JP2012023236A - Thin film solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin film solar cell which has a high light confinement effect, and in which microcrystalline silicon-based photoelectric conversion layers with excellent film quality are laminated.SOLUTION: In a thin film solar cell, on a substrate 1, a light reflective first electrode layer 2, at least one photoelectric conversion layer 4, and a light transmissive second electrode layer 5 are laminated in series, and on a surface 2 of the photoelectric conversion layer side of the first electrode layer 2, irregularity for irregularly reflecting incident light is formed. At least one of the photoelectric conversion layer 4 is a microcrystalline silicon-based photoelectric conversion layer. Between the first electrode layer 2 and the photoelectric conversion layer 4, a flattening layer 3 consisting of transmissive conductive polymer material is disposed. The surface of the photoelectric conversion layer side of the flattening layer 3 has higher smoothness than that of the surface of the photoelectric conversion layer side of the first electrode layer 2.

Description

  The present invention relates to a thin film solar cell in which irregularities for irregularly reflecting incident light are formed on the surface of an electrode layer on the back side.

  A solar cell is a power generation device that converts light energy into electric power, and in order to improve power generation efficiency, it is necessary to increase the utilization efficiency of incident light. For this reason, for example, as described in Patent Document 1 below, an uneven surface texture structure is formed on the surface of the light-reflective lower electrode layer (back electrode layer), and the lower part is not absorbed by the photoelectric conversion layer. Attempts have been made to increase the light utilization efficiency by confining light inside the solar cell by diffusely reflecting sunlight reaching the electrode layer at the junction interface between the lower electrode layer and the photoelectric conversion layer.

  In order to improve the characteristics of recent solar cells, a solar cell having a multi-junction structure in which a plurality of photoelectric conversion layers having different absorption wavelength bands are stacked has been put into practical use. For example, a photoelectric conversion layer using a semiconductor film made of microcrystalline silicon (hereinafter referred to as a microcrystalline silicon-based photoelectric conversion layer) is a photoelectric conversion layer using a semiconductor film made of amorphous silicon (hereinafter referred to as amorphous silicon). Sensitivity to long wavelength light is higher than that of the system photoelectric conversion layer), and long wavelength light that cannot be absorbed by the amorphous silicon photoelectric conversion layer can be used for power generation. For this reason, the microcrystalline silicon thick photoelectric conversion layer and the amorphous silicon photoelectric conversion layer are laminated to form a multi-junction structure so that the amorphous silicon photoelectric conversion layer is disposed on the light incident side. Improvements in efficiency have been made.

  However, when a microcrystalline silicon film is formed on an electrode layer processed into a concavo-convex structure, a large number of microcrystalline silicon film growth directions occur, and macroscopic crystal grains having different orientations collide with each other during the film growth process. There was a problem that defects occurred and the photoelectric conversion characteristics deteriorated due to the deterioration of the film quality. For this reason, when the microcrystalline silicon photoelectric conversion layer is formed on the electrode having the concavo-convex structure, it is necessary to reduce the concavo-convex structure of the electrode layer to some extent, and a sufficient light confinement effect cannot be obtained.

  On the other hand, in Patent Document 2 below, in the solid-state imaging device, when there are irregularities on the electrode layer, or when dust or the like is attached, if a photoelectric conversion layer is stacked on the electrode layer, Since the film thickness may be reduced and cracks may occur, an undercoating film made of an organic polymer material such as polyaniline, polythiophene, polypyrrole, or polycarbazole is formed on the electrode film. It is described to alleviate.

  However, the unevenness on the electrode layer in the cited document 2 is formed by adhering dust and the like, and is irregularly reflected at the junction interface between the electrode layer and the photoelectric conversion layer to confine light inside the solar cell. It was not formed for the purpose. In addition, a microcrystalline silicon-based photoelectric conversion layer is not used as the photoelectric conversion layer, and no consideration is given to the above-described problems when a microcrystalline silicon-based photoelectric conversion layer is formed on an electrode having an uneven structure. It has not been.

JP-A-8-288529 JP 2007-80936 A (paragraph number 0059)

  Accordingly, an object of the present invention is to provide a thin film solar cell in which a microcrystalline silicon photoelectric conversion layer having a high light confinement effect and a good film quality is laminated.

  In order to achieve the above object, a thin-film solar cell of the present invention includes a light-reflective first electrode layer, at least one photoelectric conversion layer, and a light-transmissive second electrode layer that are sequentially stacked on a substrate. In the thin film solar cell in which irregularities for irregularly reflecting incident light are formed on the surface of the first electrode layer on the photoelectric conversion layer side, at least one of the photoelectric conversion layers is a microcrystalline silicon photoelectric conversion layer. And a planarization layer made of a transparent conductive polymer material is disposed between the first electrode layer and the photoelectric conversion layer, and the surface of the planarization layer on the photoelectric conversion layer side is the first electrode. It is characterized by being smoother than the surface of the layer on the photoelectric conversion layer side.

  In the thin film solar cell of the present invention, the planarization layer is formed on the first electrode layer processed into the uneven structure, and the surface of the planarization layer on the photoelectric conversion layer side is the surface of the first electrode layer on the photoelectric conversion layer side. Therefore, a microcrystalline silicon film with good film quality can be formed on the planarization layer. For this reason, the microcrystalline silicon-based photoelectric conversion layer having a good film quality can be formed while increasing the light confinement effect by increasing the undulation width of the unevenness on the first electrode layer, thereby enhancing the light confinement effect. The power generation efficiency of the thin-film solar cell can be increased. Furthermore, since the light confinement effect can be utilized at a higher level, the film thickness of the photoelectric conversion layer can be made thinner, and as a result, the time required to form the photoelectric conversion layer is shortened and the productivity of the thin film solar cell is improved. And a thin film solar cell can be provided at low cost.

  In the thin film solar cell of the present invention, the average surface roughness Ra on the photoelectric conversion layer side of the first electrode layer is 25 nm or more, and the average surface roughness Ra on the photoelectric conversion layer side of the planarization layer is less than 25 nm. It is preferable that According to this aspect, the light scattering efficiency is excellent, and the utilization efficiency of the light incident in the solar cell can be increased.

  In the thin-film solar cell of the present invention, it is preferable that a transparent conductive oxide layer is disposed between the first electrode layer and the planarization layer. According to this aspect, it is possible to prevent corrosion of the first electrode layer when the planarizing layer is formed.

  In the thin film solar cell of the present invention, it is preferable that a transparent conductive oxide layer is disposed between the planarization layer and the photoelectric conversion layer. According to this aspect, even when gas is generated from the material constituting the planarization layer when the photoelectric conversion layer is formed, the transparent conductive oxide layer is disposed between the planarization layer and the photoelectric conversion layer. Thus, the influence of degassing can be prevented, and the film quality of the photoelectric conversion layer can be improved.

  According to the present invention, a thin film solar cell using a microcrystalline silicon photoelectric conversion layer having a high light confinement effect and a good film quality can be provided.

It is a schematic block diagram of the thin film solar cell of this invention.

  The thin film solar cell of this invention is demonstrated taking the solar cell shown in FIG. 1 as an example.

  The thin film solar cell shown in FIG. 1 is laminated on a substrate 1 in the order of a light-reflecting back electrode 2 having a surface with irregularities, a planarization layer 3, a photoelectric conversion layer 4, and a transparent electrode 5. The thin film solar cell shown in FIG. 1 is a substrate type solar cell, and the back electrode 2 corresponds to the “first electrode” in the present invention, and the transparent electrode 5 corresponds to the “second electrode” in the present invention. To do.

  The substrate 1 is not particularly limited. For example, an insulating plastic film substrate such as a polyimide film, a polyethylene terephthalate film, a polyethylene naphthalate film, a polyethersulfone film, an acrylic film, or an aramid film, a glass substrate, a stainless steel substrate, or the like can be used.

  There is no limitation in particular as a metal material which comprises the back surface electrode 2. FIG. A metal material having a light reflectance in a wavelength region that can be absorbed by the photoelectric conversion layer 4 is preferably 60% or more, and more preferably a metal material having 80% or more. Specifically, simple materials such as Al, Ag, Ta, Zn, Mo, W, Ni, and Cr, or alloy materials containing these as main components can be preferably used. More preferred is Ag, Al, or an alloy containing these as a main component. These metal materials can be formed by film formation by any film formation method known in the art, such as vapor deposition, sputtering, and plating.

  Irregularities for irregularly reflecting incident light are formed on the surface of the back electrode 2 on the photoelectric conversion layer side. The surface preferably has an average surface roughness Ra of 25 nm or more, more preferably 25 to 300 nm, and particularly preferably 40 to 200 nm. If the average surface roughness Ra is less than 25 nm, the light confinement effect may not be sufficient. Moreover, when average surface roughness Ra exceeds 300 nm, the case where the planarization effect cannot fully be exhibited unless the planarization layer formed on it is thickened is assumed. In such a case, the flatness can be ensured by forming the flattening layer in a plurality of times, but the film thickness is 10 times or more at the thick and thin portions of the flattening layer for flattening. To be different. Thus, if a region having a resistance 10 times or more higher in the film thickness direction is locally formed, the conversion efficiency of the solar cell may be lowered due to power loss due to the resistance, and therefore the upper limit is preferably 300 nm. In this invention, average surface roughness Ra means the value calculated | required from the uneven | corrugated data calculated | required by the surface measurement by a stylus type surface shape measuring device (profiler). Among them, it is preferable to use a value obtained by surface shape measurement using a scanning probe microscope (SPM), particularly an atomic force microscope (AFM).

In order to form irregularities on the surface of the back electrode 2, for example, an appropriate film forming condition for forming the back electrode 2 is selected, for example, 0.3 atomic% of aluminum (Al) is added to silver (Ag) It can be formed by a sputtering method using the alloy target. Specifically, a sputtering method under a discharge condition of about 300 ° C., Ar / O _{2 as} a discharge gas (flow rate ratio Ar / O ₂ = 90/10 to 95/5), and a discharge pressure of about 0.5 Pa. Can be formed. Further, after film formation, plasma treatment or etching treatment may be performed. Further, by forming an uneven structure on the film forming surface of the substrate 1 in advance and forming the back electrode 2 on the substrate on which the uneven structure is formed, the unevenness can be formed on the surface of the back electrode 2.

  A planarization layer 3 is disposed between the back electrode 2 and the photoelectric conversion layer 4, and the surface of the planarization layer 3 on the photoelectric conversion layer side is smoother than the surface of the back electrode 2 on the photoelectric conversion layer side. Has been. The surface preferably has an average surface roughness Ra of less than 25 nm, more preferably less than 20 nm, and particularly preferably less than 15 nm. When the average surface roughness Ra exceeds 25 nm, the film quality such as void generation of the microcrystalline silicon film is likely to be impaired, so that the microcrystalline silicon photoelectric conversion layer having a good film quality may not be formed on the back electrode 2 in some cases.

  In order to make the surface of the planarization layer 3 on the photoelectric conversion layer side smoother than the surface of the back electrode 2 on the photoelectric conversion layer side, a conductive polymer material in a solution or liquid in which a conductive polymer material is dissolved is used. The dispersion liquid in which the molecular material is dispersed is applied using a spin coater, bar coater, screen printer or the like, and dried by heating.

  When a liquid conductive polymer is applied on the surface of the back electrode having a concavo-convex structure, a large amount of material enters the concave portions, so that when viewed microscopically, the thickness of the flattening layer at the concave portions and convex portions is reduced. Since it is formed so as to be different (thick at the concave portion and thin at the convex portion) and is heated and dried in this state, as a result, the photoelectric conversion layer of the planarization layer 3 is more rough than the unevenness of the surface of the back electrode 2 on the photoelectric conversion layer side. The unevenness on the side surface is alleviated.

  The planarization layer 3 is formed of a transparent conductive polymer material made of a material having high conductivity and having a high transmittance for light in a wavelength region that can be absorbed by at least the photoelectric conversion layer 4. Examples of such transparent conductive polymer materials include polythiophene, polyaniline and the like doped with an oxidizing agent such as poly (styrene sulfonate), camphor sulfonic acid, dodecylbenzene sulfonic acid, and the like. Specifically, poly (3,4-ethylenedioxythiophene) doped with poly (styrenesulfonate) (PEDOT: PSS), polyaniline doped with camphorsulfonic acid (PANI: CSA), polyaniline Examples thereof include a material doped with dodecylbenzenesulfonic acid (PANI: DBSA). Using a dispersion or solution of these materials, a film can be formed by any method known in the art such as spin coating, ink jet, or printing.

  The conductivity of the planarizing layer 3 is preferably 1 S / cm or more, and more preferably 100 S / cm or more. Since the carrier transport resistance can be reduced as the conductivity of the flattening layer 3 is higher, the photoelectric conversion characteristics can be improved, and the decrease in the fill factor (FF) can be suppressed.

  The planarization layer 3 preferably has a light transmittance of 50% or more, more preferably 80% or more, for light having a wavelength of 300 to 1100 nm.

  The flattening layer 3 is preferably formed under conditions such that when formed on a substrate having a smooth surface, the film thickness is 50 nm or more, and the film thickness is 100 nm or more. It is more preferable that the film is formed by.

  The photoelectric conversion layer 4 includes an n-type silicon layer, an i-type silicon layer, and a p-type silicon layer. In the present invention, the photoelectric conversion layer 4 includes a microcrystalline silicon-based photoelectric conversion layer in which an i-type silicon layer is formed of a microcrystalline silicon film. Further, the photoelectric conversion layer 4 may have a multi-junction structure by stacking two or more layers. In the case of a multi-junction structure, an amorphous silicon photoelectric conversion layer in which an i-type silicon layer is formed of an amorphous silicon film is disposed on the transparent electrode side, and a microcrystalline silicon photoelectric conversion layer is disposed on the back electrode side. It is preferable to arrange and laminate. By laminating in this way, long-wavelength light that cannot be absorbed by the amorphous silicon-based photoelectric conversion layer can be used for power generation by the microcrystalline silicon-based photoelectric conversion layer, so that power generation efficiency can be further improved.

The transparent electrode 5 is made of ITO (indium oxide + tin oxide), Al (aluminum) or Ga (gallium) added ZnO, Nb (niobium) added TiO ₂ , F (fluorine) added SnO ₂ , IZO (indium oxide + zinc oxide). It is comprised with transparent conductive oxides, such as.

  As described above, when a microcrystalline silicon film is formed on an electrode layer that has been processed into a concavo-convex structure, a number of microcrystalline silicon film growth directions occur, and macroscopic crystal grains having different orientations form during film growth. There was a problem that defects occurred due to collision and the film quality deteriorated.

  In the solar cell according to the present invention, since the planarization layer 3 is disposed between the back electrode 2 and the photoelectric conversion layer 4, and the surface unevenness of the back electrode is relaxed by the planarization layer 3, Even if the undulation is increased, the film quality of the microcrystalline silicon film is not easily impaired. For this reason, it is possible to obtain a thin film solar cell having a high photoelectric conversion efficiency and having a microcrystalline silicon-based photoelectric conversion layer with good film quality while enhancing the light confinement effect. Moreover, since the unevenness | corrugation of the surface of a back surface electrode can be raised and the light confinement effect can be heightened, the thin film of a photoelectric converting layer is possible.

  In this embodiment, the planarization layer 3 is directly formed on the back electrode 2, but between the back electrode 2 and the planarization layer 3, ITO (indium-tin oxide), IZO (indium— First transparent conductive oxide formed of transparent conductive oxide material such as zinc oxide), IWO (indium-tungsten oxide), AZO (Al-doped zinc oxide), GZO (Ga-doped zinc oxide) A physical layer may be arranged. When the planarizing layer 3 is directly formed on the back electrode 2, depending on the material of the back electrode 2, the back electrode 2 may be corroded by the transparent conductive polymer material forming the planarizing layer 3. Corrosion of the back electrode 2 can be suppressed by disposing the first transparent conductive oxide layer between 2 and the planarizing layer 3.

  The first transparent conductive oxide layer is preferably formed on the condition that the film thickness is 10 to 1000 nm when formed on a substrate having a smooth surface. It is more preferable that the film is formed under such a condition that the film thickness is as follows. If the film thickness of the first transparent conductive oxide layer is too thin, the above effects may not be sufficiently obtained. In addition, if it is too thick, there is a possibility of deterioration in photoelectric conversion characteristics due to decrease in mass productivity due to the time required for film formation, increase in light absorption in the transparent conductive oxide layer, decrease in FF due to increase in resistance in the film thickness direction, etc. There is.

  In this embodiment, the photoelectric conversion layer 4 is directly formed on the planarization layer 3. However, between the planarization layer 3 and the photoelectric conversion layer 4, ITO (indium-tin oxide), IZO ( Second transparent conductive material formed of a transparent conductive oxide material such as indium-zinc oxide), IWO (indium-tungsten oxide), AZO (Al-doped zinc oxide), GZO (Ga-doped zinc oxide) A conductive oxide layer may be disposed.

  When the photoelectric conversion layer 4 is directly formed on the planarization layer 3, the photoelectric conversion layer 4 may be removed from the planarization layer 3 when the photoelectric conversion layer 4 is formed depending on the type of the transparent conductive polymer material used for the formation of the planarization layer 3. Although the film quality of the photoelectric conversion layer 4 may be impaired by the gas, the second transparent conductive oxide layer is disposed between the flattening layer 3 and the photoelectric conversion layer 4, so that the flattening layer Deterioration of the film quality of the photoelectric conversion layer 4 due to degassing from 3 can be prevented.

  The second transparent conductive oxide layer is preferably formed on the condition that the film thickness is 10 to 1000 nm when formed on a substrate having a smooth surface. It is more preferable that the film is formed under such a condition that the film thickness is as follows. If the thickness of the second transparent conductive oxide layer is too thin, the above effects may not be sufficiently obtained. In addition, if it is too thick, there is a possibility of deterioration in photoelectric conversion characteristics due to decrease in mass productivity due to the time required for film formation, increase in light absorption in the transparent conductive oxide layer, decrease in FF due to increase in resistance in the film thickness direction, etc. There is.

Example 1
A glass substrate (trade name “1737 glass plate”, manufactured by Corning) is introduced into a vacuum chamber, an Ar atmosphere, a pressure of 0.67 Pa, and Ag-Al containing 0.3 atomic% (hereinafter referred to as “at%”) of Al. When an alloy is used as a target material, the film forming speed is 0.56 nm / s, the film forming temperature is 250 ° C., and the film is formed on a smooth substrate by the DC magnetron sputtering method. A film was formed, and a back electrode having an average surface roughness Ra of 45 nm was formed on a glass substrate.
Next, the inside of the vacuum chamber is an Ar—O ₂ atmosphere, the pressure is 0.67 Pa, a ZnO + 2 wt% Al ₂ O ₃ target is used as a target material, a film forming speed is 0.11 nm / s, and a film forming temperature is 250 ° C. Then, when a film was formed on a smooth substrate by a DC magnetron sputtering method, the film was formed under the condition of a film thickness of 30 nm, and a first transparent conductive oxide layer was formed on the back electrode. The average surface roughness Ra of the first transparent conductive oxide layer was 44 nm.
Next, 5 weight of dimethyl sulfoxide is added to an aqueous dispersion (trade name “CLEVIOS PH500”, manufactured by HC Starck) of poly (3,4-ethylenedioxythiophene) doped with poly (styrene sulfonate). The non-film-formation portion on the substrate was previously masked with Kapton tape, and spin coated at 2000 rpm × 30 s after the material was dropped. Subsequently, it heated for 15 minutes on the hotplate set to 130 degreeC, and the Kapton tape was peeled after that, and the planarization layer was formed on the 1st transparent conductive oxide layer. The film forming condition of the planarizing layer is a condition that results in a film thickness of about 100 nm when performed on a smooth substrate. Moreover, the average surface roughness Ra of this planarization layer was 7 nm.
Next, an n-type silicon layer made of an amorphous silicon film with a thickness of 30 nm, an i-type silicon layer made of a microcrystalline silicon film with a thickness of 2000 nm, and a p-type silicon made of an amorphous silicon film with a thickness of 30 nm on the planarizing layer. The layers were sequentially formed by a CVD method to form a microcrystalline silicon-based photoelectric conversion layer.
Next, an ITO target (manufactured by Tosoh) is used on the photoelectric conversion layer, Ar + 1% O ₂ is used as a discharge gas, and a transparent electrode made of an ITO film having a thickness of 70 nm is formed by a DC magnetron sputtering method with a discharge pressure of 0.7 Pa. Formed to produce a thin film solar cell.

The open-circuit voltage, short-circuit current density, fill factor (FF), and photoelectric conversion efficiency of this thin-film solar cell were measured using a solar simulator under AM (air mass) 1.5 equivalent, illumination intensity of 100 mW / cm ² , and light irradiation conditions of 25 ° C. did. The results are shown in Table 1.

(Comparative Example 1)
In Example 1, a thin-film solar cell was manufactured in the same manner as in Example 1 except that no planarization layer was formed, and cell characteristics were measured. The results are shown in Table 1.

  As shown in Table 1, by providing the planarization layer, the film quality of the microcrystalline silicon photoelectric conversion layer was improved, and the photoelectric conversion efficiency was improved.

1: Substrate 2: Back electrode 3: Planarization layer 4: Photoelectric conversion layer 5: Transparent electrode

Claims (4)

A light-reflective first electrode layer, at least one photoelectric conversion layer, and a light-transmissive second electrode layer are sequentially stacked on the substrate, and the photoelectric conversion layer side surface of the first electrode layer is formed on the surface. In a thin film solar cell in which irregularities for irregularly reflecting incident light are formed,
At least one of the photoelectric conversion layers is a microcrystalline silicon photoelectric conversion layer,
A planarization layer made of a transparent conductive polymer material is disposed between the first electrode layer and the photoelectric conversion layer, and the surface of the planarization layer on the photoelectric conversion layer side of the first electrode layer A thin-film solar cell, which is smoother than the surface on the photoelectric conversion layer side.   The average surface roughness Ra on the photoelectric conversion layer side of the first electrode layer is 25 nm or more, and the average surface roughness Ra on the photoelectric conversion layer side of the planarization layer is less than 25 nm. Thin film solar cell.   The thin film solar cell according to claim 1, wherein a transparent conductive oxide layer is disposed between the first electrode layer and the planarizing layer.   The thin film solar cell of any one of Claims 1-3 by which the transparent conductive oxide layer is arrange | positioned between the said planarization layer and the said photoelectric converting layer.