KR20190020218A - Manufacturing mehod of nitrogen doped graphene quantum dot, nitrogen doped graphene quantum dot and solar cell comprising nitrogen doped graphene quantum dot - Google Patents

Abstract

The present invention relates to a method for manufacturing nitrogen-doped graphene quantum dots, and to a solar cell comprising the nitrogen-doped graphene quantum dots. The method for manufacturing the nitrogen-doped graphene quantum dots, according to embodiments of the present invention, comprises the following steps: preparing a mixture solution of graphene oxide and polyethylenimine; stirring the mixture solution of the polyethylenimine and the graphene oxide; and heating the stirred mixture solution of the polyethylenimine and the graphene oxide, thereby cutting graphene oxide and nitrogen-doping and reducing the cut graphene oxide. According to the present invention, it is possible to manufacture the nitrogen-doped graphene quantum dots by an environmental method based on a solution process at low process temperatures and with short process times.

Description

TECHNICAL FIELD [0001] The present invention relates to a method for producing a graphene quantum dot, a nitrogen doped graphene quantum dot, a nitrogen doped graphene quantum dot, and a nitrogen doped graphene quantum dot DOT}

The present invention relates to a method for producing a nitrogen doped graphene quantum dots and a solar cell comprising nitrogen doped graphene quantum dots.

The graphene quantum dot (GQD) is a nano-sized graphene sheet with optical properties that vary with size. Graphene quantum dots can be composed of one or several layers of graphene. Graphene is composed of sp ² hybrid carbon structure and has excellent physical properties such as electrical, thermal and mechanical properties.

Graphene is a zero-band-gap semiconductor, which is difficult to use as a fluorescent material. The two-dimensional graphene sheet can be converted into graphene quantum dots of a zero-dimensional size through chemical treatment. Generally, for the synthesis of graphene quantum dots, so-called "top-down" and "bottom-up" schemes are used. In the top-down method, a large carbon-based material such as graphene can be formed by cutting into graphene quantum dots of small size. In this regard, various process technologies have been introduced for the formation of graphene quantum dots, including organic synthesis, voltage application, electrochemical methods, multistage synthesis, high temperature / high pressure technology and hydrothermal segregation.

Doping graphene quantum dots is an effective way to convert the intrinsic properties of graphene quantum dots. Nitrogen is similar in size to carbon, but has a greater electronegativity than carbon. By doping the graphene quantum dot with nitrogen atoms, bandgap and fluorescence properties can be controlled. Thus, nitrogen doped graphene quantum dots (N-GQD) can exhibit unique properties in electrocatalytic activity, tunable luminescence and biocompatibility.

On the other hand, graphene composed only of carbon has a high conductivity with a π - π conjugation structure. However, by substituting a small amount of carbon in graphene with one electron more than carbon, it can have higher conductivity than carbon-only graphene. Thus, as a method of synthesizing graphene doped with nitrogen Methods have been disclosed. Among them, a chemical vapor deposition method, a nitrogen plasma method, and the like have been reported.

However, these methods are in various studies because they are difficult to mass-produce.

Recently, various researches have been carried out to replace existing fossil fuels in order to solve the energy problems faced. Extensive research is underway to utilize natural energy such as wind, nuclear, and solar power to replace petroleum resources that are depleted within several decades. Among these solar cells, unlike other energy sources, the resources are infinite and environmentally friendly. Since the development of Se solar cells in 1983, recently, silicon solar cells and polymer solar cells (for example, dye-sensitized solar cells ), And CIGS-based solar cells.

Such solar cells use optical coatings to reflect the spectral portion of sunlight that is not converted to electricity to improve output. In the case where the optical coating is not used, there is a problem that sunlight outside the spectrum response region (sunlight having a longer or shorter wavelength than the spectrum response region) is absorbed by the battery, thereby increasing the temperature of the battery and decreasing the light conversion efficiency . For this purpose, only the light of 350 nm or more (silicon solar cell) or 400 nm or more (polymer solar cell) in the sunlight is filtered using the filter.

Therefore, there is a demand for a method of converting sunlight in an area not electrically converted in a solar cell and simultaneously converting them into light of a wavelength that can be used in a solar cell, thereby increasing energy utilization efficiency.

 Particle & Particle Systems Characterization 32 (2015) 434-440  Scientific Reports 4 (2014) 5294

The present invention can easily and eco-friendlyly prepare nitrogen doped graphene quantum dots based on a solution process at a low process temperature and a short process time, and can reduce nitrogen doping ratio, C / N atomic ratio or C / O atomic ratio Doped graphene quantum dots which can be easily controlled by using the method of the present invention.

It is another object of the present invention to provide a nitrogen doped graphene quantum dot capable of adjusting the spectrum of incident light and improving the photoluminescence quantum yield.

Also, the present invention provides a solar cell including a graphene quantum dot doped with nitrogen that functions to move a spectrum of incident light having a wavelength of 500 nm or less to a wavelength band of 500 nm or more at which the solar cell can absorb the light. .

According to an embodiment of the present invention, there is provided a method for preparing a graphene oxide-doped graphene quantum dot comprising: preparing a mixed solution of graphene oxide and a polyethyleneimine; Stirring the mixed solution of polyethyleneimine and graphene oxide; And heating the stirred mixture of the polyethyleneimine and the graphene oxide to cut the graphene oxide and nitrogen-doping and reducing the cut graphene oxide.

Further, in the step of preparing the graphene oxide and polyethyleneimine mixed solution, the weight ratio of polyethyleneimine to graphene oxide may be 0.01 to 20.0.

In the preparation of the mixed solution of graphene oxide and polyethyleneimine, the mixed solution may contain water, ethanol, dimethylformamide (DMF), dimethylsulfoxide (DMSO), tetrahydrofuran tetrahydrofuran, THF) and acetone (acetone).

Further, in the step of stirring the mixture of the polyethyleneimine and the graphene oxide, heat treatment may be further performed, and the heat treatment temperature may be 0 to 100 ° C.

Further, in the step of heating the stirred mixture of the polyethyleneimine and the graphene oxide to cut the graphene oxide and nitrogen-doping and reducing the cut graphene oxide, the heating temperature may be 25 to 400 ° C .

The graphene quantum dot doped with nitrogen according to an embodiment of the present invention is characterized in that it absorbs light having a wavelength of 200 to 550 nm and emits it as light having a wavelength of 460 to 650 nm and has a quantum yield of 78 to 99% measured by a PL spectrophotometer .

Also, the carbon / oxygen atomic ratio of the nitrogen-doped graphene quantum dot may be 2.0 to 10.0.

In addition, the carbon / nitrogen atom ratio of the nitrogen-doped graphene quantum dot may be 2.0 to 20.0.

A solar cell comprising nitrogen doped graphene quantum dots in accordance with an embodiment of the present invention includes a substrate; A first electrode formed on the substrate; A photoactive layer formed on the first electrode; And an energy down shift layer disposed on the photoactive layer, wherein the energy down shift layer absorbs light having a wavelength of 200 to 550 nm and emits light having a wavelength of 460 to 650 nm, and PL And a PL quantum yield measured by a spectrophotometer is 78 to 99%.

The method of preparing a nitrogen doped graphene quantum dot according to an embodiment of the present invention is an eco-friendly, simple method that uses a solution process to lower the nitrogen doping rate, the C / N atomic ratio, or the C / O atom Controlled graphene quantum dot manufacturing method can be manufactured and mass production is possible.

Nitrogen-doped graphene quantum dots according to embodiments of the present invention can control the nitrogen doping rate, C / N atomic ratio, or C / O atomic ratio in a simple manner, and can control optical characteristics, Can be applied.

The solar cell including the nitrogen doped graphene quantum dot according to the embodiment of the present invention can adjust the spectrum of the wavelength of incident light, improve the photoluminescence quantum yield, improve the photoelectric efficiency, The light energy efficiency can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart of a method of manufacturing a nitrogen doped graphene quantum dot according to an embodiment of the present invention.
2 is a schematic diagram of a solar cell including nitrogen doped graphene quantum dots according to an embodiment of the present invention.
3 is a TEM photograph of the graphene quantum dot doped with nitrogen prepared according to Examples 1 to 4. FIG.
4 is a graph of X-ray diffraction analysis of nitrogen doped graphene quantum dots prepared in Examples 1 to 4.
5 is a graph of Raman spectroscopic analysis of nitrogen doped graphene quantum dots prepared in Examples 1 to 4.
6 is an X-ray photoelectron spectroscopy (XPS) analysis of C1S of nitrogen doped graphene quantum dots prepared according to Examples 1 to 4.
FIG. 7 is a graph of X-ray photoelectron spectroscopy (XPS) analysis of O1S of nitrogen doped graphene quantum dots prepared according to Examples 1 to 4. FIG.
FIG. 8 is a graph of X-ray photoelectron spectroscopy (XPS) analysis of N1S of nitrogen doped graphene quantum dots prepared according to Examples 1 to 4. FIG.
FIG. 9 shows the C / O and C / N atomic ratios of the nitrogen doped graphene quantum dots prepared in Examples 1 to 4.
10 is a UV-Vis spectrophotometric graph of nitrogen-doped graphene quantum dots prepared according to Examples 1 to 4. FIG.
FIG. 11 shows spectrofluorometer results for excitation wavelengths of 370 nm of nitrogen doped graphene quantum dots prepared according to Examples 1 to 4. FIG.
12 shows spectrofluorometer results for the excitation wavelength of 405 nm of the nitrogen-doped graphene quantum dots prepared according to Examples 1 to 4. FIG.
13 shows the photoluminescence quantum yield (PLQY) at an excitation wavelength of 370 nm of the nitrogen-doped graphene quantum dots prepared according to Examples 1 to 4.
14 is a graph showing photoluminescence decay at five emission wavelengths (excitation wavelength is 379 nm) of 380 nm, 405 nm, 450 nm, 550 nm and 650 nm of nitrogen-doped graphene quantum dots prepared according to Example 1. [ FIG.
15 shows the phtolumnscence decay at five emission wavelengths (excitation wavelength is 379 nm) of 380 nm, 405 nm, 450 nm, 550 nm, and 650 nm of nitrogen-doped graphene quantum dots prepared according to Example 2. [ FIG.
16 is a graph showing photoluminescence decay at five emission wavelengths (excitation wavelength is 379 nm) of 380 nm, 405 nm, 450 nm, 550 nm, and 650 nm of nitrogen-doped graphene quantum dots prepared according to Example 3. [ FIG.
17 shows the phtolumnscence decay at five emission wavelengths (excitation wavelength is 379 nm) of 380 nm, 405 nm, 450 nm, 550 nm, and 650 nm of the nitrogen-doped graphene quantum dots prepared according to Example 4. [ FIG.
18 shows the time-resolved fluorescence analysis of the nitrogen-doped graphene quantum dots prepared according to Examples 1 to 4.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art. Accordingly, the shapes and sizes of the elements in the drawings may be exaggerated for clarity of description, and the elements denoted by the same reference numerals in the drawings are the same elements. In the drawings, like reference numerals are used throughout the drawings. In addition, " including " an element throughout the specification does not exclude other elements unless specifically stated to the contrary.

nitrogen Doped Grapina Quantum dot  Manufacturing method

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart of a method of manufacturing a nitrogen doped graphene quantum dot according to an embodiment of the present invention.

Referring to FIG. 1, a method for preparing a graphene quantum dot having nitrogen doped according to an embodiment of the present invention includes: preparing a mixed solution of graphene oxide and polyethyleneimine; Stirring the mixed solution of polyethyleneimine and graphene oxide; And heating the stirred mixture of the polyethyleneimine and the graphene oxide to cut the graphene oxide and nitrogen-doping and reducing the cut graphene oxide.

Hereinafter, the method for producing a nitrogen doped graphene quantum dot according to the present invention will be described in detail for each step.

First, the first step of the method of preparing a graphene quantum dot doped with nitrogen according to an embodiment of the present invention is a step of preparing a mixed solution of graphene oxide and polyethyleneimine.

The graphene oxide can be prepared by chemically oxidizing and stripping graphene or graphite. In addition, a syringe filter or a centrifugation method may be used to control the size of the graphene oxide.

The polyethyleneimine (PEI, polyethyleneimine) is not particularly limited, but may be a polyethyleneimine for commercial use.

Conventionally, dimethylformamide was used as a nitrogen dopant for preparing nitrogen-doped graphene oxide or nitrogen-doped graphene quantum dots. The dimethylformamide has difficulties in controlling the nitrogen doping rate and difficulty in controlling impurities there was.

On the other hand, in the embodiment of the present invention, it is possible to control nitrogen doping only by controlling the relative ratio of polyethyleneimine to graphene oxide by using polyethyleneimine.

Since the polyethyleneimine has a highly reactive amine group and an aliphatic moiety, it can be used as a reducing agent, a surface modifier, and can improve hydrogen bonding on the surface of graphene oxide and function graphene oxide.

The weight ratio of polyethyleneimine to graphene oxide contained in the mixed solution of the first step, that is, the weight ratio of polyethyleneimine / graphene oxide may preferably be 0.01 to 20.0, more preferably 0.5 to 10.0, May be 1.0 to 5.0.

If the weight ratio of polyethyleneimine / graphene oxide in the mixed solution is less than 0.01, there is a problem that graphene oxide is difficult to be doped with nitrogen. When the weight ratio of polyethyleneimine / graphene oxide is more than 20.0, the property of graphene oxide itself decreases have.

The nitrogen doped graphene quantum dot manufacturing method according to an embodiment of the present invention can control the nitrogen doping level by controlling the mixing ratio of graphene oxide and polyethyleneimine. Furthermore, since graphene quantum dots doped with nitrogen can be prepared without containing any additive, it is environmentally friendly and can be mass-produced by a liquid phase process.

Further, by using polyethyleneimine as the nitrogen dopant, it is possible to produce graphene quantum dots doped with sufficient nitrogen even at a low temperature.

In the preparation of the mixed solution of graphene oxide and polyethyleneimine, the mixed solution may contain water, ethanol, dimethylformamide (DMF), dimethylsulfoxide (DMSO), tetrahydrofuran, THF) and acetone (Acetone). As a specific example, the mixed solution in the above step may be an aqueous solution, but is not limited thereto.

Furthermore, the mixed solution in the above step may not contain a dispersant and other additives. It is possible to produce graphene quantum dots doped with nitrogen of good quality without dispersants and other additives, thereby being environmentally advantageous.

Next, the second step of the nitrogen doped graphene quantum dot manufacturing method according to the present invention is a step of stirring the mixed solution of the polyethyleneimine and the graphene oxide. Through stirring the mixture of polyethyleneimine and graphene oxide, at least a portion of the graphene oxide can be reduced and can be nitrogen doped.

Since the polyethyleneimine is used as a nitrogen dopant in the step of stirring the polyethyleneimine and the graphene oxide mixed solution, reduction and nitrogen doping of graphene oxide can be simultaneously performed at normal pressure.

Further, the nitrogen doping ratio and the degree of reduction of graphene oxide can be controlled by controlling the ratio of the polyethyleneimine and the graphene oxide.

In the step of stirring the mixture of the polyethyleneimine and the graphene oxide, the graphene oxide may be functionalized.

Here, the functionalization refers to attaching PEI and GO. The experimental method of functionalization is to disperse graphene oxide in distilled water and adjust the concentration to 4.0 mg / mL. The solution is further dispersed by ultrasonic for 1 hour. 2000 PEI (0.5 mg / mL) with an average molecular weight in distilled water is added to the graphene oxime solution to change the PEI / GO weight ratio from 0.001 to 0.2. To attach PEI to graphene oxide, mix from 1 to 4 hours at 0 to 100 캜 using a magnetic stirrer.

The step of stirring the polyethyleneimine and the graphene oxide mixed solution may include heat treatment. At this time, the heat treatment temperature may be 0 to 100 ° C, preferably 20 to 95 ° C, and more preferably 25 to 80 ° C.

If the heat treatment temperature is less than 0 ° C, the functionalization level of graphene oxide may be lowered by polyethyleneimine. If the heat treatment temperature exceeds 100 ° C, the reaction occurs during the stirring of the polyethyleneimine and the graphene oxide mixed solution, It may not be uniform.

In the step of stirring the polyethyleneimine and the graphene oxide mixed solution, the mixture solution of the polyethyleneimine and the graphene oxide may be stirred at 100 to 1000 rpm, preferably at 250 to 650 rpm, .

Next, in the third step of the method for producing nitrogen doped graphene quantum dots according to the present invention, the graphene oxide is cut by heating the stirred solution of the polyethyleneimine and the graphene oxide, and the cut graphene oxide is dissolved in nitrogen Doping and reduction.

In the above step, the heating temperature may be 25 to 400 ° C, preferably 200 to 350 ° C, and more preferably 250 to 350 ° C.

If the heating temperature is lower than 25 占 폚 in the above step, the reaction of graphene oxide and polyethyleneimine may not occur, the graphene oxide may not function, or the graphene oxide may not be reduced. On the other hand, if the heating temperature is higher than 400 ° C in the above step, the size of the prepared graphene quantum dot and the nitrogen doping of the graphene quantum dot may not be adjusted to a desired level.

Further, in the step of heating the stirred mixture of the polyethyleneimine and the graphene oxide to cut the graphene oxide and nitrogen-doping and reducing the cut graphene oxide, the heating time may be at least 1 minute, 5 minutes to 4 hours, and more preferably 15 minutes to 2 hours.

Also, in the step of heating the mixed solution of the stirred polyethyleneimine and graphene oxide to cut the graphene oxide and nitrogen-doping and reducing the cut graphene oxide, the raising time is from 1 ° C / min to 20 ° C / Min, preferably from 5 to 10 [deg.] C / min.

If the temperature rise time is less than 1 ° C / min in the above step, there is a disadvantage in that the process time is lengthened and the process efficiency is decreased. If the temperature rise time exceeds 20 ° C / min in the above step, The cleavage of the pin oxide and the cleaved graphene oxide may not be uniformly controlled by nitrogen doping and reduction.

nitrogen Doped Grapina Qdot

The nitrogen doped graphene quantum dot according to an embodiment of the present invention absorbs light having a wavelength of 200 to 550 nm and emits light with a wavelength of 460 to 650 nm and has a PL quantum yield of 78 to 99% And is measured by a spectrophotometer.

The nitrogen-doped graphene quantum dot according to an embodiment of the present invention may be one produced by the above-described manufacturing method. That is, the N-doped graphene quantum dot may be prepared by controlling the doping ratio of nitrogen by controlling the injection ratio of polyethyleneimine and graphene oxide, and the C / N atomic ratio or C / N atomic ratio of the nitrogen- O atomic ratio.

Generally, in order to maximize the efficiency in the solar cell, it is advantageous that the range of the wavelength that can be absorbed by the light absorbing layer of the solar cell in the solar cell is wide. In a general solar cell, the light absorbing layer absorbs light in a narrow wavelength range in the visible light region to generate electrons and holes.

The graphene quantum dot doped with nitrogen according to an embodiment of the present invention is capable of absorbing light having a wide wavelength of 200 to 550 nm and emitting the light with a wavelength of 460 to 650 nm to be applied to a solar cell, particularly a CIGS solar cell And can be applied to improve solar cell performance.

The carbon / oxygen atomic ratio of the nitrogen-doped graphene quantum dot may be 2.0 to 10.0. Also, the carbon / oxygen atom ratio may preferably be 2.8 to 9.0.

When the carbon / oxygen atomic ratio of the nitrogen doped graphene quantum dots is less than 2.0, graphene quantum dots can be obtained instead of graphene quantum dots, so that the quantum yield of graphene quantum dots produced thereby can be very low. When the nitrogen / doped graphene quantum dot has a carbon / oxygen atomic ratio of 10.0 or more, a very high amount of carbon is present, so that the band gap of the graphene quantum dot becomes very low and the quantum yield can be lowered. In addition, the excitation wavelength and the emission wavelength may overlap, making it impossible to downshift the desired wavelength in the solar cell.

The carbon / nitrogen atom ratio of the nitrogen doped graphene quantum dot may be 2.0 to 20.0. Further, the carbon / nitrogen atom ratio may preferably be 4.0 to 12.0.

When the carbon / nitrogen atomic ratio of the nitrogen doped graphene quantum dots is 20.0 or more, the spectrum may be blue shifted to a short wavelength band, and the excitation spectrum and the emission spectrum may overlap each other and the desired wavelength transition may not be achieved. When the carbon / nitrogen atomic ratio of the nitrogen-doped graphene quantum dot is 2.0 or less, the ratio of Graphitic N / Oxidized N decreases and the yield of PL can decrease.

nitrogen Doped Grapina Quantum dot  Including solar cells

2 is a schematic diagram of a solar cell including nitrogen doped graphene quantum dots according to an embodiment of the present invention.

Referring to FIG. 2, a solar cell including nitrogen doped graphene quantum dots according to an embodiment of the present invention includes a substrate; A first electrode formed on the substrate; A photoactive layer formed on the first electrode; And an energy down shift layer disposed on the photoactive layer, wherein the energy down shift layer absorbs light having a wavelength of 200 to 550 nm and emits light having a wavelength of 460 to 650 nm, and PL And has a PL quantum yield of 78 to 99% as measured by a spectrophotometer.

A solar cell including nitrogen doped graphene quantum dots according to an embodiment of the present invention may have an improved light absorption rate and photoelectric conversion rate by including an energy down shift layer.

The nitrogen doped graphene quantum dot included in the energy down shift layer may be one produced by the above-described manufacturing method. That is, the N-doped graphene quantum dot may be prepared by controlling the doping ratio of nitrogen by controlling the injection ratio of polyethyleneimine and graphene oxide, and the C / N atomic ratio or C / N atomic ratio of the nitrogen- O atomic ratio.

Example

Example 1

Step 1: 10 mL of an aqueous solution of graphene oxide (GO) at a concentration of 4 mg / mL and 2 mL of an aqueous solution of polyethyleneimine (PEI) at a concentration of 0.5 mg / mL were prepared.

Step 2: A polyethyleneimine aqueous solution was added to the prepared graphene oxide aqueous solution to prepare a mixed solution (PEI (0.5 mg / mL) / GO (4 mg / mL = 0.4 (v / v), weight ratio 0.05).

The prepared mixed solution was placed on a hot plate and processed at a temperature of 90 DEG C and a stirring speed of 450 rpm for 4 hours to prepare a functionalized graphene oxide paste.

Step 3: The functionalized graphene oxide paste was placed horizontally, heated up to 350 ° C at a rate of 5 ° C per minute, and heated for 60 minutes to obtain nitrogen doped graphene quantum dots and nitrogen doped porous graphene.

Step 4: The nitrogen-doped graphene quantum dot and the nitrogen-doped porous graphene were dispersed in an ethanol solvent and filtered using a 0.1 PTFE syringe filter to obtain a yellow solution for absorption and emission analysis.

Example 2

In step 1 of Example 1, the polyethyleneimine and graphene oxide contained in the mixed solution were mixed so that the volume ratio of PEI (0.5 mg / mL) / GO (4 mg / mL = 0.16 (v / v) And graphene quantum dots doped with nitrogen were prepared in the same manner as in Example 1 except that the heat treatment temperature in the horizontal path was changed to 300 캜 in Step 3.

Example 3

In step 1 of Example 2, the polyethyleneimine and graphene oxide contained in the mixed solution were mixed so that the volume ratio of PEI (0.5 mg / mL) / GO (4 mg / mL = 0.08 (v / v) And a nitrogen doped graphene quantum dot was prepared in the same manner as in Example 2 except that the heat treatment temperature in the horizontal path was changed to 250 and the time to 15 minutes in the step 3.

Example 4

In step 1 of Example 2, the polyethyleneimine and graphene oxide contained in the mixed solution were mixed so that the volume ratio of PEI (0.5 mg / mL) / GO (4 mg / mL = 0.16 (v / v) And graphene quantum dots doped with nitrogen were prepared in the same manner as in Example 2, except that the heat treatment temperature in the horizontal furnace was changed to 250 캜 in Step 3.

Comparative Example 1

Step 1: 400 mL of graphite was mixed with 10 mL of nitric acid at a concentration of 16 mol / mL. The mixed solution was ultrasonicated at 80 W (10 kHz) for 2 hours.

Step 2: The mixed solution prepared in step 1 above was placed in an oil bath on a hot plate and heated at 120 DEG C for 24 hours. After the reaction, the mixed solution was slowly cooled to room temperature.

Step 3: The mixed solution prepared in step 2 was separated at 12,000 rpm for 10 minutes using a centrifuge.

Step 4: The mixed solution prepared in step 3 above was further heated at 200 ° C to evaporate the supernatant. As a result, a reddish brown solid was obtained. The reddish brown solid phase was dissolved in water and the pH value was adjusted by the addition of NaOH.

Comparative Example 2

Step 1: 300 mg of graphene oxide was mixed with 30 mL of dimethylformamide to prepare a mixed solution (10 mg / mL). The mixed solution was subjected to ultrasonic treatment at 200 W (100 kHz) for 1 hour.

Step 2: The mixed solution prepared in step 1 was placed in a Teflon line-treated container (50 mL), heated at 200 DEG C for 5 hours, and then cooled to room temperature.

Step 3: The mixed solution prepared in step 2 was removed with the aid of a rotary evaporator, and the nitrogen-doped graphene quantum dots thus obtained were dispersed in water or an organic solvent.

Comparative Example 3 - Solvent heat  Nitrogen by synthetic method Doped Grapina Quantum dot  Produce

Step 1: A 5 mL solution of 210 mg (1 mole) of citric acid (CA) and 180 mg (3 mole) of ethanol diamine (EDA) was prepared.

Step 2: The mixed solution prepared in step 1 was placed in a Teflon line-treated container (20 mL), heated at 160 DEG C for 4 hours, and then cooled to room temperature.

Step 3: The mixed solution prepared in step 2 was mixed with ethanol and centrifuged at 5000 rpm for 5 minutes. The resulting solution was redispersed in water again.

Experimental Example

Experimental Example 1 - Grapina Qdot  Size analysis

In order to observe the changes in the size of the nitrogen doped graphene quantum dots according to the manufacturing conditions such as the nitrogen doping ratio, the reaction temperature, and the reaction time of the nitrogen doped graphene quantum dots prepared in Examples 1 to 4 of the present invention, The results are shown in Fig.

Referring to FIG. 3, the size of the nitrogen-doped graphene quantum dot prepared by Example 1 is 2 to 3 nm, the size of the nitrogen-doped graphene quantum dot prepared by Example 2 is 3 to 4 nm, The size of the nitrogen doped graphene quantum dot prepared by Example 4 is 4 to 4 nm and the size of the nitrogen doped graphene quantum dot prepared by Example 4 is 5 to 5 nm.

It was confirmed that the size of the nitrogen doped graphene quantum dot can be controlled by changing the manufacturing conditions such as the doping ratio, the reaction temperature, and the reaction time.

Experimental Example  2

(One) X-ray diffraction (XRD)  analysis

In order to analyze the crystal structure of the nitrogen-doped graphene quantum dots prepared in Examples 1 to 4 of the present invention, an X-ray diffraction analysis was carried out and this is shown in FIG.

Referring to FIG. 4, the second-order values of the nitrogen-doped graphene quantum dots prepared in Examples 1 to 4 were 20.77, 18.21, 19.80 and 18.21 °, respectively, and the 2-theta value of conventional graphene oxide was 10.33 It can be seen that the second-order values of the nitrogen-doped graphene quantum dots prepared in Examples 1 to 4 of the present invention are higher.

(2) Raman analysis

Raman analysis was performed to analyze the optical properties of the nitrogen-doped graphene quantum dots prepared in Examples 1 to 4 of the present invention, and this is shown in FIG.

Referring to FIG. 5, three major peaks were observed in the Raman analysis graph of the nitrogen-doped graphene quantum dots prepared by Examples 1-4. It can be seen that two of the three peaks are D band and G band of graphene and the other one of the three peaks is a band associated with nitrogen. The D band is a band appearing at 1340 cm ^-1 when the carbon hexagonal network has defects and distortions, and the G band is a band appearing at 1580 cm ^-1 when the carbon hexagonal network is perfectly bonded and E 2 g vibration is present . Further, the degree of distortion of the hexagonal plane of the material from the D band and I _D / I _G can be confirmed

Experimental Example  3 - C / O Atomic ratio  And C / N Atomic ratio  analysis

The C / O atomic ratio and the C / O atomic ratio of the nitrogen-doped graphene quantum dots varied according to the manufacturing conditions such as the nitrogen doping ratio, the reaction temperature, and the reaction time of the nitrogen-doped graphene quantum dots prepared in Examples 1 to 4 of the present invention, X-ray photoelectron spectroscopy (XPS) analysis was performed to analyze the N atomic ratio, and the results are shown in FIGS. 6, 7, 8, and 9.

Referring to FIG. 8, it can be seen that the C / O atomic ratio of the nitrogen-doped graphene quantum dots prepared in Examples 1 to 4 is about 2.8 to 9.0.

Referring to FIG. 9, it can be seen that the C / N atomic ratio of the nitrogen-doped graphene quantum dots prepared in Examples 1 to 4 is about 4.0 to 12.0.

Experimental Example  4

(One) Photoluminescence  Quantum yield ( Photoluminescence quantum yield ) And spectral modulation

Doped graphene quantum dots prepared according to Examples 1 to 4 and Comparative Examples 1 to 3 of the present invention exhibited different PL densities of nitrogen doped graphene quantum dots according to changes in manufacturing conditions such as nitrogen doping ratio, UV-Vis spectroscopic analysis and spectrofluorometer analysis for the excitation wavelength of 405 nm were performed to analyze the phtolumnescence quantum yield (PLQY) and migration of the excitation spectrum. The results are shown in Figs. 10, 11 12, Fig. 13 and Table 1.

The PL quantum yield was measured with a xenon lamp and a PL spectrophotometer (Darsa, PSI TRADING CO., Ltd.).

Referring to FIG. 10 and Table 1, it can be seen that the absorption range of the nitrogen-doped graphene quantum dot prepared by Example 1 is lower than the wavelength of 500 nm. Referring to FIG. 11, the nitrogen-doped graphene quantum dot prepared by Example 1 has an emission wavelength of 510 to 650 nm at an excitation wavelength of 370 nm. Referring to FIG. 12, the nitrogen-doped graphene quantum dot prepared by Example 1 has an emission wavelength of 475 to 650 nm at an excitation wavelength of 405 nm.

Referring to FIG. 10 and Table 1, it can be seen that the absorption range of the nitrogen-doped graphene quantum dot prepared by Example 2 is lower than the wavelength of 500 nm. Referring to FIG. 12, the nitrogen-doped graphene quantum dot prepared by Example 2 has an emission wavelength of 460 to 650 nm at an excitation wavelength of 405 nm.

Nitrogen-doped graphene quantum dots according to embodiments of the present invention have a high PL quantum yield without solvent thermal synthesis. On the other hand, in the case of Comparative Example 1, since the solution process method is used instead of the solvent thermo-synthesis method, the yield of PL quantum yield is relatively low. The comparative example 2 and the comparative example 4 using the solvent thermogravimetry method were measured to have a relatively high PL quantum yield, but the manufacturing process is complicated and the manufacturing time is long, which is not suitable for mass production.

The substance (or method) Absorption spectrum (nm) Excitation wavelength (nm) Emission spectrum (nm)
Photoluminescence quantum yield (%) Example 1 Polyethylene imine, heating 200 ~ 470 405 510-550 78 Example 2 Polyethylene imine, heating 200 ~ 550 405 460-550 99 Comparative Example 1 nitric acid,
Reflux method
(reflux)
200 ~ 550 365 450 to 600 - Comparative Example 2 DMF,
Solvent thermosynthesis 200 to 600 400 400 to 650 74 Comparative Example 3 Citric acid, ethanol diamine, solvent thermosynthesis 220 ~ 420 340 to 420 400 to 550 94

13 shows the photoluminescence quantum yield (PLQY) at an excitation wavelength of 370 nm of the nitrogen-doped graphene quantum dots prepared according to Examples 1 to 4.

Referring to FIG. 13, it can be seen that the photoluminescence quantum yield at the excitation wavelength of 370 nm of the nitrogen-doped graphene quantum dots prepared in Examples 1 to 4 is 78 to 99%.

PL quantum yield was measured with a PL instrument. A comparative sample known quantum yield was used to calculate the quantum yield of the graphene quantum dot. The following formula is used to calculate the PL quantum yield.

PL quantum yield = (quantum yield of the reference ^{_{sample) X (R GQD 2 / R}} ref. 2) X (A ref. / A GQD) X (S GQD / S ref.)

R is the refractive index of the solvent, A is the absorption rate of the solution, S is the sum of the emission spectra

(2) Photoluminescence  attenuation( Photoluminescence decay )

The nitrogen doped graphene quantum dots prepared according to Examples 1 to 4 and Comparative Examples 1 to 3 of the present invention were subjected to the measurement of nitrogen doping ratio, reaction temperature, reaction time, Resolved spectroscopy system (TRSS) was used to analyze the photoluminescence attenuation at emission wavelengths of 380, 405, 450, 550, and 650 nm at the excitation wavelength of 400 nm. 15, 16, 17 and 18, respectively.

Referring to FIG. 14, it can be seen that the maximum photoluminescence decay lifetime of the nitrogen-doped graphene quantum dot prepared according to Example 1 was obtained when the emission wavelength was 550 nm.

Referring to FIG. 15, it can be seen that the maximum photoluminescence decay lifetime of the nitrogen doped graphene quantum dot prepared in Example 2 was obtained when the emission wavelength was 550 nm.

Referring to FIG. 16, it can be seen that the maximum photoluminescence decay lifetime of the nitrogen doped graphene quantum dots prepared according to Example 3 is similarly obtained when the emission wavelengths are 450, 550 and 650 nm.

Referring to FIG. 17, it can be seen that the maximum photoluminescence decay lifetime of the nitrogen-doped graphene quantum dot prepared according to Example 4 was obtained when the emission wavelength was 550 nm.

Referring to FIG. 18, it can be seen that the photoluminescence decay lifetime obtained at the emission wavelength of 550 nm of the nitrogen-doped quantum dots prepared by Examples 1 to 4 is fitted using a damping spinning formula consisting of three multiple exponents.

14 shows the respective PL decay times at emission wavelengths of 380, 405, 450, 500, 550, and 600 nm when the excitation wavelength is 379 nm. In order to improve the efficiency through the CIGS solar cell wavelength transition of the graphene quantum dots, the PL decay time of the emission wavelength should be long at a wavelength band of 500 nm or more, and the PL decay time of the emission wavelength should be short at a wavelength of less than 500 nm. 14, 15 and 17, since the maximum PL decay time was observed at a wavelength of 550 nm, the graphene quantum dots prepared by Example 1, Example 2 and Example 4 may be useful for CIGS solar cell applications have. Referring to FIG. 16, the graphene quantum dots prepared in Example 3 can be useful for silicon solar cell applications because the maximum PL decay time was obtained at wavelengths of 405, 450 and 550 nm.

The present invention is not limited to the above-described embodiment and the accompanying drawings, but is intended to be limited by the appended claims. It will be apparent to those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. something to do.

1000: Solar cell
1100: substrate 1200: first electrode
1300: electron transport layer 1400: light absorption layer
1500: hole transport layer 1600: second electrode
1700: energy down shift layer

Claims (9)

Preparing a mixed solution of graphene oxide and polyethyleneimine;
Stirring the mixed solution of polyethyleneimine and graphene oxide; And
And heating the stirred mixture of the polyethyleneimine and the graphene oxide to cut the graphene oxide and nitrogen-doped and reduced the graphene oxide to obtain a nitrogen-doped graphene quantum dot.
The method according to claim 1,
Wherein the weight ratio of polyethyleneimine to graphene oxide in the step of preparing the graphene oxide and polyethyleneimine mixed solution is 0.01 to 20.0.
The method according to claim 1,
In the preparation of the mixed solution of graphene oxide and polyethyleneimine, the mixed solution may contain water, ethanol, dimethylformamide (DMF), dimethylsulfoxide (DMSO), tetrahydrofuran, THF) and acetone (Acetone).
The method according to claim 1,
Further comprising the step of heat treating the mixture of the polyethyleneimine and the graphene oxide mixed solution, wherein the heat treatment temperature is 0 to 100 占 폚.
The method according to claim 1,
Heating the mixed solution of the polyethyleneimine and the graphene oxide with stirring to cut the graphene oxide and nitrogen doping and reducing the cut graphene oxide, wherein the heating temperature is from 25 to 400 DEG C, Pin quantum dot.
Nitrogen doped graphene quantum dots that absorb light at a wavelength of 200-550 nm and emit light at a wavelength of 460-650 nm and have a PL quantum yield of 78-99% as measured by a PL spectrophotometer.
The method according to claim 6,
Wherein the nitrogen / doped graphene quantum dot has a carbon / oxygen atomic ratio of 2.0 to 10.0.
The method according to claim 6,
Wherein the nitrogen doped graphene quantum dot has a carbon / nitrogen atomic ratio of 2.0 to 20.0.
Board;
A first electrode formed on the substrate;
A photoactive layer formed on the first electrode; And
And an energy down shift layer disposed on the photoactive layer,
Wherein the energy down shift layer comprises a nitrogen doped graphene quantum dot having a PL quantum yield of 78 to 99% as measured by a PL spectrophotometer, absorbing light at a wavelength of from 200 to 550 nm, emitting light at a wavelength of from 460 to 650 nm, Solar cells.

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