Solar cell

A solar cell includes a first substrate, a first hole transport layer, a first photoelectric conversion layer containing a perovskite compound, and a second photoelectric conversion layer containing a photoelectric conversion material in this order. A band gap of the perovskite compound is greater than a band gap of the photoelectric conversion material. With respect to an absorption wavelength of the first photoelectric conversion layer 3, a refractive index nA of the first hole transport layer 2 satisfies refractive index of the first substrate≤nA≤refractive index of the first photoelectric conversion layer. Further, with respect to a transmission wavelength of the first photoelectric conversion layer 3 and an absorption wavelength of the second photoelectric conversion layer 5, a refractive index nB of the first hole transport layer 2 satisfies refractive index of the first substrate≤nB≤refractive index of the first photoelectric conversion layer.

BACKGROUND

1. Technical Field

The present invention relates to a solar cell.

2. Description of the Related Art

In recent years, perovskite solar cells have been researched and developed as new solar cells.

With respect to perovskite solar cells, perovskite compounds denoted by a chemical formula ABX3(herein, A represents a monovalent cation, B represents a divalent cation, and X represents a halogen anion) are used as photoelectric conversion materials.

Julian Burschka et al., “Sequential deposition as a route to high-performance perovskite-sensitized solar cells”, Nature, vol. 499, pp. 316-319, 18 Jul. 2013 discloses a perovskite solar cell in which a perovskite compound denoted by a chemical formula CH3NH3PbI3(hereafter referred to as “MAPbI3”) is used as a photoelectric conversion material of a perovskite solar cell. In the perovskite solar cell disclosed in Julian Burschka et al., the perovskite compound denoted by MAPbI3, TiO2, and Spiro-OMeTAD are used as a photoelectric conversion material, an electron transport material, and a hole transport material, respectively.

Renxing Lin, et al., “Monolithic all-perovskite tandem solar cells with 24.8% efficiency exploiting comproportionation to suppress Sn(ii) oxidation in precursor ink”, Nature Energy, vol. 4, pp. 864-873, 2019 discloses a perovskite tandem solar cell. The perovskite tandem solar cell has a configuration in which a plurality of solar cells including respective perovskite compounds having band gaps that differ from each other are stacked on one another. The perovskite tandem solar cell can improve a photoelectric conversion efficiency.

SUMMARY

One non-limiting and exemplary embodiment provides a perovskite tandem solar cell having a high photoelectric conversion efficiency.

In one general aspect, the techniques disclosed here feature a solar cell including a first substrate, a first hole transport layer, a first photoelectric conversion layer, and a second photoelectric conversion layer in this order, wherein the first hole transport layer contains a p-type metal oxide semiconductor and a p-type organic semiconductor polymer, the first photoelectric conversion layer contains a perovskite compound, the second photoelectric conversion layer contains a photoelectric conversion material, a band gap of the perovskite compound is greater than a band gap of the photoelectric conversion material, with respect to an absorption wavelength of the first photoelectric conversion layer, a refractive index nAof the first hole transport layer satisfies Relational expression (1) below,
refractive index of the first substrate≤nA≤refractive index of the first photoelectric conversion layer  Relational expression (1):
and with respect to a transmission wavelength of the first photoelectric conversion layer and an absorption wavelength of the second photoelectric conversion layer, a refractive index nBof the first hole transport layer satisfies Relational expression (2) below,
refractive index of the first substrate≤nB≤refractive index of the first photoelectric conversion layer.  Relational expression (2):

It is an object of the present disclosure to provide a perovskite tandem solar cell having a high photoelectric conversion efficiency.

DETAILED DESCRIPTIONS

Definition of Term

A term “perovskite compound” used in the present specification means a perovskite crystal structure denoted by a chemical formula ABX3(herein, A represents a monovalent cation, B represents a divalent cation, and X represents a halogen anion) and a structure having a crystal similar to the perovskite crystal.

A term “perovskite solar cell” used in the present specification means a solar cell containing a perovskite compound as a photoelectric conversion material.

A term “tandem solar cell” used in the present specification means a stacked solar cell having a configuration in which a plurality of solar cells including respective photoelectric conversion materials having band gaps that differ from each other are stacked on one another.

A term “perovskite tandem solar cell” used in the present specification means a tandem solar cell in which a photoelectric conversion material used for at least one solar cell constituting the tandem solar cell is a perovskite compound.

Underlying knowledge forming basis of the present disclosure will be described below.

In general, the tandem solar cell has a structure in which a solar cell including a photoelectric conversion material having a wide band gap (hereafter referred to as “top cell”) and a solar cell including a photoelectric conversion material having a narrow band gap (bottom cell) are stacked. In the tandem solar cell, in general, the solar cell including a photoelectric conversion material having a wide band gap is disposed on the light incident side and is called a “top cell”. In general, the solar cell including a photoelectric conversion material having a narrow band gap is disposed on the opposite side of the light incident side and is called a “bottom cell”. The solar cells including respective photoelectric conversion materials having band gaps that differ from each other being stacked enables the light, of the incident light, with a wavelength unable to be absorbed in the top cell to be absorbed in the bottom cell so as to generate electricity. Consequently, the light in a wide band can be used compared with the solar cell including a single photoelectric conversion material, and the conversion efficiency of the solar cell can be improved.

When perovskite solar cells are stacked on one another and are used as a tandem solar cell, as disclosed in NPL 2, in a common structure, the bottom cell is stacked, with a recombination layer interposed therebetween, on the top cell in which the first electrode, the hole transport layer, the photoelectric conversion layer, and the electron transport layer are arranged in this order from the substrate side.

In such an instance, in general, a p-type metal oxide semiconductor or a p-type organic semiconductor polymer is used for the hole transport layer in the top cell. When the p-type metal oxide semiconductor is used, it is difficult to use the coating process akin to that for the perovskite material since, in general, the p-type metal oxide semiconductor is produced using a vacuum process. To realize the coating process, examples of the method include a method in which the p-type metal oxide semiconductor is made into nanoparticles, and a solution including the dispersed nanoparticles is applied. However, a film produced by using such a method has high resistance and is not suitable for a solar cell. In addition, the refractive index of the p-type metal oxide semiconductor in a long-wavelength region is greater than the refractive indices of the first electrode material and the perovskite material. This causes an increase in light reflection in the long-wavelength region at the interface between the hole transport layer and a layer located upstream in the moving direction of the incident light and at the interface between the hole transport layer and a layer located downstream. Consequently, the amount of the long-wavelength light passing through the top cell is decreased, and the efficiency of the bottom cell is decreased. In this regard, in the above-described common configuration described in, for example, NPL 2, a layer that adjoins the hole transport layer and that is located upstream of the hole transport layer in the moving direction of the incident light is the first electrode or the substrate when the substrate also serves as the first electrode. In this regard, a layer that adjoins the hole transport layer and that is located downstream of the hole transport layer in the moving direction of the incident light is the photoelectric conversion layer.

On the other hand, when the p-type organic semiconductor polymer is used, film formation can be readily performed using the coating process. However, the refractive index of the p-type organic semiconductor polymer is significantly lower than the refractive indices of the first electrode material and the perovskite material. This causes an increase in light reflection at the interface between the hole transport layer and a layer located upstream in the moving direction of the incident light and at the interface between the hole transport layer and a layer located downstream. Consequently, reflection is increased at the interfaces between the hole transport layer and the respective adjacent layers, and the efficiency of the solar cell is suppressed from increasing.

In consideration of these findings, the present inventors found that a low-resistance film can be realized by the coating process and that a high conversion efficiency can be realized in a tandem solar cell by realizing a hole transport layer material, in the top cell, having the refractive indices at a long wavelength and a short wavelength positioned between those of the perovskite material and the first electrode material.

The embodiments according to the present disclosure will be described below in detail with reference to the drawings.

EMBODIMENTS ACCORDING TO THE PRESENT DISCLOSURE

First Embodiment

FIG.1is a schematic sectional view illustrating a perovskite tandem solar cell according to a first embodiment. As illustrated inFIG.1, the perovskite tandem solar cell100according to the first embodiment includes a top cell101and a bottom cell102on a first substrate1.

The top cell101includes a first hole transport layer2, a first photoelectric conversion layer3, and a first electron transport layer4. The bottom cell102includes a second photoelectric conversion layer5. The solar cell100according to the first embodiment includes the first substrate1, the first hole transport layer2, first photoelectric conversion layer3, the first electron transport layer4, and the second photoelectric conversion layer5successively from the light-incident surface, that is, successively from the bottom inFIG.1. In other words, the solar cell100according to the first embodiment satisfies conditions (1) to (3) below. It is desirable that the condition (3) be satisfied, however, the condition (3) is not limited to being satisfied.(1) The first photoelectric conversion layer3is disposed between the first substrate1and the second photoelectric conversion layer5.(2) The first hole transport layer2is disposed between the first substrate1and the first photoelectric conversion layer3.(3) The first electron transport layer4is disposed between the first photoelectric conversion layer3and the second photoelectric conversion layer5.

The top cell101is not limited to including the first electron transport layer4. That is, the solar cell100according to the first embodiment may include the first substrate1, the first hole transport layer2, first photoelectric conversion layer, and the second photoelectric conversion layer in this order.

In this regard, between the above-described various types of layers constituting the solar cell100according to the first embodiment, other layers may be appropriately disposed for suppressing the various types of layers from recombining at the interface or for bonding the various types of layers to each other.

The first photoelectric conversion layer3contains a perovskite compound. The second photoelectric conversion layer5contains a photoelectric conversion material. In such an instance, the band gap of the perovskite compound is greater than the band gap of the photoelectric conversion material contained in the second photoelectric conversion layer5. According to the present configuration, the short-wavelength light is absorbed in the first photoelectric conversion layer3. Herein, the short-wavelength light is light with a wavelength of, for example, 300 nm to 700 nm. In this regard, the long-wavelength light is absorbed in the second photoelectric conversion layer5. Herein, the long-wavelength light is light with a wavelength of, for example, 700 nm to 1,200 nm. Having such a configuration enables the solar cell100to efficiently absorb the light.

With respect to an absorption wavelength of the first photoelectric conversion layer3, the refractive index nAof the first hole transport layer2satisfies Relational expression (1) below.
refractive index of the first substrate 1≤nA≤refractive index of the first photoelectric conversion layer 3  Relational expression (1):

Further, with respect to a transmission wavelength of the first photoelectric conversion layer3and an absorption wavelength of the second photoelectric conversion layer5, the refractive index nBof the first hole transport layer2satisfies Relational expression (2) below.
refractive index of the first substrate 1≤nB≤refractive index of the first photoelectric conversion layer 3  Relational expression (2):

The first hole transport layer2satisfying Relational expressions (1) and (2) above enables the solar cell100according to the first embodiment to effectively exploit the light and, as a result, enables a high photoelectric conversion efficiency to be realized.

The first hole transport layer2may contain both the p-type metal oxide semiconductor and the p-type organic semiconductor polymer. In the interior of the first hole transport layer2, it is desirable that the p-type metal oxide semiconductor and the p-type organic semiconductor polymer be uniformly dispersed. When the p-type metal oxide semiconductor and the p-type organic semiconductor polymer are uniformly dispersed, a refractive index difference is reduced in the interior of the first hole transport layer2. Consequently, since the first hole transport layer2can effectively pass the light, the photoelectric conversion efficiency can be improved.

Examples of the p-type metal oxide semiconductor include nickel oxide (NiO), copper(I) oxide (Cu2O), and Copper(II) oxide (CuO). The p-type metal oxide semiconductor may be at least one selected from the group consisting of NiO, Cu2O, and CuO.

Examples of the p-type organic semiconductor polymer include a phenyl amine containing a tertiary amine in the skeleton, a triphenylamine derivative, and a PEDOT compound having a thiophene structure. The p-type organic semiconductor polymer may be at least one selected from the group consisting of a phenyl amine containing a tertiary amine in the skeleton, a triphenylamine derivative, and a PEDOT compound having a thiophene structure.

Examples of the p-type organic semiconductor polymer include a poly[bis(4-phenyl)(2,4,6-triphenylmethyl)amine] (hereafter referred to as “PTAA”), (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene) (hereafter referred to as “Spiro-OMeTAD”), and PEDOT:PSS.

The first hole transport layer2may be realized by, for example, selecting at least one material from the above-described group of the p-type metal oxide semiconductors, selecting at least one material from the above-described group of the p-type organic semiconductor polymers, and combining the materials selected from the two groups.

It is desirable that the p-type metal oxide semiconductor is granular. The p-type metal oxide semiconductor having a granular shape enables the p-type metal oxide semiconductor to be uniformly dispersed in the p-type organic semiconductor polymer. In such an instance, the size of the p-type metal oxide semiconductor particle is smaller than the wavelength of the light absorbed by the solar cell100and is desirably less than or equal to one-quarter of the wavelength of the light. Due to the above-described configuration, the first hole transport layer2has low resistance suitable for the solar cell100and can realize a first hole transport layer capable of making the most of both the long-wavelength light and the short-wavelength light. Therefore, due to the above-described configuration, the solar cell100according to the first embodiment can effectively exploit the light and can realize a solar cell having a high photoelectric conversion efficiency.

The layers constituting the solar cell100according to the first embodiment and the configuration of the first hole transport layer2will be described later in detail.

Second Embodiment

FIG.2is a schematic sectional view illustrating a perovskite tandem solar cell according to a second embodiment. A solar cell200according to the second embodiment has a configuration in which a plurality of layers are added to the solar cell100according to the first embodiment. As illustrated inFIG.2, the solar cell200according to the second embodiment includes a top cell101, an intermediate layer10, and a bottom cell102on the first substrate1. The intermediate layer10functions as a recombination layer in which an electron and a hole are recombined. In the top cell101according to the second embodiment, a first electrode layer8, the first hole transport layer2, the first photoelectric conversion layer3, and the first electron transport layer4are disposed in this order. In this regard, in the bottom cell102according to the second embodiment, a second hole transport layer6, the second photoelectric conversion layer5, a second electron transport layer7, and a second electrode layer9are disposed in this order. The intermediate layer10is disposed between the first electron transport layer4and the second hole transport layer6. The second hole transport layer6is disposed between the intermediate layer10and the second photoelectric conversion layer5. The second electron transport layer7is disposed so that the second electron transport layer7, the second photoelectric conversion layer5, and the second hole transport layer6are arranged in this order.

The solar cell200according to the second embodiment can generate electricity by connecting the first electrode layer8to the second electrode layer9through an external circuit during light irradiation.

The first hole transport layer2, the first photoelectric conversion layer3, and the second photoelectric conversion layer5have the configurations akin to those of the first hole transport layer2, the first photoelectric conversion layer3, and the second photoelectric conversion layer5, respectively, described in the first embodiment. Therefore, the solar cell200according to the second embodiment can effectively exploit the light and, as a result, can realize a high photoelectric conversion efficiency in a manner similar to the solar cell100according to the first embodiment.

Third Embodiment

FIG.3Ais a schematic sectional view illustrating a perovskite tandem solar cell according to a third embodiment. A solar cell300A according to the third embodiment has a configuration in which a plurality of layers are added to the solar cell100according to the first embodiment. As illustrated inFIG.3A, in the solar cell300A according to the third embodiment, a top cell101is disposed on the first substrate1, and a bottom cell102is disposed on a second substrate11. The top cell101is bonded to the bottom cell102with an intermediate layer14interposed therebetween. That is, the intermediate layer14functions as a bonding layer for bonding the top cell101to the bottom cell102. In the top cell101according to the third embodiment, the first electrode layer8, the first hole transport layer2, the first photoelectric conversion layer3, the first electron transport layer4, and a third electrode layer12are disposed in this order. In this regard, in the bottom cell102, a fourth electrode layer13, the second hole transport layer6, the second photoelectric conversion layer5, the second electron transport layer7, and the second electrode layer9are disposed in this order. The intermediate layer14is disposed between the third electrode layer12and the second substrate11. The second substrate11is disposed between the intermediate layer14and the fourth electrode layer13.

The solar cell300A according to the third embodiment can generate electricity in the top cell101by connecting the first electrode layer8to the third electrode layer12through an external circuit during light irradiation. In addition, the bottom cell102can generate electricity by connecting the second electrode layer9to the fourth electrode layer13through an external circuit during light irradiation.

The first hole transport layer2, the first photoelectric conversion layer3, and the second photoelectric conversion layer5have the configurations akin to those of the first hole transport layer2, the first photoelectric conversion layer3, and the second photoelectric conversion layer5, respectively, described in the first embodiment. Therefore, the solar cell300A according to the third embodiment can effectively exploit the light and, as a result, can realize a high photoelectric conversion efficiency in a manner similar to the solar cell100according to the first embodiment.

FIG.3Bis a schematic sectional view illustrating a modified example of the perovskite tandem solar cell according to the third embodiment. As illustrated inFIG.3B, in the bottom cell102of the solar cell300B that is a modified example of the solar cell according to the third embodiment, the fourth electrode layer13, the second electron transport layer7, the second photoelectric conversion layer5, the second hole transport layer6, and the second electrode layer9maybe disposed in this order. The present configuration can also realize a high photoelectric conversion efficiency in a manner similar to the solar cell300A illustrated inFIG.3A.

Fourth Embodiment

FIG.4Ais a schematic sectional view illustrating a perovskite tandem solar cell according to a fourth embodiment. A solar cell400A according to the fourth embodiment has a configuration in which a plurality of layers are added to the solar cell100according to the first embodiment. As illustrated inFIG.4A, in the solar cell400A according to the fourth embodiment, a top cell101is disposed on the first substrate1, and a bottom cell102is disposed on a second substrate11. The top cell101is bonded to the bottom cell102with the intermediate layer14interposed therebetween. That is, the intermediate layer14functions as a bonding layer for bonding the top cell101to the bottom cell102. In the top cell101according to the fourth embodiment, the first electrode layer8, the first hole transport layer2, the first photoelectric conversion layer3, the first electron transport layer4, and the third electrode layer12are disposed in this order. In this regard, in the bottom cell102, the second electrode layer9, the second electron transport layer7, the second photoelectric conversion layer5, the second hole transport layer6, and the fourth electrode layer13are disposed in this order. The intermediate layer14is disposed between the third electrode layer12and the fourth electrode layer13.

The solar cell400A according to the fourth embodiment can generate electricity in the top cell101by connecting the first electrode layer8to the third electrode layer12through an external circuit during light irradiation. In addition, the bottom cell102can generate electricity by connecting the second electrode layer9to the fourth electrode layer13through an external circuit during light irradiation.

The first hole transport layer2, the first photoelectric conversion layer3, and the second photoelectric conversion layer5have the configurations akin to those of the first hole transport layer2, the first photoelectric conversion layer3, and the second photoelectric conversion layer5, respectively, described in the first embodiment. Therefore, the solar cell400A according to the fourth embodiment can effectively exploit the light and, as a result, can realize a high photoelectric conversion efficiency in a manner similar to the solar cell100according to the first embodiment.

FIG.4Bis a schematic sectional view illustrating a modified example of the perovskite tandem solar cell according to the fourth embodiment. As illustrated inFIG.4B, in the bottom cell102of the solar cell400B that is a modified example of the solar cell according to the fourth embodiment, the second electrode layer9, the second hole transport layer6, the second photoelectric conversion layer5, the second electron transport layer7, and the fourth electrode layer13may be disposed in this order. The present configuration can also realize a high photoelectric conversion efficiency in a manner similar to the solar cell400A illustrated inFIG.4A.

Fifth Embodiment

FIG.5is a schematic sectional view illustrating a perovskite tandem solar cell according to a fifth embodiment. A solar cell500according to the fifth embodiment has a configuration in which a plurality of layers are added to the solar cell100according to the first embodiment. As illustrated inFIG.5, in the solar cell500according to the fifth embodiment, a top cell101and a bottom cell102are disposed on the first substrate1. The top cell101is bonded to the bottom cell102with the intermediate layer14interposed therebetween. That is, the intermediate layer14functions as a bonding layer for bonding the top cell101to the bottom cell102. In the top cell101according to the fifth embodiment, the first electrode layer8, the first hole transport layer2, the first photoelectric conversion layer3, the first electron transport layer4, and the third electrode layer12are disposed in this order. In this regard, in the bottom cell102, the fourth electrode layer13, the second photoelectric conversion layer5, and the second electrode layer9are disposed in this order. The intermediate layer14is disposed between the third electrode layer12and the fourth electrode layer13.

In the solar cell500according to the fifth embodiment, an uneven shape is disposed on a light-incident-side surface of the second photoelectric conversion layer5, that is, on the lower-side surface inFIG.5. The fourth electrode layer13is disposed so as to cover the uneven shape.

The solar cell according to the fifth embodiment can generate electricity in the top cell101by connecting the first electrode layer8to the third electrode layer12through an external circuit during light irradiation. In addition, the bottom cell102can generate electricity by connecting the second electrode layer9to the fourth electrode layer13through an external circuit during light irradiation.

The first hole transport layer2, the first photoelectric conversion layer3, and the second photoelectric conversion layer5have the configurations akin to those of the first hole transport layer2, the first photoelectric conversion layer3, and the second photoelectric conversion layer5, respectively, described in the first embodiment. Therefore, the solar cell500according to the fifth embodiment can effectively exploit the light and, as a result, can realize a high photoelectric conversion efficiency in a manner similar to the solar cell100according to the first embodiment.

The layers constituting the solar cells according to the first embodiment to the fifth embodiment will be described below.

First Substrate1

The first substrate1supports the layers of the top cell101that is a perovskite solar cell. The first substrate1can be formed of a transparent material. For example, a glass substrate or a plastic substrate (including a plastic film) can be used. In this regard, when the first electrode layer8has sufficient strength, since the layers can be supported by the first electrode layer8, in such an instance, the first substrate1may also serve as an electrode layer. In other words, the first electrode layer capable of functioning as the first substrate1may be used.

First Hole Transport Layer2

The hole transport layer is a layer for transporting holes. In the first embodiment to the fifth embodiment, the first hole transport layer2contains both the p-type metal oxide semiconductor and the p-type organic semiconductor polymer. In the first embodiment to the fifth embodiment, it is desirable that the p-type metal oxide semiconductor and the p-type organic semiconductor polymer be uniformly distributed in the interior of the first hole transport layer2as described in the first embodiment. Regarding the p-type metal oxide semiconductor, CuO, Cu2O, NiO, or the like can be used. Regarding the p-type organic semiconductor polymer, PTAA, Spiro-OMeTAD, PEDOT:PSS, or the like can be used. The first hole transport layer2may be realized by selecting at least one material from the above-described group of the p-type metal oxide semiconductors, selecting at least one material from the above-described group of the p-type organic semiconductor polymers, and combining the resulting materials.

In this regard, it is desirable that the p-type metal oxide semiconductor be granular as described in the first embodiment. The p-type metal oxide semiconductor being granular enables the p-type metal oxide semiconductor to be uniformly dispersed in the p-type organic semiconductor polymer. In such an instance, the size of the particle is smaller than the wavelength of the light absorbed by the solar cell and is desirably less than or equal to one-quarter of the wavelength.

In the first hole transport layer2, the p-type metal oxide semiconductor may be NiO, and the p-type organic semiconductor polymer may be PEDOT:PSS. NiO has a high refractive index with respect to, in particular, the long-wavelength light. PEDOT:PSS has a low refractive index with respect to the short-wavelength light and the long-wavelength light. Therefore, a predetermined refractive index in a wide range can be realized by combining NiO and PEDOT:PSS and appropriately adjusting the mixing ratio (for example, volume ratio) of NiO to PEDOT:PSS.

For example, when the first hole transport layer2is composed of a mixture of NiO and PEDOT:PSS, a volume ratio of the p-type organic semiconductor polymer to a total of the p-type metal oxide semiconductor and the p-type organic semiconductor polymer in the first hole transport layer2may be greater than or equal to 0.12 and less than or equal to 0.46. That is, the volume ratio is expressed as (volume of p-type organic semiconductor polymer)/{(volume of p-type metal oxide semiconductor)+(volume of p-type organic semiconductor polymer)}.

When the above-described volume ratio is greater than or equal to 0.12 and less than or equal to 0.46, the first hole transport layer2can have refractive indices with respect to the long-wavelength light and the short-wavelength light that are refractive indices positioned between the refractive index of the perovskite material and the refractive index of the material used for the first substrate1and that are refractive indices positioned between the refractive index of the perovskite compound and the refractive index of the electrode material which may be disposed between the first substrate1and the first hole transport layer2. Therefore, according to this configuration, the light is made to efficiently reach the first photoelectric conversion layer3and the second photoelectric conversion layer5, and the photoelectric conversion efficiency of the solar cell100can be improved.

Relational expression (1) above may be satisfied with respect to the light with a wavelength of 500 nm.

Further, Relational expression (2) above may be satisfied with respect to the light with a wavelength of 1,000 nm.

With respect to the light with the above-described wavelength, the first hole transport layer2satisfying Relational expression (1) and Relational expression (2) above enables the tandem solar cells according to the first embodiment to the fifth embodiment to effectively exploit the light and, as a result, enables a high photoelectric conversion efficiency to be realized.

In the instance of the configuration in which the first electrode layer8is disposed between the first substrate1and the first hole transport layer2as in the solar cells according to the second embodiment to the fifth embodiment, with respect to the light with a wavelength of 500 nm, the refractive index nAof the first hole transport layer2may satisfy Relational expression (3) below.
refractive index of the first electrode layer≤nA≤refractive index of the first photoelectric conversion layer  Relational expression (3):

Further, in the instance of the above-described configuration, with respect to the light with a wavelength of 1,000 nm, the refractive index nBof the first hole transport layer2may satisfy Relational expression (4) below.
refractive index of the first electrode layer≤nB≤refractive index of the first photoelectric conversion layer  Relational expression (4):

The first hole transport layer2satisfying Relational expression (3) and Relational expression (4) above enables the tandem solar cells according to the second embodiment to the fifth embodiment to more effectively exploit the light and, as a result, enables a higher photoelectric conversion efficiency to be realized.

The refractive index nAof the first hole transport layer2with respect to the light with a wavelength of 500 nm may be greater than or equal to 2.00 and less than or equal to 2.61. In addition, the refractive index nBof the first hole transport layer with respect to the light with a wavelength of 1,000 nm may be greater than or equal to 1.75 and less than or equal to 2.17. When the first hole transport layer2have the refractive indices in the above-described ranges with respect to the wavelength of 500 nm and the wavelength of 1,000 nm, the first hole transport layer2readily satisfies Relational expressions (1) to (4) above with respect to the materials frequently used for the first substrate1, the first electrode layer8, and the first photoelectric conversion layer3, in general. Therefore, the refractive indices nAand nBsatisfying the above-described ranges enable the tandem solar cells according to the first embodiment to the fifth embodiment to more effectively exploit the light and, as a result, enables a higher photoelectric conversion efficiency to be realized.

The thickness of the first hole transport layer2is desirably greater than or equal to 1 nm and less than or equal to 1,000 nm, more desirably greater than or equal to 10 nm and less than or equal to 500 nm, and further desirably greater than or equal to 10 nm and less than or equal to 50 nm. The thickness of the first hole transport layer2being greater than or equal to 1 nm and less than or equal to 1,000 nm enables sufficient hole transportability to be realized. Further, when the thickness of the first hole transport layer2is greater than or equal to 1 nm and less than or equal to 1,000 nm, the light is converted to electricity with a high efficiency since the resistance of the first hole transport layer2is low.

The first hole transport layer2may contain a support electrolyte and a solvent. The support electrolyte and the solvent have an effect of, for example, stabilizing holes in the first hole transport layer2.

Examples of the support electrolyte include ammonium salts and alkali metal salts. Examples of the ammonium salt include tetrabutylammonium perchlorate, tetraethylammonium hexafluorophosphate, imidazolium salts, and pyridinium salts. Examples of the alkali metal salt include lithium perchlorate and potassium tetrafluoroborate.

The solvent contained in the first hole transport layer2may have high ionic conductivity. The solvent may be an aqueous solvent or an organic solvent. It is desirable that the solvent be an organic solvent from the viewpoint of stabilization of a solute. Examples of the organic solvent include heterocyclic compounds, such as tert-butyl pyridine, pyridine, and N-methylpyrrolidone.

The solvent contained in the first hole transport layer2may be an ionic liquid. The ionic liquid may be used alone or in combination with other solvents. The ionic liquid is desirable from the viewpoint of low volatility and high flame retardancy.

Examples of the ionic liquid include imidazolium compounds such as 1-ethyl-3-methylimidazolium tetracyanoborate, pyridine compounds, alicyclic amine compounds, aliphatic amine compounds, and azoniumamine compounds.

First Photoelectric Conversion Layer3

The first photoelectric conversion layer3contains a perovskite compound. That is, the first photoelectric conversion layer3contains the first perovskite compound composed of a monovalent cation, a divalent cation, and a halogen anion as a photoelectric conversion material. The photoelectric conversion material is a light-absorbing material.

In the present embodiment, the perovskite compound may be a compound denoted by a chemical formula ABX3(herein, A represents a monovalent cation, B represents a divalent cation, and X represents a halogen anion).

In the present specification, A, B, and X are also referred to as an A site, a B site, and an X site, respectively, in accordance with an expression idiomatically used for the perovskite compound.

In the present embodiment, the perovskite compound may have a perovskite type crystal structure denoted by the chemical formula ABX3. As an example, a monovalent cation is located at the A site, a divalent cation is located at the B site, and a halogen anion is located at the X site.

A Site

There is no particular limitation regarding the monovalent cation located at the A site. An example of the monovalent cation is an organic cation or an alkali metal cation. An example of the organic cation is a methylammonium cation (that is, CH3NH3+), a formamidinium cation (that is, NH2CHNH2+), a phenylethylammonium cation (that is, C6H5C2H4NH3+), or a guanidium cation, (that is, CH6N3+). An example of the alkali metal cation is a cesium cation (that is, CO.

For the sake of a high photoelectric conversion efficiency, the A site may include at least one selected from the group consisting of Cs+, a formamidinium cation, and a methylammonium cation.

The cation constituting the A site may be composed of a mixture of a plurality of organic cations described above. The cation constituting the A site may be a mixture of at least one of the above-described organic cations and at least one of the above-described metal cations.

B Site

There is no particular limitation regarding the divalent cation located at the B site. An example of the divalent cation A is a divalent cation of an element of group XIII to group XV. For example, the B site includes a Pb cation, that is, Pb2+.

X Site

There is no particular limitation regarding the halogen anion located at the X site.

The element, that is, the ion, located at each of the A, B, and X sites may be a plurality of types or a single type.

The first photoelectric conversion layer3may contain materials other than photoelectric conversion materials. For example, the first photoelectric conversion layer3may further contain a quencher substance for reducing the defect density of the perovskite compound. The quencher substance is a fluorine compound such as tin fluoride. The molar ratio of the quencher substance to the photoelectric conversion material may be greater than or equal to 5% and less than or equal to 20%.

The first photoelectric conversion layer3may mainly contain a perovskite compound composed of a monovalent cation, a divalent cation, and a halogen anion.

The sentence “the first photoelectric conversion layer3mainly contains a perovskite compound composed of a monovalent cation, a divalent cation, and a halogen anion” means that the first photoelectric conversion layer3contains greater than or equal to 70% (desirably greater than or equal to 80%) of the perovskite compound composed of a monovalent cation, a divalent cation, and a halogen anion.

The first photoelectric conversion layer3may contain impurities. The first photoelectric conversion layer3may further contain compounds other than the above-described perovskite compound.

The first photoelectric conversion layer3may have a thickness of, for example, greater than or equal to about 100 nm and less than or equal to about 2,000 nm. The perovskite compound contained in the first photoelectric conversion layer3may be formed using a solution coating method, a co-vapor deposition method, or the like.

In this regard, the first photoelectric conversion layer3may take on a form in which a portion is mixed with the above-described first hole transport layer2and the first electron transport layer4described later or a form in which there are large-area interfaces to the first electron transport layer4and to the first hole transport layer2in the film.

The top cell101has to pass the long-wavelength light to the bottom cell102. In other words, the top cell101is required to absorb the short-wavelength light. Therefore, the band gap of the perovskite compound contained in the first photoelectric conversion layer3is greater than the band gap of the photoelectric conversion material contained in the second photoelectric conversion layer5. For example, the perovskite compound contained in the first photoelectric conversion layer3is MAPbI3, FAPbI3, MAPbBr3, or FAPbBr3.

First Electron Transport Layer4

The first electron transport layer4is in contact with, for example, the first photoelectric conversion layer3and the third electrode layer12. The first electron transport layer4transports electrons. The first electron transport layer4contains a semiconductor. It is desirable that the first electron transport layer4is formed of a semiconductor having a band gap of greater than or equal to 3.0 eV. Examples of the semiconductor include organic or inorganic n-type semiconductors.

Examples of the organic n-type semiconductor include imide compounds, quinone compounds, fullerene, and derivatives of the fullerene. Examples of the inorganic n-type semiconductor include metal oxides, metal nitrides, and perovskite oxides. Examples of the metal oxide include oxides of Cd, Zn, In, Pb, Mo, W, Sb, Bi, Cu, Hg, Ti, Ag, Mn, Fe, V, Sn, Zr, Sr, Ga, Si, or Cr. TiO2is desirable. Examples of the metal nitride include GaN. Examples of the perovskite oxide include SrTiO3, CaTiO3, and ZnTiO3.

The first electron transport layer4may be composed of a material having a band gap of greater than 6.0 eV. Examples of the material having a band gap of greater than 6.0 eV include(i) halides of alkali metals or alkaline-earth metals, such as lithium fluoride and barium fluoride, and(ii) oxides of alkaline-earth metals, such as magnesium oxide.
In such an instance, to ensure the electron transportability of the first electron transport layer4, the thickness of the first electron transport layer4is, for example, less than or equal to 10 nm.

The first electron transport layer4may contain a plurality of layers composed of materials that differ from each other.

In this regard, when an electrode in contact with the first electron transport layer4has a property of blocking holes from the first photoelectric conversion layer3, the electron transport material is not limited to being present. Herein, the property of blocking holes means that the first photoelectric conversion layer3is not in ohmic contact with the electrode. Examples of the electrode material having such a function include aluminum.

Second Photoelectric Conversion Layer5

A photoelectric conversion material used for the second photoelectric conversion layer5has a smaller band gap than the perovskite compound that is a photoelectric conversion material used for the first photoelectric conversion layer3. Examples of the materials used for the second photoelectric conversion layer5include silicon, perovskite compounds, chalcopyrite-type compounds such as CIGS, and group III-V compounds such as GaAs.

The second photoelectric conversion layer5may take on a form in which a portion is mixed with the second electron transport layer7and the second hole transport layer6described later or a form in which there are large-area interfaces in the film.

Second Hole Transport Layer6

The second hole transport layer6contains a hole transport material. The hole transport material is a material for transporting holes. Examples of the hole transport material include organic materials and inorganic semiconductors.

Typical examples of the organic material and the inorganic semiconductor used as the hole transport material contained in the second hole transport layer6are akin to those used as the hole transport material contained in the first hole transport layer2.

The second hole transport layer6may contain a plurality of layers formed of materials that differ from each other.

The thickness of the second hole transport layer6is desirably greater than or equal to 1 nm and less than or equal to 1,000 nm, more desirably greater than or equal to 10 nm and less than or equal to 500 nm, and further desirably greater than or equal to 10 nm and less than or equal to 50 nm. The thickness of the second hole transport layer6being greater than or equal to 1 nm and less than or equal to 1,000 nm enables sufficient hole transportability to be realized. Further, the thickness of the second hole transport layer6being greater than or equal to 1 nm and less than or equal to 1,000 nm enables the light to be converted to electricity with high efficiency since the second hole transport layer6has low resistance.

Regarding a method for forming the film, known various coating methods or printing methods can be adopted. Examples of the coating method include a doctor blade method, a bar coating method, a spraying method, a dip coating method, and a spin coating method. Examples of the printing method include a screen printing method.

The second hole transport layer6may contain a support electrolyte and a solvent. The support electrolyte and the solvent have an effect of, for example, stabilizing holes in the second hole transport layer6.

Examples of the support electrolyte and the solvent which may be contained in the second hole transport layer6are akin to examples of the support electrolyte and the solvent which may be contained in the first hole transport layer2.

Second Electron Transport Layer7

The second electron transport layer7is a layer having the function akin to the function of the first electron transport layer4, and similar material and configuration can be used.

First Electrode Layer8

The first electrode layer8has electrical conductivity. In addition, the first electrode layer8has a light-transmitting property. For example, the first electrode layer8can pass the light in the visible region to the near-infrared region. The first electrode layer8may be formed using, for example, a metal oxide that is transparent and that has electrical conductivity. An Example of such a metal oxide is(i) an indium-tin complex oxide,(ii) a tin oxide doped with antimony,(iii) a tin oxide doped with fluorine,(iv) a zinc oxide doped with at least one selected from the group consisting of boron, aluminum, gallium, and indium, or(v) a complex of these.

The first electrode layer8can be formed by using an opaque material and by being formed into a pattern through which the light passes. An example of the pattern through which the light passes is a linear pattern, a wavy-line-like pattern, a grid-like pattern, or a punching-metal-like pattern in which a plurality of fine through holes are regularly or irregularly arranged. The first electrode layer8having these patterns enables the light to pass through portions with no electrode layer material. An example of the opaque material is platinum, gold, silver, copper, aluminum, rhodium, indium, titanium, iron, nickel, tin, zinc, or an alloy containing any one of these. A carbon material having electrical conductivity may be used as the opaque material.

The light transmittance of the first electrode layer8may be, for example, greater than or equal to 50%, or may be greater than or equal to 80%. The wavelength of the light which has to pass through the first electrode layer8is in accordance with the absorption wavelength of the first photoelectric conversion layer3and the second photoelectric conversion layer5. The thickness of the first electrode layer8is, for example, greater than or equal to 1 nm and less than or equal to 1,000 nm.

Second Electrode Layer9, Third Electrode Layer12, and Fourth Electrode Layer13

The second electrode layer9, the third electrode layer12, and the fourth electrode layer13are layers having the function akin to the function of the first electrode layer8, and similar material and configuration can be used.

In this regard, when the second electrode layer9is disposed at a position furthest from the light incident side (lower side inFIG.2) as in the solar cell200according to the second embodiment, the second electrode layer9is not limited to having a light-transmitting property, and an opaque material can be used without further processing.

The intermediate layer10functioning as a recombination layer is disposed between the top cell101and the bottom cell102and is a layer for realizing electrical connection. The intermediate layer10has electrical conductivity. In addition, the intermediate layer10has a light-transmitting property. The intermediate layer10passes, for example, the light in the visible region to the near-infrared region.

The intermediate layer10may be formed using, for example, a metal oxide that is transparent and that has electrical conductivity. An Example of such a metal oxide is(i) an indium-tin complex oxide,(ii) a tin oxide doped with antimony,(iii) a tin oxide doped with fluorine,(iv) a zinc oxide doped with at least one selected from the group consisting of boron, aluminum, gallium, and indium, or(v) a complex of these.

The intermediate layer10can be formed by using an opaque material and by being formed into a pattern through which the light passes. An example of the pattern through which the light passes is a linear pattern, a wavy-line-like pattern, a grid-like pattern, or a punching-metal-like pattern in which a plurality of fine through holes are regularly or irregularly arranged. The intermediate layer10having these patterns enables the light to pass through portions with no opaque material. An example of the opaque material is platinum, gold, silver, copper, aluminum, rhodium, indium, titanium, iron, nickel, tin, zinc, or an alloy containing any one of these. A carbon material having electrical conductivity may be used as the opaque material.

Intermediate Layer14Functioning as Bonding Layer

The intermediate layer14functioning as a bonding layer is disposed between the top cell101and the bottom cell102and is a layer for mechanically supporting the top cell101and the bottom cell102. The intermediate layer14has a light-transmitting property. The intermediate layer14passes, for example, the light in the visible region to the near-infrared region.

The intermediate layer14may be formed using, for example, a transparent resin material. Examples of the material for forming the intermediate layer14include polymethyl methacrylate resins, silicon resins, and epoxy resins.

Alternatively, the intermediate layer14can be formed by using an opaque material and by being formed into a pattern through which the light passes. An example of the pattern through which the light passes is a linear pattern, a wavy-line-like pattern, a grid-like pattern, or a punching-metal-like pattern in which a plurality of fine through holes are regularly or irregularly arranged. Examples of the opaque material include metal materials and opaque resin materials.

Example of Method for Producing Solar Cell

Regarding the top cell101in the solar cell according to the present embodiment, each of the first electrode layer8, the first hole transport layer2, the first photoelectric conversion layer3, the first electron transport layer4, and the third electrode layer12may be formed on the first substrate1by using, for example, a coating method represented by spin coating, spray coating, die coating, ink jet, gravure coating, flexographic coating, or the like, a physical vapor deposition method (PVD) represented by vapor deposition, sputtering, or the like, or a chemical vapor deposition method (CVD) by using heat, light, plasma, or the like. Likewise, regarding the bottom cell102, the fourth electrode layer13, the second hole transport layer6, the second photoelectric conversion layer5, the second electron transport layer7, and the second electrode layer9may be formed by using, for example, a coating method represented by spin coating, spray coating, die coating, ink jet, gravure coating, flexographic coating, or the like, PVD represented by vapor deposition, sputtering, or the like, or CVD by using heat, light, plasma, or the like. Thereafter, the solar cell can be formed by integrating the top cell101and the bottom cell102by using the intermediate layer10or the intermediate layer14.

Optimum Mixing Ratio of p-Type Metal Oxide Semiconductor to p-Type Organic Semiconductor Polymer

The first hole transport layer2in the solar cell according to the present embodiment has a configuration in which a p-type metal oxide semiconductor and a p-type organic semiconductor polymer are mixed. In such an instance, Formula (3) below applies where the refractive index of the p-type metal oxide semiconductor is denoted by N1, the refractive index of the p-type organic semiconductor polymer is denoted by N2, the volume fraction of the p-type organic semiconductor polymer in the first hole transport layer2is denoted by f, and the refractive index of the first hole transport layer2is denoted by N.
[Math. 1]
f[(N12−N2)/(N12+2N2)]+(1−f)[(N22−N2/(N22+2N2)]=0  (3)

The refractive index N of the first hole transport layer2at specific mixing ratio can be determined by solving Formula (3).

It is desirable that the refractive index of the first hole transport layer2be greater than the refractive index of the first electrode layer8and less than the refractive index of the first photoelectric conversion layer3in a long-wavelength (for example, 1,000 nm) region. In such an instance, specifying the materials for forming the first electrode layer8, the first photoelectric conversion layer3, the p-type organic semiconductor polymer, and the p-type metal oxide semiconductor enables an optimum range of the mixing ratio of the p-type metal oxide semiconductor to the p-type organic semiconductor polymer to be determined in accordance with the material configuration.

As an example, an optimum ratio range was calculated when indium oxide doped with tin was used for the first electrode layer8, a perovskite compound in which Cs, CH3NH3, and HC(NH2)2were used at the A site, Pb was used at the B site, and I and Br were used at the X site was used for the first photoelectric conversion layer3, PEDOT:PSS was used for the p-type organic semiconductor polymer, and NiO was used for the p-type metal oxide semiconductor.FIG.6is a graph illustrating dependence of the refractive index of the first hole transport layer2with respect to the light with a wavelength of 1,000 nm on a volume fraction of PEDOT:PSS. Specifically, the horizontal axis represents the volume fraction of PEDOT:PSS (that is, the volume fraction of PEDOT:PSS relative to a total of PEDOT:PSS and NiO), and the vertical axis represents the refractive index of the first hole transport layer2. It is desirable that the refractive index of the first hole transport layer2be within the range between the refractive index of the first electrode layer8and the refractive index of the first photoelectric conversion layer3. Therefore, it is desirable that the volume fraction of PEDOT:PSS be set to be within the range of greater than or equal to 0.12 and less than or equal to 0.46.

EXAMPLES

A method for producing a solar cell of Example 1 will be described below.

To begin with, a method for producing a top cell in the solar cell of Example 1 will be described.

Initially, a glass substrate having a thickness of 0.7 mm was prepared as a first substrate.

Subsequently, a tin-doped indium oxide layer having a thickness of 150 nm was formed as a first electrode layer on the first substrate by using a sputtering method.

Thereafter, a first hole transport layer was formed on the first electrode layer by applying a raw material solution for the first hole transport layer by using a spin coating method. The raw material solution for the first hole transport layer was formed by mixing a PEDOT:PSS dispersion liquid (produced by Heraeus) and a NiO nanoparticle dispersion liquid (produced by NanoGrade) at 1:1 (volume fraction), dispersing NiO nanoparticles by applying ultrasonic waves, and, thereafter, filtering the solution with a filter (openings: 0.2 μm).

Next, the first hole transport layer was coated with a raw material solution for a first photoelectric conversion layer by using a spin coating method so as to form the first photoelectric conversion layer. A raw material solution for the first photoelectric conversion layer was produced by dissolving 0.92 mol/L of PbI2(produced by TOKYO KASEI KOGYO CO., LTD.), 0.17 mol/L of PbBr2(produced by TOKYO KASEI KOGYO CO., LTD.), 0.83 mol/L of formamidinium iodide (produced by GreatCell Solar) (hereafter referred to as “FAI”), 0.17 mol/L of methylammonium bromide (produced by GreatCell Solar) (hereafter referred to as “MABr”), and 0.05 mol/L of CsI (produced by Iwatani Corporation) in a solvent mixture of dimethylsulfoxide (produced by acros) and N-dimethylformamide (produced by acros). Regarding the solvent mixture used for the raw material solution for the first photoelectric conversion layer, the mixing ratio of dimethylsulfoxide to N-dimethylformamide was 1:4 (volume ratio).

Thereafter, a first electron transport layer was formed on the first photoelectric conversion layer by successively forming films of fullerene having a thickness of 25 nm and bathocuproin (BCP) having a thickness of 5 nm by using a vapor deposition method.

Subsequently, a third electrode layer was formed on the first electron transport layer by forming a tin-doped indium oxide layer having a thickness of 200 nm by using a sputtering method.

The top cell in the solar cell of Example 1 was obtained through the above-described steps.

Next, a method for producing a bottom cell in the solar cell of Example 1 will be described.

Initially, a glass substrate having a thickness of 0.7 mm was prepared as a second substrate.

Thereafter, a tin-doped indium oxide layer having a thickness of 150 nm was formed as a fourth electrode on the second substrate by using a sputtering method.

Subsequently, a second hole transport layer was formed on the fourth electrode by applying a raw material solution for the second hole transport layer by using a spin coating method. Regarding the solution, a PEDOT:PSS dispersion liquid (produced by Heraeus) was used.

Next, the second hole transport layer was coated with a raw material solution for a second photoelectric conversion layer by using a spin coating method so as to form the second photoelectric conversion layer. A raw material solution for the second photoelectric conversion layer was produced by dissolving 0.84 mol/L of SnI2(produced by TOKYO KASEI KOGYO CO., LTD.), 0.84 mol/L of FAI (produced by GreatCell Solar), 0.69 mol/L of PbI2(produced by TOKYO KASEI KOGYO CO., LTD.), and 0.69 mol/L of methylammonium iodide (produced by GreatCell Solar) (hereafter referred to as “MAI”) in a solvent mixture of dimethylsulfoxide (produced by acros) and N-dimethylformamide (produced by acros). Regarding the solvent mixture used for the raw material solution for the second photoelectric conversion layer, the mixing ratio of dimethylsulfoxide to N-dimethylformamide was 1:4 (volume ratio).

Thereafter, a second electron transport layer was formed on the second photoelectric conversion layer by successively forming films of fullerene having a thickness of 25 nm and bathocuproin (BCP) having a thickness of 5 nm by using a vapor deposition method.

Subsequently, a second electrode layer was formed on the second electron transport layer by forming a silver film having a thickness of 100 nm by using a vapor deposition method.

The bottom cell in the solar cell of Example 1 was obtained through the above-described steps.

The solar cell of Example 1 was obtained by using an epoxy resin as an intermediate layer and bonding the third electrode layer of the top cell to the second substrate of the bottom cell. That is, the solar cell of Example 1 had a configuration akin to the configuration of the solar cell300A according to the third embodiment illustrated inFIG.3A.

Comparative Example 1

Regarding a solar cell of Comparative example 1, the PEDOT:PSS dispersion liquid (produced by Heraeus) was used as a raw material solution for the first hole transport layer. The other steps were akin to the steps in Example 1.

Comparative Example 2

Regarding a solar cell of Comparative example 2, the NiO nanoparticle dispersion liquid (produced by NanoGrade) was used as a raw material solution for the first hole transport layer. The other steps were akin to the steps in Example 1.

Performance comparisons between solar cells of Example 1, Comparative example 1, and

Comparative Example 2

The photoelectric conversion efficiencies of the top cells in the solar cells of Example 1, Comparative example 1, and Comparative example 2 were measured. A solar simulator (produced by Bunkoukeiki Co., Ltd.) and an electrochemical analyzer ALS (produced by BAS Inc.) were used for measuring the photoelectric conversion efficiency. The quasi-solar light (illumination intensity of 100 mW/cm2) which was reproduced so as to have a spectrum closely analogous to the solar light was realized by the solar simulator. The quasi-solar light was applied to the top cell. The top cell irradiated with the quasi-solar light was subjected to the measurement of the output current value while the applied voltage was changed by using the electrochemical analyzer so that the current-voltage characteristics (hereafter referred to as I-V characteristics) of the solar cell were measured. The photoelectric conversion efficiency was calculated from an open-circuit voltage, a short-circuit current, and a fill factor of the resulting I-V characteristics.

FIG.7is a graph illustrating I-V characteristics of the top cell in the solar cell of Example 1, Comparative example 1, or Comparative example 2 during quasi-solar light irradiation. Specifically, the horizontal axis inFIG.7represents the voltage, and the vertical axis represents the current density. As illustrated inFIG.7, it can be ascertained that the top cell in the solar cell of Example 1 has higher short-circuit current than Comparative example 1 and Comparative example 2. In such an instance, as presented in Table 1, regarding the conversion efficiency of the top cell in the solar cell, the solar cell of Example 1 was 15.1%, the solar cell of Comparative example 1 was 14.5%, and the solar cell of Comparative example 2 was 0.5%. Therefore, it can be ascertained that the efficiency of the solar cell is improved due to the configuration of Example 1.

Next, the transmittance of the top cell in the solar cell was measured regarding Example 1 and Comparative example 1. The transmittance was measured using UV-VIS-NIR Spectrophotometer SolidSpec-3700 (produced by SHIMADZU CORPORATION). The measurement light was applied to the top cell while the wavelength was changed within the range of 300 nm to 1,300 nm. In such an instance, after the light passed through the top cell was introduced into an integrating sphere and dispersed, the intensity was measured using a detector so that the transmittance at each wavelength was obtained.

FIG.8is a graph illustrating a transmission spectrum of the top cell in the solar cell of 1 or Comparative example 1. Specifically, the horizontal axis inFIG.8represents the wavelength, and the vertical axis represents the transmittance. As illustrated inFIG.8, it can be ascertained that the solar cell of Example 1 has higher transmittance than Comparative example 1 in the wavelength range of 900 nm to 1,300 nm. The transmittance in the wavelength region of 900 nm to 1,300 nm illustrated inFIG.8was integrated, and the area was compared. It can be ascertained that a total amount of light incident on the bottom cell in the solar cell of Example 1 is about 7.4% greater than that of the solar cell of Comparative example 1.

The solar cell according to the present disclosure is useful as various solar cells such as a solar cell to be disposed on a roof.