JP2934355B2 - Photoelectric conversion element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric conversion device.

[0002]

2. Description of the Related Art In order to improve the efficiency of a photoelectric conversion element, it is important to increase light absorption and increase short-circuit current. For example, in an amorphous solar cell, for this purpose, amorphous silicon germanium having a lower band gap is conventionally used as the i-layer.
However, since amorphous silicon germanium has a higher defect density in the film than amorphous silicon and is applied to devices, some device structure ingenuity has been required. As such a method, a method in which an amorphous silicon layer and an amorphous silicon germanium layer or an amorphous germanium layer are stacked is proposed.

[0003] The reason for such a laminated structure will be described below. For example, when amorphous silicon germanium is used as an amorphous silicon alloy material,
As the composition ratio of germanium increases in order to increase the short-circuit current, the film quality deteriorates. As a result, despite the increase in light absorption, the generated holes and electrons recombine when traveling through the film toward the p-layer or the n-layer, making it difficult to collect and increasing the short-circuit current. do not do.
For this reason, there is a limit that the short-circuit current does not increase even when the band gap is smaller than a certain value. That is, in order to increase the short-circuit current by increasing light absorption, it is necessary to sufficiently collect generated carriers. This problem is solved by combining the low film quality region and the high film quality region. In such a structure, if the region with low film quality is sufficiently thin,
Carriers generated in that region may travel a short distance in a region with low film quality, and recombination can be extremely reduced. That is, by forming a structure in which the carrier generation region and the carrier traveling region are separated, amorphous silicon germanium having a lower band gap can be used.

The improvement of the characteristics of the amorphous silicon germanium layer using such a laminated structure or the photoelectric conversion element using the same is described in, for example, JP-A-2-65121 and JP-A-3-74778. On the other hand, a structure in which a band gap is provided with a profile has been used to improve device characteristics. The bandgap profile technique is described in, for example, Japanese Patent Application Laid-Open No. 1-71182. As shown in FIG. 3, the bandgap profile technique changes the bandgap in the i-layer continuously, the bandgap is wide at both ends of the p-layer and the n-layer, and the bandgap is higher than that of the p-layer on the light receiving surface side. This is a technique for forming a drum-shaped band gap structure in which the narrowest region is provided. Such a structure can be formed by changing the composition ratio of a material such as amorphous silicon carbon and amorphous silicon germanium. With such a structure, characteristics are improved by three kinds of effects. First, the number of carriers generated on the light-receiving surface side, that is, the p-layer side, is increased to facilitate hole collection. Next, by placing a low-quality film on the p-layer side and a high-quality film on the n-layer side, hole transport characteristics are improved. Finally, carrier collection is improved because the internal electric field is enhanced by the bandgap slope.

[0005]

In the above-described band gap profile technique, the film quality is deteriorated. For example, when amorphous silicon germanium is used, the band gap can be reduced only to about 1.4 eV in a low band gap region. When amorphous silicon carbon is used, the band gap can be increased only up to about 2.0 eV in a high band gap region. Therefore, there is a limit in improving the short-circuit current, open-circuit voltage, and reliability.

On the other hand, in the above-mentioned laminated structure element, since an amorphous silicon layer is used, light absorption of this layer is reduced, and light utilization efficiency is reduced accordingly. In addition, since amorphous germanium used for the other layer tends to have a low film quality, it is difficult to effectively extract generated carriers. For this reason, the short-circuit current, the open-circuit voltage, and the improvement of the reliability were also limited. Furthermore, since different materials are laminated, there is a disadvantage that the manufacturing method is complicated.

[0007] It is an object of the present invention to improve the light use efficiency and reliability of the photoelectric conversion element and to simplify the manufacturing method by adopting a laminated structure in which the material and the film thickness are optimized.

[0008]

The photoelectric conversion element of the present invention is a photoelectric conversion element having a pin structure, wherein a substantial part of an i-layer is formed of an amorphous silicon alloy material, and the composition ratio of the alloy material is By changing the thickness, a plurality of layers having different composition ratios are stacked so as to alternately generate the magnitude of the band gap in the thickness direction of the i layer, and the thickness of the layer having a small silicon composition ratio is 1 to 8 nm. It is characterized by. Here, the substantial part of the i-layer means the i-layer except for the vicinity of the p / i interface and the n / i interface. This is because at the p / i interface and n / i interface, a region of 2 to 30 nm from the interface may be composed of a different material or band gap structure in order to reduce carrier recombination near the interface. It is. In the plurality of layers, adjacent layers have different composition ratios from each other, and there may be a region where the composition continuously changes at the interface of each layer in manufacturing, but the composition ratio is constant in each layer. It has become. The thickness of the i-layer is preferably 50 to 150
nm. Further, on the n-layer side of the substantial i-layer, a substantial
Among the layers constituting the i-layer, a layer having a silicon composition ratio equal to or higher than the layer having the largest silicon composition ratio is substantially used.
It is preferable to provide an i-layer made of the same material as the i-layer and having a high silicon composition ratio. Preferably, the i-layer is formed of amorphous silicon germanium or amorphous silicon carbon.

[0009]

In the photoelectric conversion device of the present invention, a plurality of layers having different composition ratios are laminated so as to alternately generate a band gap in the thickness direction of the i-layer. The element characteristics are improved as compared with the case where a region having a low film quality is continuously formed as a carrier traveling region. This is because, since the silicon composition ratio is small, the distance that the carrier travels in a layer having a low film quality may be short, and the recombination is extremely reduced. Note that, depending on the material forming the amorphous silicon alloy material, the band gap may be reduced or increased when the silicon composition ratio is reduced. Examples of the former material include germanium and tin that reduce the band gap, and examples of the latter material include carbon, nitrogen, and oxygen that increase the band gap.
Regardless of which material is used, the film quality decreases as the silicon composition ratio decreases. When the thickness of the layer having a low silicon composition ratio is smaller than 1 nm, the layer is alloyed and has characteristics close to a device having a uniform band gap. When the thickness is larger than 8 nm, as shown in FIG. , Exceeding the limit that can be used as an element. As shown in FIG. 4, 8 nm has a fill factor of 0.42,
For example, when used as an optical sensor, it is not used with a large positive bias, so that such characteristics can be used. Further, since a substantial part of the i-layer is formed of an amorphous silicon alloy, the device can be manufactured continuously only by changing the gas flow rate. Further, since the composition ratio of each layer is constant, the control of the gas flow rate is simplified, and the fabrication of the device is further facilitated. Further, the photoelectric conversion element of the present invention,
Since the element has the above-described laminated structure, light can be absorbed with a small i-layer thickness. Therefore, the thickness of the i-layer is set to 5
When the thickness is set to 0 to 150 nm, the thickness becomes thinner than the conventional band gap profile element of 200 nm. By thinning the i-layer, the traveling distance of the carrier is shortened, the internal electric field is increased, and the reliability is further improved. In addition, regions with a large silicon composition ratio
When a predetermined thickness is provided on the n-layer side, light absorption on the n-layer side is reduced and the travel distance of holes is reduced as compared with a structure in which different band gaps are alternately repeated in the entire i-layer. Thereby, the fill factor is further improved.

Further, when amorphous silicon germanium is used as the amorphous silicon alloy material of the i-layer, the amount of light absorption increases and the short-circuit current increases. Further, when amorphous silicon carbon is used as the amorphous silicon alloy material for the i-layer, the band gap increases and the open-circuit voltage increases.

[0011]

The present invention will be specifically described below with reference to examples.

An embodiment relating to a single-layer type amorphous silicon germanium element which is an example of the present invention will be described. FIG. 1 shows a schematic diagram of the single-layer amorphous silicon germanium element of the embodiment, and FIG. 2 shows a schematic diagram of the band gap structure. First, the structure of the single-layer amorphous silicon germanium element of the embodiment will be described. 50 nm Ti layer 2, 100 nm Ag layer 3 and 100 nm ZnO layer 4 on stainless steel substrate 1
After forming an amorphous silicon n-layer 5 with a thickness of 100 nm, an i-layer is formed. Next, the structure of the i-layer will be described. Wide band gap amorphous silicon germanium i-layer 6
Has a band gap of 1.56 eV, a film thickness of 2.5 nm, and a narrow band gap amorphous silicon germanium i-layer 7.
Has a band gap of 1.34 eV, a thickness of 2 nm, and a thickness of each boundary region of 0.25 nm. The number of times of each repetition is 12, and the film thickness of the modulation region is 60 nm in total. There is a region where the band gap linearly decreases from 1.75 eV to 1.56 eV from the p-layer side to the band gap modulation region, and its film thickness is 15 nm. The band gap is constant at 1.56 eV from the modulation region to the n-layer side, and the film thickness is 60 nm. An amorphous silicon carbon p layer 8 having a thickness of 10 nm and a transparent conductive film 9
The structure is such that ITO of 60 nm and Al of 600 nm as the metal collecting electrode 10 are sequentially laminated. In the embodiment, the substrate 1 is not particularly limited to stainless steel, but can be manufactured using a substrate such as glass. When a light-transmitting substrate such as glass is used, a structure in which light is incident from the light-transmitting substrate side may be employed. The Ti layer 2 is used for improving the adhesion to the substrate. Although the Ag layer 3 is used as a light reflection layer on the back surface, another metal having a high reflectance can be used instead. It is desirable that the surface has an uneven structure. That is, the light reflected by the back surface is scattered by the uneven structure, and the optical path length of the light in the film is extended, so that the light absorption increases. This is because the insertion of the ZnO layer 4 also has the effect of further increasing light scattering.
The amorphous layers are stacked in the order of nip,
When the translucent substrate side is used, the order is pin.
In any case, it is desirable that the light incident side be the p-layer. The transparent conductive film 9 is not limited to ITO, and may be made of SnO ₂ or ZnO. The metal collecting electrode 10 may be made of a material other than Al, but is preferably a metal having good electric conductivity.

Next, the manufacturing method of the present invention will be described. First, a Ti layer 2 is formed to a thickness of 50 nm on a stainless steel substrate 1 by electron beam evaporation. After that, the Ag layer 3 is deposited at 100 ° C. at a substrate temperature of 350 ° C. to form irregularities. A ZnO layer 4 is formed thereon to a thickness of 100 nm. An amorphous silicon n layer 5, an amorphous silicon germanium i layer,
Plasma CVD of amorphous silicon carbon p layer 8
The layers are sequentially laminated by a method. As the conditions for forming the amorphous layer, the substrate temperature is 200 ° C., the RF power is 10 W, and the pressure is 0.1 to
0.3 Torr and the gas flow conditions are shown in Table 1.

[0014]

[Table 1]

After forming the amorphous silicon n layer 5 to 100 nm, a wide band gap amorphous silicon germanium i layer 6 and a narrow band gap silicon germanium i layer 7 are formed. At this time, it is formed by automatically controlling the flow rates of the GeH ₄ gas and the SiH ₄ gas by a computer so as to form a band gap structure as shown in FIG. An amorphous silicon carbon p-layer 8 is formed to a thickness of 10 nm on the i-layer thus formed. afterwards,
IT that is transparent conductive film 9 by magnetron sputtering
O is formed to 60 nm, and a collector electrode of Al10 is formed to 600 nm by electron beam evaporation. Next, a method for forming the i-layer will be described in detail.

The flow rates of the GeH ₄ gas and the SiH ₄ gas are controlled by a flow control device connected to a mass flow controller. This is an integrated power supply and computer,
Each may be controlled separately. The ordinary photoelectric conversion element does not require such a complicated band gap structure as in the present embodiment, and thus does not require such a flow rate control device. In order to perform well, it is essential to use such a flow control device. Next, before the actual film formation, a program is executed in the flow rate control device. The program only needs to set the value of the flow rate and the time at the point where the flow rate changes. In the case where the laminated region is configured to be repeated at a constant cycle, the program can be simplified by setting the number of repetitions.
Specific changes in flow rate from the n-layer side are shown below. First S
The iH ₄ gas 5 sccm, to form 20 minutes 55 seconds GeH ₄ gas 0.6 sccm. Next, only GeH ₄ gas is increased to 1.6 sccm in 5 seconds. The state is maintained for 42 seconds, and then only GeH ₄ gas is reduced to 0.6 sccm in 5 seconds. Hold in that state for 52 seconds. After repeating this 13 times, only GeH ₄ gas is reduced to 0.03 sccm in 5 minutes and 14 seconds.

In this embodiment, the case where the i-layer is formed in one chamber has been described. However, the i-layer may be formed in a plurality of chambers.

As can be seen from this embodiment, the parameters for determining the device structure are very large, and it is very difficult to optimize by trial and error like a device having a uniform band gap. Therefore, to design the device,
It is desirable to calculate the amount of light absorption in the i-layer in advance. It is more desirable to be able to simulate the device characteristics, but for the purpose of setting a short circuit current as a guide, it is sufficient to calculate the light absorption amount which can be calculated relatively easily. When designing a device to maximize the amount of light absorption, the band gap of the narrow band gap layer is 1.45 eV.
Below, the film thickness is 8 nm or less, preferably about 2 nm. The reason why the band gap of the narrow band gap layer is set to 1.45 eV or less is to obtain a short-circuit current higher than that of an element manufactured by a conventional band gap profile technique. FIG. 4 shows a change in the fill factor when the thickness of the narrow band gap layer is changed in this embodiment. In this case, the total film thickness of the narrow band gap layer was kept constant at 30 nm, and the change in the characteristics was examined by changing the number of laminations. At a film thickness of 8 nm or more, the fill factor becomes 0.4 or less, which makes it difficult to apply the device to an element. It can be seen that a higher fill factor is better for use in solar cells and the like, and a film thickness of about 2 nm is desirable. The band gap of the wide band gap region is
The film thickness is desirably 1.5 eV or more and 10 nm or less. The number of repetitions is desirably 10 or more. Furthermore, the thickness of the entire i-layer is 1
50 nm or less is desirable. In order to further improve the fill factor, it is desirable to increase the light absorption on the p-layer side, which is the light-receiving surface side, and to reduce the light absorption on the n-layer side, specifically, not more than half of the i-layer thickness. It is desirable to provide a region having a band gap equal to or greater than the band gap of the wide band gap layer on the n-layer side with a film thickness of, for example, a constant region as in this embodiment.

Next, Table 2 shows a comparison of device characteristics between the device of this embodiment and the conventional band gap device under long-wavelength light of 650 nm or more. As a characteristic, the short-circuit current was improved, but the open-circuit voltage was reduced, so that the maximum output was almost the same value. However, since the thickness of the i-layer is almost half that of the conventional device, an improvement in reliability can be expected. Therefore, these elements were subjected to light irradiation of AM1.5, 100 mW / cm ² for 200 hours, and the device of the present example was 71% of the initial efficiency, and the conventional band gap device was 52% of the initial efficiency.
And the reliability has greatly improved.

[0020]

[Table 2]

In the present embodiment, as shown in FIG.
A similar effect can be obtained even when there is a band gap constant region on the layer side, but this region is absent or the band gap gradually increases toward the n-layer side. In FIG. 5, the bandgap increases as the narrow bandgap layer approaches the n-layer side, but with such a structure, the fill factor can be further improved. This is because the traveling distance of the hole can be reduced by increasing the carrier generation on the light incident side, so that the element characteristics can be further improved. In order to obtain the same effect, a structure in which the wide band gap layer gradually increases as approaching the n-layer side as shown in FIG. 6 may be used. Further, a similar effect can be obtained even if the structure is a combination of FIG. 5 and FIG. Further, even in the structure as shown in FIG. 7, since the carrier generation decreases as approaching the n-layer side, the same device characteristic improving effect can be obtained even with such a structure having no periodicity.

In this embodiment, one of the photoelectric conversion elements is used.
As an example, a single-layer type amorphous silicon germanium alloy element has been described, but this element may be used for a photoelectric conversion element laminated in two or three layers, and an amorphous silicon carbon alloy material may be used instead of the amorphous silicon germanium alloy material. May be used. FIG. 8 shows a band gap structure when a photoelectric conversion element is formed using amorphous silicon carbon. Amorphous silicon germanium and amorphous silicon carbon have different band gaps, but they are common in that the film quality deteriorates as the composition ratio increases with respect to silicon. Similar elements are used when a band gap profile structure is applied. Because a characteristic improvement effect is obtained,
It is clear that a similar effect is obtained even in the case of a laminated structure.

[0023]

As described above, according to the present invention, the i-layer and the n-layer
Side, an i-layer region having a substantial i-layer silicon composition ratio is provided.
The fill factor can be improved by
The efficiency of the pond could be improved. Also, the thickness of the i-layer
The reliability was improved by setting the thickness to 50 to 150 nm. Further
Further, by using amorphous silicon germanium as an amorphous silicon alloy material, light absorption was increased and short circuit current was improved. Also, by using amorphous silicon carbon as the amorphous silicon alloy material, the average band gap of the i-layer was increased, and the release voltage was improved.

[Brief description of the drawings]

FIG. 1 is a schematic structural diagram illustrating an element structure of a photoelectric conversion element according to an embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating a band gap structure of a photoelectric conversion element according to an embodiment of the present invention.

FIG. 3 is a schematic diagram illustrating a band gap structure of a conventional photoelectric conversion element.

FIG. 4 is a diagram illustrating the dependence of the fill factor on the film thickness of the narrow band gap region in the photoelectric conversion element according to the embodiment of the present invention.

FIG. 5 is a schematic diagram illustrating a band gap structure of a photoelectric conversion element according to an embodiment of the present invention.

FIG. 6 is a schematic diagram illustrating a band gap structure of a photoelectric conversion element according to an embodiment of the present invention.

FIG. 7 is a schematic diagram illustrating a band gap structure of a photoelectric conversion element according to an embodiment of the present invention.

FIG. 8 is a schematic diagram illustrating a band gap structure of a photoelectric conversion element according to an embodiment of the present invention.

[Explanation of symbols]

Reference Signs List 1 stainless steel substrate 2 Ti layer 3 Ag layer 4 ZnO layer 5 amorphous silicon n layer 6 wide band gap amorphous silicon germanium i layer 7 narrow band gap amorphous silicon germanium i layer 8 amorphous silicon carbon p layer 9 transparent conductive film 10 metal collecting electrode 11 i Layer thickness 12 Band gap minimum portion 13 Wide band gap amorphous silicon carbon i layer 14 Narrow band gap amorphous silicon carbon i layer

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-104383 (JP, A) JP-A-61-120480 (JP, A) JP-A-62-158316 (JP, A) JP-A-2- 177371 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) H01L 31/04

Claims (4)

(57) [Claims] In a photoelectric conversion element having a pin structure, a substantial part of an i-layer is formed of an amorphous silicon alloy material, and a band ratio is changed in a thickness direction of the i-layer by changing a composition ratio of the alloy material. A plurality of layers having different composition ratios are stacked so as to alternately generate the size of the gap,
Thickness of less layer of silicon composition ratio Ri 1~8nm der, the
Substantially equal to the maximum silicon composition ratio on the n-layer side of the i-layer
Or an i-layer having a silicon composition ratio higher than that
The photoelectric conversion device characterized by that. 2. The photoelectric conversion device according to claim 1, wherein the i-layer has a thickness of 50 to 150 nm. 3. An amorphous silicon alloy material comprising :
2. The photoelectric conversion element according to claim 1, wherein the photoelectric conversion element is made of fass silicon germanium . 4. An amorphous silicon alloy material comprising :
The photoelectric conversion element according to claim 1, wherein the photoelectric conversion element is fac silicon carbon .