Tetramethylammonium Iodide Additive for Enhancing the Charge Carrier Mobilities of <scp>Diketopyrrolopyrrole‐Based</scp> Conjugated Polymer in Ambipolar Organic <scp>Field‐Effect</scp> Transistors ^†

Diketopyrrolopyrrole (DPP) is one of the most promising building blocks for constructing polymer semiconductors with high charge-carrier mobilities in organic field-effect transistors (OFETs). In this study, a novel DPP-based conjugated polymer, PDPPy-BDD, was designed and synthesized. The ambipolar transistor characteristics were realized average hole electron 3.5 × 10–3 3.07 10–2 cm2 V−1 s−1, respectively. Both could be successfully enhanced by using tetramethylammonium iodide (NMe4I) additive. Such an enhancement attributed to formation stronger interchain π–π stackings, weakening face-on packing orientation thin film state, higher channel conductivities OFETs. Organic (OFETs) are some actively studied electronic components, which widely employed integrated circuits backplane switches optoelectronic displays. To meet ever-increasing demands flexible electronics, polymers have gained significant attention due their distinct advantages low-cost production, good stretchability, ease processing.[1-3] Since first diketopyrrolopyrrole (DPP)-based semiconductor OFETs reported 2008,[4] DPP planar core has been recognized as construct OFETs.[5-7] important figure-of-merit mobility (μ). Currently, optimized p- or n-channel OFET operation can achieve on order 10 cm2·V−1·s−1 unipolar p-type operation[8, 9] >1 n-type operation.[10] Given that transport inherent characteristic materials,[11] it reasonable synthesize combine single polymer. However, development high- performance polymers, particular operation, still lags behind hinders exploration circuits.[12, 13] general, obtain access high-performance materials, necessary carefully optimize molecular structure, ionization potential, affinity. DPPs intrinsically behave subunit electron-donating thiophene substitution, enhance electron-accepting property, pyridine- flanked (DPPy) developed. Copolymerization various donor groups attempted fine-tune frontier orbitals polymers. Thus, donor–acceptor (D–A) design strategy adopted approach produce low bandgap transporting performances electronics.[7, 14, 15] For instance, 2020, Shi et al. D–A PTFDFT showing excellent semiconducting properties under ambient conditions 1.08 2.23 cm2·V−1·s−1, respectively.[15] despite such successful example, best provide charge- debated. simultaneously injection efficiency sufficient air-stability OFETs, lowest unoccupied orbital (LUMO) energy level should deep –4 eV. there not many strong electron-deficient monomer units. addition, copolymerization units sometimes leads unbalanced hole/electron ratios, results reduced mobilities, current leakage on/off (Ion/Ioff) ratio.[16-18] avoid these issues, pyridine-flanked would suitable main chain block.[19-21] 1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo-[1,2-c: 4,5-c′]dithiophene-4,8-dione (BDD) unit composed electron-rich group, but electron-withdrawing carbonyl substitution effectively lowers highest occupied (HOMO) LUMO levels, utilized weak acceptor type polymers.[22] Notably, Hou synthesized photovoltaic labeled PBDTBDD BDD demonstrated its solar cells (PSCs).[23-25] interesting based applications. context, we became interested resulting so-called dual-acceptor More specifically, work PDPPy-BDD (Scheme S1). thermal, optical, systematically studied. Similar our previous study,[26] incorporation both DPPy into lowered HOMO increased barrier decreased barrier, This beneficial developing dominant semiconductors. Bottom-gate top-contact (BGTC) devices fabricated films annealed at different temperatures (150, 200, 250 °C) thoroughly evaluate electrical properties. device 200 °C showed V-shaped transfer 3.1 further improve carrier consider material processing engineering A crucial element unlocked touted potential electronics doping.[27] Among wide variety chemical species used additives, Luo al.[28] achieved remarkable 24-times charge (PDPPTTT) incorporating ionic additive, (NMe4I). We also recently adding NMe4I, role NMe4I mechanism completely understood.[6] solve molar ratios additive comprehensively investigated. Based strategy, 5.5 (a 57% increase) 3.47 13% microstructures, spectroscopic characterization, parasitic effects investigated understand origin increase introducing NMe4I. Overall, suggest (the structure shown Figure 1a synthesis route given Scheme S1 Supporting Information) Stille coupling polymerization anhydrous toluene solution Pd2(dba)3 P(o-tol)3 catalysts.[29] purified Soxhlet extraction 79.89% yield, weights estimated gel permeation chromatography (GPC, S2): Mn (number-average weight) = 62.3 kg·mol–1; Mw (weight-average 132.0 PDI 2.12. more detailed synthetic procedure characterization data described Information. Thermogravimetric analysis (TGA) differential scanning calorimetry (DSC) study thermal behavior PDPPy-BDD. TGA thermogram nitrogen atmosphere 2a. found exhibit stability decomposition temperature (Td, 5% weight loss) 363 °C, indicating practical applications adequate range annealing optimization during characterization. As 2b, DSC curve no obvious melting crystallization peaks heating cooling processes, respectively.[25] On other hand, glass-transition (Tg) ~183 observed, only relatively stable morphology solid providing structural information about rigid backbone interaction level.[30] related morphologies microstructures will discussed later section. ultraviolet-visible near-infrared (UV−vis−NIR) absorption spectra dilute chloroform as-cast pristine 2c, summarized Table 1. exhibited typical dual-band film. lower band (band I: 400—520 nm) intramolecular (ICT) transition, II: 520—750 derived from transition. clear vibronic λmax 664 nm (0-0) 614 (0-1) solution, 680 629 Compared spectrum, maximum peak state red-shifted approximately 16 nm, demonstrates effective stacking film, likely coplanarity backbone. optical calculated onset, 1.67 determine levels photoelectron spectroscopy air (PESA) measurement performed. dissolved spin-coated glass substrate PESA measurement. onset value drawing tangent baseline, 2d –5.56 By bandgap, –3.89 gaps between function electrode defined injection, determined values gold (Au) –5.1 eV efficient polymer.[26] distributions density functional theory (DFT) calculations. S12. They abridged side chains visualization. computed spectrum S13. features experimental visible region reproduced; 703 (corresponding mostly HOMO-to-LUMO 624 630 HOMO-to-LUMO+1 –5.41 –3.40 eV, fairly agreement than gap. because gap includes broadening effects. evaluated. adopt BGTC configuration (Figure S3). fabrication spin-coating onto octadecyltrimethoxysilane (OTMS)-treated N++-Si/SiO2 substrates previously reported.[6] initially changing inert atmosphere, reduce concentration traps, crystallinity films, contact source/drain electrodes.[31] displayed irrespective temperatures. Figures 3a 3b show examples p-channel curves, output curves 3, S4 S5. (μ) obtained saturation regime extracted S1. 3c S6 distribution It all generally dispersed temperature. 1.6 1.52 cm2·V–1·s–1, respectively, averaged threshold voltage (Vth) –37.5 V 35 V. expected use pyridine-substituted BDD), PDPPy-BDD-based mobilities. voltages improved treatment. Only when 150 Vth (absolute value) slightly –41.1 41.7 same (200 °C). optimal μh μe With annealing, 5.0 2.35 cm2·V–1·s–1. 3d. S7, atomic force microscopy (AFM) images obtained. All manifest fiber-like aggregated morphologies, may root mean square (RMS) 150, comparable 5.446, 5.674, 3.851 6.706 homogeneous surface roughness microstructure, leading voltage. above aiming improving performance, common salt, namely, (NMe4I), selected influence performance. Note tetraalkylammonium salt layer often improves produces memory behavior.[29, 32, 33] Three samples prepared repeating versus 48 : 1, 24 4, S8 S9. always °C. small amount (48 1) added solution. 4a 4b, (57.1% respect film) (13.0% film), respectively (Table 2). case PDPPy-BDD:NMe4I (24 1), 3.29 while significantly suppressed 3.2 cm2·V−1·s−1. tendency ratio Although almost unchanged (3.19 cm2·V−1·s−1), 1.8 Another feature addition reproducibility devices. homogeneity 4c S10. changes 4d. These 1 terms negative voltages. –34.2 –49.1 (1/48 mol%). trend worse increasing amount. reports salts tune balance Vth.[28, 34, 35] Therefore, optimistic results. Molar effect confirmed. mechanisms include contributions (i) conductivity, (ii) charge-transfer doping semiconductors, (iii) interplay microstructure: thin-film (iv) effect: resistance reduction doping.[27, 28] following section, explore possible improvement First, gain insight enhancing mobility, UV−vis−NIR characterize neat 5). did change well-defined redox events occurred 5a). depicted 5b, red-shift longer wavelength absorbance observed suggests π−π interactions noted led degree stackings result consistent Next, Fourier-transform infrared (FT-IR) measured. PDPPy-BDD: difference, S11. probably apparently corroborated integrals (Figures S13 S14). dimer redshift spectrum. agrees observation appear lack coordination dissociation solution). causes transitions iodine-centered states units, absorption. calculations stacked dimers without responsible presence how microstructure influenced AFM grazing-incidence angle X-ray scattering (GIWAXS) measurements carried out. 6a—b, contained interconnected fibrous RMS 3.85 nm. After size area 28.5 6c). ordered large fiber aggregates formed (see highlighted circle 6d), transport. 6e—hshow two-dimensional (2D) GIWAXS patterns respective cross-sectional 1D profiles along Qz Qxy directions. 6e), diffractions assignable lamellar (100) (200), diffraction structures, (010), appeared 6h) 6g) axes, predominant surface. orientated manner 6f). (100), (200) (300) axis suggesting well-ordered 6g). spacing (d-spacing) distance (dπ-π) 20.3 Å 3.65 axis, those S2). clearly induced structure. explained inhibition torsion branched alkyl chains.[28] disadvantageous OFETs.[36] less preferential (weaker (010) intensity 6g), highly reason why factor determining resistance. thus fabricating lengths. (Rc) line method (TLM),[37, 38] way extracting Rc current−voltage widths, plotted total (RT) length gate ranging 40 120 S15—S16. operated 60 (n-type) (VG V) employed. 7a shows fitted three PDPPy- BDD:NMe4I (16 1). values, including resistance, V, S3. seen (2895.2 kΩ∙cm) (3008.6 4643.3 kΩ∙cm Considering contribution addtive (Rch), contributed Generally, additives activation decrease compared (166.7 1133.5 2798.7 kΩ∙cm, after approximated required fill trap before accumulation, gradual number traps electrodes channel.[39] already associated films. summary, new taking account property Systematic characterizations revealed even though increased. concluded final combined action several factors. combination realizing μ regime, W L width length, Ci capacitance per dielectric layer, ID drain-source current, VG out Rigaku TG8120 DSC8230, flow scan rate °C·min–1. Seiko Instruments SPA-400 stiff cantilever Instrument DF-20. procedures fabrication, except source drain deposited. conducted AC-2 spectrometer (Riken-Keiki Co.). measurements. recorded JASCO V-670 spectrophotometer. solutions with/without concentration, 10-mm path quartz cuvette. Solid-state 2000 r/min s 15 min. FT/IR-4100 4000 600 cm−1. JEOL model JNM-ECZ400S/L1 (400 MHz) room Deuterated solvent. Chemical shifts NMR relative residual solvent δ 7.26 1H spectroscopy. 2D-GIWAXS BL40B2 SPring-8 (Hyogo, Japan). beam 1.0 camera 344.8 mm. 2D-scattering acquired photon counting detector (Pilatus3X 2M, Dectris, Ltd.). mounted helium cell radiation damage. measured incident 0.15°, critical external reflection silicon close samples. components vector, Q, parallel perpendicular sample Qz, DFT performed Gaussian 16. B3LYP exchange-correlation D95V basis set used. Absorption time-dependent excited states. details, see W.H. thanks JST SPRING, grant JPMJSP2106 Oversea Study Program Guangzhou Elite Project. T.M. JSPS KAKENHI 19H02786. Q.L. project funded China Postdoctoral Science Foundation (2021M701551). A.K.K.K. acknowledges National Natural (grant no. 62150610496), Department Education Guangdong Province University Innovation (2021KTSCX107), Shenzhen Science, Technology Commission (JCYJ20220530113014033). P.S. thankful QUT financial support Australian Research Council (ARC) Discovery Grant (DP210103006) funding (QUT/322120-0301/07). synchrotron experiments approval JASRI (Proposal No. 2020A0651). authors thank Dr. Hiroyasu Masunaga Noboru Ohta (Japan Synchrotron Radiation Institute: JASRI) assistance experiments. Appendix S1: Information Please note: publisher content functionality any supporting supplied authors. Any queries (other missing content) directed corresponding author article.