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Vinylene-linked oligothiophene-difluorobenzothiadiazole copolymer for transistor applications Abby Casey, Yang Han, Eliot Gann, Joshua P. Green, Christopher R. McNeill, Thomas D. Anthopoulos, and Martin Heeney ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09628 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 26, 2016

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ACS Applied Materials & Interfaces

Vinylene-linked oligothiophenedifluorobenzothiadiazole copolymer for transistor applications Abby Casey,a Yang Han,a,c Eliot Gann,b Joshua P. Green,a Christopher R. McNeillb, Thomas D. Anthopoulosc and Martin Heeneya* a

Department of Chemistry and Centre for Plastic Electronics, Imperial College London, London

SW7 2AZ, U.K. bDepartment of Materials Science and Engineering, Monash University, Wellington Road, Clayton, VIC 3800, Australia. cDepartment of Physics and Centre for Plastic Electronics Imperial College London SW7 2AZ, U.K. KEYWORDS.

Conjugated

polymers,

organic

semiconductors,

field-effect

transistor,

fluorination. donor-acceptor copolymer.

ABSTRACT. The synthesis of the novel donor-acceptor monomer 4,7-bis[(E)-2-(5-bromo-3dodecylylthiophen-2-yl)ethenyl]-5,6-difluoro-2,1,3-benzothiadiazole (FBT-V2T2) is reported. Polymerisation with 4,4'-ditetradecyl-5,5'-bistrimethylstannyl-2,2'-bithiophene afforded a highly crystalline polymer that aggregated strongly in solution. Polymer films were well ordered resulting in high performance field-effect transistors with low onset voltages, negligible hysteresis, high channel current on/off ratios and peak hole mobilities of up to 0.5 cm2V-1s-1.

ACS Paragon Plus Environment

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Notably the transistors exhibited close to ideal behaviour with extracted mobilities almost independent of gate of voltage.

1. Introduction Soluble conjugated polymers can be used as organic semiconducting materials in electronic devices such as organic field-effect transistors (OFETs).1 The attractive properties of these semiconducting polymers, such as solubility and mechanical flexibility, could potentially allow the fabrication of low cost, large area, lightweight and flexible electronic devices. Intensive efforts in polymer design and optimization over the last decades have led to materials with promising charge transport properties in OFETs. The reported charge carrier mobility of semiconducting polymers now regularly exceeds that of amorphous silicon, with mobilities often reported over 1 cm2V-1s-1.2,3 In terms of polymer design, whilst some all-donor polymers have shown high FET mobility,4,5 most high performance polymers have a donor-acceptor (D-A) structure. Benzothiadiazole (BT) is one of the most commonly used acceptor units and has been incorporated into many high performance D-A polymers. However, the majority of these polymers contain BT directly linked to ever more complex fused-aromatic donor units such as indacenodithiophene (IDT),6,7 cyclopentadithiophene

(CDT),8,9

dithienosilole10,

silaindacenodithiophene11

and

indacenodithieno[3,2-b]thiophene (IDTT)12 that require laborious multi-step syntheses. Simpler thiophene based BT co-polymers have generally suffered from lower performance, with mobilities typically on the order of 10-4 to 10-2 cm2V-1s-1 in transistor devices.13–15 The positioning and type of solubilizing side chain present on the oligiothiophene segment has a strong influence on the solid state structure and therefore charge carrier mobility in these copolymers.15 For example, by tuning the regiochemistry of the dodecyl sidechains in a


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sexithiophene co-monomer, Reichmanis and co-workers16 were able to obtain charge carrier mobilities of ~0.2 cm2V-1s-1 when spin coating films of poly(benzothiadiazole-sexithiophene). Incorporating branched 2-octyldodecyl side chains has also led to hole mobilities up to 0.2 cm2V1 -1

s , although the synthetic complexity of such sidechains is higher in comparison to readily

available straight alkyl chains.17 Other modifications such as the replacement of BT with difluorobenzothiadiazole (FBT) in thiophene based co-polymers have also led to improved FET mobilities of 0.29 cm2V-1s-1.18 More recently, Cao and co-workers19 were able to achieve impressive mobilities for FBT based materials (up to 1.92 cm2V-1s-1) by using 2-decyltetradecyl side chains and optimizing the relative positions of these side chains in comparison to previous examples.18 Clearly then, there is a large scope for improving the mobility of these simple thiophene-BT based polymers without having to resort to complex fused donor units. To add to this promising family of thiophene-BT co-polymers we here report the synthesis of the novel vinyl-flanked FBT monomer, FBT-V2T2 (see Figure 1) and its copolymer with 4,4’-ditetradecyl-2,2’-bithiophene, where the straight chain alkyl groups are positioned tail-to-tail, to produce the novel polymer P(FBT-V2T4).

Whilst vinyl-flanked BT monomers (see Figure 1) have previously been

incorporated into small molecules and polymers,20–26 these materials have almost exclusively been used for organic photovoltaic applications and their performance in OFET devices has remained relatively unexplored. Vinyl-flanked BT (BT-V2) co-polymerised with 4,8-dialkoxybenzo[1,2-b:4,5-b’]dithiophene (BDT) has been tested in OFET devices, however it exhibited a low hole mobility on the order of 10-5 cm2V−1s−1.21 BT units linked by a vinyl group (VBT2 - see Figure 1) have also been co-polymerised with diketopyrrolopyrrole (DPP) and isoindigo (IID). Both polymers demonstrated ambipolar performance, with the DPP based polymer exhibiting


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hole mobilities up to 0.32 cm2V−1s−1 and electron mobilities up to 0.13 cm2V−1s−1 and the isoindigo based polymer giving hole and electron mobilities on the order of 10-2 cm2V−1s−1.27 These are relatively modest mobilities considering the usually high performance of DPP and IID co-polymers.28 To the best of our knowledge vinyl-flanked FBT has not been previously reported or incorporated into polymeric backbones.

BT-V2

BT-V2T2

VBT2

Previously synthesised vinyl-BT monomers

FBT-V2T2

Figure 1: Reported vinyl-BT containing monomers20,22,23,25–27 and the novel fluorinated monomer FBT-V2T2.

Previous reports have investigated the incorporation of vinyl linkers into the oligiothiophene segment of thiophene-BT co-polymers. However these materials were found to be amorphous with low hole mobilities of 10-4 or less.16 In contrast we find that incorporating the vinyl spacer groups adjacent to the FBT unit results in a semi-crystalline polymer, which we rationalize on the basis that the vinyl spacer alleviates torsional twisting between the FBT and the flanking 3-


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dodecylthiophene, facilitating backbone planarization. The resulting polymer exhibits hole mobilities of up to 0.5 cm2V-1s-1 in OFET devices. Unlike many recently reported high mobility polymers which show significant deviations from ideal transistor behaviour, with high apparent mobilities only observed at low gate voltages,2,29 this polymer displays a mobility almost independent of gate voltage. This robust value is one of the highest reported mobilities for simple thiophene-BT based co-polymers and compares well with BT containing donor-acceptor polymers of much higher complexity.

2. Synthesis of monomer and polymer The synthetic procedure for the preparation of FBT-V2T2 is shown in Scheme 1. Whilst there are a number of other options for the introduction of the vinyl spacer, we chose to focus on a Suzuki coupling route since this leads to a well-defined product with no regioisomer issues. Swager and co-workers have previously reported a three step procedure to vinyl boronic esters from aromatic dibromides, via Sonogashira coupling with protected alkynes, deprotection and hydroboroylation.30 Following this strategy, we initially coupled ethynyltrimethylsilane with 5,6-difluoro-4,7diiodobenzo[c][1,2,5]thiadiazole (6) in good yield. However, subsequent de-protection proved difficult, with fluoride based de-protecting agents such as TBAF and KF leading to degradation of the product, whilst treatment with methanolic potassium hydroxide or potassium carbonate led to mixtures resulting from nucleophilic aromatic substitution of the fluorine substituents with methoxide. We therefore decided to couple ethynyltrimethylsilane with 2-bromo-3dodecylthiophene (2). The resulting thiophene-alkyne (3) was readily deprotected by treatment with excess potassium carbonate in a mixed methanol/THF solvent. Hydroboration of the


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resulting alkyne (4) with 4,4,5,5-tetramethyl-1,3,2-dioxaborolane afforded the novel trans alkenyl boronic ester 5. The trans isomer was verified by the large coupling constant of 18 Hz between the alkene protons. A Suzuki coupling between 5 and 5,6-difluoro-4,7diiodobenzo[c][1,2,5]thiadiazole (6) afforded 7 in reasonable yield. Subsequent bromination gave the final monomer FBT-V2T2 (8). This was co-polymerised with 4,4'-ditetradecyl-5,5'bistrimethylstannyl-2,2'-bithiophene via Stille coupling under microwave irradiation to yield the polymer P(FBT-V2T4).31 The crude polymer was purified by solvent washing (methanol, acetone and hexane in that order) to remove catalyst impurities and low weight material. The remaining polymer had a number average molecular weight (Mn) of 19.3 kDa (Mw = 36.6 kDa, Ð = 1.90) as determined by high temperature size exclusion chromatography in 1,2,4-trichlorobenzene (TCB) at 140 °C. The molecular weight is likely to have been limited by the relatively low solubility of the polymer in the reaction solvent (chlorobenzene) causing it to precipitate out of solution during the polymerisation process.


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Scheme 1: Synthesis of novel FBT-V2T2 and P(FBT-V2T4).

2. Optical properties The polymer was not fully soluble in either chloroform or chlorobenzene at room temperature, due to strong aggregation between chains. P(FBT-V2T4) appeared to dissolve in 1,2,4trichlorobenzene (TCB) at room temperature, with an absorption maximum at 635 nm. However a pronounced red-shifted shoulder at 710 nm was suggestive of polymer aggregation (see Figure 2A). This was further probed with variable temperature solution measurements. Hence spectra were recorded in TCB solution (16.6 mg/dm3) every 10 °C upon heating from 25 °C to 85 °C and


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upon subsequent cooling from 85 °C to 25 °C (see Figure 2C-D). The solution was equilibrated at each temperature for 15 minutes. As the solution is heated the λmax blue-shifts as the aromatic units in the backbone experience increasing rotational freedom, disrupting backbone planarity (see Table S1). The low energy shoulder peak remains at 710 nm but decreases in intensity with increasing temperature as aggregation between polymer chains decreases. This peak disappears when the polymer is heated to 75°C and above in TCB (see Figure 2B). At 85°C the polymer is fully solvated and the λmax is blue-shifted by 50 nm in comparison to the solution at 25°C. The changes are reversible upon cooling, although the aggregate peak only appears at 55°C (see Figure 2B). The absorption maximum of the polymer in thin film (as-spun from 5 mg/mL solution of P(FBT-V2T4) in hot TCB) is red-shifted by 16 nm from the room temperature TCB solution spectrum and 66 nm from the TCB solution spectrum at 85°C, likely due to increased backbone planarization in the solid state. The shoulder peak remains at 710 nm but increases in intensity relative to the main peak, suggesting an increase in aggregation between polymer chains as the polymer moves into the solid state. The intensity of shoulder further increases upon annealing the film (figure 2B) for 30 minutes under Argon at either 100°C or 200°C, suggesting a further increase enhancement in solid state ordering. This highly aggregated behaviour is characteristic of fluorinated polymers, with the introduction of fluorine substituents tending to increase the planarity and crystallinity of the polymers.18,32–35 The introduction of vinylene spacer groups is expected to also help backbone planarization by reducing steric interactions between the BT unit and the adjacent 3-dodecylthiophene.36

The optical bandgap, measured from the onset of

absorption in thin film was 1.60 eV.


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Solution (RT) Solution (85°C) Thin film

1.0 0.8

B 1.0

0.6 0.4 0.2

As-spun Annealed at 100 °C Annealed at 200 °C

Normalized absorption

Normalized absorption

A

0.8 0.6 0.4 0.2

0.0 400

500

600

700

800

900

1000

0.0 300

400

500

Wavelength (nm)

0.5

600

700

800

900

1000

Wavelength (nm)

Heating

25°C 35°C 45°C 55°C 65°C 75°C 85°C

0.4 0.3 0.2

D 0.5 Absorption (a.u.)

C Absorption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1

Cooling

85°C 75°C 65°C 55°C 45°C 35°C 25°C

0.4 0.3 0.2 0.1

0.0

0.0 400

500

600

700

800

900

1000

400

500

Wavelength (nm)

600

700

800

900

1000

Wavelength (nm)

Figure 2: (A) UV-Vis absorption spectra of P(FBT-V2T4) in room temperature TCB solution, TCB solution heated to 85°C and in thin film (as-spun from hot TCB solution); (B) influence of annealing on thin film; UV-Vis absorption spectra of P(FBT-V2T4) in TCB solution (16.6 mg/dm-3) heating from 25°C to 85°C (C) and cooling from 85°C to 25°C (D).

3. Investigating the aggregation and conformation of FBT-V2T2 and P(FBT-V2T4) In order to probe the possible planarization of the polymer backbone, the minimum energy geometry of the FBT-V2T2 monomer was initially investigated. There are ten possible conformations of the vinyl-flanked molecule 7. The optimised geometry of each conformation


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was calculated using DFT with a B3LYP level of theory and a basis set of 6-31G(d). Within the model full alkyl chains were replaced with methyl groups in order to simplify the calculations. In all ten conformations the equivalent vinyl protons adjacent to the FBT unit are labelled ‘A’ whilst the equivalent vinyl protons adjacent to the thiophene unit are labelled ‘B’. All ten conformations were predicted to be almost completely planar. In the ten conformations of molecule 7 the thiophene rings can be positioned in three different ways, either both sulfur atoms on the thiophene rings are pointing towards the ‘A’ vinyl protons (thiophene orientation AA – see Figure 3), one is pointing towards an ‘A’ vinyl proton and one is pointing towards a ‘B’ vinyl proton (thiophene orientation AB – see Figure S3), or both are pointing to the ‘B’ vinyl protons (thiophene orientation BB – see Figure S3). The flanking vinyl groups of molecule 7 can also be positioned in three different ways (see Figure 3): one in which both ‘B’ vinyl protons are closest in space to the nitrogen atoms in the BT ring (vinyl orientation 1), one in which one ‘B’ vinyl proton is closest in space to a nitrogen atom and one is closest in space to a fluorine atom (vinyl orientation 2), and one in which both ‘B’ vinyl protons are closest in space to the fluorine substituents (vinyl orientation 3). The three lowest energy conformations are shown in Figure 3 whilst the remaining conformations are shown in Figure S3. In the three lowest energy conformations the thiophene rings are all in the AA orientation. Looking at the interatomic distances and also the Mülliken partial charges on the atoms within the model suggests that moderate hydrogen type bonding could help planarise all three vinyl orientations (1, 2 and 3). Mülliken partial charges on the vinyl protons ranged between +0.145 and +0.187, whilst charges ranged between -0.567 and -0.589 for the nitrogen atoms in the BT ring and -0.276 to -0.279 for the fluorine substituents. The average interatomic distances between positively charged ‘A’ vinyl protons and negatively charged


10

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nitrogen atoms (HA•••N: 2.47 Å) or negatively charged fluorine atoms (HA•••F: 2.34 Å) are longer than for ‘B’ vinyl protons (HB•••N: 2.29 Å, HB•••F: 2.21 Å). However, all of these distances are less than the sum of the Van der Waal radii of either hydrogen and nitrogen (2.75 Å) or hydrogen and fluorine (2.67 Å), suggesting there may be electrostatic interactions contributing to the planarisation of the monomer. It appears that due to the increased negativity of the nitrogen atoms in comparison to the fluorine atoms, orientations in which the positively charged vinyl protons are closer in space to the nitrogen atoms are further stabilised, hence vinyl orientation 1 is the lowest in energy. We suggest these intramolecular electrostatic interactions are likely to contribute to backbone planarization.


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Figure 3: Optimised geometries of the three lowest energy conformations of molecule 7, conformation 1AA (vinyl orientation 1 and thiophene orientation AA), conformation 2AA (vinyl orientation 2 and thiophene orientation AA) and conformation 3AA (vinyl orientation 3 and thiophene orientation AA), including interatomic distances and relative energies calculated using DFT (B3LYP, 6-31G(d)).

In order to investigate if the predicted interactions in the monomer could be observed experimentally, two-dimensional

19

F-1H heteronuclear Overhauser effect spectroscopy nuclear

magnetic resonance (2D 19F-1H HOESY NMR) spectra of the non-brominated monomer 7 were obtained in 1,1,2,2-tetrachloroethane-d2 (TCE-d2) (see Figure 4). The downfield doublet at 8.56 ppm in the one-dimensional 1H NMR spectrum can be assigned to vinyl protons ‘A’ (those closest to the electron poor FBT unit), whilst the upfield doublet at 7.21 ppm can be assigned to vinyl protons ‘B’ (those closer to the electron rich thiophene ring). At room temperature no through space correlation was observed between the vinyl protons and the fluorine substituents on the BT unit, which we ascribe to fast conformational exchange by rapid intramolecular motion around the vinylene-aromatic single bonds on the NMR timescale. However, when the solution was cooled to -40°C a correlation was observed between the equivalent fluorine atoms and both sets of vinyl protons (A and B), with a weaker correlation observed between the downfield vinyl protons ‘A’ and the fluorine atoms and a stronger correlation between the upfield vinyl protons ‘B’ and the fluorine atoms (see Figure 4). An NOE (nuclear Overhauser effect) correlation is only observed when the interatomic distance between the atoms is less than 4-5 Å. The data therefore suggests a mixture of vinyl conformations is present in the cooled sample, in agreement with the fact that the calculated difference in relative energies between the


12

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vinyl conformations is quite small (see Figures 3 and S3). Correlations between ‘B’ vinyl protons and fluorine atoms (i.e. in vinyl conformations 2 and 3), will be stronger than correlations between ‘A’ vinyl protons and fluorine atoms (i.e. in vinyl conformations 1 and 2) as ‘B’ vinyl protons are closer in space (~2.21 Å (F···HB) compared to ~2.34 Å (F···HA)). Further evidence of the planarization effect upon cooling is given by the variable temperature 1H and

19

F NMR spectra, which both broaden upon cooling (see figures S1 and S2) in agreement

with the reduced conformational freedom.

Figure 4: 2D 19F-1H HOESY NMR spectrum of molecule 7 at -40°C.

Finally the optimised geometries and electron density plots of the polymer P(FBT-V2T4) were calculated using the lowest energy monomer as the repeat unit (conformation 1AA for molecule 7 – see Figure 3). DFT calculations were carried out with a B3LYP level of theory and a basis set of 6-31G(d) on a trimer of the repeat unit with methyl groups instead of the full alkyl chains in order to simplify the calculations. DFT calculations predict the backbone of P(FBT-V2T4) to


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be highly co-planar (see Figure 5). Sterically unfavourable head-to-head linkages are avoided due to the careful positioning of the alkyl chain in the monomer FBT-V2T2 resulting in a highly planar polymer backbone.

Figure 5: HOMO electron density plot of P(FBT-V2T4) (A), LUMO electron density plot (B) and side-view of the optimised geometry of P(FBT-V2T4) (C), calculated using DFT (B3LYP, 6-31G(d)).

4. Electronic properties The electronic properties of the polymer in the solid state were investigated using cyclic voltammetry (CV) and photoelectron spectroscopy in air (PESA). The polymer reduction and oxidation potentials were obtained on spun-cast films on fluorine doped tin oxide (see Figure S4). P(FBT-V2T4) appears to undergo two sequential one-electron oxidation and reduction processes as the cyclic voltammograms display two quasi-reversible oxidation and reduction peaks. The onset of first oxidation was 0.67 V versus Ag/AgCl, which allows the HOMO value of the polymer to be estimated as -5.04 eV (the reference ferrocene/ferrocenium potential was


14

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taken to be 4.8 eV below the vacuum level).37 Similarly the LUMO was estimated to be -3.40 eV from the onset of the first reduction peak (-0.97 V versus Ag/AgCl). The ionisation potential of the polymer in thin film was also measured using PESA, giving a value of -4.99 eV (see Figure S5) in close agreement with the CV value.

5. Thermal properties The thermal properties of powder samples of P(FBT-V2T4) were investigated using differential scanning calorimetry (DSC in a temperature range of 0-300°C (see Figure S6). Upon the first heating cycle the polymer undergoes a relatively broad endothermic transition, of enthalpy ~3 J/g, which onsets at ~45°C and peaks at ~52°C. Upon cooling an exothermic transition, of enthalpy ~21 J/g, onsets at ~49°C and peaks at ~34°C. In the second heating cycle the broad endotherm onsets earlier at ~28°C, peaks at ~43°C and had an enthalpy of ~15 J/g. The second cooling cycle shows a similar exothermic transition to the first cooling cycle, of enthalpy ~20 J/g, which onsets at ~49°C and peaks at ~34°C. The transitions of the second heating and cooling cycles are reproduced in the third heating and cooling cycles. These results suggest that after the first heating and cooling cycle the polymer solidifies into a different solid state structure that is then reproducible. We propose these transitions are the result of the tetradecyl and dodecyl side chains melting and crystallizing. Similar observations were observed for the melting of tetradecyl side chains at 35-40°C in poly(3-tetradecylthiophene)38 and dodecyl side chains at ~50°C in poly(3-dodecylthiophene).39,40 Interestingly we did not observe a high temperature peak attributable to a backbone melting, suggesting the melting point is above 300 ˚C, in agreement with a rigid backbone.


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6. Thin-Film Transistor Measurements The transistor performance of P(FBT-V2T4) was investigated in top gate, bottom contact OFET devices. Transistors were fabricated on glass substrates using Au (30 nm) source-drain electrodes and CYTOP dielectric. Gold electrodes were treated with pentafluorobenzene thiol (PFBT) SAM to increase the work function and improve hole injection.41 The polymer was spun cast from hot TCB solution (5 mg/ml) to ensure the polymer was fully dissolved. Polymer films were then investigated either as-spun, or annealed at 100 or 200°C for 30 min, prior to spin coating the dielectric layer. The device characteristics are summarised in Table 1 and the 200°C annealed device transfer and output characteristics are shown in Figure 6A. The transfer and output characteristics for the as-spun and 100°C annealed devices are shown in Figure S7. Asspun films of the polymer showed p-type charge transport with an average linear hole mobility of 0.088 cm2V-1s-1 (peak mobility of 0.13 cm2V-1s-1), and an average saturated mobility of 0.11 cm2V-1s-1 (peak mobility of 0.14 cm2V-1s-1). Annealing at 100°C for 30 minutes improved the average linear and saturated mobilities of devices to 0.18 cm2V-1s-1, with a peak linear mobility to 0.27 cm2V-1s-1 and the peak saturated mobility of 0.25 cm2V-1s-1. Annealing at 200°C led to further improvements with average linear mobility increasing to 0.35 cm2V-1s-1 (peak 0.41 cm2V1 -1

s ), whilst the average saturated mobility increased to 0.38 cm2V-1s-1 (peak 0.50 cm2V-1s-1).

Devices showed good transistor operation with negligible hysteresis and on/off ratios on the order of 105 when annealed at 200°C. It is important to note that although polymers with significantly higher charge carrier mobilities have been reported, these sometimes display significant deviations from ideal transistor behaviour, with high apparent mobilities only observed at low gate voltages, with lower mobilities when the device is fully switched on at higher gate voltages.29,42 In the present case the transistors now exhibit close to ideal behaviour.2


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Notably the extracted mobility based on the first derivative of the linear regime transfer curve and first derivative of the square root of the saturation regime transfer curve is almost gate voltage-independent (Figure 6B) in good agreement with the gradual channel approximation model employed.

Table 1: Summary of important transistor operating characteristics.

Annealing Temp µlin average (µlin max) (oC) (cm2V-1s-1)

µsat average (µsat max)

As spun

0.088 ± 0.035 (0.13)

0.11 ± 0.029 (0.14)

-13.1 ± 1.2 103~104

100

0.18 ± 0.072 (0.27)

0.18 ± 0.051 (0.25)

-12.9 ± 1.1 103~104

200

0.35 ± 0.054 (0.41)

0.38 ± 0.096 (0.50)

-13.0 ± 1.6 104~105

(cm2V-1s-1)

VTh (V)

Ion/Ioff


17


-30

7

VG = 100 V

VD = -5 V VD = -60 V

6

ID / A

10-6

5 4

10-7

3 -8

10

2 10-9 10-10 10

B

1

0.1

0

-10

-20 -30 VG / V

-40

-50

-25 ∆VG = 10 V -20 ID / µ A

10-5

ID sat 1/2 / 103 A1/2

A

Mobility (µ µ ) / cm2V-1s-1

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-15 -10

1

-5

0

0

-60

VG = 0 V

0

-10

-20

-30

-40

-50

-60

VD / V

0.01

1E-3 saturation mobility (µ µsat, VD= -60 V) linear mobility (µ µlin, VD = -5 V) 1E-4 10

0

-10

-20

-30

-40

-50

-60

VG / V

Figure 6: (A) Transfer (left) and output (right) characteristics for devices annealed at 200°C in the top gate, bottom contact configuration (channel width and length of the transistors are 1000 µm and 40 µm, respectively). (B) Mobility calculation based on first derivative of the linear regime transfer curve and first derivative of the square root of the saturation regime transfer curve.

7. Investigating the structure-property relationship using AFM and GIWAXS It is likely that the mobility is somewhat limited by the low molecular weight of the polymer (Mn = 19.3 kDa), as low molecular weight materials tend to form small crystalline domains with distinct grain boundaries that can reduce mobility.43 High performance high molecular weight polymers however, can form interconnected aggregates leading to high charge carrier mobility.44


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To examine the difference in the carrier mobilities between as-spun and annealed polymer films, their morphology was examined using atomic force microscopy (AFM). Topography images of the films are shown in Figure 7 and phase images are shown in Figure S8. The as-spun film was comprised of many small polymer grains of around 30 nm in diameter with clear grain boundaries visible in the topography images (Figure 7). These small domains with distinct grain boundaries are likely to be the result of the low molecular weight of P(FBT-V2T2). The polymer film annealed at 100°C is somewhat smoother and the grain boundaries are less distinct, whilst the polymer film annealed at 200°C has noticeably fewer grain boundaries and a more continuous morphology than the other films. The reduction in grain boundaries with annealing may help to explain the improved mobility of these devices, as well as the gate voltage independence of the mobility observed for devices annealed at 200°C. To investigate whether reduced grain boundaries in the annealed films reduce charge trapping, the deep trap concentration and surface trap densities were estimated from the onset voltages and subthreshold swing of the linear transfer curve45,46 of the as-spun, and annealed at 100°C and 200°C devices. We found that whilst the surface trap density appears to be independent of temperature, the deep trap concentration reduces upon annealing (Figure S9).


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Figure 7: AFM topography images for thin-films of P(FBT-V2T4) as-spun (a) annealed at 100°C (b) and annealed at 200°C (c).

Crystalline packing within the thin films were examined using Grazing Incidence Wide-angle X-ray Scattering (GIWAXS) at the SAXS/WAXS beamline at the Australian Synchrotron.47 14 keV photons were aligned to a grazing angle of ~0.1 degrees (close to critical angle) where the scattering was most intense, and an angle of 0.2 degrees (above critical angle) where intensity measurements can be compared between samples. Figure 8 shows the 2D GIWAXS patterns, and Table 2 records the packing parameters derived from the scattering patterns. 1D lineouts are supplied in the supporting information (see Figure S10). At all temperatures, a liquid crystalline, edge-on stacking behaviour is observed in the samples, with a clear alkyl lamella with a spacing of 2.3 nm observed out of the plane of the sample, and π-π stacking with a d-spacing of 0.35 nm observed in the plane of the sample. In addition to these well-defined peaks there is a broad amorphous halo associated with disordered polymer chains.


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Figure 8: 2D GIWAXS Patterns of films as spun (a) annealed at 100 °C (b) and 200 °C (c).

Table 2: GIWAXS Packing parameters spacing (d) coherence length (ξ) and intensity (I) in the π-π and alkyl directions. Annealing

dπ-π

ξπ-π

Iπ-π

dalk

ξalk

Ialk

Temp °C

nm

nm

a.u.

nm

nm

a.u.

As spun

0.355 ±0.001

5±1

45±5

2.3±0.1 12.2±0.2 600±10


21


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100

0.356 ±0.001

5.7±0.3 60±5

2.3±0.1 12.4±0.3 720±20

200

0.357 ±0.001

7.3±0.1 95±15

2.3±0.1 14.4±0.1 2010±20

Page 22 of 35

Upon annealing, we measure a decrease in peak width and increase in scattering intensity, indicating a corresponding increase in the crystalline coherence length (from 5 nm of coherent ππ spacing and 12 nm of coherent alkyl lamella to over 7 nm of coherent pi stacking and 14 nm of coherent alkyl lamella) and an increase in the relative crystallinity within the thin films of more than a factor of 2 in both directions. Furthermore the relative intensity of the amorphous halo decreases with annealing (see Figure S10) consistent with an increase in the level of crystallinity in the thin film. The orientation and liquid crystallinity of the film remains unchanged upon annealing. The increase in relative crystallinity and crystalline coherence length with annealing, alongside the reduction in grain boundaries observed in AFM images (see Figure 7) are likely to explain the increase in charge carrier mobility (and the reduction in deep trap concentration) measured in thermally annealed OFETs. 8. Conclusions In conclusion we have designed and prepared a new monomer FBT-V2T2 comprising a fluorinated benzothiadiazole core with flanking 3-alkylthiophene groups separated by a vinylene spacer. The inclusion of the vinylene spacer is shown to retain conjugation whilst reducing steric interactions between adjacent aromatics. This allows alkyl chains to be placed in the 3-position of the thiophenes flanking benzothiadiazole, rather than the more common 4-position, thereby minimising any potential steric interactions with co-monomers. DFT calculations in combination with temperature dependent 1H and 19F NMR spectroscopy suggest that FBT-V2T2 can from coplanar structures. Co-polymerisation of FBT-V2T2 with tail-to-tail tetradecyl-bithiophene by


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Stille coupling resulted in the semi-crystalline polymer P(FBT-V2T2) that formed well-ordered films with promising performance in OFET devices. Mobilities of up to 0.5 cm2V-1s-1 were achieved when films were annealed to 200°C, with the annealing process demonstrated to increase relative crystallinity and crystalline coherence length, as well as reduce grain boundaries. This is one of the highest reported mobilities for simple thiophene-BT based copolymers and exceeds the performance of many more complex donor-acceptor co-polymers. Importantly the observed mobility is almost gate voltage independent. Device performance may be improved further by exploring the use of longer branched alkyl chains such as 2-octyldodecyl or 2-decyltetradecyl groups, which are likely to improve solubility and therefore increase the molecular weight of the polymer. We also note that the use of vinyl spacer groups flanking the BT unit leaves scope to introduce bulkier groups, such as electron withdrawing cyano groups48,49 or solubilising alkoxy or thioalkyl chains50 onto the 5 and 6 positions of the BT unit, without causing disruptive steric interactions with adjacent thiophene rings that have previously been found to hinder both OFET and OPV device performance.

9. Experimental

9.1 General All chemical and solvents were used as purchased from Sigma-Aldrich. 2-Bromo-3dodecylthiophene,51 5,6-difluoro-4,7-diiodobenzo[c][1,2,5]thiadiazole52 and 4,4'-ditetradecyl5,5'-bistrimethylstannyl-2,2'-bithiophene53

were

synthesised

according

to

the

reported

procedures. A Biotage initiator (V2.3) in constant temperature mode was used for all microwave reactions. A Bruker AV-400 (400 MHz) spectrometer was used to record all NMR spectra.


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Molecular weights were recorded using a Polymer Labs (PL-GPC-220) running 1,2,4trichlorobenzene at 140°C and calibrated against polystyrene standards (EasiCal PS-1). UVVisible spectra were recorded in 1,2,4-trichlorobenzene (16.6 mg/dm3) using a Shimadzu UV1800 UV-Vis Spectrophotometer. Films were prepared from TCB (5 mg/ml) by spin coating at 2000 rpm for 60 seconds. PESA measurements were recorded with a Riken Keiki AC-2 PESA spectrometer, using a light intensity of 5 nW. The data was processed with a power number of 0.5. All computational work was performed with Gaussview 5.0, using a B3LYP functional and a basis set of 6-31G(d).54 CV measurements were performed at room temperature under an argon atmosphere using a Metrohm Autolab PGStat101 Potentiostat/Galvanostat. Spin coated films (from hot 5 mg/ml TCB solution) on FTO were measured in degassed acetonitrile in the presence of tetrabutylammonium hexafluorophosphate (0.1 M), using an Ag/Ag+ reference electrode and a platinum wire counter electrode, using ferrocene as the internal standard. AFM images were recorded in tapping mode with a Picoscan PicoSPM LE. Polymer films were prepared under identical conditions to transistor devices.

9.2 OFET device fabrication Top gate/bottom contact devices were fabricated on glass substrates using Au (30 nm) sourcedrain electrodes and CYTOP dielectric. Au electrodes were treated with pentafluorobenzene thiol (PFBT) SAM to increase the work function. Polymer was dissolved in 1,2,4-trichlorobenzene at a concentration of 5 mg/ ml, and spun cast at 2000 rpm from a hot solution for 60 s. The obtained polymer films were used as-spun, annealed at 100 or 200oC for 30 min, respectively, before spin coating of dielectric. The channel width and length of the transistors are 1000 µm and 40 µm, respectively. Mobility was extracted from the slope of ID1/2 vs. VG.


24

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9.3 Synthesis Trimethyl[(3-dodecylthiophen-2-yl)ethynyl]silane (3) 2-Bromo-3-dodecyl thiophene (1.99 g, 5.99 mmol), PdCl2(PPh3)2 (126 mg, 0.180 mmol), CuI (137 mg, 0.720 mmol) and PPh3 (94 mg, 0.358 mmol) were added to a 20 mL high pressure microwave vial. The vial was sealed with a septum and then flushed with argon before adding degassed toluene (8 mL), trimethylsilyl acetylene (1.18 g, 12.0 mmol) and piperidine (4.08 g, 47.9 mmol). The argon inlet was removed and the reaction was heated to 80 °C and left to stir overnight. The solvent was then removed in vacuo and the crude product purified by column chromatography using hexane to afford trimethyl[(3-dodecylthiophen-2-yl)ethynyl]silane (3) as a yellow oil (1.42 g, 4.07 mmol). Yield: 68%; 1H NMR (400 MHz, CDCl3) δ 7.14 (d, J = 5.1 Hz, 1H), 6.85 (d, J = 5.1 Hz, 1H), 2.70 (t, J = 7.8 Hz, 2H), 1.67 – 1.60 (m, 2H), 1.37 – 1.25 (m, 18H), 0.90 (t, J = 6.8 Hz, 3H), 0.27 (s, 9H);

13

C NMR (101 MHz, CDCl3) δ 148.85, 128.06,

125.87, 118.20, 100.67, 97.55, 31.95, 30.14, 29.72, 29.69 (2C), 29.64, 29.47, 29.42, 29.39, 29.28, 22.72, 14.16, 0.01; MS (EI) m/z = 348.3 [M+].

2-Ethynyl-3-dodecylthiophene (4) To a stirred solution of 3 (2.49 g, 7.14 mmol) in methanol (20 mL) and THF (5 mL) was added potassium carbonate (2.97 g, 21.5 mmol). The reaction was monitored by GC-MS until completion. The reaction mixture was diluted with CH2Cl2 (100 mL) and washed with water (2 x 100 mL) to remove excess potassium carbonate. The organic layer was dried over MgSO4, filtered and concentrated under reduced pressure to afford 4 as a clear oil which was used without further purification (1.96 g, 7.09 mmol). Yield 99%; 1H NMR (400 MHz, CDCl3) δ 7.15


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(d, J = 5.1 Hz, 1H), 6.85 (d, J = 5.1 Hz, 1H), 3.42 (s, 1H), 2.72 (t, J = 7.8 Hz, 2H), 1.60 (m, 2H), 1.37 – 1.19 (m, 18H), 0.88 (t, J = 6.7 Hz, 3H); MS (EI) m/z = 276.2 [M+].

4,4,5,5-Tetramethyl-2-[(E)-2-(3-methylthiophen-2-yl)ethenyl]-1,3,2-dioxaborolane (5) A solution of 4 (0.706 g, 2.55 mmol) in toluene (1 mL) was degassed in a septum sealed 20 mL high pressure microwave reactor vial containing a stirrer bar. 4,4,5,5-Tetramethyl-1,3,2dioxaborolane (3.28 g, 25.6 mmol) was then added and the mixture heated to 90 °C for 3 days. The solvent was then removed under reduced pressure and the crude product purified by column chromatography initially using CH2Cl2/hexane (2:8, v:v) then gradually increasing the solvent gradient to CH2Cl2/Hexane (6:2, v:v) to afford 5 as a yellow oil (0.50 g, 1.24 mmol). Yield 48%; 1

H NMR (400 MHz, CDCl3) δ 7.49 (d, J = 18.0 Hz, 1H), 7.13 (d, J = 5.1 Hz, 1H), 6.84 (d, J =

5.1 Hz, 1H), 5.86 (d, J = 18.0 Hz, 1H), 2.67 (t, J = 7.8, 2H), 162-1.55 (m, 2H), 1.34-1.22 (m, 30H), 0.88 (t, J = 6.7 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 142.78, 139.97, 137.75, 129.78, 124.97, 115.13 (br), 83.23, 31.95, 30.96, 29.68 (br, 2C), 29.61, 29.51, 29.37 (br, 2C), 28.33, 24.79 (2C), 22.71, 14.14; MS (EI) m/z = 404.3 [M+].

5,6-Difluoro-4,7-bis[(E)-2-(3-dodecylthiophen-2-yl)ethenyl]-2,1,3-benzothiadiazole ( 7) To a septum sealed, argon flushed high pressure 5 mL microwave vial containing 5 (0.375 mg, 0.93 mmol), 6 (157 mgs, 0.37 mmol), Pd(PPh3)4 (54 mg, 0.046 mmol) and aliquat 336 (2 drops) was added degassed toluene (2 mL) and 1M Na2CO3 (1.5 mL) and the argon inlet removed. The mixture was heated to 110°C for 72 hours. The reaction mixture was then cooled to RT, diluted with toluene (10 mL) and washed with water. The organic layer was dried (MgSO4), filtered and concentrated under reduced pressure. The crude product was purified by column chromatography


26

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using CH2Cl2:hexane (1:9, v:v) to afford 7 as a red solid (150 mg, 0.207 mmol). Yield 56%; mp 99-100°C; 1H NMR (400 MHz, CDCl3) δ 8.57 (d, J = 15.9 Hz, 2H), 7.33 (d, J = 15.9 Hz, 2H), 7.23 (d, J = 5.1 Hz, 2H), 6.92 (d, J = 5.1 Hz, 2H), 2.79 (t, J = 7.6 Hz, 4H), 1.71-1.62 (m, 4H), 1.45 – 1.16 (m, 36H), 0.86 (t, J = 6.9 Hz, 6H); 19F NMR (377 MHz, CDCl3) δ -134.23; 13C NMR (101 MHz, CDCl3) δ 150.41 (dd, J = 258.9, 19.6 Hz), 149.92 – 149.81 (m), 143.73 (s), 136.94 (s), 130.06 (s), 128.84 (s), 124.85 (s), 115.02 (s), 114.75 – 114.56 (m), 31.93 (s), 31.03 (s), 29.70 (2C, s), 29.66 (s), 29.64 (s), 29.46 (s), 29.36 (s), 29.35 (s), 28.64 (s), 22.70 (s), 14.13 (s); MS (ASAP) m/z = 724.7 [M+]. ASAP-HRMS (m/z): Calculated for C42H59N2F2S3 [M+H]+: 725.3809, found 725.3802.

4,7-Bis[(E)-2-(5-bromo-3-dodecylthiophen-2-yl)ethenyl]-5,6-difluoro-2,1,3-benzothiadiazole (FBT-V2T2- 8) To a solution of 7 (0.301 mg, 0.415 mmol) in chloroform (15 mL) and acetic acid (15 mL) was added N-bromosuccinimide (NBS) (155 mg, 0.872 mmol) and the resulting mixture stirred in the absence of light for 12 h. The reaction mixture was poured into a saturated solution of sodium sulfite and extracted with chloroform. The organics were combined, dried (MgSO4), filtered, and the solvent removed under reduced pressure. The crude product was then recrystallized from ethyl acetate to afford 8 as a red solid (289 mg, 0.327 mmol). Yield: 79% ; mp 127-128°C; 1H NMR (400 MHz, CDCl3) δ 8.48 (d, J = 16.0 Hz, 2H), 7.18 (d, J = 16.0 Hz, 2H), 6.88 (s, 2H), 2.73 (t, J = 7.6 Hz, 4H), 1.68 – 1.58 (m, 4H), 1.43 – 1.16 (m, 36H), 0.87 (t, J = 6.7 Hz, 6H); 19F NMR (377 MHz, CDCl3) δ -133.91;

13

C NMR (101 MHz, CDCl3) δ 144.14 (s), 138.66 (s),

132.87 (s), 127.95 (s), 115.31 (s), 114.71 – 114.50 (m), 112.50 (s), 31.92 (s), 30.81 (s), 29.67 (s), 29.64 (s), 29.58 (s), 29.39 (s), 29.35 (s), 29.21 (s), 28.55 (s), 22.69 (s), 14.11 (s) (note some


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Page 28 of 35

signals for carbons coupling to fluorine too weak to detect); MS (MALDI) isotopic cluster at m/z = 882.6 [M+]; Anal. Calcd for C42H56Br2F2N2S3: C 57.14, H 6.39, N 3.17; found: C 57.21, H 6.29, N 3.11.

P(FBT-V2T4) To a microwave reactor tube (2 mL) was added FBT-V2T2 (98.94 mg, 0.121 mmol), 4,4'ditetradecyl-5,5'-bis(trimethylstannyl)-2,2'-bithiophene (99.13 mg, 0.121 mmol), Pd2(dba)3 (2.05 mg, 0.002 mmol) and P(o-tol)3 (2.73 mg, 0.009 mmol). The tube was sealed, evacuated and backfilled with argon, before degassed chlorobenzene (1 mL) was added. After further degassing with argon for 20 min, the tube was heated in the microwave at 100°C (2 min), 140°C (2 min), 160°C (2 min), 180°C (10 min) and finally 200°C for 25 min. After cooling to RT the solution was precipitated into methanol (100 mL), stirred (30 min), filtered through a soxhlet thimble and extracted (methanol, acetone, hexane) under argon to leave a dark blue polymer (102 mg, yield: 71%). GPC (1,2,4-TCB, 140 °C) Mn: 19.32 kDa, Mw: 36.63 kDa, Mn/Mw (Ð): 1.90. 1H NMR (400 MHz, TCE, 130°C) δ 8.57 (d, J = 16.0 Hz, 2H), 7.42 (d, J = 16.0 Hz, 1H), 7.11 (s, 2H), 7.07 (s, 2H), 2.96 – 2.83 (m, 8H), 1.90 – 1.75 (m, 8H), 1.61 – 1.46 (m, 18H), 1.42 – 1.31 (m, 60H), 1.01 – 0.93 (m, 12H).

19

F NMR (376 MHz, TCE, 130°C) δ -133.81; Anal. Calcd. for

C78H116F2N2S5 C 73.19, H 9.13, N 2.19; found: C 72.95, H 9.26, N 2.24.

ASSOCIATED CONTENT Supporting Information. Additional figures and table as mentioned in the text, including NMR spectra, DFT calculations, cyclic voltammograms, DSC plots, transistor output and transfer plots, AFM images and


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experimental information for alkyne substituted BTs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT We thank the UK's Engineering and Physical Sciences Research Council (EPSRC) for financial support via the Doctoral Training Centre in Plastic Electronics EP/G037515/1 (AC, JG, MH) and the British Council (Grant Number 173601536). We gratefully acknowledge Dr Fiona Scholes (CSIRO) for the PESA measurements. CRM acknowledges funding from the Australian Research Council (DP130102616). Part of this research was undertaken on the SAXS/WAXS beamline at the Australian Synchrotron, Victoria, Australia. ASAP-HRMS measurements were carried out at the EPSRC UK National Mass Spectrometry Facility at Swansea University. REFERENCES (1)

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