KR101025240B1 - Organic Solar Cell - Google Patents

Abstract

An organic solar cell including a cathode and an anode facing each other and at least one photoactive layer interposed between the cathode and the anode is provided, the photoactive layer including a hole acceptor, an electron acceptor, and a liquid crystal material.

Description

Organic Solar Cell

The technology disclosed herein relates generally to an organic solar cell, and more particularly, to an organic solar cell having improved photoelectric conversion efficiency by controlling the morphology of the photoactive layer.

Organic solar cells are the next generation solar cells that are in the spotlight as an alternative to silicon solar cells, and the core material of the organic solar cell is a conjugated polymer. Unlike silicon solar cells, conjugated polymers have a high absorption coefficient and can absorb sunlight even with a thin film (about 100 nm). Therefore, the conjugated polymer can be manufactured as a thin device. It has the advantage that there are various application fields other than buildings where silicon solar cells are mainly used.

In 1986, C. Tang of Eastman Kodak presented the possibility of practical application of solar cell with heterojunction structure using copper phthalocyanine (CuPc) and perylene tetracarboxylic derivative (perylene tetracarboxylic derivative). Interest and research have been amplified. Later, in the early 1990s, a solar cell was reported using a mixed film of a conjugated polymer and a fullerene derivative as a photoactive layer in the Heger group. Here, the efficiency was increased to some extent by overcoming the short diffusivity of excitons, which is a disadvantage of single layer solar cells. However, even in this case, since the low solubility of fullerenes was a problem, fullerene-modified fullerene derivatives (PCBMs) were developed to improve efficiency by 3%. Since then, various studies have been conducted to obtain high efficiency organic solar cells, and photoelectric conversion efficiency of up to 6.5% has been announced. In addition, the recent prediction that the photovoltaic conversion efficiency of about 10% is theoretically possible by controlling the energy level and physical properties of organic semiconductors brightens the future of practical use for organic solar cells.

In order to commercialize organic solar cells, high efficiency must be achieved. In this context, the method of increasing the photoelectric conversion efficiency by increasing the area of the interface by the bulk heterojunction method of blending P-type and N-type materials to facilitate the separation of excitons, and heat treatment after fabrication of organic solar cell devices By adding a process, a method of improving photoelectric conversion efficiency and a method of using a tandem solar cell, which is a structure in which a single cell is connected by direct current with a DC structure, have been developed.

According to an embodiment, the organic solar cell includes a cathode and an anode facing each other, and at least one photoactive layer interposed between the cathode and the anode, wherein the photoactive layer includes a hole acceptor, an electron acceptor, and a liquid crystal material. can do.

A thin film may be formed by introducing a small amount of the liquid crystal material as an additive into the mixed solution of the electron acceptor and the hole acceptor material used for forming the photoactive layer. As a result, a morphology in which electrons or holes are easily moved through the alignment characteristics of the liquid crystal material may be provided, thereby providing an organic solar cell device having greatly improved photoelectric conversion efficiency.

An anode layer planarization layer may be further included between the photoactive layer and the anode.

The cathode or the anode may be formed on glass, plastic, metal foil, or ceramic.

The electron acceptor may be used alone or in combination of fullerenes and derivatives thereof, semiconductor nanoparticles such as CdS, CdSe, CdTe, or ZnSe.

The hole receptor may be a phthalocyanine-based pigment, an indigo, a thionigo-based pigment, a melancyanine compound, a cyanine compound, a polyparaphenylenevinylene, a polyparaphenylenevinylene derivative, a polythiophene ) Or polythiophene derivatives may be used alone or in combination.

The liquid crystal material may be used singly or in combination with nematic liquid crystal, smectic liquid crystal, discotic liquid crystal, ferroelectric liquid crystal, or cholesteric liquid crystal.

The photoactive layer may include 30 to 80 wt% of the hole acceptor, 10 to 70 wt% of the electron acceptor, and 0.1 to 20 wt% of the liquid crystal material. In the above range, an organic solar cell having excellent photoelectric conversion efficiency may be manufactured.

Hereinafter, with reference to the drawings will be described in detail for the technology disclosed herein.

1 is a view showing the structure of a conventional organic solar cell. Referring to FIG. 1, an organic solar cell includes a substrate 100, an anode (transparent electrode) layer 101, an anode layer buffer layer 102, a photoactive layer 103, and a cathode 104. . Conventional organic solar cells use indium tin oxide (ITO), which is a transparent electrode having a high work function, as a cathode material, and Al or Ca having a low work function as a cathode material.

Unlike silicon solar cells, organic solar cells consist of a junction structure of a hole acceptor (D) and an electron acceptor (A) material, and when absorbing visible light, an electron-hole pair Hole pairs are generated and electrons move to the electron acceptor, resulting in a photovoltaic effect through the separation of electron-holes, which is called the photoinduced charge transfer (PICT). This is due to very fast charge transfer between the receptor materials.

The photoactive layer 103 uses a two-layer structure of a hole acceptor (D) material and an electron acceptor (A) material or a composite thin film ((D + A) blend] structure having a thickness of about 100 nm. Also uses a mixed structure [D / (D + A) / A] in which the latter composite thin film is sandwiched between the hole acceptor and electron acceptor layers. In addition, an anode planarization layer 102 or a hole transport layer is formed between the ITO electrode, which is the anode 101, and the photoactive layer, and an electron transport layer is formed between the cathode 104 and the photoactive layer 103 as a buffer layer. Sometimes a transport layer is inserted. The operating principle of a conventional organic solar cell is as follows. When the organic solar cell is exposed to light, the light is absorbed by the hole acceptor through the substrate, the anode, and the anode layer planarization layer 102 to form an electron-hole pair in an excited state. The electron-hole pair diffuses in any direction and encounters an interface with the electron acceptor material, separating it into electrons and holes. That is, electrons move toward the electron acceptor material having a high electron affinity, and holes remain on the hole acceptor side and are separated into respective charge states. They are collected and moved to each electrode by the difference in concentration between the internal magnetic field and the accumulated charge formed by the work function difference of both electrodes, and finally flow through the external circuit in the form of current. However, the time it takes for the charge-hole pair to recombine and disappear again is about 100 picoseconds, so the distance that the charge-hole pair can move within the conjugated polymer without recombination is about 10 nm. Therefore, in order for the light-charged pair to split apart without recombination to produce electrons and holes, the interface between the electron acceptor and the hole acceptor must be within 10 nm of where the charge-hole pair was formed. This distance is highly dependent on the morphology between the electron acceptor and the hole acceptor. Therefore, controlling the morphology of the electron acceptor and the hole acceptor network of the organic solar cell device is recognized as the most important factor of efficiency improvement.

Hereinafter, by adding a small amount of the liquid crystal material to the photoactive layer, a process of manufacturing a new organic solar cell device having a morphology that is easy to move electrons or holes through the alignment characteristics of the liquid crystal material will be described in detail.

Substrate / anode layer provided

First, a substrate is provided. As a board | substrate, when provided in the light-incidence surface side of a solar cell, it may be colorless transparent or somewhat colored, having light transmittance. For example, transparent glass substrates such as soda-lime glass or alkali-free glass, and plastic films arbitrarily produced from polyester, polyamide, epoxy resin, or the like may be used. In addition to ITO (Indium Tin Oxide), the anode may be FTO (Fluorine-doped Tin Oxide), IZO (Indium Zinc Oxide), ZnO-Ga ₂ O ₃ , ZnO-Al ₂ O ₃ , SnO ₂ -Sb ₂ O _3, or the like. Transparent electrodes can be used. In the forming of the anode, sputtering or vacuum deposition may be used. The electrode formed on the substrate is immersed in acetone, alcohol, water, or a mixed solution thereof, followed by ultrasonic cleaning.

Anode layer planarization layer formation

Next, an anode layer planarization layer is formed on the substrate / anode layer. As the material constituting the anode layer planarization layer, a compound having the ability of transporting holes, having a hole movement effect in both the anode and the photoactive layer and at the same time blocking electrons as well as excellent film forming ability is preferable. Specifically, polymer materials such as conductive polymers such as phthalocyanine derivatives, naphthalocyanine derivatives, aromatic diamine compounds, triazoles, imidazoles, and polyethylenedioxythiophene (PEDOT) are used, but are not particularly limited thereto. As a method of forming the hole transport layer, a spin coating method, a spray coating method, a screen printing method, a vacuum deposition method, or the like may be used as the forming method.

Photoactive Layer Formation

Next, a photoactive layer is formed on the anode layer planarization layer. The photoactive layer is a mixture of a hole acceptor and an electron acceptor, and the hole acceptor comprises a compound such as a phthalocyanine pigment, an indigo, a thionigo pigment, a melancyanine compound, a cyanine compound, and a poly-p-phenylenevinylene. And polythiophene and the like, and are not particularly limited as an electron acceptor, but may be used alone or in combination with fullerenes and derivatives thereof, and semiconductor nanoparticles such as CdS, CdSe, CdTe, or ZnSe. . At this time, the size of the nanoparticles may be used of about 1 ~ 100 nm.

To prepare a solution for forming a photoactive layer, a solution in which a mixture of a hole acceptor and an electron acceptor is dissolved is prepared, and a liquid crystal material is added thereto. As the solvent used, polar solvents such as chloroform, chlorobenzene, 1,2-dichlorobenzene and toluene can be used. The method of forming the photoactive layer may be spin coating, spray coating, screen printing, bar coating, doctor blade method, ink jet method, or the like.

The liquid crystal material added to the photoactive layer may be used by mixing one or two or more liquid crystal materials of any type. The liquid crystal material used is 4-isothiocyanatophenyl, 4-pentylbicyclo [2.2.2] octane-1-carboxylate, 4-pentylbicyclo [2.2.2] octane-1-carboxylate, n- (4-ethoxybenzylidene) -4-butylaniline (n- (4-ethoxybenzylidene) -4-butylaniline), 4'-octyl-4-biphenylcarbonitrile (4'-octyl-4-biphenylcarbonitrile) , 4- (trans-4-pentylcyclohexyl) benzonitrile, 4-methoxycinnamic acid, 4 '-(hexyloxy) -4- Biphenylcarbonitrile (4 '-(hexyloxy) -4-biphenylcarbonitrile), 1-isothiocyanato-4- (trans-4-propylcyclohexyl) benzene (1-isothiocyanato-4- (trans-4-propylcyclohexyl) benzene), 4-cyanophenyl-4-butylbenzoate, 1- (4-transhexylcyclohexyl) -4- [2- (4-isothiocyanatophenyl ) Ethyl] benzene (1- (4-trans-hexylcyclohexyl) -4- [2- (4-isothiocyanatophenyl) ethyl] benzene) and 4'-heptyl-4-biphenylcarbonitrile (4'-heptyl-4-biphenylc nematic liquid crystal represented by arbonitrile) and the like, 4, n-hexyloxyphenyl-4, n'-decyloxybenzoate (4, n-hexyloxyphenyl-4, n'-decyloxybenzoate), 4- (decyloxy Benzoic acid (4- (decyloxy) benzoic acid), 4- (heptyloxy) benzoic acid), 4-[(S, S) -2,3-epoxyhexyloxy] phenyl] (4-[(S, S) -2,3-epoxyhexyloxy] phenyl), 4- (undecyloxy) benzoic acid, 4-octyl-4-cyanobiphenyl (4-octyl-4 Smectic represented by cyanobiphenyl), cholesteryl stearate, 4,4'-biphenyl dicarboxylic acid, and phenylpyrimidine. smectic) liquid crystal, 2,3,6,7,10,11-hexamethoxytriphenylene (2,3,6,7,10,11-hexamethoxytriphenylene), (dodecyl-D-cellobiside) hep Dodecyl-D-cellobiosideheptadodecanoate, perylenediimide 1,3,5-triethynylbenzene, phthalocyanine, Tricycloquinazoline, 2,3,6,7,10-hexahexylthiotriphenylene, 2,3,6,7,10-hexahexylthiotriphenylene, hexapentyloxytriphenylene, biphenyl Biphenyl ethynylbenzene, phenylcyclohexane, cyclohexyl cyclohexane, 2,4,6-triphenyl-1,3,5-triazine (2,4,6-triphenyl- Discotic liquid crystals represented by 1,3,5-triazine) and 2,3,6,7,10,11-hexakis (pentyloxy) triphenylene and the like. In addition, ferroelectric liquid crystals, particles having spontaneous electrical polarization that can be reversed by an electric field, can be used in the present invention. Representatives include p-dioxybenzylidene-p-amino-2-methylcinnamate, 2- (4-butyloxyphenyl) -5-octyloxypyrimidine (p-dioxybenzylidene-p-amino-2-methylcinnamate) 2- (4-butyloxyphenyl) -5-octyloxypyrimidine) and hexakis (4- (4 '-((s))-6-methyloctyloxy) biphenoxy) cyclotriphosphazene (4- (4') -((s) -6-methyloctyloxy) biphenoxy) cyclotriphosphazene), cholesteryl acetate, cholesteryl benzoate, cholesteryl caprylate, and cholesterol Cholesteryl chloride, cholesteryl hemisuccinate, cholesteryl oleate, cholesteryl chloroformate, cholesteryl oleyl carbonate ( cholesteryloleyl carbonate, cholesteryl palmitate, and cholesteryl pelagonate (cholesteryl p cholesteric liquid crystals such as elargonate) and the like, and various liquid crystal materials may be used.

The photoactive layer may include 30 to 80 wt% of the hole acceptor, 10 to 70 wt% of the electron acceptor, and 0.1 to 20 wt% of the liquid crystal material. In the above range, an organic solar cell having excellent photoelectric conversion efficiency may be manufactured.

Formation of Cathode Layer

Next, a cathode layer is formed on the photoactive layer. The cathode layer is an electrode for effectively collecting electrons generated in the photoactive layer, and it is preferable to use an electrode material composed of a metal, an alloy, an electrically conductive compound, and a mixture thereof having a small work function. As the material of such a cathode layer, an alkali metal, sodium-potassium alloy, magnesium-silver mixture, aluminum-lithium alloy, aluminum-lithium fluorine mixture, or the like can be used. These electrode materials can form a film by sputtering or a vacuum deposition method, and the formation method is not specifically limited.

Sealing process

Since organic solar cells may be degraded by moisture and oxygen during operation, it is necessary to block each layer formed from the atmosphere. First, attach a moisture absorbent that can absorb moisture in the center of the protective cap of the glass material, and the sealing material is dispensed on the rim. Next, the fabricated device (substrate / anode layer / anode leveling layer / photoactive layer / cathode) is disposed on the dispensed sealing material, and then the sealing material is cured by applying UV or heat. If the sealing material is cured using UV, UV light should not be introduced into the photoactive layer, since the photoactive layer may be deteriorated by UV. In addition, the sealing process may use a method of forming a multilayer thin film under vacuum. This sealing process is already well established in the organic light emitting device industry.

Hereinafter, a method of implementing an organic solar cell will be embodied through examples, but the present invention is only presented to aid the understanding of the technology disclosed herein, and the technology disclosed herein is not limited to the following examples.

Comparative Example 1

A transparent indium tin oxide (ITO) having a thickness of ˜1,500 kPa having a sheet resistance of 15 Ω / sq as an anode layer was formed on a glass substrate (hereinafter referred to as an ITO substrate). The ITO substrate was immersed in acetone, semicoclean (manufactured by Plough Science, Inc.), and isopropyl alcohol (manufactured by Duksan Chemical Co., Ltd.), followed by ultrasonic cleaning for 15 minutes, and then dried. Next, this ITO substrate was surface-treated for 3 minutes in an atmospheric plasma surface treatment apparatus. Next, polyethylene dioxythiophene: polystyrenesulfonate (hereinafter, abbreviated as PEDOT: PSS) was filtered using a 0.45 μm filter as an anode layer planarization layer on the top of ITO, followed by 40 nm by spin coating. The film thickness was formed. After drying this board | substrate at 140 degreeC for 15 minutes, it transferred to the glove box of the dry nitrogen atmosphere of 1 ppm or less of oxygen. 15 mg of regioregular polytrihexylthiophene (hereinafter referred to as P3HT) as an electron donor material and [6,6] -phenylC61-butyric acid methyl ester which is a fullerene derivative as electron acceptor material ( 12 mg of Nano-C, hereinafter abbreviated as PCBM) was dissolved in 1 mL of chlorobenzene to prepare a solution for the photoactive layer in advance, and the photoactive layer solution was filtered through a filter. The photoactive layer solution filtered on top of PEDOT: PSS was spin coated and dried at 50 ° C. for 40 minutes to form a photoactive layer having a thickness of 100 nm. Subsequently, this substrate was set in a vacuum deposition apparatus, and lithium fluoride was formed as a cathode on the photoactive layer with a film thickness of 0.5 nm, and an Al thin film with a film thickness of 100 nm was formed thereon using a vacuum deposition method. Next, the ITO substrate on which each layer was formed was subjected to a sealing process in a glove box of a dry nitrogen atmosphere to obtain an organic solar cell having a layer structure as shown in FIG. 1.

The fabricated organic solar cell device was heat-treated for 30 minutes on a hot plate set at 150 ° C. The photoelectric conversion efficiency was measured, and the voltage-current density of the organic solar cell device was measured using Keithley 236 Source Measurement and Solar Simulator (300W simulator models 81150 and 81250, Spectra physics Co.) under standard conditions (AM 1. 5, 100 mW). / cm ² , 25 ℃), the voltage-current density measurement results of the solar cell device is shown in FIG. The open circuit voltage (Voc) was 0.69 V, the short circuit current (Jsc) was 9.25mA / cm ² , and the fill factor was 0.70, thereby representing 4.46% photoelectric conversion efficiency.

≪ Example 1 >

A solution prepared by dissolving 15 mg of P3HT and 12 mg of PCBM, a fullerene derivative in 1 mL of chlorobenzene, was added 100 μl of Merck's low molecular liquid crystal, commercially available as BL004, to the solution prepared in Comparative Example 1. All manufactured the organic solar cell in the same manner.

The VOC value of the fabricated organic solar cell was 0.68, the Jsc value was 10.55, the curve factor was 73%, and the photoelectric conversion rate was 5.23%. The voltage-current density measurement results of the device are shown in FIG. 3.

<Example 2>

All of the methods shown in Comparative Example 1 were performed except that the photoactive layer solution was prepared by adding 4-cyano-4'-pentylbiphenyl (LC1) to the mixed solution of P3HT and PCBM. In the same manner, an organic solar cell was manufactured. The organic solar cell had a Voc value of 0.71, a Jsc value of 10.77, a curve factor of 68%, and a photoelectric conversion rate of 5.20%.

<Example 3>

All of the organic solar cells manufactured in Comparative Example 1 were prepared except that 4,4'-Azooxydianisole (LC2) was added to the P3HT and PCBM mixed solution to prepare a photoactive layer solution. In the same manner, an organic solar cell was manufactured. The fabricated organic solar cell had a Voc value of 0.69, a Jsc value of 8.92, a curve factor of 76%, and a photoelectric conversion rate of 4.70%.

<Example 4>

The P3HT content of the photoactive layer solution was changed to 10 mg and the PCBM content was changed to 15 mg in the solar cell manufacturing process shown in Example 1, and all were prepared in the same manner. The Voc value was 0.67, the Jsc value was 9.45, and the curve factor was 72%, indicating a photoelectric conversion efficiency of 4.5%.

Example 5 Fabrication of Organic Solar Cell Containing Low Molecular Liquid Crystal Material

Except for changing the amount of the liquid crystal material added to the photoactive layer solution in the solar cell manufacturing process shown in Example 1 to 500㎛ all were prepared by the same method. The fabricated organic solar cell had a Voc value of 0.69, a Jsc value of 9.55, a curve factor of 75%, and a photoelectric conversion rate of 4.94%. The voltage-current density measurement results of the device are shown in FIG. 4.

<Example 6>

Organic solar cells were manufactured in the same manner except that polyparaphenylenevinylene was used instead of P3HT in the solar cell manufacturing process shown in Example 1. The fabricated organic solar cell had a Voc value of 0.67, a Jsc value of 9.55, a curve factor of 71%, and a photoelectric conversion rate of 4.54%.

<Example 7>

Organic solar cells were manufactured in the same manner except for using the second fullerene derivative instead of PCBM in the solar cell process shown in Example 1. The second fullerene derivatives here are described in Szydłowska, K. Trzci´nska, R. Bilewicz, J. Mieczkowski, Synth. Met. 158, 75 (2008)] and synthesized in the same manner as the device fabrication. The fabricated organic solar cell had a Voc value of 0.64, a Jsc value of 9.67, a curve factor of 73%, and a photoelectric conversion rate of 4.51%. The voltage-current density measurement results of the device are shown in FIG. 5.

In the organic solar cell device fabricated according to Examples 1 to 7, the value of the open circuit voltage determined by the difference in the bandgap energy between the two materials of the photoactive layer was measured to be higher than that of Comparative Example 1 without the addition of liquid crystal. The short-circuit current determined by the ion conductivity according to the flow and good interfacial contact also showed a higher value than the case where no liquid crystal material was added. For this reason, the energy conversion efficiency also showed a higher tendency compared with the case where no liquid crystal was added.

As described above, the organic solar cell device can be manufactured in a very simple process by adding a small amount of liquid crystal to the photoactive layer solution of the organic solar cell device. By adding a liquid crystal material to the photoactive layer of the organic solar cell, a photoelectric conversion efficiency higher than that of an organic solar cell device in which a conventional liquid crystal material is not added may be obtained.

1 is a view showing the structure of a conventional organic solar cell.

2 is a voltage-current density measurement result of the solar cell device of Comparative Example 1.

3 is a voltage-current density measurement result of the solar cell device of Example 1.

4 is a voltage-current density measurement result of the solar cell device of Example 5. FIG. 5 is a voltage-current density measurement result of the solar cell device of Example 7. FIG.

Claims (7)

A cathode and an anode facing each other; And At least one photoactive layer interposed between the cathode and the anode, The photoactive layer has a junction structure of a hole acceptor and an electron acceptor, An organic solar cell further comprising 0.1 to 20% by weight of the liquid crystal material as an additive relative to the total weight of the photoactive layer. The organic solar cell of claim 1, further comprising an anode layer planarization layer between the photoactive layer and the anode. The organic solar cell of claim 1, wherein the cathode or the anode is formed on glass, plastic, metal foil, or ceramic. The organic solar cell of claim 1, wherein the electron acceptor is at least one selected from the group consisting of fullerenes and derivatives thereof, and semiconductor nanoparticles such as CdS, CdSe, CdTe, and ZnSe. The method of claim 1, wherein the hole acceptor is a phthalocyanine-based pigment, indigo, thionigo-based pigment, melancyanine compound, cyanine compound, polyparaphenylenevinylene (poly-p-phenylenevinylene) At least one organic solar cell selected from the group consisting of polyparaphenylene vinylene derivatives, polythiophene (polythiophene) and polythiophene derivatives. The liquid crystal material of claim 1, wherein the liquid crystal material is a nematic liquid crystal, a smectic liquid crystal, a discotic liquid crystal, a ferroelectric liquid crystal and a cholesteric liquid. ) At least one organic solar cell selected from the group consisting of liquid crystals. The organic solar cell of any one of claims 1 to 3, wherein the photoactive layer comprises 30 to 80 wt% of a hole acceptor, 10 to 70 wt% of an electron acceptor, and 0.1 to 20 wt% of a liquid crystal material.

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