WO2016209005A1 - Perovskite-based solar cell using graphene as conductive transparent electrode - Google Patents

Abstract

The present invention relates to a perovskite-based solar cell using graphene as a conductive transparent electrode. The perovskite-based solar cell has improved peak efficiency through an appropriate energy band combination of a graphene electrode/hole transfer layer/perovskite/electron transfer layer/metal electrode, and may represent 17% or more of the conversion efficiency value.

Description

Perovskite based solar cell using graphene as conductive transparent electrode

The present invention relates to an energy device including a solar cell, and more particularly, to replace an existing ITO and FTO transparent conductive oxide electrode which is fragile with a flexible graphene electrode and to use perovskite as an absorber. It relates to a battery.

Organic / inorganic composite perovskite has a high absorption coefficient, balanced electron / hole transport, low temperature treatment, small exciton binding energy and longer exciton diffusion lengths than organic semiconductor materials, resulting in a photoactive layer of the solar cell. It is a promising material used. High performance perovskite solar cells typically employ a nip architecture consisting of metal oxide / perovskite material / hole transport material such as TiO ₂ or Al ₂ O ₃ . However, the selection of the substrate is limited by the high temperature process of 450 ° C. or higher for the manufacture of the metal oxide thin film, and the manufacturing cost is increased due to this limitation. Organic materials used as an alternative to metal oxide layers have been utilized in pin-type perovskite solar cells as well as common nip structures. Poly (3,4-ethylenedioxythiophene): poly (styrenesulfonic acid) (PEDOT: PSS) and [6, 6] -phenyl C61-butyric acid methyl ester (PCBM), which are usually solution-processable, are each a hole transport layer (HTL). And electron transport layers (ETLs). Recently, 18.1% power conversion efficiency (PCE) has been achieved by a pin device comprising indium tin oxide (ITO) / PEDOT: PSS / CH ₃ NH ₃ PbI ₃ (MAPbI ₃ ) / PCBM / Au. This is still low compared to the nip device which shows efficiency of 20% or more in recent years by using a scaffold metal oxide as the electron transport layer (ETL). Nevertheless, pin perovskite solar cells have been studied for their advantages such as low hysteresis behavior, low processing temperature and easy manufacturing process.

In the case of a pin structure flexible perovskite substrate solar cell using a low temperature process, ITO is mainly coated on polyethylene naphthalate (PEN) film and used as an electrode, and has shown an efficiency of up to 12.2% up to now. It is reported that the decrease in efficiency of the solar cell is due to the breakability of the ITO layer having high mechanical brittleness. On the other hand, in the field of organic solar cell (OPV), flexible conductive electrodes such as graphene, carbon nanotubes, metal lattice and conductive polymers are already being used as a substitute for brittle transparent conducting oxide (TCO) applicable to flexible solar cells. Much has been studied. Among them, graphene and single layer 2D carbon materials, which are optically very transparent (about 97% in the visible region), mechanically robust, flexible and stretchable, are the most promising candidates. In the prior art, there is an example in which graphene is used as a conductive transparent electrode in a fuel-sensitized solar cell or an organic solar cell. Among the solar cells using a graphene transparent electrode reported so far, 8.48% of the highest efficiency is obtained in tandem polymer solar cells. This tandem type of structure is disadvantageous due to the large number of constituent layers, and is still lower than TCO-free perovskite solar cells showing PCE 11.0%. There are examples of graphene electrodes used in perovskite devices in recent studies, but in these studies, graphene was used as the top electrode, not as a replacement for conventional TCO electrodes.

Recently, a solar cell formed on a flexible substrate is attracting attention as a major development field of the next generation solar cell technology. For stable operation of the flexible solar cell, it is necessary to form a constituent layer of the solar cell with a less fragile material. The conductive transparent electrode widely used in solar cells is a conductive transparent oxide of indium tin oxide (ITO) or fluorine doped tin oxide (FTO). However, the material is fragile, and as the solar cell repeatedly bends, it causes a decrease in efficiency of the solar cell.

Graphene is less conductive than conductive transparent oxides, but is a readily available material, and has excellent characteristics in light transmittance, mechanical strength, and flexibility.

The organometallic halide perovskite material has a high leap forward in the efficiency increase of 3rd generation solar cells due to its high light absorption and charge mobility (at least 20% efficiency). Suitable for use in

In the present invention, in order to overcome the disadvantages of the prior art as described above, the solar cell using a permeate of the perovskite and replace the conventional ITO and FTO transparent conductive oxide electrode with a flexible and flexible graphene electrode and It aims at providing the manufacturing method.

In order to achieve the above object, the present invention provides a perovskite-based solar cell comprising a graphene layer as a conductive transparent electrode.

The conductive transparent electrode made of the graphene layer may be a transparent front electrode.

In addition, the solar cell may be one in which a transparent anode electrode, a hole transport layer, a perovskite layer, an electron transport layer and a cathode electrode made of a graphene layer are sequentially stacked on a substrate.

In the solar cell, a transparent cathode electrode, an electron transport layer, a perovskite layer, a hole transport layer, and an anode electrode made of a graphene layer may be sequentially stacked on a substrate.

In addition, the solar cell may further include a metal oxide layer deposited on the transparent anode electrode or the transparent cathode electrode made of the graphene layer.

The metal oxide layer is MoO ₃ , NiO, CoO and TiO ₂ It may be to include one or more selected from the group consisting of.

The metal oxide layer may have a thickness of about 0.5 nm to about 6 nm.

The perovskite may be a perovskite device using a lead halide adduct compound.

The lead halide adduct compound may be represented by Formula 1 below.

[Formula 1]

A PbY · ₂ · Q

In the above formula,

A is an organic halide compound or an inorganic halide compound,

Y is F ^- is a halogen ion, ^-, Cl ^-, Br ^- or I

Q is a Lewis base compound containing a functional group having an electron-pair donor as an atom having a non-covalent electron pair, and the peak of the FT-IR of the functional group is 1 to 10 cm ^- from the compound of Formula 1 to the compound of Formula 2 below. Red shifted by ¹ appears,

[Formula 2]

PbY ₂ · Q

Y and Q are the same as defined for the formula (1).

A may be CH ₃ NH ₃ I, CH (NH ₂ ) ₂ I or CsI.

The perovskite may be made by heating and drying the adduct compound to remove the Lewis base compound included in the adduct compound.

The present invention has successfully implemented a perovskite-based solar cell using graphene as a conductive transparent electrode, and has the highest efficiency through proper energy band combination of graphene electrode / hole transport layer / perovskite / electron transport layer / metal electrode. 17.1% was achieved. This is the highest efficiency of graphene electrode-based solar cell efficiency reported so far, and other transparent conductive electrodes (metal thin films or conductive organic materials such as PEDOT: PSS) to replace transparent conductive oxide electrodes such as ITO and FTO in addition to graphene electrodes. It is the highest efficiency among the solar cell using. In addition, if the structure of the present invention is formed on a polymer substrate of graphene-coated PET, PEN, etc., high efficiency flexible solar cell is expected to be less efficient in repetitive bending as well as the existing ITO electrode. It is expected to be used for the development of flexible devices in optoelectronic devices (for example, optical sensors and light emitting diodes) and perovskite-based memory devices.

1 is a graph showing conversion efficiency (PCE) of a perovskite-based solar cell using graphene as a conductive transparent electrode according to the present invention.

2 is a schematic diagram of a perovskite-based solar cell using graphene as a conductive transparent electrode.

Figure 3 shows (a) graphene, (b) 1 nm MoO ₃ deposited graphene, (c) 2 nm MoO ₃ deposited graphene, (d) ITO, (e) UVO treated ITO and (f) UVO PEDOT: PSS droplets on ITO covered with 1 nm MoO ₃ after treatment.

Figure 4 is (a) graphene (b) graphene / 1 nm MoO ₃ (c) SEM image of graphene / 2 nm MoO ₃ is shown (scale bar 100 nm).

5 is (a) graphene / 2 nm MoO ₃ Electrode and (b) ITO / 1 nm MoO ₃ SEM images measured in SE mode (left) and BSE mode (right) of the cross section of the device with electrodes are shown.

6 is a graph showing the relationship between MoO ₃ thickness and average PCE of (a) graphene electrode and (b) ITO electrode, (c) JV curve showing the highest performance of Example 2 and (d) Comparative Example 2 device, (e) the relationship between sheet resistance and MoO ₃ thickness of graphene and ITO, and (f) the transmittance of graphene and ITO with or without 2 nm thick MoO ₃ layer.

7 shows the measured JV curves of (a) Example 1 and (b) Example 2. FIG.

8 shows PCE histograms of the Example 2 and Comparative Example 2 electrodes.

9 shows EQE spectra (black lines) and J _cs (blue lines) at the highest performance of (a) Example 2 and (b) Comparative Example 2 electrodes.

10 shows the transmission of glass / graphene and glass / ITO with or without a 2 nm thick MoO ₃ layer.

11 is MoO ₃ of various thickness A diagram showing the UPS spectrum and calculated work function of (a) ITO and (b) graphene, including layers, and (c) a schematic energy level of the structure.

12 shows (a) graphene, (b) Example 2, (c) Example 2 / PEDOT: PSS, (d) Comparative Example 1, (e) Comparative Example 2 and (f) Comparative Example 2 / PEDOT: An AFM image (3 μm × 3 μm) of the PSS is shown.

FIG. 13 shows planar SEM images of MAPbI ₃ perovskite films prepared on (a) Example 2 / PEDOT: PSS and (b) Comparative Example 2 / PEDOT: PSS. (c) and (d) show high magnification images of (a) and (b).

(A) is FT-IR spectrum of DMSO (solution), PbI ₂ DMSO (powder) and MAI PbI ₂ DMSO (powder), (b) MAI PbI ₂ DMSO (powder) and FAI This is a comparison of the FT-IR spectra of PbI ₂ DMSO (powder).

15 is an FT-IR spectrum of PbI ₂ · TU (powder) and FAI · PbI ₂ · TU (powder).

As the invention allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the written description. However, this is not intended to limit the present invention to specific embodiments, it should be understood to include all transformations, equivalents, and substitutes included in the spirit and scope of the present invention. In the following description of the present invention, if it is determined that the detailed description of the related known technology may obscure the gist of the present invention, the detailed description thereof will be omitted.

Hereinafter, the present invention will be described in detail.

The present invention provides a perovskite-based solar cell comprising a graphene layer as a conductive transparent electrode.

According to a preferred embodiment, the conductive transparent electrode made of the graphene layer may be a transparent front electrode.

In addition, the solar cell may be one in which a transparent anode electrode, a hole transport layer, a perovskite layer, an electron transport layer and a cathode electrode made of a graphene layer are sequentially stacked on a substrate.

In the solar cell, a transparent cathode electrode, an electron transport layer, a perovskite layer, a hole transport layer, and an anode electrode made of a graphene layer may be sequentially stacked on a substrate.

According to an embodiment of the present invention, there is provided a solar cell in which a transparent anode electrode, a hole injection layer (metal oxide layer), a hole transport layer, a perovskite layer, an electron transport layer and a cathode electrode are sequentially stacked on a substrate.

Here, graphene is a two-dimensional carbon allotrope, and a method of manufacturing the same is a peeling method for physically separating a layer of graphene from graphite, and chemical oxidation to obtain graphene by dispersing the graphite in a dispersion and chemically reducing it. / Reduction method, pyrolysis method of obtaining graphene layer through high temperature pyrolysis on silicon carbide (SiC) substrate, and chemical vapor deposition method, among which chemical vapor deposition can be exemplified as a method for synthesizing high quality graphene. . Although not limited, graphene prepared by a chemical vapor deposition method is preferred. In this embodiment, a single layer graphene is used, but not limited thereto, and multilayer graphene may also be used.

The thickness of the graphene layer may be 0.01 to 35 nm, preferably 0.01 to 3.5 nm, more preferably 0.01 to 0.35 nm.

According to one embodiment, the graphene may exhibit a characteristic aspect ratio of 0.1 or less, graphene layer number of 100 or less, and a specific surface area of 300 m ² / g or more. The graphene refers to a single mesh plane of SP ² bonds of carbon (C) in graphite hcp structure, and recently, graphene composite layers having a plurality of layers are also classified as graphene in a broad sense.

The method of transferring the graphene to the substrate is not particularly limited and will not be described in detail because it follows a general method known in the art.

According to the present invention, an electron transport layer may be directly formed on the transparent cathode electrode (graphene layer), or a hole transport layer may be directly formed on the transparent anode electrode (graphene layer), but a metal oxide layer may be deposited on the graphene layer. It is possible. In this case, when the graphene layer is a transparent anode electrode, the metal oxide layer may serve as a hole injection layer, and when the graphene layer is a transparent cathode electrode, the metal oxide layer may serve as an electron injection layer.

According to one embodiment, the metal oxide layer may be deposited to make the graphene layer wettable. The metal oxide layer is MoO ₃ , NiO, CoO and TiO ₂ It may include one or more selected from the group consisting of, but is not limited thereto, and any material known in the art may be used without limitation. The metal oxide layer may have a thickness of about 0.5 nm to about 6 nm.

Preferably from about 1 nm to about 6 nm, more preferably from about 1 nm to about 4 nm, even more preferably from about 2 to 3 nm.

On the other hand, the hole transport layer is poly (3,4-ethylenedioxythiophene) polystyrene sulfone (PEDOT: PSS); Tetrafluoro-tetracyano-quinodimethane (CuPc: F ₄ -TCNQ); And PEDOT: PSS includes tungsten oxide (WO _x ), graphene oxide (GO), carbon nanotubes (CNT), molybdenum oxide (MoO _x ), vanadium oxide (V ₂ O ₅ ) and nickel oxide (NiO _x ) At least one selected from the group may be selected from the blended, but is not limited thereto, and any material used in the art may be used without limitation. For example, it may be one containing a hole transporting monomolecular material or a hole transporting polymer material. Spiro-MeOTAD [2,2 ', 7,7'-tetrakis- (N, N-di-p-methoxyphenyl-amine) -9,9'-spirobifluoren] may be used as the hole-transport monomolecular substance. P3HT [poly (3-hexylthiophene)] may be used as the hole transport polymer material. In addition, the hole transport layer may include a doping material, and the doping material may be selected from the group consisting of Li-based dopants, Co-based dopants, and combinations thereof, but is not limited thereto. For example, a mixture of spiro-MeOTAD, 4-tert-butyl pyridine (tBP), and Li-TFSI may be used as a material constituting the hole transport layer.

PEDOT: PSS used as the hole transport layer used in the preferred embodiment of the present invention is a polystyrene sulfonate (PSS) gel in which PEDOT (poly (3,4-ethylenedioxythiophene) (poly (3,4-ethylenedioxythiophene) 5-10 is polymerized is dispersed in an aqueous solution) In addition, the electron transport layer is at least one selected from the group consisting of fullerenes, bathocuproine (BCP) and fullerene derivatives or selected from TiO ₂ , ZnO, SrTi0 ₃ , WO ₃ or mixtures thereof. A metal oxide may be used, and any material used in the industry may be used without limitation.

Examples of fullerene derivatives include, but are not limited to, Phenyl-C61-butyric acid methyl ester (PCBM). According to a preferred embodiment, C60 / BCP can be used. In addition, the metal electrode or the back electrode used as the cathode or anode electrode is Pt, Au, Al, Ni, Cu, Ag, In, Ru, Pd, Rh, Ir, Os and C and combinations thereof It may be selected, but is not limited to these, any material known in the art can be used without limitation.

According to another embodiment of the present invention, the perovskite may be a lead halide adduct compound.

The lead halide adduct compound may be represented by Formula 1 below.

[Formula 1]

A PbY · ₂ · Q

In the above formula,

A is an organic halide compound or an inorganic halide compound,

Y is F ^- is a halogen ion, ^-, Cl ^-, Br ^- or I

Q is a Lewis base compound containing a functional group having an electron-pair donor as an atom having an unshared electron pair, and the peak of the FT-IR of the functional group is 1 to 10 cm ^- from the compound of Formula 1 to the compound of Formula 2 below. Red shifted by ¹ appears,

[Formula 2]

PbY ₂ · Q

Y and Q are the same as defined for the formula (1).

A may be CH ₃ NH ₃ I, CH (NH ₂ ) ₂ I or CsI.

The perovskite may be made by heating and drying the adduct compound to remove the Lewis base compound included in the adduct compound.

Perovskite preparation using the adduct compound may be referred to the Korean Patent Application Nos. 2015-0090139 and 2015-0164744, the other patent applications of the researchers, the entire contents of which are incorporated herein by reference.

According to one embodiment to the A in the formula (1) formula (3) or an organic cation represented by the formula (4) or Cs ⁺ cation and F ^-, Cl ^-, Br ^- and I ^- organic or inorganic consisting of a combination of a halogen ion is selected from It may be a halide compound.

 [Formula 3]

(R ₁ R ₂ N = CH-NR ₃ R ₄ ) ⁺

In the above formula,

R ₁ , R ₂ , R ₃ and R ₄ are independently selected from hydrogen and unsubstituted or substituted C1-C6 alkyl,

[Formula 4]

(R ₅ R ₆ R ₇ R ₈ N) ⁺

In the above formula,

R ₅ , R ₆ , R ₇ and R ₈ are hydrogen, unsubstituted or substituted C1-C20 alkyl or unsubstituted or substituted aryl.

More specifically, the A may be selected from CH ₃ NH ₃ I (MAI, Methyl Ammonium Iodide, methyl ammonium iodide), CH (NH ₂ ) ₂ I (FAI, Formamidinium Iodide, formamidinium iodine) or CsI have.

Q is a Lewis base compound having a functional group in which nitrogen (N), oxygen (O) or sulfur (S) atoms are electron pair donors, and more specifically Q is nitrogen, oxygen, sulfur atoms in electron pair donors Thioamide group, thiocyanate group, thioether group, thioketone group, thiol group, thiophene group, thiourea group, thiosulfate group, thioacetamide group, carbonyl group, aldehyde group, carboxyl group, ether group, Ester group, sulfonyl group, sulfo group, sulfinyl group, thiocyanato group, pyrrolidinone group, peroxy group, amide group, amine group, amide group, imide group, imine group, azide group, pyridine group, pyrrole group, nitro It may be a Lewis base compound containing at least one functional group selected from the group consisting of a group, a nitroso group, a cyano group, a nitrooxy group, and an isocyano group, and a thioamide group having a S atom as an electron pair donor, Ney Compounds containing at least one selected from the group consisting of a group, a thioether group, a thioketone group, a thiol group, a thiophene group, a thiourea group, a thioacetamide group and a thiosulfate group as a functional group have stronger bonds with lead halides. It may be more preferable to the present invention.

For example, dimethylsulfoxide (DMSO), N, N-dimethylacetamide (DMA), N-methyl-2-pyrrolidione (N-Methyl-2-pyrrolidione (N-Methyl-2-pyrrolidione) MPLD)), N-Methyl-2-pyridine (MPD), 2,6-dimethyl-γ-pyrone (2,6-Dimethyl-γ-pyrone (DMP)), acetamide (Acetamide), Urea, Thiourea (Thiourea (TU)), N, N-Dimethylthioacetamide (NTA), Thioacetamide (TAM), Ethylenediamine (Ethylenediamine (EN)), Tetramethylethylenediamine (TMEN), 2,2'-Bipyridine (BIPY), 1,10-Piperidine , At least one compound selected from the group consisting of aniline, pyrrolidine, diethylamine, N-methylpyrrolidine, and n-propylamine Thiourea (TU), preferably containing an S electron pair donor, N, It may be selected from N-dimethylthioacetamide (N, N-Dimethylthioacetamide (DMTA)), thioacetamide (Thioacetamide (TAM)).

The FT-IR peak corresponding to the functional group of the electron-pair donor atom in which the Lewis base compound represented by Q corresponds to Pb is red shifted by 10 to 30 cm ⁻¹ from the compound represented by Formula 2 than the Q compound (red shift) may appear.

The Lewis base compound may be in a liquid state, preferably nonvolatile or low volatility, and a boiling point of 120 ° C. or higher, for example, a boiling point of 150 ° C. or higher may be used.

In the method for producing a lead halide adduct compound represented by Formula 1,

Preparing a precursor solution by dissolving a lead halide, an organic halide compound or an inorganic halide compound, and a Lewis base compound containing nitrogen (N), oxygen (O), or sulfur (S) atoms in an electron pair donor in a first solvent;

It provides a method for producing a lead halide adduct compound comprising the step of filtering the precipitate produced by adding a second solvent to the precursor solution.

The lead halide, a halogenated compound containing a divalent cation, and an organic material including a ligand may be mixed in a molar ratio of 1: 1: 1 to 1: 1: 1.5, and may be mixed in a molar ratio of 1: 1: 1. Most preferred.

According to one embodiment, the first solvent comprises an organic material including the lead halide, an organic halide compound or an inorganic halide compound and a functional group having an electron pair donor of nitrogen (N), oxygen (O) or sulfur (S) atoms. All are organic solvents that can dissolve, propanediol-1,2-carbonate (PDC), ethylene carbonate (EC), diethylene glycol, propylene carbonate (PC), propylene carbonate (PC), hexamethyl phosphate triamide (HMPA ), Ethyl acetate, nitrobenzene, formamide, γ-butyrolactone (GBL), benzyl alcohol, N-methyl-2-pyrrolidone (NMP), acetophenone, ethylene glycol, trifluorophosphate, benzonitrile ( BN), valeronitrile (VN), acetonitrile (AN), 3-methoxy propionitrile (MPN), dimethyl sulfoxide (DMSO), dimethyl sulfate, aniline, N-methylformamide (NMF), phenol, 1,2-dichlorobenzene, tri-n-butyl phosphate, o-di Chlorobenzene, selenium oxychloride, ethylene sulfate, benzenethiol, dimethyl acetamide, diethyl acetamide, N, N-dimethylethaneamide (DMEA), 3-methoxypropionnitrile (MPN), diglyme, cyclo Hexanol, bromobenzene, cyclohexanone, anisole, diethylformamide (DEF), dimethylformamide (DMF), 1-hexanethiol, hydrogen peroxide, bromoform, ethyl chloroacetate, 1-dodecanethiol, di-n-butylether, dibutyl ether, acetic anhydride, m-xylene, p-xylene, chlorobenzene, morpholine, diisopropyl ethamine , Diethyl carbonate (DEC), 1-pentanediol, n-butyl acetate 1-hexadecanethiol, and the like, and the organic solvent may be used alone or in combination of two or more thereof.

The first solvent may be added in excess, and preferably, may be added in a weight ratio of 1: 1 to 1: 3 (lead halide: first solvent) with respect to the weight of the lead halide.

According to one embodiment, the second solvent may be a non-polar or weak polar solvent that can selectively remove the first solvent, for example, acetone, C1-C3 alcohol, ethyl acetate, diethyl ether And solvents selected from the group consisting of alkylene chlorides, cyclic ethers, and mixtures thereof.

According to one embodiment, perovskite prepared from a lead halide adduct compound may exhibit low reproducibility when using toluene and chlorobenzene, which are used as common volatile solvents, which indicates that the perovskite quality is This is because when using one volatile solvent, the amount of dripping and / or the spinning speed of the cleaning liquid and the solubility difference between the cleaning liquid and the precursor solution can be greatly influenced. However, when using a second solvent according to the present invention, preferably a diethyl ether solvent, a perovskite having high reproducibility by using a sufficient amount of the second solvent in the first solvent completely dissolved regardless of the spin coating conditions. You can get the sky curtain.

According to a preferred embodiment of the present invention, the adduct compound includes peaks positioned in the range of 2θ values of XRD diffraction peaks of 7 to 8.5 and 9.8 to 10.5, and specifically 2θ values of XRD diffraction peaks of 6 to 7 , Peaks located in the range of 7 to 8.5 and 9.8 to 10.5, respectively, or 2θ values of XRD diffraction peaks in the range of 7 to 8.5, 9.8 to 10.5, 11 to 12.5, and 13 to 14, respectively. (See FIGS. 14 and 15). These peaks do not appear in compounds prepared by other methods, and may be characteristic peaks in adduct compounds.

According to one embodiment, the lead halide duct compound thin film prepared as described above may form a transparent thin film as shown in Figure 2 (a).

The lead halide adduct compound formed of the thin film may be subjected to a heating step at a temperature of 30 ° C. or higher, preferably, may be heated at a temperature of 40 ° C. or 50 ° C. or higher, for example, 30 ° C. or higher and 150 ° C. It can be heated in the following temperature range to form perovskite. In addition, the heating process may be heated in a stepwise manner of being heated at a temperature of 30 ℃ to 80 ℃ and further heated at 90 ℃ to 150 ℃, perovskite having a more compact structure by the additional heating process Get a decision. In the annealing process, the perovskite is formed by removing the ligand organic material represented by Q of Formula 1 from the crystal structure of the lead halide adduct compound. According to one embodiment, the manufactured perovskite thin film is dark brown and Likewise, a thin film having a dark color may be formed.

The substrate may be a flexible substrate made of a polymer material or a non-flexible substrate such as glass.

According to a preferred embodiment, the transparent anode electrode is graphene, the metal oxide layer is MoO ₃ (molybdenum trioxide), the hole transport layer is PEDOT: PSS, the perovskite layer is MAPbI ₃ (Methyl Ammonium Lead Iodide), the electron transport layer is C _{A 60} (fullerene) / bathocuproine (BCP), cathode electrode is provided with a high efficiency TCO-free perovskite based solar cell comprising lithium fluoride (LiF) / Al (aluminum). Hereinafter, one preferred embodiment of the present invention will be described in more detail, but the scope of the present invention is not limited thereto, and various modifications are possible.

MoO ₃ formed to a thickness of several nanometers  A layer is formed between the graphene and the PEDOT: PSS layer, which provides hydrophilicity to the graphene surface and can raise the low work function (4.23 eV) to a high level (4.71 eV) through hole doping. The wettability and device properties of PEDOT: PSS are influenced by the thickness of the MoO ₃ layer. In a more preferred embodiment, a conversion efficiency of up to about 17.1% can be achieved by introducing a MoO ₃ interfacial layer about 2 nm thick (see FIG. 1).

The structure of a solar cell device according to an embodiment of the present invention is schematically shown in FIG. In the MAPbI ₃ perovskite-based solar cell structure using graphene as a conductive transparent electrode, PEDOT: PSS and C ₆₀ / BCP were used as the hole transport layer (HTL) and the electron transport layer (ETL), respectively. The structure is capable of low temperature processing, making it suitable for next generation applications applied on flexible plastic substrates.

Single-layer graphene grown through chemical vapor deposition (CVD) increases the work function (~ 4.3 eV) by P-type doping and consequently improves conductivity, as well as the highest occupancy of the preferred hole transport layer. Since it induces an energy level of the highest occupied molecular orbital (HOMO) level (e.g., ~ 5.2 eV for PEDOT: PSS), it is preferable to use it as a transparent anode electrode. However, the present invention is not limited thereto and may be used as a cathode by n-doped graphene as necessary. In the present invention, MoO ₃ between the graphene and PEDOT: PSS layer by vacuum thermal deposition  After depositing the layer, heat treatment was performed at 150 ° C. on a hot plate to prevent loss during spin coating. In the present invention, MoO ₃ By changing the thickness of the layer to 0-4 nm, the interfacial properties of the graphene electrode, such as wettability and doping level of PEDOT: PSS, were adjusted. The wettability of PEDOT: PSS on graphene is a very important factor in the fabrication of high performance devices.

Hereinafter, the perovskite-based solar cell using the graphene according to the present invention as a conductive transparent electrode from the Examples and Experimental Examples in more detail. However, the following examples are only examples of the present invention and should not be construed as limiting the protection scope of the present invention.

<Examples 1 to 3> Fabrication of graphene-based perovskite solar cell

Single layer graphene coated glass substrate (Graphene Square Inc.> 1 kΩ cm ² , 15 x 15 mm ² transferred CVD-grown graphene onto a cleaned glass substrate (AMG, 25 x 25 mm ² ) on copper foil  ) To produce a graphene-based perovskite solar cell.

Ultra-thin MoO ₃ layers having various thicknesses of 1 nm, 2 nm and 4 nm were coated on a single layer graphene coated glass substrate by vacuum thermal evaporation at a deposition rate of 0.1 Ås ^-1 using a vacuum thermal evaporator, followed by heat treatment at 150 ° C. for 10 minutes. . The deposition rate and thickness during the deposition were monitored using a quartz crystal sensor. The MoO ₃ layer was intended to change the hydrophobic graphene surface to hydrophilic.

Subsequently, the substrate was first soaked in deionized water, and then 50 μl of PEDOT: PSS solution (Clevios P VP Al 4083) was spin coated at 5000 rpm for 30 seconds, followed by heat treatment at 150 ° C. for 20 minutes to form a hole transport layer. It was.

A perovskite layer was formed on the hole transport layer as an absorber. The perovskite layer is MAI, PbI ₂ DMSO was mixed at a molar ratio of 1: 1: 1 and dissolved in a 50 wt% DMF solution without heating, followed by spin coating of 50 μl of completely dissolved MAI.PbI ₂ .DMSO solution for 3500 rpm for 20 seconds, and the spin coating. After 8 seconds after the start, 0.3 ml of diethyl ether was slowly dropped to remove excess dimethylformamide to form a CH ₃ NH ₃ I · PbI ₂ · DMSO adduct compound film. The CH ₃ NH ₃ I.PbI ₂ .DMSO adduct compound film was heat treated at 65 ° C. for 1 minute and at 100 ° C. for 4 minutes to form a dark brown perovskite film.

Thereafter, the substrate was deposited with C ₆₀ (20 nm), BCP (10 nm), LiF (0.5 nm), and Al (150 nm) inside a vacuum thermal evaporator of less than 10 ⁻⁶ Torr. All of the above spin coatings were performed under atmospheric conditions.

Comparative Examples 1 to 4 Fabrication of Perovskite Solar Cells Based on ITO

In the fabrication of ITO-based perovskite solar cells, the ITO devices used were fabricated on commercially available ITO-coated glass substrates (AMG, 9.5 Ω cm ² , 25 × 25 mm ² ).

The ITO-coated glass substrate was washed with acetone, isopropanol and deionized water for 15 minutes each using an ultrasonic bath, dried over nitrogen gas, stored in an oven at 120 ° C., and used.

The ultra-thin MoO ₃ layer having various thicknesses of 0 nm, 1 nm, 2 nm and 4 nm on the washed and dried ITO-coated glass substrate was vacuum-heated at a deposition rate of 0.1 s s ^-1 using a vacuum thermal evaporator. After evaporation, heat treatment was performed at 150 ° C. for 10 minutes. The deposition rate and thickness during the deposition were monitored using a quartz crystal sensor.

Thereafter, the substrate was first soaked in deionized water, and then 50 µl of PEDOT: PSS solution was spin coated at 5000 rpm for 30 seconds, followed by heat treatment at 150 ° C. for 20 minutes to form a hole transport layer.

A perovskite layer was formed on the hole transport layer as an absorber. The perovskite layer is MAI, PbI ₂ And dissolving DMSO in a molar ratio of 1: 1: 1 without heating in a 50 wt% DMF solution, followed by spin coating a fully dissolved 50 µl MAI.PbI ₂ .DMSO solution for 3500 rpm for 20 seconds, and spin coating After 8 seconds after the start, 0.3 ml of diethyl ether (DE) was slowly dropped to remove excess dimethylformamide to form a CH ₃ NH ₃ I · PbI ₂ · DMSO adduct compound film. CH ₃ The NH ₃ I.PbI ₂ .DMSO adduct compound film was heat treated at 65 ° C. for 1 minute and at 100 ° C. for 4 minutes to form a dark brown perovskite film. Thereafter, the substrate was deposited with C ₆₀ (20 nm), BCP (10 nm), LiF (0.5 nm), and Al (150 nm) inside with a vacuum thermal evaporator of less than 10 ⁻⁶ Torr. All of the above spin coatings were performed under atmospheric conditions.

Experimental Example 1 Characterization of Solar Cell

SEM images of the perovskite-based solar cells of Examples 1 to 3 and Comparative Examples 1 to 4, J- V curve, external quantum efficiency (EQE) spectrum, sheet resistance, Transmittance, UPS and AFM images were analyzed.

SEM images were analyzed using a field emission scanning electron microscope (AURIGA, Zeiss) and Oriel Sol3A solar simulator calibrated to 100 mWcm ^-2 using standard Si solar cells (RC-1000-TC-KG5-N, VLSI Standards). Solar simulations were carried out with AM 1.5G daylight produced by.

J- V curves were recorded by Keithley 2400 source meter, the forward and reverse scan rates were set to 200 ms per 20 mV and the active area of the device was 1.77 mm ² .

EQE spectra were measured with a Newport IQE200 system with a 300mW Xenon light source and a lock-in amplifier.

Sheet resistance was measured using 4 needle method (CMT-SERIES, Advanced Instrument Technology).

The transmittance was measured using UV-vis spectroscopy (Cary 5000, Agilent).

UPS measurements were made using a helium discharge lamp (He I 21.2 eV, AXIS-NOVA, Kratos), and AFM images were taken using a XE-100 (Park Systems) scanning probe microscope in contactless mode.

MoO ₃  Due to the deposition of the layer PEDOT: PSS  Wettability and Graphene  Improved hydrophilicity of the electrode

The wettability of PEDOT: PSS of graphene-based and ITO-based devices is very important for the fabrication of high performance devices, and the contact angle measurements are used to determine the difference in wettability with and without the MoO ₃ layer.

3 shows an optical microscope image of a PEDOT: PSS layer in which water droplets were dropped on the surfaces of graphene and ITO. MoO ₃ as shown in FIG. 3A The contact angle of PEDOT: PSS on the layerless graphene surface was measured as 90.4 ± 0.3 °, and as a result, successive PEDOT: PSS / MAPbI ₃ layers are unlikely to be formed by the spin-coating process (FIG. 3A inset). ). However, as shown in FIGS. 3B and 3C, the contact angle is reduced to 46.6 ± 1.3 ° when there is a 1 nm thick MoO ₃ layer on the graphene, and the contact angle is 30.0 ± 1.6 ° when there is a 2 nm thick MoO ₃ layer. Shows a decrease. It can be seen in the insets of FIGS. 3b and 3c that the wettability also improved with the reduction in contact angle as described above.

As shown in FIG. 3C, dark brown MAPbI ₃ The film was formed into a rectangle in the center of a pre-heat-deposited MoO ₃ layer glass substrate, in particular a very clear rectangular model MAPbI ₃ The film was formed on a 2 nm thick MoO ₃ layer and the PEDOT: PSS showed better wetting on a thick MoO ₃ layer.

Figure 4 is a SEM image, it is not sufficiently covered with a 1nm thick MoO ₃ layer, and clearly shows the hydrophobic graphene surface is completely covered with a 2nm thick MoO ₃ layer.

For comparison, the contact angle of PEDOT: PSS on the surface of ITO was determined by UV / ozone treatment and MoO ₃ as shown in FIGS. 3D-3F. It was measured before and after the combined treatment of the deposition. As a result, similar to graphene. The ITO surface showed no wettability with respect to PEDOT: PSS, which forms a continuous film by spin coating, and the contact angle on the UVO treated ITO surface was significantly from 84.0 ± 1.3 ° (FIG. 3D) to 16.9 ± 1.8 ° (FIG. 3E). Reduced and slightly reduced to 9.3 ± 0.6 ° (FIG. 3F) by the 1 nm thick MoO ₃ layer, which means improved wettability of the ITO surface.

5 shows graphene electrodes of 2 nm thick MoO ₃ (FIG. 5A) and 1 nm-thick MoO _3. The SEM image of the device cross section produced using the ITO electrode of is shown. The left image of FIG. 5 was measured in secondary electron (SE) mode and the right image was measured in back-scattered electron (BSE) mode. PEDOT similar thickness (~ 50 nm) and morphology of graphene, and is formed by spin coating on the ITO: PSS layer of hydrophilic MoO ₃ It can be maintained smoothly and continuously by the interfacial layer. In addition, as shown in FIG. 5, the surface of the perovskite film was observed to be very smooth with uniform thickness (˜510 nm) on both graphene and ITO base.

The smooth and dense perovskite film described above is PbI ₂ The nip perovskite solar cell, which is manufactured through Lewis base adducts and exhibits the highest conversion efficiency of 19.7%, was recently developed by the researchers. The manufacturing method may refer to Korean Patent Application No. 2015-0164744. MAI · PbI ₂ · DMSO adduct film was formed by spin coating with diethyl ether (DE) dropwise to wash excess dimethylformamide (DMF) solvent and then converted to perovskite film by heat treatment. It became.

Device In performance MoO ₃ Influence of layer thickness

MoO ₃ on device performance To determine the effect of layer thickness, MoO ₃ of various thicknesses on graphene and ITO electrodes Open circuit voltage ( V _oc ), short circuit current ( J _sc ), charge rate (FF), conversion efficiency (PCE) for the perovskite solar cells of Examples 1 to 3 and Comparative Examples 1 to 4, in which layers are used. And the results measured for the highest conversion efficiency is shown in Table 1.

electrode MoO ₃ thickness [nm] V _oc [V] J _sc [mA cm ² ] FF PCE [%] Top PCE [%] Example 1 (G-M1) Graphene One 0.72 ± 0.36 17.6 ± 6.3 0.45 ± 0.09 6.7 ± 4.2 12.1 Example 2 (G-M2) 2 1.03 ± 0.02 21.9 ± 0.4 0.72 ± 0.02 16.1 ± 0.6 17.1 Example 3 (G-M4) 4 1.00 ± 0.01 22.9 ± 0.4 0.70 ± 0.02 15.9 ± 0.5 16.2 Comparative Example 1 (ITO-M0) ITO 0 0.96 ± 0.01 21.4 ± 0.5 0.83 ± 0.02 17.0 ± 0.4 17.6 Comparative Example 2 (ITO-M1) One 0.97 ± 0.01 22.6 ± 0.4 0.83 ± 0.01 18.2 ± 0.5 18.8 Comparative Example 3 (ITO-M2) 2 0.95 ± 0.01 22.2 ± 0.4 0.76 ± 0.01 16.1 ± 0.4 16.9 Comparative Example 4 (ITO-M4) 4 0.94 ± 0.01 21.0 ± 0.4 0.74 ± 0.01 14.7 ± 0.6 15.7

Average PCE and MoO₃ The relationship between the layer thicknesses is shown in FIGS. 6A and 6B.

As shown in FIG. 6, in the case of the graphene-based device, the PEDOT: PSS solution or the perovskite solution forms the film after the spin coating because the device that does not deposit the MoO ₃ layer does not wet the hydrophobic graphene surface. PCE could not be evaluated (see also FIG. 3A insertion). As well as the case of Example 1 by depositing the MoO ₃ layer of thickness 1nm irregular PEDOT: PSS were by irregular coating of PCE represents a large change in the 0% ~ 12.1%, 1nm thick These results MoO ₃ It can be seen that the layer does not completely cover the hydrophobic graphene surface. As a result, the current density and voltage (JV) characteristics between the elements were not constant (FIG. 7A).

However, MoO ₃ thicker than 1 nm  The device of Example 2 and Example 3 having a layer significantly reduced the performance variation between devices with average PCE values of 16.1% and 15.9%, respectively (FIGS. 6A and 7B). In particular, the highest PCE of 17.1% was achieved in Example 2, which is the first achievement for graphene electrode perovskite-based solar cells replacing ordinary TCO electrodes, as well as the highest conversion efficiency among TCO-free solar cells. It represents.

As shown in FIG. 6B, in the comparative examples of ITO-based devices, PCE was significantly affected by MoO ₃ thickness change. Comparing the electrode of Comparative Example 1 with the electrode of Comparative Example 2, the average PCE increased from 17.0% to 18.2%, and the average PCE of Comparative Example 3 and Comparative Example 4 having a MoO ₃ layer thicker than 1 nm was 16.1% and Decreased to 14.7%.

8 shows a histogram of PCE for each electrode type of Example 2 and Comparative Example 2. FIG.

The highest performance JV curves of Example 2 and Comparative Example 2 at 100 mW cm ² AM 1.5G one solar irradiation, measured via reverse and forward bias sweeps, are shown in FIGS. 6C and 6D, respectively. Neither Example 2 nor Comparative Example 2 showed significant hysteresis along the scan direction.

In addition, compared with Comparative Example 2, Example 2 showed high series resistance and low shunt resistance. To understand the electrical properties of MoO ₃ -modified graphene and ITO electrodes, sheet resistance was measured by a four-point probe. 6E shows the relationship between sheet resistance and the thickness of the MoO ₃ layer in graphene and ITO. As shown, the yes, the initial high sheet resistance (> ² k Ω cm 2) only were significantly reduced to ~ 780 Ω cm ² by the deposition of MoO ₃ layer 0.5 nm thick, the thickness of the MoO ₃ layer of the pin As it increased to 2 nm, it further decreased to ˜500 Ω cm ² . The initial sheet resistance of the ITO is 9.5 Ω was measured in cm ^2, it was slightly reduced to 9.2 Ω cm ² by evaporation of the first and 2 nm of the MoO ₃ layer. More than four times the MoO ₃ doping significantly reduced the sheet resistance of the initial monolayer graphene, but it was still much higher than ITO. Thus, Example 2 showed the results of the high series resistance, low shunt resistance and low filling rate (FF) for Comparative Example 2.

Transparency of the device

As shown in FIG. 6F, the single atomic thickness single layer graphene (˜97% transmittance) exhibits high transparency in the visible wavelength range compared to ITO (˜89% transmittance), and Example 2 device is compared with Comparative Example 2 device. Short-circuit current ( J _sc ) level is shown.

8 shows an external quantum efficiency (EQE) spectrum of Example 2 and Comparative Example 2. FIG. The integrated photocurrent was calculated above at 20.2 and 21.0 mA cm ⁻¹ , respectively. Compared to the ITO anode, the EQE of graphene- and TIO-based devices is similar because the low carrier collection efficiency of the graphene anode is compensated for by higher light transmittance (FIGS. 6F and 9). In addition, despite the conductivity of graphene, which is much lower than ITO, Example 2 shows a high open voltage (V _oc ) level compared to Comparative Example 2, and the result contributes to the high PCE of Example 2 ( 90% or more than Comparative Example 2.

Work function difference of the device

Gives the same device structure, but V _oc along the electrode  The difference of is related to the work function difference of the electrodes. Ultra-thin MoO ₃ at work function of ITO and graphene electrodes  In order to investigate the effect of the layer, ultraviolet photoelectron spectroscopy (UPS) measurements were made.

10 is MoO ₃ UPS spectra of ITO and graphene electrodes are shown as the thickness of the layer changes. As shown in FIG. 10A, 0.5 nm thick MoO ₃ The deposition of the layer rapidly shifted to high kinetic energy due to the blocking of the secondary battery of ITO. As a proof, the work function increased from 4.29 eV to 4.65 eV. MoO ₃ Further deposition of the layers to 1 nm and 2 nm thickness raised the work function to 4.69 eV and 4.72 eV, respectively. Therefore, it is desirable to minimize the energy barrier between the anode and the hole transport layer (HTL) for efficient hole collection. As shown in Table 1, the high J _sc and PCE of Comparative Example 2 compared to Comparative Example 1 is due to an increase in the work function of the center electrode leading to improved hole trapping efficiency. Meanwhile, MoO ₃ PCE decreased as the thickness of the layer increased to 2 nm and 4 nm. Thin MoO ₃ between ITO and Organic Semiconductor in Device Structure  The layer behaves like an insulating layer and the holes are moO ₃ due to the tunnel effect, which is a gradual reduction of films thicker than ~ 3 nm.  Can pass through layers. The hole collection efficiency of Comparative Examples 3 and 4 is likely to be improved by an increase in the work function of the electrode, but may be reduced by a decrease in the tunneling probability of the major due to a sudden decrease in FF and PCE compared to Comparative Example 2.

UPS spectrum of graphene-based devices, also MoO ₃ It exhibited similar behavior as the ITO device with increased layer. As shown in FIG. 10B, 0.5 nm thick MoO ₃ deposited on graphene  The rapid change in photoelectron initiation in the layer showed an increase in work function from 4.23 eV to 4.61, MoO ₃ Further deposition of the layers to 1 nm and 2 nm thickness was found to raise the work function to 4.67 eV and 4.71 eV, respectively. As shown in FIG. 10 c, the ultra-thin MoO ₃ layer deposited on graphene not only promoted hole collection from the hole transport layer to the graphene anode by reducing the energy barrier at the interface, but also hydrophobic graphene Successful spin coating of PEDOT: PSS film on the surface was made possible.

Comparing UPS data from ITO and graphene electrodes shows the same MoO ₃ The work function for thickness was found to be nearly identical. The work function values of Comparative Example 2 and Example 2 were also not significantly different. This means that higher V _oc of Example 2 than Comparative Example 2 is not accounted for in terms of energy barrier differences at the anode / HTL interface.

Also, Since V _oc can be affected by interface quality, MoO ₃ by atomic force microscopy (AFM) measurements  And the morphology of the electrode after formation of the PEDOT: PSS film.

As shown in Figs. 11A to 11F, the root-mean-square roughnesses of Comparative Example 1, Comparative Example 2, and Comparative Example 2 / PEDOT: PSS were found to be 2.06, 1.95, and 1.2, respectively, and Example 2 It was confirmed that the roughness of exhibits a surface rms roughness of 0.29 nm, which is 6 times lower than that of Comparative Example 2 (1.95 nm rms roughness). Herein, Comparative Example 2 / PEDOT: PSS means having a MoO ₃ layer having the same thickness as Comparative Example 2 and further having a PEDOT: PSS layer. Example 2 / PEDOT: PSS can also be interpreted in an equivalent sense.

In general, it is known that the surface roughness of the underlying layer is smaller, so that when V _oc increases, a better interface can be created between the stacks. In this regard, the electrode of Example 2, which contributes to the high V _oc of Example 2, appears to have established a better PEDOT: PSS interface than the Comparative Example 2 / PEDOT: PSS interface.

Further, as shown in FIGS. 12 and 13, the particle size of Example 2 in the SEM image of the perovskite surface on Example 2 / PEDOT: PSS and Comparative Example 2 / PEDOT: PSS was shown. It was confirmed that the larger.

Since the nano-scale edges of the surface can function as nucleation sites, the surface roughness of the underlying layer plays an important role in determining the particle size of PEDOT: PSS in this study. Thus, the particles of the perovskite film on the smooth surface of Example 2 / PEDOT: PSS can grow to a larger size than the particles of the surface of Comparative Example 2 / PEDOT: PSS. The larger particles of Example 2 reduced the voltage loss due to charge recombination at the particle boundary. Thus, it is given as a factor that gives a higher V _oc in Example 2 compared to Comparative Example 2.

The present invention not only uses graphene as a transparent conductive anode but also provides a high efficiency TCO-free solar cell. MoO ₃ introduced to the anode surface of the solar cell of the present invention  The layers form a better interface and allow for the alignment of the desired energy levels between the anode and the hole transport layer. In particular, MoO ₃ on graphene Deposition serves a better role as a conductive electrode and provides optimum MoO ₃ In layer thickness, the highest PCE of 17.1% and 18.8% was achieved for graphene based devices and ITO based devices, respectively. Graphene electrodes have low conductivity compared to ITO electrodes, but have similar J _sc , high V _oc and high transparency and low surface roughness.

Claims (11)

Perovskite-based solar cell comprising a graphene layer as a conductive transparent electrode. The method of claim 1, The conductive transparent electrode is a solar cell that is a transparent front electrode. The method of claim 1, The solar cell of which a transparent anode electrode, a hole transport layer, a perovskite layer, an electron transport layer and a cathode electrode made of a graphene layer are sequentially stacked on a substrate. The method of claim 1, A transparent cathode electrode, an electron transport layer, a perovskite layer, a hole transport layer and an anode electrode made of a graphene layer are sequentially stacked on a substrate. The method according to claim 3 or 4, The solar cell further comprises a metal oxide layer deposited on the transparent anode electrode or transparent cathode electrode consisting of the graphene layer. The method of claim 5, The metal oxide layer is MoO ₃ , NiO, CoO and TiO ₂ Solar cell comprising one or more selected from the group consisting of. The method of claim 5, The metal oxide layer has a thickness of about 0.5 nm to about 6 nm. The method of claim 1, The perovskite is a solar cell prepared using a lead halide adduct compound. The method of claim 8, The lead halide adduct compound is a solar cell represented by the following formula (1): [Formula 1] A PbY · ₂ · Q In the above formula, A is an organic halide compound or an inorganic halide compound, Y is F ^- is a halogen ion, ^-, Cl ^-, Br ^- or I Q is a Lewis base compound containing a functional group having an electron-pair donor as an atom having a non-covalent electron pair, and the peak of the FT-IR of the functional group is 1 to 10 cm ^- from the compound of Formula 1 to the compound of Formula 2 below. Red shifted by ¹ appears, [Formula 2] PbY ₂ · Q Y and Q are the same as defined for the formula (1). The method of claim 9, Wherein A is CH ₃ NH ₃ I, CH (NH ₂ ) ₂ I or CsI. The method of claim 9, The perovskite is made by heating and drying the adduct compound to remove the Lewis base compound contained in the adduct compound.

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