KR20160149874A - Naphthalene diimide based copolymers with high electron mobility and synthesizing method of the same - Google Patents

Abstract

The present invention relates to a novel NDI-based copolymer comprising thiophene, bithiophene, or thienylene-vinylene-thienylene. According to the present invention, a novel NDI-based copolymer having high electron mobility is confirmed to be synthesized. The copolymer synthesized according to the present invention substantially adjusts intermolecular interactions and flatness of a polymer acceptor, and causes a change in the electron mobility. As a result, when applied to a solar cell, the NDI copolymer can enhance efficiency of the solar cell. Also, a solar cell having high efficiency can be prepared without a separate additive.

Description

[0001] The present invention relates to an NDI-based copolymer having high electron mobility and a method for synthesizing the same. More specifically,

The present invention relates to novel naphthalene diimide based (NDI) copolymers with high electron mobility and their synthesis. Specifically, the present invention relates to an NDI-based copolymer improved in electron mobility by including an electron-rich unit in an electron deficiency unit of NDI, and a polymer solar cell comprising the same.

Polymer solar cells (PSCs) are well suited for power sources for next generation flexible and portable devices due to their advantages such as low cost, flexibility, light weight and large area production (M. Kaltenbrunner et al ., Nat Commun ., 2012, 3; R. Sondergaard et al ., Mater. Today , 2012, 15, 36). Polymer / fullerene bulk-heterojunction (BHJ) solar cells are at the forefront of efficiency tables with power conversion efficiencies (PCEs)> 9% recently (TL Nguyen et al .., Energ Environ Sci 2014, 7, 3040;.. C. Cabanetos et al, J. Am Chem Soc 2013, 135, 4656;........ TE Kang et al, J. Mater Chem A 2014 , 2, 15252). This success is largely due to the development of novel low-band-gap polymers with reasonable molecular design, but the fullerene derivatives, namely phenyl-C ₆₁ or C ₇₁ -butyric acid methyl ester (PCBM) and fullerene bisadduct, is used as an electron acceptor (electron acceptor) (M. Lenes et al, Adv Mater 2008, 20, 2116;. KH Kim et al, Chem Mater 2012, 24, 2373;.... Y. He et al, J Am. Chem. Soc ., 2010, 132, 1377). However, in order to further improve the stability of the PCE value and the PSCs, there is a desperate need to develop a substitute thereof. This is because conventional fullerene derivatives typically have the disadvantage of including low light absorption at long wavelengths, limited controllability of energy levels, and a lack of mechanical and thermal stability (E. Zhou et al ., Adv. Mater 2013, 25, 6991;. ... IH Jung et al, Chem Mater 2014, 26, 3450). The use of active layers in PSCs, consisting of polymer donors and polymer acceptors, called so-called all-polymer solar cells (all-PSCs), has emerged as a new theme is (A. Facchetti, Mater Today 2013, 16, 123;.. T. Earmme et al, Adv Mater 2014, 26, 6080;...... WW Li et al, Adv Mater 2014, 26, 3304). The notable advantages of all-PSCs are: (1) versatility in molecular design of both polymer donors and polymeric acceptors to adjust energy levels towards higher open-circuit voltages (V _OC ); and (2) and the thermal properties (Y. Zhou et al, Adv Mater 2014, 26, 3767;... D. Mori et al, Adv Energy Mater 2014, 4;...... H. Huang et al, Adv Funct. Materials 2014, 24, 2782). However, its low performance (3-5% of PCEs) was a major hurdle, which is typically due to limited photocurrent (J _SC ) <10 mAcm ^-2 and fill factor (FF) &Lt; 50%). Its low photocurrent and fill factor is primarily due to the undesirable blend form in the active layer and the electron mobility of the polymer acceptor much lower than fullerene (H. Kang et al ., J. Am. Chem. Soc . 2015, 137 , 2359; M. Schubert et al, Adv Funct Mater 2014, 24, 4068;..... N. Zhou et al, Adv Energy Mater 2014, 4, n / a). In addition, the orientation of the polymeric acceptor in the film must be carefully controlled to improve the J _SC and FF values of all-PSCs. The anisotropic charge transfer characteristics of the polymeric acceptor with respect to the orientation direction (e.g., face-on or edge-on) of the polymer in the thin film are less sensitive to the charge transfer direction, (M. Schubert et al ., Adv. Funct. Mater . 2014, 24, 4068), to a much greater extent than the PSCs containing acceptors.

In recent years, extensive efforts have been made to develop new n-channel polymeric semiconductors with high electron mobility, and naphthalene diimide (NDI) based copolymers are among the most successful polymer acceptors (M. Sommer, J. Mater. Chem. , 2014, 2, 3088, R. Steyrleuthner et al ., Adv. Mater . 2010, 22, 2799; S. Vasimalla et al ., Chem. Y. Kim et al ., Chem.Materials 2013, 25, 3251). This is because the NDI-based polymer has a high electron affinity at the center of NDI caused by (1) two strong electron-withdrawing diimide groups, (2) a high expansion to produce strong? -N molecule interactions Conjugated structure and (3) controllable physicochemical properties by introducing various alkyl chains onto the N-site of the diimide group (S. Vasimalla et al. F. Wurthner et al., Chem. Commun., 2011, 47, 5109), J. Chem., 2014, 26, 4030; Thanks to these advantages, organic field-effect transistors (OFETs) made using NDI-based copolymers produced high electron mobilities of up to 1.8 cm ² V ^-1 s ^-1 (H. Yan et al., Nature 2009, 457, 679; R. Kim et al., Adv. Funct. Despite the highly improved electron mobility of NDI-based copolymers in recently reported OFETs, it has so far been found that poly [[N, N'-bis (2-octyldodecyl) (NDI2OD-T2)) containing only a limited number of NDI-based copolymers, such as bis (dicarboximide) -2,6-diyl] -alta-5,5'- (2,2'-bithiophene) all of which have been used to produce high PCEs in all PSCs (C. Mu et al., Adv. Mater. 2014, 26, 7224; D. Mori et al., Energ. H. Kang et al., J. Am. Chem. Soc. 2015, 137, 2359). This fact leads to the introduction of various NDI-based polymer acceptors into all-PSCs.

Accordingly, the present inventors have completed the present invention after having repeatedly conducted experiments to develop a novel NDI-based copolymer capable of improving PCE by improving electron mobility and applying it to a polymer solar cell.

Accordingly, it is an object of the present invention to provide a novel NDI-based copolymer improved in electron mobility by introducing an electron-rich unit and a method for synthesizing the same. It is another object of the present invention to provide a novel NDI-based electron acceptor capable of improving the efficiency of a battery by introducing it into a polymer solar cell.

In order to achieve the above object, the present invention provides an NDI-based copolymer comprising thiophene, bithiophene or thienylene-vinylene-thienylene in an electron-rich unit.

In the present invention, the NDI (Naphthalene diimide) -based copolymer refers to a polymer synthesized on the basis of NDI. In the present invention, the high electron affinity of the NDI center caused by the diimide group of the NDI polymer, conjugated structure to generate intermolecular interactions with each other and the possibility of introduction of the diimide groups of various dielectic chains into the N-position, etc. As a result, Respectively.

In the present invention, the electron rich unit refers to a relatively electron-rich unit. In the present invention, NDI is an electron deficient unit that is relatively short of electrons. In accordance with the present invention, the incorporation of thiophene, bithiophene or thienylene-vinylene-thienylene units as electron-rich units significantly modulates the intermolecular interaction of NDI and the planarity of the polymer acceptor, resulting in different electron mobility Can be synthesized.

In one embodiment of the present invention, the NDI-based copolymer may be used as an electron acceptor. The electron acceptor can accept electrons from other particles and can be used for electronic devices such as solar cells and transistors. Also known as an electron acceptor or an electron acceptor.

In one embodiment of the present invention, the NDI-based copolymer may have a polydispersity index of 1.5-3.0. The polydispersity index (PDI) means a molecular weight distribution. When the polydispersity index is 1, the polydispersity index is monodispersed. When the polydispersity index is greater than 1, the polydispersity index is polydisperse.

The present invention also provides an organic field-effect transistor (OFET) comprising the NDI-based copolymer of claim 1 as an active layer.

In the present invention, the transistor means a semiconductor device that performs amplification and switching. In recent years, an organic field-effect transistor (OFET) using an organic semiconductor technology is mainly used for a channel of a transistor . New materials have been actively developed to improve the efficiency, including various molecules with various directions and bonding materials as the active layer and small molecules such as rubrene, tetracene, pentacene, TCMQ and various polymers. have.

The present invention also provides a solar cell comprising a substrate, an organic thin film, an active layer and an electrode, wherein the active layer comprises a polymer solar cell (PSCs) comprising the NDI-based copolymer as an electron acceptor to provide.

In the present invention, the solar cell is a semiconductor device that converts solar energy into electrical energy. Examples of the solar cell include an inorganic solar cell, an organic monomolecular solar cell, and a polymer solar cell. And the low power efficiency of the polymer solar cell has attracted attention. In order to improve the power conversion efficiency (PCE) of polymer solar cells, it has been required to develop new polymers in batteries. Various factors such as electron mobility, Voc and FF are involved in improving the PCE of a solar cell. In the present invention, PCE of a solar cell can be improved by introducing a polymer acceptor capable of improving electron mobility, It was possible to manufacture a solar cell without a separate additive.

In one embodiment of the present invention, the active layer is made of PTB7-Th (poly [[4,8-bis [5- (2-ethylhexyl) thiophen- 3,4-b] thiophendiyl]])) was prepared in accordance with the procedure described in Example 1, step b) from ethyl (3-fluoro-2- It can be included as a donor.

In one embodiment of the present invention, the organic thin film may include poly- (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT: PSS).

In the present invention, the PEDOT: PSS is a polymer in which two ionomers are mixed, and is a transparent conductive polymer having high flexibility. Dispersed as gelled particles in water, and dispersed in a substrate such as glass in the same manner as spin coating.

The present invention also relates to a process for producing a polymer electrolyte membrane, comprising: a) degassing a solution containing a thiophene monomer added to toluene and DMF; b) adding and degassing triphenylphosphine and tris (dibenzylideneacetone) dipalladium to the solution degassed in step a); c) heating the solution degassed in step b) and cooling to room temperature to obtain a gel; d) diluting the gel obtained in the step c) and refluxing it; e) removing the solvent of the gel refluxed in step d), and then adding methanol to obtain a precipitate; And f) purifying the precipitate obtained in step e). The present invention also provides a method for synthesizing an NDI-based copolymer.

In the present invention, the synthesis method is based on a steel coupling reaction in which two kinds of compounds cause a condensation reaction between different functional groups. In the present invention, microwave irradiation is simultaneously performed. In the present invention, the heating in step c) may be performed at 150 ° C. The gel obtained in the step d) may be diluted with chloroform, and the reflux in the step e) may be carried out at 110 ° C.

In one embodiment of the present invention, the thiophene-based monomer in step a) may be any one selected from the group consisting of the following chemical formulas (1) to (4).

In the present invention, the thiophene-based monomer is a monomer to which the NDI copolymer is added so as to contain thiophene, bithiophene, and thienylene-vinylene-thienylene, respectively, through the following reaction formula.

[Reaction Scheme 1]

[Reaction Scheme 2]

[Reaction Scheme 3]

[Reaction Scheme 4]

In one embodiment of the present invention, the purification of step f) may be successively Soxhlet extraction with methanol, acetone, hexane, dichloromethane and chloroform.

According to the present invention, it has been confirmed that a novel NDI-based copolymer having a high electron mobility can be synthesized. The copolymer synthesized according to the present invention remarkably controls the intermolecular interaction and the planarity of the polymer acceptor and changes the electron mobility. As a result, when the NDI copolymer according to the present invention is applied to a solar cell, It was confirmed that the efficiency can be improved and a solar cell with high efficiency can be produced without any additive.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a synthetic schematic diagram of an NDI-based copolymer. FIG.
2 is a view showing the chemical structure of the donor and acceptor polymers used in the present invention.
3 is a cyclic potential diagram of P (NDI2OD-T) (FIG. 3A), P (NDI2OD-T2) (FIG. 3B) and P (NDI2OD-TVT) (FIG. Each thin film was prepared by drop casting onto a gold platinum electrode and tested in a solution of acetonitrile (0.1 M Bu ₄ NPF ₆ ) (scan rate, 20 mVs ^-1 ).
Figure 4a is the UV-visible absorption spectrum of the film, Figure 4b is the side view and back angle of the calculated molecular structure of the three polymeric acceptors, where [theta] represents the plane-to-plane angle between the NDI units.
Figure 5 shows the UV-photometric coefficients of P (NDI2OD-T) (Figure 5a), P (NDI2OD-T2) (Figure 5b) and P (NDI2OD-TVT) (Figure 5c) in chloroform solutions and in thin films, FIG. All three polymers show similar extinction coefficients.
6 shows measurement results for P (NDI2OD-T) (FIG. 6A), P (NDI2OD-T2) (FIG. 6B) and P (NDI2OD-TVT) (FIG.
FIG. 7 is a graph showing migration characteristics of NDI copolymerization system FETs in electron-enhanced operation with V _DS = +100 V, which are P (NDI2OD-T) (FIG. 7A), P (NDI2OD- And P (NDI2OD-TVT) (Fig. 7C). Polymer thin films were prepared by drop casting in a nitrogen atmosphere and annealing at 220 ℃.
Fig. 8 shows the output characteristics of the NDI copolymerization system FETs in an electron-enhanced operation with V _DS = +100 V, which are P (NDI2OD-T) (Fig. 8A), P (NDI2OD- (NDI2OD-TVT) (Fig. 8C). Polymer thin films were prepared by drop casting in a nitrogen atmosphere and annealing at 220 ℃.
9 is a graph showing current density-voltage (JV) characteristics (FIG. 9A) and EQE spectrum (FIG. 9B) of all-PSCs.
10 is the R-SoXS profile of PTB7-Th: P (NDI2OD-T), PTB7-Th: P (NDI2OD-T2) and PTB7-Th: P (NDI2OD-TVT) measured at 287.5 eV.
11 is a GIXS pattern of P (NDI2OD-T) (FIG. 11A), P (NDI2OD-T2) (FIG. 11B) and P (NDI2OD-TVT) (FIG.
Figs. 11D and 11E also show that the GIXS patterns of PTB7-Th: P (NDI2OD-T), PTB7-Th: P (NDI2OD-T2), and PTB7-Th: P (NDI2OD- TVT) (Fig. 11D) and out of plane (Fig. 11E). All samples were prepared under the conditions used to make the highest efficiency solar cell spin-coated from chloroform solution.
12 shows the in-plane (FIG. 12A) and out-of-plane (FIG. 12B) cutting lines of the GIXS pattern of P (NDI2OD-T), P (NDI2OD-T2) and P (NDI2OD-TVT) All samples were prepared by spin coating from a chloroform solution under the same conditions.
13 (a) and 13 (b) show PTB7-Th: P (NDI2OD-T) (FIG. 13A), PTB7-Th: P GIXS pattern.
Fig. 14 is a graph showing the results of measurement for the electron-only device of the all-polymer blend film (Fig. 14B) of the NDI-based copolymer raw film (Fig. 14A) and PTB7- And the measured space-charge-limiting JV characteristic. The thickness of the all-polymer blend was ~110 nm.

Hereinafter, the present invention will be described in detail with reference to examples. However, these examples are intended to further illustrate the present invention, and the scope of the present invention is not limited to these examples.

<Experimental Examples> Preparation and characterization of experimental materials

1. Experimental material

PTB7-Th and 4,9-dibromo-2,7-bis (2-octyldodecyl) benzo [lmn] [3,8] phenanthroline- Tetraon were purchased from 1-Materials and Sunatech Inc., respectively, and used without purification. Unless otherwise noted, all chemicals were purchased from Aldrich and used as such. All reactions were carried out under an argon atmosphere.

2. Monomer synthesis

Synthesis of 2,5-bis (trimethylstannyl) thiophene

2,5-bis (trimethylstannyl) thiophene was synthesized according to Scheme 1 below.

[Reaction Scheme 1]

The cooled dried over -78 ℃ tetrahydrofuran (THF) of 2,5-di-bromothiophene solution (0.1 M concentration) was slowly added to (1.6 M in THF), n- butyllithium in 2.1 equiv of, N ₃ &Lt; / RTI &gt; for 1 hour. Then 2.2 equivalents of trimethyltin chloride (18.2 mL, 1.0 M in THF) was added and the reaction was heated to ambient temperature. The mixture was stirred overnight. The reaction mixture was quenched with _{NH 4 Cl (aq) (quenched} ), it was extracted with Et ₂ O. The organic layer was washed with water and brine; It was dried over MgSO _4, was concentrated in vacuo. The resulting compound was purified by recrystallization using EtOH to provide the corresponding product (2.55 g, 75.3% yield) as an acicular white powder: ¹ H-NMR (400 MHz, CDCl ₃ , 隆) 7.37 s, 2H), 0.36 (s, 18H).

Synthesis of 5,5'-bis (trimethylstannyl) -2,2'-bithiophene

5,5'-bis (trimethylstannyl) -2,2'-bithiophene was synthesized according to the following reaction formula 2.

[Reaction Scheme 2]

A solution of 2,2'-bithiophene (0.1 M concentration) in dry THF cooled to -78 ° C was slowly added to 2.1 equivalents of n-butyl lithium (1.6 M in THF) and stirred under argon for 1 hour. When trimethyltin chloride (2.2 eq.) Was added, the solution was heated to 0 &lt; 0 &gt; C and the reaction was heated to ambient temperature. The mixture was stirred overnight. The reaction mixture was diluted with ethyl acetate. The organic layer was washed with water and brine; It was dried over MgSO _4, was concentrated in vacuo. The resulting compound was purified by recrystallization using EtOH to give the corresponding product (96% yield): ¹ H-NMR (400 MHz, CDCl ₃ , 隆) 7.28 (d, 2H), 7.09 , &Lt; / RTI &gt; 2H), 0.39 (s, 18H).

Synthesis of (E) -1,2-di (thiophen-2-yl) ethane

(E) -1,2-di (thiophen-2-yl) ethane was synthesized according to the following reaction formula 3.

[Reaction Scheme 3]

The solution of thiophene-2-carbaldehyde in dry THF cooled to -18 ℃ (5.6g, 50mmol) was slowly added to TiCl ₄ (6.5mL), was stirred under argon for 30 minutes. Zinc powder (7.8 mg) was slowly added to the solution and stirred at -18 [deg.] C for 30 minutes. The mixture was heated to room temperature and then refluxed for 3.5 hours. The reaction was quenched by the addition of ice water (100 mL) and the solid mixture was collected by filtration and dried in a vacuum oven. After the solid was dissolved in methylene chloride (80 mL), the insoluble material was removed by filtration and evaporated. Was purified by being screen is the compound was recrystallized from hexane (56% ^{yield): 1 H-NMR (400} MHz, CDCl 3, δ) 7.19 (d, 2H), 7.05 (s, 2H), 7.04 (d, 2H) , &Lt; / RTI &gt; 7.00 (m, 2H)

Synthesis of (E) -1,2-bis (5- (trimethylstannyl) thiophen-2-yl)

(E) -1,2-bis (5- (trimethylstannyl) thiophen-2-yl) ethane was synthesized according to Reaction Scheme 4 below.

[Reaction Scheme 4]

(0.1 M concentration) of (E) -1,2-di (thiophen-2-yl) ethane in dry THF cooled to -78 ° C was slowly added to 2.1 equivalents of n-butyl lithium (2.5 M in hexane) And stirred under argon for 1 hour. When trimethyltin chloride (2.2 eq.) Was added, the solution was heated to 0 &lt; 0 &gt; C and the reaction was heated to ambient temperature. The mixture was stirred overnight. The reaction mixture was diluted with ethyl acetate. The organic layer was washed with water and brine; It was dried over MgSO _4, was concentrated in vacuo. The residue was recrystallized using EtOH to provide the corresponding product (44% yield): ¹ H-NMR (400 MHz, CDCl ₃ , 隆) 7.12 (d, 2H), 7.09 7.07 (d, 2H), 0.26 (s, 18H).

3. Characterization

¹ H-NMR spectra were performed on a liquid 400 NB NMR apparatus using CDCl ₃ as d- solvent. The peak was given in ppm relative to CDCl ₃ (7.26 ppm). (Flow rate: 1 mL / min) at a molecular weight of 80 &lt; 0 &gt; C with a Waters 1515 isocratic HPLC pump, temperature control module and a Waters 2414 refractive index meter. o-Dichlorobenzene (o-DCB) was used as the eluent to the polystyrene standards. UV-visible absorption spectra were obtained at room temperature using a UV spectrophotometer UV-1800 (Shimadzu Scientific Instruments). The electrochemical circulation transfer method (CV) (CHI 600C electrochemical analyzer) was carried out in anhydrous acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (Bu ₄ NPF ₆ ), Pt disk working electrode, Pt counter electrode and Ag wire With a potential scan rate of 50 mV / s with a quasi-reference electrode. For electrochemical measurements, the polymer membrane was coated with a chloroform solution having a concentration of approximately 1 mg / mL. (GIXS) measurement was performed at Beamline 3C of Pohang Accelerator Laboratory (Korea). GIXS samples were prepared by spin coating on PEDOT: PSS / Si substrates with optimized active layer conditions. An X-ray having a wavelength of 1.1179 A was used. The incident angle (~ 0.12 °) was chosen such that the X-rays could penetrate completely into the film. Soft X-ray resonance scattering (R-SoXS) was performed in an Advanced Light Source (ALS). RSoXS samples were prepared on PEDOT: PSS / Si substrates under the same optimized active layer conditions. The active layer was then floated in water and transferred to a 1.0 mm X 1.0 mm, 100 nm thick Si ₃ N ₄ film supported by a 5 mm x 5 mm, 200 μm thick Si frame (Norcada Inc.).

4. General polymerization procedure by steel coupling reaction

All polymers were synthesized by microwave - assisted steel coupling reactions. Three monomers were weighed and added to a microwave vial equipped with a magnetic stirrer. Dry toluene and dry DMF were added and the solution was degassed with argon for 20 minutes. After the triphenylphosphine (PPh ₃ , 8 mol%) and tris (dibenzylideneacetone) dipalladium (Pd ₂ (dba) ₃ , 2 mol%) were added, the cap was sealed. The mixture was degassed for an additional 20 minutes, after which the vial was placed in a microwave reactor and stirred at 150 &lt; 0 &gt; C for 3 hours. The trimethylstannyl- and bromo-terminal groups in the polymer were then reacted with 2-bromothiophene (0.2 mL) and 2- (tributylstannyl) thiophene (0.25 mL) in a microwave reactor at 150 &Lt; / RTI &gt; After cooling to room temperature, the obtained gel was diluted with chloroform and refluxed with ethylenediaminetetraacetic acid (EDTA, 300 mg) in water at 110 ° C for 1 hour. The organic layer was washed with water, and dried over MgSO _4. The solvent was removed in vacuo and after precipitating into methanol, the resulting solid was purified via Soxhlet extraction successively with methanol, acetone, hexane, dichloromethane and chloroform. The filtrate in each fraction was concentrated under reduced pressure and precipitated in cold methanol. The polymer was dried under vacuum for 24 hours.

P (NDI2OD-T), P (NDI2OD-T2) and P (NDI2OD-TVT) were synthesized according to the polymerization procedure as described above. First, polymerization of P (NDI2OD-T) was carried out by using 4,9-dibromo-2,7-bis (2-octyldodecyl) benzo [lmn] [3,8] phenanthroline- (125 mg, 1 eq), tris (dibenzyllithinacetone) dipalladium (Pd ₂ (dba (dibenzylideneacetone) ) ₃ ) (5.57 mg) and tri (o-tolyl) phosphine (7.4 mg) in toluene (2.0 mL) and dimethylformamide (DMF) (0.2 mL) to synthesize P (NDI2OD- Lt; / RTI &gt; Chloroform fraction (fraction) was 300 mg, and the yield 70%. The GPC results _{M n = 44K, PDI = 2.15} . ^{1 H-NMR (400 MHz,} 1,2-dichlorobenzene-d4): δ (ppm) 8.93 (s, 2H), 7.57 (s, 2H), 4.27-4.24 (br, 4H), 2.25-2.19 (br, 2H), 1.53-1.11 (br, 64H), 0.85-0.83 (m, 12H).

Next, the polymerization of P (NDI2OD-T2) was carried out by using 4,9-dibromo-2,7-bis (2-octyldodecyl) benzo [lmn] [3,8] phenanthroline- Bis (trimethylstannyl) -2,2'-bithiophene (149.8 mg, 1 eq), Pd ₂ (dba) ₃ (5.57 mg) and tri (o-tolyl) phosphine (7.4 mg) was prepared in toluene (2.0 mL) and DMF (0.2 mL) to synthesize P (NDI2OD-T2). And the chloroform fraction is 318 mg, yield 75%. The GPC results were _{M n = 52K, PDI = 2.01} . ^{1 H-NMR (400 MHz,} 1,2-dichlorobenzene-d4): δ (ppm) 8.93 (s, 2H), 7.41-7.29 (br, 4H), 4.27-4.25 (s, br, 4H), 2.23- 2.20 (s, br, 2H), 1.53-1.10 (br, 64H), 0.85-0.84 (m, 12H).

Finally, the polymerization of P (NDI2OD-TVT) was carried out by using 4,9-dibromo-2,7-bis (2-octyldodecyl) benzo [lmn] [3,8] phenanthroline- (TVT) (157.4 mg, 1 &lt; RTI ID = 0.0 &gt; eq), Pd ₂ (dba) ₃ (5.57 mg) and tri (o-tolyl) phosphine (7.4 mg) were dissolved in toluene (2.0 mL) and DMF (0.2 mL) &Lt; / RTI &gt; Chloroform fraction is 310 mg and the yield 73%, GPC result was _{M n = 104K, PDI = 2.73} . ^{1 H-NMR (400 MHz,} 1,2-dichlorobenzene-d4): δ (ppm) 8.80 (s, 2H), 7.37-7.11 (br, 6H), 4.29-4.20 (s, br, 4H), 2.22- 2.17 (s, br, 2H), 1.50-1.16 (br, 64H), 0.85-0.84 (m, 12H)

&Lt; Example 1 > Properties of NDI-based polymers

A series of NDI based copolymers were prepared according to the above experimental example using 4,9-dibromo-2,7-bis (2-octyldodecyl) benzo [lmn] [3,8] phenanthroline- Bis (trimethylstannyl) thiophene, referred to as P (NDI2OD-T), P (NDI2OD-T2) and P (NDI2OD- Supported steel of (E) -1,2-bis (5-trimethylstannyl) -2,2'-bithiophene and (E) -1,2-bis (5- (trimethylstannyl) thiophen- Stille coupling reaction. A detailed synthetic procedure of the NDI-based copolymer is disclosed in FIG.

Figure 2 shows the chemical structures of the three polymer acceptors and PTB7-Th polymer donors used in this study. The number average molecular weight (M _n ) and polydispersion index (PDI) of the polymer were determined by size exclusion chromatography (SEC) at 80 ° C using o-dichlorobenzene as the eluent. All polymers had considerably high M _n (greater than 40 kg / mol) and similar PDI values (see Table 1, below).

Table 1 summarizes the polymer information used in the present invention, wherein ^a was measured by SEC using o-DCB as the eluent at 80 ° C and ^b was measured in dilute chloroform solution. ^c is a film in a state of being spin-cast from a chloroform solution, and ^d is the start of absorption of UV-visible light in the polymer film. ^e is measured by cyclic voltammetry and ^f is calculated by HOMO = LUMO -E _g ^opt .

1.1 Comparison of electrochemical properties

In the present invention, the electrochemical characteristics of NDI-based polymeric acceptors were compared (FIG. 3). The three polymers had a similar lowest unoccupied molecular orbital (LUMO) level of approximately 4.0 eV, due to the major contribution of the strong electron-accepting NDI moiety. -5.54, and -5.48 eV, respectively, in the highest occupied molecular orbital (HOMO) of P (NDI2OD-T), P (NDI2OD-T2) and P , Which was mainly influenced by the electronic abundance unit.

1.2 UV-Visible Absorption Spectrum Comparison

4A shows the UV-visible absorption spectrum of the polymeric acceptor of the thin film, and Table 1 summarizes the absorption characteristics. All of the polymers had strong absorption in the range of 500 to 800 nm due to efficient intramolecular charge transfer from electron rich to electron deficient NDI units. The optical bandgap (E _g ^opt ) value gradually decreased in the order of P (NDI2OD-T) (1.70 eV), P (NDI2OD-T2) (1.50 eV) and P (NDI2OD-TVT) This is because P (NDI2OD-T2) and P (NDI2OD-TVT) have a π-conjugated length that is more extended than P (NDI2OD-T). Importantly, P (NDI2OD-T), P (NDI2OD-T2) and P (NDI2OD-TVT) exhibited different solution-film behavior in UV-visible absorption (Figure 5). The degree of the red shift of the maximum absorption peak from the solution to the thin film was found to be P (NDI2OD-T) (545? 600 nm), P (NDI2OD-T2) (688? 702 nm) 703 nm), for three polymer acceptors (λ _max ^sol → λ _max ^film ). Changes in the smaller red shifts in P (NDI2OD-T2) and P (NDI2OD-TVT) may have been due to pre-aggregation of the two polymeric acceptors, possibly induced by strong intermolecular interactions (M. Schubert et al., Adv. Energy. Mater . 2012, 2, 369).

1.3 Density function theory  Calculation

The structural properties of the three different NDI based copolymers were investigated by density functional theory (DFT) calculations and performed at the B3LYP / 6-31G (d, p) level. The NDI-thiophene-NDI structure was simulated and the alkyl chain located at the N (nitrogen atom) of the NDI was replaced with the simplest alkyl chain, methyl group, to simplify the simulation. The calculated molecular structure (side view) of the three polymers is shown in Figure 4b. The three polymers have similar dihedral angles between NDI and thiophene groups in each electron rich unit: P (NDI2OD-T): 50.23 DEG, P (NDI2OD-T2): 52.92 DEG and P NDI2OD-TVT): 49.05 DEG (Fig. 6). However, P (NDI2OD-TVT) polymers with vinyl linkages between adjacent thiophene units exhibit significantly improved stereoregular and planar structures of conjugated backbones compared to P (NDI2OD-T) and P (NDI2OD-T2) . As shown in Fig. 4B, the plane-to-plane angle between two different NDI units of the P (NDI2OD-TVT) polymer was very low at 1.98 [deg.], (NDI2OD-T2) (19.89 DEG). The introduction of the TVT unit can extend conjugation length and improve molecular stiffness so that adjacent thiophene rings are not rotated and adjacent thiophene rings do not promote strong intermolecular interactions. This can lead to a more crystalline structure by reducing the distance between polymer chains in the thin film state (R. Kim et al ., Adv. Funct. Mater . 2013, 23, 5719).

&Lt; Example 2 > Preparation and effect of OFET

To illustrate the effect of the structural modification of the NDI copolymer on the charge transfer characteristics, the present inventors used the P (NDI2OD-T), P (NDI2OD-T2) and P (NDI2OD-TVT) An organic field-effect transistor (OFET) device having a bottom-gate top-contact configuration was fabricated.

2.1 Manufacture of OFET devices

NDI copolymer system OFETs were fabricated with very high n-doped Si wafers (<0.004 Ω cm) covered with 300 nm thick SiO ₂ dielectrics (C _i = 10 nF cm ^-2 ). SiO ₂ / Si wafers were treated in solution phase with n-octadecyltrimethoxysilane (OTS). A 3 mM solution of OTS in trichlorethylene (TCE) was added to a UV / ozone treated SiO ₂ / Si wafer after washing with a pyran solution (7: 3 volume ratio mixture of H ₂ SO ₄ and H ₂ O ₂ ) Spin-coated. The wafer was then exposed to ammonia vapor in a vacuum desiccator at room temperature for 12 hours. Subsequently, the substrate was rinsed with toluene, acetone and isopropyl alcohol, followed by N ₂ blowing. A drop-cast thin film of polymer was deposited on an OTS-treated SiO ₂ / Si substrate using a chlorobenzene solution (2 mg mL ^-1 ). Then, it was annealed on a hot plate at 200 ℃ for 30 minutes in an N ₂ atmosphere, a polymer film (annealed). A gold electrode with a thickness of 40 nm was thermally deposited on the semiconductor layer through a shadow mask. The device's current-voltage (IV) characteristics were then measured using a Keithley 4200 semiconductor parameter analyzer in a glove box filled with N ₂ .

2.2 Electrical performance of OFET devices

The electrical performance of the NDI copolymer system OFET device is summarized in Table 2 below.

Table 2 summarizes the OFET performance of the NDI-based copolymer thin film, where ^a is the NDI copolymer film FET performance tested in a nitrogen atmosphere, and its n-channel characteristics were measured at V _DS = +100 V. Also it indicates the maximum of ^b and ^c the average mobility of the FET device 20 having more than the channel length (L) and 1000 ㎛ of the channel width (W) of 50 ㎛ track ^d is the standard deviation.

The three different NDI copolymers exhibited n-channel properties and the electron mobility (μ _{e, FET} ) values were improved as the degree of coplanarity of the polymer backbone increased, indicating that P (NDI 2 OD- for ^{T) 0.25 cm 2 V -1 s} -1, P (NDI2OD-T2) in cm ² V ^-1 s ^-1 and 1.28 P (NDI2OD-TVT) of 2.31 cm ² V ^-1 s ^-1 for about Maximum μ _{e, and FET} value. This mobility is significantly improved compared to previously reported values. The movement and output characteristics are shown in Figs. 7 and 8, respectively. In particular, P (NDI2OD-TVT) polymers exhibited significantly higher μ _{e, FET} values, which may be due to improved π-π interactions between adjacent P (NDI2OD-TVT) chains induced by planar structures. The charge transfer capacities of the three different NDI copolymers were in good agreement with the results obtained from the molecular simulations and the above-mentioned optical properties.

&Lt; Example 3 > Manufacture and effect of all-polymer solar cell

In the present invention, conventional all-PSCs (ITO / PEDOT: PSS / all-polymer blend / LiF / Al) were prepared using PTB7-Th as an electron donor and three NDI copolymers as an electron acceptor .

3.1 Manufacture of All-PSC devices

The conventional type of all-PSC device is indium tin oxide (ITO) / poly- (3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS) / PTB7-Th: NDI- copolymer / LiF / Al structure. The ITO-coated glass substrate was ultrasonicated in acetone and 2% Helmanex soap in water and then extensively washed with deionized water. It was also sonicated in deionized water and then sonicated in isopropyl alcohol. Thereafter, the substrate was dried in an oven at 80 캜 for several hours. The ITO substrate was UV-ozone treated prior to PEDOT: PSS deposition. The filtered dispersion (PH 500) of PEDOT: PSS in water was spin coated at 3,000 rpm for 40 seconds and heated at 150 ° C for 30 minutes in air. After the application of the PEDOT: PSS layer, all subsequent procedures were carried out in a glove box under N ₂ atmosphere. Each active layer of the PTB7-Th: NDI polymer (1.3: 1 w / w) in chloroform (CF) solution was spin-cast on the ITO / PEDOT: PSS substrate at 3000 rpm. The volumetric fraction of the 1,8-diiodooctane additive in the CF solution was about 1 volume%. The final thickness of each film was 100-110 nm. Subsequently, the substrate was placed in a deposition chamber and maintained for> 1 hour under high vacuum (less than 10 ^-6 Torr) before depositing approximately 0.9 nm of LiF and 100 nm of Al. The configuration of the shadow mask produced four independent devices for each substrate. Table 3 below shows the average device performance of optimized all-PSCs.

3.2 Checking All-PSC Device Performance

The active area of the fabricated device was 0.09 cm ² , which was carefully measured by optical microscopy. The device's current density-voltage (JV) characteristics were measured under simulated AM 1.5G solar irradiance (100 mWcm ^-2 , Peccell: PEC-L01) at ambient conditions. These solar simulator systems meet the Class AAB and ASTM standards. The intensity of the solar simulator was carefully calibrated using a standard silicon reference cell and a KG-5 visible color filter. JV behavior was collected using a Keithley 2400 SMU (Figure 5). EQE and IQE curves were obtained by a spectroscopic measurement system (K3100 IQX, McScience Inc.). A light source (300-W xenon arc lamp) was used with a spectrometric measurement system (K3100 IQX, McScience Inc.). (or all-polymer blend) / Au / ITO / ZnO / polymer only (or all-polymer blend) / LiF / PEDOT: PSS / Al device structure.

The optimized ratio of PTB7-Th: NDI-copolymer in the blend was 1.3: 1.0 (weight ratio), and the optimized film thickness of the spin-coated active layer from solution in chloroform was approximately 100 nm. Figure 9 shows the current density-voltage (JV) curve and the external quantum efficiency (EQE) spectrum of all-PSCs. Table 4 summarizes the device capabilities. PTB7-Th: P (NDI2OD-T) and PTB7-Th: P (NDI2OD-T2) devices were 3.03% (V _OC = 0.80 V; J _SC = 8.85 mA cm ^-2 ; FF = 0.43) and 3.22% V _OC = 0.79 V; J _SC = 10.99 mA cm ^-2 ; FF = 0.37). Conversely, the PTB7-Th: P (NDI2OD-TVT) device had a much larger PCE value of 4.25% (V _OC = 0.84 V; J _SC = 11.40 mA cm ^-2 ; FF = 0.43). These high PCE values were mainly due to high J _SC values exceeding 11 mA cm ^-2 . We have found that the J _SC value of all-PSCs is well within 2% error with the corresponding integrated J _SC obtained from the external quantum efficiency (EQE) spectrum (FIG. 9).

<Example 4> Morphology and crystallinity analysis of NDI copolymer

To obtain a deeper understanding of the effect of structural changes of NDI-based copolymers on photovoltaic properties, the blend form of P (NDI2OD-T), P (NDI2OD-T2) Was measured using resonant soft X-ray scattering (R-SoXS) and grazing incidence wide angle X-ray scattering (GIWAXS).

4.1 Analysis of R-SoXs

First, R-SoXS results of P (NDI2OD-T), P (NDI2OD-T2) and P (NDI2OD-TVT) membranes blended with PTB7-Th were compared under optimized conditions for the optimized device. Figure 10 shows the R-SoXS scattering profile of the all-polymer blend of PTB7-Th: NDI based copolymer, obtained at 287.5 eV, where the maximum scattering contrast between the two polymers was observed (HP Yan et al . , 2002 , 6, 677, BA Collins et al ., Adv. Energy , Materials, 2013, 3, 65, S. Swaraj et al ., Nano. Lett . 2010, 10, 2863).

For better recognition of the relative strength at low q, the data is shown as intensity Xq ² vs q. As shown in FIG. 10, the PTB7-Th: P (NDI2OD-TVT) blend film showed the maximum scattering intensity at q = 0.004A- ¹ . In comparison, the PTB7-Th: P (NDI2OD-T) and PTB7-Th: P (NDI2OD-T2) films had larger maximum scattering intensities at q = 0.005 and 0.007A- ¹ , respectively. (Mu et al ., Adv. Mater . 2014, 26, 7224), since the domain spacing length from the q-peak of the R-SoXS profile (d = 2? / Q) The size of the phase-separated domain of Th: P (NDI2OD-TVT) is somewhat larger than PTB7-Th: P (NDI2OD-T) (63 nm) and PTB7-Th: nm.

However, in the present invention, all three blends have a relatively well mixed BHJ domain, and when considering other reported all-PSC blend systems. It was noted that its domain size was sufficient to generate efficient exciton dissociation and charge transfer (H. Kang et al ., J. Am. Chem. Soc . 2015, 137, 2359). As a result, the PTB7-Th: P (NDI2OD-TVT) device had the highest PCE value despite slightly larger domain size, as shown in Table 4 below.

Table 4 summarizes the characteristics of the all-PSC device, where ^a represents the average PCE value of more than twelve devices.

4.2 Measurement of GIXS

Next, the microstructures of P (NDI2OD-T), P (NDI2OD-T2) and P (NDI2OD-TVT) membranes were compared and blend films with PTB7-Th were investigated by GIXS measurement under optimized device conditions. Figures 11a-c compare the GIXS results of P (NDI2OD-T), P (NDI2OD-T2) and P (NDI2OD-TVT) pristine membranes prepared from the chloroform solution by spin coating under the same conditions. As a result, all of each polymer is P (NDI2OD-T), P (NDI2OD-T2) and 23.6 Å with respect to P (NDI2OD-TVT) (q in = 0.266 Å -1), 24.9 Å (q in = 0.252 Å - ¹ ) and a sharp (100) scattering peak _{in the} in-plane direction (q _in ) with a similar lamellar spacing of 24.6 Å (q _in = 0.255 Å ^-1 ) (FIG. 12). P (NDI2OD-TVT) films showed clearly distinct diffraction peaks up to the third order (300), while P (NDI2OD-T) and P (NDI2OD-T2) ). &Lt; / RTI &gt; In addition, all three polymer films showed preferential phase-on orientation, but the (010) peak intensities of the face-on oriented π-π laminate were about P (NDI2OD-T2) and P (NDI2OD-TVT) . When the orientation of the polymer in the in-plane direction and the face-on direction coexist, it has been reported that the performance of the organic electronic device can be improved by improving the charge mobility in the three-dimensional direction (Sun, K et al ., Nat.Commun . 2015, 6 , 6013 ; Mei, JG et al ., J. Am . Chem . Soc ., 2011 , 133 , 20130-20133 , Nguyen, TL et al ., Energ . Lee, J. et al ., J. Am . Chem . Soc . 2013, 135 , 9540-9547; Kim, R. et al ., Adv.Funct.Mater . 2013, 23, 5719-5727)

The peak positions for the π-π stack were different and corresponded to different distances of 4.27, 3.95 and 3.92 Å for P (NDI2OD-T), P (NDI2OD-T2) and P (NDI2OD-TVT), respectively. Thus, the P (NDI2OD-TVT) polymer had the shortest π-π lap distance, indicating the formation of a denser chain-to-chain packing, which caused increased electron mobility due to better orbital overlap between adjacent polymer chains C. Lee et al ., Adv. Mater . 2015, 27 , 2466-2471)

Taking these results into account, the P (NDI2OD-TVT) film can provide the most efficient 3-D charge transfer path, which is very beneficial in improving the charge carrier mobility in OFETs (J Lee et al ., J Am Chem Soc 2013, 135, 9540).

13A-C show 2D GIXS patterns of all-polymer blend films of PTB7-Th: P (NDI2OD-T), PTB7-Th: P (NDI2OD-T2) and PTB7-Th: P (NDI2OD-TVT). 11D and 11E show the in-plane and out-of-plane cut lines of the GIXS pattern. For reference, we added the GIXS pattern of the original PTB7-Th film showing strong face-on geometry. All samples were prepared under optimized device conditions (spin coating from chloroform solution). The peak intensities of the p-on-aligned π-π stacks were PTB7-Th: P (NDI2OD-T2) and PTB7-Th: P (NDI2OD-TVT) blend film. Also, as shown in Figures 11d and 11e, the crystalline character of the polymeric acceptor was well preserved after blending with the PTB7-Th polymer.

&Lt; Embodiment 5 > Measurement of charge-carrier mobility of All-PSCs

To illustrate the role of the polymer microstructure on the electrical properties and performance of all-PSCs, we measured and compared the charge-carrier mobility of all-PSCs using the space-charge-current limited (SCLC) .

The charge-carrier mobility of the original film of the NDI-based copolymer and the blend film with PTB7-Th is summarized in Table 5 below and its J-V curve is shown in FIG.

For both the circular n-type polymer membrane and the blend film with PTB7-Th, due to the strong face-on pi-pi laminate interaction with the shortest packing distance in the membrane promoting charge transfer through the aggregated chains, P The highest electron mobility (μ _e ) value was achieved (NDI2OD-TVT) (M. Schubert et al ., Adv. Funct . For example, for a all- polymer ^{blend, 6.7X10 -6 (P (NDI2OD-} T)), 8.2X10 -6 (P (NDI2OD-T2)) and ^{7.0X10 -5 cm 2 V -1 s -1} ( while the different μ _e value significantly in P (NDI2OD-TVT)) observed, it had a very similar values of μ _h ^{2X10 -4 cm 2 V -1 s -1} . In spite of the relatively large domain polymer phase separation to the other blends and, PTB7-Th: P (NDI2OD -TVT) had a 10-fold higher than any other blend μ _e. The ratio of μ _h / μ _e was 29.0 (P (NDI2OD-T)), 22.8 (P (NDI-2T)) and 2.3 Blend, which suggests efficient charge extraction with minimized charge recombination at PTB7-Th: P (NDI2OD-TVT) (T. Earmme et al ., J. Am. Chem. Soc , 2013, 135, 14960) blends exhibited high PCE values due to their high μ _e values and well-balanced charge transfer. In order to observe the charge mobility in the horizontal direction (the above SCLC method is a direction perpendicular to the charge mobility direction), an electron mobility was confirmed by additionally forming an OFET device using a blend. PTB7-Th: P (NDI-TVT) shows the highest electron mobility as shown in Table 5 in Table 6 below (Table 6 was changed to Table 7 while adding Table 6) . It can be seen that P (NDI-TVT) in the blend efficiently forms a channel capable of moving charges in three-dimensional directions.

<Example 6> Measurement of electron mobility of all-PSCs according to additives

Recently, solvent additives such as 1,8-diiodooctane (DIO) have been considered to be important in optimizing the performance of all-PSC devices by adjusting the polymer flocculation behavior and the BHJ blend form (H. Kang et al ., J. Am. Chem. Soc . 2015, 137, 2359). As summarized in Table 6 below, we also found that PTB7-Th: P (NDI2OD-T), PTB7-Th: P (NDI2OD-T2) and PTB7-Th: P NDI2OD-TVT) all-PSCs.

Table 7 shows that a summary of the characteristics of the all-optimized PSCs with DIO additive ^a is the average value of the PCE apparatus of 12 is exceeded. As shown in the above table, it was observed that the DIO additive improved the electron mobility and performance of the PTB7-Th: P (NDI2OD-T) and PTB7-Th: P (NDI2OD-T2) devices, (H. Kang et al ., J. Am. Chem. Soc ., 2015, 137, 2359). As a clear contrast, the PCE values of PTB7-Th: P (NDI2OD-TVT) all-PSCs were not affected by the solvent additive. This result was probably due to strong intermolecular interactions of the P (NDI2OD-TVT) chain without the use of additives. For example, the μ _e and μ _h / μ _e values of PTB7-Th: P (NDI2OD-TVT) were not changed by the DIO additive, and the other PTB7-Th: P (NDI2OD- P (NDI2OD-T2) blends. Therefore, PTB7-Th: P (NDI2OD-TVT) can produce high performance all-PSCs in the absence of additives. This feature is very important because the use of solvent additive often results in an uncertain process in device manufacturing due to the difficulty of removing the high boiling additive and the residual solvent additive causes excessive phase separation leading to poor thermal and long term stability (DH Wang et al ., Adv. Funct. Mater . 2013, 23, 1297).

That is, in the present invention, a series of NDI-based copolymers having different electron mobility were developed, and all-PSCs without the high-efficiency additives were prepared. In particular, the P (NDI2OD-TVT) polymer has the highest electron mobility (μ _{e, FET} = 1) among the three NDI polymers due to the significantly improved coplanarity, increased intermolecular interactions _, 2.31 cm ² V ^-1 s ^-1 ). In addition, the PTB7-Th: P (NDI2OD-TVT) device showed the highest PCE value of 4.25% without the help of any solvent additive. The present inventors have found that PTB7-Th: P (NDI2OD-TVT) has the highest SCLC electron mobility (μ _e ) in the vertical direction, which has improved planarity and stronger phase-on π- &Lt; / RTI &gt; The high μ _e value of the PTB7-Th: P (NDI2OD-TVT) blend resulted in a well-balanced μ _h / μ _e ratio with minimized charge recombination.

Therefore, according to the present invention, the structural design of a polymer acceptor having a high electron mobility can promote the 3-D charge transfer and produce high performance additive free all-PSCs.

The present invention has been described with reference to the preferred embodiments. It will be understood by 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. Therefore, the disclosed embodiments should be considered in an illustrative rather than a restrictive sense. The scope of the present invention is defined by the appended claims rather than by the foregoing description, and all differences within the scope of equivalents thereof should be construed as being included in the present invention.

Claims (10)

An NDI-based copolymer containing an electron-rich unit such as thiophene, bithiophene or thienylene-vinylene-thienylene. The NDI-based copolymer according to claim 1, wherein the NDI-based copolymer is used as an electron acceptor. The NDI-based copolymer according to claim 1, wherein the NDI-based copolymer has a polydispersity index of 1.5-3.0. An Organic field-effect transistor (OFET) comprising the NDI-based copolymer of claim 1 as an active layer. 1. A solar cell comprising a substrate, an organic thin film, an active layer, and an electrode, wherein the active layer comprises the NDI-based copolymer of claim 1 as an electron acceptor. 6. The method of claim 5, wherein the active layer is selected from the group consisting of PTB7-Th (poly [[4,8-bis [5- (2-ethylhexyl) thiophen- ] Dithiophene-2,6-diyl] [3-fluoro-2 - [(2-ethylhexyl) carbonyl] thieno [3,4- b] thiophendi]])) as an electron donor Wherein the polymer solar cell is a polymer solar cell. The polymer solar cell according to claim 5, wherein the organic thin film comprises poly- (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT: PSS). A method for synthesizing an NDI-based copolymer comprising the steps of:
a) degassing a solution in which thiophene monomer is added to toluene and DMF;
b) adding and degassing triphenylphosphine and tris (dibenzylideneacetone) dipalladium to the solution degassed in step a);
c) heating the solution degassed in step b) and cooling to room temperature to obtain a gel;
d) diluting the gel obtained in the step c) and refluxing it;
e) removing the solvent of the gel refluxed in step d), and then adding methanol to obtain a precipitate; And
f) purifying the precipitate obtained in step e). The method according to claim 8, wherein the thiophene-based monomer in step a) is any one selected from the group consisting of compounds represented by the following formulas (1) to (4).
[Chemical Formula 1]

(2)

(3)

[Chemical Formula 4]

The method according to claim 8, wherein the purification of step f) is successively Soxhlet extraction with methanol, acetone, hexane, dichloromethane and chloroform.

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