Article pubs.acs.org/cm
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, London SW7 2AZ, U.K. Department of Materials and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, U.K. § 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, London SW7 2AZ, U.K. ‡
S Supporting Information *
ABSTRACT: Molecular doping, under certain circumstances, can be used to improve the charge transport 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 indenopyrazine 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 transistors (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 attracted tremendous research interest during recent years due to their broad potential applications in next-generation electronics. Of special interest are solution processable OTFTs which offer the prospect to fabricate large area flexible devices by a range of printing techniques.1−3 The drive to optimize transistor performance has led to the synthesis of many novel organic semiconductor (OSC) materials of various molecular structures, as well as focused efforts to optimize 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 material. 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 re-design and synthesis of new materials. To date, several different approaches for extrinsically doping (i.e., the deliberate addition of a dopant molecule) organic semiconductors have been developed.12−29 One of the most © 2016 American Chemical Society
well investigated is the concept of integer charge transfer between the dopant and the OSC. The latter approach 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 localized 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 example of p-dopant with a suitably high electron affinity is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), whose electon affinity of 5.2 eV is close to the ionization potential 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 ionization 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 frontier molecular orbitals of the OSC and Received: September 5, 2016 Revised: September 30, 2016 Published: October 4, 2016 8016
DOI: 10.1021/acs.chemmater.6b03761 Chem. Mater. 2016, 28, 8016−8024
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Chemistry of Materials
and performance of OTFTs can be optimized leading to an 11fold enhancement of the hole mobility. The channel current on/off ratio was also increased by at least 1 order of magnitude, and the threshold voltages were reduced with increasing dopant concentration.
the p-dopant in a supramolecular or charge transfer complex, rather than by integer charge transfer.28,40,41 In this mechanism, the frontier-orbital hybridization of the complex leads to empty levels within the band gap of the surrounding matrix. Here the supramolecular complex effectively acts as the dopant, and while 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 mechanism also suggested that increasing the electron affinity of the dopant was not sufficient to enhance 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 reported by Zalar et al.42 It was shown that the addition of the bulky Lewis acid tris(pentafluorophenyl)borane [B(C6F5)3] to a polymer containing a Lewis basic pyridyl comonomer 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 photoluminescence of conjugated materials via the donation of electron density from the conjugated system 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 incorporated in thin-film transistors for the purpose of chemical sensing,46 as far as we are aware the potential and efficiency of this doping motif to improve device performance has not been investigated in field-effect transistors until now. In order to demonstrate the potential of this approach, 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 copolymers of indenopyrazine as suitable candidates. Indenopyrazine is an analogue of the well-studied fused aromatic indenofluorene, in which the central benzene ring is replaced by an electron deficient pyrazine. This results in an increase in both ionization potential and electron affinity compared to the indenofluorene analogue.47 The basic pyrazine has two available lone pairs of electrons that may bind to the tris(pentafluorophenyl)borane. Indenopyrazine and its copolymers have previously been investigated as deep blue emitters in organic light-emitting diode (OLED) applications,48,49 as well as donor polymers for high voltage organic solar cells.50 However, the performance of indenopyrazine copolymers in OTFTs has been moderate thus far, likely due to problems with charge injection 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 topgate/bottom-contact OTFT architectures. Importantly the doped OTFT devices are prepared via spin-coating directly from solution as the polymers, 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 doping is performed through exposure of the semiconducting layer to liquids or gaseous dopants and/or at the charge injection contact interface. By adjusting the concentration of dopants, the doping efficiency
2. EXPERIMENTAL SECTION General Methods. 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 Agilent Technologies 1200 series GPC running in chlorobenzene at 80 °C, using two PL mixed B columns in series and calibrated against narrow polydispersity polystyrene standards. UV−vis spectra 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). Photoelectron 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. Samples for PESA were prepared on glass substrates by spin-coating. Differential scanning calorimetry (DSC) measurements were conducted under nitrogen at scan rate of 20 °C/ min with a TA DSC-Q20 instrument. The molecular packing was characterized by wide-angle X-ray diffraction (XRD, PANalytical X’Pert Pro MPD) using the Cu Kα radiation. θ/2θ scans were performed at room temperature to the films drop-cast from the corresponding complex solutions onto silicon substrates. Fermi levels were measured using a KP Technology scanning Kelvin probe system SKP5050 in nitrogen environment at room temperature. General Procedure for Salt Wash Purification for IP-diBr. The monomer IP-diBr (200 mg) was dissolved in dry THF (50 mL). Potassium tert-butoxide (1M solution in THF, 4.5 mL) was added dropwise and the reaction 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 purified product. This was repeated twice. Synthesis of Poly(6,6,12,12-tetraoctyldiindeno[1,2-b:1,2-e]pyrazine-co-2,5-thiophene) (IP-T). 2,8-Dibromo-6,6,2,2tetraoctyldiindeno[1,2-b:1,2-e]pyrazine (IP-diBr; 0.20 g, 0.23 mmol), 2,5-bis(trimethylstannyl)thiophene (0.10 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 microwave 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 (100 mL of MeOH/5 mL of HCl), filtered into a Soxhlet thimble, and extracted (Soxhlet) using methanol, acetone, and hexane. The remaining polymer was removed from the thimble and 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 fibers. Yield = 125 mg, 69%. GPC: Mn = 23 kDa, Đ = 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-tetraoctyldiindeno[1,2-b:1,2-e]pyrazine-co-2,5-thieno[3,2-b]thiophene) (IP-TT). 2,8-Dibromo6,6,12,12-tetraoctyldiindeno[1,2-b:1,2-e]pyrazine (0.2 g, 0.23 mmol), 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene (0.11 g, 0.23 mmol), Pd2(dba)3 (4.2 mg, 0.0046 mmol equiv), 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 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 (100 mL of MeOH/5 mL of HCl), filtered into a Soxhlet thimble, and extracted (Soxhlet) using methanol, acetone, and 8017
DOI: 10.1021/acs.chemmater.6b03761 Chem. Mater. 2016, 28, 8016−8024
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Chemistry of Materials Scheme 1. Synthesis of IP-T, IP-TT, and Structure of B(C6F5)3
Table 1. Optical and Energetic Properties of IP-T and IP-TT λmax (nm)
a
λonset (nm)
polymer
Mn (kg/mol)
Đ
chlorobenzene
film
film
IPa (eV)
Egoptb (eV)
IP-T IP-TT
23 20
2.3 2.1
487 495
492 498
515 525
5.78 5.82
2.41 2.36
Ionization potential was measured by PESA (error, ±0.05 eV). bEgopt was estimated from the onset of film absorption. AFM. AFM images were obtained with a Picoscan PicoSPM LE scanning probe in tapping mode. Samples were prepared by spincoating polymer solutions on plain glass substrates, and solvent residue was removed under vacuum, following the same procedures for fabrication of transistor devices except that dielectric and gate electrodes were not applied.
hexane. The remaining polymer was removed from the thimble and 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 and dried (MgSO4), concentrated, and precipitated from chlorobenzene into cold methanol to yield the polymer as dark red fibers. Yield = 94 mg, 48%. GPC: Mn = 20 kDa, Đ = 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-dichlorobenzene respectively to prepare stock solutions at the concentration of 10 mM. To a 100 μL aliquot of polymer stock solution was added the desired amount of B(C6F5)3 stock solution and o-dichlorobenzene solvent to dilute if needed to reach a final volume of 150 μL, which corresponds to a constant concentration of polymer at 6.67 mM. The undoped solution was prepared by adding 50 μL of o-dichlorobenzene to 100 μL of polymer stock solution. The polymer concentration and molar equivalents of B(C6F5)3 were calculated with respect to the mass of repeat units of each polymer. Fabrication of OTFT Devices. Top-gate/bottom-contact 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 deposited source/drain electrodes were treated with a selfassembled monolayer (SAM) of pentafluorobenzenethiol (PFBT) to improve work function before applying a semiconductor layer. The aforementioned undoped or doped polymer solutions were then spuncast at 2000 rpm for 60 s onto prepatterned substrates. The obtained semiconductor 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, followed by annealing at 100 °C for 30 min to form a dielectric layer of 900 nm. Al (50 nm) gate electrodes 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, respectively. Transistor characterization was carried out under nitrogen using a Keithley 4200 parameter analyzer. The saturation mobility was extracted from the slope of ID1/2 vs VG: 2 2L ⎛ ∂ IDsat ⎞ ⎜⎜ ⎟⎟ μsat = WCi ⎝ ∂VG ⎠
3. RESULTS AND DISCUSSION Synthesis. The indenopyrazine monomer IP-diBr was synthesized according to the previously reported method.49,50 Since the incomplete alkylation of analogous fluorene monomers is known to be a cause of oxidative instability, we carefully removed any partially alkylated product by treatment of the monomer with a strong base, followed by filtration through alumina following the method reported for fluorene.52 The resulting monomer was polymerized by Stille crosscoupling with 2,5-bis(trimethylstannyl)thiophene or 2,5- bis(trimethylstannyl)thieno[3,2-b]thiophene under microwave accelerated conditions (Scheme 1).53 The resulting polymers (designated IP-T and IP-TT) were purified by precipitation and extraction with methanol, acetone, and hexane to remove oligomers and catalysts residues. The remaining polymers were washed with diethyldithiocarbamate to remove Pd residues.54 Following a final precipitation, both polymers were isolated as red fibers 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 absorption spectroscopy, both in solution and as spun-cast thin films. The results are shown in Figure 1 and summarized in Table 1. IP-T has an absorption 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 ionization 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
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Figure 1. UV−vis spectra of IP-T and IP-TT in solution (chlorobenzene) and as thin films.
solution, and further red shifts to 498 nm with a shoulder at 471 nm upon film formation. The absorption onset in film is also shifted to the longer wavelength side, 525 nm, which corresponds to a slightly smaller band gap of 2.36 eV. The ionization 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 difficult. 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 equiv of F4TCNQ, which suggests no effective charge transfer interaction between the guest molecule and polymer matrix, in agreement with the measured energy levels. In considering the use of B(C6F5)3 as a dopant, it is apparent that its bulky structure may cause disturbance of polymer chain packing upon blending, potentially reducing the transistor performance 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 (Supporting Information Figure S1), suggesting an amorphous solid state. The molecular packing of the polymers was also characterized by wide-angle X-ray diffraction (XRD) of dropcast films (Figure S3). Neither of the polymers displayed any distinct diffraction peaks, consistent with largely amorphous films. IP-T exhibited three very broad and weak diffraction bands at 2θ = 7.6° (d = 11.6 Å), 2θ = 9.5° (d = 9.3 Å), and 2θ = 12.4° (d = 7.1 Å), respectively. IP-TT also afforded three weak diffractions at 2θ = 9.7° (d = 9.1 Å), 2θ = 13.8° (d = 6.4 Å), and 2θ = 20.7° (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 equivalents of dopant yet constant polymer concentration 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
Figure 2. UV−vis spectra of undoped and doped IP-T (a) and IP-TT (b) films with different equivalents of B(C6F5)3.
were calculated with respect to the mass of repeat units of each polymer. 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 attributed to an interaction between the dopant and the polymer. The undoped 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 between the indenopyrazine lone pair and B(C6F5)3. Such a complex would be expected to withdraw electron density from the indenopyrazine, resulting in the formation of zwitterionic-type complexes with the dopant and therefore a lower energy charge-transfer-type band.45 Compared to the band gap of the undoped 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 electrons from the indenopyrazine unit to the Lewis acid dopant by adduct formation.44 Similar behavior 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 IPT 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 equiv of B(C6F5)3. This is in contrast to the solution processing of films 8019
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Figure 3. Representative plots of the transfer characteristics of transistors based on undoped 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 IPTT (d).
of polymers doped with F4TCNQ, which requires careful processing in many cases due to the reduced solubility of the oxidized polymer and/or the formation of aggregates of charge transfer salts.56,57 The nature of the interaction between the polymer 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 conventional integer charge transfer mechanism. Rather coordination of one of the N lone pairs on the indenopyrazine, 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 resulting “pyrazinium” like cations are known to be strongly electrophilic. Thus, the resulting complex is expected to 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 mechanism is similar to that described by Méndez et al.,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 unoccupied molecular orbital) of dopant. The resultant complex has a lower HOMO than the surrounding matrix but also a lower LUMO. The difference in our case is that it is not the delocalized 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 perspective, than the intramolecular charge transfer complex, where the intermolecular resonance integral was shown to be both important and subtly dependent on steric interactions.
To further establish that a Lewis acid−base interaction is occurring between the indenopyrazine polymers and B(C6F5)3 rather than an ICT or charge transfer complex (or frontier molecular hybridization of OSC and dopant), we treated the doped polymers with pyridine, which is a stronger base than (indeno)pyrazine but is substantially harder to oxidize. Thus, if an ICT transition or charge transfer interaction was occurring, the presence of pyridine should not prevent it. However, 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. Indeed we observe the latter, with the optical properties of the film recovered upon treatment with pyridine (see Figure S5), consistent with the observations 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 absorption peak in the near-infrared region until 1100 nm before and after exposure to pyridine, a feature that should be present for the conventional 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 equiv 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 toward the HOMO level reaching a stable value of 5.14 eV, as expected for a p-type dopant. Transistor Fabrication and Characterization. After confirming the complex formation between dopant and hosts, the electrical performance 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 spin-cast on substrates with prepatterned Au source/drain electrodes, to obtain the pristine or doped polymer OTFTs. The spin-cast films were stored under vacuum for 30 min to remove solvent 8020
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Chemistry of Materials residue and used without thermal annealing. After deposition of the dielectric and gate electrodes, the two sets of OTFTs were electrically characterized. In Figure 3a we show the transfer characteristics of OTFTs based on the pristine (undoped) and p-doped IP-T films with different dopant concentrations. The performance of the full series of devices is summarized in Table S1. The undoped IP-T device displays characteristics of a typical hole transporting transistor, with field-effect hole mobility of 0.086 ± 0.034 cm2 V−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 ionization 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 1 order of magnitude while the off current remains largely unchanged. The hole mobility is also found to increase sharply with the addition of just 0.01 equiv of dopant and continues to increase with dopant concentration up to a maximum of 0.62 ± 0.16 cm2 V−1 s−1 for 0.075 equiv of dopant. Further increase in the dopant level to 0.5 equiv reduces the hole mobility to values similar to those obtained for the undoped OTFTs. The threshold voltage of the resulting OTFTs is also found to shift toward more positive gate voltages with increasing dopant concentration, yielding −75.8 ± 5.1 V for 0.01 equiv of B(C6F5)3 and −34.3 ± 1.7 V for 0.5 equiv of B(C6F5)3, as shown in Figure 3b. The steep change in characteristics upon addition of a relatively small concentration of dopant is attributed to the all-important hole trap filling effect.31,32 Similar dopant concentration-dependent performance is also observed 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 cm2 V−1 s−1, OTFTs based on 0.075 equiv of B(C6F5)3-doped IP-TT exhibit maximum hole mobility of 0.22 ± 0.025 cm2 V−1 s−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 equiv of B(C6F5)3-doped devices. The on/ off ratio is also increased by more than 1 order of magnitude, highlighting a potential advantage of the particular doping approach explored here. To determine whether the density of trap states influences the device performance, we calculated the interface trap density (Ntr) and trap concentration (Dtr, per unit area and unit energy) from the corresponding transistor transfer characteristics using Ntr =
Ci |VTh − Von| e
⎞ C⎛ eS Dtr = 2i ⎜ − 1⎟ ⎠ e ⎝ kT ln(10)
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.
appears to improve for B(C6F5)3 concentrations in the range 0.01−0.075 equiv and reduce for concentrations >0.075 equiv. 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 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+ =
(2)
Ci |VTh(doped) − VTh(pristine)| e
(4)
Here we consider only the free holes accumulated 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 concentration for both IP-T and IP-TT and increases with B(C6F5)3 concentration. Noteworthy is the fact that although VTh is further reduced at high dopant concentration, and hence the injection barrier as well, the hole mobility reduces too (Figure 3d). We believe this to be due to the introduction of structural defects and/or energetic disorder 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
(3)
where e is the elementary charge, Ci the geometric capacitance of gate dielectric, Von the onset voltage, k the Boltzmann constant, T the measuring temperature , and S the subthreshold swing.60,61 As shown in Figure 4, for both IP-T and IP-TT, the Ntr appears to be relatively 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 performance and in particular the hole mobility evolution. Specifically, the performance of the resulting OTFTs 8021
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peak emerging at 2θ = 7.6°. These new peaks suggest that high loadings of dopant induce some morphological changes, but there are no obvious peaks which could be assigned to domains of phase segregated B(C6F5)3 or its adducts (for example upon complexation with atmospheric water which is known to afford a crystalline material64), further confirming the good dispersion of the dopant into the polymer.
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 associated with structural and/or energetic disorder hinder the hole transport and reduce the hole mobility 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 efficiency.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 morphology of the solid polymer films using AFM and XRD methods. Figure 5a shows
4. CONCLUSION In conclusion, doping of the Lewis basic indenopyrazine containing polymers with the Lewis acid B(C6F5)3 is shown to dramatically affect the optical and electrical properties of the polymer. Incorporation 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 degrades. Based on these findings we propose that moderate p-doping leads to effective trap filling and a positive impact on device operation while higher dopant concentrations lead to defect formation and unwanted structural disorder with adverse effects on transistor performance. We believe that the proposed p-doping approach can be exploited as a generic route for the development of OTFTs with improved operating characteristics.
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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.
ASSOCIATED CONTENT
S Supporting Information *
the AFM topography images of the spin-coated pristine (undoped) and doped (0.075 and 0.5 equiv) IP-T films. The layers were deposited using identical experimental conditions to those used to fabricate the OTFTs. As can be seen, the undoped IP-T layer appears amorphous and very smooth. Optimized IP-T films doped with 0.075 equiv of B(C6F5)3, appear to retain the smoothness and continuity. However, increasing the doping to 0.5 equiv 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 evidence 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 segregated. The topography AFM images of pristine and doped IP-TT films display similar surface morphology 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 equiv of B(C6F5)3, respectively. This is most likely due to the change of interchain packing mode of the polymers after insertion of sterically bulky dopant molecules. To investigate this possibility, wide-angle XRD was performed on drop-cast films from pristine and complex solutions on polished silicon wafers (Figure S3). Although the polymers are largely amorphous, for IP-T the broad diffraction peak at 2θ = 7.6° disappears when 0.075 and 0.5 equiv of B(C6F5)3 are added, perhaps due to some disruption of the backbone packing. An additional weak peak emerges at 2θ = 13.8° (d = 6.4 Å) at the high dopant loading. For IP-TT no significant changes are observed for the film doped with 0.075 equiv of B(C6F5)3. The addition of 0.5 equiv of B(C6F5)3 resulted in changes; however, with the weak peaks at 2θ = 9.7° and 2θ = 20.7° dissapearing, the peak around 2θ = 13.8° (d = 6.4 Å) becoming better defined and a new broad
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03761. Additional tables and figures as described in the text (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Scott E. Watkins for the PESA measurements. This work was made possible by a NPRP Award [NPRP 6-4521-089] from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the authors.
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