Isoindigo dye incorporated copolymers with naphthalene and anthracene: promising materials for stable organic field effect transistors† Prashant Sonar, * a Huei-Shuan Tan, a Shuangyong Sun, b Yeng Ming Lam * b and Ananth Dodabalapur * ac In this paper, we report the design and synthesis of isoindigo based low band gap polymer semiconductors, poly{N,N 0 -(2-octyldodecyl)-isoindigo-alt-naphthalene} (PISD-NAP) and poly{N,N 0 -(2-octyldodecyl)- isoindigo-alt-anthracene} (PISD-ANT). A series of donor–acceptor (D–A) copolymers can be prepared where donor and acceptor conjugated blocks can be attached alternately using organometallic coupling. In these polymers, an isoindigo dye acceptor moiety has been attached alternately with naphthalene and anthracene donor comonomer blocks by Suzuki coupling. PISD-NAP and PISD-ANT exhibit excellent solution processibility and good film-forming properties. Gel permeation chromatography exhibits a higher molecular mass with lower polydispersity. UV-vis-NIR absorption of these polymers exhibits a wide absorption band ranging from 300 nm to 800 nm, indicating the low band gap nature of the polymers. Optical band gaps calculated from the solid state absorption cutoff value for PISD-NAP and PISD-ANT are around 1.80 eV and 1.75 eV, respectively. Highest occupied molecular orbital (HOMO) values calculated respectively for PISD-NAP and PISD-ANT thin films on glass substrate by photoelectron spectroscopy in air (PESA) are 5.66 eV and 5.53 eV, indicative of the good stability of these materials in organic electronic device applications. These polymers exhibit p-channel charge transport characteristics when used as the active semiconductor in organic thin-film transistor (OTFT) devices in ambient conditions. The highest hole mobility of 0.013 cm 2 V 1 s 1 is achieved in top contact and bottom-gate OTFT devices for PISD-ANT , whereas polymer PISD-NAP exhibited a hole mobility of 0.004 cm 2 V 1 s 1 . When these polymer semiconductors were used as a donor and PC 71 BM as an acceptor in OPV devices, the highest power conversion efficiency (PCE) of 1.13% is obtained for the PISD-ANT polymer. 1 Introduction Diketopyrrolopyrrole (DPP) and isoindigo (ID) dyes possess a high photo- and mechanical/thermal stability, and were applied in the form of inks or paints for colouring bers, plastics and surface coatings earlier. 1–6 Recently, DPP 7–22 and ID 23–34 dyes have been proven to be very promising fused aromatic conju- gated blocks for making low band gap donor–acceptor (D–A) polymers. These moieties or dyes are gaining signicant attention in the organic electronics community due to their excellent performances both in organic thin lm transistors (OTFTs) and organic photovoltaic (OPV) devices. DPP and ID are both strong electron withdrawing moieties due to their two lactam rings and can be used as strong electron acceptors for making D–A based low band gap polymer semiconductors with deep HOMO levels. The insolubility of the DPP and ID core is due to the strong p–p stacking and the hydrogen atoms at the lactam nitrogens, resulting in strong intermolecular hydrogen bonding interactions. It has been shown that DPP and ID based small molecules and polymers can be solution processable by replacing the hydrogen atom on lactam with straight or branched alkyl side chains. Additionally, the optical and elec- tronic properties of these materials can be tuned by incorpo- rating suitable conjugated blocks (either electron donor or acceptor) adjacent to the DPP or the ID moiety. Recently, we have reported 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole- 1,4(2H,5H)-dione 14–17 and 3,6-di(furan-2-yl)pyrrolo[3,4-c]pyrrole- 1,4(2H,5H)-dione 18,19 (DPP derivatives) based D–A based low band gap organic semiconductors with hole mobilities of 1 to 1.54 cm 2 V 1 s 1 in OTFT devices and 5% power conversion efficiency (PCE) in OPV devices. Following recent reports on 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 School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798. E-mail: [email protected]c Microelectronics Research Centre, The University of Texas at Austin, Austin, TX, 78758, USA. E-mail: [email protected]† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2py20942j Cite this: Polym. Chem., 2013, 4, 1983 Received 7th November 2012 Accepted 3rd January 2013 DOI: 10.1039/c2py20942j www.rsc.org/polymers This journal is ª The Royal Society of Chemistry 2013 Polym. Chem., 2013, 4, 1983–1994 | 1983 Polymer Chemistry PAPER Downloaded by McMaster University on 05 March 2013 Published on 07 January 2013 on http://pubs.rsc.org | doi:10.1039/C2PY20942J View Article Online View Journal | View Issue
Received 7th November 2012Accepted 3rd January 2013
DOI: 10.1039/c2py20942j
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Isoindigo dye incorporated copolymers withnaphthalene and anthracene: promising materials forstable organic field effect transistors†
Prashant Sonar,*a Huei-Shuan Tan,a Shuangyong Sun,b Yeng Ming Lam*b
and Ananth Dodabalapur*ac
In this paper, we report the design and synthesis of isoindigo based low band gap polymer semiconductors,
poly{N,N0-(2-octyldodecyl)-isoindigo-alt-naphthalene} (PISD-NAP) and poly{N,N0-(2-octyldodecyl)-isoindigo-alt-anthracene} (PISD-ANT). A series of donor–acceptor (D–A) copolymers can be prepared
where donor and acceptor conjugated blocks can be attached alternately using organometallic
coupling. In these polymers, an isoindigo dye acceptor moiety has been attached alternately with
naphthalene and anthracene donor comonomer blocks by Suzuki coupling. PISD-NAP and PISD-ANT
exhibit excellent solution processibility and good film-forming properties. Gel permeation
chromatography exhibits a higher molecular mass with lower polydispersity. UV-vis-NIR absorption of
these polymers exhibits a wide absorption band ranging from 300 nm to 800 nm, indicating the low
band gap nature of the polymers. Optical band gaps calculated from the solid state absorption cutoff
value for PISD-NAP and PISD-ANT are around 1.80 eV and 1.75 eV, respectively. Highest occupied
molecular orbital (HOMO) values calculated respectively for PISD-NAP and PISD-ANT thin films on glass
substrate by photoelectron spectroscopy in air (PESA) are 5.66 eV and 5.53 eV, indicative of the good
stability of these materials in organic electronic device applications. These polymers exhibit p-channel
charge transport characteristics when used as the active semiconductor in organic thin-film transistor
(OTFT) devices in ambient conditions. The highest hole mobility of 0.013 cm2 V�1 s�1 is achieved in top
contact and bottom-gate OTFT devices for PISD-ANT, whereas polymer PISD-NAP exhibited a hole
mobility of 0.004 cm2 V�1 s�1. When these polymer semiconductors were used as a donor and PC71BM
as an acceptor in OPV devices, the highest power conversion efficiency (PCE) of 1.13% is obtained for
the PISD-ANT polymer.
1 Introduction
Diketopyrrolopyrrole (DPP) and isoindigo (ID) dyes possess ahigh photo- and mechanical/thermal stability, and were appliedin the form of inks or paints for colouring bers, plastics andsurface coatings earlier.1–6 Recently, DPP7–22 and ID23–34 dyeshave been proven to be very promising fused aromatic conju-gated blocks for making low band gap donor–acceptor (D–A)polymers. These moieties or dyes are gaining signicantattention in the organic electronics community due to theirexcellent performances both in organic thin lm transistors
gineering (IMRE), Agency for Science,
search Link, Singapore 117602. E-mail:
ring, Nanyang Technological University,
.sg
iversity of Texas at Austin, Austin, TX,
engr.utexas.edu
tion (ESI) available. See DOI:
Chemistry 2013
(OTFTs) and organic photovoltaic (OPV) devices. DPP and ID areboth strong electron withdrawing moieties due to their twolactam rings and can be used as strong electron acceptors formaking D–A based low band gap polymer semiconductors withdeep HOMO levels. The insolubility of the DPP and ID core isdue to the strong p–p stacking and the hydrogen atoms at thelactam nitrogens, resulting in strong intermolecular hydrogenbonding interactions. It has been shown that DPP and ID basedsmall molecules and polymers can be solution processable byreplacing the hydrogen atom on lactam with straight orbranched alkyl side chains. Additionally, the optical and elec-tronic properties of these materials can be tuned by incorpo-rating suitable conjugated blocks (either electron donor oracceptor) adjacent to the DPP or the ID moiety. Recently,we have reported 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione14–17 and 3,6-di(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione18,19(DPP derivatives) based D–A based lowband gap organic semiconductors with hole mobilities of 1 to1.54 cm2 V�1 s�1 in OTFT devices and 5% power conversionefficiency (PCE) in OPV devices. Following recent reports on
(E)-1H,10H-[3,30]bindolylidene-2,20-dione or ID based D–A lowband gap organic semiconductors for high performance organicelectronic devices by Reynolds and other groups,35 we have alsostarted exploring this potential moiety for designing novel D–Amaterials. ID is a fused planar aromatic conjugated moietywhere two indole heterocycles are combined with a centralcarbon–carbon double bond in E-conguration. The planar IDacceptor building block has been used comprehensively tocombine with other donor building blocks such as thiophene,uorene, carbazole, phenylene, bithiophene, propylenedioxy-thiophene, thienothiophene, terthiophene, benzodithiopheneand dithienosilole for synthesizing various D–A copolymers.23–34
These copolymers have been used in OTFT and OPV devices dueto their higher mobility and higher PCE, respectively. Incorpo-ration of such donor aromatic blocks with ID is expected toenhance intermolecular interactions through p–p stacking andmay enhance charge delocalization. Optical, electrochemical,electrical and morphological properties of such D–A copolymersemiconductors are strongly dependant on the nature of thedonor moiety inserted in the polymer backbone. In the abovecases, ID has been combined mostly with thiophene or thio-phene based derivatives. Naphthalene and anthracene are twoof the most common and easily available fused acene donoraromatic blocks, which have not been hitherto widely used ascomonomers. We recently developed polymeric semiconductorswith naphthalene and anthracene combined with DPP furanand these copolymers exhibited mobilities as high as 0.11 cm2
V�1 s�1 in OTFTs and a promising PCE around 2.6% in OPVdevices.19 Inspired by this performance, we decided to use theelectron rich ID conjugated block for making D–A based lowband gap copolymers with weak electron decient blocksnaphthalene and anthracene. 2,6-Attachment of naphthalene
Scheme 1 Synthesis of isoindigo based PISD-NAP and PISD-ANT copolymers.
1984 | Polym. Chem., 2013, 4, 1983–1994
and anthracene gives the most planar conformation compare toother conrmations (1,4- or 1,5-attachment) and it can achievethe highest extended p-conjugation. Combining the dibromoID compound with 2,6-bisboronic esters of naphthalene andanthracene via Suzuki coupling gave us poly{N,N0-(2-octyldo-decyl)-isoindigo-alt-naphthalene} (PISD-NAP) and poly{N,N0-(2-octyldodecyl)-isoindigo-alt-anthracene} (PISD-ANT) polymers,respectively. PISD-ANT on octyltrichlorosilane (OTS) treated Si/SiO2 substrate showed the highest hole mobility of 0.013 cm2
V�1 s�1 using gold for source and drain electrodes. Bulk het-erojunction OPV devices fabricated using PISD-ANT as a donorcopolymer with [6,6]phenyl-C71-butyric acid methyl ester(PC70BM) as the acceptor gave the highest power conversionefficiencies of 1.13% using a polymer to fullerene ratio of 1 : 2.This result clearly denotes how commonly available aceneblocks can be incorporated into a conjugated backbone usingpotential electron accepting dyes. The polymer synthesis,optical, electrochemical characterization, device performance(OTFT and OPV) and structure–property correlation areexplained in detail.
2 Results and discussion2.1 Syntheses and characterization of polymers
The starting monomer 6,60-dibromo-N,N0-(2-octyldodecyl)-iso-indigo (1) was synthesized according to the literaturemethod.34,35 The synthetic route for novel alternating copolymerspoly{N,N0-(2-octyldodecyl)-isoindigo-alt-naphthalene} (PISD-NAP) and poly{N,N0-(2-octyldodecyl)-isoindigo-alt-anthracene}(PISD-ANT) involving an isoindigo unit with naphthalene andanthracene is shown in Scheme 1. Copolymerization was con-ducted at 80 �C under argon atmosphere for 72 h using
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compound 6,60-dibromo-N,N0-(2-octyldodecanyl)-isoindigo (1)with 2,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxabrolan-2-yl)naphtha-lene (2) in the presence of Aliquat 336 as a phase transfer cata-lyst, Pd(PPh3)4 as an organometallic catalyst, and 2 M K2CO3 asbase in toluene via Suzuki coupling. The reaction was termi-nated by adding the bromobenzene and phenyl boronic acid endcapping reagents, respectively, and stirring the reaction mixturefor a further 3 h. The polymer reaction mixture was then pouredinto a mixture of methanol and 2 M HCl and stirred for a fewhours to get precipitation of the polymer PISD-NAP with a goodyield. The crude polymer PISD-NAP was then subjected to puri-cation in a cellulose thimble via Soxhlet extraction usingmethanol, acetone, and hexane, respectively, for 24 hours each.The Soxhlet extraction technique removes oligomers and cata-lytic impurities effectively from the crude polymer sample. Thenal residue was then extracted with chloroform and precipi-tated again from methanol, ltered, washed with methanol anddried. Another copolymer, PISD-ANT, was synthesized andpuried in exactly the same way as the above method, but using2,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxabrolan-2-yl)anthracene (3)and 6,60-dibromo-N,N0-(2-octyldodecanyl)-isoindigo (1) startingcomonomers. Both copolymers displayed good solutionprocessibility in common organic solvents such as dichloro-methane, THF, chloroform, chlorobenzene and dichloroben-zene. These polymers also displayed nice lm formingproperties and large akes of polymer lms can be formed aerprecipitating the polymers in methanol as a non-solvent. Puri-ed polymers PISD-NAP and PISD-ANT showed number averagemolecular weights (Mn) of 74 190 g mol�1 and 55 270 g mol�1
with polydispersity indices (PDI) of 2.91 and 2.40, respectively,measured by gel permeation chromatography (GPC) at acolumn temperature of 40 �C with THF as eluent and poly-methylmethacrylate (PMMA) as standard. GPC elution curves forboth polymers and their molecular weight details are given inFig. S5† (Table 1). Molecular weight disparities between PISD-NAP and PISD-ANT polymers are due to the solubility differencearising from shorter to longer conjugatedmoieties (naphthaleneand anthracene) incorporated in the backbone. The thermalstability of the polymers was analyzed by thermogravimetricanalysis (TGA) under nitrogen ow. PISD-NAP and PISD-ANTpolymers exhibited a 5% weight loss at 364 �C and 348 �C,respectively, which indicates the excellent thermal stability ofthe polymer semiconductors (see Fig. S6†). DSC characterizationwas carried out up to 300 �C and no thermal transitions wereobserved (see Fig. S7†).
Table 1 Polymerization results and thermal stability of the copolymers
a Number-average molecular weight. b Weight-average molecularweight equivalent to PS interacting with the column. c Decompositiontemperature (with 5% weight loss) determined by TGA under N2.
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2.2 Optical properties
The UV-vis absorption spectra of PISD-NAP and PISD-ANTpolymers were measured in DCB solutions and as spin coatedthin lms on glass. The spectra are shown in Fig. 1 (Table 2). Allcopolymers exhibit wide absorption bands extending from300 nm to 800 nm. Two absorption peaks in both regions areassociated with the p–p* transition band and intramolecularcharge transfer between naphthalene/anthracene and IDmoieties. Most donor–acceptor based materials exhibit suchfeatures in their absorption spectra. PISD-NAP shows twoabsorption maxima (lmax) at 468 nm and 582 nm in DCB solu-tion whereas PISD-ANT exhibits lmax at 418 nm and 600 nm.Another shoulder peak at 630 nm is observed for PISD-ANTpolymer. A wider absorption spectrum prole for PISD-ANT,together with a 18 nm red shi in lmax compared to PISD-NAPcopolymer, is indicative of the better light harvesting capabil-ities of this polymer due to the extended conjugation length ofthe backbone (anthracene vs. naphthalene). Solid stateabsorption measurements of these copolymers were measuredby spin coating their thin lms on glass substrates using DCBsolution and are quite similar with those of the solution spectra,as shown in Fig. 2 (Table 2). A slight difference compared tosolution data is that the solid-state absorption spectra of thepolymers have broader long wavelength edges, indicating weakintermolecular interactions related to p–p stacking. The iso-indigo conjugated block contains a weak donating phenylenering. The absorption maxima of PISD-NAP and PISD-ANT (bothin solution and solid state) are blue shied compared to otherreported isoindigo based copolymers in which the ID block iscombined with thiophene, bithiophene, propylenediox-ythiophene, thienothiophene, terthiophene, benzodithiopheneand dithienosilole.23–34 Thiophene and its derivatives are strongdonors by nature and once they are combined with strongelectron acceptors, such as ID, they result in strong donor–strong acceptor low band gap materials, which causes adecrease in the conjugation. However in the case of PISD-NAPand PISD-ANT, we have used weak electron donating
Fig. 1 Normalized UV-vis absorption spectrum of PISD-NAP and PISD-ANT inDCB and thin film layer deposited on glass from DCB solution.
a Diluted solution in DCB. b Optical band gap calculated from the absorption onset of the thin lms. c Measured from the oxidation onset of the CV.d Calculated from the reduction onset. e Obtained from photoelectron spectroscopy in air. f Determined from the HOMO–LUMO difference.
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conjugated moieties (weak donors – naphthalene and anthra-cene) combined with a strong ID acceptor, which produces weakdonor–strong acceptor low band gap semiconductors. Theabsorption maxima of PISD-NAP and PISD-ANT are wellmatched with other weak donor–ID based low band gap poly-mers where uorene and carbazole blocks have been used ascomonomers.24 Band gap engineering is strongly dependant onthe nature and strength of the donor/acceptor blocks involvedin the conjugated backbone according to their ionization
Fig. 2 Cyclic voltammograms showing the first scan of cathodic and anodiccycles of (a) PISD-NAP and (b) PISD-ANT spin coated polymers on workingelectrode at a scan rate 50 mV s�1 in acetonitrile. The electrolyte was 0.1 MTBAPF6.
1986 | Polym. Chem., 2013, 4, 1983–1994
potential and electron affinity values, respectively. The opticalband gaps, calculated from solid state absorption cut offmeasurements are found to be 1.80 eV and 1.75 eV for PISD-NAPand PISD-ANT, respectively.
2.3 Redox properties and photoelectron spectroscopy in air(PESA) measurements
An investigation of the electrochemical properties of PISD-NAPand PISD-ANT is important in order to determine the highestoccupied molecular orbital (HOMO) and lowest unoccupiedmolecular orbital (LUMO) energy levels. These energy levels arecritical to the functioning of OTFT and OPV devices. The elec-trochemical properties were studied by cyclic voltammetry (CV),as shown in Fig. 2, by using 0.1 M tetrabutylammonium tetra-uoroborate (Bu4NBF4) solution of the supporting electrolyte inacetonitrile at room temperature under nitrogen with a scanrate of 50 mV s�1. Ag/AgCl and polymer coated Pt electrodeswere used as reference and working electrodes, respectively. TheHOMO and LUMO of the PISD-NAP and PISD-ANT were calcu-lated from the oxidation and reduction onset potentials recor-ded by voltammogram. Oxidation and reduction onsetpotentials were determined from the cathodic and anodic peaksvia intersecting two tangents drawn with respect to the risingcurrent and baseline charging current. The HOMO and LUMOvalues were calculated with reference to the ferrocene oxidationonset potential using HOMO ¼ 4.4 eV + Eoxd.onset and LUMO ¼4.4 eV + Ered.onset equation for the calculations. The HOMOvalues for PISD-NAP and PISD-ANT are 5.70 eV and 5.62 eV,respectively, indicating the deep nature of the energy levels withrespect to the vacuum level, which is benecial for oxidativestable devices. A slightly lower HOMO value of PISD-NAPcompared to PISD-ANT is due to the electron rich nature ofanthracene compared to naphthalene. The LUMO values forPISD-NAP and PISD-ANT from reduction onsets are 3.61 eV and3.70 eV, respectively. These energy values are quite comparableto the other ID based copolymers and the lower HOMO energyvalues could be attributed to the electron decient lactam ringpresent in the ID unit.5,6 The band gap calculated from theHOMO–LUMO difference of CV data for PISD-NAP and PISD-ANT are 2.09 eV and 1.92 eV, respectively, which are higher thanthe optical band gaps (1.80 eV and 1.75 eV) obtained from thethin lm UV-vis-NIR spectrum. This difference in band gap isattributable to the exciton binding energy of the polymer.36,37 Tocorroborate the HOMO values calculated from the CV, we alsoconducted photoelectron spectroscopy in air (PESA) measure-ments using a spin coated thin lm of polymers on glasssubstrates, as shown in Fig. 3. The calculated HOMO values for
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Fig. 3 Photoelectron spectroscopy in air (PESA) measurements of (a) PISD-NAPand (b) PISD-ANT spin copolymer thin films spin coated on glass.
Fig. 4 2D-XRD pattern intensity graphs and 2D-XRD images obtained with theincident X-ray perpendicular to the thin film stack of (a) PISD-NAP and (b) PISD-ANT copolymers.
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PISD-NAP and PISD-ANT by PESA are 5.66 eV and 5.53 eV,respectively, which are in agreement with the CV data. LUMOvalues calculated using the optical band gap and HOMO values(calculated by PESA) difference for PISD-NAP and PISD-ANT arearound 3.86 eV and 3.79 eV, respectively, and these HOMOvalues calculated by PESA are more practical than CV data dueto the use of solid state measurements. We decided to use thesematerials as donor polymers for OPV application because oftheir low band gap and lower HOMO energies.
2.4 X-Ray diffraction (XRD) analysis
X-ray diffraction (XRD) is an important tool to investigate themolecular packing and ordering of polymer chains in the solidstate. We conducted 2D-XRD experiments on polymer akes inorder to get clear information about p–p stacking and theinterlayer spacing distances. Polymer akes were prepared bydissolving the polymer in chloroform and making its dilutesolution in a round bottom ask; later on, the solvent wasremoved on a rotary evaporator and then a thin layer of polymerdeposited on the ask wall was rinsed off with a non-solventsuch as methanol. The 2D-XRD images and 2D-XRD diffractionpatterns of the PISD-NAP and PISD-ANT copolymers weremeasured when the incident X-ray was normal to the polymer
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akes and are shown in Fig. 4. Both diffractograms exhibitprimary (100) and secondary (010) peaks which correspond tothe interlayer d-spacing and p–p stacking. The primary peakslocated at 2q¼ 4.08� and 2q¼ 4.22� for PISD-NAP and PISD-ANTcorrespond to a d-spacing of 21.63 A and 20.91 A, respectively.The secondary diffraction peaks for both polymers are ratherbroad and range from 15 to 28 A, which may be attributed to theshort range order arising from the amorphous regions andsome randomly ordered crystalline domains in the polymer.Secondary diffraction maxima peaks for PISD-NAP and PISD-ANT were measured at 2q ¼ 23.70� and 2q ¼ 24.42� which areassigned to p–p stacking distances of 3.8 A and 3.7 A, respec-tively. Lower d-spacing and p–p stacking distances for PISD-ANT compared to PISD-NAP are due to the higher conjugationlength of the isoindigo–anthracene repeating unit compared tothe isoindigo–naphthalene block, which induces better p–p
stacking. The lower p–p stacking distance of PISD-ANT isexpected to result in a higher performance in OTFT devices. Thep–p stacking distances for PISD-NAP and PISD-ANT are slightlyhigher than earlier reported isoindigo–bithiophene basedcopolymer PII-2T (3.4 A to 3.5 A),26 which is related to the weak
electron donating nature of naphthalene and anthracenecompared to the strong electron donating bithiophene blockused in the conjugated backbone.
Table 3 Summary of field effect mobilities (m), on/off ratios (Ion/Ioff) andthreshold voltages (Vt) for the copolymers (top contact bottom gate devices)prepared on OTS-treated substrates
Polymersmsat(cm2 V�1 s�1) Vt (V) Ion/Ioff ratio
PISD-NAP 0.004 �25.00 5.00 � 104
PISD-ANT 0.013 �17.30 9.80 � 106
2.5 Organic thin lm transistors (OTFTs) performance andmorphology study
PISD-NAP and PISD-ANT based OTFT devices were fabricatedusing top-contact, bottom-gate device geometry and the transferand output characteristics for PISD-NAP and PISD-ANT OTFTsmeasured in air are shown in Fig. 5. The PISD-NAP and PISD-ANT based thin lms spin coated from chloroform solutionOTFT devices exhibit hole mobilities of 0.003 cm2 V�1 s�1 and0.013 cm2 V�1 s�1 on OTS treated Si/SiO2 substrate in air,respectively. When dichlorobenzene solvent was used for thespin coating of PISD-NAP and PISD-ANT on OTS treated Si/SiO2
substrates, 0.0038 cm2 V�1 s�1 and 0.0032 cm2 V�1 s�1 mobilitywas recorded, respectively. The higher mobility of PISD-ANT
Fig. 5 OTFT device schematics with field-effect transistor behavior of (a) PISD-NAteristics (left) for 60 nm thick film spin-coated from chloroform at room temperatu
1988 | Polym. Chem., 2013, 4, 1983–1994
compared to PISD-NAP is due to the smaller p–p stackingdistance of PISD-ANT calculated from the XRD measurements(see the XRD analysis section). The on/off ratio, thresholdvoltage (Vth) andmobility values at various conditions have beenlisted in Table 3. It is vital to note that the mobility valuesmeasured in air indicate the high stability of these materials,whichmight be due to the lower HOMO values of both polymers
P and (b) PISD-ANT polymers; output characteristics (right) and transfer charac-re and ambient conditions (channel length 15 mm, channel width ¼ 4 mm).
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Fig. 6 AFM (height and phase) images of 60 nm thick film spin-coated of (a)PISD-NAP and (b) PISD-ANT on OTS–HMDS-treated native Si substrate.
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(PISD-NAP �5.66, PISD-ANT �5.53). In most of the earlierreported papers, the isoindigo strong acceptor block iscombined with strong donors such as thiophene and bithio-phene in their conjugated backbone, which exhibits highermobility (varying from 0.019 to 0.2 cm2 V�1 s�1), whereas inthese materials we used weak donors such as naphthalene andanthracene, respectively. Compared to the earlier report on DPPbased polymers with naphthalene and anthracene, this perfor-mance is lower, which may be due to the large dihedral anglebetween the isoindigo phenylene unit with naphthalene andanthracene (phenylene–phenylene dihedral angle is larger thanthiophene–phenylene). The weak intramolecular interactionsbetween the strong acceptor and weak donor makes thesematerials more amorphous than strong acceptor and strongdonor based conjugated materials. Additionally, distortion inplanarity is also caused due to a high dihedral angle betweenthe isoindigo unit and the six member naphthalene/anthracenering (less orbital overlapping) compared to a ve memberthiophene or bithiophene heterocyclic ring (more orbital over-lapping). A small red shi from solution to solid state absorp-tion of PISD-NAP and PISD-ANT is also the signature of weakintramolecular interactions. Strong D–A interaction and back-bone planarity is expected to show better p–p stacking andfavors charge carrier transportation. Our observations are inclose agreement with the effect of polymer symmetry andbackbone curvature on interchain “molecular docking” of iso-indigo-based polymers and their device performance.38 Themorphological behavior of PISD-NAP and PISD-ANT wereinvestigated by atomic force microscopy (AFM) using spincoated thin lms (similar to the device fabrication conditions)on Si/SiO2 substrates. The AFM height and phase images of thethin lms for PISD-NAP and PISD-ANT are shown in Fig. 6a andb, respectively. AFM images of both polymers exhibit a moreamorphous nature with small crystalline domains, whichresemble a nodular morphology. PISD-ANT shows more biggrains with fewer voids, which make the interconnectingnetwork of nodular morphology more pronounced compared tothe PISD-NAP polymer. Such morphological observations aremore similar to an earlier reported isoindigo based IIDT poly-mer where the isoindigo unit was combined with thiopheneblocks.26 The conjugated backbones of PISD-NAP and PISD-ANTare constructed with strong acceptors and weak donors, whichmight be responsible for the amorphous nature. Additionally,distortion in the conjugated backbone plane causes slightdeviation in molecular orbital overlapping. Strong intermolec-ular p–p interactions and donor–acceptor intramolecularinteractions are usually responsible for getting very highmobility in OTFT devices.
2.6 Organic photovoltaic (OPV) device study and blendmorphology
Aer evaluating PISD-NAP and PISD-ANT polymers for stableOTFT applications, we decided to use these materials as activesemiconductors for OPV devices due to their wide absorptionrange and promising mobility values. The photovoltaic deviceswere fabricated and investigated by using a blend of PISD-NAP
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and PISD-ANT with PC71BM (1 : 2) using various conditions,such as thermal annealing and the usage of additives. The UV-vis absorption spectra of PISD-NAP and PISD-ANT copolymersblended with PCBM as a pristine, chloronaphthalene (CN), anddiiodooctane (DIO) additive added spin coated thin lm areshown in Fig. 7. From this optical data, it is clear that theabsorption prole of the polymer:PCBM blend is quite differentfrom that of the polymer. Aer addition of DIO as a processingadditive, the relative intensity ratio of the two peaks in theabsorption spectrum is higher than the neat lm and the lmwith CN additive. Such kind of change in the absorption proleis related to the external quantum efficiency (EQE) enhance-ment. The IPCE spectra of the devices are depicted in Fig. 8 andwe have clearly noticed the effect of processing additives forenhancing EQE response. PISD-ANT based OPV devicesexhibited three times higher EQE response than PISD-NAPbased devices due to the better light harvesting capability andhigher charge carrier mobility. This has been also proved byshowing the differences in hole mobility values of both poly-mers when they are used in OTFT devices. Photovoltaic resultsof the respective devices are summarized in Table 4 and theircurrent density–voltage (J–V) characteristics are shown in Fig. 9.In general, PISD-ANT based devices showed much higher powerconversion efficiency (PCE) than those devices based on thePISD-NAP polymer. The higher performance was mainlyattributed to the larger short circuit current (Jsc) generated,which was related to the better absorption and higher mobilityof the PISD-ANT polymer.
As photovoltaic performance is known to be affected by lmmorphologies, next, the morphology of the various lms were
Fig. 7 UV-vis absorption spectrum of (a) PISD-NAP and (b) PISD-ANT copoly-mers blended with PCBM as a pristine, chloronaphthalene, and diiodooctaneadditive added spin coated thin film.
Fig. 8 IPCE spectra of PISD-NAP and PISD-ANT devices, as blended with [70]PCBM in a 1 : 2 ratio using various conditions such as pristine, chloronaphthalene,and diiodooctane additives.
Table 4 Solar cell performance of PISD-NAP/PCBM and PISD-ANT/PCBMdevices
Fig. 9 Current density–voltage (J–V) characteristics of PISD-NAP and PISD-ANTdevices, as blended with [70]PCBM in a 1 : 2 ratio using various conditions suchas pristine, chloronaphthalene, and diiodooctane additives.
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studied using AFM operating in the tapping mode. The topo-logical and phase images were shown in Fig. 10. It was foundthat devices based on PISD-ANT/PCBM showed smaller domainsize than that of PISD-NAP/PCBM. Smaller domain sizes implythat there is more interface available in the PISD-ANT/PCBMlms for exciton dissociation. This can be translated to higherJsc compared to PISD-NAP devices. Hence, the better deviceperformance given by the PISD-ANT/PCBM lms can be corre-lated with this improvedmorphology. Processing additives suchas CN and DIO were also added to ne tune the nanostructuresof the active layers. The best performance device was based onPISD-ANT/PCBM in DCB (with 2.5% v/v CN), which had a PCE of1.13%, with a Jsc of 4.51 mA cm�2, Voc of 0.70, and FF of 0.36.
3 Conclusion
A series of copolymers containing isoindigo strong acceptorcombined with naphthalene and anthracene weak donors havebeen designed and synthesized by palladium-catalyzed classicalSuzuki polycondensation. Poly{N,N0-(2-octyldodecyl)-isoindigo-alt-naphthalene} (PISD-NAP) and poly{N,N0-(2-octyldodecyl)-isoindigo-alt-anthracene} (PISD-ANT) were obtained in a goodyield with good lm forming properties. The optical and elec-trochemical band gaps calculated for PISD-NAP and PISD-ANT
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Fig. 10 Height and phase AFM images of PISD-NAP and PISD-ANT blended with PCBM thin films spin coated on ITO/PEDOT coated glass using pristine anddiiodooctane additives.
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are 1.80 eV and 1.75 eV respectively. The HOMO values for PISD-NAP and PISD-ANT were determined using cyclic voltammetryand photoelectron spectroscopy in air (PESA) and found to be in
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good agreement. The HOMO values are in the range of 5.53 eVto 5.66 eV for both polymers, indicating the oxidative stability ofthese materials. Upon testing these polymers in OTFT devices at
ambient conditions (in air), the polymer exhibits excellentp-channel charge transport characteristics. The highest holemobility of 0.013 cm2 V�1 s�1 is achieved in top contact andbottom-gate OTFT devices for PISD-ANT, whereas the polymerPISD-NAP showed a hole mobility of 0.004 cm2 V�1 s�1. Addi-tionally, these polymers also demonstrated promising perfor-mance in organic photovoltaic devices when combined withPC71BM as an acceptor. The highest power conversion efficiency(PCE) of 1.13% is obtained for the PISD-ANT polymer. Thepromising performances of these materials in both OTFTs andOPV devices at ambient conditions makes isoindigo a verypromising conjugated moiety for constructing a variety of newstable polymer semiconductors.
4 Experimental4.1 General
All the reagents and solvents were purchased from Strem, Acros,and Sigma-Aldrich and used without further purication. Allreactions were carried out using the Schlenk technique in anargon or nitrogen atmosphere with anhydrous solvents.
4.2 Characterization1H and 13C NMR data were performed on a Bruker DPX 400MHzspectrometer with chemical shis referenced to residual CHCl3and H2O in CDCl3. Gel permeation chromatography (GPC)analysis against polymethylmethacrylate (PMMA) standardswas performed in THF at 40 �C on a Waters high pressure GPCassembly with an M590 pump (model 2690), m-Styragel columnsof 105, 104, 103, 500 and 100 A and a refractive index (RI)detector. A typical concentration of 1.5 mg weight of polymerdissolved in 1 mL of THF was used for running GPC samples.UV-vis spectra were recorded on a Shimadzu model 2501-PC.Cyclic voltammetry experiments were performed using anAutolab potentiostat (model PGSTAT30) by Echochimie. All CVmeasurements were recorded in solid state with 0.1 M tetra-butylammonium hexauorophosphate as the supportingelectrolyte (scan rate of 100 mV s�1). The experimentswere performed at room temperature with a conventionalthree-electrode conguration consisting of a platinum discworking electrode, a gold counter electrode, and an Ag/AgClreference electrode. Differential scanning calorimetry (DSC) wascarried out under nitrogen on a TA Instrument DSC Q100instrument (scanning rate of 10 �Cmin�1). Thermal gravimetricanalysis (TGA) was carried out using a TA Instrument TGA Q500instrument (heating rate of 10 �C min�1).
Core compound 6,60-dibromoisoindigo is the fully conju-gated andmost important functional derivative for synthesizingvarious conjugated semiconductors. This compound can beobtained easily in quantitative yield from the acid catalyzedaldol condensation of 6-bromoisatin and 6-bromooxindolereuxed in acetic acid under argon, reported by Reynolds et al.35
In order to induce good solubility, an octyldodecyl group wasattached to the ID core. The nitrogen atom of the compoundwas alkylated by using branched 2-octyldodecyl bromide inanhydrous dimethylformamide (DMF) solvent in the presence
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of potassium carbonate (K2CO3), producing compound 6,60-dibromo-N,N0-(2-octyldodecanyl)-isoindigo in 74% yield aercolumn chromatography (see 1H and 13C NMR Fig. S1 and S2spectra†). This dibromo derivative (compound 1) was used as acommon block for polymerization with other bisboronic estersof naphthalene and anthracene, respectively. Naphthalene andanthracene bisboronic esters were prepared from their dibromoanalogues using bis(pinacolato)diboron and potassium acetatein the presence of Pd(II), according to the literature method(see 1H and 13C NMR Fig. S3 and S4 spectra†).19
4.3 Synthesis of poly{N,N0-(2-octyldodecyl)-isoindigo-alt-naphthalene} (PISD-NAP)
To a 50 mL Schlenk ask, 6,60-dibromo-N,N0-(2-octyldodecanyl)-isoindigo (4) (0.300 g, 0.30 mmol) and 2,6-bis(4,4,5,5-tetra-methyl-1,3,2-dioxabrolan-2-yl)naphthalene (5) (0.114 g 0.30mmol), 2 M aqueous K2CO3 solution (5 mL) and 2 drops ofAliquat 336 were dissolved in toluene (15 mL). The solutionwas purged with argon for 30 min, and then tetrakis-(triphenylphosphine)palladium (20 mg, 0.012 mmol) was addedin the above solution quickly. The reaction mixture was stirredat 80 �C for 3 days. Then, a toluene solution of phenyl boronicacid was added and the mixture was stirred for an additional4 h, followed by addition of a few drops of bromobenzene andstirring overnight. The resulting mixture was poured into amixture of methanol and water and stirred for a few hours. Theresulting solid was ltered off and subjected to Soxhlet extrac-tion for 2 days in methanol, acetone, and hexane for theremoval of oligomers and catalytic impurities. The remainingpolymer was extracted with chloroform and precipitated againfrom methanol, ltered, washed with methanol, and driedunder vacuum at room temperature (78% yield).Mw/Mn (GPC)¼74 190/21 5969. UV-vis: 468, 582 nm (in chloroform).
4.4 Synthesis of poly{N,N0-(2-octyldodecyl)-isoindigo-alt-anthracene} (PISD-ANT)
To a 50 mL Schlenk ask, 6,60-dibromo-N,N0-(2-octyldodecanyl)-isoindigo (4) (0.300 g, 0.30 mmol) and 2,8-bis(4,4,5,5-tetra-methyl-1,3,2-dioxabrolan-2-yl)anthracene (6) (0.124 g, 0.30mmol), 2 M aqueous K2CO3 solution (6 mL) and 2 drops ofAliquat 336 were dissolved in toluene (12 mL). The solutionwas purged with argon for 30 min, and then tetrakis-(triphenylphosphine)palladium (20 mg, 0.017 mmol) wasadded. The reaction was stirred at 80 �C for 3 days. Then, atoluene solution of phenyl boronic acid was added and themixture was stirred for an additional 4 h, followed by additionof a few drops of bromobenzene and stirring overnight. Theresulting mixture was poured into a mixture of methanol andwater and stirred for a few hours. The resulting solid wasltered off and subjected to Soxhlet extraction for 2 days inmethanol, acetone, and hexane for the removal of oligomersand catalytic impurities. The remaining polymer was extractedwith chloroform and precipitated again from methanol,ltered, washed with methanol, and dried under vacuum atroom temperature (62% yield). Mw/Mn (GPC) ¼ 55 276/132 786.UV-vis: 418, 600 nm (in chloroform).
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Top-contact, bottom-gate OTFT test devices were prepared on asilicon wafer in ambient conditions without taking any specialprecautions to exclude air, moisture and ambient light. Aheavily n-doped silicon wafer with a 220 nm surface layer ofsilicon dioxide (SiO2) was used as the substrate/gate electrode,with the top SiO2 layer serving as the gate dielectric (capacitance15.6 nF cm�2). The SiO2 surface of the Si wafer substrate wascleaned by sonication in acetone, and then immersed inpiranha solution (volume ratio of H2SO4 to H2O2 was 2 : 1) for 8minutes at room temperature. The cleaned wafer was thenimmersed in a 0.1 M solution of octyltrichlorosilane (OTS-8) intoluene at 60 �C for 30 min, followed by rinsing with tolueneand then drying with N2. It has been shown bymany groups thatthe self assembled monolayer (SAM) of OTS on Si/SiO2 substrateregulates the surface energy and is oen responsible for bettermolecular organization and ordering of appropriate organicand polymeric semiconductors. Additionally, such surfacetreatment has been shown to reduce the trap density of stateswhich controls charge transport. The semiconductor layer(PISD-NAP and PISD-ANT polymeric thin lms of 8 mg mL�1 inchloroform) was deposited on top of the OTS-8-modied SiO2
surface by spin coating. PISD-NAP and PISD-ANT thin lmsdeposited on Si/SiO2 were optionally annealed on a hotplate at100 and 150 �C, respectively at ambient conditions. Subse-quently, on top of the polymer active layer, 50 nm thick gold(Au) was deposited for source (S) and drain (D) electrodesthrough a shadow mask. Silicon oxide at the backside of thesilicon wafer of the TFT device was removed with sandpaper toprovide a conductive gate contact. The TFT devices were thencharacterized using a Keithley SCS-4200 probe station under anambient environment with a relative humidity level of 65%. Theeld effect mobility (m) was calculated from the saturationregime of the transfer characteristics. The charge carriermobility values were calculated from the saturation regime ofthe OTFT transfer characteristics from eqn (1).
msat ¼vId
vVg
����Vd¼const
L
WCins
�Vg � Vo
� (1)
whereW and L are the channel width and length, respectively, Ci
is the capacitance per unit area of the insulation layer. Channelwidth and channel length for these devices are 15 mm and4 mm, respectively.
4.6 OPV fabrication and its characterization
For solar cell device fabrication, indium tin oxide (ITO)-coatedglass substrates were purchased from Kintec. The glass/ITOsubstrates were cleaned by ultrasonication in subsequent bathsof detergent (15 min), de-ionized water (15 min), acetone (15min), and isopropanol (15 min). The substrates were thensubjected to air plasma cleaning for 2 minutes prior to the spincoating of a 30 nm thick PEDOT:PSS hole transporting layer(Clevios P VP AI 4083). The ITO/glass substrates withPEDOT:PSS were baked at 140 �C for 10 min in a glovebox. Theactive layers of, PISD-NAP:[70]PCBM and PISD-ANT:[70]PCBMusing 1 : 2 ratios were spin coated on PEDOT:PSS deposited ITO
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glass using dichlorobenzene. An aluminum cathode wasdeposited by thermal evaporation through a shadow maskunder a vacuum pressure of 10�6 Torr to complete the devices,giving a device area of 0.07 cm2. The IPCE (incident photon-to-current conversion efficiency) was measured using a Merlinradiometer (Newport) with a monochromator-calibrated wave-length control; the incident light intensity was determined by acalibrated Si photodiode (Hamamatsu). The current–voltagecharacteristics were measured using an Agilent 4155C Semi-conductor Parameter Analyzer under simulated AM 1.5G illu-mination, while irradiance was provided by a 96000 solarsimulator (Xenon lamp, Newport Oriel). The simulator lampintensity was set using a single crystalline silicon reference cell.
Acknowledgements
The authors acknowledge the Visiting Investigatorship Pro-gramme (VIP) of the Agency for Science, Technology andResearch (A*STAR), Republic of Singapore for nancial support.We are also thankful to Mr Lim Poh Chong for 2D-XRDmeasurement study.
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