RU2789132C2 - Conjugated polymer based on substituted benzodithiophene, 5,6-difluorobenzo[c][1,2,5]thiadiazole and thiophene and its application in perovskite solar cells - Google Patents

Abstract

FIELD: power industry.

SUBSTANCE: invention can be used in the manufacture of solar panels. The conjugated polymer based on substituted benzodithiophene, 5,6-difluorobenzo[c][1,2,5]thiadiazole and thiophene has the following structure:

,

where n=5-200. The use of a conjugated polymer as a hole-transport material in perovskite solar cells is also proposed.

EFFECT: increase in the conversion of solar energy.

2 cl, 7 dwg, 2 ex

Description

The invention relates to new organic compounds, semiconductor materials based on these compounds and their use in perovskite solar cells. Perovskite solar cells (PSBs) have been intensively studied all over the world in recent years and are considered as a promising technology for converting solar energy into electrical energy. In less than 10 years, the efficiency of converting sunlight into PSB has increased to 25.5%, which is close to the parameters of solar cells based on crystalline silicon (26.7%).

In the manufacture of perovskite solar cells, special attention is paid to the layers located between the photoactive layer and the electrodes, since they largely determine the efficiency and stability of the devices [T.-W. Lee et al., Energy Environ. Sc., 2016, 9, 12-30; C.-Z. Li et al., Chin. Chem. Lett., 2017, 28, 503-511]. These buffer layers are represented by electron-transport and hole-transport materials capable of selectively extracting negative and positive charge carriers, respectively, from the perovskite layer and ensuring their efficient transport to the corresponding electrodes in the device.

In most cases, spiro-OMeTAD, 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene is used as a hole transport material (Fig. 1 ) [Y. Qi et al., Adv. mater. Interfaces, 2018, 5, 1700623]. However, spiro-OMeTAD has poor charge-transport properties and therefore requires doping (doping) with, for example, oxygen and additional reagents such as LiTFSI (lithium bis(trifluorosulfone)imide) and tBuPy (4-tert-butylpyridine) [A. Sellinger et al., Chem. Sci., 2019, 10, 1904]. Both the doping process itself and the use of LiTFSI and tBuPy negatively affect the stability of perovskite solar cells; in particular, spiro-OMeTAD radical cations are capable of oxidizing T anions in the active layer, leading to its decomposition [T. Miyasaka et al., J. Mater. Chem. A, 2018, 6, 2219; C. Adachi et al., Sol. RRL, 2020, 4, 2000305; G. Chen et al., J. Renew. Sust. Energy, 2018, 10, 043702]. In addition, spiro-OMeTAD is a low molecular weight compound capable of crystallization at elevated temperatures, which leads to the loss of the integrity of its films and the failure of solar cells.

In view of the foregoing, there is an acute problem of developing fundamentally new hole-transport materials based on amorphous polymer compounds. This problem is partly solved by using polymeric aromatic amines such as PTAA-poly[bis(4-phenyl)(2,4.6-trimethylphenyl)amine (FIG. 2). However, this polymer has a non-conjugated structure, therefore, it has a low mobility of charge carriers and does not work well in undoped perovskite solar cells: the light conversion efficiency usually does not exceed 13% [C. Jia et al., Energy Environ. Sci., 2018, 11, 2035]. Higher efficiency achieved by doping the polymer, for example, with atmospheric oxygen [J. Fang et al., Adv. Sci., 2018, 5, 1800159], which also has a bad effect on the stability of devices. Thus, the use of polymeric aromatic amines such as PTAA does not solve the problem of creating efficient and stable perovskite solar cells. Solving this problem requires obtaining and using conjugated polymer structures with good hole-transport properties without additional doping.

This invention proposes the use of a new conjugated polymer P1 based on benzodithiophene, 5,6difluorobenzo[c][1,2,5]thiadiazole and thiophene of the following structure:

where n=5-200,

as a hole-transport material in perovskite solar cells. The repeating unit of polymer P1 consists of the following blocks: )-[2,2'-bithiophen]-5-yl)benzo[1,2-b:4,5-b']dithiophene alternating with electron-withdrawing 2,5-bis(5,6-difluorobenzo[c][1 ,2,5] thiadiazol-4-yl)thiophene linked at positions 7 and 7' of the benzothiadiazole moieties and positions 5' and 5'' of the peripheral thiophene units of the bithiophene moieties. The polymer chain length can vary from 5 to 200 units (FIG. 3).

The use of polymer P1 in the composition of perovskite solar cells as a hole-transport layer allows:

• form continuous insulating hole-transport coatings over the granular photoactive material, preventing its reaction with the metal of the upper electrode;

• ensure efficient transport of positive charge carriers (holes) to the upper hole-collecting electrode of the device;

• Ensure high efficiency of perovskite solar cells (efficiency>18%) without doping the hole-transport layer.

Polymer P1 was prepared by a standard Stille polycondensation reaction according to the scheme shown in FIG. 4. The Stille polycondensation reaction is widely used for the synthesis of materials for organic electronics, in particular, conjugated polymers for organic solar cells [N. Mori, R. Takahashi, K. Hyodo, S. Nishinaga, Y. Sawanaka, and Y. Nishihara, Macromolecules 2018, 51, 1357-1369; C. Gu, D. Liu, J. Wang, Q. Niu, C. Gu, B. Shahid, B. Yu, H. Cong and R. Yang, J. Mater. Chem. A, 2018, 6, 2371-2378].

The polymer was purified from low molecular weight compounds in a Soxhlet apparatus by sequential washing with acetone, heptane, methylene chloride, and chlorobenzene. The high molecular weight fraction obtained from chlorobenzene was used for further research. The weight average molecular weight of the polymer was 246,000 g/mol, and the polydispersity coefficient was 2.7. Polymer P1 is readily soluble in many organic solvents, such as chlorobenzene, toluene, and 1,2-dichlorobenzene, which indicates the prospects for its further use.

An important feature of the new P1 polymer is its good optoelectronic characteristics. From the data of cyclic voltammetry, the rise potential of the polymer oxidation wave was determined (relative to the Fc/Fc ⁺ pair, E _Fc / _Fc+ = -4.8 eV), from which the energy of the highest occupied molecular orbital of the polymer Р1 HOMO was estimated as -5.4 eV. The energy of the lowest free molecular orbital (LUMO) was calculated as E _g ^opt -E _HOMO (eV) and amounted to -3.7 eV. The HOMO energy for P1 corresponds very well to the position of the valence band of the perovskite material (-5.4 eV) [Q. Chen, N. De Marco, Y. Yang, T.-W. Song, S.-S. Chen, H. Zhao, Z. Hong, N. Zhou, Y. Yang Nano Today 2015, 10, 355], which should ensure efficient extraction of positive charge carriers. Materials with similar characteristics are optimal for use in perovskite solar cells as hole-transport layers.

The design of a perovskite solar cell of the classical type (n-i-p structure) using polymer P1 as a hole-transport layer is shown in Fig. 5. It consists of a transparent electrically conductive substrate based on indium oxide doped with tin (ITO), which is also an electron-collecting electrode 0, an electron-selective layer 1, a photoactive layer 2, a hole-transport layer 3, represented by a polymer film P1, an electron -blocking layer 4 and upper hole-collecting electrode 5.

An electron-selective layer designed to block holes and transfer electrons from the active layer of the solar battery to the electrode layer 0. The electron-selective layer may include metal oxides TiO ₂ , SnO ₂ , ZnO, In ₂ O ₃ , WO ₃ , CeO ₂ , Zn ₂ SnO ₄ , Nb ₂ O ₅ , Zn ₂ Ti ₃ O ₈ , BaSnO ₃ , BaTiO ₃ , SrSnO ₃ and others, metal chalcogenides CdS, CdSe, PbS, PbSe, PbTe, ZnS, ZnSe, Sb ₂ S ₃ , Bi ₂ S ₃ , In ₂ S ₃ , MnS, SnS, SnS ₂ , organic compounds from a number of carboxylic and phosphonic acids, fullerene derivatives, derivatives of perylenediimide, naphthalenediimide, acenes, oxydiazoles, and any n-type organic semiconductors. The thickness of the electron selective layer can be from 1 to 200 nm.

The photoactive layer 2 is any perovskite semiconductor material of the general formula ABX ₃ , where A is a monovalent cation, B is Sn ²⁺ , Pb ²⁺ , X is a halogen atom (Br ^- , I ^- ). Preferred organic cations And are methylammonium (MA) - CH ₃ NH ₃ ⁺ and formamidinium (FA) - [H ₂ NCHNH ₂ ] ⁺ . The preferred inorganic cation is Cs ⁺ . Variants of the composition of the photoactive layer are also possible, where combinations of organic cations and halogen atoms are used. For example, MA _x FA _1-x PbI _y Br _3-y , where x=0÷1, y=0÷3. Changing the x and y indices can affect the performance and stability of devices. The most preferred composition of the photoactive layer is Cs _0.12 FA _0.88 PbI ₃ . The thickness of the photoactive layer can be from 100 to 1000 nm.

The hole-transport layer 3 is represented by a film of conjugated polymer P1 with a thickness of 5 to 100 nm.

The electron-blocking layer 4 is represented by p-type metal oxides, such as MoO ₃ or MoO _x (x~3), V ₂ O ₅ or VO _x (x~2.5), CuO _x (x=0.5-1.0) with a thickness of 1 up to 100 nm.

The hole-collecting electrode 5 with a thickness of 30 to 300 nm may be translucent or opaque for visible spectral radiation. The translucent hole-collecting electrode can be made using transparent electrically conductive oxides: tin-doped indium oxide, fluorine-doped tin oxide, aluminum-doped zinc oxide, and other conductive oxides. Films of electrically conductive polymers such as PEDOT:PSS polyethylenedioxythiophene:polystyrenesulfonate, polyanilines and polypyrroles can also be used as translucent electrode materials. In addition, translucent electrodes can be formed on the basis of metals, i.e. metal microgrids, nanowires and ultra-thin films of gold, silver, copper, nickel, aluminum or other metals are used. A translucent electrode can also be made on the basis of carbon materials: graphene, carbon nanotubes, nanofibers, etc. As a translucent electrode layer, both individual materials from the above, and any combination of them can be used. To form an opaque hole-collecting electrode, films of metals (for example, Ag, Cu, Ni, Cr, Al, Au, Pt, etc.) or their alloys (nichrome, chromel, etc.), as well as other materials, can be used, having the properties of metals or semi-metals (for example, titanium nitride, graphite, various soot options).

A perovskite solar cell using polymer P1 as a hole-transport layer showed the following characteristics:

Open circuit voltage: 1093 mV;

Short circuit current density: 23.4 mA/cm ² ;

Fill factors: 77%;

Efficiency (COP) of light conversion: 19.7%

The reference device fabricated using PTAA as the hole transport layer material under the same conditions showed the worst performance:

Open circuit voltage: 1070 mV;

Short circuit current density: 22.1 mA/cm ² ;

Fill factors: 72%;

Efficiency (COP) of light conversion: 17.1%

The current-voltage characteristic for a perovskite solar cell using polymer P1 is shown in FIG. 6.

The present invention is illustrated by the following examples.

Example 1. Synthesis of polymer P1 Monomers M1 (210 mg; 0.155 mmol), M2 (193 mg; 0.155 mmol), catalyst Pd ₂ (dba) ₃ were placed in a 50 ml three-necked flask, where dba is dibenzylideneacetone (5 mg; 0.005 mmol ), additional ligand tri(o-tolyl)phosphine (5 mg; 0.015 mmol) and toluene (20 ml). The reaction mass was degassed three times and filled with argon. The flask was immersed in an oil bath and heated to 110°C, after which the reaction was monitored by gel permeation chromatography (GPC) every 30 minutes. When Mw ~ 200000 g/mol was reached (usually 4-5 h is required), the reaction was stopped by sequentially introducing trimethyl(thiophen-2-yl) stannan and bromothiophene into the reaction mixture with an interval of 30 minutes. Then the reaction mass was poured into methanol (50 ml), the precipitated polymer was filtered off on a Buchner funnel and dried in a vacuum desiccator. After that, the polymer was dissolved in 1,2-dichlorobenzene (40 ml) with stirring for 3 hours at 90°C. The polymer solution was then poured into isopropanol (300 ml) and the precipitated polymer was filtered through an extraction thimble. Then the sleeve with the polymer was placed in a Soxhlet apparatus and washed successively with acetone, methylene chloride, and chlorobenzene. The chlorobenzene fraction was evaporated to a volume of 20 ml and precipitated with isopropanol (150 ml). The precipitate was separated on a Buchner funnel and dried in a vacuum desiccator. The weight of the chlorobenzene fraction was 248 mg, the yield of polymer P1 was 78%. The analysis of the purified polymer was carried out on a GPC column in comparison with a series of F8BT standards (poly[(9,9-di-n-octylfluorenyl-2,7-diyl)@(benzo[2,1,3]thiadiazole-4,8-diyl )]). The weight average molecular weight was M _w =206000 g/mol, polydispersity index PDI=2.3.

Example 2. Manufacture of a perovskite solar cell using polymer P1 as a hole-transport layer.

The perovskite solar cell has the design shown in FIG. 5. Glasses (25x25 mm) with a deposited film of conductive indium oxide doped with tin oxide (ITO) with a resistance of 10-12 Ohm/ and a thickness of the conductive layer up to 125 nm were used for the manufacture of the solar battery. The substrates were washed sequentially in distilled water, toluene (high purity), and acetone (high purity), and then purified in water, acetone (high purity), and isopropanol (high purity) using ultrasound. Immediately before applying the charge-transport layers, the substrates were additionally kept in air plasma for 5 min. The cleaned substrates were coated with a SnO ₂ precursor solution (50 µl) prepared by diluting a 15% aqueous colloidal dispersion of SnO ₂ (Alfa-Aesar) 1.5 times with distilled water at a substrate rotation speed of 4000 rpm for 40 seconds. Next, the films were heated at 175°С in air for 15 min and then for another 10 min at 120°С in an inert nitrogen atmosphere in a glove box. The SnO ₂ films were passivated by applying a solution of phenyl-C ₆₁ -butyric acid (0.1 mg/ml) in chlorobenzene (3500 rpm, 30 s) followed by annealing at 100°C for 10 min. MAPbI ₃ perovskite films were spin-coated from a 1.4 M precursor solution (methylammonium iodide and PbI ₂ in equimolar ratios, 60 µl) in a mixture of N,N-dimethylacetamide and N-methyl pyrrolidone in a 4:1 ratio by volume at 4000 rpm /min 10 seconds after the application of this solution, 120 μl of toluene was poured onto a rotating substrate, which caused accelerated perovskite crystallization. Then the films were laid out on the supply at 50°С, heated to 100°С, and kept at this temperature for 5 min in an inert nitrogen atmosphere. A solution of polymer P1 (6 mg/ml in chlorobenzene) was spin-coated onto MAPbI ₃ films at 1000 rpm. An electron-blocking layer (30 nm V ₂ O ₅ ) and a hole-collecting electrode (120 nm Ag) were deposited by evaporation of the starting materials in a vacuum (10 ⁶ mm Hg).

A perovskite solar cell using polymer P1 as a hole-transport layer showed the following characteristics:

Open circuit voltage: 1093 mV;

Short circuit current density: 23.4 mA/cm ² ;

Fill factors: 77%;

Efficiency (COP) of light conversion:

19.7% A reference device made using PTAA as the hole transport layer material under the same conditions showed the worst performance:

Open circuit voltage: 1070 mV;

Short circuit current density: 22.1 mA/cm ² ;

Fill factors: 72%;

Efficiency (COP) of light conversion: 17.1%

The current-voltage characteristics for a perovskite solar cell using polymer P1 and reference PTAA are shown in FIG. 6 and the external quantum efficiency spectra in FIG. 7.

Claims (4)

1. A conjugated polymer based on benzodithiophene, 5,6-difluorobenzo[c][1,2,5]thiadiazole and thiophene, having the following structure:

where n=5-200. 2. The use of a conjugated polymer based on benzodithiophene, 5,6-difluorobenzo[c][1,2,5]thiadiazole and thiophene according to claim 1 as a hole-transport material in perovskite solar cells.

Images