JP2016192426A - Photoelectric conversion device, substrate with transparent electrode layer, and manufacturing method of photoelectric conversion device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photoelectric conversion device capable of reducing a defect that is generated between a transparent electrode layer and a photoelectric conversion layer, relatively to the prior arts while maintaining a light scattering performance.SOLUTION: On a surface of a transparent electrode layer 3 closer to a photoelectric conversion layer 5, a plurality of coating-side projections 17 are provided, and each of the coating-side projections 17 is peak-shaped and includes a round apex 11. Marginal ends of the coating-side projections 17 are settled within a first specific range 20. The first specific range 20 is a range of which the diameter is 0.1 μm or more and 1.5 μm or less, with the apex of the coating-side projection 17 defined as a center. On a surface of the coating-side projection 17, a plurality of coating-side protruding parts 18 are formed and marginal ends of the coating-side protruding parts 18 are settled within a second specific range 21. The second specific range 21 is a range of which the diameter is 10 nm or more and 150 nm or less, with an apex of the coating-side protruding part 18 defined as a center.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a photoelectric conversion device such as a solar cell and a manufacturing method thereof. Moreover, this invention relates to the board | substrate with a transparent electrode layer used for the said photoelectric conversion apparatus.

Conventionally, a photoelectric conversion device that performs photoelectric conversion between light energy and electric energy is known. A specific example of this photoelectric conversion device is a solar cell that converts light energy into electrical energy.
This solar cell is formed by laminating a transparent electrode layer, a photoelectric conversion layer, and a back electrode layer in this order, taking light from the transparent electrode layer side into the photoelectric conversion layer, and converting it into charge carriers. And it is the structure taken out from a back surface electrode layer.

By the way, conventionally, in order to increase the photoelectric conversion efficiency of the solar cell, fine irregularities are formed on the transparent electrode layer, the reflected light from the photoelectric conversion layer and the back electrode layer is scattered, and the light taken in from the transparent electrode layer is photoelectrically converted. Efforts have been made to confine as much as possible on the conversion layer side.
For example, in the solar cell described in Patent Document 1, a light scattering effect is obtained by forming a pyramidal uneven structure on the surface of the transparent electrode layer on the photoelectric conversion layer side. In addition to the above, Patent Documents 2 and 3 are documents describing prior art related to the present invention.

International Publication No. 2010/090142 International Publication No. 2003/036657 Japanese Patent Laid-Open No. 03-125481

In general, it is known that the light scattering performance is enhanced by increasing the surface roughness of the transparent electrode layer.
However, the pyramid structure described in Patent Document 1 has a structure in which the tips of the convex portions forming the surface irregularities are pointed. For this reason, if the surface irregularities are too large, the photoelectric conversion layer that covers the surface irregularities will crack. As a result, defects may occur at the bonding interface between the transparent electrode layer and the photoelectric conversion layer. And when the said defect arises, since interface resistance will increase, there exists a problem that photoelectric conversion efficiency will fall as a result.

  Therefore, an object of the present invention is to provide a photoelectric conversion device that can reduce the occurrence of defects that occur between the transparent electrode layer and the photoelectric conversion layer as compared with the prior art while maintaining light scattering performance.

  The invention described in claim 1 for solving the above problem is a photoelectric conversion device in which a transparent electrode layer, a back electrode layer, and a photoelectric conversion layer are sandwiched between the transparent electrode layer and the back electrode layer. In the photoelectric conversion device in which the surface on the photoelectric conversion layer side of the transparent electrode layer has zinc oxide as a main component, the surface on the photoelectric conversion layer side of the transparent electrode layer includes a plurality of convex portions, The convex portion has a mountain-like shape and has a rounded apex, and the edge portion of the convex portion falls within a first prescribed range, and the first prescribed range is equal to the convex portion. Centering on the apex, the diameter is in the range of 0.1 μm or more and 1.5 μm or less, and a plurality of protrusions are formed on the surface of the protrusion, and the edge of the protrusion is defined as a second prescribed. The second specified range has a diameter of 10 nm or more with the apex of the protrusion as the center. It is a photoelectric conversion device characterized by being in a range of 150 nm or less.

The “edge portion of the convex portion” here refers to a portion that forms an outline when the convex portion is viewed in plan, and is an end portion in the surface direction of the convex portion, and has an angle of 10 when viewed in cross section. A part that changes more than once.
Similarly, the “edge portion of the protruding portion” herein refers to a portion that forms an outline when the protruding portion is viewed in plan, and is an end portion in the surface direction of the protruding portion when viewed in cross section. A part where the angle changes by 10 degrees or more.
The term “main component” as used herein refers to a component that occupies 80% or more of all components.

According to the structure of this invention, since the front-end | tip part of a convex part is roundish, it can prevent that the photoelectric converting layer which coat | covers on it contacts a convex part, and is not damaged locally.
Further, according to the configuration of the present invention, in addition to the plurality of convex portions, since the fine protruding portion is formed on the surface of the convex portion, the reflected light from the photoelectric conversion layer and the back electrode layer is appropriately scattered. be able to. Therefore, the reflected light from the photoelectric conversion layer or the back electrode layer can be reflected again to the photoelectric conversion layer side, and the light can be kept on the photoelectric conversion layer side.
Thus, according to the structure of this invention, generation | occurrence | production of the defect of the photoelectric converting layer resulting from the surface unevenness | corrugation of a transparent electrode layer can be suppressed, maintaining the light scattering effect by a surface unevenness | corrugation. Therefore, for example, when the photoelectric conversion device is a solar cell, the open circuit voltage and the fill factor can be improved, and the photoelectric conversion efficiency can also be improved. Further, for example, when the photoelectric conversion device is an organic EL device, a good light scattering effect can be obtained, so that illuminance can be improved while reducing color unevenness.

  The invention according to claim 2 is the photoelectric conversion device according to claim 1, wherein the protruding portion has a rounded tip portion in the protruding direction.

  According to the configuration of the present invention, the tip portion of the protruding portion is also rounded, so that the photoelectric conversion layer coated thereon is less likely to crack and less likely to be defective.

In the photoelectric conversion device according to claim 1 or 2, preferably, the number of vertices per 1 μm ² on the surface of the transparent electrode layer on the photoelectric conversion layer side is 35 or more (claim 3).

That is, it is preferable that the transparent electrode layer has a vertex number density of 35 [1 / μm ² ] or more at the interface between the transparent electrode layer and the photoelectric conversion layer.

  The photoelectric conversion device according to claim 1, wherein the transparent electrode layer, the photoelectric conversion layer, and the back electrode layer are preferably laminated on a transparent insulating substrate, and the transparent The developed area ratio of the interface between the electrode layer and the photoelectric conversion layer is 35% or less (claim 4).

  The “development area ratio of the interface” here refers to how much the development area (surface area) of the definition region is increased with respect to the area of the definition region. That is, “the development area ratio of the interface between the transparent electrode layer and the photoelectric conversion layer” is the same as the surface of the transparent electrode layer on the photoelectric conversion layer side, with respect to the uniform plane. This represents how much the area increases with respect to the uniform plane when the surface on the conversion layer side is developed into a uniform plane.

The invention according to claim 5 is a photoelectric conversion device in which a transparent electrode layer, a photoelectric conversion layer, and a back electrode layer are laminated on a transparent insulating substrate, and the surface of the transparent electrode layer on the photoelectric conversion layer side is In the photoelectric conversion device mainly composed of zinc oxide, the surface of the transparent electrode layer on the photoelectric conversion layer side includes a plurality of convex portions, and the convex portions are mountain-shaped and rounded. The edge of the convex portion is within a first specified range, and the first specified range has a diameter of 0.1 μm to 1.5 μm with the vertex of the convex as the center. A plurality of protrusions are formed on the surface of the convex portion, and an edge portion of the protrusion is within a second specified range, and the second specified range is The transparent electrode has a diameter in the range of 10 nm to 150 nm with the apex of the protrusion as the center. The surface on the photoelectric conversion layer side of the layer has a number of vertices per 1 μm ² of 35 or more, and the development area ratio of the interface between the transparent electrode layer and the photoelectric conversion layer is 35% or less. A photoelectric conversion device is characterized.

  According to the structure of this invention, generation | occurrence | production of the defect of the photoelectric converting layer resulting from the surface shape of a transparent electrode layer can be suppressed, maintaining the light-scattering effect by surface unevenness | corrugation. Therefore, for example, when the photoelectric conversion device is a solar cell, the open circuit voltage and the fill factor can be improved, and the photoelectric conversion efficiency can also be improved.

  The invention according to claim 6 is a substrate with a transparent electrode layer in which a transparent electrode layer is laminated on a transparent insulating substrate, wherein a photoelectric conversion layer and a back electrode layer are laminated on the transparent electrode layer to form a photoelectric conversion device. In the substrate with a transparent electrode layer for forming the transparent electrode layer, when the photoelectric conversion device is formed, the surface on the photoelectric conversion layer side is mainly composed of zinc oxide and has a plurality of convex portions. The convex portion has a mountain shape and has a rounded apex, and the edge of the convex portion falls within a first prescribed range, and the first prescribed range is the convex portion. Centering on the apex of the portion, the diameter is in the range of 0.1 μm or more and 1.5 μm or less, and a plurality of protrusions are formed on the surface of the protrusion, and the edge of the protrusion is 2 is within a specified range, and the second specified range is centered on the apex of the protruding portion. A substrate with a transparent electrode layer having a diameter in the range of 10 nm to 150 nm.

According to the configuration of the present invention, since the tip portion of the convex portion is rounded, when the photoelectric conversion device is formed, the photoelectric conversion layer covering the convex portion is locally damaged by contacting the convex portion. Can be prevented.
In addition, according to the configuration of the present invention, in addition to the plurality of convex portions, since the fine protruding portion is formed on the surface of the convex portion, the light is appropriately scattered and the light is confined on the photoelectric conversion layer side. Can do.
Thus, if a photoelectric conversion device is formed using the substrate with a transparent electrode layer of the present invention, the defects of the photoelectric conversion layer due to the surface irregularities of the transparent electrode layer are maintained while maintaining the light scattering effect due to the surface irregularities. A photoelectric conversion device in which generation is suppressed can be formed. Therefore, for example, when the photoelectric conversion device is a solar cell, the open circuit voltage and the fill factor can be improved, and the photoelectric conversion efficiency can also be improved.

  Invention of Claim 7 is a manufacturing method of the photoelectric conversion apparatus in any one of Claims 1-5, Comprising: It is a manufacturing method of the photoelectric conversion apparatus which forms a transparent electrode layer into a film using a CVD apparatus. A transparent electrode layer forming step for forming the transparent electrode layer, wherein the transparent electrode layer forming step includes a primary film forming step and a secondary film forming step; And a secondary film-forming process, including a stop process for stopping the film-forming process.

According to the method of the present invention, a transparent electrode layer is formed using a CVD apparatus. That is, the transparent electrode layer is formed using a chemical reaction such as nucleus growth.
According to the method of the present invention, at the time of forming the transparent electrode layer, at least two film forming steps of the primary film forming step and the secondary film forming step are performed, and these primary film forming step and secondary film forming step are performed. A stop process is performed during the film forming process. By doing so, the reaction such as the nucleus growth of the transparent electrode layer in the primary film forming process is stopped by the stopping process, so that it is different from the nucleus of the transparent electrode layer in the primary film forming process in the second film forming process. Nuclei are generated at these sites, and nuclei grow easily in the nuclei. Therefore, the reaction is easy to occur evenly so as to level the unevenness formed in the first film forming step, and the tip of the convex portion can be rounded. Therefore, generation of defects due to surface irregularities at the interface between the transparent electrode layer and the photoelectric conversion layer can be suppressed, and a photoelectric conversion device with high conversion efficiency can be manufactured.

  In the method for producing a photoelectric conversion device according to claim 7, preferably, the transparent electrode layer forming step forms a transparent electrode layer using a reaction gas containing diethyl zinc gas as an active ingredient, The component amount of diethyl zinc gas in the reaction gas used in the second film-forming step is larger than the component amount of diethyl zinc gas in the reaction gas used in the first film-forming step. Item 8).

  In the above-described invention, the component amount of diethyl zinc gas in the reaction gas used in the second film-forming step is 1. The component amount of diethyl zinc gas in the reaction gas used in the first film-forming step. It is preferably 3 times or more.

  The invention according to claim 9 is the method of manufacturing a photoelectric conversion device according to claim 7 or 8, wherein the stopping step stops film formation for 50 seconds to 150 seconds.

  According to the method of the present invention, it is possible to secure a sufficient time to stop or end the nuclear growth reaction of the transparent electrode layer in the primary film forming process, and the transparent electrode layer forming process is not too long.

  The invention according to claim 10 is the method for manufacturing a photoelectric conversion device according to claim 7 or 8, wherein in the stopping step, 90% or more of the gas in the film forming chamber is exhausted.

  The term “exhaust” as used herein refers to discharging the gas in the film forming chamber to the outside of the film forming chamber, and not only suctioning the gas in the film forming chamber by vacuuming or the like, Substituting this gas with another inert gas.

  According to the method of the present invention, since the reaction gas in the primary film-forming process is exhausted by the stop process, the nuclear growth of the transparent electrode layer in the primary process can be forcibly stopped or terminated.

  In the above-described invention, in the stopping step, 90% or more of the gas in the film forming chamber may be replaced with an inert gas not containing zinc.

According to the photoelectric conversion device of the present invention, it is possible to reduce defects generated between the transparent electrode layer and the photoelectric conversion layer as compared with the conventional one while maintaining the light scattering performance. Therefore, the photoelectric conversion efficiency can be improved as compared with the conventional case.
According to the substrate with a transparent electrode layer of the present invention, it is possible to form a photoelectric conversion device having improved photoelectric conversion efficiency as compared with the conventional one.
According to the method for manufacturing a photoelectric conversion device of the present invention, the surface shape of the transparent electrode layer can be easily controlled.

1 is a cross-sectional view schematically showing a photoelectric conversion device according to a first embodiment of the present invention, and hatching is omitted for easy understanding. It is the perspective view which showed typically the surface shape of the transparent electrode layer of FIG. It is sectional drawing of the principal part of the surface of the transparent electrode layer of FIG. 1, and hatching is abbreviate | omitted for easy understanding. It is the perspective view which extracted the principal part of the surface of the transparent electrode layer of FIG. It is a top view of the principal part of the surface of the transparent electrode layer of FIG. It is sectional drawing which showed the surroundings of the photoelectric converting layer of the photoelectric conversion apparatus of FIG. 1 in detail, and hatching is abbreviate | omitted in order to make an understanding easy. It is explanatory drawing of the transparent electrode layer formation process of the photoelectric conversion apparatus of FIG. 1, (a) is a schematic diagram which shows the state of a primary film forming process, (b) is a schematic diagram which shows the state of a stop process. Yes, (c) is a schematic diagram showing the state of the secondary film-forming step. It is an observation result of a board | substrate with a transparent electrode layer, (a) is an atomic force microscope image of Example 1, (b) is an atomic force microscope image of the comparative example 1. FIG. It is an observation result of the board | substrate with a transparent electrode layer of Example 1, (a) is an atomic force microscope image, (b) is the schematic diagram which traced (a). It is an observation result of the board | substrate with a transparent electrode layer of the comparative example 1, (a) is an atomic force microscope image, (b) is the schematic diagram which traced (a).

  Hereinafter, embodiments of the present invention will be described in detail. In the following description, physical properties are based on standard conditions (25 ° C., 1 atm) unless otherwise specified. The surface structure such as roughness is a value according to ISO 25178 (surface roughness measurement) unless otherwise specified.

  The photoelectric conversion device 1 according to the first embodiment of the present invention is a photoelectric conversion device that converts light energy such as sunlight into electric energy. Specifically, the photoelectric conversion device 1 is a solar cell that converts light energy into electric energy and extracts electricity to the outside.

  As shown in FIG. 1, the photoelectric conversion device 1 according to the first embodiment includes a photoelectric conversion element in which a transparent electrode layer 3, a photoelectric conversion layer 5, and a back electrode layer 6 are stacked on a transparent insulating substrate 2. It is.

The photoelectric conversion device 1 according to the first embodiment is characterized by the surface structure of the transparent electrode layer 3.
Therefore, prior to description of each component part of the photoelectric conversion device 1, the transparent electrode layer 3 which is a main characteristic part will be described.

The transparent electrode layer 3 is an electrode layer mainly composed of zinc oxide, and is a conductive layer having transparency and conductivity.
The content of zinc oxide in the transparent electrode layer 3 is 80% or more, preferably 90% or more and 100% or less, and more preferably 95% or more and 100% or less.
The transparent electrode layer 3 may be doped with another different dopant such as aluminum.

As shown in FIG. 3, the transparent electrode layer 3 is configured by laminating a base electrode layer 10 and a covering electrode layer 11 in this order from the transparent insulating substrate 2 side.
The base electrode layer 10 is a layer that is laminated on the transparent insulating substrate 2 and mainly serves as a base for the coated electrode layer 11.
The base electrode layer 10 has irregularities on the surface opposite to the transparent insulating substrate 2 (surface on the photoelectric conversion layer 5 side), and includes a plurality of base-side convex portions 15.
The base-side convex portion 15 is a convex portion that protrudes in a direction that intersects the main surface of the transparent insulating substrate 2 on the transparent electrode layer 3 side. The base side convex portions 15 are formed on the main surface of the transparent insulating substrate 2 (the surface on the photoelectric conversion layer 5 side) almost uniformly distributed.

The covered electrode layer 11 is a layer that constitutes the outermost surface of the transparent electrode layer 3, and is a cap layer formed on the surface irregularities of the base electrode layer 10. The covering electrode layer 11 is a covering layer that leveles the tip of the base-side convex portion 15 of the base electrode layer 10 and rounds it.
The covered electrode layer 11 is formed along the surface unevenness of the base electrode layer 10, and the surface of the cover electrode layer 11 generally reflects the unevenness of the surface unevenness of the base electrode layer 10. That is, the covered electrode layer 11 is formed along the shape of the base-side convex portion 15.
In addition, the surface of the coated electrode layer 11 has not only unevenness reflecting the unevenness of the surface unevenness of the base electrode layer 10, but also fine unevenness formed from the surface unevenness.
That is, the covered electrode layer 11 has large unevenness reflecting the surface unevenness of the base electrode layer 10 and small unevenness formed on the surface of the large unevenness. And the uneven | corrugated shape of the covering electrode layer 11 is formed from the small unevenness | corrugation and the arc-shaped ridgeline of the large unevenness exposed from the small unevenness | corrugation.

  Further, when the coated electrode layer 11 is viewed from another point of view, the coated electrode layer 11 includes a coated side convex portion 17 reflecting the base side convex portion 15 and a coated side protruding portion protruding from the surface of the coated side convex portion 17. 18 is provided.

The covering-side convex portion 17 is a convex portion having a shape substantially similar to the base-side convex portion 15 of the base electrode layer 10. That is, as shown in FIG. 2, the covering-side convex portions 17 protrude in a direction intersecting the main surface of the transparent insulating substrate 2 on the transparent electrode layer 3 side, and are distributed in a planar shape.
Moreover, the coating | coated side convex part 17 is mountain-like, when it sees in cross section, and is provided with the rounded vertex 22. FIG. That is, the covering side convex part 17 protrudes toward the photoelectric conversion layer 5 side, and the tip part in the protruding direction is rounded. Moreover, the coating | coated side convex part 17 has spread gradually toward the skirt from the vertex 22. FIG.

When the covering-side convex portion 17 is viewed in plan from the apex 22 side, the edge portion of the covering-side convex portion 17 is within the first specified range 20 as shown in FIG. That is, the contour formed by the base end portion of the covering-side convex portion 17 is within the first specified range 20. In other words, the boundary between the coating-side convex portion 17 and the other coating-side convex portion 17 is within the first specified range 20.
The first specified range 20 is a range having a diameter of 0.1 μm or more and 1.5 μm or less around the vertex 22 of the covering-side convex portion 17, and a diameter of 0.2 μm or more and 1.2 μm or less. Preferably, the diameter is in the range of 0.3 μm to 1 μm.

The covering side protrusion 18 is a protrusion protruding in the direction intersecting the covering side protrusion 17 as shown in FIG. The covering side protrusions 18 are scattered on the surface of the covering electrode layer 11.
The covering-side protruding portion 18 is a fine arc-shaped convex portion and includes a rounded apex 23. That is, the covering-side protruding portion 18 protrudes from the surface of the covering-side convex portion 17, and the tip portion in the protruding direction is rounded in an arc shape and gradually spreads toward the bottom.
When the covering-side protruding portion 18 is viewed in plan from the apex 23 side, the edge portion of the covering-side protruding portion 18 is within the second specified range 21 as shown in FIG. That is, the contour formed by the base end portion of the covering-side protruding portion 18 is within the second specified range 21. In other words, the boundary of the covering side protrusion 18 is within the second specified range 21.
The second specified range 21 is a range of 10 nm to 150 nm in diameter with the apex 23 of the covering-side protruding portion 18 as the center, preferably a range of 25 nm to 120 nm in diameter, and a diameter of 40 nm to 100 nm. More preferably, it is the range.

When attention is paid to the physical properties of the transparent electrode layer 3, the average film thickness of the transparent electrode layer 3 is preferably 1500 nm or more, and more preferably 1700 nm or more, from the viewpoint of securing sufficient conductivity as an electrode.
The average film thickness of the transparent electrode layer 3 is preferably 2500 nm or less, and more preferably 2000 nm or less, from the viewpoint of sufficiently incorporating light into the photoelectric conversion layer 5.

The average film thickness of the base electrode layer 10 is preferably 7 to 20 times the average film thickness of the coated electrode layer 11.
The average film thickness of the covered electrode layer 11 is preferably 200 nm or more and 500 nm or less. If it is this range, the surface asperity of the base electrode layer 10 can be leveled sufficiently.

The sheet resistance of the transparent electrode layer 3 is preferably 20Ω / □ or less, and more preferably 18Ω / □ or less.
The haze according to JIS K7136 of the transparent electrode layer 3 is preferably 25% or less, and more preferably 20% or less.

The arithmetic average roughness (Sa) of the transparent electrode layer 3 is preferably 20 nm or more and 40 nm or less. The root mean square height (Sq) of the transparent electrode layer 3 is more preferably 35 nm or more and 50 nm or less.
From the viewpoint of securing a sufficient light scattering effect while suppressing generation of defects between the surface and the photoelectric conversion layer 5, the root mean square height (Sq) is 20 nm or more and 32 nm or less. And more preferably 25 nm or more and 40 nm or less.
When the arithmetic average roughness (Sa) of the transparent electrode layer 3 is not less than 36 nm and not more than 38 nm from the viewpoint of suppressing the generation of defects with the photoelectric conversion layer 5 due to surface irregularities, the root mean square height is (Sq) is more preferably 45 nm or more and 48 nm or less.

The number density (Sds) of the vertices of the transparent electrode layer 3 is preferably 35 [1 / μm ² ] or more, and 40 [1 / μm ² ] or more from the viewpoint of securing a sufficient light scattering effect. Is more preferably 45 [1 / μm ² ] or more, and particularly preferably 48 [1 / μm ² ] or more.
On the other hand, the number density (Sds) of the vertices of the transparent electrode layer 3 is preferably 110 [1 / μm ² ] or less, and 100 [1 / μm ² ] or more, from the viewpoint of ensuring light transmittance. More preferably, it is 95 [1 / μm ² ] or less.

The development area ratio (Sdr) at the interface between the transparent electrode layer 3 and the photoelectric conversion layer 5 is preferably 35% or less from the viewpoint of suppressing the occurrence of defects between the photoelectric conversion layer 5 due to surface irregularities. , 33% or less is more preferable, and 31% or less is more preferable.
The development area ratio (Sdr) at the interface between the transparent electrode layer 3 and the photoelectric conversion layer 5 is preferably 10% or more, more preferably 15% or more, from the viewpoint of securing a sufficient light scattering effect. More preferably, it is 18% or more.

  Next, each component of the photoelectric conversion device 1 will be described.

The transparent insulating substrate 2 is a substrate having transparency and insulation, and is a support substrate that supports each layer.
The transparent insulating substrate 2 is not particularly limited as long as it has transparency and insulating properties. For example, a glass substrate or a transparent resin substrate can be used.
The transparent insulating substrate 2 may be a flexible substrate having flexibility.

  As shown in FIG. 6, the photoelectric conversion layer 5 includes an amorphous silicon photoelectric conversion unit 25, an amorphous silicon germanium photoelectric conversion unit 26, and crystalline silicon from the transparent electrode layer 3 side (light incident side). The photoelectric conversion unit 27 is laminated. That is, the photoelectric conversion device 1 is a three-junction solar cell in which three photoelectric conversion units 25, 26, and 27 are joined.

  The amorphous silicon photoelectric conversion unit 25 functions as a top cell of the photoelectric conversion layer 5, and includes a p-type amorphous semiconductor layer 30, an i-type amorphous semiconductor layer 31, and an n-type crystalline semiconductor layer 32. It is composed of

The p-type amorphous semiconductor layer 30 is a layer that functions as a p-type semiconductor, for example, a p-type amorphous silicon carbide layer.
The i-type amorphous semiconductor layer 31 is a layer that functions as an i-type semiconductor (substantially intrinsic semiconductor), and is, for example, an i-type amorphous silicon layer.
The n-type crystalline semiconductor layer 32 is a layer that functions as an n-type semiconductor, for example, an n-type crystalline silicon layer.

The amorphous silicon germanium photoelectric conversion unit 26 functions as a middle cell of the photoelectric conversion layer 5 and is a photoelectric conversion unit including the i-type amorphous silicon germanium semiconductor layer 36.
The amorphous silicon germanium photoelectric conversion unit 26 includes a p-type amorphous semiconductor layer 35, an i-type amorphous silicon germanium semiconductor layer 36, and an n-type crystalline semiconductor layer 37.
The p-type amorphous semiconductor layer 35 is a layer that functions as a p-type semiconductor, for example, a p-type amorphous silicon layer.
The i-type amorphous silicon germanium semiconductor layer 36 is a layer that functions as an i-type semiconductor, for example, an i-type amorphous silicon germanium layer.
The n-type crystalline semiconductor layer 37 is a layer that functions as an n-type semiconductor, for example, an n-type crystalline silicon layer.

  The crystalline silicon photoelectric conversion unit 27 functions as a bottom cell of the photoelectric conversion layer 5 and includes a p-type crystalline semiconductor layer 40, an i-type crystalline semiconductor layer 41, and an n-type crystalline semiconductor layer 42. .

The p-type crystalline semiconductor layer 40 is a layer that functions as a p-type semiconductor, for example, a p-type crystalline silicon layer.
The i-type crystalline semiconductor layer 41 is a layer that functions as an i-type semiconductor, for example, an i-type crystalline silicon layer.
The n-type crystalline semiconductor layer 42 is a layer that functions as an n-type semiconductor, for example, an n-type crystalline silicon layer.

The back electrode layer 6 is an electrode layer that forms a pair with the transparent electrode layer 3 and functions as an electrode.
The back electrode layer 6 is not particularly limited as long as it has electrical conductivity. For example, metals such as aluminum, silver, gold, copper, platinum, and chromium, metal alloys, and metal composites can be used.
The back electrode layer 6 may have a multilayer structure. The back electrode layer 6 may have a multilayer structure of, for example, a transparent conductive oxide layer and a metal layer.
For example, the back electrode layer 6 may have a multilayer structure in which zinc oxide (AZO) doped with aluminum and silver are laminated.

  Then, the manufacturing method of the photoelectric conversion apparatus 1 is demonstrated. In particular, the transparent electrode layer forming method, which is one of the features of the present embodiment, will be described and the remaining layers will be briefly described.

First, the transparent insulating substrate 2 is introduced into a CVD apparatus to form the transparent electrode layer 3 (transparent electrode layer forming step).
In the transparent electrode layer forming step, the transparent electrode layer 3 is formed a plurality of times.
In the transparent electrode layer forming process of the present embodiment, the secondary film forming process is performed after the primary film forming process with a stop process interposed therebetween.

  Specifically, first, as shown in FIG. 7A, the base electrode layer 10 is formed on the transparent insulating substrate 2 by a low pressure CVD method (first film forming step).

The reaction gas 48 at this time is preferably a mixed gas of diethyl zinc (DEZn), water vapor (H ₂ O), diborane (B ₂ H ₆ ), and hydrogen (H ₂ ).
Moreover, it is preferable that the film forming temperature at this time is 140 degreeC or more and 180 degrees C or less. In this embodiment, it is about 160 degreeC.
The pressure at this time is atmospheric pressure or less, preferably 10 Pa or more and 100 Pa or less, more preferably 15 Pa or more and 50 Pa or less, and further preferably 20 Pa or more and 30 Pa or less.

  When the base electrode layer 10 is formed to a predetermined thickness, the film formation is stopped and, as shown in FIG. 7B, the reaction gas 48 filled in the film formation chamber is exhausted for a predetermined time. Leave it (stop process). That is, the stopping step is a step of removing the reaction gas 48 in the vicinity of the surface of the base electrode layer 10 to make the atmosphere of the surface of the base electrode layer 10 an inert atmosphere.

The evacuation time at this time is appropriately set according to the evacuation function of the CVD apparatus and the size of the film forming chamber, but is preferably 50 seconds to 150 seconds. If it is this range, the gas in a film forming chamber can fully be exhausted.
Further, it is preferable to exhaust 90% or more of the gas in the film forming chamber, more preferably 95% or more, and particularly preferably 99% or more.

  Then, when a predetermined time has elapsed since the start of the stop step, as shown in FIG. 7C, the coated electrode layer 11 is formed on the base electrode layer 10 by the low pressure CVD method (secondary film formation). Process).

In this secondary film forming step, it is preferable to use a reaction gas 49 having a concentration of diethylzinc and diborane components higher than that in the first film forming step. That is, in this secondary film forming step, the component amounts of diethyl zinc gas and diborane gas in the reaction gas 49 are larger than the component amounts of diethyl zinc gas and diborane gas in the reaction gas 48 used in the primary film forming step. large.
Specifically, the flow rate of diethyl zinc in the second film forming step is preferably 1.3 to 2.5 times the flow rate of diethyl zinc in the first film forming step. More preferably, it is 4 to 2 times. The flow rate of diborane in the second film forming step is preferably 1.3 to 2.5 times the flow rate of diborane in the first film forming step, and is 1.4 to 2 times. More preferably.
In addition, the concentration of diethyl zinc in the reaction gas 49 during the second film formation step is 1.3 to 2.5 times the concentration of diethyl zinc gas in the reaction gas 48 during the first film formation step. Is preferably double, more preferably 1.4 to 2 times. The concentration of diborane gas in the reaction gas 49 during the second film formation step is 1.3 to 2.5 times the concentration of diborane in the reaction gas 48 during the first film formation step. Preferably, it is 1.4 to 2 times.
Moreover, it is preferable that the film forming temperature at this time is 140 degreeC or more and 180 degrees C or less. In the present embodiment, the temperature is about 160 ° C., similar to the film formation temperature of the base electrode layer 10. In addition, although the film forming temperature of the base electrode layer 10 and the film forming temperature of the covering electrode layer 11 may differ, it is preferable that it is a comparable temperature.
The pressure at this time is atmospheric pressure or less, preferably 10 Pa or more and 100 Pa or less, more preferably 15 Pa or more and 50 Pa or less, and further preferably 20 Pa or more and 30 Pa or less. In addition, although the pressure at the time of film-forming of the base electrode layer 10 and the pressure at the time of film-forming of the coating electrode layer 11 may differ, it is preferable that it is a comparable pressure.

  When the transparent electrode layer forming step is completed, each photoelectric conversion unit 25, 26, 27 is formed by a CVD apparatus to form the photoelectric conversion layer 5 (photoelectric conversion layer forming step).

  When the photoelectric conversion layer forming step is completed, the back electrode layer 6 is formed on the substrate on which the photoelectric conversion layer 5 is formed by a sputtering method, a vacuum evaporation method, or the like (back electrode layer forming step).

  And wiring members, such as a tab wire, are connected as needed, and the photoelectric conversion apparatus 1 is manufactured.

  According to the photoelectric conversion device 1 of the present embodiment, irregularities are formed on the surface of the transparent electrode layer 3 on the photoelectric conversion layer 5 side, and the tips of the irregularities are rounded, so that the photoelectric conversion layer 5 has a defect. It can be suppressed.

  In addition, according to the photoelectric conversion device 1 of the present embodiment, the coating-side convex portion 17 and the coating-side protruding portion 18 are formed on the surface of the transparent electrode layer 3 on the photoelectric conversion layer 5 side. Incident light can be kept in the photoelectric conversion layer 5.

  Moreover, according to the manufacturing method of the photoelectric conversion apparatus 1 of this embodiment, since a stop process is implemented between a 1st film forming process and a 2nd film forming process, in a 2nd film forming process, Nuclei grow easily by nuclei different from the nuclei grown in the next film-forming process. Therefore, the tip end portion of the covering-side convex portion 17 can be rounded.

  In the above-described embodiment, the three-junction photoelectric conversion layer 5 is formed by bonding the three photoelectric conversion units 25, 26, and 27. However, the present invention is not limited to this, and one photoelectric conversion unit is used. May be sufficient, and two photoelectric conversion units may be joined. Four or more photoelectric conversion units may be joined.

  In the above-described embodiment, the case of a solar cell has been described as an example of the photoelectric conversion device 1. However, the present invention is not limited to this, and the photoelectric conversion device 1 includes an organic light emitting layer as a photoelectric conversion layer. Organic EL devices may also be used.

  In the embodiment described above, in the stopping step, the reaction gas 48 filled in the film forming chamber is exhausted from the film forming chamber by being left for a predetermined time while being exhausted. However, the present invention is limited to this. However, the reactive gas 48 is generated from the film forming chamber by introducing an inert gas such as argon or nitrogen or a gas not containing zinc (for example, hydrogen gas or water vapor) into the film forming chamber. May be discharged from.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.

  The manufacturing procedure of the photoelectric conversion device of the specific Example of this invention and a comparative example, and these evaluation results are demonstrated.

Example 1
In the photoelectric conversion device 1 of Example 1, a glass substrate having a thickness of 1.8 mm was used as the transparent insulating substrate 2. A transparent electrode layer 3 was formed on the transparent insulating substrate 2 to produce a substrate with a transparent electrode layer.
Specifically, first, the first film forming step was performed, and the base electrode layer 10 was formed on the transparent insulating substrate 2.
In this primary film forming step, the base electrode layer 10 containing zinc oxide (ZnO ₂ ) as a main component was formed under the condition of 160 ° C. and 25 Pa.
As the reactive gas 48 at this time, diethyl zinc (DEZn), water vapor (H ₂ O), diborane (B ₂ H ₆ ), and hydrogen (H ₂ ) were used. The respective flow ratios of diethyl zinc, diborane, and hydrogen when the flow rate of water vapor was 1 were 0.44, 0.22, and 0.88.
The film formation time of the base electrode layer 10 at this time was 760 seconds.

When the film formation of the base electrode layer 10 in the first film formation process was completed, a stop process was subsequently performed.
In this stop process, the reaction gas 48 in the film forming chamber was exhausted by leaving it for 100 seconds.

When the stop process was completed, the secondary film forming process was subsequently performed, and the coated electrode layer 11 was formed on the base electrode layer 10.
In this secondary film forming step, the coated electrode layer 11 containing ZnO ₂ as a main component was formed under the condition of 160 Pa and 25 Pa.
As the reaction gas 49 at this time, diethyl zinc (DEZn), water vapor (H ₂ O), diborane (B ₂ H ₆ ), and hydrogen (H ₂ ) were used. The respective flow ratios of diethyl zinc, diborane, and hydrogen when the flow rate of water vapor was 1 were 0.87, 0.44, and 0.22. That is, in the second film formation step, the flow rates of diethylzinc and diborane were approximately doubled and the hydrogen flow rate was reduced to approximately 1/4 compared to the first film formation step.
The film formation time of the coated electrode layer 11 at this time was 100 seconds.

  The thus obtained substrate with a transparent electrode layer was measured for average film thickness, sheet resistance, and haze according to JIS K7136, which were 1884 nm, 16.2 Ω / □, and 19.0%, respectively.

An amorphous silicon photoelectric conversion unit 25 was produced on the substrate with the transparent electrode layer using a plasma CVD apparatus.
Silane (SiH ₄ ), hydrogen (H ₂ ), methane (CH ₄ ), and diborane (B ₂ H ₆ ) are introduced as reaction gases, and a p-type amorphous silicon carbide layer is formed as the p-type amorphous semiconductor layer 30. Was formed so as to have a film thickness of 15 nm.
Thereafter, silane was introduced as a reaction gas, and an i-type amorphous silicon layer was formed as the i-type amorphous semiconductor layer 31 so as to have a thickness of 80 nm.
Thereafter, silane, hydrogen and phosphine (PH ₃ ) were introduced as reaction gases, and an n-type crystalline silicon layer was formed as the n-type crystalline semiconductor layer 32 so as to have a thickness of 10 nm. In this way, an amorphous silicon photoelectric conversion unit 25 was formed.

After the amorphous silicon photoelectric conversion unit 25 was formed, an amorphous silicon germanium photoelectric conversion unit 26 was produced.
Specifically, silane, hydrogen, and diborane were introduced as reaction gases, and a p-type amorphous silicon layer was formed as the p-type amorphous semiconductor layer 35 so as to have a thickness of 10 nm.
Thereafter, silane, germane (GeH ₄ ), and hydrogen were introduced as reaction gases to form an i-type amorphous silicon germanium layer as the i-type crystalline semiconductor layer 41 with a thickness of 150 nm.
Thereafter, silane, hydrogen, and phosphine were introduced as reaction gases, and an n-type crystalline silicon layer was formed as the n-type crystalline semiconductor layer 37 so as to have a thickness of 10 nm.
In this way, an amorphous silicon germanium photoelectric conversion unit 26 was formed.

After the amorphous silicon germanium photoelectric conversion unit 26 is formed, silane, hydrogen and diborane are introduced as reaction gases, and a p-type crystalline silicon layer is formed as the p-type crystalline semiconductor layer 40 so as to have a film thickness of 10 nm. did.
Thereafter, silane and hydrogen were introduced as reaction gases, and an i-type crystalline silicon layer was formed as an i-type amorphous semiconductor layer 41 so as to have a film thickness of 1.5 μm.
Thereafter, silane, hydrogen and phosphine were introduced as reaction gases, and an n-type crystalline silicon layer was formed as the n-type crystalline semiconductor layer 42 so as to have a film thickness of 15 nm. In this way, a crystalline silicon photoelectric conversion unit 27 was formed.

  Thereafter, Al-doped ZnO (AZO) with a thickness of 30 nm and silver (Ag) with a thickness of 300 nm were sequentially formed by sputtering, and this was used as the back electrode layer 6.

After the back electrode layer 6 was formed, each layer formed on the transparent electrode layer 3 was partially removed by a laser scribing method and separated into a size of 1 cm ² . The photoelectric conversion device 1 obtained in this way was referred to as Example 1.

(Comparative Example 1)
As Comparative Example 1, a transparent electrode layer-equipped substrate was produced by continuously forming a transparent electrode layer in Example 1 without performing a stopping step. The other processes were the same as in Example 1, and a photoelectric conversion device was produced. This was used as Comparative Example 1.

(Example 2)
In Example 2, a photoelectric conversion device was produced in the same manner as in Example 1 except for the prescription of the coated electrode layer 11 in the secondary film forming step, and this was designated as Example 2.
In the second film formation step of Example 2, the flow rates of diethylzinc and diborane were increased by about 1.4 times and the flow rate of hydrogen was increased by about 0.7 times compared to the case of the first film formation step. .

(Comparative Example 2)
As Comparative Example 2, a substrate with a transparent electrode layer was produced by continuously forming a transparent electrode layer in Example 2 without performing a stopping step. The other steps were the same as in Example 2, and a photoelectric conversion device was produced. This was used as Comparative Example 2.

[Evaluation by Atomic Force Microscope (AFM)]
For each of Example 1 and Comparative Example 1, Example 2 and Comparative Example 2, the surface of the substrate with a transparent electrode layer was observed with an atomic force microscope (manufactured by Pacific Nanotechnology). The results are shown in Table 1. Further, typical AFM images of Example 1 and Comparative Example 1 are shown in FIG. 8, FIG. 9, and FIG.

As shown in FIG. 8, FIG. 9, FIG. 10, and Table 1, a large difference in surface structure was observed between the photoelectric conversion device of Example 1 and the photoelectric conversion device of Comparative Example 1.
As can be seen from FIG. 8, the particle shape is different between Example 1 and Comparative Example 1. In Example 1, the particle shape is approximately bowl-shaped, whereas in Comparative Example 1, the particle shape is approximately rhomboid. It was a particle shape. As shown in FIG. 10, in Comparative Example 1, the tip of the convex portion was sharply sharpened, whereas in Example 1, the tip of the convex portion was rounded as shown in FIG.

  Moreover, the edge part of the convex part was in the range of 0.1 μm or more and 1.5 μm or less with the apex of the convex part as the center, as can be seen from FIG. 9. A plurality of protrusions are formed on the surface of the protrusion, and the edge of the protrusion has a diameter in the range of 10 nm to 150 nm with the apex of the protrusion as the center. The tip of the protrusion was rounded.

As shown in Table 1, the arithmetic average roughness (Sa) of Example 1 takes a value smaller than the arithmetic average roughness of Comparative Example 1, and the root mean square height (Sq) of Example 1 is compared. A value smaller than the root mean square height of Example 1 was taken. The arithmetic average roughness (Sa) of Example 2 is smaller than the arithmetic average roughness of Comparative Example 2, and the root mean square height (Sq) of Example 2 is the root mean square height of Comparative Example 2. A smaller value was taken.
These results indicate that in Examples 1 and 2 where the stopping process was performed, the surface unevenness of the base electrode layer 10 was leveled by the covering electrode layer 11 covering the base-side convex portion 15 of the base electrode layer 10. It is considered based.

As shown in Table 1, the vertex number density (Sds) of Example 1 was larger than the vertex density of Comparative Example 1. Further, the number density (Sds) of vertices in Example 2 was larger than the number density of vertices in Comparative Example 2.
These results are considered to be based on the fact that in Examples 1 and 2, the number of vertices increased due to the presence of the coating-side protrusions 18 formed on the surface of the coating-side convex portions 17 of the coating electrode layer 11.
Looking at these results from another viewpoint, it was found that Example 1 and Comparative Example 1, Example 2 and Comparative Example 2 had different numbers of nuclei forming protrusions. From these facts, in Examples 1 and 2, it can be considered that in the secondary film-forming process, even nuclei different from the nuclei growing in the primary film-forming process are grown.

Furthermore, as shown in Table 1, the developed area ratio (Sdr) of the interface of Example 1 was smaller than the developed area ratio of the interface of Comparative Example 1. Further, the developed area ratio (Sdr) of the interface of Example 2 was smaller than the developed area ratio of the interface of Comparative Example 2.
This is probably because, in Examples 1 and 2, the surface area was reduced as much as the height of the unevenness was reduced by the covering electrode layer 11.

[Photoelectric conversion characteristics evaluation]
For each of the photoelectric conversion devices of Example 1 and Comparative Example 1, and Example 2 and Comparative Example 2, simulated solar light was 100 mW / cm at 25 ° C. using a solar simulator having a spectral distribution of AM1.5. Irradiation was performed at an energy density of ² , and photoelectric conversion characteristics were measured. The results of Example 1 and Comparative Example 1 are shown in Table 2, and the results of Example 2 and Comparative Example 2 are shown in Table 3. The values in Table 2 are normalized with the values in Comparative Example 1, and the values in Table 3 are normalized with the values in Comparative Example 2.

As shown in Table 2, although the short-circuit current density decreased in Example 1 compared with Comparative Example 1, the open-circuit voltage and the fill factor increased to compensate for it, and the photoelectric conversion efficiency improved. .
Similarly, as shown in Table 3, in Example 2, although the short circuit current density was decreased as compared with Comparative Example 2, the open circuit voltage and the fill factor increased to compensate for the decrease in the photoelectric conversion efficiency. Improved.
The decrease in the short-circuit current density in Examples 1 and 2 is considered to have occurred because the diffused light decreased due to the rounded convex portions due to the presence of fine irregularities on the surface of the convex portions.

  From the above results, a clear change in the surface structure is observed depending on whether or not there is a stop process between the primary film forming process and the secondary film forming process, and the photoelectric conversion efficiency is increased by performing the stop process. Confirmed to do.

DESCRIPTION OF SYMBOLS 1 Photoelectric conversion apparatus 2 Transparent insulating substrate 3 Transparent electrode layer 5 Photoelectric conversion layer 6 Back surface electrode layer 10 Base electrode layer 11 Covering electrode layer 17 Covering side convex part (convex part)
18 Covering side protrusion (protrusion)
20 First prescribed region 21 Second prescribed region 22 Vertex 23 Vertex 48 Reactive gas 49 Reactive gas

Claims (10)

A transparent electrode layer, a back electrode layer, and a photoelectric conversion device in which a photoelectric conversion layer is sandwiched between the transparent electrode layer and the back electrode layer,
In the photoelectric conversion device whose surface on the photoelectric conversion layer side of the transparent electrode layer is mainly composed of zinc oxide,
The surface of the transparent electrode layer on the photoelectric conversion layer side includes a plurality of convex portions,
The convex portion has a mountain shape and has a rounded apex,
The edge of the convex portion is within the first specified range,
The first specified range is a range having a diameter of 0.1 μm or more and 1.5 μm or less with the vertex of the convex portion as the center.
A plurality of protrusions are formed on the surface of the protrusion,
The edge of the protrusion is within the second specified range,
The second prescribed range is a photoelectric conversion device having a diameter of 10 nm or more and 150 nm or less with the apex of the protrusion as a center.   The photoelectric conversion device according to claim 1, wherein the protruding portion has a rounded tip portion in a protruding direction. 3. The photoelectric conversion device according to claim 1, wherein the number of vertices per 1 μm ² on the surface of the transparent electrode layer on the photoelectric conversion layer side is 35 or more. The transparent electrode layer, the photoelectric conversion layer, and the back electrode layer are laminated on a transparent insulating substrate,
The photoelectric conversion device according to claim 1, wherein a developed area ratio of an interface between the transparent electrode layer and the photoelectric conversion layer is 35% or less. A photoelectric conversion device in which a transparent electrode layer, a photoelectric conversion layer, and a back electrode layer are laminated on a transparent insulating substrate,
In the photoelectric conversion device whose surface on the photoelectric conversion layer side of the transparent electrode layer is mainly composed of zinc oxide,
The surface of the transparent electrode layer on the photoelectric conversion layer side includes a plurality of convex portions,
The convex portion has a mountain shape and has a rounded apex,
The edge of the convex portion is within the first specified range,
The first specified range is a range having a diameter of 0.1 μm or more and 1.5 μm or less with the vertex of the convex portion as the center.
A plurality of protrusions are formed on the surface of the protrusion,
The edge of the protrusion is within the second specified range,
The second specified range is a range having a diameter of 10 nm or more and 150 nm or less with the apex of the protrusion as a center.
The surface of the transparent electrode layer on the photoelectric conversion layer side has 35 or more vertices per 1 μm ² ,
Furthermore, the development area ratio of the interface of the said transparent electrode layer and the said photoelectric converting layer is 35% or less, The photoelectric conversion apparatus characterized by the above-mentioned. A substrate with a transparent electrode layer in which a transparent electrode layer is laminated on a transparent insulating substrate,
In the substrate with a transparent electrode layer for forming a photoelectric conversion device by laminating a photoelectric conversion layer and a back electrode layer on the transparent electrode layer,
When the transparent electrode layer is formed with a photoelectric conversion device, the surface on the photoelectric conversion layer side is mainly composed of zinc oxide, and includes a plurality of convex portions,
The convex portion has a mountain shape and has a rounded apex,
The edge of the convex portion is within the first specified range,
The first specified range is a range having a diameter of 0.1 μm or more and 1.5 μm or less with the vertex of the convex portion as the center.
A plurality of protrusions are formed on the surface of the protrusion,
The edge of the protrusion is within the second specified range,
The second prescribed range is a substrate with a transparent electrode layer, wherein the diameter is in the range of 10 nm to 150 nm with the apex of the protrusion as the center. It is a manufacturing method of the photoelectric conversion device according to any one of claims 1 to 5,
A method of manufacturing a photoelectric conversion device that forms a transparent electrode layer using a CVD device,
A transparent electrode layer forming step of forming the transparent electrode layer;
The transparent electrode layer forming step includes a primary film forming step and a secondary film forming step,
A method for manufacturing a photoelectric conversion device, comprising: a stop step of stopping film formation between a primary film formation step and a secondary film formation step. The transparent electrode layer forming step is to form a transparent electrode layer using a reaction gas containing diethyl zinc gas as an active ingredient,
The component amount of diethyl zinc gas in the reaction gas used in the second film forming step is larger than the component amount of diethyl zinc gas in the reaction gas used in the first film forming step. The manufacturing method of the photoelectric conversion apparatus of Claim 7.   The method for manufacturing a photoelectric conversion device according to claim 7 or 8, wherein the stopping step stops film formation for 50 seconds to 150 seconds.   The method for manufacturing a photoelectric conversion device according to claim 7 or 8, wherein in the stopping step, 90% or more of the gas in the film forming chamber is exhausted.