WO2011066954A1

WO2011066954A1 - π-CONJUGATED POLYMERS CONTAINING FLUOROARYLVINYLEDENE UNITS AND RELATIVE PREPARATION PROCESS

Info

Publication number: WO2011066954A1
Application number: PCT/EP2010/007285
Authority: WO
Inventors: Riccardo Po'; Maria Caldararo; Giuliana Schimperna
Original assignee: Eni SpA
Current assignee: Eni SpA
Priority date: 2009-12-04
Filing date: 2010-11-30
Publication date: 2011-06-09
Anticipated expiration: 2012-06-04
Also published as: IT1396648B1; ITMI20092150A1

Abstract

The present invention relates to π- conjugated polymers containing fluoroarylvinylidene units and the relative preparation process. Photovoltaic devices comprising these alternating π-conjugated polymers are also described.

Description

-CO JUGATED POLYMERS CONTAINING FLUOROARYLVINYLEDENE UNITS AND RELATIVE PREPARATION PROCESS

The present invention relates to π-conjugated polymers containing fluoroarylvinylidene units and the relative preparation process.

The present invention falls within the field of photoactive materials which can be used in the production of photovoltaic devices.

Photovoltaic devices are devices capable of con- verting the energy of a light radiation into electric energy. At present, most photovoltaic devices which can be used for practical applications exploit the physico- chemical properties of photo-active materials of the inorganic type, in particular high-purity crystalline silicon. As a result of the high production costs of crystalline silicon, scientific research has been orienting its efforts towards the development of alternative organic materials having a conjugated, oligomeric or polymeric structure. Unlike high-purity crystalline silicon, in fact, organic materials having a conjugated structure are characterized by a relative synthesis facility, a low production cost, a reduced weight of the relative photovoltaic device, in addition to allowing the recycling of said polymer at the end of the life- cycle of the device in which it is used. The functioning of organic and polymer photovoltaic cells is based on the combined use of an electron acceptor compound and an electron donor compound. The most widely-used electron donor and acceptor compounds in devices described in scientific and patent literature are π-conjugated polymers belonging to the groups of polyparaphenylene vinylenes and polythiophenes , and fullerene derivatives, respectively.

The basic conversion process of light into electric current in a polymer photovoltaic cell takes place through the following steps:

1. absorption of a photon on the part of the donor compound with the formation of an exciton, i.e. a pair of "electron-hole" charge carriers;

2. diffusion of the exciton in a region of the donor compound in which its dissociation can take place;

3. dissociation of the exciton in the two charge carriers (electron (-) and hole (+) ) separated;

4. transporting of the charges thus formed to the cathode (electron, through the acceptor compound) and anode (hole, through the donor compound) , with the generation of an electric current in the circuit of the device .

The photo-absorption process with the formation of the exciton and subsequent transfer of the electron to the acceptor compound consists in the transfer of an electron from the orbital HOMO (Highest Occupied Molecular Orbital) to the orbital LUMO (Lowest Unoccupied Molecular Orbital) of the donor and subsequently the transfer from this to the LUMO of the acceptor.

As the efficiency of an organic or polymer photovoltaic cell depends on the number of free electrons which are generated by dissociation of the excitons, one of the structural characteristics of the donor com- pounds which mostly influences said efficiency is the difference in energy existing between the HOMO and LUMO orbitals of the donor (so-called band-gap) . The wavelength of the photons which the donor compound is capable of collecting and effectively converting into elec- trie energy (so-called "photo harvesting" or "light- harvesting" process) depends, in particular, on this difference. Another important characteristic is the mobility of the electrons in the acceptor and electronic gaps in the donor, which determines the facility with which the electric charges, once photo-generated, reach the electrodes. This, in addition to being an intrinsic property of the molecules, is also strongly influenced by the morphology of the photoactive layer, which in turn depends on the reciprocal miscibility of the com- ponents and their solubility. Finally, a further funda- mental characteristic is the resistance to thermo- oxidative and photo-oxidative degradation of the materials, which must be stable under the operating conditions of the device.

In order to obtain acceptable electric currents, the band-gap between HOMO and LUMO must not be too high, but at the same time, it must not be too low, as an excessively low band-gap would negatively affect the voltage obtained at the electrodes of the device.

In the simplest way of operating, photovoltaic cells are produced by introducing a thin layer (about 100 nanometers) of a mixture of the acceptor and donor, between two electrodes. To obtain a layer of this type, a solution of the two components is prepared. A photo- active film is then created on the first electrode starting from the solution, using suitable deposition techniques such as "spin-coating" , " spray-coating" , "ink-jet printing" , etc. Finally, the counter-electrode is deposited on the dried film.

The donor material most commonly used in the production of polymer solar cells is regioregular poly (3- hexylthiophene) (P3HT) . This polymer has interesting electronic and optical characteristics (relatively low band-gap; a good absorption coefficient) , a good solu- bility in the solvents used for producing the photo- voltaic cells and a reasonable mobility of the electronic gaps .

The flow of photons of solar radiation which reaches the surface of the Earth, however, is maximum for a wavelength of about 700 nm, corresponding to energy values of around 1.8 eV, much lower than the band- gap values (generally higher than 2-3 eV) which characterize many of the polymeric materials currently known and used as donor compounds in photovoltaic devices . The light harvesting process of this spectral field is consequently not very efficient and only a fraction of the overall solar energy is converted into electric energy. As already mentioned, this is one of the main factors which cause low efficiencies in photovoltaic devices .

Among the polymers most widely-used as donor compounds, for example, the polymer MDMO-PPV (poly [2- methoxy-5- (3 , 7 -dimethyloctyloxy) -1, 4-phenylene] -alt- (vinylene) ) and especially the polymer P3HT, used in combination with acceptor compounds based on fullere- nes, are capable of obtaining maximum conversion efficiencies of solar radiation not higher than 5.4%.

In order to improve the yield of the light harvesting process and consequently the efficiency of photo- voltaic devices, it is consequently fundamental to find new donor compounds capable of effectively capturing and converting solar radiations, transporting the charges to the electrodes more efficiently and resisting oxidative degradation processes.

An objective of the present invention is to overcome the drawbacks revealed by the state of the art.

An object of the present invention therefore relates to an alternating π-conjugated polymer comprising: at least one fluoroarylvinylidene electron-acceptor unit A having general formula (I)

(I)

wherein the substituents X₁-X5, the same or different, are hydrogen atoms, fluorine atoms or an alkyl group containing from 1 to 12 carbon atoms, preferably from 1 to 4 carbon atoms, and with the condition that at least one, preferably at least two, more preferably at least three, of the substituents Xi-X₅ is a fluorine atom or a -CF₂ group, wherein R is H, F or a hydrocarbyl group, possibly fluorinated, having from 1 to 10 carbon atoms, at least one conjugated electron-donor structural unit B connected to the unit A in the points indicated by the dashed lines in general formula (I) .

The structural units A are preferably substituted with at least three F atoms or three -CF₃ groups .

The electron-donor structural units B can be selected, for example, from the following list:

-8-

wherein the Ri-R₈ substituents, the same or different, are hydrogen atoms,

C1-C37 alkyl groups, possibly branched,

OC₁-OC₁₆ alkoxy groups on the condition that the substituents Ri-Rs are bonded to a carbon atom.

Examples of Ri-Ra alkyl groups are the following: methyl, ethyl, propyl, butyl, hexyl, octyl, decyl, do- decyl, tetradecyl, hexadecyl, octadecyl, eicosyl, 2- ethylhexyl, 2 ethyloctyl, 2-ethyldecyl, 2 -ethyldodecyl , 4 -butylhexyl , 4 -buthyloctyl , 4 -butyldecyl , 4- butyldodecyl , 2-hexyloctyl, 2-hexyldecyl, 4-hexyldecyl , isopropyl, 1-ethylpropyl , 1-butylpentyl , 1 -hexylheptyl , 1-octylnonyl , 1-dodecyltridecyl , 1-hexadecylheptadecyl and 1-octadecylnonadecyl . The groups butyl, hexyl, octyl, decyl, dodecyl, 2-ethylhexyl, tetradecyl, hexadecyl, 4-hexyldecyl, 1-octylnonyl, are preferred.

In order to guarantee the solubility of the copolymers according to the present invention, the ratio between the sum of all the carbon atoms of the alkyl chains variably present in the repetitive base unit of the alternating π-conjugated polymer and the number of aromatic rings present in the same units is preferably within the range of 2.5 - 12.

The alternating π-conjugated polymers object of the present invention preferably have a structure of the repetitive base unit of the type (A-B)_n, wherein A and B have the meaning previously defined and n is an integer ranging from 1 to 1,000, preferably from 2 to 500, even more preferably from 3 to 50.

Each unit A is bound to two B units, the same or different, except for when unit A or unit B represent terminal units of the polymeric chain. In this latter case, the terminal unit A or unit B are bound to one unit only, B or A, respectively, and the remaining valence is saturated by a terminal substituent whose structure depends on the preparation method of the polymer and can be easily identified by an expert in the field. In most cases, this substituent is H or Br.

The following alternating π- conjugated polymers are particularly preferred: poly [9, 9-bis (2-ethylhexyl) - fluorene—alt- 2- (p- fluorophenyl) -1, 1-vinylidene]

(polymer 1) , poly [9 , 9-bis (2 -ethylhexyl) fluorene—al t-2- (pentafluoro-phenyl) -1, 1-vinylidene] (polymer 2) and poly [N-octyl-3 , 7 -phenothiazine—alt-2-

(pentaf luorophenyl) -1, 1-vinylidene] (polymer 3) , for the light harvesting properties shown.

When the alternating π-conjugated polymer according to the present invention is the following (polymer 1) :

a possible method for obtaining it is that corresponding to the process indicated in the following scheme A:

Scheme A

The above polymer 1 preferably has a value of the n index varying from 3 to 50.

When the alternating π-conjugated polymer according to the present invention is the following (polymer 2) :

a possible method for obtaining it is that corresponding to the process indicated in the following scheme B :

Scheme B .

The above polymer 2 preferably has a value of the n index varying from 3 to 50.

When the alternating π-conjugated polymer according to the present invention is the following (polymer 3)

a possible method for obtaining it is that corre- 285

sponding to the process indicated in the following scheme C:

Scheme C.

The above polymer 3 preferably has a value of the n index varying from 3 to 50.

Alternating π-conjugated polymers of the present invention having structures different from those previously indicated can be obtained by means of processes of the radical-cationic or redox type pertaining to those described or in any case which can be easily inferred by experts on the basis of known methods of organic chemistry.

The alternating π-conjugated polymers according to the present invention have favourable physico-chemical properties which allow them to be used as photoactive materials, in particular as electron-donor compounds within photovoltaic devices. They are characterized by band-gap values lower than 3.2 eV and are therefore particularly suitable for exploiting solar radiation with a higher wavelength. Thanks to the thermal oxidative stability conferred by the presence of fluorinated units, these materials can be advantageously used in the production of photovoltaic devices having a longer duration under conditions of high environmental stress and exposure to intense light radiation and with a significant ultraviolet component.

A further object of the present invention therefore relates to a photovoltaic device comprising any of the alternating π-conjugated polymers of the present invention .

As will be better illustrated in the following examples, the above alternating π-conjugated polymers can be easily synthesized according to the process schemes previously illustrated.

The following embodiment examples are provided for illustrative purposes of the present invention and should not be considered as limiting its protection scope.

The alternating π-conjugated polymers according to the present invention were characterized by means of UV-Vis-NIR spectroscopy to determine the energy entity of the HOMO-LU O band-gap according to the following procedure. The polymer is dissolved in toluene at a concentration of about 10^"4 , transferred to a quartz Suprasil 1.0 cm cuvette and analyzed in transmission by means of a double-beam UV-Vis-NIR spectrophotometer and Perkin Elmer λ 19 double monochromator, within the range of 190-900 nm with a pass-through band of 2.0 nm, a scanning rate of 120 nm/min and step of 1 nm, using an identical cuvette filled with the solvent alone, as reference .

The band-gap is estimated from the diffuse reflectance spectra by measuring the absorption edge corresponding to the transition from the valence band (VB) to the conduction band (CB) . For determining the edge, the intersection with the axis of the abscissa of the tangent line at the absorption band in the flexpoint was considered.

The flexpoint (λ_Ρ, y_F) is determined on the basis of the coordinates of the minimum of the spectrum in first derivative, indicated with λ' min S y min ·

The equation of the tangent line at the UV-Vis spectrum in the flexpoint ( _Fl y_F) is:

y = y' min + y_F ^_ y' min λ' _min

Finally, from the intersection condition with the axis of the abscissa ψ = 0, the following is obtained: EDGE = (y'min λ' _mi_n - yF⁾/y'min Therefore, by measuring the coordinates of the minimum of the spectrum in first derivative and the corresponding absorbance value y_F from the UV-Vis spectrum, λ_ΕϋσΕ is obtained directly by substitution.

The corresponding energy is :

¾DGE ⁼ VEDGE — h c/ gDGE

wherein h = 6.626 10^"34 J s

c = 2.998 10⁸ m s^"1 i.e. EEDGE = 1.988 10^~16 J/λ_EDGE(nm).

Finally, remembering that 1 J = 6.24 10¹⁸ eV, the following is obtained:

EEDGE = 1240 eV / A,EDGE (ntn)

EXAMPLE 1

Alternating π- conjugated polymer poly [9, 9-bis (2- ethylhexyl) fluorene—alt- 2- (p- fluorophenyl) -1, 1-vinyl- idene] , (polymer 1)

The following products were introduced, in order, into a 250 ml three-necked flask equipped with a magnetic stirrer and reflux condenser, in an inert atmos- phere:

1,289 mg (2.696 mmoles) of 9,9-bis(2'- ethylhexyl) -2 , 7-diboronic acid;

755 mg (2.696 mmoles) of p- fluoro-β,β- dibromostyrene (1, l-dibromo-2- (p- fluorophenyl) ethylene) , dissolved in about 20 ml of deaerated toluene;

- 250 mg of Aliquat 336 (5.9 mmoles) dissolved in about 50 ml of deaerated toluene;

12 ml of potassium carbonate 1.036 M (12.4 mmoles) (deaerated water) ;

66 mg (0.054 mmoles) of palladium (0) tetrakis (triphenylphosphine) .

The reaction mixture was heated to reflux temperature (90°C) and under vigorous stirring for 26 hours.

At the end of the period indicated, the mixture was concentrated to about 30 ml and poured into 400 ml of methanol. The precipitate obtained was filtered and washed, in order, with methanol, water and methanol again.

The product was dissolved in a minimum quantity of toluene and re-precipitated in methanol. 650 mg of yellow-greenish polymer were obtained.

The optical band-gap measured on the solid film for the copolymer thus obtained is 2.5 eV, HOMO and LUMO values of -5.7 and -2.2 eV, respectively, a weight average molecular weight of 6,400 and a weight loss at 250 °C of less than 1%, determined by means of thermogravimetric analysis in air.

Schematically, the reaction which took place is the following :

EXAMPLE 2

Alternating π-conjugated polymer poly [9 , 9-bis (2- ethylhexyl) fluorene—alt-2- (pentafluorophenyl) -1,1- vinyl-idene] , (polymer 2)

- 1,143 mg (2.39 mmoles) of 9 , 9-bis (2 ' -ethylhexyl) - 2 , 7-diboronic acid;

841 mg (2.39 mmoles) of pentafluoro-β,β- dibromostyrene (1, l-dibromo-2- (pentafluorophenyl) - ethylene) dissolved in about 20 ml of deaerated toluene ;

- 221 mg of Aliquat 336 (5.2 mmoles) dissolved in about 50 ml of deaerated toluene;

10.8 ml of potassium carbonate 1.033 M (11.1 mmoles) (deaerated water)

58 mg (0.047 mmoles) of palladium (0) tetrakis ( triphenylphosphine . ) The reaction mixture was heated to reflux temperature (90°C) and under vigorous stirring for 33 hours .

At the end of the period indicated, the mixture was concentrated until dry and poured into 200 ml of methanol. The precipitate obtained was filtered and washed, in order, with methanol, water, a mixture of wa- ter/ethanol and finally ethanol .

The product was dissolved in a minimum quantity of toluene and re-precipitated in methanol. 1,252 mg of grey-coloured polymer were obtained.

The optical band-gap measured on the solid film for the copolymer thus obtained is 3.1 eV, HOMO and LUMO values of -6.0 and -2.5 eV, respectively, a weight average molecular weight of 6,600 and a weight loss at 250°C of less than 1%, determined by means of thermogravimetric analysis in air.

Schematically, the reaction which took place is the following :

EXAMPLE 3

Alternating π- conjugated polymer poly [N-octyl-3 , 7- phenothiazine—alt-2- (pentafluorophenyl) -1, 1- vinylidene] , (polymer 3)

The following products were introduced, in order, into a 250 ml three-necked flask equipped with a mag- netic stirrer and reflux condenser, in an inert atmosphere :

- 1,021 mg (2.558 mmoles) of N-octylphenothiazo- 3 , 7-diboronico acid;

900 mg (2.39 mmoles) of pentafluoro-β,β- dibromostyrene ( 1 , l-dibromo-2-

(pentafluorophenyl) ethylene) dissolved in about 20 ml of deaerated toluene;

- 238 mg of Aliquat 336 (5.6 mmoles) dissolved in about 50 ml of deaearated toluene;

- 11.5 ml of potassium carbonate 1.033 M (11.9 mmoles) (deaerated water)

- 61 mg (0.050 mmoles) di palladium ( 0 ) tetrakis- ( triphenylphosphine)

The reaction mixture was heated to reflux temperature (90°C) under vigorous stirring for 39 hours .

At the end of the period indicated, the mixture was concentrated until dry and poured into 200 ml of methanol. The precipitate obtained was filtered and washed, in order, with methanol, water, a mixture of wa- ter/ethanol and finally methanol.

The product was dissolved in a minimum quantity of toluene and re-precipitated in methanol. 1,150 mg of brown-greenish-coloured polymer were obtained.

The optical band-gap measured on the solid film for the copolymer thus obtained is 2.6 eV, a weight average molecular weight of 7,000 and a weight loss at 250°C of less than 3%, determined by means of thermogravimetric analysis in air.

Schematically, the reaction which took place is the following :

Claims

1. An alternating π-conjugated polymer comprising: a fluoroarylvinylidene electron-acceptor unit A having general formula (I)

(I)

wherein the substituents Xi-X₅, the same or different, are hydrogen atoms, fluorine atoms or an alkyl group containing from 1 to 12 carbon atoms, preferably from 1 to 4 carbon atoms, and with the condition that at least one, preferably at least two, more preferably at least three, of the substituents X1-X5 is a fluorine atom or a -CF₂R group, wherein R is H, F or a hydrocarbyl group, possibly fluorinated, having from 1 to 10 carbon atoms, at least one conjugated electron-donor structural unit B connected to the A unit in the points indicated by the dashed lines in general formula (I) .

2. The alternating π-conjugated polymer according to claim 1, wherein the average number of A units in the polymer ranges from 2 to 1,000, preferably from 5 to 1,000.

3. The alternating π-conjugated polymer according to claim 1 or 2, wherein the conjugated electron-donor structural units B are selected from the following group :

-25-

šre hydrogen atoms,

C₁-C37 alkyl groups, possibly branched,

- OC₁-OC₁₆ alkoxy groups on the condition that the substituents Ri-R₈ are bonded to a carbon atom.

4) The alternating π-conjugated polymer according to claim 3, wherein the substituents Ri-Re in the above- mentioned structures are selected from the following group of substituents: methyl, ethyl, propyl, butyl, hexyl, octyl, decyl, dodecyl, tetradecyl, hexadecyl, octadecyl, eicosyl, 2-ethylhexyl, 2 ethyloctyl, 2- ethyldecyl, 2-ethyldodecyl , 4 -butylhexyl , 4- buthyloctyl, 4 -butyldecyl , 4 -butyldodecyl , 2- hexyloctyl, 2-hexyldecyl, 4 -hexyldecyl , isopropyl, 1- ethylpropyl, 1-butylpentyl, 1-hexylheptyl , 1- octylnonyl, 1-dodecyltridecyl , 1-hexadecylheptadecyl and 1-octadecylnonadecyl.

5. The alternating π-conjugated polymer according to any of the previous claims, wherein the structure of the repeating base unit is represented by the formulae (A-B)n wherein A and B have the meaning specified above and n is an integer ranging from 1 to 1,000, preferably from 2 to 500.

6. The alternating π-conjugated polymer according to any of the previous claims, wherein the ratio between the sum of all the carbon atoms of the alkyl chains present in the repeating base unit and the number of aromatic rings present in the same unit, ranges from 3.5 to 12.

7. The alternating π-conjugated polymer according to claim 1, having the following structure:

wherein n ranges from 3 to 50.

8. The alternating π-conjugated polymer according to claim 1, having the following structure

wherein n ranges from 3 to 50.

9. The alternating π-conjugated polymer according to claim 1, having the following structure:

wherein n ranges from 3 to 50.

10. A photovoltaic device comprising an alternating π- conjugated polymer according to any of the claims from 1 to 9.