition, the substrate 14 including the active region 12 is placed in a conventional vapor deposition apparatus to deposit a metal thin film 18 on the active region 12, and further to deposit a light transmitting layer 20 on the metal thin film 18.

FIG. 4 shows a second embodiment of the photovoltaic device of the present invention.
Indicated by 0.

As explained in the first embodiment, the second embodiment is also assumed to be a Schottky barrier type solar cell.

The body 111 of the solar cell 110 has an active region 112 of a thin film semiconductor material with good light absorption properties.

In this case as well, for convenience of explanation, it is assumed that the active region 112 is made of amorphous silicon formed by glow discharge in silane.
12 is a first layer 113 of undoped amorphous silicon formed by glow discharge in silane, and adjacent to this first layer 113, formed by glow discharge in a mixed gas of silane and doping gas. a second layer 115 of doped amorphous silicon.

The second layer 115 is preferably highly doped and contains 0.1 atomic percent phosphorous.

If the first layer 113 is about 3 ooo long, the second layer 115
is usually about 200 people thick.

The active region 112, so to speak, the first layer 113 of the substrate 1
14, this substrate 114 is also made of a material that reflects solar radiation like the substrate 14 of the first embodiment, but in this case it also forms a Schottky barrier or surface barrier type junction with the active region 112. It is also a material that can

Suitable materials for the substrate 114 include rhodium, iridium,
Examples include metals such as platinum.

On the opposite side of the second layer 115 from the substrate 114 is a metal layer 117 made of a gold material capable of making good electrical or ohmic contact to the second layer 115.

Platinum is one example of this material. Metal layer 117 is also substantially transparent to solar radiation and is of relatively thin thickness, typically on the order of about 50 amps.

A solar radiation transmitting layer 120 having an incident surface 122 is formed on the surface of the metal layer 1170.

Similar to the transparent layer 20 of the first embodiment, this transparent layer 120
is also substantially transparent to solar radiation while being able to make good electrical contact with metal layer 117.

An example of a material that can satisfy the conditions for the light transmitting layer 1200 is indium tin oxide.

Metal layer 1170 purpose is light transmission layer 120 and active region 112
The purpose of this is to form electrical contact between the two and avoid creating a rectifying junction.

Furthermore, since there is a highly doped layer such as the second layer 115 adjacent to the metal layer 117, the metal layer 117 and the active region 11
A good electrical contact is made between the two.

Since the first layer 113 of undoped amorphous silicon is slightly N-type, the second layer 115 must be of the same conductivity type and heavily doped.

For example, second layer 115 can be doped with an N-type impurity such as phosphorous.

Similar to the first embodiment of the present invention, the light transmitting layer 120 of the solar cell 110 has a thickness that satisfies the first non-reflection condition at the incident surface 122 for a first predetermined wavelength, and furthermore, the light transmitting layer 120 has a thickness that satisfies the first non-reflection condition at the incident surface 122 for a first predetermined wavelength. The combined thickness of layer 120 and active region 112 is selected to satisfy a second non-reflection condition at entrance surface 122 for a second predetermined wavelength.

As in the case of the first embodiment, the thickness of the metal layer 117 is the same as that of the first embodiment.
This has a slight influence on the thickness of the light transmitting layer 120 that satisfies the non-reflection condition.

In the second embodiment, a metal layer 117 for preventing electrical rectification was provided between the light transmitting layer 120 and the active region 112.
It is easily conceivable that this rectifying effect can be prevented by the doped second layer 115 alone, ie, without the metal layer 117.

Although the first and second embodiments of the invention differ in the location of the Schottky barrier within the body, their operation is essentially the same.

When using an active region 112 of amorphous silicon formed by glow discharge in silane and a substrate 114 of platinum, the thickness of the metal layer 117 is approximately 50 Å, and the thickness of the light transmitting layer 120 of indium tin oxide is approximately 525 mm. Then, the first non-reflection condition is satisfied at a wavelength of about 5000 people.

Further, the thickness of the active region 112 is about 320 OA, and the total thickness of the active region 112 and the light transmission layer 120 is about 6500 OA.
A second non-reflection condition is formed at the entrance surface 122 for solar radiation at the wavelength of the hole.

When manufacturing the solar cell 110, first the platinum substrate 11
4 is placed in a conventional glow discharge apparatus and a glow discharge is performed in a substantial silane atmosphere to deposit the first layer 113, and then a doping gas such as phosphine is introduced into the glow discharge atmosphere to deposit the second layer 113. 115 is deposited, and then the substrate 114 with the active region 112 is placed in a conventional deposition apparatus to deposit a metal layer 117 and a light transmitting layer 120.

In the first embodiment of the invention, the means for making electrical contact to the active region 12 is the substrate 14 and the thin metal film 18, whereas in the second embodiment, the means for making the electrical contact to the active region 112 is the substrate 14. 114 and a light transmitting layer 120 in series with the metal layer 117.

The invention readily shows that in the first and second embodiments of the invention, the ability of the entrance surface to capture unabsorbed solar radiation can be improved by roughening or tilting the substrate surface in contact with the active region. It can be predicted.

The photovoltaic device of the present invention provides a design standard for non-reflection conditions to realize a device with excellent solar photon conversion efficiency.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a first embodiment of the photovoltaic device of the present invention. FIG. 2 is a graph of solar radiation wavelength versus reflectance for a photovoltaic device of the present invention in which the first predetermined wavelength is about 5,000 and the second predetermined wavelength is about 6,500. FIG. 3 is a chart showing a comparison of solar radiation absorption efficiency between a conventional solar cell and a photovoltaic device of the present invention. FIG. 4 is a sectional view showing a second embodiment of the photovoltaic device of the present invention. 10.110...Solar cell, 11,111...
...Main body, 12,112...Active region, 1
4,114... Board, 18,117...
・Metal thin film, 20°120...Light transmission layer, 22
, 122... Light incidence surface, 113... First layer of active region, 115... Second layer of active region.

Claims (1)

[Scope of Claims] 1. Having an entrance surface through which solar radiation can be incident, the entrance surface having a thickness such that the entrance surface exhibits substantially non-reflection for incident solar radiation at a first predetermined wavelength of the solar radiation spectrum. a solar radiation transmitting layer having a thickness of about 100 nm, and a total thickness of the transmitting layer such that the incident surface has a solar radiation transmitting layer having a thickness such that the total thickness of the solar radiation transmitting layer is such that the incident surface has a thickness that is substantially equal to the incident solar radiation having a second predetermined wavelength longer than the first predetermined wavelength in the solar radiation spectrum. A photovoltaic device having a main body including an active region exhibiting non-reflective properties. 2. A solar width pair having an entrance surface through which solar radiation can be incident, and having a thickness such that the entrance surface exhibits substantially non-reflection to the incident solar radiation at the first predetermined wavelength of the solar radiation spectrum. a transmissive layer and a combined thickness of the transmissive layer such that the incident surface is substantially non-reflective to incident solar radiation at a second predetermined wavelength in the solar radiation spectrum that is longer than the first predetermined wavelength; and a metal layer located between and adjacent to the transparent layer and the active region,
The metal layer is comprised of a material capable of forming a surface barrier type junction with the active region. 3. The photovoltaic device according to claim 1, wherein the active region includes a first layer and a second layer of the same conductivity type, and the second layer has a higher impurity concentration than the first layer. .