Solar cell

Abstract

The present invention aims to provide a solar cell that includes a photoelectric conversion layer containing an organic-inorganic perovskite compound and that can exhibit high photoelectric conversion efficiency and high heat resistance. Provided is a solar cell including, in the stated order: a cathode; a photoelectric conversion layer; and an anode, the photoelectric conversion layer containing an organic-inorganic perovskite compound represented by the formula R-M-X.sub.3 where R is an organic molecule, M is a metal atom, and X is a halogen atom or a chalcogen atom, and a polymer having an acid dissociation constant pKa of 3 or less.

Claims

1. A solar cell comprising, in the stated order: a cathode; a photoelectric conversion layer; and an anode, the photoelectric conversion layer containing an organic-inorganic perovskite compound represented by the formula R-M-X.sub.3 where R is an organic molecule, M is a metal atom, and X is a halogen atom or a chalcogen atom, and a polymer having an acid dissociation constant pKa of 3 or less, wherein the polymer having an acid dissociation constant pKa of 3 or less is a halogen-containing polymer that has a structure of the following formula (X.sup.H) containing a halogen atom and electron-withdrawing groups bonded to a hetero atom: ##STR00008## wherein R.sup.1 and R.sup.2 are each an electron-withdrawing group, R.sup.H is a group containing a halogen atom, and R.sup.1 and R.sup.2 may be the same as or different from each other.

2. The solar cell according to claim 1, wherein the polymer having an acid dissociation constant pKa of 3 or less has a weight average molecular weight of 2,000 or more and 1,000,000 or less.

3. The solar cell according to claim 1, wherein the polymer having an acid dissociation constant pKa of 3 or less is a fluorine-containing polymer containing a structural unit that has a structure of the following formula (X.sup.F) containing a fluorine atom and electron-withdrawing groups bonded to a hetero atom: ##STR00009## wherein R.sup.1 and R.sup.2 are each an electron-withdrawing group, R.sup.F is a group containing a fluorine atom, and R.sup.1 and R.sup.2 may be the same as or different from each other.

4. The solar cell according to claim 3, wherein on an anode-side surface, α and β calculated by the following steps (A) to (C) satisfy α>0.6 and β<0.2: (A) performing sputtering on the anode-side surface n times, where n is an integer including 0, and measuring a ratio F(n) of the intensity of a fluoride ion to the total ion intensity (fluoride ion intensity/total ion intensity) and a ratio I(n) of the intensity of an iodide ion to the total ion intensity (iodide ion intensity/total ion intensity) on the surface by time-of-flight secondary ion mass spectrometry (TOF-SIMS) after each sputtering; (B) based on the relation between n and F(n) and I(n) obtained in the step (A), calculating sputtering cumulative time N, a ratio F(N) of the intensity of the fluoride ion to the total ion intensity (fluoride ion intensity/total ion intensity) at the sputtering cumulative time N, and a ratio I(N) of the intensity of the iodide ion to the total ion intensity (iodide ion intensity/total ion intensity) at the sputtering cumulative time N; and (C) based on the sputtering cumulative time N, F(N), and I(N) obtained in the step (B), plotting a graph with the sputtering cumulative time N on a horizontal axis and F(N) and I(N) values normalized to their maximum values as 1 on a vertical axis, and determining α as the value of I(N) and F(N) at an intersection of I(N) and F(N) at which N is closest to Nmax among intersections of I(N) and F(N) in a region of N<Nmax, and β as the value of I(N) and F(N) at an intersection of I(N) and F(N) at which N is closest to Nmax among intersections of I(N) and F(N) in a region of N≥Nmax, with Nmax being N at which I(N) reaches its maximum value.

5. The solar cell according to claim 1, wherein the polymer having an acid dissociation constant pKa of 3 or less is polystyrenesulfonyl-trifluoromethanesulfonimide.

6. The solar cell according to claim 1, comprising a hole transport layer between the anode and the photoelectric conversion layer.

7. The solar cell according to claim 6, wherein the hole transport layer contains the polymer having an acid dissociation constant pKa of 3 or less.

8. The solar cell according to claim 2, wherein the polymer having an acid dissociation constant pKa of 3 or less is a fluorine-containing polymer containing a structural unit that has a structure of the following formula (X.sup.F) containing a fluorine atom and electron-withdrawing groups bonded to a hetero atom: ##STR00010## wherein R.sup.1 and R.sup.2 are each an electron-withdrawing group, R.sup.F is a group containing a fluorine atom, and R.sup.1 and R.sup.2 may be the same as or different from each other.

9. The solar cell according to claim 8, wherein on an anode-side surface, α and β calculated by the following steps (A) to (C) satisfy α>0.6 and β<0.2: (A) performing sputtering on the anode-side surface n times, where n is an integer including 0, and measuring a ratio F(n) of the intensity of a fluoride ion to the total ion intensity (fluoride ion intensity/total ion intensity) and a ratio I(n) of the intensity of an iodide ion to the total ion intensity (iodide ion intensity/total ion intensity) on the surface by time-of-flight secondary ion mass spectrometry (TOF-SIMS) after each sputtering; (B) based on the relation between n and F(n) and I(n) obtained in the step (A), calculating sputtering cumulative time N, a ratio F(N) of the intensity of the fluoride ion to the total ion intensity (fluoride ion intensity/total ion intensity) at the sputtering cumulative time N, and a ratio I(N) of the intensity of the iodide ion to the total ion intensity (iodide ion intensity/total ion intensity) at the sputtering cumulative time N; and (C) based on the sputtering cumulative time N, F(N), and I(N) obtained in the step (B), plotting a graph with the sputtering cumulative time N on a horizontal axis and F(N) and I(N) values normalized to their maximum values as 1 on a vertical axis, and determining α as the value of I(N) and F(N) at an intersection of I(N) and F(N) at which N is closest to Nmax among intersections of I(N) and F(N) in a region of N<Nmax, and β as the value of I(N) and F(N) at an intersection of I(N) and F(N) at which N is closest to Nmax among intersections of I(N) and F(N) in a region of N≥Nmax, with Nmax being N at which I(N) reaches its maximum value.

10. The solar cell according to claim 2, wherein the polymer having an acid dissociation constant pKa of 3 or less is polystyrenesulfonyl-trifluoromethanesulfonimide.

11. The solar cell according to claim 2, comprising a hole transport layer between the anode and the photoelectric conversion layer.

12. The solar cell according to claim 11, wherein the hole transport layer contains the polymer having an acid dissociation constant pKa of 3 or less.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic view of an exemplary crystalline structure of an organic-inorganic perovskite compound.

(2) FIG. 2 is a schematic view illustrating a graph plotted with sputtering cumulative time N on the horizontal axis and F(N) and I(N) values normalized to their maximum values as 1 on the vertical axis.

(3) FIG. 3 is a schematic cross-sectional view of an exemplary solar cell of the present invention.

DESCRIPTION OF EMBODIMENTS

(4) Embodiments of the present invention are more specifically described with reference to, but not limited to, the following examples.

Example 1

(5) (1) Synthesis of Acidic Polymer

(6) An amount of 15 g of p-styrenesulfonic acid and 30 mL of thionyl chloride were reacted in 70 mL of DMF for three hours, followed by separation to give styrene sulfonyl chloride. The obtained styrene sulfonyl chloride and 10 g of trifluoromethanesulfonamide were then added to a solution obtained by adding 0.23 g of dimethylaminopyridine to 13 mL of triethylamine, and they were reacted. Thereafter, 17 g of silver oxide was added to give precipitate, whereby a fluorine atom-containing monomer was obtained.

(7) The obtained fluorine atom-containing monomer was then reacted in an argon atmosphere at 65° C. for 18 hours using azobisisobutyronitrile as a polymerization initiator. This produced a silver salt of an acidic polymer of the following formula (wherein m is an integer of 2 or more) having the structure of the formula (1.sup.H) wherein R.sup.H is CF.sub.3, that is, a silver salt of poly(N-styrenesulfonyl-trifluoromethanesulfonimide) (poly-TFSI).

(8) The pKa of the obtained acidic polymer was determined by determining the equilibrium constant of the compound and a conjugate acid of the compound in an aqueous solution by ultraviolet-visible spectroscopy. Specifically, the pKa was determined by a method in accordance with the method disclosed in the following document. The acid dissociation constant pKa determined by this method was −5.0. “Steric Effects in Displacement Reactions. III. The Base Strengths of Pyridine, 2,6-Lutidine and the Monoalkylpyridines” HERBERT C. BROWN AND XAVIER R. MIHM, J. Am. Chem. Soc. 1955, Vol. 77, pp 1723-1726

(9) The weight average molecular weight of the obtained acidic polymer was 50,000 as measured by gel permeation chromatography (GPC) using HSPgel RT MB-M (available from Waters Corporation) as a column and dimethylsulfoxide as a solvent.

(10) ##STR00006##

(11) The obtained silver salt of the acidic polymer was mixed with methylamine. The mixture was refined to give a methylamine salt of the acidic polymer. Similarly, the obtained silver salt of the acidic polymer was mixed with Spiro-OMeTAD. The mixture was refined to give a Spiro-OMeTAD salt of the acidic polymer.

(12) (2) Preparation of Solar Cell

(13) A FTO film having a thickness of 1,000 nm as a cathode was formed on a glass substrate. The FTO film was subjected to ultrasonic cleaning using pure water, acetone, and methanol in the stated order, each for 10 minutes. The FTO film was then dried to form a cathode formed of a FTO film.

(14) To a surface of the obtained cathode was applied, by a spin coating method, a titanium isopropoxide solution in ethanol adjusted to 2%, followed by firing at 400° C. for 10 minutes to give a thin-film electron transport layer having a thickness of 20 nm. To the thin-film electron transport layer was applied, by a spin coating method, a titanium oxide paste containing polyisobutyl methacrylate as an organic binder and titanium oxide (mixture of particles having an average particle size of 10 nm and particles having an average size of 30 nm). The titanium oxide paste was then fired at 500° C. for 10 minutes to form a porous electron transport layer having a thickness of 100 nm.

(15) PbI.sub.2 was dissolved in a solvent mixture of N,N-dimethylformamide (DMF) and dimethylsulfoxide (DMSO). The solution was applied to the electron transport layer by spin coating. Subsequently, a mixture of CH.sub.3NH.sub.3I and the methylamine salt of the acidic polymer at a weight ratio of 9:1 was dissolved in isopropanol. The solution was applied by spin coating and fired at 150° C. for five minutes to form a photoelectric conversion layer having a thickness of 400 nm.

(16) An amount of 5 mg of the obtained Spiro-OMeTAD salt of the acidic polymer, 30 μL of t-butylpyridine, and 15 mg of Spiro-OMeTAD were dissolved in 1 mL of chlorobenzene to prepare a coating solution for hole transport layer formation.

(17) The coating solution for hole transport layer formation was applied to the photoelectric conversion layer by a spin coating method to a thickness of 50 nm. Immediately after the formation of the hole transport layer, firing (pre-firing) was performed at 100° C. for 10 minutes to form a hole transport layer.

(18) An ITO film having a thickness of 100 nm as an anode was formed on the obtained hole transport layer by vacuum evaporation to form a solar cell including the cathode, the electron transport layer, the photoelectric conversion layer, the hole transport layer, and the anode stacked together.

Example 2

(19) A solar cell was obtained as in Example 1 except that polystyrenesulfonic acid was used as the acidic polymer.

(20) The polystyrenesulfonic acid used had an acid dissociation constant pKa of −2.8 and a weight average molecular weight of 50,000.

Comparative Examples 1 to 7

(21) A solar cell was obtained as in Example 1 except that instead of the acidic polymer, a polymer or monomer shown in Table 1 other than an acidic polymer was used.

(22) <Evaluation>

(23) The solar cells obtained in the examples and the comparative examples were evaluated as follows. Table 1 shows the results.

(24) (1) Measurement of Photoelectric Conversion Efficiency

(25) A power source (model 236, available from Keithley Instruments Inc.) was connected between the electrodes of the solar cell. A current-voltage curve was drawn using a solar simulator (available from Yamashita Denso Corp.) at an intensity of 100 mW/cm.sup.2, and the photoelectric conversion efficiency was calculated.

(26) The obtained photoelectric conversion efficiency was evaluated as “5” when it was 15% or higher, “4” when it was 13% or higher and lower than 15%, “3” when it was 11% or higher and lower than 13%, “2” when it was 9% or higher and lower than 11%, and “1” when it was lower than 9%.

(27) (2) Evaluation of Heat Resistance

(28) The obtained solar cell was put in an 85° C. oven. The photoelectric conversion efficiency after 500 hours was measured.

(29) The heat resistance was evaluated as “5” when the conversion efficiency after 500 hours was 90% or higher of the initial conversion efficiency, “4” when it was 80% or higher and lower than 90% of the initial conversion efficiency, “3” when it was 60% or higher and lower than 80% of the initial conversion efficiency, “2” when it was 40% or higher and lower than 60% of the initial conversion efficiency, and “1” when it was lower than 40% of the initial conversion efficiency.

(30) TABLE-US-00001 TABLE 1 Photoelectric conversion layer Weight Evaluation Polymer average Conver- Heat Acidic other than Acidic molecular sion resist- polymer acidic polymer monomer pKa weight efficiency ance Example 1 Poly-TFSI — — −5.0 50000 5 6 Example 2 Polystyrene- — — −2.8 50000 4 4 sulfonic acid Comparative — Polyacrylic — 4.3 25000 3 5 Example 1 acid Comparative — Polyvinylidene — 14.0 60000 2 5 Example 2 fluoride Comparative — Methyl poly- — 13.0 100000  2 5 Example 3 methacrylate Comparative — Polyacrylamide — 4.0 50000 2 5 Example 4 Comparative — Polyamic acid — 5.0 30000 1 4 Example 5 Comparative — — TFSI −5.0 — 5 2 Example 6 Comparative — — p-Toluene- −2.8 — 4 1 Example 7 sulfonic acid

Example 3

(31) (1) Synthesis of Fluorine-Containing Polymer

(32) An amount of 15 g of p-styrenesulfonic acid and 30 mL of thionyl chloride were reacted in 70 ml of DMF for three hours, followed by separation to give styrene sulfonyl chloride. The obtained styrene sulfonyl chloride and 10 g of trifluoromethanesulfonamide were then added to a solution obtained by adding 0.23 g of dimethylaminopyridine to 13 mL of triethylamine, and they were reacted. Thereafter, 17 g of silver oxide was added to give precipitate, whereby a fluorine atom-containing monomer was obtained.

(33) The obtained fluorine atom-containing monomer was then reacted in an argon atmosphere at 65° C. for 18 hours using azobisisobutyronitrile as a polymerization initiator. This produced a silver salt of a fluorine-containing polymer of the following formula (wherein m is an integer of 2 or more) having the structure of the formula (1.sup.F) wherein R.sup.F is CF.sub.3, that is, a silver salt of poly(N-styrenesulfonyl-trifluoromethanesulfonimide) (poly-TFSI).

(34) The pKa of the obtained fluorine-containing polymer was −5.0 as measured by determining the equilibrium constant of the compound and a conjugate acid of the compound in an aqueous solution by ultraviolet-visible spectroscopy.

(35) The weight average molecular weight of the obtained fluorine-containing polymer was 50,000 as measured by gel permeation chromatography (GPC) using HSPgel RT MB-M (available from Waters Corporation) as a column and dimethylsulfoxide as a solvent.

(36) ##STR00007##

(37) The obtained silver salt of the fluorine-containing polymer was mixed with methylamine. The mixture was refined to give a methylamine salt of the fluorine-containing polymer. Similarly, the obtained silver salt of the fluorine-containing polymer was mixed with 2,2′,7,7′-tetrakis-(N,N-di-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD). The mixture was refined to give a Spiro-OMeTAD salt of the fluorine-containing polymer.

(38) (2) Preparation of Solar Cell

(39) A FTO film having a thickness of 1,000 nm as a cathode was formed on a glass substrate. The FTO film was subjected to ultrasonic cleaning using pure water, acetone, and methanol in the stated order, each for 10 minutes. The FTO film was then dried to form a cathode formed of a FTO film.

(40) To a surface of the obtained cathode was applied, by a spin coating method, a titanium isopropoxide solution in ethanol adjusted to 2%, followed by firing at 400° C. for 10 minutes to give a thin-film electron transport layer having a thickness of 20 nm. To the thin-film electron transport layer was applied, by a spin coating method, a titanium oxide paste containing polyisobutyl methacrylate as an organic binder and titanium oxide (mixture of particles having an average particle size of 10 nm and particles having an average size of 30 nm). The titanium oxide paste was then fired at 500° C. for 10 minutes to form a porous electron transport layer having a thickness of 100 nm.

(41) PbI.sub.2 was dissolved in a solvent mixture of N,N-dimethylformamide (DMF) and dimethylsulfoxide (DMSO). The solution was applied to the electron transport layer by spin coating. Subsequently, a mixture of CH.sub.3NH.sub.3I and the methylamine salt of the fluorine-containing polymer at a weight ratio of 9:1 was dissolved in isopropanol. The solution was applied by spin coating and fired at 150° C. for five minutes to form a photoelectric conversion layer having a thickness of 400 nm.

(42) An amount of 5 mg of the obtained Spiro-OMeTAD salt of the fluorine-containing polymer, 30 μL of t-butylpyridine, and 15 mg of Spiro-OMeTAD were dissolved in 1 mL of chlorobenzene to prepare a coating solution for hole transport layer formation.

(43) The coating solution for hole transport layer formation was applied to the photoelectric conversion layer by a spin coating method to a thickness of 50 nm. Immediately after the formation of the hole transport layer, firing (pre-firing) was performed at 100° C. for 10 minutes to form a hole transport layer.

(44) An ITO film having a thickness of 100 nm as an anode was formed on the obtained hole transport layer by vacuum evaporation. Immediately after the formation of the anode, firing (post-firing) was performed at 100° C. for 10 minutes to form a solar cell including the cathode, the electron transport layer, the photoelectric conversion layer, the hole transport layer, and the anode stacked together.

(45) (3) Calculation of α and β

(46) α and β were calculated by the steps (A) to (C) described above. α was 0.90. β was 0.10.

Examples 4 and 5

(47) A solar cell was obtained as in Example 3 except that the method of forming the hole transport layer was changed as shown in Table 2. α and β were calculated by the steps (A) to (C).

Example 6

(48) A solar cell was obtained as in Example 3 except that a trifluoromethanesulfonylimide monomer, which is a fluorine-containing monomer, was used in a coating solution for hole transport layer formation instead of the fluorine-containing polymer. α and β were calculated by the steps (A) to (C).

Example 7

(49) A solar cell was obtained as in Example 3 except that the method of forming the hole transport layer was shown in Table 2. α and β were calculated by the steps (A) to (C).

Comparative Example 8

(50) A solar cell was obtained as in Example 3 except that a trifluoromethanesulfonylimide monomer as a fluorine-containing monomer was used in a coating solution for photoelectric conversion layer formation instead of the fluorine-containing polymer. α and β were calculated by the steps (A) to (C).

Comparative Examples 9 to 11

(51) A solar cell was obtained as in Comparative Example 8 except that the method of forming the hole transport layer was as shown in Table 2. α and β were calculated by the steps (A) to (C).

Comparative Example 12

(52) A solar cell was obtained as in Example 3 except that neither the fluorine-containing polymer nor the fluorine-containing monomer was added to the coating solution for photoelectric conversion layer formation. A solar cell was obtained as in Example 1 except that the coating solution for photoelectric conversion layer formation was used. α and β were calculated by the steps (A) to (C).

Comparative Examples 13 to 18

(53) A solar cell was obtained as in Comparative Example 12 except that the method of forming the hole transport layer was shown in Table 2. α and β were calculated by the steps (A) to (C).

(54) (Evaluation)

(55) The solar cells obtained in the examples and the comparative examples were evaluated as follows.

(56) Table 2 shows the results.

(57) (1) Evaluation of Photoelectric Conversion Efficiency

(58) A power source (model 236, available from Keithley Instruments Inc.) was connected between the electrodes of the solar cell. A current-voltage curve was drawn using a solar simulator (available from Yamashita Denso Corp.) at an intensity of 100 mW/cm.sup.2, and the photoelectric conversion efficiency was calculated.

(59) The obtained photoelectric conversion efficiency was evaluated as “5” when it was 15% or higher, “4” when it was 13% or higher and lower than 15%, “3” when it was 11% or higher and lower than 13%, “2” when it was 9% or higher and lower than 11%, and “1” when it was lower than 9%.

(60) (2) Evaluation of Heat Resistance

(61) The obtained solar cell was put in an 85° C. oven. The photoelectric conversion efficiency after 500 hours was measured.

(62) The heat resistance was evaluated as “5” when the conversion efficiency after 500 hours was 90% or more of the initial conversion efficiency, “4” when it was 80% or higher and lower than 90% of the initial conversion efficiency, “3” when it was 60% or higher and lower than 80% of the initial conversion efficiency, “2” when it was 40% or higher and lower than 60% of the initial conversion efficiency, and “1” when it was lower than 40% of the initial conversion efficiency.

(63) (3) Evaluation of Light Resistance

(64) The solar cell was put in Sunshine Xenon Weather Meter (available from Suga Test Instruments Co., Ltd.) and irradiated with light corresponding to an intensity of 100 mW/cm.sup.2 at 60° C. for 100 hours. The photoelectric conversion efficiency before and after the light irradiation was calculated by the same method as above.

(65) The light resistance was evaluated as “5” when the conversion efficiency after the light irradiation was 90% or higher of the conversion efficiency before the light irradiation (initial conversion efficiency), “4” when it was 80% or higher and lower than 90% of the initial conversion efficiency, “3” when it was 60% or higher and lower than 80% of the initial conversion efficiency, “2” when it was 40% or higher and lower than 60% of the initial conversion efficiency, and “1” when it was lower than 40% of the initial conversion efficiency.

(66) TABLE-US-00002 TABLE 2 Photoelectric conversion layer Evaluation Organic- Hole transport layer Photo- inorganic Fluorine- Fluorine- Fluorine- Fluorine- electric Heat Light perovskite containing- containing t-Butyl- containing containing Pre- Post- conversion resist- resist- compound polymer monomer pyridine polymer monomer firing firing α β efficiency ance ance Example 3 ◯ ◯ x ◯ ◯ x ◯ ◯ 0.90 0.10 5 5 5 Example 4 ◯ ◯ x ◯ ◯ x ◯ x 0.70 0.10 4 5 5 Example 5 ◯ ◯ x ◯ x x ◯ ◯ 0.90 0.10 5 5 5 Example 6 ◯ ◯ x ◯ x ◯ ◯ ◯ 0.90 0.18 5 3 4 Example 7 ◯ ◯ x x x ◯ x x 0.80 0.13 5 3 4 Comparative ◯ x ◯ ◯ ◯ x ◯ ◯ 0.90 0.21 5 2 3 Example 8 Comparative ◯ x ◯ ◯ x ◯ ◯ ◯ 0.90 0.23 5 1 2 Example 9 Comparative ◯ x ◯ ◯ x ◯ x x 0.90 0.30 5 1 1 Example 10 Comparative ◯ x ◯ ◯ x x ◯ ◯ 0.90 0.30 5 2 1 Example 11 Comparative ◯ x x ◯ ◯ x ◯ ◯ 0.40 0.10 3 5 5 Example 12 Comparative ◯ x x ◯ ◯ x ◯ x 0.30 0.10 2 5 5 Example 13 Comparative ◯ x x ◯ x ◯ ◯ ◯ 0.70 0.20 4 3 3 Example 14 Comparative ◯ x x ◯ x ◯ ◯ x 0.20 0.14 1 3 4 Example 15 Comparative ◯ x x ◯ x ◯ x x 0.70 0.30 4 2 1 Example 16 Comparative ◯ x x ◯ x ◯ x x 0.20 0.14 1 2 4 Example 17 Comparative ◯ x x ◯ x x ◯ ◯ 0.20 0.10 1 5 5 Example 18

INDUSTRIAL APPLICABILITY

(67) The present invention can provide a solar cell that includes a photoelectric conversion layer containing an organic-inorganic perovskite compound and that can exhibit high photoelectric conversion efficiency and high heat resistance.

REFERENCE SIGNS LIST

(68) 1 solar cell 2 cathode 3 electron transport layer 31 thin-film electron transport layer 32 porous electron transport layer 4 photoelectric conversion layer containing organic-inorganic perovskite compound and acidic polymer 5 hole transport layer 6 anode (patterned electrode)