JP2011023520A - P-type semiconductor film and solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a p-type semiconductor film readily controlling a forbidden band width or an electron affinity distribution in the p-type semiconductor film constituted of a p-type semiconductor and used as a light absorbing layer of a solar cell, so as to provide a high conversion efficiency solar cell. <P>SOLUTION: The p-type semiconductor film 1 is constituted of the p-type semiconductor and is used as the light absorbing layer of the solar cell A. The p-type semiconductor film 1 is constituted of N (N is a natural number meeting N≥2) layers laminated in a light incident direction. Out of the N layers, in M layers counted from the layer disposed the closest to the light incident side to the M-th (M is a natural number meeting 2≤M≤N) layer, the forbidden band width of one of mutually adjacent optional two layers which is disposed on the light incident side is larger than that of the other layer and a change in forbidden band width generated between the layers is of a step-like form, or the electron affinity of one layer disposed on the light incident side is lower than that of the other layer and a change in electron affinity generated between the layers is of a step-like form. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a p-type semiconductor film used as a light absorption layer of a solar battery and a solar battery provided with the p-type semiconductor film.

  Conventionally, it is known that the photoelectric conversion efficiency of a solar cell is improved by gradually changing the forbidden band width or electron affinity inside the light absorption layer of the solar cell. For example, as disclosed in the device simulation of Non-Patent Document 1, in a structure in which the forbidden band width of the light absorption layer is gradually reduced from the light incident side, recombination of injected carriers is performed by increasing the forbidden band width. Is reduced, the open circuit voltage of the solar cell A is increased as compared with the case where the light absorption layer in which the forbidden bandwidth is uniformly distributed is used. In addition, if the forbidden band width of the light absorption layer is distributed so as to gradually expand from the light absorbing side, an internal electric field is generated due to a change in the forbidden band width, so that photoexcited minority carriers drift in the internal electric field. By moving, recombination is reduced and the short-circuit current density of the solar cell is improved. Furthermore, by combining the two forbidden band width distributions, the forbidden band width inside the light absorption layer is gradually reduced from the light incident side, and the forbidden band width is gradually increased when a certain depth is exceeded. When distributed in a double-graded distribution, both open-circuit voltage and short-circuit current density increase compared to solar cells that use a light-absorbing layer with a uniform distribution of the forbidden bandwidth, providing solar cells with high conversion efficiency. it can.

  Further, in a solar cell based on a pn junction, it is advantageous to use a p-type semiconductor, which is an electron having a long lifetime of photoexcited minority carriers, for the light absorption layer.

  As a conventional technique in which a light absorption layer is formed of a p-type semiconductor and the forbidden band width inside the light absorption layer is changed, a technique disclosed in Patent Documents 1-3 has been proposed.

Patent Document 1 discloses that a forbidden band width is changed in a Cu (In, Ga) Se ₂ thin film. In the technique described in Patent Document 1, In, Ga, and Se are vapor-deposited in the first first stage to form an (In, Ga) ₂ Se ₃ film, and then Cu and Se are vapor-deposited in the second stage. Then, a CIGS film with an excessive Cu composition is formed, and In, Ga, and Se are further evaporated in the final third stage to form a CIGS film with an insufficient Cu composition. In this method, by controlling the amounts of vapor deposition of In and Ga in the first stage and the third stage, the ratio of In and Ga gradually changes in the depth direction as In and Ga diffuse in the film forming process. Thus, a thin film in which the forbidden bandwidth gradually changes is formed.

Patent Document 2 discloses that a forbidden band width is changed in a Cu (In, Al) Se ₂ film. In the technique described in Patent Document 2, an AlSe film and a Cu film are deposited on a substrate, and then a Cu-excess Cu (In, Al) Se ₂ film is deposited while heating in a selenium atmosphere, followed by heating in a selenium atmosphere. Thus, a Cu (In, Al) Se ₂ film in which the Al / (In + Al) ratio continuously changes is formed.

  In Patent Document 3, a chalcopyrite structure semiconductor film is configured by a first semiconductor layer and a second semiconductor layer, and the first semiconductor layer has a Ga (In + Ga) ratio toward the second semiconductor layer side. The forbidden band width decreases as the distance from the second semiconductor layer approaches the second semiconductor layer, and the second semiconductor layer has a forbidden band width larger than the minimum forbidden band width in the first semiconductor layer. Thus, it is disclosed to form a double graded distribution.

However, in the above prior art, when forming a light absorption layer film with a p-type semiconductor, the forbidden band width is changed by naturally diffusing elements in the film or adjusting the material supply amount. Therefore, it is difficult to control the distribution state of the forbidden bandwidth, and it is difficult to optimize the distribution of the forbidden bandwidth. For this reason, the conversion efficiency of the solar cell A is significantly lower than predicted from the design of the forbidden bandwidth. In particular, as disclosed in Non-Patent Document 2, in a Cu (In, Ga) S ₂ semiconductor thin film, since diffusion of In and Ga is insufficient, it is difficult to control the graded distribution.

Japanese National Patent Publication No. 10-513606 JP-A-11-274526 JP 2007-335792 A

C.H. Huang, Sheng S. Lia and, T. J. Anderson, “DEVICE MODELING AND SIMULATION OF CIS-BASED SOLAR CELLS”, Proceedings of 29th IEEE Photovoltaic Specialists Conference, New Orleans, p.748 (2002). Supervised by Takahiro Wada, “Latest Technology of Compound Thin Film Solar Cell”, CMC Publishing, 2007, p.182

  The present invention has been made in view of the above points, and easily controls the forbidden band width or electron affinity distribution in a p-type semiconductor film made of a p-type semiconductor and used as a light absorption layer of a solar cell. An object of the present invention is to provide a p-type semiconductor film capable of obtaining a solar cell with high conversion efficiency and a solar cell including the p-type semiconductor film.

  The p-type semiconductor film 1 according to the present invention is composed of a p-type semiconductor and is used as a light absorption layer of the solar cell A. The p-type semiconductor film 1 is composed of N layers (N is a natural number satisfying N ≧ 2) stacked along the light incident direction. Among the N layers, any two layers adjacent to each other in the M layers from the layer arranged closest to the light incident side to the Mth layer (M is a natural number satisfying 2 ≦ M ≦ N). Among them, the forbidden band width of one layer arranged on the light incident side is larger than the forbidden band width of the other layer, and the change of the forbidden band width generated between the layers is stepped. Alternatively, the electron affinity of one layer disposed on the light incident side is smaller than the electron affinity of the other layer, and the change in electron affinity generated between the layers is stepped. The step-like change means that the forbidden band width or the electron affinity value changes discontinuously at the interface between the layers.

  For this reason, by adjusting the forbidden band width or electron affinity of each layer constituting the p-type semiconductor film 1, the distribution of the forbidden band width or electron affinity in the p-type semiconductor film 1 can be easily controlled. When the p-type semiconductor film 1 is used as a light absorption layer of the solar cell A, the recombination of carriers in the light absorption layer is reduced and the carrier is transported by the forbidden band width or electron affinity distribution in the light absorption layer. Can be inhibited.

  Of the M layers, the forbidden band width (Eg (n)) of an arbitrary nth (where 1 ≦ n ≦ M−1) layer from the light incident side and the forbidden bandwidth of the (n + 1) th layer ( Eg (n + 1)) preferably satisfies the relationship of the following formula (1).

0 (eV) <Eg (n) −Eg (n + 1) ≦ 0.1 (eV) (1)
In this case, it is possible to further reliably prevent the carrier transport from being inhibited between the M layers.

  Among the M layers, the electron affinity (χ (n)) of an arbitrary nth (where 1 ≦ n ≦ M−1) layer from the light incident side (χ (n)) and the electron affinity (χ (n) of the (n + 1) th layer. It is also preferable that n + 1)) satisfy the relationship of the following formula (2).

−0.1 (eV) ≦ χ (n) −χ (n + 1) <0 (eV) (2)
Also in this case, it is possible to further suppress the inhibition of carrier transport between each of the M layers.

  Of the N layers, the total thickness of the M−1 layers from the layer disposed closest to the light incident side to the M−1st layer is preferably less than 150 nm.

  In this case, the retention of carriers in the layer having the smallest forbidden band width or the largest electron affinity in the p-type semiconductor film 1 can be suppressed, and the recombination of carriers can be further reduced.

  When M is a natural number satisfying 2 ≦ M <N, among the N layers, N−M + 1 layers from the Mth layer to the Nth layer from the light incident side are adjacent to each other. The forbidden band width of one of the two layers arranged on the light incident side is smaller than the forbidden band width of the other layer and the change of the forbidden band width generated between the layers is stepped, or the light incident side It is preferable that the electron affinity of one layer disposed in the layer is larger than the electron affinity of the other layer, and the change in electron affinity generated between the layers is stepped.

  In this case, carriers excited in the Mth layer having the smallest forbidden band width or the largest electron affinity in the p-type semiconductor film 1 are adjacent to the (M + 1) th layer having the larger forbidden band width or the smallest electron affinity. Reflection by the (M + 1) th layer makes the carrier transport direction constant, improves the carrier collection efficiency, and increases the short-circuit current density.

  The thickness of the Mth layer from the light incident side is preferably 150 nm or more and less than 450 nm.

  In this case, the retention of carriers in the layer having the smallest forbidden band width or the largest electron affinity in the p-type semiconductor film 1 can be reduced, and carrier recombination can be further suppressed.

In any of the N layers, the carrier concentration is preferably greater than 1 × 10 ¹⁵ / cm ³ .

  In this case, the p-type semiconductor film 1 and a window layer made of an n-type semiconductor are joined to form a pn junction, whereby a diffusion potential suitable for the solar cell A can be generated and A suitable depletion layer can be generated.

  The N layers preferably have the same crystal structure. The same crystal structure means that both crystals have the same type of crystal structure such as a zinc blende structure, a wurtzite structure, a sodium chloride structure, a spinel structure, a chalcopyrite structure, and the like.

  In this case, structural defects at the interface between the layers constituting the p-type semiconductor film 1 can be suppressed, and carrier recombination can be further suppressed.

  In particular, the crystal structure of the N layers is preferably a chalcopyrite structure.

  In this case, the forbidden band width or the electron affinity of each layer is controlled to a desired value by changing the type or composition ratio of the constituent element of the chalcopyrite structure crystal in each layer constituting the p-type semiconductor film 1. Can do.

  The main elements constituting the N layers are preferably composed of the same element group. The main element means an element excluding a small amount of elements (such as Group 1 elements such as Na and K) that can be included as dopants and unavoidable impurities among the constituent elements of each layer. Moreover, being comprised by the same element group means that the combination of the group of the elements which comprise each layer is the same between each layer.

  In this case, the forbidden band width or the electron affinity of each layer can be controlled to a desired value by changing the composition ratio of the constituent elements of each layer constituting the p-type semiconductor film 1.

  The main elements constituting the N layers are preferably a group 11 element, a group 13 element, and a group 16 element.

  In this case, the forbidden band width or electron affinity of each layer can be controlled to a desired value by changing the composition ratio of the Group 11 element, Group 13 element, and Group 16 element of each layer constituting the p-type semiconductor film 1. it can.

  The main elements constituting the N layers are a group 11 element, In, Ga, and a group 16 element, and Ga / (In + Ga) in an arbitrary nth layer (where 1 ≦ n ≦ M) from the light incident side. ) Ratio X (n) and Ga / (In + Ga) ratio X (n + 1) in the (n + 1) th layer preferably satisfy the relationship of the following formula (3).

0 <X (n) −X (n + 1) ≦ 0.2 (3)
In this case, it is possible to further reliably prevent the carrier transport from being inhibited by preventing the difference in the forbidden band width or the difference in electron affinity between the layers from becoming too large.

  The main elements constituting the N layers are a group 11 element, In, Al, and a group 16 element, and Al / (in an arbitrary nth layer (where 1 ≦ n ≦ M) from the light incident side. It is also preferable that the ratio Y (n) of In + Al) and the ratio Y (n + 1) of Al / (In + Al) of the (n + 1) th layer satisfy the relationship of the following formula (4).

0 <Y (n) −Y (n + 1) ≦ 0.1 (4)
In this case, it is possible to further reliably prevent the carrier transport from being inhibited by preventing the difference in the forbidden band width or the difference in electron affinity between the layers from becoming too large.

  The main elements constituting the N layers are Cu, Zn, a group 13 element and a group 16 element, and Zn / (in the nth (where 1 ≦ n ≦ M) layer is arbitrary from the light incident side. It is also preferable that the ratio Z (n) of Cu + Zn and the ratio Zn (n + 1) of Zn / (Cu + Zn) in the (n + 1) th layer satisfy the relationship of the following formula (5).

0 <Zn (n) −Zn (n + 1) ≦ 0.4 (5)
In this case, it is possible to further reliably prevent the carrier transport from being inhibited by preventing the difference in the forbidden band width or the difference in electron affinity between the layers from becoming too large.

  The solar cell A according to the present invention includes the p-type semiconductor film 1 as a light absorption layer.

  For this reason, by adjusting the forbidden band width or electron affinity of each layer constituting the p-type semiconductor film 1, the distribution of the forbidden band width or electron affinity in the p-type semiconductor film 1 can be easily controlled. By the forbidden band width or electron affinity distribution in the light absorption layer, it is possible to reduce the recombination of carriers in the light absorption layer and to inhibit the carrier transport from being inhibited.

  According to the present invention, it becomes easy to control the forbidden band width or electron affinity distribution in the p-type semiconductor film, and by using this p-type semiconductor film as the light absorption layer of the solar cell, the photoelectric conversion efficiency of the solar cell is improved. Can be improved.

It is a schematic sectional drawing which shows Embodiment 1 of this invention. It is an energy band figure in the p-type semiconductor film in the said Embodiment 1. FIG. It is a graph which shows the change of the conversion efficiency with respect to the forbidden bandwidth difference Eg (a) -Eg (b) of the 1st layer and the 2nd layer which were derived | led-out about the said Embodiment 1. FIG. It is an energy band figure in the p-type semiconductor film in Embodiment 2 of this invention. It is a graph which shows the change of the conversion efficiency with respect to the forbidden bandwidth difference (DELTA) Eg between the 1st layer and the 2nd layer derived | led-out by the numerical simulation about the said Embodiment 2. FIG. It is an energy band figure in the p-type semiconductor film in Embodiment 3 of this invention. It is a graph which shows the change of the conversion efficiency with respect to the total film thickness of the three layers which were derived | led-out by the numerical simulation about the said Embodiment 3, and existed in the light incident side rather than the layer where the forbidden bandwidth becomes the minimum. (A) is the energy band figure in the p-type semiconductor film in Embodiment 4 of this invention, (b) is the energy band figure in the p-type semiconductor film in Embodiment 5 of this invention. It is a graph which shows the conversion efficiency of the solar cell derived | led-out by numerical simulation about Embodiment 1, Embodiment 4, and Embodiment 5. FIG. It is. It is a graph which shows the change of the conversion efficiency of the solar cell derived | led-out by the numerical simulation at the time of changing the film thickness x of the 2nd layer from which the forbidden bandwidth becomes the minimum about the said Embodiment 5. FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to only the embodiments described herein.

  In the embodiment shown in FIG. 1, a transparent electrode film 3, an n-type semiconductor film 4, a p-type semiconductor film 1, and a back electrode 5 are sequentially stacked on one surface side of a substrate 2. Thereby, the solar cell A is configured.

  The substrate 2 is made of a material having high light transmittance, for example, glass or light transmissive resin.

The transparent electrode film 3 is made of, for example, a conductive metal oxide. Examples of the conductive metal oxide include SnO ₂ : F, ZnO: Al, ZnO: Ga, IXO (In ₂ O ₃ : X, where X is Sn, Mn, Mo, Ti, Zn, etc.). It is done. The transparent electrode film 3 may be formed of only one kind of conductive metal oxide as described above, or formed as a multilayer film formed by laminating a plurality of kinds of conductive metal oxide layers. May be.

The n-type semiconductor film 4 functions as a window layer of the solar cell A and forms a pn junction with the p-type semiconductor film 1. The n-type semiconductor film 4 includes, for example, CdS, ZnO, Zn (O, S), (Zn _1-x Mg _x ) O (0 <x <1), TiO ₂ , Zn _x In _2-2x S _3-2x ( It is formed from an n-type semiconductor such as 0 <x <1), Zn _x In _2-2x Se _3-2x (0 <x <1), or In ₂ S ₃ . The n-type semiconductor film 4 may be formed of only one type of n-type semiconductor, or a plurality of types of n-type semiconductor layers may be formed by stacking, for example, a ZnO layer and a CdS layer. It is good also as a multilayer film constituted by laminating. The n-type semiconductor film 4 is formed by a CVD method such as a sputtering method or a thermal chemical vapor deposition method.

  The p-type semiconductor film 1 functions as a light absorption layer. The p-type semiconductor film 1 has a configuration in which N layers (N is a natural number satisfying N ≧ 2) having different compositions are stacked.

  The p-type semiconductors constituting the N layers in the p-type semiconductor film 1 preferably have the same crystal structure, that is, all of the N layers have a zinc blende structure, a wurtzite structure, sodium chloride It is preferable that they have the same type of crystal structure, such as a mold structure, a spinel structure, or a chalcopyrite structure. In this case, structural defects at the interface between the layers constituting the p-type semiconductor film 1 can be suppressed, and carrier recombination can be further suppressed. In particular, it is preferable that all of the N layers are made of a semiconductor having a chalcopyrite structure.

  The p-type semiconductor is preferably a semiconductor whose band gap or electron affinity is easily adjusted by changing the composition ratio of the constituent elements, and the p-type semiconductor having a different constituent ratio of the constituent elements. It is preferable that the interdiffusion between the constituent elements is small.

  In particular, the main elements of the p-type semiconductor constituting each of the N layers are all composed of the same element group, and the composition ratio of the constituent elements of the p-type semiconductor in each layer constituting the p-type semiconductor film 1 is changed. This makes it easy to control the forbidden band width or electron affinity of each layer to a desired value. In this case, the content of elements other than the main element contained in each layer, that is, a small amount of elements that can be contained as dopants (Group 1 elements such as Na and K) and unavoidable impurities is 5 atomic% or less. Is preferred.

As the p-type semiconductor, a semiconductor whose main element is a group 11 element, group 13 element or group 16 element is particularly preferable. A chalcopyrite type semiconductor composed of a group 11 element, a group 13 element and a group 16 element is preferred. Such p-type _{semiconductor, (Cu 1-x Ag x} ) (In 1-y Ga y) (S 1-z Se z) 2, (Cu 1-x Ag x) (In 1-y Al y _{) (S 1-z Se z} ) 2, (Cu 1-x Ag x) (Ga 1-y Al y) (S 1-z Se z) 2, (Cu 1-x Ag x) (In 1-y _{Ga y) (S 1-z} Te z) 2, (Cu 1-x Ag x) (In 1-y Al y) (S 1-z Te z) 2, (Cu 1-x Ag x) (Ga 1 _{-y Al y) (S 1-} z Te z) 2, (Cu 1-x Ag x) (In 1-y Ga y) (Se 1-z Te z) 2, (Cu 1-x Ag x) ( _{In 1-y Al y) (} Se 1-z Te z) 2, (Cu 1-x Ag x) (Ga 1-y Al y) (Se 1-z T _{z) 2,} and the like. Here, x, y, and z are numbers from 0 to 1.

Among them, particularly _{(Cu 1-x Ag x)} (In 1-y Ga y) (S 1-z Se z) 2, (Cu 1-x Ag x) (In 1-y Ga y) (S 1- _{z Te z) 2, (Cu} 1-x Ag x) (in 1-y Ga y) (Se 1-z Te z) 2 the main element 11 group element such as, in, with Ga, and group 16 elements there p-type _{semiconductor, (Cu 1-x Ag x} ) (In 1-y Al y) (S 1-z Te z) 2, (Cu 1-x Ag x) (In 1-y Al y) (S 1 _{-z Se z) 2, (Cu} 1-x Ag x) (in 1-y Al y) (Se 1-z Te z) main element 11 group element such as _2, in, Al, and group 16 A p-type semiconductor which is an element is preferable.

As the p-type semiconductor, a chalcopyrite-type semiconductor in which the main element is composed of a group 11 element, a group 12 element, a group 13 element, and a group 16 element is also preferable. In particular, the main element is Cu, Zn, a group 13 element, and A semiconductor that is a group 16 element is preferred. As such a p-type semiconductor, (Cu _1-x Zn _x ) (In _1-y Ga _y ) (S _1-z Se _z ) ₂ , (Cu _1-x Zn _x ) (In _1-y Al _{y) ) (S 1-z Se z} ) 2, (Cu 1-x Zn x) (Ga 1-y Al y) (S 1-z Se z) 2, (Cu 1-x Zn x) (In 1-y _{Ga y) (S 1-z} Te z) 2, (Cu 1-x Zn x) (In 1-y Al y) (S 1-z Te z) 2, (Cu 1-x Zn x) (Ga 1 _{-y Al y) (S 1-} z Te z) 2, (Cu 1-x Zn x) (In 1-y Ga y) (Se 1-z Te z) 2, (Cu 1-x Zn x) ( _{In 1-y Al y) (} Se 1-z Te z) 2, (Cu 1-x Zn x) (Ga 1-y Al y) (Se 1-z T _{z) 2,} and the like. Here, x, y, and z are numbers from 0 to 1.

Further, as a p-type semiconductor, a main element is a semiconductor having a chalcopyrite structure composed of a group 12 element and a group 16 element (Cd _1-x Zn _x ) (Te _1-y Se _y ), (Cd _1-x Zn _x ) (Te _1-y S _y ), (Cd _1-x Zn _x ) (Se _1-y S _y ), and the like. Here, x and y are numbers from 0 to 1.

Further, as a p-type semiconductor, a main element is a zincblende type or wurtzite type semiconductor composed of a group 13 element and a group 15 element (Ga _1-x Al _x ) (As _1-y Py), (Ga _{1-x In x) (As} 1-y P y), (Al 1-x In x) (As 1-y P y), (Ga 1-x In x) (P 1-y N y), ( _{Ga 1-x Al x) (} P 1-y N y), (Al 1-x In x) (P 1-y N y), (Ga 1-x In x) (As 1-y N y), _{(Ga 1-x Al x)} (As 1-y N y), also include such _{(Al 1-x In x)} (As 1-y N y). Here, x and y are numbers from 0 to 1.

  Among the N layers constituting the p-type semiconductor film 1, in the M layers from the layer arranged closest to the light incident side to the Mth layer (M is a natural number satisfying 2 ≦ M ≦ N), While the forbidden band width is gradually reduced from the light incident side layer, the change in the forbidden band width between layers is discontinuous stepwise, or the electron affinity is sequentially increased from the light incident side layer. The change in electron affinity at is discontinuous stepwise. Since the forbidden band width is the gap between the valence band and the conduction band, and the electron affinity is the gap between the vacuum level and the conduction band, the valence band level relative to the vacuum level in each layer. If the forbidden band width is sequentially reduced from the light incident side layer, the electron affinity gradually increases from the light incident side layer at the same time.

  In this case, the difference in the forbidden bandwidth between adjacent layers (Eg (n) −Eg (n + 1)) preferably satisfies the following formula (1).

0 (eV) <Eg (n) −Eg (n + 1) ≦ 0.1 (eV) (1)
Moreover, it is preferable that the difference in electron affinity (Eg (n) −Eg (n + 1)) between adjacent layers satisfies the following formula (2).

−0.1 (eV) ≦ χ (n) −χ (n + 1) <0 (eV) (2)
As described above, when the M layers satisfy the formula (1) or the formula (2), the difference in the forbidden band width or the difference in electron affinity between the M layers is too large. In this way, it is possible to more reliably suppress the inhibition of carrier transport between layers.

  Of the N layers constituting the p-type semiconductor film 1, the total thickness of the M−1 layers from the layer disposed closest to the light incident side to the M−1st layer is less than 150 nm. Is preferred. In this case, the retention of carriers in the layer having the smallest forbidden band width or the largest electron affinity in the p-type semiconductor film 1 can be suppressed, and the recombination of carriers can be further reduced.

  Further, when the thickness of the Mth layer from the light incident side is 150 nm or more and less than 450 nm, the retention of carriers in the layer having the smallest forbidden band width or the largest electron affinity in the p-type semiconductor film 1 is reduced. Carrier recombination can be further suppressed.

  In the case of M = N, in all the layers (N layers) constituting the p-type semiconductor film 1, the forbidden band width sequentially decreases from the layer closest to the light incident side, and the forbidden band width changes between the layers. Becomes stepwise discontinuous, or the electron affinity gradually increases from the layer on the light incident side, and the change in electron affinity between layers becomes stepwise discontinuous.

  If M <N, the N-M + 1 layers from the M-th layer to the N-th layer from the light incident side out of the N layers are forbidden, contrary to the above. The width gradually increases from the light incident side layer, and the change in the forbidden band width between layers becomes discontinuous stepwise, or the electron affinity gradually decreases from the light incident side layer, It is preferable to make the change in electron affinity discontinuous stepwise.

  Such adjustment of the forbidden band width or electron affinity of each layer can be performed by adjusting the composition of each layer. In particular, when each layer is formed of a p-type semiconductor having a chalcopyrite type structure in which the main elements are Group 11 elements, In, Ga, and Group 16 elements, the In and Ga composition ratios are sequentially changed between the layers. The forbidden bandwidth and electron affinity can be changed discontinuously. When each layer is formed of a p-type semiconductor having a chalcopyrite structure in which the main elements are Group 11 elements, In, Al, and Group 16 elements, by sequentially changing the composition ratio of In, Al between the layers, The forbidden band width and electron affinity can be changed discontinuously between the layers. In addition, when each layer is formed of a p-type semiconductor having a chalcopyrite structure in which the main elements are Cu, Zn, a group 13 element and a group 16 element, the composition ratio of Cu and Zn is sequentially changed between the layers. The forbidden band width and electron affinity can be changed discontinuously between the layers.

  Here, when the main elements of the p-type semiconductor constituting the N layers are a group 11 element, In, Ga, and a group 16 element, the Mth element (M is 2 from the layer arranged closest to the light incident side). In M layers up to ≦ M ≦ N, a Ga / (In + Ga) ratio X (n) in an arbitrary n-th (where 1 ≦ n ≦ M) layer from the light incident side And the Ga / (In + Ga) ratio X (n + 1) in the (n + 1) th layer preferably satisfy the relationship of the following formula (3).

0 <X (n) −X (n + 1) ≦ 0.2 (3)
When the main elements of the p-type semiconductor constituting the N layers are a group 11 element, In, Al, and a group 16 element, the Mth element (M is 2 from the layer arranged closest to the light incident side). In M layers up to ≦ M ≦ N, a ratio of Al / (In + Al) in an arbitrary nth layer (where 1 ≦ n ≦ M) from the light incident side Y (n) And the Al / (In + Al) ratio Y (n + 1) of the (n + 1) th layer preferably satisfies the relationship of the following formula (4).

0 <Y (n) −Y (n + 1) ≦ 0.1 (4)
Further, when the main elements of the p-type semiconductor constituting the N layers are Cu, Zn, a group 13 element and a group 16 element, an arbitrary nth element from the light incident side (where 1 ≦ n ≦ M) It is preferable that the ratio Z (n) of Zn / (Cu + Zn) in this layer and the ratio Zn (n + 1) of Zn / (Cu + Zn) in the (n + 1) th layer satisfy the relationship of the following formula (5).

0 <Zn (n) −Zn (n + 1) ≦ 0.4 (5)
When the composition ratio between layers is adjusted as in the above formulas (3) to (5), the difference in the forbidden band width or the difference in electron affinity between the layers does not become too large, and carrier transport is inhibited. Can be more reliably suppressed.

Further, it is preferable that the N layers constituting the p-type semiconductor film 1 have a carrier concentration higher than 1 × 10 ¹⁵ / cm ³ . In this case, the p-type semiconductor film 1 and a window layer made of an n-type semiconductor are joined to form a pn junction, whereby a diffusion potential suitable for the solar cell A can be generated and A suitable depletion layer can be generated.

  The plurality of layers constituting the p-type semiconductor film 1 can be sequentially formed by spray coating pyrolysis or atomic layer deposition (ALD). In particular, in the spray coating pyrolysis method, a p-type semiconductor whose composition ratio is uniformly controlled based on the composition of the solution can be sequentially formed, and the forbidden band width or the electron affinity is discontinuous as described above. Changes can be made easily.

The back electrode 5 can be formed of a metal film, for example. Examples of the metal include Au, Pt, and Ag. Further, the back electrode 5 can be formed of carbon. Further, as in the case of the upper solar cell A in the tandem solar cell A, when it is necessary to transmit the long-wavelength sunlight that is not absorbed by the p-type semiconductor film 1 serving as the light absorption layer through the back electrode 5, the back electrode 5 can also be formed of a light-transmitting conductive metal oxide. As the conductive metal oxide, for example, the same conductive metal oxide as in the case of forming the transparent electrode film 3, an oxide containing copper such as Cu ₂ O and CuSr ₂ O ₄ , Ag ₂ O, and the like are used. Can be mentioned. The back electrode 5 can be formed by an appropriate film forming method such as a sputtering method.

[Embodiment 1]
In the present embodiment, N and M are in a relationship of 2 = M = N. That is, the forbidden band of the first layer 6a which is formed by laminating two layers of the first layer 6a and the second layer 6b constituting the p-type semiconductor film 1 and is disposed on the light incident side of the two layers. The width is larger than the forbidden band width of the second layer 6b and the change in the forbidden band width generated between the layers is discontinuous, or the electron affinity of the first layer 6a is smaller than the electron affinity of the second layer 6b and between the layers. The resulting change in electron affinity is made discontinuous.

In this embodiment, the substrate 2 is formed of glass, the transparent electrode film 3 is formed of ZnO: Al, the n-type semiconductor film 4 is formed of In ₂ S ₃ , and the p-type semiconductor film 1 is Cu (In _{1− x} Ga _x ) S ₂ and the back electrode 5 is made of Au.

  In the p-type semiconductor film 1, the thickness of the first layer 6a on the light incident side (n-type semiconductor film 4 side) is 50 nm, and the side opposite to the light incident side with respect to the first layer 6a (n-type). The thickness of the second layer 6b on the opposite side of the semiconductor film 4 is 1950 nm.

The forbidden band width and the electron affinity of each layer constituting the p-type semiconductor film 1 are adjusted by changing the ratio of In and Ga in each layer. For example, in Cu (In _1-x Ga _x ) S ₂ constituting the p-type semiconductor film 1, if x = 0.2, the forbidden band width is about 0.1 eV. In the case of Cu (In _1-x Al _x ) S ₂ , if x = 0.1 and in the case of (Cu _1-x Zn _x ) InS ₂ , x = 0.4, the forbidden band width is about 0. .1 eV.

In Cu (In _1-x Ga _x ) S ₂ and Cu (In _1-x Al _x ) S ₂ , the conduction band level is mainly determined by the group 13 element, so even if the solid solution ratio x changes. The conduction band level is almost constant, and the expansion of the forbidden band width accompanying an increase in x is almost consistent with a decrease in electron affinity.

  If the forbidden band widths of the first layer 6a and the second layer 6b in the p-type semiconductor film 1 are Eg (a) and Eg (b), respectively, and the electron affinity is χ (a) and χ (b), respectively, Satisfies the following relationship: FIG. 2 is an energy band diagram showing this relationship.

Eg (a)> Eg (b)
χ (a) <χ (b)
Eg (a) −Eg (b) = χ (b) −χ (a)
FIG. 3 shows the result of deriving the characteristics of the solar cell A configured as described above by numerical simulation in which the Poisson equation and the current continuity equation are calculated by the finite difference method. FIG. 3 shows a change in conversion efficiency with respect to the forbidden bandwidth difference Eg (a) −Eg (b) between the first layer 6a and the second layer 6b. The forbidden band difference 0 eV corresponds to the case where the p-type semiconductor film 1 is formed as a single layer, and the conversion efficiency in this case is 14.1%. According to this result, the conversion efficiency increases as the forbidden bandwidth difference increases until the forbidden bandwidth difference reaches about 0.05 eV. This is because the forbidden band width of the first layer 6a in contact with the n-type semiconductor film 4 is increased, so that recombination of injected carriers from the n-type semiconductor film 4 side is reduced and the open circuit voltage is increased.

  On the other hand, when the forbidden bandwidth difference is 0.05 eV or more, the conversion efficiency decreases. This is because if the forbidden band width difference is increased, the forbidden band width difference (electron affinity difference) is too large, thereby causing a discontinuous barrier in the conduction band, thereby inhibiting the transport of photocarriers excited in the second layer 6b. This is because recombination of carriers staying in the second layer 6b occurs and the photocurrent decreases. According to FIG. 3, when the forbidden band difference is in the range of 0.1 eV or less, particularly 0.08 eV or less, the conversion efficiency is higher than that in the case where the p-type semiconductor film 1 is a single layer (forbidden band difference of 0 eV). Becomes higher.

[Embodiment 2]
In the present embodiment, N and M are in a relationship of 3 = M = N. That is, the first layer, the second layer, and the third layer are laminated in order from the light incident side (n-type semiconductor film 4 side) to form the p-type semiconductor film 1 with three layers, and in these three layers, The forbidden band width of one of the two adjacent layers disposed on the light incident side is larger than the forbidden band width of the other layer and the change of the forbidden band width generated between the layers is discontinuous, or the light The electron affinity of one layer arranged on the incident side is smaller than the electron affinity of the other layer, and the change in electron affinity generated between the layers is made discontinuous.

  The present embodiment has the same configuration as that of the first embodiment except that the p-type semiconductor film 1 has a configuration in which three layers of a first layer, a second layer, and a third layer are stacked. The forbidden band width and the electron affinity of each layer constituting the p-type semiconductor film 1 are adjusted by changing the ratio of In and Ga in each layer.

  The forbidden band widths of the first layer, the second layer, and the third layer in the p-type semiconductor film 1 are Eg (a), Eg (b), and Eg (c), respectively, and the electron affinity is χ (a), Assuming that χ (b) and χ (c), these satisfy the following relationship. FIG. 4 is an energy band diagram showing this relationship.

Eg (a)> Eg (b)> Eg (c)
χ (a) <χ (b) <χ (c)
Eg (a) −Eg (b) = χ (b) −χ (a)
Eg (b) −Eg (c) = χ (c) −χ (b)
FIG. 5 shows the result of deriving the characteristics of the solar cell A configured as described above by numerical simulation in which the Poisson equation and the current continuity equation are calculated by the finite difference method. FIG. 5 shows the change in conversion efficiency with respect to the forbidden bandwidth difference ΔEg between the first layer and the second layer. Here, Eg (b) -Eg (c) and the film thickness of each layer are calculated in several combinations. Therefore, the efficiency may be different even with the same ΔEg. The forbidden band difference 0 eV corresponds to the case where the p-type semiconductor film 1 is formed of a single layer, and the configuration of the p-type semiconductor film 1 in this case is the same as the case of the forbidden band difference 0 eV in the first embodiment. Therefore, the conversion efficiency is 14.1%, which is the same as in the first embodiment. According to FIG. 5, if the forbidden band difference is in the range of 0.1 eV or less, particularly 0.08 eV or less, the conversion is more than in the case where the p-type semiconductor film 1 is a single layer (forbidden band difference of 0 eV). Increases efficiency.

[Embodiment 3]
In the present embodiment, N and M are in a relationship of 4 = M = N. That is, the p-type semiconductor film 1 is formed of four layers by laminating a first layer, a second layer, a third layer, and a fourth layer in order from the light incident side (n-type semiconductor film 4 side), and In these four layers, the forbidden band width of one layer arranged on the light incident side of the two adjacent layers is larger than the forbidden band width of the other layer, and the change of the forbidden band width generated between the layers becomes discontinuous. Alternatively, the electron affinity of one layer arranged on the light incident side is smaller than the electron affinity of the other layer and the change in electron affinity generated between the layers is discontinuous.

  This embodiment has the same configuration as that of Embodiment 1 except that the p-type semiconductor film 1 has a configuration in which four layers of a first layer, a second layer, a third layer, and a fourth layer are stacked. is doing. The forbidden band width and the electron affinity of each layer constituting the p-type semiconductor film 1 are adjusted by changing the ratio of In and Ga in each layer.

  The forbidden band widths of the first layer, the second layer, the third layer, and the fourth layer in the p-type semiconductor film 1 are Eg (a), Eg (b), Eg (c), and Eg (d), respectively. Assuming that the electron affinity is χ (a), χ (b), χ (c), and χ (d), these satisfy the following relationship. FIG. 6 is an energy band diagram showing this relationship.

Eg (a)> Eg (b)> Eg (c)> Eg (d)
χ (a) <χ (b) <χ (c) <χ (d)
Eg (a) −Eg (b) = χ (b) −χ (a)
Eg (b) −Eg (c) = χ (c) −χ (b)
Eg (c) −Eg (d) = χ (d) −χ (c)
FIG. 7 shows the result of deriving the characteristics of the solar cell A configured as described above by numerical simulation in which the Poisson equation and the current continuity equation are calculated by the finite difference method. FIG. 7 shows the change in conversion efficiency with respect to the total film thickness of three layers (first layer, second layer, and third layer) existing on the light incident side from the fourth layer where the forbidden band width is minimum. Yes. Here, the forbidden band width (electron affinity) and film thickness of each layer are calculated in several combinations. As in the first and second embodiments, since the conversion efficiency when the p-type semiconductor film 1 is formed as a single layer is 14.1%, the light incident side is higher than the fourth layer where the forbidden band width is minimum. It can be seen that if the total thickness of the layers present in the layer is 150 nm or less, higher conversion efficiency can be obtained than in the case of a single layer. This is because when a layer with a large forbidden band is thinned, a layer with a large forbidden band exists in the depletion layer, and carriers stay in the fourth layer with the smallest forbidden band width or the largest electron affinity. This is because the carrier recombination can be reduced. Here, in order for a layer having a large forbidden band to exist in the depletion layer, the carrier concentration of each layer constituting the p-type semiconductor film 1 is preferably 10 ¹⁵ / cm ³ or more.

  In the first to third embodiments, the difference in the forbidden band width and the difference in the electron affinity are all the same, but it is not always necessary to match, and either the forbidden band width or the electron affinity is What is necessary is just to satisfy predetermined magnitude relationship.

[Embodiments 4 and 5]
In the fourth embodiment, N = 3 and M = 2. That is, the p-type semiconductor film 1 is formed by laminating three layers of a first layer, a second layer, and a third layer in order from the light incident side (n-type semiconductor film 4 side). Among them, in the first layer and the second layer on the light incident side, the forbidden band width of the first layer is larger than the forbidden band width of the second layer and the change of the forbidden band width generated between the layers is discontinuous, or The electron affinity of the first layer is smaller than the electron affinity of the second layer, and the change in electron affinity generated between the layers is made discontinuous. Further, in the second layer and the third layer opposite to the light incident side, the forbidden band width of the second layer is smaller than the forbidden band width of the third layer, and the change of the forbidden band width generated between the layers is discontinuous. Alternatively, the electron affinity of the second layer is larger than the electron affinity of the third layer, and the change in electron affinity generated between the layers is made discontinuous.

  The fourth embodiment has the same configuration as that of the first embodiment except that the p-type semiconductor film 1 has a configuration in which three layers of a first layer, a second layer, and a third layer are stacked. The forbidden band width and the electron affinity of each layer constituting the p-type semiconductor film 1 are adjusted by changing the ratio of In and Ga in each layer.

  When the forbidden band widths of the first layer, the second layer, and the third layer in the p-type semiconductor film 1 are Eg (a), Eg (b), and Eg (c), they satisfy the following relationship. That is, the p-type semiconductor film 1 is formed so that the forbidden band width of the second layer is minimized and the forbidden band width of the first layer and the third layer is larger than that. FIG. 8A is an energy band diagram showing this relationship.

Eg (a) = Eg (c)> Eg (b)
Eg (a) (= Eg (c)) − Eg (b) = 0.05 eV
In the fifth embodiment, N = 4 and M = 2. That is, the p-type semiconductor film 1 is formed by laminating four layers of a first layer, a second layer, a third layer, and a fourth layer in order from the light incident side (n-type semiconductor film 4 side). Of the four layers, in the first layer and the second layer on the light incident side, the forbidden band width of the first layer is larger than the forbidden band width of the second layer and the change of the forbidden band width generated between the layers is discontinuous. Alternatively, the electron affinity of the first layer is smaller than the electron affinity of the second layer, and the change in electron affinity generated between the layers is discontinuous. In the second to fourth layers opposite to the light incident side, in the two adjacent layers, the forbidden band width of the light incident side layer is smaller than the forbidden band width of the other layer, and is forbidden between the layers. The change in the bandwidth is made discontinuous, or the electron affinity of the layer on the light incident side is larger than the electron affinity of the other layer, and the change in the electron affinity generated between the layers is made discontinuous.

  The fifth embodiment has the same configuration as that of the first embodiment except that the p-type semiconductor film 1 has a configuration in which four layers of a first layer, a second layer, a third layer, and a fourth layer are stacked. is doing. The forbidden band width and the electron affinity of each layer constituting the p-type semiconductor film 1 are adjusted by changing the ratio of In and Ga in each layer.

  When the forbidden band widths of the first layer, the second layer, the third layer, and the fourth layer in the p-type semiconductor film 1 are Eg (a), Eg (b), Eg (c), and E (d), respectively. These satisfy the following relationship. That is, the forbidden band width of the second layer is minimized, the forbidden band width is larger for the first layer and the third layer, and the forbidden band width is larger for the fourth layer than the third layer. A type semiconductor film 1 is formed. FIG. 8B is an energy band diagram showing this relationship.

Eg (d)> Eg (a) = Eg (c)> Eg (b)
Eg (a) (= Eg (c)) − Eg (b) = 0.05 eV
Eg (d) -Eg (c) = 0.05 eV
FIG. 9 shows a first embodiment (where Eg (a) −Eg (b) = 0.05 eV), a fourth embodiment, and a fourth embodiment, which are derived by numerical simulation that calculates the Poisson equation and the current continuity equation by the finite difference method. The conversion efficiency of the solar cell A in the form 5 is shown. As shown in this figure, the p-type semiconductor film 1 has a three-layer structure and a forbidden band width that is the smallest forbidden than the second layer, as compared with the first embodiment. The conversion efficiency is higher in the fourth embodiment in which the third layer having a larger band width is formed. Further, when the fourth layer having a larger forbidden band width than the third layer is formed in the third layer as in the fifth embodiment, the conversion efficiency is further increased. This is because, in the fourth and fifth embodiments, carriers photoexcited in the second layer having the smallest forbidden band width are reflected at the interface with the third layer adjacent to the second layer having a larger forbidden band width, so that the carrier collection efficiency is improved. This is because the short-circuit current density is increased. Similarly, in the fifth embodiment, the carriers photoexcited in the third layer are reflected at the interface with the fourth layer having a large forbidden band adjacent to the third layer, thereby further improving the carrier collection efficiency and the short-circuit current density. Will further increase.

  Also, FIG. 10 is derived by numerical simulation in Embodiment 5 in which the Poisson equation and the current continuity equation are calculated by the finite difference method when the thickness x of the second layer that minimizes the forbidden bandwidth is changed. The change of the conversion efficiency of the solar cell A is shown. When the p-type semiconductor film 1 has a two-layer structure as in the first embodiment, the conversion efficiency is 14.6% (see FIG. 9), whereas in the fifth embodiment, as shown in FIG. A higher conversion efficiency than that of the first embodiment can be obtained when the film thickness x of the layer 6b is in the range of 150 to 450 nm. This is because carrier retention is reduced and carrier recombination is suppressed by optimizing the thickness of the second layer having the smallest forbidden band width.

  In FIG. 8, the forbidden bandwidths of the first layer and the third layer are the same, but they are not necessarily the same. In FIG. 8A, it is only necessary to satisfy Eg (a)> Eg (b) and Eg (b) <Eg (c). Similarly, in FIG. 8B, Eg (a)> Eg (b), Eg (b) <Eg (c), and Eg (c) <Eg (d) may be satisfied.

  Moreover, although the valence band level is the same in all layers in each layer, it is not necessarily the same. For example, the electron affinity χ (a) to χ (c) of the first layer to the third layer in FIG. 8A satisfies χ (a) <χ (b), χ (b)> χ (c). do it. In the case of FIG. 8B, the electron affinity χ (a) to χ (d) of each layer is represented by χ (a) <χ (b), χ (b)> χ (c), χ (c)> χ What is necessary is just to satisfy (d).

  Hereinafter, specific examples of the present invention will be described.

Example 1
First, SnO ₂ : F was deposited on a soda-lime glass substrate 2 by a thermal chemical vapor deposition (CVD) method to form a transparent electrode film 3 having a thickness of about 0.4 μm.

A window layer (n-type semiconductor film 4) having a two-layer structure of a TiO ₂ film and an In ₂ S ₃ film was formed on the transparent conductive film 12. The TiO ₂ film was formed to a thickness of about 0.1 μm by sputtering. Sputtering was performed by applying RF 400 W in an Ar atmosphere using a TiO ₂ sintered body as a target. The In ₂ S ₃ film was formed to a thickness of 0.1 μm by spray coating pyrolysis. In the spray coating thermal decomposition method, while heating the TiO ₂ film formed previously 370 ° C., in the TiO ₂ film, a InCl ₃ 2 mmol / l, an aqueous solution containing 6 mmol / l thiourea Sprayed.

_Next, a p-type semiconductor film 1 of the two-layer structure having a composition of _{Cu (In 1-x Ga x} ) S 2, changing the rate of solid solution x in each layer. The composition of the first layer 6a is _{Cu (In 0.8 Ga 0.2) S} 2, the thickness is 50 nm, the composition of the second layer 6b is CuInS _2, a thickness of 2000 nm,. In a p-type semiconductor having such a composition of Cu (In _1-x Ga _x ) S ₂ , increasing the Ga solid solution ratio x increases the forbidden band width and decreases the electron affinity.

The first layer 6a and the second layer 6b were formed by spray coating pyrolysis. The first layer 6a is formed by heating an aqueous solution containing CuCl ₂ at a concentration of ₂ mmol / l, InCl ₃ at 1.6 mmol / l, GaCl ₃ at 0.4 mmol / l, and thiourea at a concentration of 6 mmol / l to 300 ° C. The surface of the mold semiconductor film 4 was formed by spray coating. For the second layer 6b, an aqueous solution containing CuCl ₂ at a concentration of ₂ mmol / l, InCl ₃ at a concentration of 2 mmol / l, and thiourea at a concentration of 6 mmol / l is spray-coated on the surface of the first layer 6a heated to 300 ° C. Was formed.

When the p-type semiconductor film 1 is formed in this manner, the p-type semiconductor having a composition of Cu (In _1-x Ga _x ) S ₂ has a small interdiffusion of In and Ga between layers having different compositions. The diffusion of In and Ga between the layer 6a and the second layer 6b is suppressed, and the change in the forbidden band width and the electron affinity between the first layer 6a and the second layer 6b is the same as shown in FIG. It becomes discontinuous in a staircase shape.

  Next, an Au film having a thickness of about 0.2 μm was formed as the back electrode 5 by vapor deposition.

Thus, it was possible to obtain a solar cell having a p-type semiconductor film 1 of the two-layer structure having a composition of _{Cu (In 1-x Ga x} ) S 2 as a suitable light-absorbing layer.

(Example 2)
First, ZnO: B was deposited on a soda-lime glass substrate 2 by a thermal chemical vapor deposition (CVD) method to form a transparent electrode film 3 having a thickness of about 1.0 μm. In this thermal CVD method, Zn (C ₂ H ₅ ) ₂ and H ₂ O were supplied with a carrier gas, diluted B ₂ H ₆ gas was supplied as a dopant, and the substrate 2 was heated to 200 ° C.

A window layer (n-type semiconductor film 4) having a two-layer structure of a TiO ₂ film and an In ₂ S ₃ film was formed on the transparent conductive film 12 in the same manner as in Example 1.

_Next, a p-type semiconductor film 1 of the three-layer structure having a composition of _{Cu (In 1-x Al x} ) S 2, changing the rate of solid solution x in each layer. The composition of the first layer is Cu (In _0.94 Al _0.06 ) S ₂ , the thickness is 50 nm, the composition of the second layer is Cu (In _0.97 Al _0.03 ) S ₂ , and the thickness is 25 nm. The composition of the third layer is CuInS ₂ and the thickness is 2000 nm. In a p-type semiconductor having such a composition of Cu (In _1-x Al _x ) S ₂ , the forbidden band width increases and the electron affinity decreases as the solid solution ratio x of Al increases.

The first layer, the second layer, and the third layer were formed by spray coating pyrolysis. In the first layer, an aqueous solution containing CuCl ₂ at a concentration of ₂ mmol / l, InCl ₃ at 1.88 mmol / l, AlCl ₃ at 0.12 mmol / l, and thiourea at a concentration of 6 mmol / l was heated to 350 ° C. The film was formed by spray coating on the surface of the mold semiconductor film 4. The second layer was heated to 350 ° C. with an aqueous solution containing CuCl ₂ at 2 mmol / l, InCl ₃ at 1.94 mmol / l, AlCl ₃ at 0.06 mmol / l mmol / l, and thiourea at 6 mmol / l. A film was formed by spray coating on the surface of the first layer. The third layer is formed by spray-coating an aqueous solution containing CuCl ₂ at a concentration of ₂ mmol / l, InCl ₃ at a concentration of 2 mmol / l, and thiourea at a concentration of 6 mmol / l on the surface of the second layer heated to 300 ° C. Filmed.

When the p-type semiconductor film 1 is formed in this way, the p-type semiconductor having a composition of Cu (In _1-x Al _x ) S ₂ has a small interdiffusion of In and Al between layers having different compositions. The diffusion of In and Al between the layers of the layer, the second layer, and the third layer is suppressed, and the change of the forbidden bandwidth and the electron affinity is discontinuous stepwise between the layers as in the case shown in FIG.

Next, a ZnO: Al film having a thickness of about 0.8 μm, which is a light-transmitting metal oxide film, was formed as the back electrode 5 by sputtering. Sputtering was performed by applying RF 400 W in an Ar atmosphere using a ZnO: Al sintered body containing 2% by mass of Al ₂ O ₃ as a target.

As described above, a solar cell having a p-type semiconductor film 1 having a three-layer structure having a composition of Cu (In _1-x Al _x ) S _{2 serving} as a suitable light absorption layer could be obtained.

(Example 3)
First, SnO ₂ : F is deposited on a soda-lime glass substrate 2 by the same method as in Example 1 to form a transparent electrode film 3 having a film thickness of about 0.4 μm, and this transparent conductive film 12 is formed. A window layer (n-type semiconductor film 4) having a two-layer structure of a TiO ₂ film and an In ₂ S ₃ film was formed thereon.

Next, a p-type semiconductor film 1 having a four-layer structure having a composition of (Cu _1-x Zn _x ) InS ₂ was formed, and the solid solution ratio x in each layer was changed. The composition of the first layer is (Cu _0.7 Zn _0.3 ) InS ₂ , the thickness is 50 nm, the composition of the second layer is CuInS ₂ , the thickness is 300 nm, and the composition of the third layer is (Cu _{0.3 . 7} Zn _0.3) InS _2, the thickness is 100 nm, the composition of the fourth layer is _{(Cu 0.6 Zn 0.4) InS 2} , a thickness of 1500 nm. In a p-type semiconductor having such a composition of (Cu _1-x Zn _x ) InS ₂ , the forbidden band width increases and the electron affinity decreases as the solid solution ratio x of Zn increases.

The first layer, the second layer, the third layer, and the fourth layer were formed by spray coating pyrolysis. In the first layer, an aqueous solution containing CuCl ₂ at 1.4 mmol / l, ZnCl ₂ at 0.6 mmol / l, InCl ₃ at 2 mmol / l, and thiourea at a concentration of 6 mmol / l was heated to 300 ° C. n The film was formed by spray coating on the surface of the mold semiconductor film 4. The second layer is formed by spray coating an aqueous solution containing CuCl ₂ at a concentration of ₂ mmol / l, InCl ₃ at a concentration of 2 mmol / l, and thiourea at a concentration of 6 mmol / l on the surface of the first layer heated to 300 ° C. Filmed. The third layer is an aqueous solution containing CuCl ₂ at a concentration of 1.4 mmol / l, ZnCl ₂ at 0.6 mmol / l, InCl ₃ at 2 mmol / l, and thiourea at a concentration of 6 mmol / l. A film was formed by spray coating on the surface of the two layers. The fourth layer is an aqueous solution containing CuCl ₂ at a concentration of 1.2 mmol / l, ZnCl ₂ at 0.8 mmol / l, InCl ₃ at 2 mmol / l, and thiourea at a concentration of 6 mmol / l, heated to 300 ° C. A film was formed by spray coating on the surface of the three layers.

When the p-type semiconductor film 1 is formed in this way, the p-type semiconductor having the composition of (Cu _1-x Zn _x ) InS ₂ has a small interdiffusion of Cu and Zn between layers having different compositions, and thus the first layer To the fourth layer, the diffusion of Cu and Zn between the layers is suppressed, and the change in the forbidden band width and the electron affinity is discontinuous in a stepped manner in the same manner as shown in FIG. 8B.

Next, a carbon film having a thickness of about 10 μm was formed as the back electrode 5 by a screen printing method. As described above, a p-layer structure having a composition of (Cu _1-x Zn _x ) InS ₂ which becomes a suitable light absorption layer. A solar cell having the type semiconductor film 1 could be obtained.

1 p-type semiconductor film A solar cell

Claims (15)

  A p-type semiconductor film composed of a p-type semiconductor and used as a light absorption layer of a solar cell, and composed of N layers (N is a natural number satisfying N ≧ 2) stacked in the light incident direction. Among the N layers, the M layers from the layer arranged closest to the light incident side to the Mth layer (M is a natural number satisfying 2 ≦ M ≦ N) are arbitrary adjacent to each other. Of the two layers, the forbidden band width of one layer arranged on the light incident side is larger than the forbidden band width of the other layer, and the change of the forbidden band width generated between the layers is stepped or arranged on the light incident side. A p-type semiconductor film characterized in that the electron affinity of one layer is smaller than the electron affinity of the other layer and the change in electron affinity generated between the layers is stepped. Of the M layers, the forbidden band width (Eg (n)) of an arbitrary nth (where 1 ≦ n ≦ M−1) layer from the light incident side and the forbidden bandwidth of the (n + 1) th layer ( 2. The p-type semiconductor film according to claim 1, wherein each of Eg (n + 1)) satisfies the relationship of the following formula (1).
0 (eV) <Eg (n) −Eg (n + 1) ≦ 0.1 (eV) (1) Among the M layers, the electron affinity (χ (n)) of an arbitrary nth (where 1 ≦ n ≦ M−1) layer from the light incident side (χ (n)) and the electron affinity (χ (n) of the (n + 1) th layer. 2. The p-type semiconductor film according to claim 1, wherein n + 1)) satisfies the relationship of the following formula (2).
−0.1 (eV) ≦ χ (n) −χ (n + 1) <0 (eV) (2)   2. The total thickness of the M−1 layers from the layer disposed closest to the light incident side to the M−1th layer among the N layers is less than 150 nm. 4. The p-type semiconductor film according to any one of 3 above. M is a natural number satisfying 2 ≦ M <N;
Of the N layers, N−M + 1 layers from the M-th layer to the N-th layer from the light incident side are arranged on the light incident side of any two adjacent layers. The forbidden band width of one layer is smaller than the forbidden band width of the other layer and the change of the forbidden band width occurring between the layers is stepped, or the electron affinity of one layer disposed on the light incident side is 4. The p-type semiconductor film according to claim 1, wherein the p-type semiconductor film has a step-like change in electron affinity that is greater than the electron affinity and that occurs between layers. 5.   6. The p-type semiconductor film according to claim 1, wherein the thickness of the Mth layer from the light incident side is 150 nm or more and less than 450 nm. 7. The p-type semiconductor film according to claim 1, wherein a carrier concentration is higher than 1 × 10 ¹⁵ / cm ³ in any of the N layers.   The p-type semiconductor film according to claim 1, wherein the N layers have the same crystal structure.   9. The p-type semiconductor film according to claim 8, wherein the crystal structure of the N layers is a chalcopyrite structure.   The p-type semiconductor film according to claim 1, wherein the N layers are composed of the same element group.   11. The p-type semiconductor film according to claim 10, wherein main elements constituting the N layers are a group 11 element, a group 13 element, and a group 16 element. The main elements constituting the N layers are a group 11 element, In, Ga, and a group 16 element, and Ga / (In + Ga) in an arbitrary nth layer (where 1 ≦ n ≦ M) from the light incident side. 12) and the ratio X (n + 1) of Ga / (In + Ga) in the (n + 1) th layer satisfy the relationship of the following formula (3). p-type semiconductor film.
0 <X (n) −X (n + 1) ≦ 0.2 (3) The main elements constituting the N layers are a group 11 element, In, Al, and a group 16 element, and Al / (in an arbitrary nth layer (where 1 ≦ n ≦ M) from the light incident side. 12. The ratio Y (n) of In + Al) and the ratio Y (n + 1) of Al / (In + Al) of the (n + 1) th layer satisfy the relationship of the following formula (4). P-type semiconductor film.
0 <Y (n) −Y (n + 1) ≦ 0.1 (4) The main elements constituting the N layers are Cu, Zn, a group 13 element and a group 16 element, and Zn / (in the nth (where 1 ≦ n ≦ M) layer is arbitrary from the light incident side. 11. The ratio Z (n) of Cu + Zn and the ratio Zn (n + 1) of Zn / (Cu + Zn) in the (n + 1) th layer satisfy the relationship of the following formula (5). P-type semiconductor film.
0 <Zn (n) −Zn (n + 1) ≦ 0.4 (5)   A solar cell comprising the p-type semiconductor film according to claim 1 as a light absorption layer.

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