By considering the qualitative benefits associated with solution rheology and mechanical properties of polymer semiconductors, it is expected that polymer-based electronic devices will soon enter our daily lives as indispensable elements in a myriad of flexible and ultra low-cost flat panel displays. Despite more than a decade of research focused on designing and synthesizing state-of-the-art polymer semiconductors for improving charge transport characteristics, the current mobility values are still not sufficient for many practical applications. The confident mobility in excess of ∼10 cm(2)/V·s is the most important requirement for enabling the realization of the aforementioned near-future products. We report on an easily attainable donor-acceptor (D-A) polymer semiconductor: poly(thienoisoindigo-alt-naphthalene) (PTIIG-Np). An unprecedented mobility of 14.4 cm(2)/V·s, by using PTIIG-Np with a high-k gate dielectric poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)), is achieved from a simple coating processing, which is of a magnitude that is very difficult to obtain with conventional TFTs by means of molecular engineering. This work, therefore, represents a major step toward truly viable plastic electronics.
Structure−property relationships associated with a hybrid siloxaneterminated hexyl chain (SiC6), photophysics, molecular packing, thin-film morphology, and charge carrier transport are reported for two novel naphthalene diimide (NDI)-based polymers; P(NDI2SiC6-T2) consists of NDI and bithiophene (T2) repeating units, while for P(NDI2SiC6-TVT), the (E)-2-(2-(thiophen-2-yl)-vinyl)thiophene (TVT) units are introduced into the NDI-based backbone. The analysis of the optical spectra shows that the preaggregation of these polymers in solution is highly sensitive to the choice of solvent such that the films prepared by using different solvents can be "tuned" with regard to their degrees and types of the aggregates. In-depth morphology investigations (atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXD), and near-edge X-ray absorption fine structure (NEXAFS)) combined with device optimization studies are used to probe the interplay between molecular structure, molecular packing, and OFET mobility. It is found that the polymer films cast as a coating from chloroform (CF) solvent favor a mixed face-on and edge-on orientation, while 1-chloronaphthalene (CN)-cast films favor an almost entirely edge-on orientation, resulting in a difference in mobility between CF-and CN-cast devices. Within this work, the annealed P(NDI2SiC6-T2) device fabricated from CF, despite showing a less densely packed organization, shows the highest electron mobility of up to 1.04 cm 2 /V· s due to a highly balanced face-on to edge-on ratio. This work, for the first time, advances our understanding for how the balanced face-on to edge-on ratio plays a dramatic role in facilitating charge transport, opening a new charge-transport mechanism in electronic devices. ■ INTRODUCTIONSignificant efforts have been made toward the development of solution-processable polymeric materials for organic field-effect transistors (OFETs) because of their facile processability in solution at a relatively low temperature, which can open a new paradigm in application for flexible electronics and device manufacturing through cost-effective graphic art printing processes. 1−9 Current state-of-the-art polymers have been developed for use in p-channel OFETs with hole mobilities surpassing 10 cm 2 /V·s; 1,10−12 however, the distinct lack of highperformance, ambient-stable, solution processed n-channel OFETs has hindered the development of low cost organic complementary circuits. 13−23 Therefore, not only is the development of reliable n-channel polymeric semiconductors a crucial issue but also a challenge for this type of polymers is a deep understanding and control of the layer morphology in organic electronics.A breakthrough in n-channel polymers occurred with the development of P(NDI2OD-T2) polymer, containing naphthalene diimide (NDI) and bithiophene (T2) repeating units, by Facchetti and co-workers, demonstrating unprecedented OFET characteristics with electron mobility of up to 0.85 cm 2 / V·s in the device architecture. 24,25 Interestingly, P(NDI2OD-...
Two acceptor-acceptor (A-A) type copolymers (PIIG-BT and PIIG-TPD) with backbones composed exclusively of electron-deficient units are designed and synthesized. Both copolymers show unipolar n-type operations. In particular, PIIG-BT shows electron mobility of up to 0.22 cm(2) V(-1) s(-1). This is a record value for n-type copolymers based on lactam cores.
Recognizing the importance of molecular coplanarity and with the aim of developing new, ideal strong acceptor-building units in semiconducting polymers for highperformance organic electronics, herein we present a simplified single-step synthesis of novel vinylene-and acetylene-linked bis-benzothiadiazole (VBBT and ABBT) monomers with enlarged planarity relative to a conventionally used acceptor, benzothiadiazole (BT). Along these lines, four polymers (PDPP-VBBT, PDPP-ABBT, PIID-VBBT, and PIID-ABBT) incorporating either VBBT or ABBT moieties are synthesized by copolymerizing with centro-symmetric ketopyrrole cores, such as diketopyrrolopyrrole (DPP) and isoindigo (IID), and their electronic, physical, and transistor properties are studied. These polymers show relatively balanced ambipolar transport, and PDPP-VBBT yields hole and electron mobilities as high as 0.32 and 0.13 cm 2 V −1 s −1 , respectively. Interestingly, the acetylenic linkages lead to enhanced electron transportation in ketopyrrole-based polymers, showing a decreased threshold voltage and inverting voltage in the transistor and inverter devices, respectively. The IID-based BBT polymers exhibit the inversion of the dominant polarity depending on the type of unsaturated carbon bridge. Owing to their strong electron-accepting ability and their highly π-extended and planar structures, VBBT and ABBT monomers should be extended to the rational design of high-performance polymers in the field of organic electronics.
Two donor− (D−) acceptor (A) type polymers based on a soluble chromophore of phenothiazine (PT) unit that is a tricyclic nitrogen−sulfur heterocycle, have been synthesized by introducing an electron-deficient benzothiadiazole (BT) building block copolymerized with either PT or phenothiazine-S,S-dioxide (PT-SS) unit as an oxidized form of PT. The resulting polymers, PPTDTBT and PPTDTBT-SS are fully characterized by UV−vis absorption, electrochemical cyclic voltammetry, Xray diffraction (XRD), and DFT theoretical calculations. We find that the maximum absorption of PPTDTBT is not only markedly red-shifted with respect to that of PPTDTBT-SS but also its band gap as well as molecular energy levels are readily tuned by the insertion of S,S-dioxides into the polymer. The main interest is focused on the electronic applications of the two polymers in organic field-effect transistors (OFETs) as well as conventional and inverted polymeric solar cells (PSCs). PPTDTBT is a typical p-type polymer semiconductor for OFETs and conventional PSCs based on this polymer and PC 71 BM show a power conversion efficiency (PCE) of 1.69%. In case of PPTDTBT-SS, the devices characteristics result in: (i) 1 order of magnitude higher hole mobility (μ = 6.9 × 10 −4 cm 2 V −1 s −1 ) than that obtained with PPTDTBT and (ii) improved performance of the inverted PSCs (1.22%), compared to its conventional devices. Such positive features can be accounted for in terms of closer packing molecular characteristics owing either to the effects of dipolar intermolecular interactions orientated from the sulfonyl groups or the relatively high coplanarity of PPTDTBT-SS backbone.
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