Influence of Semiconductor Thickness and Molecular Weight on the Charge Transport of a Naphthalenediimide-Based Copolymer in Thin-Film Transistors

The N-type semiconducting polymer, P­(NDI2OD-T2), with different molecular weights (MW = 23, 72, and 250 kg/mol) was used for the fabrication of field-effect transistors (FETs) with different semiconductor layer thicknesses. FETs with semiconductor layer thicknesses from ∼15 to 50 nm exhibit similar electron mobilities (<i>μ</i>’s) of 0.2–0.45 cm² V^–1 s^–1. Reduction of the active film thickness led to decreased <i>μ</i> values; however, FETs with ∼2 and ∼5 nm thick P­(NDI2OD-T2) films still exhibit substantial <i>μ</i>’s of 0.01–0.02 and ∼10^–4 cm² V^–1 s^–1, respectively. Interestingly, the lowest molecular weight sample (P-23, MW ≈ 23 kg/mol, polydispersity index (PDI) = 1.9) exhibited higher <i>μ</i> than the highest molecular weight sample (P-250, MW ≈ 250 kg/mol, PDI = 2.3) measured for thicker devices (15–50 nm). This is rather unusual behavior because typically charge carrier mobility increases with MW where improved grain-to-grain connectivity usually enhances transport events. We attribute this result to the high crystallinity of the lowest MW sample, as confirmed by differential scanning calorimetry and X-ray diffraction studies, which may (over)­compensate for other effects

Efficient Tin-Free Route to a Donor–Acceptor Semiconducting Copolymer with Variable Molecular Weights

For the fabrication of efficient photovoltaic devices and thin-film transistors, π-conjugated polymers with high molecular weight are desirable as they frequently show superior charge transport, morphological, and film-forming properties. Herein, we present an extremely fast tin-free method to polymerize a naphthalene diimide-dithiophene-based anion-radical monomer in the presence of Pd catalyst having bulky and electron-rich tritert-butylphosphine ligands (Pd/P^<i>t</i>Bu₃). With this method, the corresponding semiconducting polymer, PNDIT2 (also known as P­(NDI2OD-T2 or N2200) with a molecular weight in excess of 1000 kg/mol can be obtained quickly at room temperature and at rather low catalyst concentrations. In general, molecular weights of resulting polymer can be regulated by reaction conditions (e.g., catalyst concentration and reaction time). Besides high molecular weight PNDIT2 (e.g., with <i>M</i>_N ∼ 350 kg/mol, <i>Đ</i>_M =2.9), PNDIT2 with moderate molecular weight (e.g., <i>M</i>_N ∼ 110 kg/mol, <i>Đ</i>_M = 2.3) and low molecular weight (e.g., <i>M</i>_W ∼ 12 kg/mol, <i>Đ</i>_M = 1.9), can also be obtained. It was found that thus-prepared PNDIT2 exhibits field-effect electron mobilities of up to ∼0.31 cm²/(V s), similar to the Stille-derived N2200 control polymer (up to ∼0.33 cm²/(V s)). Preliminary studies demonstrated that Pd/P^<i>t</i>Bu₃ catalyst is remarkably efficient in polymerizing of other anion-radical monomers, such as isoindigo-, and diketopyrrolopyrrole-based ones, although conventional Ni and Pd catalysts (e.g., Ni­(dppp)­Cl₂, Ni­(dppp)­Cl₂, Pd­(PPh₃)₄) failed to polymerize these monomers

Molecular Doping of a High Mobility Diketopyrrolopyrrole–Dithienylthieno[3,2‑<i>b</i>]thiophene Donor–Acceptor Copolymer with F6TCNNQ

Herein we present a molecular doping of a high mobility diketopyrrolopyrrole–dithienylthieno­[3,2-<i>b</i>]­thiophene donor–acceptor copolymer poly­[3,6-(dithiophene-2-yl)-2,5-di­(6-dodecyl­octadecyl)­pyrrolo­[3,4-<i>c</i>]­pyrrole-1,4-dione-<i>alt</i>-thieno­[3,2-<i>b</i>]­thiophene], PDPP­(6-DO)₂TT, with the electron-deficient compound hexafluoro­tetracyano­naphthoquino­dimethane (F6TCNNQ). Despite a slightly negative HOMO_donor–LUMO_acceptor offset of −0.12 eV which may suggest a reduced driving force for the charge transfer (CT), a partial charge CT was experimentally observed in PDPP­(6-DO)₂TT:F6TCNNQ by absorption, vibrational, and electron paramagnetic resonance spectroscopies and predicted by density functional theory calculations. Despite the modest CT, PDPP­(6-DO)₂TT:F6TCNNQ films possess unexpectedly high conductivities up to 2 S/cm (comparable with the conductivities of the benchmark doped polymer system P3HT:F4TCNQ having a large positive offset). The observation of the high conductivity in doped PDPP­(6-DO)₂TT films can be explained by a high hole mobility in PDPP­(6-DO)₂TT blends which compensates a lowered (relatively to P3HT:F4TCNQ) concentration of free charge carriers. We also show that F6TCNNQ-doped P3HT, the system which has not been reported so far to the best of our knowledge, exhibits a conductivity up to 7 S/cm, which exceeds the conductivity of the benchmark P3HT:F4TCNQ system