SOLAR CELL

Abstract

The present invention aims to provide a solar cell excellent in photoelectric conversion efficiency, in which reduction in the photoelectric conversion efficiency due to continuous irradiation with light (photodegradation) is suppressed and the photoelectric conversion layer is less likely to suffer corrosion. The present invention provides a solar cell having a structure including: a cathode; an electron transport layer; a photoelectric conversion layer;

and an anode stacked in the stated order, the photoelectric conversion layer including an organic-inorganic perovskite compound represented by the formula: R-M-X.sub.3 where R represents an organic molecule, M represents a metal atom, and X represents a halogen or chalcogen atom, the cathode being formed of a titanium material and having an oxide layer on at least one surface.

Claims

1. A solar cell having a structure comprising: a cathode; an electron transport layer; a photoelectric conversion layer; and an anode stacked in the stated order, the photoelectric conversion layer comprising an organic-inorganic perovskite compound represented by the formula: R-M-X.sub.3 where R represents an organic molecule, M represents a metal atom, and X represents a halogen or chalcogen atom, the cathode being formed of a titanium material and having an oxide layer on at least one surface.

2. The solar cell according to claim 1, wherein the titanium material is titanium metal, a mixture of titanium metal and another metal, or a titanium alloy.

3. The solar cell according to claim 1, wherein the oxide layer has a thickness of 1 nm or more and 1,000 nm or less.

4. The solar cell according to claim 1, wherein the oxide layer includes a gradient oxide layer in which the ratio of titanium atoms to oxygen atoms gradiently increases in the thickness direction toward a portion formed of the titanium material.

5. The solar cell according to claim 4, wherein the gradient oxide layer has a thickness of 5 nm or more and 150 nm or less.

6. The solar cell according to claim 2, wherein the oxide layer has a thickness of 1 nm or more and 1,000 nm or less.

7. The solar cell according to claim 2, wherein the oxide layer includes a gradient oxide layer in which the ratio of titanium atoms to oxygen atoms gradiently increases in the thickness direction toward a portion formed of the titanium material.

8. The solar cell according to claim 3, wherein the oxide layer includes a gradient oxide layer in which the ratio of titanium atoms to oxygen atoms gradiently increases in the thickness direction toward a portion formed of the titanium material.

9. The solar cell according to claim 6, wherein the oxide layer includes a gradient oxide layer in which the ratio of titanium atoms to oxygen atoms gradiently increases in the thickness direction toward a portion formed of the titanium material.

10. The solar cell according to claim 7, wherein the gradient oxide layer has a thickness of 5 nm or more and 150 nm or less.

11. The solar cell according to claim 8, wherein the gradient oxide layer has a thickness of 5 nm or more and 150 nm or less.

12. The solar cell according to claim 9, wherein the gradient oxide layer has a thickness of 5 nm or more and 150 nm or less.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0071] FIG. 1 is a schematic view illustrating an exemplary crystal structure of an organic-inorganic perovskite compound.

[0072] FIG. 2 is a cross-sectional view schematically illustrating an exemplary solar cell of the present invention.

DESCRIPTION OF EMBODIMENTS

[0073] Embodiments of the present invention are more specifically described in the following with reference to, but not limited to, examples.

Example 1

[0074] A titanium oxide paste prepared by dispersing titanium oxide nanoparticles (mixture of particles with an average particle size of 10 nm (F6A available from Showa Denko K.K.) and particles with an average particle size of 30 nm (F4A available from Showa Denko K.K.)) in ethanol was applied by spin coating to a surface of a titanium metal thin film (with a naturally oxidized surface) having a thickness of 200 nm as a cathode, followed by irradiation with UV light at 80 mW/cm.sup.2 for one minute. Thus, a porous electron transport layer was formed.

[0075] Separately, lead iodide was reacted with dimethyl sulfoxide (DMSO) in advance to prepare a lead iodide-dimethyl sulfoxide complex. The lead iodide-dimethyl sulfoxide complex was dissolved in N,N-dimethylformamide (DMF) to obtain a 60% by weight coating solution. The resulting coating solution was applied to the electron transport layer by spin coating to a thickness of 200 nm, and an isopropanol solution of methyl ammonium iodide (CH.sub.3NH.sub.3I) adjusted to 8% was further applied thereto by spin coating to react the methyl ammonium iodide with lead iodide, followed by firing at 180 C. Thus, a photoelectric conversion layer containing an organic-inorganic perovskite compound was formed.

[0076] Next, a solution was prepared by dissolving, in 25 L of chlorobenzene, 68 mM of Spiro-OMeTAD (having a spirobifluorene skeleton), 55 mM of t-butylpyridine, and 9 mM of bis(trifluoromethylsulfonyl)imide-lithium salt. The solution was applied by spin coating to form a hole transport layer.

[0077] On the obtained hole transport layer was formed an ITO film having a thickness of 100 nm as an anode by electron beam deposition. Thus, a solar cell in which a cathode, an electron transport layer, a photoelectric conversion layer, a hole transport layer, and an anode were stacked was obtained.

Examples 2 to 5

[0078] A solar cell was obtained in the same manner as in Example 1, except that the cathode was heated under the firing temperature condition as shown in Table 1 before formation of the porous electron transport layer.

Example 6

[0079] A solar cell was obtained in the same manner as in Example 5, except that the thickness of the titanium metal thin film as a cathode was changed as shown in Table 1.

Examples 7 and 8

[0080] A solar cell was obtained in the same manner as in Example 1, except that the thickness of the titanium metal thin film as a cathode was changed as shown in Table 1 and the cathode was heated under the firing temperature condition as shown in Table 1 before formation of the porous electron transport layer.

Example 9

[0081] A solar cell was obtained in the same manner as in Example 1, except that a thin-film titanium oxide electron transport layer (thickness of 30 nm) was formed by RF sputtering on the surface of the cathode before formation of the porous electron transport layer.

Examples 10 and 11

[0082] A solar cell was obtained in the same manner as in Example 1, except that the cathode was heated under the firing temperature condition as shown in Table 1 and a thin-film titanium oxide electron transport layer (thickness of 30 nm) was formed by RF sputtering before formation of the porous electron transport layer.

Example 12

[0083] A solar cell was obtained in the same manner as in Example 1, except that a metal thin film as shown in Table 1 was used, instead of the titanium metal thin film as a cathode.

Examples 13 and 14

[0084] A solar cell was obtained in the same manner as in Example 12, except that the cathode was heated under the firing temperature condition as shown in Table 1 before formation of the porous electron transport layer.

Example 15

[0085] A solar cell was obtained in the same manner as in Example 12, except that the cathode was heated under the firing temperature condition as shown in Table 1 and then a thin-film titanium oxide electron transport layer (thickness of 30 nm) was formed by RF sputtering on the surface of the cathode before formation of the porous electron transport layer.

Examples 16 to 19

[0086] A solar cell was obtained in the same manner as in Example 1, except that a metal thin film as shown in Table 1 was used, instead of the titanium metal thin film as a cathode.

Comparative Example 1

[0087] A substrate having a titanium metal thin film with a thickness of 200 nm was placed in a glove box in which an oxygen concentration was 10 ppm or less and subjected to hydrofluoric acid treatment in the glove box, whereby an oxidation film on the surface of the titanium metal thin film was removed. Then, a solar cell was obtained in the same manner as in Example 1, except that the operations were performed in the glove box.

Comparative Examples 2 to 5

[0088] A solar cell was obtained in the same manner as in Example 1, except that a metal thin film as shown in Table 2 was used, instead of the titanium metal thin film as a cathode.

Comparative Example 6

[0089] A solar cell was obtained in the same manner as in Comparative Example 3, except that a thin-film titanium oxide electron transport layer (thickness of 30 nm) was formed by RF sputtering on the surface of the cathode before formation of the porous electron transport layer.

Comparative Example 7

[0090] A solar cell was obtained in the same manner as in Example 3, except that the cathode was heated under the firing temperature condition as shown in Table 2 and then a thin-film titanium oxide electron transport layer (thickness of 30 nm) was formed by RF sputtering on the surface of the cathode before formation of the porous electron transport layer.

Comparative Examples 8 to 29

[0091] A solar cell was obtained in the same manner as in Example 1, except that a metal thin film as shown in Table 2 was used, instead of the titanium metal thin film as a cathode.

Comparative Example 30

[0092] A solar cell was obtained in the same manner as in Example 1, except that a transparent electrode thin film as shown in Table 2 was used, instead of the titanium metal thin film as a cathode and an Au film (thickness of 100 nm) was formed as an anode on the hole transport layer by resistance heating deposition.

Comparative Example 31

[0093] A solar cell was obtained in the same manner as in Comparative Example 30, except that a thin-film titanium oxide electron transport layer (thickness of 30 nm) was formed on the surface of the cathode by RF sputtering before formation of the porous electron transport layer.

Comparative Examples 32 to 34

[0094] A solar cell was obtained in the same manner as in Example 1, except that a transparent electrode thin film as shown in Table 2 was used, instead of the titanium metal thin film as a cathode and an Au film (thickness of 100 nm) was formed as an anode on the hole transport layer by resistance heating deposition.

Comparative Example 35

[0095] A solar cell was obtained in the same manner as in Comparative Example 34, except that a thin-film titanium oxide electron transport layer (thickness of 30 nm) was formed on the surface of the cathode by RF sputtering before formation of the porous electron transport layer.

Evaluation

[0096] The solar cells obtained in the examples and comparative examples were evaluated as follows. Tables 1 and 2 show the results.

Confirmation of Oxide Layer and Measurement of Thickness Thereof

[0097] The cathode of each obtained solar cell was subjected to X-ray photoelectron spectroscopy (XPS), while Ar sputtering was performed for etching in the thickness direction (depth direction). The ratio of signals of titanium (Ti) to signals of oxygen (O) was increased and then became constant. The thickness (depth) at which the ratio became constant from the surface of the cathode was measured, and the obtained thickness was taken as the thickness of the oxide layer. Also, the region where the ratio of the signals of titanium (Ti) to the signals of oxygen (O) gradiently increased was measured as the thickness of a gradient oxide layer.

Measurement of Photoelectric Conversion Efficiency

[0098] A power source (model 236 available from Keithley Instruments Inc.) was connected between the electrodes of each solar cell. The solar cell was irradiated with light at an intensity of 100 mW/cm.sup.2 using a solar simulator (Yamashita Denso Corp.) and the photoelectric conversion efficiency was measured. The resulting photoelectric conversion efficiency values were standardized with the photoelectric conversion efficiency of the solar cell obtained in Example 1 set to 1.

Photodegradation Test

[0099] A power source (model 236 available from Keithley Instruments Inc.) was connected between the electrodes of each solar cell. The solar cell was irradiated with light at an intensity of 100 mW/cm.sup.2 using a solar simulator (Yamashita Denso Corp.). The photoelectric conversion efficiency was measured right after the start of the irradiation with light and after irradiation with light for one hour. The maintenance rate of the photoelectric conversion performance after the irradiation with light (photoelectric conversion efficiency after irradiation with light for one hour/photoelectric conversion efficiency right after the start of the irradiation with light) was obtained.

Evaluation of Corrosion in Photoelectric Conversion Layer

[0100] An evaluation sample was prepared by forming a photoelectric conversion layer containing an organic-inorganic perovskite compound on a cathode and firing the photoelectric conversion layer at 180 C. in the same manner as in each of the examples and comparative examples. The evaluation sample in which the color of the photoelectric conversion layer was changed from brown (original color of the organic-inorganic perovskite compound) was rated x (Poor). The evaluation sample in which the color of the photoelectric conversion layer was kept brown was rated 0 (Good).

Comprehensive Evaluation

[0101] When the solar cell had a photoelectric conversion efficiency higher than that of the solar cell obtained in Example 1, kept a maintenance rate of the photoelectric conversion performance of 90% or higher in the photodegradation test, and was rated 0 (Good) in the evaluation of corrosion in the photoelectric conversion layer, such a solar cell was rated 0 (Good). The solar cell that failed to satisfy any one of these parameters was rated x (Poor).

TABLE-US-00001 TABLE 1 Titanium oxide-containing Cathode oxide layer Gradient oxide layer Thickness Firing Presence or Thickness Presence or Thickness Type (nm) temperature absence (nm) absence (nm) Example 1 Ti 200 Not fired Present 4 Present 4 Example 2 Ti 200 100 C. Present 6 Present 6 Example 3 Ti 200 200 C. Present 11 Present 11 Example 4 Ti 200 300 C. Present 35 Present 35 Example 5 Ti 200 400 C. Present 80 Present 60 Example 6 Ti 500 400 C. Present 80 Present 60 Example 7 Ti 500 500 C. Present 200 Present 100 Example 8 Ti 500 600 C. Present 280 Present 120 Example 9 Ti 200 Not fired Present 33 Present 3 Example 10 Ti 200 100 C. Present 36 Present 6 Example 11 Ti 200 200 C. Present 41 Present 11 Example 12 Al/Ti 200/100 Not fired Present 3 Present 3 Example 13 Al/Ti 200/100 100 C. Present 6 Present 6 Example 14 Al/Ti 200/100 200 C. Present 11 Present 11 Example 15 Al/Ti 200/100 200 C. Present 41 Present 11 Example 16 Co/Ti 200/100 Not fired Present 3 Present 3 Example 17 Cr/Ti 200/100 Not fired Present 3 Present 3 Example 18 Mo/Ti 200/100 Not fired Present 3 Present 3 Example 19 W/Ti 200/100 Not fired Present 3 Present 3 Maintenance rate of Thin-film electron photoelectric conversion Corrosion in transport layer Photoelectric performance after photoelectric Thickness conversion irradiation with light conversion Comprehensive Composition (nm) efficiency (%) Rating layer evaluation Example 1 Absent 1 90 Example 2 Absent 1.2 95 Example 3 Absent 1.4 96 Example 4 Absent 1.5 98 Example 5 Absent 1.3 98 Example 6 Absent 1.5 98 Example 7 Absent 1.4 98 Example 8 Absent 1.1 98 Example 9 Present 30 1.4 90 Example 10 Present 30 1.5 95 Example 11 Present 30 1.6 96 Example 12 Absent 1.4 90 Example 13 Absent 1.5 94 Example 14 Absent 1.7 96 Example 15 Present 30 2 97 Example 16 Absent 1.2 91 Example 17 Absent 1.2 91 Example 18 Absent 1 93 Example 19 Absent 1.1 93

TABLE-US-00002 TABLE 2 Titanium oxide-containing Cathode oxide layer Gradient oxide layer Thickness Firing Presence or Thickness Presence or Thickness Type (nm) temperature absence (nm) absence (nm) Comparative Example 1 Ti 200 Not fired Absent Absent Comparative Example 2 Mo 500 Not fired Absent Absent Comparative Example 3 Al/Mo 200/100 Not fired Absent Absent Comparative Example 4 Co/Mo 200/100 Not fired Absent Absent Comparative Example 5 Cr/Mo 200/100 Not fired Absent Absent Comparative Example 6 Al/Mo 200/100 Not fired Absent 30 Absent Comparative Example 7 Al/Mo 200/100 200 C. Absent 30 Absent Comparative Example 8 Al 200 Not fired Absent Absent Comparative Example 9 Cr 200 Not fired Absent Absent Comparative Example 10 Co 200 Not fired Absent Absent Comparative Example 11 Zn 200 Not fired Absent Absent Comparative Example 12 Al/Zn 200/100 Not fired Absent Absent Comparative Example 13 Co/Zn 200/100 Not fired Absent Absent Comparative Example 14 Cr/Zn 200/100 Not fired Absent Absent Comparative Example 15 Mo/Zn 200/100 Not fired Absent Absent Comparative Example 16 W/Zn 200/100 Not fired Absent Absent Comparative Example 17 Cu 200 Not fired Absent Absent Comparative Example 18 Ni 200 Not fired Absent Absent Comparative Example 19 Zr 200 Not fired Absent Absent Comparative Example 20 Nb 200 Not fired Absent Absent Comparative Example 21 Ag 200 Not fired Absent Absent Comparative Example 22 In 200 Not fired Absent Absent Comparative Example 23 Sn 200 Not fired Absent Absent Comparative Example 24 Sb 200 Not fired Absent Absent Comparative Example 25 Hf 200 Not fired Absent Absent Comparative Example 26 Ta 200 Not fired Absent Absent Comparative Example 27 W 200 Not fired Absent Absent Comparative Example 28 Pb 200 Not fired Absent Absent Comparative Example 29 Bi 200 Not fired Absent Absent Comparative Example 30 ITO 200 Not fired Absent Absent Comparative Example 31 ITO 200 Not fired Absent 30 Absent Comparative Example 32 GZO 200 Not fired Absent Absent Comparative Example 33 SUS 500 Not fired Absent Absent Comparative Example 34 FTO 500 Not fired Absent Absent Comparative Example 35 FTO 500 Not fired Absent 30 Absent Maintenance rate of Thin-film electron photoelectric conversion Corrosion in transport layer Photoelectric performance after photoelectric Thickness conversion irradiation with light conversion Comprehensive Composition (nm) efficiency (%) Rating layer evaluation Comparative Example 1 Absent 0 x Comparative Example 2 Absent 0 x Comparative Example 3 Absent 0 x Comparative Example 4 Absent 0 x Comparative Example 5 Absent 0 x Comparative Example 6 Present 30 0.2 x Comparative Example 7 Present 30 0.2 x Comparative Example 8 Absent 0.2 x x Comparative Example 9 Absent 0.1 x x Comparative Example 10 Absent 0.1 x x Comparative Example 11 Absent 0 x x Comparative Example 12 Absent 0 x x Comparative Example 13 Absent 0 x x Comparative Example 14 Absent 0 x x Comparative Example 15 Absent 0 x x Comparative Example 16 Absent 0 x x Comparative Example 17 Absent 0 x x Comparative Example 18 Absent 0 x Comparative Example 19 Absent 0.2 x Comparative Example 20 Absent 0.1 x Comparative Example 21 Absent 0 x x Comparative Example 22 Absent 0 x x Comparative Example 23 Absent 0 x x Comparative Example 24 Absent 0 x x Comparative Example 25 Absent 0.1 x Comparative Example 26 Absent 0.1 x Comparative Example 27 Absent 0.2 x Comparative Example 28 Absent 0 x Comparative Example 29 Absent 0 x x Comparative Example 30 Absent 0.8 44 x x Comparative Example 31 Present 30 1.5 78 x x Comparative Example 32 Absent 0.5 33 x x Comparative Example 33 Absent 0 x x Comparative Example 34 Absent 0.7 66 x x Comparative Example 35 Present 30 1.8 73 x x

INDUSTRIAL APPLICABILITY

[0102] The present invention can provide a solar cell excellent in photoelectric conversion efficiency, in which reduction in the photoelectric conversion efficiency due to continuous irradiation with light (photodegradation) is suppressed and the photoelectric conversion layer is less likely to suffer corrosion.

REFERENCE SIGNS LIST

[0103] 1: Solar cell

[0104] 2: Cathode

[0105] 3: Electron transport layer

[0106] 31: Thin-film electron transport layer

[0107] 32: Porous electron transport layer

[0108] 4: Photoelectric conversion layer containing an organic-inorganic perovskite compound

[0109] 5: Hole transport layer

[0110] 6: Anode (patterned electrode)