CONJUGATED COPOLYMERS OF
DIKETOPYROLLOPYROLE AS ACTIVE LAYER FOR
OPTOELECTRONIC DEVICES

FIELD OF INVENTION

The invention generally relates to the field of solid state chemistry and particularly to synthesis of conjugated copolymers of diketopyrollopyrole, wherein the conjugated copolymers are employed as active layer for optoelectronic devices

BACKGROUND

Diketopyrollopyroles, hereinafter referred to as DPP, are chromophores belonging to the class of high performing pigments. Specifically, DPP containing polymers have shown light emitting and photo voltaic properties. Various types of polymers containing DPP in the main chain have been synthesized in the prior art. One such synthesis is disclosed in US Patent No. 6451459 assigned to Ciba Specialty Chemicals Corporation. The Ciba patent provides a DPP polymer, wherein the polymer chain units are formed through aryl substitutions on either side of the fused lactam ring. However, the absorption peaks of the synthesized polymers are around 600nm to 750nm. Further, there is no characterization of electron mobility which is a key factor in determining conductivity of the polymer. Further the conductivity of the polymer as claimed in Ciba Patent is dependent on co polymerization of the DPP polymer obtained with polymers known to enhance conductivity. US Patent

application no.2011/10284826 filed by Pascal Hayoz.et.al., discloses synthesis of diketopyrollopyrole polymers for use in organic semiconductor devices. However, the synthesized polymers exhibit electron mobility value of 0.01cm2/Vs. One conjugated polymer synthesized by Hugo Bronstein.et.al., published in the Journal of the American Chemical Society and incorporated herein by reference, has the structural formula

However, the polymer exhibits an electron mobility value of 0.063cm2/Vs. Another conjugated polymer synthesized by Bijleveld, J. C.et.al., published in the Journal of the American Chemical Society and incorporated herein by reference

and referred to as PDPP3T, exhibits an electron mobility value of 0.008cm2/Vs. To date very high hole mobilities for organic semiconductor are reported in field-effect transistor (FET) devices but FET based on n-channel semiconductors are rare and very poor in performance as compare to p-channel molecular semiconductors. Hence there is a need for
synthesizing an organic polymer that exhibits high electron mobility for development of new high performance n-type organic semiconductors.

BRIEF DESCRIPTION OF DRAWINGS:

So that the manner in which the recited features of the invention
can be understood in detail, some of the embodiments are
illustrated in the appended drawings. It is to be noted, however,
that the appended drawings illustrate only typical embodiments
of this invention and are therefore not to be considered limiting
of its scope, for the invention may admit to other equally
effective embodiments.

FIG.1 shows the reaction steps for obtaining a conjugated
polymer of DPP according to an embodiment of the invention.

FIG. 2 shows the chemical structure of DPP-DPP copolymers
according to an embodiment of the invention.

FIG.3a shows the absorption spectra of different conjugated
copolymers of DPP measured in solution, according to an
embodiment of the invention.

FIG.3b shows the absorption spectra of different conjugated
copolymers of DPP measured by incorporating the conjugated
copolymers in thin film, according to an embodiment of the
invention.

FIG.3c shows combined absorption spectra of a conjugated
copolymer of DPP N-CS2DPP-OD-TEG measured in solution,
according to an embodiment of the invention.

FIG. 4 shows a representative construction of an organic field effect transistor OFET with the conjugated copoloymer of DPP as the active layer, according to an embodiment of the invention.

FIG.5 shows a representative set of the transfer characteristics obtained from a TG-BC transistor, according to an embodiment of the invention.

SUMMARY OF THE INVENTION

One aspect of the invention provides a method for obtaining conjugated copolymer of DPP. The method includes mixing equal concentration of a first moiety and a second moiety in a reaction chamber; purging the reaction chamber at least once with a nitrogen gas in presence of a catalyst precursor; subjecting the purged mixture to a reflux cycle to form a copolymer; extracting the copolymer in an organic solvent to form a concentrated copolymer solution; and precipitating the concentrated copolymer solution in presence of methanol to obtain dried copolymer.

Another aspect provides a conjugated copolymer of DPP, and derivatives thereof, exhibiting absorption in the near IR region and having high electron mobility value of >1cm2/Vs.

DETAIL DESCRIPTION OF THE INVENTION:

One embodiment of the invention provides a method for synthesis of conjugated copolymers comprising DPP unit. The DPP-DPP based conjugated co-polymers are obtained by Suzuki coupling reaction. The chemical structures of copolymers

obtained through the Suzuki coupling reaction are shown in FIG. 1. Initially 1 millimolar concentration of a first moiety having the formula 3,6-Di(2-bromothien-5-yl)-2,5-di(2-octyldodecyl)-pyrrolo [3,4-c]pyrrole-1,4-dione, - F1
and 1 millimolar concentration of a second moiety having the formula 3,6-bis-(5-bromo-thiophen- 2-yl)- N,N′-bis(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-1,4-dioxo-pyrrolo[3,4-c]pyrrole- F2, are taken in 40ml of degassed dry Toluene. To the solution comprising the first moiety and the second moiety two drops of aliquat336 is added. The mixture comprising of the first moiety and the second moiety is then loaded onto a reaction container. 20mg of a ligand (o-tol)3P and 10mg of a catalyst precursor Pd2(DBA)3 are then loaded to the reaction chamber. The reaction container is purged with nitrogen for 10 min to remove dissolved oxygen. 1ml of degassed aqueous solution of K3PO4 is added through syringe to the nitrogen purged mixture and the mixture is subjected to a second purging cycle with nitrogen for 10 min. At the end of the nitrogen purging cycle, the mixture is refluxed at 90oC for 48 h. At the end of the refluxing cycle, the reaction mixture is cooled to room temperature, washed with water and extracted with CHCl3.

The CHCl3 extracted solvent is evaporated to obtain a concentrated polymer solution. The concentrated polymer solution is precipitated through slow addition of the concentrated polymer solution into a vigorously stirring methanol. The precipitates are collected by filtration and then washed initially with hot methanol and subsequently with acetone by soxhlet

extraction to remove low molecular weight oligomers. The polymer is then dried under vacuum for overnight to obtain a 66% yield. The conjugated copolymer synthesized is represented by the structure STR1 shown below

The backbone of the conjugated copolymer comprises of two units of DPP in alternate conformation linked together by two aryl unit Ar and Ar’. In one embodiment of the invention, the aryl groups Ar and Ar’ are identical units. Alternatively, Ar and Ar’ can be distinct units. The aryl unit Ar comprises of cyclic hydrocarbons which include but are not limited to homocyclic hydrocarbons, heterocyclic hydrocarbons, sulphur substituted five membered unsaturated ring, selenium substituted five membered unsaturated ring, fused heterocyclic rings. A representative set of structures incorporated into the DPP structure is as shown below:

The aryl unit Ar’ comprises of cyclic hydrocarbons which include but are not limited to homocyclic hydrocarbons, heterocyclic hydrocarbons, sulphur substituted five membered unsaturated ring, selenium substituted five membered

unsaturated ring, fused heterocyclic rings. A representative set of structures incorporated into the DPP structure is as shown below:

The R groups which form the side chains of the copolymer includes but are not limited to linear alkyl chain compounds, branched alkyl chain compounds, carboxy alkanes and alkyl chains which can be interrupted by one or more oxygen atoms. The R groups are represented by the following structures:

The R’ groups which form the side chains of the copolymer includes but are not limited to linear alkyl chain compounds, branched alkyl chain compounds, carboxy alkanes and alkyl chains which can be interrupted by one or more oxygen atoms. The R’ groups are represented by the following structures:

In one embodiment of the invention, four distinct conjugated copolymers of DPP were synthesized. The conjugated copolymers of DPP are classified based on the predominant R`

group present on STR1 which is the core repetitive unit of the copolymer. Accordingly the conjugated copolymers are given the nomenclatures of PTDPPOD for the core copolymer. Based on the R` group the conjugated copolymers are given the nomenclatures of PTDPPOD-EH, PTDPPOD-OD, PTDPPOD-HE and PTDPPOD-TEG, as shown in Fig.2 Example 1: Synthesis of PTDPPOD-TEG Copolymer of DPP In one embodiment of the invention, a derivative of the copolymer of DPP is synthesized according to the method described herein above. The synthesis of the derivative of the copolymer of DPP shall be explained in detail herein. Initially, the Suzuki coupling polymerization between
3,6-bis(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl)-N,N-bis(2-octyldodecyl)-1,4-dioxopyrrolo[3,4-c]pyrrole and 3,6-bis(5-bromothiophen-2-yl)-N,N-bis(2-(2-(2-methoxyethoxy)ethoxy) ethyl)-1,4-dioxopyrrolo[3,4-c] pyrrole is carried out in the presence of a catalyst palladium dibenzylideneacetone Pd2(DBA)3 and the active ligand P(o-tol)3. The copolymer is then purified by precipitation in methanol followed by Soxhlet extraction using methanol, acetone, and hexane, which ensured removal of catalytic impurities and undesired low-molecular weight oligomers. Finally, re-precipitation is done in methanol to obtain PTDPPOD-TEG Copolymer. The synthesized conjugated copolymers of DPP showed solubility in common organic solvents which include but are not limited to chloroform, chlorobenzene, dichlorobenzene and toluene.

Optical characterization of polymers of DPP: The ability of each of the conjugated copolymer of DPP synthesized, to perform in the near IR region are determined by measuring the absorption spectra. In one embodiment of the system, the absorption spectra of each of the conjugated copolymer of DPP are measured in solution. In an example of the invention, the solution for dissolving the conjugated copolymer of DPP is chloroform (CHCl3) solution. In another embodiment of the invention, the absorption spectra of each of the conjugated copolymer of DPP are measured by incorporating the synthesized conjugated copolymer of DPP in a thin film. Fig. 3a, 3b and 3c show the absorption spectra of different conjugated copolymers of DPP measured in solution and measured by incorporating the conjugated copolymers in thin film. Each of the conjugated copolymer of DPP displays an absorption edge up to 1000 nm. The absorption maximum is located around 900 nm. In dilute chloroform solution, all the polymers exhibits a typical π-π* transition at 400 nm and intramolecular charge transfer band at 900 nm (as shown in Fig. 3a and 3c). The absorption tail extends till 1μm which covers the near-IR region. Thin film absorption spectrum (as shown in 3b and 3c) of all the polymers PTDPPOD-EH, PTDPPOD-OD, PTDPPOD-HE and PTDPPOD-TEG shows a considerable red shift of 20 nm, 12 nm, 26 nm and 45 nm respectively which is due to the strong inter molecular interactions. The presence of planar DPP units in the polymer backbone increases the degree of co-planarity and provides a longer effective conjugation length

which lowers the band gap drastically. Band gap is calculated from the low energetic edge of the absorption spectrum. In an embodiment of the invention, the band gap was estimated to be 1.2eV for all the polymers irrespective of the different alkyl chains.

Industrial Application:

The conjugated copolymer of DPP and derivatives can be incorporated as an active layer in formation of organic semiconductor devices. The semiconductor devices include but are not limited to field effect transistors FET, solar panels and all such devices where the regular semiconductor materials can be replaced with organic semiconductor materials for enhanced performance. Specifically, the semiconductor devices mentioned herein include n-type devices.

One embodiment of the invention provides an organic semiconductor device having the synthesized conjugated copolymer of DPP as an active layer. FIG. 4 shows a representative construction of an organic field effect transistor OFET with the conjugated copolymer of DPP as the active layer, according to an embodiment of the invention. A bottom-gate, bottom-contact (BG-BC) is formed from Si/SiO2 substrates with pre-patterned Au source and/or drain electrodes 401. The organic semiconductor layer 402 comprising of the conjugated copolymer of DPP is spun on top of the substrate to complete the transistor fabrication. The spin cast substrate is subjected to thermal annealing at 140°C in presence of nitrogen. The BG-BC thus obtained exhibit ambipolar characteristics with electron and

hole mobilities of ~0.01 cm2/Vs. A Top-gate bottom-contact (TG-BC) 403 is made on top of glass substrate with pre-patterned Al source and/or drain electrodes. The organic semiconductor layer is spin-casted on top of the substrates. A transparent fluoropolymer is used as the dielectric layer 404 of the semiconductor device comprising the TG-BC and BG-BC. In an example of the invention, the fluoropolymer is a commercially available fluorophore CYTOP-type A. The transistor fabrication is completed with the evaporation of the top Al gate electrode by vacuum thermal sublimation. FIG.5 shows a representative set of the transfer characteristics obtained from a TG-BC transistor, according to an embodiment of the invention. The devices show high channel currents as a direct result of the high electron mobility which for some “hero” devices exceeds 3 cm2/Vs. The channel current on/off ratio is also high and typically in the order of 104 or higher.

The invention as described herein above provides a method for synthesizing conjugated copolymers of DPP. Representative copolymers synthesized include PTDPPOD-EH, PTDPPOD-OD, PTDPPOD-HE and PTDPPOD-TEG. The conjugated copolymers exhibit broad absorption tail extending into near-IR region till 1µm. The presence of two DPP units in an alternative fashion in the backbone of the copolymer effectively reduces the optical band gap. Due to their bulkier and branched side chains, all the synthesized copolymers exhibit solubility in common organic solvents including tetrahydrofuran, chloroform, toluene

and hexane. The electron mobility values of the synthesized copolymers are in the range of 1cm2/Vs to 4cm2/Vs. The foregoing description of the invention has been set for merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to person skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.

We Claim:

1. A method for obtaining conjugated copolymer of DPP
comprising the steps of

mixing equal concentration of a first moiety and a second moiety in a reaction chamber; purging the reaction chamber at least once with a nitrogen gas in presence of a catalyst precursor; subjecting the purged mixture to a reflux cycle to form a copolymer;
extracting the copolymer in an organic solvent to form a concentrated copolymer solution; and precipitating the concentrated copolymer solution in presence of methanol to obtain dried copolymer.

2. The method for obtaining conjugated copolymer of DPP of claim 1, wherein first moiety is represented by the F1 and second moiety is represented by F2.

3. The method for obtaining conjugated copolymer of DPP of claim 1, wherein the catalyst precursor is palladium dibenzyledene acetone.

4. The method for obtaining conjugated copolymer of DPP of claim 1, wherein the reflux cycle includes heating the mixture comprising F1and F2 to about 98 oC for about 48 hours, the heated mixture then cooled to a temperature in the range 20oC - 30 oC.

5. The method for obtaining conjugated copolymer of DPP of claim 1, wherein organic solvent is selected from a group

comprising chloroform, benzene, chlorobenzene, dichlorobenzene and toluene.

6. The method for obtaining conjugated copolymer of DPP of
claim 1, wherein the step of precipitation comprises of
a first precipitation in a vigorously stirring methanol; a second precipitation by washing at least once in hot methanol; and vacuum drying to obtain the copolymer of DPP.

7. A conjugated copolymer of DPP having a structure STR 1.

8. The conjugated copolymer according to claim 7, wherein the Ar is selected from a group comprising of cyclic hydrocarbons, wherein the cyclic hydrocarbons comprises of homocyclic hydrocarbons, heterocyclic hydrocarbons, sulphur substituted five membered unsaturated ring, selenium substituted five membered unsaturated ring and fused heterocyclic rings.

9. The conjugated copolymer according to claim 7, wherein the Ar’ is selected from a group comprising of cyclic hydrocarbons, wherein the cyclic hydrocarbons comprises of homocyclic hydrocarbons, heterocyclic hydrocarbons, sulphur substituted five membered unsaturated ring, selenium substituted five membered unsaturated ring and fused heterocyclic rings.

10. The conjugated copolymer according to claim 7, wherein
the R comprises of linear alkyl chain compounds, branched
alkyl chain compounds, carboxy alkanes and alkyl chains
which can be interrupted by one or more oxygen atoms.

10. The conjugated copolymer according to claim 7, wherein the
R’ comprises of comprises of linear alkyl chain compounds,
branched alkyl chain compounds, carboxy alkanes and alkyl
chains which can be interrupted by one or more oxygen
atoms.

12. The conjugated copolymer of DPP of claim 7, wherein the
conjugated copolymer of DPP have electron mobility value of
in the range of about 1cm2/Vs to about 4cm2/Vs.