Doping of Large Ionization Potential Indenopyrazine Polymers via Lewis Acid complexation with tris(pentafluorophenyl)borane: A Simple Method for Improving the Performance of Organic Thin-Film Transistors Yang Han,†, § George Barnes,† Yen-Hung Lin,§ Jaime Martin,‡ Mohammed Al-Hashimi,∥ Siham Y. AlQaradawi,⊥ Thomas D. Anthopoulos,§ and Martin Heeney*,†

†Department of Chemistry and Centre for Plastic Electronics, Imperial College London, SW7 2AZ, UK‡Department of Materials and Centre for Plastic Electronics, Imperial College London, SW7 2AZ, UK∥Department of Chemistry, Texas A&M University at Qatar, P.O. Box 2713, Doha, Qatar⊥Department of Chemistry & Earth Sciences, Qatar University, P.O. Box 110003 Doha, Qatar §Department of Physics and Centre for Plastic Electronics, Imperial College London, SW7 2AZ, UKKEYWORDS molecular doping, p-type doping, organic thin-film transistors, conjugated polymers

ABSTRACT: Molecular doping, under certain circumstances, can be used to improve the charge trans-port in organic semiconductors through the introduction of excess charge carriers which can in turn negate unwanted trap states often present in organic semiconductors. Here, two Lewis basic indenopy-razine copolymers with large ionization potential (5.78 and 5.82 eV) are prepared to investigate the p-doping efficiency with the Lewis acid dopant, tris(pentafluorophenyl)borane using organic thin-film tran-sistors (OTFTs). The formation of Lewis acid-base complex between the polymer and dopant molecules is confirmed via optical spectroscopy and electrical field-effect measurements, with the latter revealing a dopant concentration-dependent device performance. By adjusting the amount of p-dopant, the hole mobility can be increased up to 11 fold while the OTFTs’ threshold voltages are reduced. The work demonstrates an alternative doping mechanism other than the traditional charge transfer model, where the energy level matching principle can limit the option of dopants.

1. INTRODUCTION Organic thin-film transistors (OTFTs) have at-tracted tremendous research interest during re-cent years due to their broad potential applica-tions in next-generation electronics. Of special in-terest are solution processable OTFTs which offer the prospect to fabricate large area flexible de-vices by a range of printing techniques.1–3 The drive to optimise transistor performance has led to the synthesis of many novel organic semicon-ductor (OSC) materials of various molecular struc-tures, as well as focussed efforts to optimise their device fabrication procedures to achieve high performance OTFTs.4–8 An alternative approach to improve the device performance of a given OSC is through the blending of a suitable dopant mate-rial. Controlling the dopant concentration can yield improved yet controllable performance.9–11

The clear advantage of this approach is that the charge transport of semiconductor matrix can be enhanced by negating unwanted trap states

hence bypassing time-consuming redesign and synthesis of new materials.

To date, several different approaches for extrin-sically doping (i.e. the deliberate addition of a dopant molecule) organic semiconductors have been developed.12–29 One of the most well investi-gated is the concept of integer charge transfer between the dopant and the OSC. The latter ap-proach relies upon matching the electron affinity (EA) of the dopant with the ionization potential (IP) of the OSC (for p-doping, and vice versa for n-doping) to generate localised charges on the dopant molecule and a free counter charge in the semiconductor matrix. The excess charges can then fill traps and under certain circumstances enhance the charge carrier mobility.9,30–32 An ex-ample of p-dopant with a suitably high electron affinity is 2,3,5,6-tetrafluoro-7,7,8,8-tetra-cyanoquinodimethane (F4TCNQ), whose electon affinity of 5.2 eV is close to the ionization poten-tial of many OSCs.33–37 However, there is a limited

choice of available dopants that have sufficient electron affinity to p-dope OSCs of larger ionisa-tion energies.13,30,34,35,37–39

Recently an alternative doping mechanism has been found to operate in some cases, in which doping proceeds via the hybridization of the fron-tier molecular orbitals of the OSC and the p-dopant in a supramolecular or charge transfer complex, rather than by integer charge trans-fer.28,40,41 In this mechanism, the frontier-orbital hybridization of the complex leads to empty lev-els within the band gap of the surrounding matrix. Here the supramolecular complex effectively acts as the dopant and whilst the general matching of the ionization energy of the OSC with the electron affinity of the dopant remained a necessity in this complex formation model, the proposed mecha-nism also suggested that increasing the electron affinity of the dopant was not sufficient to en-hance the doping efficiency and that reducing the intermolecular resonance integral by suppressing overlap of the frontier molecular overlap was also important. Another approach to the formation of a charge-transfer complex in conjugated systems was re-ported by Zalar et al.42 It was shown that the addi-tion of the bulky Lewis acid tris(pentafluo-rophenyl)borane [B(C6F5)3] to a polymer contain-ing a Lewis basic pyridyl co-monomer formed an adduct with improved mobility in hole-only diode devices compared to the pristine polymer (i.e. the B(C6F5)3 acts as an extrinsic dopant). Earlier work had already demonstrated that such adducts could modulate the optical band gap and photolu-minescence of conjugated materials via the dona-tion of electron density from the conjugated sys-tem to the Lewis acid. They also observed that the Lewis acid preferably bound to the most basic nitrogen atom when mixed binding sites were available.43–45 Although B(C6F5)3 was also incorpo-rated in thin film transistors for the purpose of chemical sensing,46 as far as we are aware the po-tential and efficiency of this doping motif to im-prove device performance has not been investi-gated in field-effect transistors until now.

In order to demonstrate the potential of this ap-proach we chose to study p-type polymers of high ionization potential which would be difficult to p-dope via the traditional integer charge transfer method due to the absence of suitable oxidants. As such we identified co-polymers of indenopy-razine as suitable candidates. Indenopyrazine is an analogue of the well-studied fused aromatic in-denofluorene, in which the central benzene ring is replaced by an electron deficient pyrazine. This results in an increase in both ionisation potential and electron affinity compared to the indenofluo-rene analogue.47 The basic pyrazine has two avail-able lone pairs of electrons that may bind to the tris(pentafluorophenyl)borane. Indenopyrazine and its co-polymers have previously been investi-gated as deep blue emitters in organic light-emit-

ting diode (OLED) applications,48,49 as well as donor polymers for high voltage organic solar cells50. However the performance of indenopy-razine co-polymers in OTFTs has been moderate thus far, likely due to problems with charge injec-tion and trapping related to the large ionization potential.47,51

Here we report the synthesis of two polymers of indenopyrazine with thiophene and thieno[3,2-b]thiophene, and demonstrate for the first time that their OTFT performance can be tuned by complexation with B(C6F5)3. We investigate the doping efficiency using carefully engineered top-gate/bottom-contact OTFT architectures. Impor-tantly the doped OTFT devices are prepared via spin coating directly from solution as the poly-mers, dopant and resulting complexes are all readily soluble in common organic solvents. This allows a facile control of the dopant level, which is more difficult to achieve in systems where dop-ing is performed through exposure of the semi-conducting layer to liquids or gaseous dopants and/or at the charge injection contact interface. By adjusting the concentration of dopants, the doping efficiency and performance of OTFTs can be optimised leading to an 11 fold enhancement of the hole mobility. The channel current on/off ra-tio was also increased by at least one order of magnitude and the threshold voltages were re-duced with increasing dopant concentration.

2. EXPERIMENTAL SECTION General. All chemicals were purchased from

commercial suppliers unless otherwise specified. 1H NMR and 13C NMR spectra were recorded on BRUKER 400 spectrometer in CDCl3 solution at 298 K. Number-average (Mn) and weight-average (Mw) molecular weight were determined by Agi-lent Technologies 1200 series GPC running in chlorobenzene at 80 °C, using two PL mixed B col-umns in series, and calibrated against narrow polydispersity polystyrene standards. UV-vis spec-tra were recorded on a UV-1601 Shimadzu UV-vis spectrometer. Flash chromatography (FC) was performed on silica gel (Merck Kieselgel 60 F254 230-400 mesh). Photo Electron Spectroscopy in Air (PESA) measurements were recorded with a Riken Keiki AC-2 PESA spectrometer with a power setting of 5 nW and a power number of 0.5. Sam-ples for PESA were prepared on glass substrates by spin-coating. Differential scanning calorimetry (DSC) measurements were conducted under ni-trogen at scan rate of 20 ˚C/min with a TA DSC-Q20 instrument. The molecular packing was char-acterized by wide-angle X-ray diffraction (XRD, PANalytical X'Pert Pro MPD) using the Cu Kα radia-tion. θ/2θ scans were performed at room temper-ature to the films drop cast from the correspond-ing complex solutions onto silicon substrates. Fermi levels were measured using a KP Technol-

ogy scanning Kelvin probe system SKP5050 in ni-trogen environment at room temperature. General procedure for salt wash purifica-tion for IP-diBr. The monomer IP-diBr (200 mg) was dissolved in dry THF (50 mL). Potassium tert-bu-toxide (1M solution in THF, 4.5 mL) was added and the re-action stirred for 15 min. THF was removed under reduced pressure and the remaining precipitate was dissolved in dry hexane (25 mL). Filtration through alumina and subsequent removal of the hexane under reduced pressure yielded the puri-fied product. This was repeated twice.

Synthesis of poly(6,6,12,12-tetraoctyldiin-deno[1,2-b:1,2-e]pyrazine-co-2,5-thiophene) (IP-T). 2,8-Dibromo-6,6,12,12-tetraoctyldiindeno[1,2-b:1,2-e]pyrazine (IP-diBr) (0.20 g, 0.23 mmol), 2,5-bis

Scheme 1. Synthesis of IP-T, IP-TT and structure of B(C6F5)3.

-(trimethylstannyl)thiophene (0.10 g, 0.23 mmol), Pd2(dba)3 (4.2 mg, 0.0046) and P(o-tol)3 (5.6 mg, 0.018 mmol) were added to a microwave vial. Dry chlorobenzene (4 mL) was added and the mixture headed under microwave irradiation for 5 min at 100 °C, 5 min at 120 °C, 10 min at 160 °C and 20 min at 180 °C. After cooling to 50 °C the resulting solution was poured into cold acidic methanol (MeOH 100 mL/ HCl 5 mL), filtered into a soxhlet thimble and extracted (soxhlet) using methanol, acetone and hexane. The remaining polymer was removed from the thimble, dried and dissolved in chloroform/water solution (100 mL/100 mL) and sodium diethyldithiocarbamate trihydrate (0.50 g) was added. The solution was stirred vigorously at 50 °C for 3 h. The chloroform solution was washed with water, dried (MgSO4), concentrated and precipitated from chlorobenzene into cold methanol to yield the polymer as dark red fibres. Yield = 125 mg, 69 %. GPC: Mn = 23 kDa, Ð = 2.3. 1H NMR (400 MHz, CDCl3, δ): 8.15 (d, J = 7.6 Hz, 2H), 7.81 (d, J = 7.6 Hz, 2H), 7.76 (s, 2H), 7.53 (b, 2H), 2.43 - 2.27 (b, 4H), 2.19 - 2.01 (b, 4H), 1.20 - 1.10 (b, 48H) 0.81 (t, J = 6.6 Hz, 12H).

Synthesis of poly(6,6,12,12-tetraoctyldiin-deno[1,2-b:1,2-e]pyrazine-co-2,5-thieno[3,2-b]thiophene) (IP-TT). 2,8-Dibromo-6,6,12,12-tetraoctyldiindeno[1,2-b:1,2-e]pyrazine (IP-diBr) (0.2 g, 0.23 mmol), 2,5-bis(trimethyl-

stannyl)thieno[3,2-b]thiophene (0.11 g, 0.23 mmol), Pd2(dba)3 (4.2 mg, 0.0046 mmol) and P(o-tol)3 (5.6 mg, 0.018 mmol) were added to a mi-crowave vial. Dry chlorobenzene (4 mL) was added and the mixture heated under microwave irradiation for 5 min at 100 °C, 5 min at 120 °C, 10 min at 160 °C and 20 min at 180 °C. After cooling to 50 °C the resulting solution was poured into cold acidic methanol (MeOH 100 mL/ HCl 5 mL), filtered into a soxhlet thimble and extracted (soxhlet) using methanol, acetone and hexane. The remaining polymer was removed from the thimble, dried under vacuum and dissolved in chloroform/water solution (100 mL/100 mL) and sodium diethyldithiocarbamate trihydrate (0.50 g) was added. The solution was stirred vigorously at 50 °C for 3 h. The chloroform solution was washed with water, dried (MgSO4), concentrated and precipitated from

chlorobenzene into cold methanol to yield the polymer as dark red fibres. Yield = 94 mg, 48 %. GPC: Mn = 20 kDa, Ð = 2.1. 1H NMR (400 MHz, CDCl3, δ): 8.16 (b, 2H), 7.75 - 7.70 (b, 6H), 2.43 - 2.27 (b, 4H), 2.17 - 2.01 (b, 4H), 1.20 - 1.10 (b, 48H), 0.81 (t, J = 7.2 Hz, 12H).

Preparation of complex solutions. IP-T, IP-TT and B(C6F5)3 were dissolved in o-dichloroben-zene respectively, to prepare stock solutions at the concentration of 10 mM. 100 µL of polymer stock solution was added the desired amount of B(C6F5)3 stock solution and o-dichlorobenzene sol-vent to dilute if needed to reach a final volume of 150 µL, which is corresponding to a constant con-centration of polymer at 6.67 mM. The un-doped solution was prepared by adding 50 µL o-dichlorobenzene to 100 µL polymer stock solu-tion. The polymer concentration and molar equiv-alents of B(C6F5)3 were calculated with respect to the mass of repeat units of each polymer.

Fabrication of OTFT devices. Top gate/bot-tom contact (TG/BC) configuration was employed to fabricate transistor devices based on pristine

and doped polymers. Bottom contact substrates were prepared by thermal evaporation of Au (60 nm) to glass through shadow mask. The de-posited source/drain electrodes were treated with self-assembled monolayer (SAM) of pentafluo-robenzenethiol (PFBT) to improve work function before applying semiconductor layer. The above mentioned un-doped or doped polymer solutions were then spun cast at 2000 rpm for 60s onto pre-patterned substrates. The obtained semicon-ductor films were stored under vacuum (~10-6

mbar) for 30 min to remove solvent residue and used without thermal annealing. CYTOP (Asahi Glass) dielectric was then spin coated on top, fol-lowed by annealing at 100 °C for 30 min to form a dielectric layer of 900 nm. Al (50 nm) gate elec-trodes were evaporated on top of dielectric through shadow mask to complete the TG/BC transistor devices. The channel width and length of the final transistors were 1 mm and 40 μm, re-spectively. Transistor characterization was carried out under nitrogen using a Keithley 4200 parame-ter analyzer. The saturation mobility was ex-tracted from the slope of ID1/2 vs. VG:

Table 1. Optical and energetic properties of IP-T and IP-TT.

Polymer Mn

[kg/mol] Ð λmax

[nm] λonset [nm] I.P.a)

[eV] Egopt b)

[eV]Chloroben-zene Film Film

IP-T 23 2.3 487 492 515 5.7

8 2.41

IP-TT 20 2.1 495 498 525 5.8

2 2.36

a)Ionisation potential was measured by PESA (error ±0.05 eV); b) Egopt was estimated from the onset of

film absorption

µsat¿2LWC i

(∂√ ID sat∂V G

)2

(1) AFM. AFM images were obtained with a Pi-

coscan PicoSPM LE scanning probe in tapping mode. Samples were prepared by spin coating polymer solutions on plain glass substrates and solvent residue was removed under vacuum, fol-lowing the same procedures for fabrication of transistor devices except that dielectric and gate electrodes were not applied.

3. RESULTS AND DISCUSSIONSynthesis. The indenopyrazine monomer 2,8-di-bromo-6,6,12,12-tetraoctyldiindeno[1,2-b:1,2-e]pyrazine (IP-diBr) was synthesised according to the previously reported method.49,50 Since the incomplete alkylation of analogous fluorene

monomers is known to be a cause of oxidative in-stability, we carefully removed any partially alky-lated product by treatment of the monomer with a strong base, followed by filtration through alu-mina following the method reported for fluo-rene.52 The resulting monomer was polymerised by Stille cross-coupling with 2,5-bis(trimethylstan-nyl)thiophene or 2,5- bis(trimethylstannyl)thieno[3,2-b]thiophene un-der microwave accelerated conditions (Scheme 1).53 The resulting polymers (designated IP-T and IP-TT) were purified by precipitation and extrac-tion with methanol, acetone and hexane to re-move oligomers and catalysts residues. The re-maining polymers were washed with di-ethyldithiocarbamate to remove Pd residues.54

Following a final precipitation, both polymers were isolated as red fibres in reasonable yield. The polymers exhibited similar molecular weight

and dispersity, as measured by gel permeation chromatography in chlorobenzene (Table 1). Physical Properties. The optical properties of the polymers were investigated by UV-Vis absorp-tion spectroscopy, both in solution and as spun-cast thin films. The results are shown in Figure 1 and summarised in Table 1. IP-T has an absorp-tion maximum at 487 nm with a shoulder at 461 nm in dilute chlorobenzene solution, and its main absorption peak red shifts slightly to 492 nm with a shoulder at 463 nm in solid state. The optical band gap of IP-T is estimated to be 2.41 eV from the onset of absorption in film. The measured ion-ization potential of a film of IP-T was 5.78 (±0.05) eV via photoelectron spectroscopy in air (PESA). Compared to IP-T, IP-TT shows a slight red shift in absorption in both solution and film. The main absorption peaks at 495 nm with a shoulder at 468 nm in solution, and further red shifts to 498 nm with a shoulder at 471 nm upon film forma-tion. The absorption onset in film is also shifted to the longer wavelength side, 525 nm, which corre-sponds to a slightly smaller band gap of 2.36 eV. The ionisation potential of IP-TT is similar, with a value of 5.82 (±0.05) eV measured by PESA. The ionization potential of both polymers is therefore significantly larger than the measured electron affinity of F4TCNQ (5.2 eV)55, which makes doping in accordance with integer charge transfer diffi-cult. To examine whether the charge transfer could occur between the pyrazine polymers and F4TCNQ, films of IP-T with varying amounts of F4TCNQ were investigated by UV-Vis absorption (Figure S4). In all cases there was no obvious shift or intensity changes in the absorption peak of IP-T even with 0.5 eq. of F4TCNQ, which suggests no effective charge transfer interaction between the guest molecule and polymer matrix, in agreement with the measured energy levels.

0.0

0.5

1.0

350 400 450 500 5500.0

0.5

1.0

IP-T Solution IP-T Film

Wavelength (nm)

Nor

mal

ised

Abs

. (a.

u.)

IP-TT Solution IP-TT Film

Figure 1. The UV-Vis spectra of IP-T and IP-TT in solution (chlorobenzene) and as thin films.

In considering the use of B(C6F5)3 as a dopant, it is apparent that its bulky structure may cause dis-turbance of polymer chain packing upon blend-

ing, potentially reducing the transistor perfor-mance of material if the polymer itself is in a well-ordered state. In that respect less ordered, loosely packed conjugated polymers may be more suitable for doping via formation of B(C6F5)3 adducts. To evaluate whether IP-T and IP-TT are suitable hosts for doping, differential scanning calorimetry (DSC) traces of both polymers were recorded. The DSC traces show no obvious heat flow that can be ascribed to any crystalline or mesophase transition from 10 to 300 °C (Figure S1), suggesting an amorphous solid state. The molecular packing of the polymers was also char-acterized by wide-angle X-ray diffraction (XRD) of drop-cast films (Figure S3). Neither of the poly-mers displayed any distinct diffraction peaks, consistent with largely amorphous films. IP-T ex-hibited three very broad and weak diffraction bands at 2θ = 7.6o (d = 11.6 Å), 2θ = 9.5o (d = 9.3 Å) and 2θ = 12.4o (d = 7.1 Å), respectively. IP-TT also afforded three weak diffractions at 2θ = 9.7o (d = 9.1 Å), 2θ = 13.8o (d = 6.4 Å) and 2θ = 20.7o (d = 4.3 Å), the last of which possibly corresponds to a loose π-π stacking. The lack of any distinctive semicrystallinity in IP-T or IP-TT suggests they are ideal candidates for Lewis acid doping.

To establish the binding of dopant B(C6F5)3 to the indenopyrazine unit of IP-T and IP-TT in the solid state, solutions with varying molar equiva-lents of dopant yet constant polymer concentra-tion were spun cast onto glass substrates and the film UV-Vis spectra were measured, as shown in Figure 2. The polymer concentration and molar equivalents of B(C6F5)3 were calculated with re-spect to the mass of repeat units of each poly-mer. As the dopant is known not to absorb in the wavelength range from 400 to 800 nm,44 any change in the absorption spectra can be attrib-uted to an interaction between the dopant and the polymer.

Figure 2. The UV-Vis spectra of un-doped and doped IP-T (a) and IP-TT (b) films with different equivalents of B(C6F5)3.

The un-doped IP-T film shows an absorption maximum at 492 nm with a strong shoulder at 463 nm. With addition of dopant, the intensity of the absorption maximum is obviously reduced and a new peak at 590 nm appears. The intensity of this peak increases in accordance with the dopant amount. The emergence of this new peak is consistent with the formation of a complex be-tween the indenopyrazine lone pair and B(C6F5)3. Such a complex would be expected to withdraw electron density from the indenopyrazine, result-ing in the formation of zwitterionic type com-plexes with the dopant and therefore a lower en-ergy charge transfer type band.45 Compared to the band gap of the un-doped polymer at 2.41 eV, the onset of the new longer wavelength peak at 640 nm corresponds to a narrower band gap of 1.94 eV, which is ascribed to withdrawing of elec-trons from indenopyrazine unit to the Lewis acid dopant by adduct formation.44 Similar behaviour is observed for IP-TT blend films. The intensity of the absorption at 498 nm is noticeably reduced with addition of the Lewis acid dopant. A new peak at 604 nm with onset of 675 nm emerges, which corresponds to a narrower optical band gap of 1.84 eV compared to 2.36 eV of the original IP-TT. It should also be noted that within the range of dopant concentration as listed in Figure 2, there is an isosbestic point, at 512 and 520 nm for IP-T and IP-TT, respectively, which suggests two separate species exist in both systems in solid state, the uncomplexed and the complexed indenopyrazine units.43 We further note that the relative intensity of the polymer peaks at 492 and 463 nm for IP-T (and 498/471 nm for IP-TT) change differently upon addition of the dopant (the longer wavelength peak decreases in relative intensity compared to the shorter wavelength peak). We believe this may be due to changes in short range aggregation between the polymer backbones upon complexation. It is also worth highlighting that the solution of polymer remains fully solvated even upon addition of 0.5 equiva-lents of B(C6F5)3. This is in contrast to the solution processing of films of polymers doped with F4TCNQ, which requires careful processing in

many cases due to the reduced solubility of the oxidised polymer and/or the formation of aggre-gates of charge transfer salts.56,57

The nature of the interaction between the poly-mer and B(C6F5)3 is worth commenting upon. The electron affinity of B(C6F5)3 is estimated to be in the range of 3-3.5 eV, based upon the measured reduction potential versus ferrocene/ferrocenium (-1.79 to -1.65 eV).58 Due to the large offset with the ionization potential of the indenopyrazine polymers reported here (-5.78 and -5.82 eV), we do not expect doping to occur via the conven-tional integer charge transfer mechanism. Rather co-ordination of one of the N lone pairs on the in-denopyrazine, which are located in a sp2 orbital orthogonal to the main conjugated backbone, to the tris(pentafluorophenyl)borane is expected i.e. formation of a Lewis acid-base complex. The re-sulting ‘pyrazinium’ like cations are known to be strongly electrophilic. Thus the resulting complex is expected to

-120-100 -80 -60 -40 -20 0

10-9

10-8

10-7

10-6

10-5

|I D| (

A)

VG (V)

eq. of B(C6F5)3 0 0.025 0.075 0.5

(a)

0.0 0.1 0.2 0.3 0.4 0.510-3

10-2

10-1

100

-100

-200

sat (

cm2 V-1

s-1)

eq. of B(C6F5)3

(b)

V Th (V

)

-120-100 -80 -60 -40 -20 0

10-9

10-8

10-7

10-6

10-5

|I D| (

A)

VG (V)

eq. of B(C6F5)3 0 0.025 0.075 0.5

(c)

0.0 0.1 0.2 0.3 0.4 0.5

10-3

10-2

10-1

-100

-200

eq. of B(C6F5)3

sat (

cm2 V-1

s-1)

V Th (V

)

(d)

Figure 3. Representative plots of the transfer characteristics of transistors based on un-doped and doped IP-T (a) and IP-TT (c) at VD=-120 V; and dependence of saturation mobility (navy squares) and threshold voltage (orange circles) on doping equivalents of B(C6F5)3 with IP-T (b) and IP-TT (d).

exhibit a significant increase in both the IP and EA, such that empty molecular orbitals now form within the band gap of the bulk semiconductor, leading to the doping effect. The proposed mech-anism is similar to that described by Méndez and Salzmann,28 in which an intramolecular charge transfer complex is formed by interaction of the HOMO (highest occupied molecular orbital) of the organic semiconductor and the LUMO (lowest un-occupied molecular orbital) of dopant. The resul-tant complex has a lower HOMO than the sur-rounding matrix but also a lower LUMO. The dif-ference in our case is that it is not the delocalised HOMO of the semiconductor which is interacting with the dopant, but rather a specific lone pair, which leads to an overall increase in IP and EA (analogous to lowering of the HOMO and LUMO). We suggest this interaction is significantly easier to design from a molecular engineering perspec-tive, than the intramolecular charge transfer com-plex, where the intermolecular resonance integral was shown to be both important and subtly de-pendent on steric interactions.

To further establish that a Lewis acid-base inter-action is occurring between the indenopyrazine polymers and B(C6F5)3 rather than an ICT or charge transfer complex (or frontier molecular hy-bridization of OSC and dopant), we treated the

doped polymers with pyridine, which is a stronger base than (indeno)pyrazine but is substantially harder to oxidise. Thus if an ICT transition or charge transfer interaction was occurring, the presence of pyridine should not prevent it. How-ever if the main interaction was a reversible Lewis acid-base interaction via the N lone pair, then the presence of excess pyridine would be expected to reverse the complexation of the IP backbone. In-deed we observe the latter, with the optical prop-erties of the film recovered upon treatment with pyridine (see Figure S5), consistent with the ob-servations of Bazan et al..45 We further note that as shown in Figure 2, the UV-Vis spectra of the doped indenopyrazine polymers show no new ab-sorption peak in the near-infrared region until 1100 nm before and after exposure to pyridine, a feature that should be present for the conven-tional integer charge transfer doping.24,35,57,59 The p-doping effect could be further confirmed by measurement of the Fermi energy level using the Kelvin probe method for pristine films of IP-T and those doped with 0.5 eq. of B(C6F5)3. The Fermi level (EF) of the pristine IP-T film was 4.34 eV, close to the middle of the band gap estimated from the optical spectra. Upon treatment with B(C6F5)3 the Fermi level shifts significantly to-wards the HOMO level reaching a stable value of 5.14 eV, as expected for a p-type dopant.

Transistor Fabrication and Characterisa-tion. After confirming the complex formation be-tween dopant and hosts, the electrical perfor-mance of the pristine and doped polymers was assessed using top-gate OTFTs. The dopant and polymers were dissolved in o-dichlorobenzene separately to make stock solutions. The latter were diluted with either pure solvent or the target amount of dopant solution, and then spincast on substrates with pre-patterned Au source/drain electrodes, to obtain the pristine or doped poly-mer OTFTs. The spincast films were stored under vacuum for 30 min to remove solvent residue and used without thermal annealing. After deposition of the dielectric and gate electrodes, the two sets of OTFTs were electrically characterised.

In Figure 3a we show the transfer characteris-tics of OTFTs based on the pristine (un-doped) and p-doped IP-T films with different dopant con-centrations. The performance of the full series of devices is summarised in Table S1. The un-doped IP-T device displays characteristics of a typical hole transporting transistor, with field-effect hole mobility of 0.086 (±0.034) cm2V-1s-1, current on/off ratio in the range of 103-104, and a rather large threshold voltage (VTh) of -102 (±1.8) V. The latter feature is attributed to the rather large ion-ization potential and the difficulty in injecting holes. The addition of B(C6F5)3 has a significant impact on device performance. For example, from the transfer curves in Figure 3a it is evident that the channel on current is increased by at least one order of magnitude while the off current re-mains largely unchanged. The hole mobility is also found to increase sharply with the addition of just 0.01 equivalance (eq.) of dopant and contin-ues to increase with dopant concentration up to a maximum of 0.62 (±0.16) cm2V-1s-1 for 0.075 eq. of dopant. Further increase in the dopant level to 0.5 eq. reduces the hole mobility to values similar to those obtained for the un-doped OTFTs.

The threshold voltage of the resulting OTFTs is also found to shift towards more positive gate voltages with increasing dopant concentration, yielding -75.8 (±5.1) V for 0.01 eq. of B(C6F5)3, and -34.3 (±1.7) V for 0.5 eq. of B(C6F5)3, as shown in Figure 3b. The steep change in charac-teristics upon addition of a relatively small con-centration of dopant is attributed to the all-impor-tant hole trap filling effect.31,32 Similar dopant con-centration-dependent performance is also ob-served for IP-TT, as shown in Figure 3c, 3d and Table S2. Compared to an average hole mobility for the pristine IP-TT of 0.019 (±0.0049) cm2V-1s-1, OTFTs based on 0.075 eq. B(C6F5)3 doped IP-TT exhibit maximum hole mobility of 0.22 (±0.025) cm2V-1s-1, with a significantly reduced threshold voltage i.e. from -101.3 (±1.2) V for pristine IP-TT OTFTs to 45.4 (±1.0) V for 0.5 eq. B(C6F5)3 doped devices. The on/off ratio is also increased by more than one order of magnitude, highlighting a po-

tential advantage of the particular doping ap-proach explored here.

To determine whether the density of trap states influences the device performance, we calculated the interface trap density (Ntr) and trap concen-tration (Dtr, per unit area and unit energy) from the corresponding transistor transfer characteris-tics using:

Ntr¿Cie

∨V Th−V on∨¿

(2)and

Dtr¿Cie2 ( eSkTln (10 )

−1)

(3) where e is the elementary charge, Ci the geomet-ric capacitance of gate dielectric, Von the onset voltage, k the Boltzmann constant, T the measur-ing temperature and S

0

2

4

6

8

0.50.250.10.0750.050.0250.01

h+ , N

tr (x

1011

cm-2)

eq. of B(C6F5)3

h+

Ntr

0

(a)

0

1

2

Dtr (x

1012

eV-1cm

-2)

Dtr

0

1

2

3

4

5

Dtr (x

1012

eV-1cm

-2)

(b)

eq. of B(C6F5)3

Dtr

0.50.250.10.0750.050.0250.010 0

2

4

6

8

h+ , N

tr (X

1011

cm-2)

h+

Ntr

Figure 4. Additional free charges generated by addition of B(C6F5)3 (Δh+), interface trap density (Ntr) and trap concentration per energy unit (Dtr) of transistors based on doped IP-T (a) and IP-TT (b) as a function of equivalents of B(C6F5)3.

the subthreshold swing.60,61 As shown in Figure 4, for both IP-T and IP-TT, the Ntr appears to be rela-tively independent of dopant concentration, whereas Dtr decreases upon addition of small amounts of B(C6F5)3 but increases again for higher

dopant contents. The evolution of Dtr is consistent with the observed changes in transistor perfor-mance and in particular the hole mobility evolu-tion. Specifically, the performance of the resulting OTFTs appears to improve for B(C6F5)3 concentra-tions in the range 0.01-0.075 eq. and reduce for concentrations >0.075 eq. On the basis of these findings we argue that it is the energetically deep trap states located in the bulk rather than surface trap states, that influences the performance of devices the most. The incorporation of the dopant helps to passivate those deep traps and hence enhance the overall device performance.

Doping of the polymers with B(C6F5)3 is also found to reduce the VTh, another important device operating parameter. The latter observation is most likely attributed to the excess number of holes in the channel which leads to trap screening and/or to the reduction of the injection barrier width upon p-doping. Using the data from Figure 3 the total number of extra holes introduced by complexation can be calculated from the shift in threshold voltage according to the following equation:10,62

Δh+¿Cie |V Th (doped )−V Th (pristine )|

(4)Here we consider only the free holes accumu-

lated at the interface and not those being trapped in the bulk of the semiconductor layer. As shown in Figure 4, the amount of free holes introduced by B(C6F5)3 is largely dependent on dopant con-centration for both IP-T and IP-TT, and increases with B(C6F5)3 concentration. Noteworthy is the fact that although the VTh is further reduced at high dopant concentration, and hence the injec-tion barrier as well, the hole mobility reduces too (Figure 3d). We believe this to be due to the intro-duction of structural defects and/or energetic dis-order to the host semiconductor.34 Both of these effects are expected to perturb charge transport across the semiconducting channel with adverse effects on the hole mobility of the device. On the basis of these results we conclude that moderate p-doping of the two polymers with B(C6F5)3 leads to a significant enhancement in the overall device performance by introducing extra free holes and the subsequent screening of deep traps, while at higher dopant concentrations adverse effects as-sociated with structural and/or energetic disorder hinder the hole transport and reduce the hole mo-bility of the devices.

Film Morphology. From the literature it is known that phase segregation of the dopant and the polymer can lead to poor doping effi-ciency.56,63 In an effort to understand whether such unwanted effects are at play here, we studied the influence of the B(C6F5)3 dopant on the film mor-phology of the solid polymer films using AFM and XRD methods. Figure 5a shows the AFM topogra-phy images of the spin-coated pristine (un-doped)

and doped (0.075 eq. and 0.5 eq.) IP-T films. The layers were deposited using identical experimen-tal conditions to those used to fabricate the OTFTs. As can be seen, the un-doped IP-T layer appears amorphous and very smooth. Optimised IP-T films doped with 0.075 eq. of B(C6F5)3, ap-pear to retain the smoothness and continuity. However, increasing the doping to 0.5 eq. of B(C6F5)3 results in a slight increase in the root mean square (RMS) surface roughness of the IP-T layer from 0.276 to 0.312 nm, but with no evi-dence of material segregation/crystallization. The lack of such features suggests that most of the B(C6F5)3 molecules are dispersed into the IP-T as polymer adducts rather than being phase segre-gated. The topography AFM images of pristine and doped IP-TT films display similar surface mor-phology and change in roughness (Figure 5b) upon doping with B(C6F5)3. Specifically, the RMS increases slightly from 0.274 to 0.315 and 0.350 nm for IP-TT films doped with 0, 0.075 and 0.5 eq. of B(C6F5)3, respectively. This is most likely due to the change of inter-chain packing mode of the polymers after insertion of sterically bulky dopant molecules. To investigate this possibility, wide-angle XRD was performed on dropcast films from pristine and complex solutions on polished silicon wafers (Figure S3). Although the polymers are largely amorphous, for IP-T the broad diffrac-tion peak at 2θ = 7.6o disappears when 0.075 and 0.5 eq. of B(C6F5)3 are added, perhaps due to some disruption of the backbone packing. An ad-ditional weak peak emerges at 2θ = 13.8 (d = 6.4 Å) at the high dopant loading. For IP-TT no signifi-cant changes are observed for the film doped with 0.075 eq. of B(C6F5)3. The addition of 0.5 eq. of B(C6F5)3 resulted in changes however, with the weak peaks at 2θ = 9.7o and 2θ = 20.7o dissa-pearing, the peak around 2θ = 13.80 (d = 6.4 Å) becoming better defined and a new broad peak emerging at 2θ = 7.6. These new peaks suggest that high loadings of dopant induce some mor-phological changes, but there are no obvious peaks which could be assigned to domains of phase segregated B(C6F5)3 or its adducts (for ex-ample upon complexation with atmospheric water which is known to afford a crystalline material64), further confirming the good dispersion of the dopant into the polymer.

RMS = 0.276 nm

0 eq.

RMS = 0.275 nm

0.075 eq.

RMS = 0.312 nm

0.5 eq.

(a)

RMS = 0.274 nm

0 eq.

RMS = 0.315 nm

0.075 eq.

RMS = 0.350 nm

0.5 eq.

(b)

Figure 5. AFM topography images of doped IP-T (a) and IP-TT (b) films with different equivalents of B(C6F5)3. Scan size: 1×1 µm2.

4. ConclusionIn conclusion, doping of the Lewis basic indenopy-razine containing polymers with the Lewis acid B(C6F5)3 is shown to dramatically affect the optical and electrical properties of the polymer. Incorpo-ration of moderately p-doped polymer layers as the channel materials in thin-film transistors, leads to an enhanced hole mobility as compared to transistors based on pristine polymers. On the other hand, increasing the dopant concentration above a critical level appears to adversely affect the hole mobility and the device performance de-grades. Based on these findings we propose that moderate p-doping leads to effective trap filling and a positive impact on device operation whilst higher dopant concentrations leads to defect for-mation and unwanted structural disorder with ad-verse effects on transistor performance. We be-lieve that the proposed p-doping approach can be exploited as a generic route for the development of OTFTs with improved operating characteristics.

ASSOCIATED CONTENT Supporting Information. The Supporting Informa-tion is available free of charge via the Internet at http://pubs.acs.org. Additional tables and figures as described in the text (PDF)

AUTHOR INFORMATIONCorresponding Author* E-mail: m.heeney@imperial.ac.uk NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Dr. Scott E Watkins for the PESA measure-ments. This work was made possible by a NPRP award [NPRP 6-452-1-089] from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the responsi-bility of the authors.

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