JP2013197514A - Solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve conversion efficiency of a solar cell including SnS.SOLUTION: A solar cell 10 comprises: a photoelectrode 12 that has a transparent conductive substrate 14 having conductivity, a buffer layer 16 which is adjacent to the transparent conductive substrate 14 and transports electrons, and a light absorption layer 18 which includes SnS, is adjacent to the buffer layer 16, and absorbs light; a counter electrode 20; and a hole transport layer 22 formed between the photoelectrode 12 and the counter electrode 20. In the solar cell 10, the outer peripheral side of the region between the transparent conductive substrate 14 and the counter electrode 20 is covered with sealing materials 24. In the solar cell 10, a conduction-band offset value is in a range of -0.2 eV or more but 0.1 eV or less, which is a difference between lower ends of conductive bands of the light absorption layer 18 and the buffer layer 16 when the lower end of the conductive band of the light absorption layer 18 is set as a reference.

Description

  The present invention relates to a solar cell.

Conventionally, as a solar cell, between a photoelectrode provided with a buffer layer (electron transport layer) covered with a light absorption layer on a transparent conductive substrate, and a counter electrode arranged to face this photoelectrode, A material in which a hole transport layer is interposed has been proposed. Especially, what used SnS for the light absorption layer is proposed (for example, refer nonpatent literatures 1-3). SnS has an optical absorption coefficient of 10 ⁵ cm ⁻¹ and a band gap of 1.1 to 1.4 eV, and has suitable optoelectronic properties for use in the light absorption layer. Such SnS solar cells are manufactured using an n-type semiconductor as a buffer layer.

Thin Solid Film 515 (2007) 5771 Solar Energy Materials and Solar Cells 91 (2007) 774 J. et al. Phys. Chem. C 114 (2010) 3256

However, in a solar cell in which CdS or CdZnS, which is a Cd-based semiconductor, and SnS are combined, the maximum values of the short circuit current density J _sc are 6 mA / cm ² (Non-patent Document 1) and 9.6 mA / cm ² ( Non-Patent Document 2). In TiO ₂ that is a non-Cd semiconductor, the short-circuit current density J _sc is as low as 0.3 mA / cm ² (Non-patent Document 3). For industrial applications, the use of Cd is not preferable, and the current problem is that the conversion efficiency is low in a non-Cd buffer layer.

  This invention is made | formed in view of such a subject, and it aims at providing the solar cell which can improve conversion efficiency more in the thing containing SnS.

  As a result of diligent research to achieve the above-described object, the present inventors have found that the conversion efficiency of a solar cell containing SnS is considered to be within a suitable range of the conduction band lower end (conduction band) of the buffer layer that is an n-type semiconductor. Has been found to be able to be further improved, and the present invention has been completed.

That is, the solar cell of the present invention is
A photoconductive electrode comprising: a conductive conductive transparent substrate; a buffer layer that is adjacent to the transparent conductive substrate and transports electrons; and a light absorption layer that includes SnS and is adjacent to the buffer layer and absorbs light;
With the counter electrode,
A hole transport layer formed between the photoelectrode and the counter electrode,
The conduction band offset value, which is the difference between the lower end of the conduction band of the light absorption layer and the buffer layer, with respect to the lower end of the conduction band of the light absorption layer is in the range of −0.2 eV to 0.1 eV. .

  The solar cell of this invention can improve conversion efficiency more. The reason why such an effect is obtained is not clear, but is presumed as follows. For example, in this solar cell, a buffer layer is used for the purpose of preventing recombination of electrons in a transparent conductive substrate and holes of SnS as a light absorption layer. When electrons photoexcited by SnS are transported through the buffer layer to the transparent conductive substrate, the difference between the lower end of the conduction band between the buffer layer and the light absorption layer (SnS) (also referred to as conduction band offset, CBO) However, if the value is a large negative value, it is considered that recombination easily occurs and the characteristics deteriorate. On the other hand, when CBO is a large positive value, it is considered that electrons are hardly transported from SnS to the transparent conductive substrate, and the characteristics are deteriorated. In the present invention, it is presumed that the conversion efficiency can be further improved by employing the buffer layer having the optimum CBO.

FIG. 2 is a schematic cross-sectional view showing the configuration of solar cell 10. Zn _(1-X) diagram of the relationship between the Mg _X O and the bottom of the conduction band in SnS and (CBM) and the X value. FIG. 2 is a schematic cross-sectional view showing a configuration of a battery module 100. Zn _(1-X) graph showing the relationship between X values and the optical band gap of Mg _X O. Zn relationship diagram between CBO values for _(1-X) X value and SnS of Mg _X O. The power generation characteristics of the solar cells of Example 3 and Comparative Example 3. Zn _(1-X) relationship diagram between the X value and the solar cell characteristics of Mg _X O.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing the configuration of the solar cell 10 of the present embodiment. The solar cell 10 of the present invention includes a transparent conductive substrate 14 having conductivity, a buffer layer 16 that is adjacent to the transparent conductive substrate 14 and transports electrons, and light that is adjacent to the buffer layer 16 and contains SnS and absorbs light. A photoelectrode 12 having an absorption layer 18, a counter electrode 20, and a hole transport layer 22 formed between the photoelectrode 12 and the counter electrode 20 are provided. In this solar cell 10, the outer periphery of the region between the transparent conductive substrate 14 and the counter electrode 20 is covered with a sealing material 24.

The transparent conductive substrate 14 has a configuration in which a transparent conductive film 14b is laminated on the buffer layer 16 side in a transparent substrate 14a such as a glass substrate. Examples of the material of the transparent conductive substrate 14 include fluorine-doped SnO ₂ coated glass (FTO), ITO coated glass, ZnO: Al coated glass, antimony doped tin oxide (SnO ₂ —Sb) coated glass, and gallium doped zinc oxide. Coated glass (GZO) etc. are mentioned. In addition, tin oxide or indium oxide doped with cations or anions with different valences, or a metal electrode with a structure capable of transmitting light, such as a mesh or stripe, provided on a glass substrate is also transparent. It can be used as the substrate 14. As the transparent substrate 14a, in addition to a transparent glass substrate, a substrate that transmits light, such as a glass substrate semi-transparent glass substrate that prevents light reflection by appropriately roughening the surface of the glass substrate, transparent Although a plastic plate, a transparent plastic film, an inorganic transparent crystal body, etc. can be used, a transparent glass substrate is preferable.

The buffer layer 16 is a layer that transports electrons generated in the light absorption layer 18 to the transparent conductive substrate 14. Examples of the material of the buffer layer 16 include n-type semiconductor materials typified by TiO ₂ , ZnO, SnO _2, and Zn _(1-X) Mg _X O (where 0 <X <1). Of these, Zn _(1-X) Mg _X O (hereinafter also referred to as ZMO) is preferable. Hereinafter, the solar cell 10 using mainly ZMO will be described. The ZMO of the buffer layer 16 is, for example, that _{X of} Zn _(1-X) Mg _X O preferably satisfies 0.1 <X <0.2, and satisfies 0.15 ≦ X ≦ 0.17. More preferably. Here, the difference between the lower end of the conduction band between the buffer layer and the light absorption layer (SnS) based on the light absorption layer (SnS) is defined as a conduction band offset (also referred to as CBO). At this time, when X exceeds 0.1, CBO becomes a smaller negative value, and recombination between electrons in the transparent conductive substrate and SnS holes as the light absorption layer can be further suppressed, which is preferable. Moreover, when X is less than 0.2, CBO becomes a smaller positive value, and electrons are easily transported from the light absorption layer 18 (SnS) to the transparent conductive substrate 14, which is preferable.

The light absorption layer 18 is a layer that covers the buffer layer 16 and includes SnS that is a metal sulfide having absorption in the visible light region and / or the infrared light region. SnS has a light absorption coefficient of 10 ⁵ cm ⁻¹ and a band gap of 1.1 to 1.4 eV, and has optoelectronic properties suitable as a light absorber for use in solar cells.

  In the solar cell 10 of the present invention, the conduction band offset value CBO, which is the difference between the lower end of the conduction band of the light absorption layer and the buffer layer, with respect to the lower end of the conduction band of the light absorption layer is −0.2 eV or more and 0.1 eV or less. It is a range. When CBO is −0.2 eV or more, recombination between electrons in the transparent conductive substrate 14 and holes of SnS as the light absorption layer 18 can be further suppressed. Moreover, when CBO is 0.1 eV or less, electrons are easily transported from the light absorption layer 18 (SnS) to the transparent conductive substrate 14, which is preferable. The CBO is more preferably in the range of −0.1 eV to 0 eV. In this range, the conversion efficiency can be further improved.

Here, the relationship between X of Zn _(1-X) Mg _X O and CBO will be described. FIG. 2 is an explanatory view of the relationship between X and the conduction band bottom (CBM) of Zn _(1-X) Mg _X O and SnS. The optical band gap can be obtained by UV-vis measurement. For example, the ionization potential IP of the SnS thin film can be measured by a photoelectron spectrometer (AC-2 manufactured by Riken Keiki), and is 4.9 eV (actual measurement value). The optical band gap Eg is 1.2 eV (actual measurement value). Assuming that the ionization potential IP is equal to the upper end of the valence band, the lower conduction band CBM of SnS is − (IP−Eg) = − 3.7 eV. The lower end of the conduction band of ZnO is reported to be -4.1 eV, and it is reported that the upper end position of the valence band hardly changes even when Mg is doped into ZnO. Here, CBO is defined as the difference from the CMO of ZMO with reference to the conduction band lower end (CBM) of SnS. That is, as shown in FIG. 2, when X = 0.25, the CBM of Zn _0.75 Mg _0.25 O is higher than the SnS CBM, so the CBO is about +0.15 eV. On the other hand, when X = 0, CBO is −0.4 eV. Further, when X = 0.17, CBO is 0 eV. Thus, there is a correlation between the X value of Zn _(1-X) Mg _X O and CBO for SnS, and X satisfies 0.1 <X <0.2, or CBO More preferably, it is in the range of −0.2 eV or more and 0.1 eV or less.

  The counter electrode 20 is a conductive layer through which electrons can pass. For example, a metal thin film such as Au or Pt or a porous carbon thin film can be used, and the counter electrode 20 has the same configuration as the above-described transparent conductive substrate 14 ( In this case, the transparent conductive film is disposed so as to be in contact with the hole transport layer 22).

The hole transport layer 22 receives electrons from the counter electrode 20 in a state where loads are connected to both electrodes of the solar cell 10, while the metal sulfide cation generated in the light absorption layer 18 by absorbing light from the counter electrode 20. It is a layer that returns to the original neutralized state with the received electrons. Examples of the material of the hole transport layer 22 include copper compounds such as CuI, CuSCN, CuO, Cu ₂ O, and Cu, and p-type semiconductor materials typified by Li-doped NiO.

  The sealing material 24 is for preventing the buffer layer 16, the light absorption layer 18, and the hole transport layer 22 from coming into contact with the outside air. For example, a thermoplastic resin film such as polyethylene or an epoxy adhesive can be used as the sealing material 24.

  Next, the operation of the solar cell 10 will be described. When light is applied to the transparent conductive substrate 14 with a load connected to both electrodes of the solar cell 10, electrons and holes are generated in the light absorption layer 18, electrons are conducted to the buffer layer 16, and holes are formed. Holes are conducted to the transport layer 22. The emitted electrons move to the transparent conductive film 14b of the transparent conductive substrate 14 via the buffer layer 16 and flow to the load. The hole transport layer 22 receives electrons from the counter electrode 20, and returns the holes to the original neutralized state with the received electrons. Due to such a series of reactions, when the solar cell 10 is irradiated with sunlight, a current flows through the load.

Next, the manufacturing method of the solar cell 10 is demonstrated. First, the transparent conductive substrate 14 is obtained by forming the transparent conductive film 14b on the transparent substrate 14a such as a glass substrate using a known thin film manufacturing technique such as spray coating. Subsequently, the buffer layer 16 is formed on the transparent conductive film 14 b of the transparent conductive substrate 14. Specifically, a dispersion liquid in which n-type semiconductor particles having a predetermined size (for example, a particle diameter of about 20 to 400 nm) are dispersed is prepared, and this dispersion liquid is applied to the transparent conductive film 14b by a bar coater method or a printing method. The buffer layer 16 may be formed by coating by drying, etc., and baking after drying. Alternatively, a thin-film buffer made of an n-type semiconductor on the transparent conductive film 14b by physical vapor deposition such as electron beam vapor deposition, resistance heating vapor deposition, sputter vapor deposition, or cluster ion beam vapor deposition, or chemical vapor deposition such as CVD (Chemical Vapor Deposition). Layer 16 may be formed. In the formation of the buffer layer 16, a conductive band offset value CBO having a range of −0.2 eV to 0.1 eV is used. For example, it is preferable to use Zn _(1-X) Mg _X O (provided that 0.1 <X <0.2). Here, changing the value of X, for example, may be adjusted by sputtering using _{Zn (1-X) Mg X} O target was used _{Zn (1-X) Mg X} O targets and ZnO targets two It may be adjusted by original simultaneous sputtering or the like. Subsequently, the light absorption layer 18 is formed on the buffer layer 16. Specifically, a thin film made of SnS is formed on the buffer layer 16 by physical vapor deposition such as electron beam vapor deposition, resistance heating vapor deposition, sputter vapor deposition, or cluster ion beam vapor deposition, or chemical vapor deposition such as CVD or CBD (Chemical Bath Deposition). Then, the light absorption layer 18 is formed by heating in the atmosphere under the condition of 180 to 210 ° C. The vapor deposition may be repeated a plurality of times.

  Subsequently, the hole transport layer 22 is formed on the light absorption layer 18. Specifically, a dispersion in which p-type semiconductor particles having a predetermined size (for example, a particle diameter of about 20 to 400 nm) are dispersed is prepared, and this dispersion is applied to the light absorption layer 18 by a bar coater method or a printing method. Alternatively, the hole transport layer 22 may be formed by applying, for example, and baking after drying. Alternatively, a thin hole transport layer 22 made of a p-type semiconductor is formed on the light absorption layer 18 by physical vapor deposition such as electron beam vapor deposition, resistance heating vapor deposition, sputter vapor deposition, cluster ion beam vapor deposition, or chemical vapor deposition such as CVD. It may be formed. Thereafter, the counter electrode 20 is formed on the hole transport layer 22. Specifically, a counter electrode made of a metal thin film such as Au or Pt on the hole transport layer 22 by physical vapor deposition such as electron beam vapor deposition, resistance heating vapor deposition, sputter vapor deposition, or cluster ion beam vapor deposition, or chemical vapor deposition such as CVD. 20 may be formed. Or the same board | substrate as the transparent conductive substrate 14 mentioned above may be prepared, and it may laminate | stack so that the transparent conductive film 14b may contact the hole transport layer 22. FIG. Finally, the side surfaces of the buffer layer 16, the light absorption layer 18, and the hole transport layer 22 are covered with the sealing material 24 to complete the solar cell 10.

  In the solar cell 10 of the present embodiment described in detail above, the conduction band offset value CBO, which is the difference between the lower end of the conduction band between the light absorption layer 18 and the buffer layer 16, with the conduction band lower end of the light absorption layer 18 as a reference is -0. Since it is in the range of not less than 0.2 eV and not more than 0.1 eV, the conversion efficiency can be further improved. In a solar cell, in general, when electrons photoexcited with SnS are transported to the transparent conductive substrate 14 via the buffer layer 16, if CBO has a large negative value, electrons and holes are easily recombined. It is thought that the characteristics deteriorate. On the other hand, when CBO is a large positive value, it is considered that electrons are hardly transported from SnS to the transparent conductive substrate 14 and the characteristics are deteriorated. In the present invention, the conversion efficiency can be further improved by employing the buffer layer 16 having the optimum CBO.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  For example, although one solar cell 10 has been described in the above-described embodiment, a battery module 100 in which a plurality of solar cells (hereinafter referred to as single cells) 110 are connected in series as shown in FIG. 3 may be used. The single cell 110 is a portion surrounded by a one-dot chain line in FIG. This single cell 110 is similar to the buffer layer 16, the light absorption layer 18, and the hole transport layer 22 of the solar cell 10 of the above-described embodiment between the transparent conductive film 114 b of the transparent conductive substrate 114 and the counter electrode 120. The buffer layer 116, the light absorption layer 118, and the hole transport layer 122 are included. The counter electrode 120 of one single cell 110 is bent in the thickness direction of the cell and is electrically connected to the transparent conductive film 114b of one adjacent single cell 110, but the transparent conductive property of the other adjacent single cell 110 is transparent. The membrane 114b and the counter electrode 120 of both adjacent unit cells 110 are electrically insulated by a sealing material 124. Of the transparent conductive substrate 114, the transparent substrate 114 a is a member common to all the single cells 110, but the transparent conductive film 114 b is formed for each single cell 110. This battery module 100 is effective when high output is required. Further, it can be arranged in a planar space. Depending on the space in which the batteries are arranged, a plurality of solar cells 10 may be stacked in the vertical direction and connected in series. Of course, parallel connection is also possible.

Hereinafter, an example in which the solar cell of the present invention (hereinafter also referred to as an element) is specifically manufactured will be described as examples. Here, an element in which the light absorption layer is SnS and the buffer layer is Zn _(1-X) Mg _X O is fabricated, and the characteristics thereof are examined.

[Example 1]
The multilayer element shown in FIG. 1 was produced according to the following procedure. ITO thin film (140 nm) as a transparent conductive substrate / Zn _(1-X) Mg _x O (ZMO, X = 0.17) thin film (100 nm) as a buffer layer on a glass substrate by RF magnetron sputtering Then, an SnS thin film (50 nm) as a light absorption layer and a Cu thin film as a hole transport layer were sequentially formed. However, after the ZMO thin film was formed, heat treatment was performed at 300 ° C. in an Ar atmosphere, and after the SnS thin film was formed, heat treatment was performed at 200 ° C. in the atmosphere, thereby performing surface modification. ZMO film formation was performed by binary simultaneous sputtering using a Zn _0.8 Mg _0.2 O target and a ZnO target. The amount of Mg was controlled by the power applied to the ZnO target. Here, sputtering was performed by adjusting so that X = 0.17. The obtained device was referred to as Example 1.

[Example 2]
An element obtained through the same process as in Example 1 except that a thin film (100 nm) of Zn _(1-X) Mg _X O (X = 0.15) was formed as the buffer layer was set as Example 2. .

[Example 3]
An element obtained through the same steps as in Example 1 except that the transparent conductive substrate was gallium-doped zinc oxide coated glass was designated as Example 3.

[Comparative Example 1]
A device obtained through the same steps as in Example 1 except that a thin film (100 nm) of ZnO (Zn _(1-X) Mg _X O (X = 0)) was formed as a buffer layer was compared with Comparative Example 1. did.

[Comparative Example 2]
A device obtained through the same process as in Example 1 except that a thin film (100 nm) of Zn _(1-X) Mg _X O (X = 0.25) was formed as the buffer layer was used as Comparative Example 2. .

[Comparative Example 3]
A device obtained through the same steps as in Example 1 except that a TiO ₂ thin film (100 nm) was formed as a buffer layer was used as Comparative Example 3.

(Evaluation results)
FIG. 4 is a relationship diagram between X of Zn _(1-X) Mg _X O and the optical band gap. Here, ZMO of X = 0, 0.025, 0.06, 0.07, 0.17, 0.25 was produced, and the optical band gap was measured by UV-vis. As a result, it was found that a linear relationship was obtained between the X value and the optical band gap. From this relationship and the feature that the top position of the valence band does not change even if Mg is doped in ZnO, the bottom of the conduction band of the light absorption layer in Zn _(1-X) Mg _X O (X = 0) The relationship between the conduction band offset (CBO) value, which is the difference between the lower end of the conduction band between the light absorption layer and the buffer layer, and the X value is a linear relationship. FIG. 5 is a relationship diagram between the X value of Zn _(1-X) Mg _X O and the CBO value for SnS. FIG. 6 is a graph showing the power generation characteristics of the solar cells of Example 3 and Comparative Example 3. Even in the element of Example 3 in which the buffer layer was GZO, the short-circuit current density Jsc (mA / cm ² ) did not change, and the result that the open circuit voltage Voc (V) and the fill factor FF were improved was obtained. While the conversion efficiency of Comparative Example 3 was 0.89%, the conversion efficiency of Example 3 was improved to 1.1%.

FIG. 7 is a graph showing the relationship between the X value of Zn _(1-X) Mg _X O and the solar cell characteristics in Examples 1 and 2 and Comparative Examples 1 and 2, and FIG. FIG. 7B is a relationship diagram of the Voc value with respect to the X value, FIG. 7C is a relationship diagram of the FF value with respect to the X value, and FIG. 7D is a relationship diagram of the conversion efficiency (%) with respect to the X value. It is. As shown in FIG. 7, Jsc was good when the X value satisfied 0.1 <X <0.2, and highest when 0.15 ≦ X ≦ 0.17. Voc was saturated when the X value was 0.15 or more, and FF showed a moderate increasing trend with respect to the X value. Since the conversion efficiency is defined by the product of Jsc, Voc, and FF, it was inferred that the conversion efficiency largely depends on Jsc. When the X value satisfies 0.1 <X <0.2, the conversion efficiency is good. When the X value satisfies 0.15 ≦ X ≦ 0.17, the conversion efficiency is the highest. In addition, it can be seen from the results of this experiment that the conversion efficiency can be further improved when the CBO value is in the range of −0.2 eV to 0.1 eV, and the highest in the range of −0.1 eV to 0 eV. It was.

  The solar cell of the present invention can be used, for example, as a power source for various electric appliances for home use, office use, and factory use, batteries for electric vehicles, hybrid vehicles, electric bicycles, and solar panels.

DESCRIPTION OF SYMBOLS 10 Solar cell, 12 Photoelectrode, 14 Transparent conductive substrate, 14a Transparent substrate, 14b Transparent conductive film, 16 Buffer layer, 18 Light absorption layer, 20 Counter electrode, 22 Hole transport layer, 24 Sealing material, 100 Battery module, 110 Single cell, 114 transparent conductive substrate, 114a transparent substrate, 114b transparent conductive film, 116 buffer layer, 118 light absorption layer, 120 counter electrode, 122 hole transport layer, 124 sealing material.

Claims (6)

A photoconductive electrode comprising: a conductive conductive transparent substrate; a buffer layer that is adjacent to the transparent conductive substrate and transports electrons; and a light absorption layer that includes SnS and is adjacent to the buffer layer and absorbs light;
With the counter electrode,
A hole transport layer formed between the photoelectrode and the counter electrode,
A solar cell having a conduction band offset value in a range of −0.2 eV or more and 0.1 eV or less, which is a difference between the conduction band lower ends of the light absorption layer and the buffer layer with reference to the lower end of the conduction band of the light absorption layer. . _2. The sun according to claim 1, wherein the buffer layer is made of any one of TiO ₂ , ZnO, SnO ₂ , and Zn _(1-X) Mg _X O (where 0 <X <1 is satisfied). battery. The solar cell according to claim 1, wherein the buffer layer is Zn _(1-X) Mg _X O (provided that 0.1 <X <0.2 is satisfied). The solar cell according to claim 1, wherein the buffer layer is Zn _(1-X) Mg _X O (provided that 0.15 ≦ X ≦ 0.17 is satisfied). The solar cell according to claim 1, wherein the hole transport layer is made of any one of CuI, CuSCN, CuO, Cu ₂ O, and Cu.   The solar cell according to any one of claims 1 to 5, wherein the conduction band offset value is in a range of not less than -0.1 eV and not more than 0 eV.