JP5783984B2 - Photoelectric conversion element, solar cell, and production method thereof - Google Patents

Description

  Embodiments described herein relate generally to a photoelectric conversion element, a solar cell, and a method for manufacturing the same.

The development of compound photoelectric conversion devices using semiconductor thin films as light absorption layers has progressed. Among them, thin film photoelectric conversion devices using a p-type semiconductor layer having a chalcopyrite structure as a light absorption layer exhibit high conversion efficiency, Expected. Specifically, high conversion efficiency is obtained in a thin film photoelectric conversion element using Cu (In, Ga) Se ₂ made of Cu—In—Ga—Se as a light absorption layer. In general, a thin film photoelectric conversion element using a p-type semiconductor layer composed of Cu—In—Ga—Se as a light absorption layer is formed of a molybdenum back electrode, a p-type semiconductor layer, and an n-type semiconductor on a blue plate glass serving as a substrate. A layer, an insulating layer, a transparent electrode, an upper electrode, and an antireflection film are stacked. The conversion efficiency η uses an open circuit voltage Voc, a short circuit current density Jsc, an output factor FF, and an incident power density P.
η = Voc · Jsc · FF / P · 100
It is represented by

At present, the CIGS layer which is a p-type compound semiconductor layer (light absorption layer) in a high conversion efficiency CIGS thin film solar cell has a band gap of about 1.2 eV, for example, represented by CuIn _0.7 Ga _0.3 Se _2. A material having a Ga content of about 30% is used. In order to achieve matching with the sunlight spectrum and to obtain a thin film solar cell with higher conversion efficiency, it is necessary to increase the band gap of the CIGS light absorption layer to about 1.4 eV and the Ga content to 70%. Moreover, tandemization is necessary to produce a solar cell with higher conversion efficiency, and the CIGS light absorption layer as the top layer has, for example, a wide gap with a band gap of about 1.7 eV and a Ga amount of 100%. It is necessary to use a layer. However, in CIGS with high Ga concentration, it is difficult to produce a high-quality CIGS thin film, and high conversion efficiency as expected is not obtained.

JP2005-228975

  An object of the embodiment is to provide a highly efficient photoelectric conversion element, a solar cell, and a method for manufacturing the same.

The photoelectric conversion element of the embodiment is a compound semiconductor satisfying Cu (In _y Ga _1-y ) Se ₂ (where y satisfies 0 ≦ y ≦ 0.35 ) having a band gap of 1.39 eV or more and 1.68 eV or less. (Cu _x Ag _1-x ) (In _y Ga _1-y ) Se _{2 having} a layer or band gap of 1.28 eV or more and 1.68 eV or less, x satisfies 0 <x < 1, and y satisfies 0 ≦ p-type light absorption, which is a compound semiconductor layer satisfying y ≦ 1, wherein the (maximum value / minimum value) of the Ga / (In + Ga) molar ratio in the thickness direction of the compound semiconductor layer is 1.0 or more and 1.5 or less And a transparent electrode disposed on the n-type semiconductor layer. The n-type semiconductor layer has a pn junction with the p-type light absorption layer.

In the manufacturing method of the photoelectric conversion element according to the embodiment, a compound composed of Cu or Ag and Se is deposited on the back electrode, then the compound is heated to a melting point or higher, Ga and Se are deposited, and then In, Ga And Se, and then Ga and Se are deposited to form a p-type light absorption layer, an n-type semiconductor layer is formed on the p-type light absorption layer, and a transparent electrode is formed on the n-type semiconductor layer. It is characterized by that.
The p-type light absorption layer is a compound semiconductor layer satisfying Cu (In _y Ga _1-y ) Se ₂ (y satisfies 0 ≦ y ≦ 0.35 ) having a band gap of 1.39 eV or more and 1.68 eV or less, Alternatively, (Cu _x Ag _1-x ) (In _y Ga _1-y ) Se ₂ (x is 0 <x < 1, y is 0 ≦ y ≦ 1) with a band gap of 1.28 eV or more and 1.68 eV or less. It is preferable that the compound semiconductor layer satisfy

In another embodiment, a method for producing a photoelectric conversion element includes depositing a compound composed of Cu or Ag and Se on a back electrode, then heating the compound to a melting point or higher, then depositing Ga and Se, , In and Se are deposited to form a p-type light absorption layer, an n-type semiconductor layer is formed on the p-type light absorption layer, and a transparent electrode is formed on the n-type semiconductor layer.
The p-type light absorption layer is a compound semiconductor layer satisfying Cu (In _y Ga _1-y ) Se ₂ (y satisfies 0 ≦ y ≦ 0.35 ) having a band gap of 1.39 eV or more and 1.68 eV or less, Alternatively, (Cu _x Ag _1-x ) (In _y Ga _1-y ) Se ₂ (x is 0 <x < 1, y is 0 ≦ y ≦ 1) with a band gap of 1.28 eV or more and 1.68 eV or less. It is preferable that the compound semiconductor layer satisfy

1 is a conceptual cross-sectional view of a photoelectric conversion element 100 according to an embodiment of the present invention. It is a section conceptual diagram of photoelectric conversion element 200 concerning one embodiment of the present invention. It is a triangular phase diagram of CIGS used for the photoelectric conversion element of an embodiment. It is a binary phase diagram of CIGS used for the photoelectric conversion element of an embodiment. It is a binary phase diagram of AIGS used for the photoelectric conversion element of embodiment. It is a conceptual diagram of Ga / (In + Ga) composition distribution in the thickness direction of the CIGS layer used for the photoelectric conversion element of embodiment.

(Embodiment 1)
Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings.
(Photoelectric conversion element)
A photoelectric conversion element 100 according to this embodiment shown in the conceptual diagram of FIG. 1 includes a substrate 1, a back electrode 2 formed on the substrate 1, a p-type light absorption layer 3 formed on the back electrode 2, an n-type semiconductor layer 4a formed on the p-type light absorption layer 3, a transparent electrode 5 disposed on the n-type semiconductor layer 4a, an upper electrode 6 and an antireflection film 7 formed on the transparent electrode 5, Are thin film photoelectric conversion elements 100. Specific examples of the photoelectric conversion element 100 include a solar battery.

(substrate)
As the substrate 1 of the embodiment, blue plate glass is desirably used, and a metal plate such as stainless steel, Ti (titanium) or Cr (chromium), or a resin such as polyimide can also be used.

(Back electrode)
The back electrode 2 of the embodiment is an electrode of a photoelectric conversion element, and is a metal film formed on the substrate 1. As the back electrode 2, a conductive metal film such as Mo or W can be used. Among these, it is desirable to use a Mo film for the back electrode 2. The film thickness of the back electrode 2 is, for example, not less than 500 nm and not more than 1000 nm.

(P-type light absorption layer)
The light absorption layer 3a of the embodiment is a p-type compound semiconductor layer. The p-type light absorption layer 3 a is a layer formed on the main surface facing the substrate 1 on the back electrode 2. The p-type light absorption layer 3a of the embodiment includes at least one type Ib group element of Cu and Ag, at least one type IIIb element of In and Ga, and at least one type of Se, O, S, and Te. And a compound semiconductor layer having a chalcopyrite type structure. Specifically, the compound semiconductor layer having a chalcopyrite structure has (Cu _x Ag _1-x ) (In _y Ga _1-y ) Se ₂ , x satisfies 0 ≦ x ≦ 1, and y satisfies 0 ≦ It is preferable to use a compound satisfying y ≦ 1. Cu (In, Ga) and Se ₂ or less, and wherein the CIGS, Ag (In, Ga) and Se ₂ or less, AIGS, (Cu, Ag) (In, Ga) and Se ₂ hereinafter referred to as ACIGS.

  The group IIIb element of the light absorption layer 3a has a concentration distribution in the thickness direction of the layer. In the case of a single inclination, the concentration distribution changes monotonously, and the value of the Ga / (In + Ga) molar ratio of the group IIIb element on the transparent electrode 5 side of the compound semiconductor layer becomes smaller than that on the back electrode 2 side. In the case of a double gradient, there exists a concentration gradient in the layer so that a valley of the concentration distribution (a valley formed by positive and negative gradients) can be formed.

  When a light-absorbing layer with a high Ga ratio is formed by a selenization method, a three-step method, or a five-step method, it is difficult to increase the particle size, and a single-tilt slope with a deep band-double-slope valley The large amount was a factor that lowered the conversion efficiency. The light absorption layer 3a of the embodiment is a light absorption layer having a high Ga content such that the Ga / (In + Ga) molar ratio is 0.5 or more by adding a small amount of In near the surface while having a large particle diameter. But the double sloped valley is shallow. In the case of a single inclination, the amount of inclination is small. The (maximum value / minimum value) of the Ga / (In + Ga) molar ratio in the light absorption layer 3a in which the valley of the double inclination is shallow or the amount of inclination of the single inclination is small is 1.0 to 1.5. From the viewpoint of improving carrier collection efficiency, the (maximum value / minimum value) of the Ga / (In + Ga) molar ratio in the light absorption layer is preferably 1.1 or more and 1.4 or less. If the (maximum value / minimum value) of the Ga / (In + Ga) molar ratio is smaller than 1.1, it is not preferable that the band double slope valley becomes too shallow, making it difficult for carriers to move. Further, if the (maximum value / minimum value) of the Ga / (In + Ga) molar ratio is smaller than 1.1, it is not preferable that the movement of carriers becomes difficult because the amount of inclination of the single band inclination becomes too small. When the (maximum value / minimum value) of the Ga / (In + Ga) molar ratio is larger than 1.4, it is not preferable that the carriers recombine at the valley of the band double slope. In addition, when the (maximum value / minimum value) of the Ga / (In + Ga) molar ratio is larger than 1.4, the amount of inclination of the band single inclination becomes too large, so that carriers can easily recombine at the pn junction interface. It is not preferable. Moreover, the particle size of the compound constituting the light absorption layer 3a can be increased, and the average crystal particle size (volume average particle size) can be 1000 nm or more and 2500 nm or less.

  A cross section of the central portion of the photoelectric conversion element is obtained by SIMS (Secondary Ion Mass Spectrometry) or TEM-EDX (Over Electron Microscopy-Energy Dispersion X-ray Spectroscopy: Transmission Electron Microscope-Energy X-ray Energy Analysis). The Ga / (In + Ga) molar ratio can be obtained by quantitative analysis of each composition of the light absorption layer 3a by Spectroscopy). It is possible to confirm that the photoelectric conversion element satisfies the above range by performing quantitative analysis continuously in the thickness direction of the layer. Note that the value of the Ga / (In + Ga) molar ratio and its maximum and minimum values are not only measured in the thickness direction of the layer at one measurement point, but also in the thickness direction of the layers at a plurality of nearby measurement points. You may obtain | require from the value which averaged the measured value.

  The average crystal grain size of the p-type light absorption layer 3a can be known from the SEM cross-sectional image at the center of the photoelectric conversion element. When a cross-sectional image is obtained, it can be confirmed by observation of the cross-sectional SEM (Scanning Electron Microscope). The crystal grain size in an observation image having a width of 2 μm is measured by 20,000 times cross-sectional SEM observation of the central portion of the light absorption layer 3a, and the average crystal grain size is calculated.

  The band gap size can be adjusted to a target value by a combination of In and Ga in the light absorption layer 3a. When CIGS is used for the light absorption layer 3a, the band gap can be set to 1.04 eV or more and 1.68 eV or less. When AIGS is used for the light absorption layer 3a, the band gap can be set to 1.24 eV or more and 1.83 eV or less. The light absorption layer 3a of the embodiment can be used for a single junction cell or a multi-junction cell having a tandem structure. The wide gap light absorption layer 3a is preferably used for the top layer of the multi-junction cell.

A preferred combination of In and Ga when CIGS is used as the light absorption layer 3a will be described. In order to match the solar spectrum, it is desirable that the band gap is about 1.4 eV in a single junction cell, and the Ga / (In + Ga) molar ratio is 0.5 or more and 1.0 or less, The band gap is preferably 1.28 eV or more and 1.68 eV or less, and the band gap is 1.35 eV or more and 1.59 eV or less by setting the Ga / (In + Ga) molar ratio to 0.6 or more and 0.9 or less. Therefore, it is more preferable to set the Ga / (In + Ga) molar ratio to 0.65 or more and 0.85 or less because the band gap becomes 1.39 eV or more and 1.55 eV or less. Further, it is preferable to use CuGaSe ₂ having a band gap of 1.68 eV as the top layer of the tandem structure.

A preferred combination of In and Ga when AIGS is used as the light absorption layer 3a will be described. In order to match with the solar spectrum, it is desirable that the band gap is about 1.4 eV in a single junction cell, and the Ga / (In + Ga) molar ratio is 0.07 or more and 0.75 or less, The band gap is preferably 1.28 eV or more and 1.68 eV or less, and is preferably set to a Ga / (In + Ga mole) ratio of 0.19 or more and 0.6 or less, whereby the band gap becomes 1.35 eV or more and 1.59 eV or less. Therefore, it is more preferable to set the Ga / (In + Ga) molar ratio to 0.25 or more and 0.53 or less because the band gap becomes 1.39 eV or more and 1.55 eV or less. In addition, it is preferable to use AgIn _0.25 Ga _0.75 Se ₂ having a band gap of 1.68 eV as the top layer of the tandem structure.

  A preferred combination of In and Ga when ACIGS is used as the light absorption layer 3a will be described. In order to achieve matching with the solar spectrum, it is desirable that the band gap be about 1.4 eV in a single junction cell, and the Ga / (In + Ga) molar ratio should be a value larger than 0.07 and smaller than 1.0. For example, the band gap is preferably 1.28 eV or more and 1.68 eV or less.

Summarizing the preferable combinations of In and Ga described above, in order to set the band gap of the p-type light absorption layer 3a to 1.28 eV or more and 1.68 eV or less which is suitable for matching with the solar spectrum, (Cu _x Ag _{1 -x) (when} the _{in y Ga 1-y) Se} 2 of x = 0 (AIGS), Ga / (in + Ga) molar ratio is preferably 0.07 or more 0.53 or less, when 0 <x <1 (ACIGS), Ga / (In + Ga) molar ratio is preferably greater than 0.07 and smaller than 1, and when x = 1 (CIGS), the Ga / (In + Ga) molar ratio is 0.5 or more and 1.0. The following is preferred.

  CIGS, AIGS, and ACIGS can control a band gap by the composition ratio of In and Ga. That is, this means that the band gap distribution can be inclined by adjusting the composition distribution of In and Ga in the thickness direction of the layer. When the p-type light absorption layer 3 and the n-type semiconductor layer 4a are heterojunction, in the single gradient distribution in which the Ga composition ratio on the Mo back electrode side is high and the Ga composition ratio on the n-type semiconductor layer 4a side is low, Therefore, photoexcited carriers (electrons) move from the Mo side on the back surface to the CIGS interface side of the pn junction surface, thereby improving carrier collection efficiency. In the double gradient distribution in which the Ga composition ratio of the pn junction interface is slightly increased to form a notch in this single gradient distribution, the band at the pn junction interface is improved in addition to the improvement of the carrier collection efficiency due to the internal electric field of the single gradient distribution. By increasing the gap, recombination in the depletion layer is reduced and the open-circuit voltage is improved.

(N-type semiconductor layer)
The n-type semiconductor layer 4a of the embodiment is a layer that is pn heterojunction with the light absorption layer 3a formed on the main surface side facing the back electrode 2 on the p-type light absorption layer 3a. For the n-type semiconductor layer 4a, ZnO added with CdS, Zn (O, S, OH), and Mg can be used. The n-type semiconductor layer 4a is used as a buffer layer of a photoelectric conversion element, and is preferably an n-type semiconductor whose Fermi level is controlled so that a photoelectric conversion element with a high open voltage can be obtained. The thickness of the n-type semiconductor layer 4a is preferably 30 nm or more and 100 nm or less, and by setting the thickness to 40 nm or more and 60 nm or less, a transparent electrode is deposited while suppressing short-wavelength light absorption in the n-type semiconductor layer 4a. This is more preferable because it can avoid film formation damage.

(Transparent electrode)
The transparent electrode 5 of the embodiment is a film that transmits light such as sunlight and has conductivity. For the transparent electrode 5, for example, ZnO: Al containing 2 wt% of alumina (Al ₂ O ₃ ) or ZnO: B using B from diborane as a dopant can be used. For example, a (Zn, Mg) O layer can be provided as a semi-insulating layer between the transparent electrode 5 and the n-type semiconductor layer 4a.

(Antireflection film)
The antireflection film 6 according to the embodiment is a film for easily introducing light into the p-type light absorption layer 3 and is formed on the transparent electrode 5. For example, MgF ₂ is desirably used as the antireflection film 6. In order to form an extraction electrode or the like on the transparent electrode 5, it is preferable that a part of the transparent electrode 5 has a region not covered with the antireflection film 6.

(Extraction electrode)
An extraction electrode may be provided on the back electrode 2 and the transparent electrode 5. As the extraction electrode, Al, Ag, Au, or the like can be used. Furthermore, in order to improve the adhesiveness with the transparent electrode 5, an extraction electrode may be provided on the deposited film of Ni or Cr.

(Embodiment 2)
A photoelectric conversion element 200 according to this embodiment shown in the conceptual diagram of FIG. 2 includes a substrate 1, a back electrode 2 formed on the substrate 1, a p-type light absorption layer 3b formed on the back electrode 2, A thin film comprising an n-type semiconductor layer 4b formed on the p-type light absorption layer 3b, a transparent electrode 5 formed on the n-type semiconductor layer 4b, and an antireflection film 6 formed on the transparent electrode 5. Type photoelectric conversion element 200. A specific example of the photoelectric conversion element 200 is a solar cell.

  The photoelectric conversion element of Embodiment 2 is common to Embodiment 1 except that the p-type light absorption layer 3b and the n-type semiconductor layer 4b form a pn junction by homojunction. Therefore, since the configuration other than the light absorption layer 3 and the n-type semiconductor layer 4b is the same as that of the first embodiment, the description thereof is omitted.

(P-type light absorption layer n-type semiconductor layer)
The p-type light absorption layer 3b of the embodiment is a p-type compound semiconductor layer. The p-type light absorption layer 3 b is a layer formed on the main surface facing the substrate 1 on the back electrode 2. The n-type semiconductor layer 4b is a layer in a region formed by forming a part of the p-type light absorption layer on the transparent electrode 5 side into an n-type, and forms a pn junction by homojunction with the p-type light absorption layer 3b. To do. The n-type semiconductor layer 4b is a compound semiconductor layer having a conductivity type different from that of the p-type light absorption layer and having the same element as the p-type light absorption layer. The p-type light absorption layer 3b and the n-type semiconductor layer 4b of the embodiment are similar to the p-type light absorption layer 3a in that at least one type Ib group element of Cu and Ag and at least one type IIIb group of In and Ga. A compound semiconductor layer having a chalcopyrite structure containing an element and at least one VIb group element of Se, O, S, and Te is preferable. The thickness of the p-type semiconductor layer 3b is preferably 1500 nm or more and 3000 nm or less. The thickness of the n-type semiconductor layer is preferably 100 nm or more and 500 nm or less.

  In the homojunction element structure, the buffer layer can be omitted, so that short-wavelength light absorption by the buffer layer can be eliminated, and carrier recombination at the junction interface that occurs in the heterojunction structure can be avoided. The conversion efficiency of the battery is improved.

  The group IIIb element of the light absorption layer 3b has a concentration distribution with a single gradient or a double gradient in the thickness direction of the layer. In the case of a single inclination, the value of the Ga / (In + Ga) molar ratio of the group IIIb element on the transparent electrode 5 side of the n-type semiconductor layer 4b is smaller than that on the back electrode 2 side of the p-type light absorption layer 3b. In the case of a double gradient, there exists a concentration gradient in the compound semiconductor layer so that a valley of the concentration distribution is generated. The bottom of the double inclined valley exists in either the light absorption layer 3b, the interface between the light absorption layer 3b and the n-type semiconductor layer 4b, or the n-type semiconductor layer 4b.

When a compound semiconductor layer of a p-type light absorption layer and an n-type semiconductor layer having a high Ga ratio is formed by a selenization method, a three-step method, or a five-step method, it is difficult to increase the particle size. The fact that the sloped valley is deep has been a factor in reducing the conversion efficiency. Since the light absorption layer 3b and the n-type semiconductor layer 4b of the embodiment are Ga-diffused, even if the light absorption layer has a high Ga content such that the Ga / (In + Ga) molar ratio is 0.5 or more, the double gradient The valley is shallow. The (maximum value / minimum value) of the Ga / (In + Ga) molar ratio in the light absorption layer 3b and the n-type semiconductor layer 4b is 1.0 or more and 1.5 or less. From the viewpoint of improving carrier collection efficiency, the (maximum value / minimum value) of the Ga / (In + Ga) molar ratio in the light absorption layer 3b and the n-type semiconductor layer 4b is preferably 1.1 or more and 1.4 or less. If the (maximum value / minimum value) of the Ga / (In + Ga) molar ratio is smaller than 1.1, it is not preferable that the band double slope valley becomes too shallow, making it difficult for carriers to move. When the (maximum value / minimum value) of the Ga / (In + Ga) molar ratio is larger than 1.4, it is not preferable that the carriers recombine at the valley of the band double slope. Further, it is possible to increase the particle size of the compound constituting the light absorption layer 3b and the n-type semiconductor layer 4b, and the average crystal particle size can be set to 1000 nm to 2500 nm.
The preferred combination of In and Ga in the light absorption layer 3b is the same as in the first embodiment.

  CIGS, AIGS, and ACIGS can control a band gap by the composition ratio of In and Ga. That is, this means that the band gap distribution can be inclined by adjusting the composition distribution of In and Ga in the thickness direction of the layer. When the p-type light absorption layer 3b and the n-type semiconductor layer 4a are heterojunction, in the single gradient distribution in which the Ga composition ratio on the Mo back electrode side is high and the Ga composition ratio on the n-type semiconductor layer 4a side is low, Therefore, photoexcited carriers (electrons) move from the Mo side on the back surface to the CIGS interface side of the pn junction surface, thereby improving carrier collection efficiency. In the double gradient distribution in which the Ga composition ratio of the pn junction interface is slightly increased to form a notch in this single gradient distribution, the band at the pn junction interface is improved in addition to the improvement of the carrier collection efficiency due to the internal electric field of the single gradient distribution. By increasing the gap, recombination in the depletion layer is reduced and the open-circuit voltage is improved. The band double sloped valley may be on either the p-layer side or the n-layer side, but more preferably on the n-layer side in terms of carrier collection.

  Next, the manufacturing method of the photoelectric conversion element of the embodiment will be described focusing on the manufacturing method of the p-type light absorption layers 3a and 3b.

  In the method of manufacturing the band-double-tilted p-type light absorption layer according to the embodiment, a compound comprising Cu, Ag, and Se is deposited on the back electrode, and then the compound is heated to a melting point or more to deposit Ga and Se. Then, In, Ga, and Se are deposited, and then Ga and Se are deposited.

  In the manufacturing method of the band-single-gradient p-type light absorption layer of the embodiment, a compound composed of Cu or Ag and Se is deposited on the back electrode, and then the compound is heated to the melting point or higher, and then Ga and Se. And then deposit In and Se.

(Embodiment 3)
Embodiment 3 shows the manufacturing method of the photoelectric conversion element 100 of Embodiment 1. FIG.
[Step of forming back electrode on substrate]
A back electrode 2 is formed on the substrate 1. Examples of the film forming method include a sputtering method.

[Step of forming light absorption layer on back electrode]
A compound semiconductor thin film to be the light absorption layer 3 a is formed on the back electrode 2.

Conventionally, a vacuum deposition method called a three-stage method and a sputtering method called a selenization method have been used as a method for forming the light absorption layer 14. In the three-step method, first, In and Ga that are Group IIIb elements and Se that is a Group VIb element are vacuum-deposited, then Cu and Se that are Group Ib elements are vapor-deposited, and finally, In and Ga again. Se is a technique for evaporating Se, and the selenization method is a technique for performing heat treatment in an H ₂ Se gas atmosphere after Cu and IIIb group elements are deposited by sputtering. The selenization method is an advantageous film formation method for increasing the area, but it is difficult to increase the particle size, and is also a disadvantageous film formation method for forming a double inclined band. The three-stage method is used as a conventional film formation method for CIGS having a Ga / (In + Ga) molar ratio of less than 0.5. However, when the Ga / (In + Ga) molar ratio is greater than 0.5, the diffusion of Ga Since the coefficient is small, it is difficult to increase the particle size, and the double tilt of the band also has a Ga / (In + Ga) molar ratio (maximum value / minimum value) of more than 1.5, which lowers the carrier collection efficiency. By doing so, the conversion efficiency of the solar cell is lowered.

  In the Tri-phase method shown in FIG. 3, even when the Ga / (In + Ga) molar ratio is 0.5 or more and 1.0 or less, the increase in the particle size and the Ga / (In + Ga) molar ratio (maximum value / minimum value). ) Is a film forming technique that satisfies both of the double slopes of the band of 1.0 to 1.5. In the Tri-phase method, first, Cu that is an Ib group element and Se that is a VIb group element are vacuum-deposited, and then heated to a melting point or higher to form a Cu-Se liquid layer. The formation of the liquid layer can be confirmed by observation using reflection high energy electron diffraction (RHEED) or detection of a value at which the temperature change when the substrate temperature is gradually heated is detected. To do. On this Cu-Se liquid layer, Ga and Se which are IIIb group elements are vacuum-deposited. Since crystal growth proceeds through the melt, an increase in particle size is promoted. When a target film thickness necessary for light absorption is obtained, in order to form a double slope of the band, In, Ga and Se are once vacuum-deposited, and finally Ga and Se are again evaporated. The raw material for forming the first liquid layer may be Cu-Se alone, but it is more preferable to mix about 30% of Ga because eutectic and melting point decrease as shown in FIG. In the process of vacuum evaporation of In, Ga, and Se, if the amount of In deposited is increased too much, the valley of the band double slope becomes too deep, so the Ga / (In + Ga) molar ratio here is preferably about 0.8. . As can be seen from the binary phase diagram of FIG. 5, the same film formation method can be applied to AIGS using Ag instead of Cu. Similarly, the Tri-Phase method can be employed for the ACIGS film forming method. The graph of FIG. 6 shows the difference in Ga / (In + Ga) molar ratio in the thickness direction of the light absorption layer 3a due to the difference in the Tri-Phase method, the three-step method, and the five-step method. Thus, by forming the light absorption layer 3a by the Tri-Phase method, the (maximum value / minimum value) of the Ga / (In + Ga) molar ratio is the (maximum value) of the molar ratio of the three-step method or the five-step method. / Minimum value), and the valley of the molar ratio can be made shallower. As a result, it is possible to form a wide-gap light absorption layer with good band inclination and good crystallinity.

  The light absorption layer having a single inclination can be formed by vacuum-depositing In and Se after Ga and Se are vacuum-deposited on the Cu-Se liquid layer in the double inclination formation process of CGS. Further, it can be easily produced by adding a little In to CGS. The process of forming a Cu—Se liquid layer by first vacuum-depositing Cu and Se and then heating to the melting point or higher is the same as the process of forming the double gradient. The formation of the liquid layer is confirmed by observation using reflection high-energy electron diffraction (RHEED) or by detecting a value at which the temperature change becomes constant when the substrate temperature is gradually heated. Ga and Se are vacuum-deposited on this Cu-Se liquid layer. Also in this case, since the crystal growth proceeds via the melt, the increase in the particle size is promoted. Finally, in order to adjust the inclination, Se and a small amount of In are vacuum deposited. Due to the difference in the diffusion coefficient between Ga and In, a single slope of the band is formed. It should be noted that the same method can be adopted for single slope AIGS and ACIGS.

[Step of forming an n-type semiconductor layer on the light absorption layer]
An n-type semiconductor layer 4a is deposited on the obtained light absorption layer 3a.
As a method for forming the n-type semiconductor layer 4a, sputtering, vapor deposition, chemical vapor deposition (CVD), liquid layer deposition (CBD), molecular beam epitaxy (MBE), and molecular beam epitaxy (MBE) are used. (Beam Epitaxy) and the like.

  When the n-type semiconductor layer 4a is formed by sputtering, the substrate temperature is preferably 10 ° C. or higher and 300 ° C. or lower, and more preferably 200 ° C. or higher and 250 ° C. or lower. If the substrate temperature is too low, the crystallinity of the n-type semiconductor layer 4 to be formed deteriorates. Conversely, if the temperature is too high, a material having a target crystal structure cannot be obtained. It becomes difficult to form.

[Step of forming semi-insulating layer on n-type semiconductor layer]
A semi-insulating layer is deposited on the obtained n-type semiconductor layer 4a.
As a method for forming a semi-insulating layer, a method of forming a thin film by spin coating or the like by applying a mixed solution of a target raw material and a solvent, a sputtering process of a vacuum process, a vacuum deposition method, or metal organic chemical vapor deposition (MOCVD) ) Method. This step can be omitted.

[Step of forming transparent electrode layer on semi-insulating layer or n-type semiconductor layer]
Subsequently, the transparent electrode layer 5 is deposited on the semi-insulating layer or the n-type semiconductor layer 4a.
Examples of the film forming method include a sputtering method in a vacuum process, a vacuum deposition method, or a metal organic chemical vapor deposition (MOCVD) method.

[Step of taking out electrode on back electrode and transparent electrode layer]
The take-out electrode is deposited on a portion excluding at least the portion where the light absorption layer 3 is formed on the back electrode 2.
The take-out electrode is deposited on a portion excluding at least a portion where the antireflection film 6 is formed on the transparent electrode layer 5.
Examples of the film forming method include a sputtering method and a vacuum deposition method.
The film formation of the extraction electrode may be performed in one step, or may be performed after any step as a separate step.

[Step of forming an antireflection film on the transparent electrode layer]
Finally, an antireflection film 6 is deposited on a portion excluding at least a portion where the extraction electrode is formed on the transparent electrode 5.
Examples of the film forming method include a sputtering method and a vacuum deposition method.
The photoelectric conversion element 100 (compound thin film solar cell) shown in the conceptual diagram of FIG. 1 is produced through the above steps.
When producing a module of a compound thin film solar cell, after the step of forming the back electrode 2 on the substrate 1, the step of dividing the back electrode 2 with a laser, and further forming the n-type semiconductor layer 4a on the light absorption layer 3a After the step of forming and the step of forming the transparent electrode 5 on the n-type semiconductor layer 4a, integration can be performed by sandwiching a step of dividing the sample by mechanical scribing.

(Embodiment 4)
Embodiment 4 shows the manufacturing method of the photoelectric conversion element 200 of Embodiment 1. FIG. The fourth embodiment is the same as the manufacturing method of the third embodiment except for the manufacturing method of the n-type semiconductor layer 4b. Description of the common manufacturing method is omitted.

  A homojunction element structure can be obtained by converting a part of the surface region on the side where the transparent electrode 5 of CIGS / AIGS / ACIGS which is the light absorption layer 3b is formed into an n-type. The light absorption layer 3b can be made to be n-type by replacing the Cu • Ag (Ib group element) site of CIGS • AIGS • ACIGS with at least one element of Mg, Zn, or Cd. Replacement with Cd is not recommended from the viewpoint of environmental load, but it is possible in principle. Further, the n-type semiconductor layer 4b can be formed by replacing the Se (VIb group element) site with Sb or Bi to make the light absorption layer 3b n-type.

  Substitution of the Cu / Ag site is a substrate 1 deposited up to a p-type light absorption layer 3b in a solution (for example, sulfate) of 60 ° C. or more and 80 ° C. or less containing any one of n dopants such as Mg, Zn, and Cd. (Lower electrode 2) Immerse for about 20 minutes. The treated member is preferably removed from the solution, washed with water and then dried.

The substitution of Se (VIb group element) site can be formed by using Bi or Sb as an evaporation source together with Cu, In, Ga and Se and depositing an n layer by a vacuum evaporation method. By replacing the Se site, which is a VIb group element, with a small amount of Bi or Sb, which is a Vb group element, n-type conversion is possible.
Hereinafter, based on an Example, this invention is demonstrated more concretely.

Example 1
A blue glass substrate is used as the substrate 1, and a Mo thin film to be the back electrode 2 is deposited by about 700 nm by sputtering. Sputtering is performed by applying RF 200 W in an Ar gas atmosphere using Mo as a target.
After the Mo thin film to be the back electrode 2 is deposited, a CuGaSe ₂ thin film to be the light absorption layer 3b is deposited by about 2 μm by the Tri-Phase method. First, after depositing a Cu ₂ Se film by vacuum deposition, the substrate is heated to a temperature at which it becomes liquid phase while confirming a reflection image of RHEED. After confirming the formation of the melt, Ga and Se are deposited. Thereafter, In, Ga and Se (Ga / (In + Ga) = 0.8) are deposited for fine adjustment of the band inclination, and finally Ga and Se are again deposited. To deposit.

In order to make a part of the obtained light absorption layer 3b n-type, the member deposited up to the light absorption layer 3b is immersed in a 0.08 mM cadmium sulfate solution and reacted at 80 ° C. for 22 minutes. As a result, an n-type semiconductor layer 4b doped with Cd on the surface side of the light absorption layer 3b of about 100 nm is formed. A semi-insulating layer ZnO thin film to be the protective film 5 is deposited on the n-type semiconductor layer 4b by sputtering. Film formation is performed with an RF (high frequency) output of 50 W in consideration of plasma damage at the interface. Subsequently, about 1 μm of ZnO: Al containing 2 wt% of alumina (Al ₂ O ₃ ) to be the transparent electrode 6 is deposited on the protective film 5. Further, Al is deposited as an extraction electrode 7 by an evaporation method. The film thickness is 300 nm. Finally, by depositing MgF ₂ as the antireflection film 8 by sputtering, the photoelectric conversion element 200 of the embodiment can be obtained.

  Ga / (In + Ga) molar ratio distribution, particle size, open-circuit voltage (Voc), and short-circuit current density (Jsc) in the thickness direction of the layer of the obtained photoelectric conversion element 200 were measured.

  From the SEM analysis, the average particle size was 2 μm. Further, from the SIMS analysis, the (maximum value / minimum value) of the Ga / (In + Ga) molar ratio in the thickness direction of the light absorption layer 3b was 1.25.

  Using a solar simulator under a simulated sunlight irradiation of AM1.5, using a voltage source and a multimeter, changing the voltage of the voltage source, measuring the voltage at which the current under simulated sunlight irradiation is 0 mA, and measuring the open circuit voltage (Voc) was obtained, and the current when no voltage was applied was measured to obtain the short circuit current density (Jsc).

(Example 2)
Except using _CuIn 0.2 _Ga 0.8 Se ₂ thin film in the light-absorbing layer 3b is prepared a thin film solar cell in the same manner as in Example 1. In order to finely adjust the band inclination, the composition of the deposition of In, Ga and Se is adjusted so that Ga / (In + Ga) = 0.6, and finally, In, Ga where Ga / (In + Ga) = 0.8 again. And Se are deposited.

(Example 3)
Except using _CuIn 0.5 _Ga 0.5 Se ₂ thin film in the light-absorbing layer 3b is prepared a thin film solar cell in the same manner as in Example 1. In order to finely adjust the band inclination, the composition of the deposition of In, Ga and Se is adjusted so that Ga / (In + Ga) = 0.4, and finally, In, Ga where Ga / (In + Ga) = 0.5 again. And Se are deposited.

Example 4
Except using _AgIn 0.5 _Ga 0.5 Se ₂ thin film in the light-absorbing layer 3b is prepared a thin film solar cell in the same manner as in Example 1. In order to finely adjust the band inclination, the composition of the deposition of In, Ga and Se is adjusted so that Ga / (In + Ga) = 0.4, and finally, In, Ga where Ga / (In + Ga) = 0.5 again. And Se are deposited.

(Example 5)
A thin film solar cell is manufactured in the same manner as in Example 1 except that an AgIn _0.75 Ga _0.25 Se ₂ thin film is used for the light absorption layer 3b. In order to finely adjust the band inclination, the composition of the deposition of In, Ga and Se is adjusted so that Ga / (In + Ga) = 0.2, and finally, In, Ga where Ga / (In + Ga) = 0.25 is obtained again. And Se are deposited.

(Example 6)
Except using _AgIn 0.93 _{Ga 0.07} Se ₂ thin film in the light-absorbing layer 3b is prepared a thin film solar cell in the same manner as in Example 1. In order to finely adjust the band tilt, the composition of In, Ga and Se is adjusted so that Ga / (In + Ga) = 0.05, and finally, In, Ga where Ga / (In + Ga) = 0.07 is obtained again. And Se are deposited.

(Example 7)
In order to obtain a heterojunction type, the same method as in Example 1 is used except that the n-type semiconductor layer 4a is deposited on the light absorption layer 3a instead of the n-type doping step for forming the n-type semiconductor layer 4b. A thin film solar cell is manufactured. As the n-type semiconductor layer 4a, 50 nm of CdS is deposited. As a film forming method, a CBD method is used and liquid phase growth is performed.

(Example 8)
In order to obtain a heterojunction type, the same method as in Example 2 is applied except that the n-type semiconductor layer 4a is deposited on the light absorption layer 3a in place of the n-type doping step for forming the n-type semiconductor layer 4b. A thin film solar cell is manufactured. As the n-type semiconductor layer 4a, 50 nm of CdS is deposited. As a film forming method, a CBD method is used and liquid phase growth is performed.

Example 9
In order to obtain a heterojunction type, the same method as in Example 3 is applied except that the n-type semiconductor layer 4a is deposited on the light absorption layer 3a instead of the n-type doping step for forming the n-type semiconductor layer 4b. A thin film solar cell is manufactured. As the n-type semiconductor layer 4a, 50 nm of CdS is deposited. As a film forming method, a CBD method is used and liquid phase growth is performed.

(Example 10)
In order to obtain a heterojunction type, the same method as in Example 4 is applied except that the n-type semiconductor layer 4a is deposited on the light absorption layer 3a in place of the n-type doping step for forming the n-type semiconductor layer 4b. A thin film solar cell is manufactured. As the n-type semiconductor layer 4a, 50 nm of CdS is deposited. As a film forming method, a CBD method is used and liquid phase growth is performed.

(Example 11)
In order to obtain a heterojunction type, the same method as in Example 5 is applied except that the n-type semiconductor layer 4a is deposited on the light absorption layer 3a instead of the n-type doping step for forming the n-type semiconductor layer 4b. A thin film solar cell is manufactured. As the n-type semiconductor layer 4a, 50 nm of CdS is deposited. As a film forming method, a CBD method is used and liquid phase growth is performed.

(Example 12)
In order to obtain a heterojunction type, the same method as in Example 6 is used except that the n-type semiconductor layer 4a is deposited on the light absorption layer 3a instead of the n-type doping step for forming the n-type semiconductor layer 4b. A thin film solar cell is manufactured. As the n-type semiconductor layer 4a, 50 nm of CdS is deposited. As a film forming method, a CBD method is used and liquid phase growth is performed.

(Example 13)
Except using _{Cu 0.3 Ag 0.7 In 0.5 Ga 0.5} Se 2 thin film in the light-absorbing layer 3b is prepared a thin film solar cell in the same manner as in Example 1. Cu _0.6 Ag _1.4 Se is deposited instead of the Cu ₂ Se film to form a melt layer for promoting crystal growth.

(Example 14)
Except using _{Cu 0.3 Ag 0.7 In 0.7 Ga 0.3} Se 2 thin film in the light-absorbing layer 3b is prepared a thin film solar cell in the same manner as in Example 1. Cu _0.6 Ag _1.4 Se is deposited instead of the Cu ₂ Se film to form a melt layer for promoting crystal growth.

(Example 15)
A thin film solar cell is manufactured by the same method as in Example 1 except that a thin film having a single slope of Ga / (In + Ga) is used for the light absorption layer 3b. Finally, only In and Se are deposited for fine adjustment of the band inclination.

(Comparative Example 1)
Except using _CuIn 0.6 _Ga 0.4 Se ₂ thin film in the light-absorbing layer 3b is prepared a thin film solar cell in the same manner as in Example 2. Due to the fine adjustment of the band tilt, the composition of the deposition of In, Ga and Se was adjusted so that Ga / (In + Ga) = 0.2, and finally, In, Ga where Ga / (In + Ga) = 0.4 again. And Se are deposited. Since the band gap is narrow and becomes about 1.3 eV, Voc is lowered.

(Comparative Example 2)
A thin film solar cell is manufactured in the same manner as in Example 4 except that an AgIn _0.95 Ga _0.05 Se ₂ thin film is used for the light absorption layer 3b. In order to finely adjust the band inclination, In and Se are deposited, and finally, In, Ga and Se are again deposited so that Ga / (In + Ga) = 0.05. A band gap becomes narrow and Voc falls.

(Comparative Example 3)
A thin-film solar cell is manufactured by the same method as in Example 8 except that a CuIn _0.6 Ga _0.4 Se ₂ thin film is used for the light absorption layer 3b. Due to the fine adjustment of the band tilt, the composition of the deposition of In, Ga and Se was adjusted so that Ga / (In + Ga) = 0.2, and finally, In, Ga where Ga / (In + Ga) = 0.4 again. And Se are deposited. Since the band gap is narrow and becomes about 1.3 eV, Voc is lowered.

(Comparative Example 4)
A thin film solar cell is manufactured in the same manner as in Example 10 except that an AgIn _0.95 Ga _0.05 Se ₂ thin film is used for the light absorbing layer 3b. In order to finely adjust the band inclination, In and Se are deposited, and finally, In, Ga and Se are again deposited so that Ga / (In + Ga) = 0.05. A band gap becomes narrow and Voc falls.

(Comparative Example 5)
A thin-film solar cell is manufactured by the same method as in Example 2 except that a three-stage method is used for the manufacturing method of the light absorption layer 3b. Although the particle size is as small as about 1 μm and a double slope of the band is formed, the (maximum value / minimum value) of the Ga / (In + Ga) molar ratio is 2.25, and the recombination of carriers is increased to increase Voc. , Jsc decreases.
Table 1 summarizes the results. The measurement result is expressed as A: good → B → C: bad.

The molar ratio represents a molar ratio of Ga / (In + Ga).
η = Voc · Jsc · FF / P · 100

Even if the photoelectric conversion element of an Example was either a homo-type or a hetero type, the thing excellent in the conversion efficiency by which the maximum value and minimum value of the molar ratio of Ga / (In + Ga) were controlled suitably was obtained. .
Deviating from the composition range and reducing the band gap is not preferable because Voc decreases. On the other hand, if the maximum value and the minimum value of the molar ratio of Ga / (In + Ga) are too small, the carrier cannot move easily and Jsc is lowered. Conversely, if it is too large, the double inclined valley of the band or the pn junction interface Thus, carrier recombination occurs and Jsc decreases.

In the specification, some elements are represented only by element symbols.
The embodiment of the present invention has been described above. However, the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying constituent elements without departing from the scope of the invention in the implementation stage. Moreover, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, you may combine suitably the component covering different embodiment like a modification.

DESCRIPTION OF SYMBOLS 1 ... Board | substrate, 2 ... Back electrode, 3 ... p-type light absorption layer, 4 ... n-type semiconductor layer, 5 ... Transparent electrode, 6 ... Antireflection film, 100, 200 ... Photoelectric conversion element

Claims (10)

A compound semiconductor layer satisfying Cu (In _y Ga _1-y ) Se ₂ having a band gap of 1.39 eV or more and 1.68 eV or less (where _y satisfies 0 ≦ y ≦ 0.35 ), or a band gap of 1 (Cu _x Ag _1-x ) (In _y Ga _1-y ) Se ₂ (in which x satisfies 0 <x < 1 and y satisfies 0 ≦ y ≦ 1) which is not less than .28 eV and not more than 1.68 eV. A p-type light absorption layer, which is a compound semiconductor layer to be satisfied, wherein (maximum value / minimum value) of Ga / (In + Ga) molar ratio in the thickness direction of the compound semiconductor layer is 1.0 or more and 1.5 or less;
An n-type semiconductor layer pn-junction with the p-type light absorption layer;
A photoelectric conversion element comprising a transparent electrode disposed on the n-type semiconductor layer.   2. The photoelectric conversion element according to claim 1, wherein (maximum value / minimum value) of the Ga / (In + Ga) molar ratio is 1.1 or more and 1.4 or less.   3. The photoelectric conversion element according to claim 1, wherein an average crystal grain size of the p-type light absorption layer is 1000 nm or more and 2500 nm or less.   4. The photoelectric device according to claim 1, wherein a Ga / (In + Ga) molar ratio has a double slope having a valley of a concentration distribution in a thickness direction of the p-type light absorption layer. 5. Conversion element. The n-type semiconductor layer is a compound semiconductor layer having the same element as the p-type light absorption layer,
The Ga / (In + Ga) molar ratio (maximum value / minimum value) of the p-type light absorption layer and the n-type semiconductor layer is 1.0 or more and 1.5 or less. The photoelectric conversion element of Claim 1. The compound semiconductor layer is Cu (In _y Ga _1-y ) Se ₂ , and the Ga / (In + Ga) molar ratio is 0.65 or more and 1 or less , or
The x is a value larger than 0 and smaller than 1, and a Ga / (In + Ga) molar ratio is a value larger than 0.07 and smaller than 1. 6. Photoelectric conversion element.   A solar cell comprising the photoelectric conversion element according to any one of claims 1 to 6. A p-type light absorbing layer is deposited on the back electrode with a compound comprising Cu or Ag and Se, then the compound is heated above its melting point, then Ga and Se are deposited, then In, Ga and Deposit Se, then form Ga and Se, or deposit a compound consisting of Cu or Ag, and Se on the back electrode, then heat the compound above the melting point, then Ga And Se, and then deposit In and Se to form a p-type absorber layer,
Forming an n-type semiconductor layer on the p-type absorber layer;
Forming a transparent electrode on the n-type semiconductor layer;
The p-type light absorption layer is a compound semiconductor satisfying Cu (In _y Ga _1-y ) Se ₂ (where y satisfies 0 ≦ y ≦ 0.35 ) having a band gap of 1.39 eV or more and 1.68 eV or less. (Cu _x Ag _1-x ) (In _y Ga _1-y ) Se _{2 in which the} layer or the band gap is 1.28 eV or more and 1.68 eV or less (where x is 0 <x < 1, and y is 0) A method of manufacturing a photoelectric conversion element, which is a compound semiconductor layer satisfying (≦ y ≦ 1).   The said n-type semiconductor layer is formed by depositing a compound thin film, or formed by making the said p-type light absorption layer into n-type, The manufacture of the photoelectric conversion element of Claim 8 characterized by the above-mentioned. Method.   A method for producing a solar cell, wherein a photoelectric conversion element is produced by the production method according to claim 8 or 9.