JP2012124279A - Photoelectric element and solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain higher power generation efficiency in a photoelectric element in which a sulfide-based compound semiconductor such as CZTS is used in a light absorbing layer.SOLUTION: A photoelectric element 10 comprises a light absorbing layer 16 being a p-type semiconductor layer, a buffer layer 18, and a window layer 20, and the light absorbing layer 16, the buffer layer 18, and the window layer 20 are provided in this order. The light absorbing layer 16 is a film of a sulfide-based compound semiconductor containing Cu, Zn, Sn, and S. The photoelectric element 10 further comprises a ZnMgO film 19 between the buffer layer 18 and the window layer 20.

Description

  The present invention relates to a photoelectric element and a solar cell.

  A photoelectric element converts photon energy into an electrical signal (photoelectric conversion) through some physical phenomenon. Photoelectric elements are used for solar cells, photoconductive cells, photodiodes, phototransistors, and the like.

A solar cell is a kind of photoelectric element, and can efficiently convert light energy of sunlight into electric energy. A general solar cell has a structure in which a p-type semiconductor layer and an n-type semiconductor layer are joined. The p-type semiconductor used in solar cells, monocrystalline Si, polycrystalline Si, amorphous _{Si, GaAs, InP, CdTe,} CuIn 1-x Ga x Se 2 (CIGS) and _Cu 2 ZnSnS ₄ (CZTS) Hitoshigachi It has been. Among these, chalcopyrite compound semiconductors typified by CIGS and CZTS have a large light absorption coefficient, and thus can be thinned to reduce costs. In particular, a thin film solar cell using CIGS has the highest conversion efficiency, and conversion efficiency exceeding that of polycrystalline Si is also obtained. However, CIGS has a problem that it contains elements with high environmental impact and rare elements. On the other hand, CZTS has a band gap energy (1.4 to 1.5 eV) suitable for solar cells, does not contain elements with high environmental impact and rare elements, has abundant material resources, and is manufactured at low cost. Since it can be done, it has been attracting attention recently (for example, Patent Document 1).

JP 2009-26891 A

  However, photoelectric elements using sulfide compound semiconductors such as CZTS in the light absorption layer have lower power generation efficiency than conventional photoelectric elements using CIGS, and further improvement in this respect is necessary for practical use. It has been demanded.

  Therefore, an object of the present invention is to obtain higher power generation efficiency in a photoelectric device using a sulfide compound semiconductor such as CZTS in a light absorption layer.

The present invention relates to a photoelectric device including a light absorption layer that is a p-type semiconductor layer, a buffer layer, and a window layer, and the light absorption layer, the buffer layer, and the window layer are provided in this order. In the photoelectric device according to the present invention, the light absorption layer is a film of a sulfide compound semiconductor containing Cu, Zn, Sn, and S. The photoelectric element according to the present invention further includes a Zn _1-x Mg _x O film provided between the buffer layer and the window layer.

As a result of intensive studies by the present inventors, by providing a Zn _1-x Mg _x O film between the buffer layer and the window layer in a photoelectric element using a sulfide compound semiconductor such as CZTS as a light absorption layer. It was revealed that higher power generation efficiency can be obtained.

Since the lower end of the conduction band (CBM) is different between CIGS and CZTS, the combination of the light absorption layer of CZTS and the conventional buffer layer and window layer is the only way to combine the light absorption layer, buffer layer, and window layer. It is considered that the conduction band offset (CBO) was not adequately optimized. By providing the Zn _1-x Mg _x O film between the buffer layer and the window layer, the conduction band offset is optimized, and as a result, the power generation efficiency is considered to be improved. Furthermore, it is considered that suppression of leakage current by the Zn _1-x Mg _x O film also contributes to improvement of power generation efficiency.

The window layer is preferably a Zn _1-x Mg _x O film doped with at least one doping element selected from Ga, Al and B. By using Zn _1-x Mg _x O doped with Ga or the like as the material of the window layer, further excellent power generation efficiency can be obtained.

  The Mg ratio in the window layer is preferably 50 atomic% or less based on Zn.

  Furthermore, this invention provides a solar cell provided with the said photoelectric element.

  ADVANTAGE OF THE INVENTION According to this invention, still higher electric power generation efficiency can be acquired in the photoelectric element using sulfide type compound semiconductors, such as CZTS, for the light absorption layer.

It is a perspective view which shows one Embodiment of a photoelectric element. It is a graph which shows the result of the IV measurement in an Example.

  Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments. For the convenience of the drawings, the dimensional ratios in the drawings do not necessarily match those described.

FIG. 1 is a perspective view showing an embodiment of a photoelectric element. 1 includes a substrate 12, a lower electrode 14 provided on the substrate 12, a light absorption layer 16 provided on the lower electrode 14, and a buffer layer provided on the light absorption layer 16. 18, a Zn _1-x Mg _x O film 19 provided on the buffer layer 18, a window layer 20 provided on the Zn _1-x Mg _x O film 19, and an upper part provided on the window layer 20 And an electrode 22. The light absorption layer 16, the buffer layer 18, the Zn _1-x Mg _x O film 19, and the window layer 20 are laminated in this order.

  The substrate 12 supports each member formed thereon. The substrate 12 may be a conductor or an insulator. The substrate 12 is preferably selected from quartz glass, non-alkali glass, and low alkali glass (LAG), and metals, semiconductors, oxides, sulfides, nitrides, carbides, silicides, carbon, or combinations thereof. It is a plate-shaped body of material.

The lower electrode 14 is formed of a material having high electrical conductivity and good adhesion to the substrate 12. The lower electrode 14 is formed of, for example, a material selected from Mo, In—Sn—O, In—Zn—O, ZnO: B, SnO ₂ : F, SnO ₂ : Sb, and TiO ₂ : Nb. The expression “ZnO: B” means ZnO doped with B. Others are the same. When a glass substrate is used as the substrate 12, Mo is preferable from the viewpoints of adhesion, electrical conductivity, incident light reflectance, and difficulty of sulfidation.

The light absorption layer 16 is a p-type semiconductor layer containing a sulfide compound semiconductor containing Cu, Zn, Sn, and S. As the sulfide compound semiconductor, Cu ₂ ZnSnS ₄ (CZTS) is preferable. Since CZTS has a large light absorption coefficient, if CZTS is used, the thickness of the light absorption layer 16 can be extremely reduced. The light absorption layer 16 is generally a thin film, but may be in the form of particles. When the light absorption layer 16 is a thin film, the thickness is preferably 0.2 μm to 2.0 μm.

  The ratio of Cu in CZTS is preferably slightly smaller than the stoichiometric composition because high conversion efficiency can be obtained. Specifically, Cu / (Zn + Sn) is preferably 0.69 to 0.99, more preferably 0.8 to 0.9 in terms of atomic ratio.

By providing the buffer layer 18, the light absorption layer 16 and the window layer 20 are well connected, and the power generation efficiency is improved. Examples of the semiconductor constituting the buffer layer 18 include CdS, ZnO, Zn (O, OH), Zn (O, S), Zn (O, S, OH) _x , Zn _1-x Mg _x O, and In. ₂ S ₃ is mentioned. When the light absorption layer 16 is a CZTS film, CdS is preferable. The thickness of the buffer layer 18 can be appropriately set according to the purpose, but is preferably 10 nm to 100 nm.

The Zn _1-x Mg _x O film 19 (x is a numerical value greater than 0 and less than 1) is not substantially doped with a doping element such as Ga. When MgO is added to ZnO, a band gap wider than the band gap (3.3 eV) of ZnO alone can be obtained. That is, the band gap of the Zn _1-x Mg _x O film 19 can be adjusted by changing the ratio of Mg to Zn. Therefore, CBO in a photoelectric element using CZTS can be easily optimized.

The Mg ratio in the Zn _1-x Mg _x O film 19 is preferably 50 atomic percent or less, more preferably 5 atomic percent to 40 atomic percent, based on Zn. If the Mg ratio is low, a band gap having a sufficient width tends not to be obtained, and if it exceeds 50 atomic%, the crystal structure changes and the effect of improving the power generation efficiency tends to decrease.

The thickness of the Zn _1-x Mg _x O film 19 is preferably 10 nm to 100 nm. When the Zn _1-x Mg _x O film becomes thicker, the electric resistance tends to be too high. When the thickness of the Zn _1-x Mg _x O film 19 is less than 10 nm, the effect of improving the power generation efficiency tends to be reduced. From the same viewpoint, the thickness of the Zn _1-x Mg _x O film 19 is more preferably 20 nm to 80 nm.

  The window layer 20 is a light-transmitting layer provided to collect photocurrent generated by the photoelectric effect. The window layer 20 has a low resistance and transmits most of light in the visible to near infrared region. The thickness of the window layer 20 is preferably 100 nm to 2000 nm.

The window layer 20 may be, for example, a Zn _1-x Mg _x O film doped with Ga (hereinafter sometimes referred to as “ZMO: Ga film”), ZnO: B, ZnO: Al (AZO), In—Sn—O. , In—Zn—O, SnO ₂ : Sb, TiO ₂ : Nb, and ZnO: Ga (GZO). From the viewpoint of optimizing the conduction band offset, the window layer 20 is preferably a Zn _1-x Mg _x O film doped with at least one doping element selected from Ga, Al, and B. Among these, a ZMO: Ga film is particularly preferable.

When the window layer 20 is a Zn _1-x Mg _x O film doped with a doping element, the Mg ratio is preferably 50 atomic percent or less, more preferably 5 atomic percent to 40 atomic percent, based on Zn. If the Mg ratio is low, a band gap having a sufficient width tends not to be obtained, and if it exceeds 50 atomic%, the crystal structure changes and the effect of improving the power generation efficiency tends to decrease.

  The ratio of the doping element is preferably 0.5 atomic% to 12 atomic% based on Zn. When the ratio of the doping element is less than 0.5 atomic%, the electric resistance tends to be too high, and when it exceeds 12 atomic%, the electric resistance becomes high.

  The upper electrode 22 is provided to efficiently extract the current collected by the window layer 20 to the outside. The upper electrode 22 is usually formed in a comb shape. Examples of the material of the upper electrode 22 include Al, Cu, Ag, and Au, and Al is preferable. The upper electrode 22 may be an alloy containing one or more metals selected from Al, Cu, Ag, and Au. Specific examples of such alloys include Al-Ti alloys, Al-Mg alloys, Al-Ni alloys, Cu-Ti alloys, Cu-Sn alloys, Cu-Zn alloys, Cu-Au alloys, and Ag-Ti. There are an alloy, an Ag—Sn alloy, an Ag—Zn alloy, an Ag—Au alloy, and the like.

  The photoelectric element according to this embodiment can be manufactured by, for example, a method of sequentially forming a lower electrode, a light absorption layer, a buffer layer, a window layer, and an upper electrode on a substrate. Each film can be formed by a method usually employed as a method for forming a thin film, such as a sputtering method, a vacuum evaporation method, and a pulse laser deposition (PLD) method.

The CZTS film as the light absorption layer 16 forms a precursor film in which Cu, Sn, and ZnS are laminated in a predetermined order on a substrate by using a sputtering method, a vacuum deposition method, a PLD method, or the like. It can be formed by a method of sulfiding the precursor in an H ₂ S-containing atmosphere (for example, in a mixed gas atmosphere of 5% to 20% by volume H ₂ S and N ₂ ). The temperature during sulfiding is about 500 ° C to 600 ° C. When producing the precursor, various CZTS films having different compositions can be formed by changing the thickness of Cu, Sn, or ZnS.

The Zn _1-x Mg _x O film 19 can be formed, for example, by sputtering using ZnO and MgO simultaneously as targets. By changing the quantity ratio of ZnO and MgO used as a target, the ratio of Zn and Mg in the Zn _1-x Mg _x O film can be adjusted.

The ZMO: Ga film as the window layer 20 can be formed, for example, by sputtering using ZnO, MgO, and Ga ₂ O ₃ simultaneously as targets. By changing the quantitative ratio of ZnO, MgO and Ga ₂ O ₃ used as targets, the ratio of Zn, Mg and Ga in the ZMO: Ga film can be adjusted.

  Examples of film forming methods other than sputtering, vacuum deposition, and PLD include (1) hydrolysis by coating a solution of an organic metal or the like on the substrate 12 and drying in air. A sol-gel + sulfurization method in which a light absorption layer 16 is formed by causing a degenerative reaction to form a metal oxide thin film and heat-treating the metal oxide thin film in a hydrogen sulfide atmosphere, (2) an aqueous solution in which metal ions are dissolved There is a chemical bath film formation (CBD) method in which a buffer layer 18 is formed by dissolving thiourea or the like in the substrate and immersing and heating the substrate in the substrate.

The present invention is not limited to the embodiment described above, and various modifications can be made without departing from the gist of the present invention. For example, an additional layer other than the substrate, the lower electrode, the light absorption layer, the buffer layer, the Zn _1-x Mg _x O film, the window layer, and the upper electrode may be provided between the layers. As additional layers, for example, an adhesive layer, a light scattering layer, and an antireflection layer may be provided. A solar cell module including a plurality of photoelectric elements connected in series or in parallel can also be configured.

(Example)
According to the following procedure, an evaluation solar cell including a photoelectric element having the same configuration as the photoelectric element 10 of FIG. 1 was produced.
(1) A Mo film as the lower electrode 14 was formed on a quartz glass (QZG) substrate by sputtering.
(2) A CZTS precursor film was formed on the Mo film by sputtering.
(3) CZTS film (light absorption layer 16) is formed from the precursor film by sulfiding treatment at 550 ° C. to 580 ° C. for 3 hours in a mixed gas atmosphere of atmospheric pressure and 20% by volume of H ₂ S and N _2. Formed.
(4) The CZTS film was immersed in an aqueous solution containing cadmium acetate and thiouric acid at 70 ° C. for 15 minutes to form a CdS film (buffer layer 18) on the light absorption layer 16. The film thickness of the CdS film was 30 nm.
(5) Undoped Zn ₁₋ on the CdS film by RF magnetron sputtering (back pressure 3.0 × 10 ⁻⁴ Pa or less, Ar gas pressure 0.2 Pa, power 50 W) using ZnO and MgO as targets simultaneously. the _x Mg _x O layer 19 was formed. When analyzed by fluorescent X-ray analysis, the ratio of Mg to Zn was 15 atomic%. The thickness of the Zn _1-x Mg _x O film was 50 nm.
(6) By RF magnetron sputtering (back pressure 3.0 × 10 ⁻⁴ Pa or less, Ar gas pressure 0.2 Pa, power 50 W) using ZnO, MgO and Ga ₂ O ₃ simultaneously as targets, Zn _1-x A ZMO: Ga film as the window layer 20 was formed on the Mg _x O film 19. The volume resistivity of the ZMO: Ga film was 4.95 × 10 ⁻⁴ Ωcm. The visible light average transmittance (380 nm to 780 nm) of the ZMO: Ga film was 90.8%. When analyzed by fluorescent X-ray analysis, the ratio of Mg to Zn was 15 atomic%, and the ratio of Ga to Zn was 5 atomic%. The thickness of the window layer was 500 nm.
(7) Twelve comb-shaped electrodes (Al films) arranged in a matrix were formed on the window layer 20 by sputtering.
(8) The window layer 20, the buffer layer 18 and the light absorption layer 16 other than the columnar portion located under the 4 mm × 5 mm square surrounding one comb electrode are removed to expose the lower electrode 14. It was. On the lower electrode 14, 12 columnar laminates each including a comb-shaped electrode, a window layer 20, a Zn _1-x Mg _x O film 19, a buffer layer 18, and a light absorption layer 16 are arranged in a matrix of 4 rows × 3 columns. Arranged in a shape.
(9) A silver paste was applied to the exposed end surface of the lower electrode 14.

(Comparative example)
A Zn _1-x Mg _x O film made of ZnO and MgO was not formed between the window layer 20 and the buffer layer 18 by sputtering, and the window layer 20 was replaced with a ZnO: Ga film instead of a ZMO: Ga film. A solar cell for evaluation was produced in the same procedure as in the example except that a Ga (GZO) film was formed.

Evaluation of voltage-current characteristics (IV measurement)
Using simulated sunlight (AM = 1.5, 1 sun), each solar cell was subjected to IV measurement. FIG. 2 is a graph showing the relationship between voltage and current. From these measurement results, the short-circuit current density (J _sc ), the open circuit voltage (V _oc ), and the fill factor (FF) are obtained, and the power generation efficiency (η) is further calculated as η = V _oc · J _sc · FF / (unit area Per simulated solar energy). The obtained results are shown in Table 1.

The V _oc and FF of the photoelectric element of the example were improved as compared with the comparative example, and it was confirmed that high power generation efficiency η was developed reflecting this.

  The photoelectric element according to the present invention can be used for solar cells, photoconductive cells, photodiodes, phototransistors and the like.

10 ... photoelectric element, 12 ... substrate, 14 ... lower electrode, 16 ... light absorption layer, 18 ... buffer _{layer, 19 ... Zn 1-x Mg} x O layer, 20 ... window layer, 22 ... upper electrode.

Claims (4)

In a photoelectric device comprising a light absorption layer that is a p-type semiconductor layer, a buffer layer, and a window layer, wherein the light absorption layer, the buffer layer, and the window layer are provided in this order.
The light absorption layer is a film of a sulfide compound semiconductor containing Cu, Zn, Sn and S;
The photoelectric device, further comprising a Zn _1-x Mg _x O film provided between the buffer layer and the window layer. 2. The photoelectric device according to claim 1, wherein the window layer is a Zn _1-x Mg _x O film doped with at least one doping element selected from Ga, Al, and B. 3.   The photoelectric device according to claim 2, wherein a ratio of Mg in the window layer is 50 atomic% or less based on Zn. A solar cell provided with the photoelectric element as described in any one of Claims 1-3.

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