JP2013214712A - Photoelectric conversion material and organic thin-film solar cell using the same - Google Patents

Abstract

【選択図】図１A photoelectric conversion material exhibiting excellent characteristics as an electron donor (donor) or an electron acceptor (acceptor) is obtained.
A BHJ solar cell includes a photoelectric conversion layer in which a donor domain and an acceptor domain are mixed. The donor domain 26 is made of, for example, a polymer having graphene represented by the following general formula (1) as a structural unit, and the polymer acts as a donor (photoelectric conversion material). Note that the acceptor constituting the acceptor domain 28 is, for example, phenyl C ₆₁ butyric acid methyl ester (PCBM).
[Chemical 1]

[Selection] Figure 1

Description

  The present invention relates to a photoelectric conversion material composed of a polymer having a condensed aromatic ring as a structural unit, and an organic thin film solar cell using the photoelectric conversion material.

  As a solar cell that can be easily manufactured by a low-cost process such as a roll-to-roll method, an organic thin-film solar cell using an organic material has attracted attention. As this type of organic thin film solar cell, a bulk heterojunction type organic thin film solar cell (hereinafter also referred to as “BHJ solar cell”) is known.

  A BHJ solar cell is a mixture of a donor domain made of a photoelectric conversion material acting as an electron donor (donor) and an acceptor domain made of a photoelectric conversion material acting as an electron acceptor (acceptor), and converts light into electricity. A photoelectric conversion layer having a function is provided. Specifically, the photoelectric conversion layer is interposed between a positive electrode and a negative electrode, and sunlight enters the photoelectric conversion layer through the positive electrode, thereby generating excitons.

  When the exciton reaches the interface between the donor domain and the acceptor domain, it is separated into electrons and holes. Among these, the electrons move in the acceptor domain and reach the negative electrode. On the other hand, the holes move in the donor domain and reach the positive electrode. These holes and electrons become electrical energy that energizes an external circuit electrically connected to the negative and positive electrodes.

As a typical example of the photoelectric conversion material in the photoelectric conversion layer having the above functions, that is, a donor and an acceptor, as shown in Patent Document 1, poly-3-hexylthiophene (P3HT, see FIG. 6), Examples include phenyl C ₆₁ butyric acid methyl ester (PCBM, see FIG. 7).

  Here, the energy levels of the highest occupied orbit (HOMO) and the lowest unoccupied orbit (LUMO) of P3HT and PCBM are as shown in FIG. When light enters the photoelectric conversion layer as described above, first, electrons make a transition from HOMO to LUMO of the donor P3HT. That is, the energy level difference between HOMO and LUMO of P3HT corresponds to the band gap (Eg).

  The electrons that have transitioned to the LUMO of the P3HT then move to the LUMO of the PCBM that is the acceptor, thereby generating electrons and holes. That is, energy loss occurs due to the energy level difference between the LUMOs of P3HT and PCBM, and the energy level difference between the HOMO of P3HT and the LUMO of PCBM corresponds to the open circuit voltage (Voc).

  By the way, in a solar cell, when it is going to acquire the same electric power generation amount, an area can be made small, so that photoelectric conversion efficiency is large. As a result, the weight is reduced and the installation area is reduced, so that there is an advantage that the degree of freedom of the installation layout is increased.

  In an organic thin film solar cell such as a BHJ solar cell, in order to further improve the photoelectric conversion efficiency, (a) increase the amount of light absorption and activate the generation of excitons. (B) long wavelength (near infrared side) ) Light absorption to increase the utilization efficiency of sunlight, (c) increase the open circuit voltage Voc, etc. In order to realize these (a) to (c), (A) a large extinction coefficient, (B) a small energy level difference (band gap Eg) between HOMO and LUMO, (C) an energy level of LUMO It is conceivable to select a donor whose position is close to the LUMO level of the acceptor.

  As a substance that may satisfy the above (A) to (C), a condensed aromatic ring described in Patent Documents 2 to 5, that is, a π electron conjugated compound is conceived. Note that this type of condensed aromatic ring is sometimes referred to as “graphene” (see Patent Document 4).

JP 2007-273939 A Japanese Patent No. 4005571 JP 2010-56492 A JP 2007-19086 A Special table 2010-508677 gazette

  The technique described in Patent Document 2 intends to obtain a so-called nanotube-like aggregate by bonding a functional group to hexabenzocoronene (HBC) and self-assembling via the functional group. Therefore, the number of steps for obtaining the final semiconductor is large, and it is not clear whether the obtained integrated body is p-type (donor) or n-type (acceptor).

  Patent Document 3 suggests that nanotubes, which are HBC aggregates, have hole and electron conduction paths at the same time. In the technique described in Patent Document 3, the inner surface and the outer surface of the nanotube are coated with fullerene, and the hole mobility in HBC is controlled by the coverage ratio. The techniques described in a few cannot improve the characteristics of HBC itself as a donor.

  In the technique described in Patent Document 4, a functional group having a fluorine atom is bonded to a graphene derivative, whereby an n-type semiconductor is obtained. That is, with this technique, an acceptor is obtained, and a donor cannot be obtained.

  Furthermore, in any of Patent Documents 2 to 5, only low molecular organic compounds are disclosed. As is well known, a low molecular organic compound is difficult to dissolve in a solvent, and therefore, it has become difficult to perform a roll-to-roll method or the like when obtaining a photoelectric conversion layer.

  The present invention has been made to solve the above-described problems, and exhibits excellent characteristics as a donor (electron donor) or acceptor (electron acceptor), and can easily and easily obtain a photoelectric conversion layer. It aims at providing the organic thin-film solar cell which comprises the photoelectric converting material used as this, and the photoelectric converting layer containing this photoelectric converting material.

  In order to achieve the above object, the present invention relates to a photoelectric conversion material that functions as an electron donor (donor) that donates electrons or an electron acceptor (acceptor) that accepts electrons. It is characterized by comprising a polymer whose structural unit is at least one of graphene represented by

  The photoelectric conversion material of the present invention comprises a π-conjugated polymer having a condensed aromatic ring in which a π electron cloud spreads as a whole as a structural unit (hereinafter, also referred to as “nanographene polymer”). In this polymer, since the π electron cloud spreads along the main chain, the extinction coefficient increases and excitons are actively generated.

  In particular, this polymer has a small energy level difference between HOMO and LUMO. That is, the band gap Eg is small. For this reason, since the maximum absorption wavelength is shifted to the long wavelength side, the light of the long wavelength (near infrared side) is favorably absorbed, and accordingly, the utilization efficiency of sunlight is improved.

  Furthermore, the LUMO energy level of this polymer is lower (deeper) than P3HT or the like. Therefore, when the polymer is used as a donor and PCBM is used as an acceptor, the energy loss is smaller than when P3HT is used as a donor. As a result, in the organic thin film solar cell, the open circuit voltage Voc increases.

  For the above reasons, the conversion efficiency is improved in the organic thin film solar cell using this polymer as a donor.

  In the present invention, as in Patent Document 4, the condensed aromatic ring represented by the general formulas (1) to (4) is referred to as “graphene”. In addition, “nanographene” indicates that one constituent unit is a nanometer scale.

  The degree of polymerization of the polymer (number of structural units) is preferably 10 to 150. If it is less than 10, it is not easy to make the extinction coefficient sufficiently high or to make Eg sufficiently small. On the other hand, when it is larger than 150, the time required for the polymerization becomes long, and the production efficiency of the polymer is lowered.

  In other words, the polymer which shows the characteristic outstanding as a donor can be obtained efficiently by setting the polymerization degree of a polymer in said range.

  Here, when the structural unit is the graphene described above, the molecular weight of the polymer is 7400 to 111500.

  Moreover, this invention is an organic thin-film solar cell using the said photoelectric conversion material, Comprising: The photoelectric conversion layer which contains this photoelectric conversion material as an electron donor is comprised, It is characterized by the above-mentioned.

  In this organic thin film solar cell, when PCBM is used as an acceptor, the absorption coefficient of the photoelectric conversion layer is larger than when P3HT is used as a donor, and the energy level difference between HOMO and LUMO of the donor ( The band gap Eg) is reduced. In addition, the LUMO energy level of the donor is close to the LUMO level of the PCBM that is the acceptor.

  Therefore, in the organic thin film solar cell, the generation of excitons is activated and the utilization efficiency of sunlight is improved. Further, the open circuit voltage Voc increases. For the reasons described above, the conversion efficiency is improved.

  Thus, an organic thin-film solar cell with large conversion efficiency can make an area small compared with the solar cell from which the same electric power generation amount is obtained. Accordingly, since the weight is reduced, the load applied to the installation place is reduced. Further, since the installation area is reduced, the degree of freedom of the installation layout is also improved.

  A preferred example of the organic thin film solar cell is a bulk heterojunction type including a photoelectric conversion layer in which a donor domain and an acceptor domain are mixed. In this case, for example, the contact area between the donor domain and the acceptor domain is larger than that of a planar heterojunction type in which a donor layer and an acceptor layer are individually formed. In organic thin-film solar cells, excitons are mainly separated into electrons and holes at the interface between the donor domain and the acceptor domain, and are involved in power generation. By configuring as a bulk heterojunction type with a large contact area between the two, Conversion efficiency can be improved.

  According to the present invention, it is composed of a π-conjugated polymer having a large π electron cloud spread, so that the absorption coefficient is increased, the energy level difference (band gap Eg) between HOMO and LUMO is small, and the energy of HOMO It becomes a donor with a low level. As a result, the LUMO energy level is close to the PCBM LUMO energy level.

  For this reason, when an organic thin film solar cell is constituted using the polymer as a donor and PCBM as an acceptor, excitons are actively generated. Moreover, the long wavelength (near infrared side) light will be absorbed favorably and the utilization efficiency of sunlight will improve. Further, the open circuit voltage Voc increases. Therefore, an organic thin-film solar cell having a large conversion efficiency and a small area can be configured.

It is a typical longitudinal section of a bulk heterojunction type organic thin film solar cell concerning this embodiment. It is explanatory drawing which shows the structural unit of polyphenylene. It is a Raman spectrum of the photoelectric conversion material (nanographene polymer) which concerns on this Embodiment obtained by making polyphenylene and polyphenylene react. It is a graph which shows together each characteristic of the photoelectric conversion material (nanographene polymer) which concerns on this Embodiment, and P3HT. It is an energy level diagram which shows the energy level of each said HOMO and LUMO of said photoelectric conversion material (nanographene polymer), P3HT, and PCBM. It is explanatory drawing which shows the structural formula of P3HT. It is explanatory drawing which shows the structural formula of PCBM. It is a schematic explanatory view showing that electrons transition from HOMO of P3HT to LUMO and further to LUMO of PCBM.

  Hereinafter, preferred embodiments of the photoelectric conversion material according to the present invention will be described in detail in relation to a BHJ solar cell including a photoelectric conversion layer including the photoelectric conversion material, with reference to the accompanying drawings.

  FIG. 1 is a schematic vertical sectional view of a main part of a BHJ solar cell 10 according to the present embodiment. The BHJ solar cell 10 is configured such that a hole transport layer 14, a photoelectric conversion layer 16, and a back electrode 18 are superimposed on the transparent electrode 12 in this order from below.

  The transparent electrode 12 functions as a positive electrode. That is, the holes 24 move to the transparent electrode 12. In addition, as the transparent electrode 12, a material that sufficiently transmits light such as sunlight, such as indium-tin composite oxide (ITO), is selected.

  The hole transport layer 14 is a layer that supports the movement of the holes 24 generated in the photoelectric conversion layer 16 to the transparent electrode 12, and is generally poly (3,4) doped with polystyrene sulfonic acid. -Ethylenedioxythiophene), i.e. so-called PEDOT: PSS.

  The photoelectric conversion layer 16 is formed as a layer in which a donor domain 26 made of a photoelectric conversion material acting as an electron donor (donor) and an acceptor domain 28 made of a photoelectric conversion material acting as an electron acceptor (acceptor) are mixed. ing. Among these, the above-mentioned PCBM is mentioned as a suitable example of the photoelectric conversion material used as an acceptor.

  On the other hand, the p-type semiconductor serving as a donor, that is, the photoelectric conversion material according to the present embodiment is made of a polymer containing graphene represented by general formulas (1) to (4) as a structural unit.

  This polymer can be produced, for example, by further reacting the polyphenylene shown in the general formula (5) and FIG.

  As shown in FIG. 2, in the structural unit 30 of polyphenylene, the benzene nucleus 39 can be bonded to either 32 a or 32 b of the benzene nucleus 32. For this reason, the structural unit 30 includes structural isomers.

  Further, the structural unit 30 has a structure in which the triphenylbenzene skeletons 36 and 38 are single-bonded to the central benzene nucleus 34. These triphenylbenzene skeletons 36 and 38 can take different conformations depending on the rotational position around a single bond with respect to the benzene nucleus 34.

  Specific examples include the structures U1 to U4 shown below. Ultimately, polyphenylene contains at least one of these structures.

  That is, the photoelectric conversion material according to the present embodiment is a polymer obtained by reacting polyphenylene having the above structure as the structural unit 30. Therefore, the structural unit (repeating unit) of the polymer differs depending on whether the polyphenylene before the reaction is U1 to U4. That is, when each of U1-U4 reacts, the nano graphene shown by the following general formula (1)-(4) becomes a structural unit.

  As described above, the photoelectric conversion material according to the present embodiment is a polymer including at least one of the nanographenes represented by the general formulas (1) to (4) as a structural unit. That is, the polymer is not limited to one in which only one of the nanographenes represented by the general formulas (1) to (4) is bonded to each other. For example, the nanographene represented by the general formulas (1) to (4) May be randomly combined.

In addition, it is preferable that the polymerization degree of this polymer is 10-150. When the degree of polymerization is less than 10, that is, when the number of nanographenes bonded to each other is less than 10, the extinction coefficient tends to be small. In addition, when the degree of polymerization exceeds 150, that is, the number of nanographenes bonded to each other exceeds 150, the time required for polymerization to obtain a polymer as a photoelectric conversion material becomes long, and the production efficiency is lowered.
In other words, by setting the degree of polymerization within the above range, a photoelectric conversion material having a sufficiently improved absorption coefficient can be efficiently produced.

  In any of the nano graphenes represented by the general formulas (1) to (4), the molecular weight is 743. Accordingly, when the degree of polymerization of the polymer is 10 to 150, the molecular weight of the polymer is in the range of 7400 to 111500.

  In the BHJ solar cell 10 (see FIG. 1), the back electrode 18 is superimposed on the photoelectric conversion layer 16 including a photoelectric conversion material made of such a polymer. The back electrode 18 functions as a negative electrode to which the electrons 40 reach.

  The BHJ solar cell 10 according to the present embodiment is basically configured as described above. Next, the function and effect will be described.

  When light (for example, sunlight) is irradiated onto the transparent electrode 12 of the BHJ solar cell 10, the light passes through the hole transport layer 14 and reaches the photoelectric conversion layer 16. As a result, excitons 42 are generated in the photoelectric conversion layer 16.

  The generated excitons 42 move in the donor domain 26 and reach the interface between the donor domain 26 and the acceptor domain 28. At this interface, the electrons 40 and the holes 24 are separated. Similarly to the above, the electrons 40 in this move in the acceptor domain 28 and reach the back electrode 18 which is a negative electrode. On the other hand, the holes 24 move in the donor domain 26, pass through the hole transport layer 14, and then reach the transparent electrode 12 that is the positive electrode.

  Here, in this Embodiment, the donor domain 26 in the photoelectric converting layer 16 consists of a polymer which uses at least any one of the nano graphene shown by General formula (1)-(4) as a structural unit.

  As understood from the general formulas (1) to (4), in the nano graphene, the π electron cloud spreads over the whole. That is, the polymer forming the donor domain 26 is a π-conjugated polymer. Inevitably, in the polymer, the π electron cloud spreads over a wide range as compared with nanographene alone (monomer).

  In such a polymer having a large spread of the π electron cloud, the maximum absorption wavelength is shifted to the longer wavelength side and the absorption coefficient is increased, whereby the band gap (Eg) corresponding to the energy level difference between HOMO-LUMO. Becomes smaller. Accordingly, in the donor domain 26, the generation of excitons 42 becomes active and the utilization efficiency of sunlight is improved.

  In this case, the LUMO energy level of the polymer as a donor is about -3.9 eV, which is deeper than the LU3 energy level of P3HT (about -2.5 eV). Therefore, the LUMO energy level of the polymer is close to the LUMO energy level of PCBM (fullerene derivative) forming the acceptor domain 28.

  The reason for this is presumably because nanographene, which is a constituent unit of the polymer, is a condensed aromatic ring having a hydrocarbon aromatic ring as a basic skeleton, and is similar to the structure of PCBM. As a result, the open circuit voltage Voc of the BHJ solar cell 10 increases.

  In combination with the above, the BHJ solar cell 10 exhibits excellent photoelectric conversion efficiency. For this reason, in this BHJ solar cell 10, an area can be made small compared with the other solar cell from which the same electric power generation amount is obtained. Therefore, since the weight is reduced, the load applied to the installation place is reduced, and the degree of freedom in the installation layout is improved.

  Next, a method for producing the photoelectric conversion material according to the present embodiment will be described in relation to the polymer production method.

  As described above, this polymer can be obtained as a reaction product of polyphenylene.

  Polyphenylene can be obtained, for example, by reacting biscyclopentadienone with diacetylene as shown in the following reaction formula (6).

  Note that biscyclopentadienone can be produced by a known method. For example, first, 1,4-bisbenzyl and dibenzyl ketone (1,3-diphenyl-2-propanone) and n-butanol are mixed to prepare a mixed solution. Then, a methanol solution of Triton B (benzyltrimethylammonium hydroxide) may be added while heating the mixed solution.

The polyphenylene thus obtained is then reacted using iron chloride (FeCl ₃ ) as a catalyst. When the case where the structure of polyphenylene is any one of the above structures U1 to U4 is exemplified, as shown in the following reaction formulas (7) to (10), condensed aromatic rings represented by general formulas (1) to (4) A polymer having nanographene as a structural unit is obtained.

  As described above, the degree of polymerization of the polymer is preferably 10 to 150. When it is less than 10, the extinction coefficient is not improved so much. On the other hand, if it exceeds 150, it takes a long time for the polymerization to end, so the production efficiency of the polymer decreases.

  FIG. 3 is a Raman spectrum of polyphenylene and the polymer obtained as described above. As shown in FIG. 3, a peak derived from the side chain phenyl group was detected in the polyphenylene spectrum, but the peak was not detected in the polymer spectrum. From this, it can be judged that a polymer having nanographene as a structural unit is generated by the reaction of the phenyl group in the side chain of polyphenylene.

  For this polymer, the band gap Eg between HOMO-LUMO determined from the absorption edge of the UV-visible absorption spectrum (UV-Vis) and the energy level of HOMO determined by photoelectron yield spectroscopy (PYS) are expressed as P3HT. 4 together with the above characteristics.

  FIG. 4 shows that this polymer has a small band gap Eg. In addition, since the band gaps Eg of the polymer and P3HT are 2.0 eV and 2.2 eV, and the energy levels of each HOMO are −5.9 eV and −4.7 eV, the energy levels of each LUMO of the polymer and P3HT are Is -3.9 eV and -2.5 eV.

  The energy levels of HOMO and LUMO of the above polymers, P3HT and PCBM are shown in FIG. FIG. 5 shows that the HOMO energy level of the polymer is lower (deeper) than P3HT.

  5 that the energy level between the LUMO of the polymer and the LUMO of the PCBM is smaller than the energy level between the LUMO of the P3HT and the LUMO of the PCBM.

For the above reasons, in the BHJ solar cell 10 according to the present embodiment in which the photoelectric conversion layer 16 is formed using this polymer as a donor, the energy is higher than that of the conventional BHJ solar cell using P3HT as a donor. Loss decreases and the open circuit voltage Voc increases.
In addition, the photoelectric converting layer 16 containing this polymer can be formed as follows.

  First, the polymer and PCBM are added individually or a mixture of the polymer and PCBM to a suitable solvent such as toluene, chloroform, or chlorobenzene. The polymer and PCBM are dissolved in a solvent to form a solution.

  Next, this solution is applied onto the hole transport layer 14 by any method such as spin coating, ink jet printing, roller casting, or roll-to-roll method.

  Next, when the solution on the hole transport layer 14 is heated, the solution is cured and the photoelectric conversion layer 16 is obtained. If necessary, the phase separation between the donor domain 26 and the acceptor domain 28 can be further promoted by annealing. As a result, the area of the junction interface between the donor domain 26 and the acceptor domain 28 increases, and the performance can be improved.

  When a monomer is used as a donor, it is difficult to employ the method as described above when obtaining the photoelectric conversion layer 16 because the monomer is difficult to dissolve in a solvent. In contrast, in this embodiment, a polymer is used as a donor. Since the polymer is soluble in a predetermined solvent, the photoelectric conversion layer 16 can be easily and easily formed at low cost by the above-described process.

  The present invention is not particularly limited to the above-described embodiment, and various modifications can be made without departing from the scope of the invention.

  For example, in this embodiment, a bulk heterojunction type organic thin film solar cell (BHJ solar cell) 10 in which donors and acceptors are mixed in the photoelectric conversion layer 16 is described as an example, but the present invention is particularly limited to this. Instead, it may be a planar heterojunction organic thin-film solar cell in which a donor layer and an acceptor layer are individually formed. In this case, the donor layer may be formed from the polymer.

  In this embodiment, an example in which the polymer (photoelectric conversion material) is used as a donor for an organic thin film solar cell has been described. However, the present invention is not particularly limited thereto. It is also possible to employ the polymer as an acceptor for an organic thin film solar cell.

  Furthermore, the use of the polymer (photoelectric conversion material) is not limited to the photoelectric conversion layer 16 of the organic thin film solar cell. For example, it is also possible to employ for an optical sensor.

DESCRIPTION OF SYMBOLS 10 ... Bulk heterojunction type organic thin film solar cell 12 ... Transparent electrode 14 ... Hole transport layer 16 ... Photoelectric conversion layer 18 ... Back surface electrode 24 ... Hole 26 ... Donor domain 28 ... Acceptor domain 30 ... Structural unit 32, 34, 39 ... Benzene nucleus 40 ... electron 42 ... exciton

Claims (5)

In a photoelectric conversion material that functions as an electron donor that donates electrons or an electron acceptor that accepts electrons,
A photoelectric conversion material comprising a polymer having at least one of graphene represented by general formulas (1) to (4) as a structural unit.

  The photoelectric conversion material according to claim 1, wherein the polymer has a polymerization degree of 10 to 150.   The photoelectric conversion material according to claim 2, wherein the polymer has a molecular weight of 7400 to 111500.   It is an organic thin-film solar cell using the photoelectric conversion material of any one of Claims 1-3, Comprising: The organic thin film characterized by comprising the photoelectric conversion layer which contains the said photoelectric conversion material as an electron donor. Solar cell.   5. The organic thin film solar cell according to claim 4, wherein a bulk heterojunction structure comprising a photoelectric conversion layer in which the electron donor and an electron acceptor that accepts an electron donated from the electron donor are mixed is formed. An organic thin film solar cell characterized by

Images