This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys., 2013, 15, 7475--7478 7475 Cite this: Phys. Chem. Chem. Phys., 2013, 15, 7475 Synthesis, characterization and organic field effect transistor performance of a diketopyrrolopyrrole– fluorenone copolymer† Prashant Sonar,* a Tae-Jun Ha b and Ananth Dodabalapur* ab A diketopyrrolopyrrole (DPP) with fluorenone (FN) based low band gap alternating copolymer (PDPPT-alt-FN) has been synthesized via Suzuki coupling. PDPPT-alt-FN exhibits a deep HOMO level with a lower band gap. Fabricated organic thin film transistors using PDPPT-alt-FN as a channel semiconductor show p-channel behaviour with the highest hole mobility of 0.083 cm 2 V 1 s 1 measured in air. Polymeric semiconductors are potentially very important active materials for solution processable organic electronic devices such as organic field effect transistors (OFETs), organic photo- voltaic cells (OPVs) and organic light emitting diodes (OLEDs). 1 Compared to small molecules, polymers are beneficial for large area applications due to their easy film forming properties, high viscosity, mechanical/thermal stability and printability. In recent years, donor–acceptor (D–A) based polymeric semi- conductors have been proven to be very good candidate materials for making high performance OPV and OFET devices. 2 These donor and acceptor conjugated building blocks can be primarily classified into two main types, strong and weak, based on their ionization potential and electron affinity values respectively. Several new building blocks have been explored and utilized for making new polymeric structures in order to evaluate their performances in organic electronic devices. Among strong acceptors, diketopyrrolopyrrole (DPP) is one of the best alter- natives for making high performance materials due to its straightforward synthesis, fused planar nature for better charge transportation, and diverse possibilities to tune the opto-electronic properties. 1–3 DPP can be combined with various heterocyclic rings such as thiophene, furan and selenophene for making a D–A–D core building block for designing and synthesizing new polymeric structures. 4 Thiophene substituted DPP has shown huge potential in the synthesis of high performance alternating copolymers combined with various fused aromatic comonomer blocks such as thienothiophene, thienylene–vinylene, and dithienothiophene. 5 These fused aromatic comonomers belong to the strong donor category due to their electron rich nature. Weak donors such as naphthalene, carbazole and fluorene have been also used for synthesizing DPP copolymers. 6 There are very few reports of using a weak electron acceptor as a comonomer block. Among weak acceptors, fluorenone is one of the fused condensed aromatic promising building blocks for making new polymeric structures. Hitherto, there has been only one report utilizing this block as a comonomer for synthesizing a fluorenone based alternating copolymer by our group. 7 Fluorenone is a capable moiety for constructing potential polymeric structures due to its high chemical stability. The C Q O group substituted at the 9th position of fluorenone may support better p–p stacking, and interestingly, this material can be scaled up due to easy synthesis. 8,9 This block can also be useful for designing new n-type polymeric semiconductors. Additionally, incorporation of the mild electron accepting nature of fluorenone will lower the HOMO value of the copolymer for inducing air stability. In this communication, a thiophene substituted DPP combined with fluorenone based D–A alternating copolymer, PDPPT-alt-FN, has been designed and synthesized as shown in Scheme 1. First 2,5-dihydro-1,4-dioxo-3,6-dithienylpyrrolo [3,4-c]-pyrrole (1, DPP core) was synthesized using a thiophene carbonitrile precursor according to an established procedure. 3 Scheme 1 Reagents and conditions: (I) K 2 CO 3 , 2-decyl-1-tetradecyl bromide, anhydrous DMF, 120–130 1C, overnight; (II) bromine, chloroform, room temp., overnight; (III) PdCl 2 (dppf), 1,4-dioxane, 80 1C for 20 h; (IV) Pd(PPh 3 ) 4 , aliquat 336, 2 M K 2 CO 3 , toluene, 80 1C for 72 h, phenylboronic acid, bromobenzene. a Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology, and Research (A*STAR), 3 Research Link, Singapore 117602. E-mail: [email protected]b Microelectronics Research Centre, The University of Texas at Austin, Austin, TX 78758, USA. E-mail: [email protected]† Electronic supplementary information (ESI) available: Experimental details and NMR data of compounds 2 and 5. See DOI: 10.1039/c3cp50286d Received 22nd January 2013, Accepted 26th March 2013 DOI: 10.1039/c3cp50286d www.rsc.org/pccp PCCP COMMUNICATION Published on 28 March 2013. Downloaded by Washington University in St. Louis on 20/09/2013 06:21:11. View Article Online View Journal | View Issue
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This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys., 2013, 15, 7475--7478 7475
Cite this: Phys. Chem.Chem.Phys.,2013,15, 7475
Synthesis, characterization and organic field effecttransistor performance of a diketopyrrolopyrrole–fluorenone copolymer†
Prashant Sonar,*a Tae-Jun Hab and Ananth Dodabalapur*ab
A diketopyrrolopyrrole (DPP) with fluorenone (FN) based low band
gap alternating copolymer (PDPPT-alt-FN) has been synthesized via
Suzuki coupling. PDPPT-alt-FN exhibits a deep HOMO level with a
lower band gap. Fabricated organic thin film transistors using
PDPPT-alt-FN as a channel semiconductor show p-channel behaviour
with the highest hole mobility of 0.083 cm2 V�1 s�1 measured in air.
Polymeric semiconductors are potentially very important activematerials for solution processable organic electronic devicessuch as organic field effect transistors (OFETs), organic photo-voltaic cells (OPVs) and organic light emitting diodes (OLEDs).1
Compared to small molecules, polymers are beneficial for largearea applications due to their easy film forming properties,high viscosity, mechanical/thermal stability and printability.In recent years, donor–acceptor (D–A) based polymeric semi-conductors have been proven to be very good candidate materialsfor making high performance OPV and OFET devices.2 Thesedonor and acceptor conjugated building blocks can be primarilyclassified into two main types, strong and weak, based on theirionization potential and electron affinity values respectively.Several new building blocks have been explored and utilizedfor making new polymeric structures in order to evaluate theirperformances in organic electronic devices. Among strongacceptors, diketopyrrolopyrrole (DPP) is one of the best alter-natives for making high performance materials due to itsstraightforward synthesis, fused planar nature for better chargetransportation, and diverse possibilities to tune the opto-electronicproperties.1–3 DPP can be combined with various heterocyclic ringssuch as thiophene, furan and selenophene for making a D–A–Dcore building block for designing and synthesizing new polymericstructures.4 Thiophene substituted DPP has shown huge potential
in the synthesis of high performance alternating copolymerscombined with various fused aromatic comonomer blocks such asthienothiophene, thienylene–vinylene, and dithienothiophene.5
These fused aromatic comonomers belong to the strong donorcategory due to their electron rich nature. Weak donors such asnaphthalene, carbazole and fluorene have been also used forsynthesizing DPP copolymers.6 There are very few reports of usinga weak electron acceptor as a comonomer block. Among weakacceptors, fluorenone is one of the fused condensed aromaticpromising building blocks for making new polymeric structures.Hitherto, there has been only one report utilizing this block as acomonomer for synthesizing a fluorenone based alternatingcopolymer by our group.7 Fluorenone is a capable moiety forconstructing potential polymeric structures due to its highchemical stability. The CQO group substituted at the 9th positionof fluorenone may support better p–p stacking, and interestingly,this material can be scaled up due to easy synthesis.8,9 Thisblock can also be useful for designing new n-type polymericsemiconductors. Additionally, incorporation of the mild electronaccepting nature of fluorenone will lower the HOMO value of thecopolymer for inducing air stability.
In this communication, a thiophene substituted DPP combinedwith fluorenone based D–A alternating copolymer, PDPPT-alt-FN,has been designed and synthesized as shown in Scheme 1. First2,5-dihydro-1,4-dioxo-3,6-dithienylpyrrolo [3,4-c]-pyrrole (1, DPPcore) was synthesized using a thiophene carbonitrile precursoraccording to an established procedure.3
Scheme 1 Reagents and conditions: (I) K2CO3, 2-decyl-1-tetradecyl bromide,anhydrous DMF, 120–130 1C, overnight; (II) bromine, chloroform, room temp.,overnight; (III) PdCl2(dppf), 1,4-dioxane, 80 1C for 20 h; (IV) Pd(PPh3)4, aliquat336, 2 M K2CO3, toluene, 80 1C for 72 h, phenylboronic acid, bromobenzene.
a Institute of Materials Research and Engineering (IMRE), Agency for Science,
Technology, and Research (A*STAR), 3 Research Link, Singapore 117602.
E-mail: [email protected] Microelectronics Research Centre, The University of Texas at Austin, Austin,
7476 Phys. Chem. Chem. Phys., 2013, 15, 7475--7478 This journal is c the Owner Societies 2013
Compounds N,N0-bis(2-octy1-dodecyl)-3,6-dithienyl-1,4-diketo-pyrrolo[3,4-c]-pyrrole (2) and 3,6-bis-(5-bromo-thiophen-2-yl)-N,N0-bis(2-octyldodecyl)-1,4-dioxo-pyrrolo[3,4-c]-pyrrole (3) were obtainedusing compound 1 via alkylation and bromination respectively.The comonomer compound 2,7-bis (4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-fluoren-9-one (5) was produced fromcommercially available 2,7-dibromo-9H-fluoren-9-one (4) usingPdCl2(dppf), KOAc and bis(pinacolato)diboron in 1,4-dioxanesolvent using our previously reported procedure. Suzuki poly-condensation of compounds 3 and 5 resulted in a polymer(PDPPT-alt-FN) in 68% yield (Scheme 1). PDPPT-alt-FN waspurified using Soxhlet extraction with methanol, acetone andhexane solvent sequentially. Washing with these solventsremoves low molecular weight oligomer fractions and catalyticimpurities. Lastly, the pure polymer was extracted with chloro-form. Polymer solution in chloroform was removed on a rotavapand then finally precipitated in methanol as a thick film withsome flakes. The number average (Mn) and weight averagemolecular weight (Mw) of PDPPT-alt-FN were calculated usinghigh temperature gel permeation chromatography (HT-GPC) andwere found to be 42 600 and 135 600 g mol�1, respectively (seeESI† for GPC). The thermal stability of PDPPT-alt-FN was studiedusing thermogravimetric analysis (TGA). TGA showed 5% weightloss at 356 1C under nitrogen, indicating the high thermal stabilityof this polymer. In order to check the thermal transitions ofPDPPT-alt-FN, heating and cooling cycles were performed startingfrom room temperature to 350 1C using differential scanningcalorimetry (DSC). No transitions were observed during heating–cooling cycles which could be related to the rigid nature of thispolymer (see ESI† for TGA and DSC curves).
The optical properties of PDPPT-alt-FN were studied usingUV-vis absorption spectroscopy. The solution (in chlorobenzene)and the thin film (spin coated from chlorobenzene solution)absorption spectra for PDPPT-alt-FN are shown in Fig. 1. Theabsorption maxima (lmax) for both solution and thin film arelocated at 751 nm. The solid state absorption spectrum isbroader than that in solution indicating solid state interactionsor ordering of the polymer chains. Additionally, the absorptioncut off value in the solid state (813 nm) is slightly red shiftedcompared to that in solution (795 nm). The optical band gapcalculated from the solid state absorption cut-off is around1.52 nm.
The highest occupied molecular orbital (HOMO) and lowestunoccupied molecular orbital (LUMO) energy levels of PDPPT-alt-FN are important for fabricating electronic devices. Photoelectronspectroscopy in air (PESA) was used for the determination of theHOMO value of the PDPPT-alt-FN spin coated thin film on glassand is shown in Fig. 2. From the onset energy, the calculatedHOMO value is 5.42 eV. Such a deep HOMO value the polymer isappropriate for fabricating stable organic electronic devices. Thedifference between the optical band gap and HOMO gives theLUMO value and is found to be 3.9 eV. Intramolecular interactionor p–p stacking of the organic semiconducting material in thesolid state is crucial for co-relating the performance in electronicdevices. 2-D X-ray diffraction (XRD) study was performed usingX-ray irradiation parallel to the PDPPT-alt-FN flakes. The corres-ponding diffractogram and image are shown in Fig. 3. Theprimary diffraction peak (100) which corresponds to the interlayerd-spacing was found at 2y = 3.791. The interlayer spacing distancecalculated by Braggs equation using the above y value is 23.34 nm.The secondary peak (010) located from the 2y value 15 to 301 isquite broad and might be due to the mixed edge on and face onorientation of the polymeric chains on the substrate.
The performance of PDPPT-alt-FN as a channel semiconductorwas tested using bottom-gate, bottom-contact OFET devices. Forthe fabrication of complete sets of devices, n-doped silicon with200 nm silicon oxide was used. The n-silicon substrate acts as thegate electrode and the SiO2 acts as the gate dielectric material. Inorder to enhance the better adhesion of source–drain electrodesto the gate dielectric layer, a 2.5 nm thick chromium layer wasdefined by photolithography and evaporation techniques.
Fig. 1 UV-vis absorption spectra of PDPPT-alt-FN in chlorobenzene solution andspin coated thin film from chlorobenzene solution.
Fig. 2 Photoelectron spectroscopy in air (PESA) measurement of the PDPPT-alt-FN thin film spin coated on a glass substrate using chlorobenzene solution.
Fig. 3 2-D XRD pattern intensity graphs (left) and 2-D XRD image (right)obtained with the incident X-ray parallel to the thin film stack of the PDPPT-alt-FN copolymer (right).
This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys., 2013, 15, 7475--7478 7477
In order to minimize the contact resistance, the source–drainelectrodes were treated with a layer of pentafluorobenzenethiol(PFBT). An active layer of PDPPT-alt-FN of B40 nm thickness wasdeposited via spin coating on the top of octadecyltrimethoxysilane(OTMS) treated SiO2/Si using 8 mg mL�1 polymer concentration(chloroform as a solvent) at 1500 rpm for 40 seconds. The PDPPT-alt-FN thin films were annealed for 30 min on a hot plate under anitrogen atmosphere at 140 1C, 200 1C and 300 1C respectively.The schematic of the OFET device structure, output/transfercharacteristics and dependence of field-effect mobility on gatevoltage are shown in Fig. 4. The VDS vs. IDS characteristics of thedevice in which a PFBT self-assembled monolayer (SAM) annealedat 300 1C exhibits p-channel conductance are shown in Fig. 4b.The transfer characteristics for OFET devices annealed at varioustemperatures clearly show enhancement in source–drain currentwith respect to gate voltages. The effect of annealing temperatureon the field effect mobility annealed at 140 1C, 200 1C and 300 1Cand device parameters are shown in Table 1.
The mobility calculated from the characteristics of OFETdevices annealed at 200 1C and 300 1C exhibits hole mobility of0.075 cm2 V�1 s�1 and 0.083 cm2 V�1 s�1, respectively, whichare almost identical. The on/off ratio (Ion/Ioff) determinedfor these devices is in the range of 105. The mobility calculatedfor 140 1C annealed devices showed a lower mobility of
0.075 cm2 V�1 s�1. The mobility was measured in air for allof the devices which demonstrates that these OFETs are stablein air. The good air stability of PDPPT-alt-FN is due to its lowerHOMO value with respect to vacuum and the rigid buildingblocks incorporated in the backbone. In order to correlate theeffect of annealing on the charge carrier mobility, atomic forcemicroscopy (AFM) was employed.
AFM height images of the PDPPT-alt-FN polymer thin filmsannealed at 140 1C, 200 1C and 300 1C are shown in Fig. 5. At alower annealing temperature (140 1C), nodular morphology isobserved. Such morphology is a typical feature of high molecularweight semicrystalline conjugated polymers. After annealing thinfilms at higher temperatures of 200 1C and 300 1C, the morphologylooks more organized and the grain size gets bigger which isfavorable for effective charge transport. At high annealingtemperatures, crystallization of polymer chains begins anddensely packed crystalline grains are formed. The higher mobilityof the samples annealed at 200 1C and 300 1C is due to theenhancement in crystallinity which is in good agreement with theAFM study.
Conclusions
A low band gap solution processable thiophene substituteddiketopyrrolopyrrole–fluorenone based alternating polymersemiconductor, PDPPT-alt-FN, was designed, synthesized andcharacterized. Two condensed aromatic moieties such as thiophenesubstituted DPP and fluorenone aromatic building blocks tune theenergy levels and give a lower HOMO value which is useful formaking stable electronic devices. PDPPT-alt-FN has been employedas the active semiconductor layer in OFET devices and the highesthole mobility of 0.083 cm2 V�1 s�1 is measured in air for 300 1Cannealed devices. So far, there have been very few reports of usingfluorenone based organic semiconducting materials for OFETapplications. Fluorenone is one of the highly promising conjugatedbuilding blocks for making stable semiconducting materials anddevices.
Notes and references
1 (a) C. Guo, W. Hong, H. Aziz and Y. Li, Rev. Adv. Sci. Eng.,2012, 1, 200; (b) Y.-J. Cheng, S.-H. Yang and C.-S. Hsu, Chem.Rev., 2009, 109, 5868; (c) A. C. Grimsdale, K. L. Chan,
Fig. 4 (a) Schematic of bottom gate/bottom contact OFET device geometry;(b) output (VDS vs. IDS) characteristics of PDPPT-alt-FN OFET devices annealed at300 1C; (c) transfer characteristics (VGS vs. IDS) and (d) field effect mobility vs. gatevoltages of PDPPT-alt-FN OFET devices annealed at various temperatures. Devicedimensions: channel length = 4 mm; channel width = 80 mm. All devices aremeasured in air.
Table 1 PDPPT-alt-FN active channel semiconductor based OFETdevice parameters
Annealingtemp. (1C)
Mobility(cm2 V�1 s�1)
Thresholdvoltage (V)
On/offratio
140 0.038 �18.90 1.9 � 104
200 0.075 �16.80 9.0 � 105
300 0.083 �12.00 5.3 � 105
Fig. 5 PDPPT-alt-FN polymeric thin film AFM phase images annealed at 140 1C,200 1C and 300 1C on OTMS treated Si/SiO2 substrates.
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R. E. Martin, P. G. Jokisz and A. B. Holmes, Chem. Rev., 2009,109, 897.
2 L. Biniek, B. C. Schroeder, C. B. Nielsen and I. McCulloch,J. Mater. Chem., 2012, 22, 14803.
3 (a) B. Tieke, A. R. Rabindranath, K. Zhang and Y. Zhu,Beilstein J. Org. Chem., 2010, 6, 830; (b) S. Qu and H. Tian,Chem. Commun., 2012, 48, 3039; (c) Y. Zu, PhD dissertation,Koln University, Koln, 2006.
4 (a) C. B. Nielsen, M. Turbiez and I. McCulloch, Adv. Mater., 2013,25, 1859; (b) P. Sonar, S. P. Singh, Y. Li, M. S. Soh andA. Dodabalapur, Adv. Mater., 2010, 22, 5409; (c) P. Sonar,S. P. Singh, E. L. Williams, Y. Li, M. S. Soh and A. Dodabalapur,J. Mater. Chem., 2012, 22, 4425; (d) P. Sonar, J. M. Zhuo,L. H. Zhao, K. M. Lim, J. Chen, A. J. Rondinone, S. P. Singh,L. L. Chua and A. Dodabalapur, J. Mater. Chem., 2012, 22, 17284;(e) P. Sonar, T. R. B. Foong, S. P. Singh, Y. Li and A. Dodabalapur,Chem. Commun., 2012, 48, 8383; ( f ) Y. Li, P. Sonar, S. P. Singh,W. Zeng and M. S. Soh, J. Mater. Chem., 2011, 21, 10829.
5 (a) Y. Li, S. P. Singh and P. Sonar, Adv. Mater., 2010, 22, 4862;(b) H. J. Chen, Y. L. Guo, G. Yu, Y. Zhao, J. Zhang, D. Gao,
H. T. Liu and Y. Q. Liu, Adv. Mater., 2012, 24, 4618;(c) J. W. Jung, F. Liu, T. P. Russell and W. H. Jo, EnergyEnviron. Sci., 2012, 5, 6857.
6 (a) P. Sonar, S. P. Singh, Y. Li, Z.-E. Ooi, T.-J. Ha,I. Wong, M. S. Soh and A. Dodabalapur, Energy Environ.Sci., 2011, 4, 2288; (b) E. J. Zhou, S. P. Yamakawa,K. Tajima, C. H. Yang and K. Hashimoto, Chem. Mater.,2009, 21, 4055.
7 P. Sonar, T. J. Ha and A. Dodabalapur, Chem. Commun., 2013,49, 1588.
8 (a) W. Porzio, S. Destri, U. Giovanella, M. Pasini, T. Motta,D. Natali, M. Sampietro and M. Campione, Thin Solid Films,2005, 492, 212; (b) M. Linares, L. Scifo, R. Demadrille,P. Brocornes, D. Beljonne, R. Lazzaroni and B. Grevin,J. Phys. Chem. C, 2008, 112, 6850.
9 (a) F. Lincker, A. J. Attias, F. Mathevet, B. Heinrich,B. Donnio, J. L. Fave, P. Rannoua and R. Demadrille, Chem.Commun., 2012, 48, 3209; (b) T. Lee, C. A. Landis, B. M. Dhar,B. J. Jung, J. Sun, A. Sarjeant, H. J. Lee and H. E. Katz, J. Am.Chem. Soc., 2009, 131, 1692.
