JPS61183974A - Amorphous solar cell using superlattice structure for window layer - Google Patents

Abstract

PURPOSE:To contrive the inhibition of reverse diffusion of carriers generated in an I-layer, increase in yielding effect of photo-generated carriers, and improvement in release voltage, by a method wherein a selection doping superlattice structure having a high electrical conductivity and a large forbidden band width is used for the window layer of the P-I-N photo cell. CONSTITUTION:The window layer of the P-I-N amorphous photo cell is a P-type layer 4. Its superlattice structure is formed by alternately laminating well layers 4a of 5-50Angstrom thin films made of the same amorphous semiconductor, e.g. a-Si:H or a-Si1-xGex:H, as that of an I-layer 6 or a substance such as several crystal Si (muc-Si:H) of higher doping efficiency and barrier layers 4b of 5-50Angstrom thin films made of an amorphous semiconductor, such as a-Si1-xCx:H, having a larger forbidden band width or an insulating substance such as Si3H4 or BN.

Description

DETAILED DESCRIPTION OF THE INVENTION Product-1 The present invention relates to a photovoltaic cell having a superlattice structure, and more specifically, a superlattice structure that realizes a window layer through which light enters has high electrical conductivity and a wide forbidden band width. The present invention relates to a highly efficient amorphous photovoltaic cell constructed of.

・-1. Recently, amorphous semiconductor 1 such as hydrogenated amorphous Si
(a-St:H) and hydrogenated amorphous 5iGe (
a-SiGe:H) has attracted attention as a thin-film photovoltaic material because it can be made thinner than crystalline semiconductors, and because it is easy to manufacture and has good productivity. A photovoltaic cell has been proposed.

In this type of pin-type amorphous photovoltaic cell, an increase in the amount of light irradiated to the i-layer, an effective collection of carriers photogenerated in the i-layer, and an improvement in the open-circuit voltage are important factors for improving the efficiency. Therefore, the window layer through which light enters requires a semiconductor material that has high electrical conductivity and a wider forbidden band width than the i-layer. Conventional amorphous semiconductor materials have a problem in that doping efficiency is low in materials with a wide forbidden band width, and sufficiently high electrical conductivity cannot be obtained, and the open circuit voltage does not increase as expected.

According to No. 1, an object of the present invention is to increase the amount of light irradiation to the i layer,
Among the carriers generated in the layer, the carriers that become minority carriers in the window layer (electrons for the p-type window layer, holes for the n-type window layer) are suppressed from back-diffusion, and the collection efficiency of photogenerated carriers is improved. It is an object of the present invention to provide a pin-type amorphous photovoltaic cell with higher efficiency by increasing the open voltage and improving the open circuit voltage.

As a result of repeated research on pin-type amorphous photovoltaic cells, the inventor of the present invention has discovered that the window layer through which light enters has an effective forbidden band width wider than that of the i-layer.
By using a heterojunction superlattice structure with high electrical conductivity, it is possible to increase the amount of light irradiated to the i-layer-2, increase the collection efficiency of photogenerated carriers, and improve the open circuit voltage.
It has been discovered that a highly efficient photovoltaic cell can be obtained.The present invention is based on such novel findings.

To summarize the present invention, the pin-type amorphous photovoltaic cell of the present invention is a heterojunction supercell made by alternately stacking extremely thin films of two types of semiconductors with different forbidden bands on the window side layer through which light enters. It is characterized by having a lattice structure.

Next, the structure of the pin-type amorphous photovoltaic cell according to the present invention will be explained in more detail with reference to the drawings.

FIG. 1 is a schematic cross-sectional view of the structure of a pin-type photovoltaic cell l having a superlattice structure of the present invention in the p-window layer, and FIG. 2 shows an energy band diagram of the photovoltaic cell. For example, 1. Two layers 4, 1, 6 and 8 are sequentially deposited on an insulating substrate 2 having a transparent conductive film 2a according to a normal glow discharge decomposition method of raw material gas, and a back contact metal 1O is further provided. Consisted of. In FIG. 1, the window layer of the pin-type amorphous photovoltaic cell is the p-type layer 4.
This part is formed by a superlattice structure.

FIG. 3 shows an energy band diagram of the p-type window layer 4 having this superlattice structure.

The superlattice structure in the p-type layer 4 is a-5i:H or a-
Si +-xGex: Microcrystalline silicon (gc
-5 to 5 made of an amorphous semiconductor material such as Si:H)
A well layer 4a formed by an os @ film and a-S i I-X
NX: H or a-S i 1-11 Cx
The barrier layer 4b is formed by alternately laminating barrier layers 4b made of a thin film of 5 to 5 os made of an amorphous semiconductor having a wider bandgap such as :H or an insulating material such as 5ilN4 or BN. A preferable combination constituting the superlattice structure 4 is poron-doped a-5i:H and semiconducting a-S i +-x N x :H1 semiconducting a-S
i layer-xcX: H(xc7) value is 0°3 or more) or insulating Si3N4, and the preferred film thickness of each well layer and barrier layer is 5 to 50 μm, most preferably 10 to 2
It is 5th.

Furthermore, as shown in Figure 3, this heterojunction superlattice structure is configured such that the potential well generated inside it is deep for electrons, but shallow or non-existent for holes. Therefore, in the present invention, preferably, the superlattice structure 4 has a-
Si:H (25s), as a barrier layer and a −S i +
-x N x :H (x=0.3) will be configured to be deposited alternately.

In the photovoltaic cell according to the present invention, the two layers 4 of the superlattice structure preferably have 100 to 500 well layers 4a and 4b,
More preferably, it is formed by laminating layers to a thickness of 100 to 200 mm.

In addition, one layer 6 is as described above <a-5i:H or a-Sir-
xGe)(:H, the 0 layer 8 is phosphorus-doped a
-Si: Silicon containing Hl or microcrystalline phase (Lec-S
i:H). A preferred combination of 1 layer 6 and 0 layer 8 is a-Si:H and phosphorus-doped ILc-5i:H.
It is. Further, the preferable film thicknesses of the single layer 6 and nM8 are 4000 to 8000 s and 200 to SOO s, respectively, and the most preferable are 4500 to 7000 s and 200 to 30 s.
It is 0s.

Further, the insulating substrate 2 can be a transparent substrate such as a glass plate, and the back contact metal IO can be At or Al.
It can be formed by depositing g/Ti.

The high efficiency of the amorphous photovoltaic cell according to the present invention constructed as described above is believed to be due to the following three reasons.

First, in this heterojunction superlattice structure, acceptor impurity doping is mainly performed in the well layer 4a where doping efficiency is high, and carriers are carried out in the barrier layer 4a.
Since the p-type layer can easily pass through b by the tunnel effect, a p-type layer having sufficiently high electrical conductivity as a whole is formed.

Second, as a result of carriers existing in the potential well formed in the superlattice structure being quantized by the effect on the quantum size, the band edge moves to the high energy side, and the effective forbidden band width is reduced to the bulk state. Since the attenuation of the incident light in the two layers is suppressed, the i-layer is effectively irradiated with light.

Thirdly, the potential barrier layer 4b prevents electrons photogenerated in the i-layer from diffusing back into the p-layer, so photogenerated carriers effectively become photocurrent.

In the above explanation, a pin type photovoltaic cell was explained in which the 2nd layer 4 is a window layer, but as shown in FIG. By using a superlattice structure having a deep potential well in the zero layer, a highly efficient photovoltaic cell can be realized for the same reason as mentioned above.

To explain the embodiment of FIG. 4 more specifically, n
The superlattice structure in type R8 is a-Si:H or a-
For example, microcrystalline silicon (4 cm
5-50 made of amorphous semiconductor material such as Si:H)
A well layer 8a made of a thin film of
x: H or amorphous semiconductor with a wider bandgap such as a-S i l-1FCx:H, or a 5-50 S film made of an insulating material such as Si3N4 or BN. It is formed by alternately stacking barrier layers 8b. A preferable combination constituting the superlattice structure 8 is phosphorus-doped a-Si:H and semiconducting a--S i l-X N x :H or semiconducting a-Si:H.
-S i l-X CX :H (value of x is around 0.3), and the preferred thickness of each well layer and barrier layer is 5~
50 people, most preferably 10-25 people.

Further, in the 0 layer 8 of the superlattice structure, the well layers 8a and 8b are preferably 10
It is formed by laminating layers to a thickness of 0 to 500 mm, more preferably 100 to 200 mm.

Furthermore, this heterojunction superlattice structure is configured such that the potential well generated inside it is deep for holes and shallow or non-existent for electrons, as seen in Figure 3. .

The first layer 6 is as described above <a-Si:H or a-Si 1-X
The bilayer 4 is formed of poron-doped a-Si:H or microcrystalline St (terminal c-Si:H) surrounding a microcrystalline phase.
Si:H). A preferred combination of the first layer 6 and the second layer 4 is a-Si:H and poron-doped p-c-5i.
:H. Further, the preferable film thicknesses of the first layer 6 and the second layer 4 are 4000 to 200 to 500, and most preferably 4500 to 7000 and 200 to 500, respectively.
It is 3oo2.

In the above explanation, a transparent substrate such as a glass plate having a transparent conductive film was used as the substrate, but stainless steel was used as the substrate and layers were stacked in the order of p, i, n or n, i, p. It is also possible to provide a structure in which a transparent conductive film is provided thereon as an upper electrode. In this case, the top n layer or p
The layers have a superlattice structure. FIG. 5 shows an embodiment in which two layers 4, one layer 6, and nR8 are sequentially laminated on a stainless steel substrate 2, and a transparent electrode 11 is provided thereon.

Support 1 Hereinafter, the present invention will be specifically explained with reference to Examples.

Example 1 Here, a-5i: H/ a-Si I-XNX
: pin type a using H superlattice SS as p type window layer
-Describe a Si photovoltaic cell. This photovoltaic cell can be easily manufactured by a normal glow discharge decomposition method of raw material gas.

The manufacturing procedure will be explained below.

First, the ITO/glass substrate was kept at a temperature of 250°C, and H2 diluted lO%SiH+ was added as the first layer of the p-type superlattice thin film.
Gas and H2 diluted 1000p1000pp gas were flowed at a flow rate of 30 sec and 3 to 30 sec, respectively, and the pressure was 0.
．． 2 TOrr, high frequency power 5W (13,56MH2)
Glow discharge decomposition is performed for about 1 minute at
BZ H6/S i H4= 10-3~I 0-2(
1) P-type a-Si:H thin 1ll (well layer 4a)
Deposit 0 s.

Next, to form the second layer, further H2 dilution 1O%NH3
P-type a-SiI-INx:H(X~
0.3) Deposit 20 layers of thin film (barrier layer 4b).

After the deposition of the a-Si 1-XNX:H film, the residual NH3 gas in the reaction vessel is purged, and the third layer is again deposited with the same p-type a-5i:H film (well layer) as the first layer. 4a) is deposited for 20 times.

Thereafter, by repeating the same procedure, five layers of 20 s thin films were alternately deposited, and a 1 g p-type superlattice thin film with a film thickness of 200 s having potential wells as shown in Fig. 3 was created. 4 is formed by glow discharge decomposition.

Next, a-
One layer 6 consisting of Si:H and phosphorus-doped a-Si
:N+ layer 8 consisting of H is 4500 and 200
After the back contact metal An is deposited to a thickness of 100 nm, the back contact metal An is deposited.

In this structure, the photoelectric conversion efficiency under AMI irradiation is 8%.
photovoltaic cells were realized. The photoelectric conversion efficiency of a pin-type photovoltaic cell using ordinary poron-doped p-type a-Si:HJij (200 s thick) as the p-type window layer was 6%, and therefore a significant improvement was observed with the configuration of the present invention. I understand that. The open circuit voltage was also improved from 0.83V to 0.90V by replacing the normal p-type a-3t:H window layer with a superlattice structure.

Example 2 Next, a photovoltaic cell shown in FIG. 5 in which the n-layer is a window layer with a superlattice structure will be described.

Similar to Example 1, this battery can be easily manufactured by the usual glow discharge decomposition method of raw material gas. The procedure will be explained below.

First, a stainless steel substrate 2 was kept at a temperature of 250°C, and poron-doped a-Si was prepared using a conventional glow discharge decomposition method.
After depositing a P+ layer 4 of H and one layer 6 of a-Si:H to a thickness of 200 and 4500 μs, respectively, H2 diluted 10% Si was deposited as the first layer of the n-type superlattice thin film.
H4 gas and H2 diluted 1000 to 1000 pp gas were flowed at a flow rate of 30 sec and 3 to 30 sec, respectively, at a pressure of 0.2 torr and a high frequency power of 5 W (13,56
Glow discharge decomposition was performed for about 1 minute at
-2(7)n+ type a-Si:H thin film (well layer 8a)
Deposit o.

Next, the second layer was formed by adding 10% NH diluted with H2 and gas at a flow rate of 9 sec, and performing similar glow discharge decomposition to form n-type a-S i l-X N x :H(
x-0,3) Deposit 2os of thin film (barrier layer ab). After the deposition of the a-StxNx:H film, the residual NH3 gas in the reaction vessel is purged, and the first layer is again deposited on the third layer.
Twenty layers of the same n+ type a-5i:H film (well layer 8a) as the th layer are deposited.

Thereafter, by repeating the same procedure, five layers of 20 thin films were deposited alternately, and 200 thin films having potential wells as shown in FIG. ! ! A superlattice thin $8 is formed by glow discharge decomposition.

After that, a transparent conductive film ITO is deposited. In this structure, a photovoltaic cell with a photoelectric conversion efficiency of 7.7% under AMI irradiation was realized. Normal phosphorus-doped n as n-type window layer
The photoelectric conversion efficiency of a pin type photovoltaic cell using a type a-Si:H layer (200-layer thick nitrided layer) was 5.9%, which indicates that a significant improvement was achieved by the configuration of the present invention.

According to the present invention, by using a selectively doped superlattice structure with high electrical conductivity and a wide forbidden band width in the window layer of a pin-type photovoltaic cell, carriers generated in one layer can be reversely A highly efficient pin-type amorphous photovoltaic cell can be manufactured by suppressing diffusion, increasing the yield effect of photogenerated carriers, and improving the release voltage.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a pin-type non-amorphous photovoltaic cell according to an embodiment of the present invention. FIG. 2 is an energy band diagram of the pin type non-pre-amorphous photovoltaic cell of FIG. FIG. 3 is an energy band diagram of the two-layer photovoltaic cell of FIG. 1 having a superlattice structure. FIG. 4 is a schematic diagram of a pin-type non-amorphous photovoltaic cell according to another embodiment of the present invention. FIG. 5 is a schematic diagram of a pin-type non-amorphous photovoltaic cell according to still another embodiment of the present invention. FIG. 6 is an energy band diagram of an n-layer having a superlattice structure of the photovoltaic cell of FIG. 5. FIG. 22 substrate 4: 2 layers 4a, 8a: potential well layers 4b, 8b = potential barrier layer 6: i layer 8: n layer IO: back contact metal 1: transparent conductive film Figure 3 destination

Claims (1)

[Claims] 1) In a pin-type amorphous photovoltaic cell, a heterojunction superlattice manufactured by alternately stacking extremely thin films of two types of semiconductors with different forbidden band widths on the window layer side where light enters. A pin-type amorphous photovoltaic cell characterized by having a structure. 2) The photovoltaic cell according to claim 1, wherein the superlattice structure is formed in a p-type window layer. 3) The photovoltaic cell according to claim 1, wherein the superlattice structure is formed in an n-type window layer. 4) The superlattice structure includes a potential well layer made of a-Si:H, a-Si_1_-_xGex:H or an amorphous semiconductor material with high doping efficiency;
a-Si_1_-_xNx:H or a-Si_1_- having a wider forbidden band width than the well layer
_xCx:H The photovoltaic cell according to any one of claims 1 to 3, which is formed by alternately laminating potential barrier layers made of an amorphous semiconductor or an insulating material. 5) Each well layer and barrier layer has a film thickness of 5 to 50 Å,
5. The photovoltaic cell according to claim 1, wherein the superlattice structure is laminated so that the total film thickness is 100 to 500 Å. 6) The photovoltaic cell according to claim 5, wherein each well layer and barrier layer have a thickness of 10 to 25 Å, and are laminated so that the total thickness of the superlattice structure is 100 to 200 Å. .