FLEXIBLE SOLAR CELL

Abstract

The present invention aims to provide a flexible solar cell having excellent high-temperature, high-humidity durability and excellent initial performance. The present invention relates to a flexible solar cell including, on a flexible substrate: an electrode; a transparent electrode; and a photoelectric conversion layer disposed between the electrode and the transparent electrode, the photoelectric conversion layer containing an organic-inorganic perovskite compound, the flexible substrate including an aluminum foil and an aluminum oxide film formed on the aluminum foil, the flexible substrate having a ratio of the thickness of the aluminum oxide film to the total thickness of the aluminum foil and the aluminum oxide film of 0.1% or higher and 15% or lower.

Claims

1. A flexible solar cell comprising, on a flexible substrate: an electrode; a transparent electrode; and a photoelectric conversion layer disposed between the electrode and the transparent electrode, the photoelectric conversion layer containing an organic-inorganic perovskite compound, the flexible substrate including an aluminum foil and an aluminum oxide film formed on the aluminum foil, the flexible substrate having a ratio of the thickness of the aluminum oxide film to the total thickness of the aluminum foil and the aluminum oxide film of 0.1% or higher and 15% or lower.

2. The flexible solar cell according to claim 1, wherein the thickness of the aluminum oxide film is 0.1 μm or greater and 20 μm or smaller.

3. The flexible solar cell according to claim 1, wherein the organic-inorganic perovskite compound is 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 or chalcogen atom.

4. The flexible solar cell according to claim 2, wherein the organic-inorganic perovskite compound is 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 or chalcogen atom.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0087] FIG. 1 is a schematic view of an exemplary crystal structure of an organic-inorganic perovskite compound.

[0088] FIG. 2 is a schematic cross-sectional view of an exemplary flexible solar cell of the present invention.

[0089] FIG. 3 is an electron micrograph of aluminum oxide having the boehmite crystal structure.

[0090] FIG. 4 is an electron micrograph of aluminum oxide having the bayerite crystal structure.

DESCRIPTION OF EMBODIMENTS

[0091] Embodiments of the present invention are more specifically described in the following with reference to, but not limited to, examples.

Example 1

(1) Anodization of Aluminum Foil

[0092] An aluminum foil (available from UACJ, thickness 100 μm) was anodized with sulfuric acid to form an aluminum oxide film on a surface of the aluminum foil. Thus, a flexible substrate was obtained.

[0093] A cross section of the obtained flexible substrate was observed with an electron microscope (S-4800, available from Hitachi, Ltd.) and the contrast of the obtained photograph was analyzed to measure the thickness of the flexible substrate (the total thickness of the aluminum foil and the aluminum oxide film) and the thickness of the aluminum oxide film. The ratio of the thickness of the aluminum oxide film to the total thickness of the aluminum foil and the aluminum oxide film was calculated from the obtained thicknesses. A surface of the obtained flexible substrate was observed with an electron microscope (S-4800, available from Hitachi, Ltd.) and the crystal structure of the aluminum oxide in the aluminum oxide film was determined to be the boehmite crystal structure.

(2) Production of Flexible Solar Cell

[0094] On the aluminum oxide film side of the obtained flexible substrate were formed, with a deposition device, a 100-nm-thick electrode containing aluminum and a 100-nm-thick, thin-film electron transport layer containing titanium. To the thin-film electron transport layer was further 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 particles size of 30 nm). The paste was then fired at 200° C. for 10 minutes and irradiated with UV for 10 minutes. Thus, a 500-nm-thick porous electron transport layer was formed.

[0095] Subsequently, lead iodide as a metal halide compound was dissolved in N,N-dimethylformamide (DMF) to prepare a 1 M solution. The solution was formed into a film on the porous electron transport layer by a spin coating method. Separately, methylammonium iodide as an amine compound was dissolved in 2-propanol to prepare a 1 M solution. The above sample with the lead iodide film formed thereon was immersed in this solution to form a layer containing CH.sub.3NH.sub.3PbI.sub.3, which is an organic-inorganic perovskite compound. The obtained sample was annealed at 120° C. for 30 minutes.

[0096] Then, a solution was prepared by dissolving, in 25 μL of chlorobenzene, 68 mM of Spiro-OMeTAD (having a spirobifluorene skeleton), 55 mM of t-butylpyridine, and 9 mM of bis(trifluoromethanesulfonyl)imide-silver salt. The solution was applied to the photoelectric conversion layer by a spin coating method. Thus, a 150-nm-thick hole transport layer was formed.

[0097] A 100-nm-thick ITO film as a transparent electrode was formed on the obtained hole transport layer by vacuum evaporation. Thus, a flexible solar cell was obtained.

Examples 2 to 9

[0098] A flexible solar cell was obtained as in Example 1 except that the thickness of the aluminum oxide film and the ratio thereof were adjusted as shown in Table 1 by adjusting the thickness of the aluminum foil used and the treatment time or treatment temperature in anodization.

Comparative Examples 1, 2, 8, and 9

[0099] A flexible solar cell was obtained as in Example 1 except that the thickness of the aluminum oxide film and the ratio thereof were adjusted as shown in Table 1 by adjusting the thickness of the aluminum foil used and the treatment time in anodization.

Comparative Example 3

[0100] A flexible solar cell was obtained as in Example 1 except that a polyimide resin layer was formed instead of the aluminum oxide film by applying polyimide on the surface of the aluminum foil and firing the polyimide.

Comparative Example 4

[0101] A flexible solar cell was obtained as in Example 1 except that a PEN film was used instead of the flexible substrate including the aluminum foil and the aluminum oxide film.

Comparative Example 5

[0102] A flexible substrate was obtained as in Example 2 and a 300-nm-thick ITO film was formed by sputtering on the aluminum oxide film side of the flexible substrate.

[0103] Subsequently, a 50-nm-thick polyethylenedioxidethiophene:polystyrene sulfonate (PEDOT:PSS) as a hole transport layer was formed on the ITO film by a spin coating method.

[0104] To the obtained hole transport layer was then applied, by spin coating, a 4% by weight solution in chlorobenzene of a fullerene derivative (PC60BM) and a conductive polymer (PTB-7) mixed at a weight ratio of 1:1. Thus, a photoelectric conversion layer was obtained.

[0105] Subsequently, a solution of titaniumtetraisopropyl in ethanol was applied to the obtained photoelectric conversion layer by a spin coating method. Thus, a 10-nm-thick electron transport layer was obtained.

[0106] A 100-nm-thick ITO film as a transparent electrode was formed on the obtained electron transport layer by vacuum evaporation. Thus, a flexible solar cell was obtained.

Comparative Examples 6 and 7

[0107] A flexible solar cell was obtained as in Comparative Example 5 except that the same flexible substrate as that of Comparative Example 3 or 4 was used.

<Evaluation>

[0108] The flexible solar cells obtained in the examples and comparative examples were evaluated as follows. Table 1 shows the results.

(1) Initial Performance (Initial Conversion Efficiency)

[0109] A power source (236 model, available from Keithley Instruments Inc.) was connected between the electrodes of the flexible solar cell. The photoelectric conversion efficiency was measured at an exposure area of 0.01 cm.sup.2 using a solar simulator (available from Yamashita Denso Corp.) having an intensity of 100 mW/cm.sup.2, and the obtained photoelectric conversion efficiency was taken as the initial conversion efficiency.

∘ (Good): The initial conversion efficiency was 8% or higher.
x (Poor): The initial conversion efficiency was lower than 8%.

(2) Submodule Performance

[0110] Flexible solar cell submodules were produced using the same layer structures as those of the examples and comparative examples.

[0111] Each submodule was produced as follows: After the electrode was formed on the aluminum oxide film side of the flexible substrate, the electrode was divided into six sections by mechanical scribing. The production process after the division to the formation of the hole transport layer was the same as that of the corresponding example or comparative example. After the formation of the hole transport layer, mechanical scribing was performed to cut through the hole transport layer and other layers to expose the electrode. Then, the transparent layer was formed as in the corresponding example or comparative example. After formation of the transparent electrode, the transparent electrode was divided into six sections by mechanical scribing. Thus, the layers were patterned such that six cells were serially connected, whereby a submodule was obtained.

[0112] A power source (236 model, available from Keithley Instruments Inc.) was connected between the electrodes of the obtained flexible solar cell submodule. The photoelectric conversion efficiency was measured at an exposure area of 9 cm.sup.2 using a solar simulator (available from Yamashita Denso Corp.) having an intensity of 100 mW/cm.sup.2. The ratio of the obtained photoelectric conversion efficiency to the initial photoelectric conversion efficiency obtained in (1) was calculated. The evaluation was based on the following criteria.

∘∘∘ (Outstanding): The ratio was 0.8 or higher and 1 or lower to the initial conversion efficiency.
∘∘ (Excellent): The ratio was 0.6 or higher and lower than 0.8 to the initial conversion efficiency.
∘ (Good): The ratio was 0.4 or higher and lower than 0.6 to the initial conversion efficiency.
x (Poor): The ratio was lower than 0.4 to the initial conversion efficiency.

(3) Insulation Test

[0113] Resistance was measured with a tester at randomly selected 10 sites on the flexible substrate.

∘∘ (Excellent): The resistance did not reach 50 MΩ or higher at zero site.
∘ (Good): The resistance did not reach 50 MΩ or higher at one to three sites.
x (Poor): The resistance did not reach 50 MΩ or higher at four or more sites.

(4) Resistivity Test

[0114] An electrode containing aluminum and an electron transport layer were formed on the flexible substrate and fired at 200° C. for 10 minutes. The resistance was then measured with a tester.

∘∘ (Excellent): The resistance was lower than 10Ω.
∘ (Good): The resistance was 10Ω or higher and lower than 50Ω.
x (Poor): The resistance was 50Ω or higher.
(5) Corrosion of Photoelectric Conversion Layer Due to Contact with Aluminum Foil

[0115] The flexible solar cell submodule obtained above was put in a darkroom environment at a temperature of 25° C. and a dew-point of −10° C. for 150 hours. The photoelectric conversion layer containing the organic-inorganic perovskite compound was observed to determine the presence or absence of a portion that changed in color from black or brown to transparent or pale yellow in the photoelectric conversion layer.

∘ (Good): The discolored portion was absent.
x (Poor): The discolored portion was present.

(6) High-Temperature, High-Humidity Durability

[0116] A power source (236 model, available from Keithley Instruments Inc.) was connected between the electrodes of the obtained flexible solar cell submodule. The photoelectric conversion efficiency was measured at an exposure area of 9 cm.sup.2 using a solar simulator (available from Yamashita Denso Corp.) having an intensity of 100 mW/cm.sup.2. The obtained photoelectric conversion efficiency was taken as the initial conversion efficiency. Thereafter, the flexible solar cell submodule was placed in an environment at 85° C. and a humidity of 85% for 100 hours to perform a high-temperature, high-humidity durability test. The photoelectric conversion efficiency after the high-temperature, high-humidity durability test was measured in the same manner as the initial conversion efficiency. The value of (the photoelectric conversion efficiency after the high-temperature, high-humidity durability test)/(the initial conversion efficiency) was determined.

∘∘ (Excellent): The value of (the photoelectric conversion efficiency after the high-temperature, high-humidity durability test)/(the initial conversion efficiency) was 0.9 or higher.
∘ (Good): The value of (the photoelectric conversion efficiency after the high-temperature, high-humidity durability test)/(the initial conversion efficiency) was 0.8 or higher and lower than 0.9.
x (Poor): The value of (the photoelectric conversion efficiency after the high-temperature, high-humidity durability test)/(the initial conversion efficiency) was lower than 0.8.

TABLE-US-00001 TABLE 1 Flexible substrate Thickness Substrate Insulation layer of flexible Film substrate Thickness Thickness Crystal production (μm) Type Type (μm) ratio (%) structure method Example 1 100 A C 0.7 0.7 E G Example 2 103 A C 5 4.9 E G Example 3 105 A C 10 9.5 E G Example 4 108 A C 15 13.9 E G Exarnple 5 103 A C 5 4.9 F G Example 6 300 A C 1 0.3 E G Example 7 302 A C 3 1.0 E G Example 8 61 A C 2 3.3 E G Example 9 43 A C 5 11.6 E G Comparative 1000 A C 0.5 0.05 E G Example 1 Comparative 110 A C 20 18.2 E G Example 2 Comparative — A D 50 — — — Example 3 Comparative — B — — — — — Example 4 Comparative 103 A C 5 4.9 E G Example 5 Comparative — A D 50 — — — Example 6 Comparative — B — — — — — Example 7 Comparative 300 A C 0.2 0.07 E G Example 8 Comparative 55 A C 10 18.2 E G Example 9 Note *Total A: C: E: G: thickness Aluminum Aluminum Boehmite Anodization of foil oxide film F: aluminum B: D: Bayerite foil and PEN film Polyimide aluminum resin layer oxide film Evaluation High- Corrosion of temperature, Photoelectric photoelectric high- conversion Initial Submodule Insulation Resistivity conversion humidity layer performance performance test test layer durability Example 1 A o o o oo o o Example 2 A o ooo oo oo o o Example 3 A o oo oo oo o o Example 4 A o o oo o o o Exarnple 5 A o o oo o o o Example 6 A o oo oo oo o o Example 7 A o ooo oo oo o o Example 8 A o ooo oo oo o o Example 9 A o oo oo oo o o Comparative A x x x oo x o Example 1 Comparative A x x oo x x o Example 2 Comparative A o x o oo o x Example 3 Comparative A o x oo oo o x Example 4 Comparative B x ooo oo oo o o Example 5 Comparative B x o o oo o o Example 6 Comparative B x ooo oo oo o o Example 7 Comparative A x x x oo x o Example 8 Comparative A x x oo x x o Example 9 Note A: Organic- inorganic perovskite compound B: Conductive polymer and fullerene derivative

INDUSTRIAL APPLICABILITY

[0117] The present invention can provide a flexible solar cell having excellent high-temperature, high-humidity durability and excellent initial performance.

REFERENCE SIGNS LIST

[0118] 1 flexible solar cell [0119] 2 flexible substrate [0120] 3 electrode [0121] 4 transparent electrode [0122] 5 photoelectric conversion layer [0123] 6 aluminum foil [0124] 7 aluminum oxide film