EDOT‐diketopyrrolopyrrole copolymers for high bulk hole mobility and near infrared absorption

To obtain novel low‐bandgap materials with tailored hole‐transport properties and extended absorption, electron rich 3,4‐ethylenedioxythiophene is introduced as a comonomer in diketopyrrolo[3,4‐c]pyrrole copolymers with different aryl flanking units. The polymers are characterized by absorption and photoluminescence spectroscopy, dynamic scanning calorimetry, cyclic voltammetry, and X‐ray diffraction. The charge transport properties of these new materials are studied carefully using an organic field effect transistor geometry where the charge carriers are transported over a narrow channel at the semiconductor/dielectric interface. These results are compared to bulk charge carrier mobilities using space‐charge limited current (SCLC) measurements, in which the charge carrier is transported through the complete film thickness of several hundred nanometers. Finally, charge carrier mobilities are correlated with the electronic structure of the compounds. We find that in particular the thiophene‐flanked copolymer PDPP[T]₂‐EDOT is a very promising candidate for organic photovoltaics, showing an absorption response in the near infrared region with an optical bandgap of 1.15 eV and a very high bulk hole mobility of 2.9 × 10^?4 cm² V^?1 s^?1 as measured by SCLC. This value is two orders of magnitudes higher than SCLC mobilities reported for other polydiketopyrrolopyrroles and is in the range of the well‐known hole transporting polymer poly(3‐hexylthiophene). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 639–648     

Enhanced mobility and environmental stability in all organic field‐effect transistors: The role of high dipole moment solvent

Low‐operating voltage, high mobility, and stable organic field‐effect transistors (OFETs) using polymeric dielectrics such as pristine poly(4‐vinyl phenol) (PVP) and poly(methyl methacrylate) (PMMA), dissolved in solvents of high dipole moment, have been achieved. High dipole moment solvents such as propylene carbonate and dimethyl sulfoxide used for dissolving the polymer dielectric enhance the charge carrier mobilities by three orders of magnitude in pentacene OFETs compared with low dipole moment solvents. Fast switching circuits with patterned gate PVP‐based pentacene OFETs demonstrated a switching frequency of 75 kHz at input voltages of |5 V|. The frequency response of the OFETs is attributed to a high degree of dipolar‐order in dielectric films obtained from high‐polarity solvents and the resulting energetically ordered landscape for transport. Remarkably, these pentacene‐based OFETs exhibited high stability under bias stress and in air with negligible shifts in the threshold voltage. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1533–1542     

Theoretical study on charge transport of quinacridone polymorphs

The charge carrier transporting ability in the polymorphism of quinacridone (QA) has been studied using density‐functional theory and Marcus charge transport theory. The theoretical results indicated quinacridone has good electron transport ability and electron mobilities of all the polymorphism are at 10^?2 magnitude. But its hole mobility, which varied with the different molecular packing, is at range of 10^?1–10^?3 magnitude. The difference of charge carrier mobilities among the polymorphism is originated from the different packing mode. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Effect of Crystallization Kinetics on Microstructure and Charge Transport of Polythiophenes

We have examined the effects of crystallization kinetics of poly(3‐hexylthiophene) and poly[2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno(3,2‐b)thiophene] on microstructure and charge transport. Rapid crystallization increases the density of tie molecules in polythiophenes. As a consequence, ordered regions are better connected resulting in higher charge carrier mobilities. Our results suggest that controlling the crystallization kinetics might be an important factor for maximizing the charge mobility in semicrystalline polythiophene thin films.     

Super‐ and Ferroelastic Organic Semiconductors for Ultraflexible Single‐Crystal Electronics

Like silicon, single crystals of organic semiconductors are pursued to attain intrinsic charge transport properties. However, they are intolerant to mechanical deformation, impeding their application in flexible electronic devices. Such contradictory properties, namely exceptional molecular ordering and mechanical flexibility, are unified in this work. We found that bis(triisopropylsilylethynyl)pentacene (TIPS‐P) crystals can undergo mechanically induced structural transitions to exhibit superelasticity and ferroelasticity. These properties arise from cooperative and correlated molecular displacements and rotations in response to mechanical stress. By utilizing a bending‐induced ferroelastic transition of TIPS‐P, flexible single‐crystal electronic devices were obtained that can tolerate strains (?) of more than 13 % while maintaining the charge carrier mobility of unstrained crystals (μ>0.7 μ₀). Our work will pave the way for high‐performance ultraflexible single‐crystal organic electronics for sensors, memories, and robotic applications.     

Influence of lattice dynamics on charge transport in the dianthra[2,3-b:2',3'-f]-thieno[3,2-b]thiophene organic crystals from a theoretical study

The influence of lattice dynamics on carrier mobility has received much attention in organic crystalline semiconductors, because the molecular components are held together by weak interactions and the transfer integrals between neighboring molecular orbitals are extremely sensitive to small nuclear displacements. Recently, it has been shown that the dynamic disorder has little effect on hole mobility in the ab plane of pentacene, but a reasonable explanation is absent for such a puzzle. To better understand the effect of lattice vibrations on carrier transport, a further study is required for other organic materials. In this work, a mixed molecular dynamic and quantum-chemical methodology is used to assess the effect of nuclear dynamics on hole mobility in the dianthra[2,3-b:2',3'-f]-thieno[3,2-b]thiophene (DATT) crystals which exhibit high air stability with the hole mobility as large as that in rubrene-based devices. It is found that the lattice vibrations lead to an increasing encumbrance for hole transport in the ab plane of the DATT crystals as the temperature increases. By comparing the crystal structures of DATT and pentacene, the reduced hole mobility in DATT is attributed to the unsymmetric arrays of nearest-neighboring molecular dimers in the ab plane, because the electronic coupling exhibits unbalanced thermal fluctuations for the nearest-neighboring dimers which then induces a stronger oscillation for carriers along the directions with asymmetric packing. To further relate the dynamic disorder with hole transport, the variations of anisotropic mobilities are also analyzed. As a result, the negligible effect of lattice dynamics on the hole mobility in pentacene is explained by the centrosymmetric molecular packing of the nearest-neighboring molecular pairs in the ab plane.     

First‐Principles Investigation of Anisotropic Electron and Hole Mobility in Heterocyclic Oligomer Crystals

Based on quantum chemistry calculations combined with the Marcus–Hush electron transfer theory, we investigated the charge‐transport properties of oligothiophenes (nTs) and oligopyrroles (nPs) (n=6, 7, 8) as potential p‐ or n‐type organic semiconductor materials. The results of our calculations indicate that 1) the nPs show intrinsic hole mobilities as high as or even higher than those of nTs, and 2) the vertical ionization potentials (VIPs) of the nPs are about 0.6–0.7 eV smaller than the corresponding VIPs of the nTs. Based on their charge‐transport ability and hole‐injection efficiency, the nPs have potential as p‐type organic semiconducting materials. Furthermore, it was also found that the maximum values of the electron‐transfer mobility for the nTs are larger by one‐to‐two orders of magnitude than the corresponding maximum values of hole‐transfer mobility, which suggests that the nTs have the potential to be developed as promising n‐type organic semiconducting materials owing to their electron mobility.     

Influence of Backbone Chlorination on the Electronic Properties of Diketopyrrolopyrrole (DPP)‐Based Dimers

Chlorination of π‐conjugated backbones is garnering great interest because of fine‐tuning electronic properties of conjugated materials for organic devices. Herein we report a synthesis of thiophene‐based diketopyrrolopyrrole (DPP) dimers and their chlorinated counterparts by introducing a chlorine atom in the outer thiophene ring to investigate the influence of the chlorination on charge transport. The backbone chlorination lowers both the HOMO and the LUMO of the dimers and leads to a blue‐shift of maximum absorption in compared to unsubstituted counterparts. X‐ray analysis reveals that the chlorine atom prompts the outer thiophene ring out of the planarity of the backbone with a relatively large torsional angle. The chlorinated dimers exhibit slipped one‐dimensional packing decorated with multiple intermolecular interactions, because of a combination of a negative inductive effect and a positive mesomeric effect of the halogen atom, which might facilitate charge transport within the oligomeric backbones. The mobility in the single‐crystal OFET devices of the chlorinated dimers is up to 1.5 cm² V^?1 s^?1, which is two times higher than that of the non‐chlorinated DPP dimers. Our results indicate that the chlorine atom plays a key role in directing non‐covalent interactions to lock the slipped stacks, enabling electronic coupling between adjacent molecules for efficient charge transport. In addition, our results also demonstrate that these DPP dimers with straight n‐octyl chains exhibit higher mobilities than the dimers with branched 2‐ethylhexyl chains.     

Simultaneous morphological stability and high charge carrier mobilities in donor–acceptor block copolymer/PCBM blends

Donor–acceptor block copolymers (BCP), incorporating poly(3‐hexylthiophene) (P3HT), and a polystyrene copolymer with pendant fullerenes (PPCBM) provide desired stable nanostructures, but mostly do not exhibit balanced charge carrier mobilities. This work presents an elegant approach to match hole and electron transport in BCP by blending with molecular PCBM without causing any macrophase separation. An insufficient electron mobility of PPCBM can be widely compensated by adding PCBM which is monitored by the space‐charge limited current method. Using X‐ray diffraction, atomic force microscopy, and differential scanning calorimetry, we verify the large miscibility of the PPCBM:PCBM blend up to 60 wt % PCBM load forming an amorphous, molecularly mixed fullerene phase without crystallization. Thus, blending BCP with PCBM substantially enhances charge transport achieving an electron mobility of μ_e=(3.2 ± 1.7) × 10^?4 cm²V^?1s^?1 and hole mobility of μ_h=(1.8 ± 0.6) × 10^?3 cm²V^?1s^?1 in organic field‐effect transistors (OFET). The BCP:PCBM blend provides a similarly high ambipolar charge transport compared to the established P3HT:PCBM system, but with the advantage of an exceptionally stable morphology even for prolonged thermal annealing. This work demonstrates the feasibility of high charge transport and stable morphology simultaneously in a donor–acceptor BCP by a blend approach. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1125–1136     

Interplay of structural dynamics and electronic effects in an engineered assembly of pentacene in a metal–organic framework

Charge carrier mobility is an important figure of merit to evaluate organic semiconductor (OSC) materials. In aggregated OSCs, this quantity is determined by inter-chromophoric electronic and vibrational coupling. These key parameters sensitively depend on structural properties, including the density of defects. We have employed a new type of crystalline assembly strategy to engineer the arrangement of the OSC pentacene in a structure not realized as crystals to date. Our approach is based on metal–organic frameworks (MOFs), in which suitably substituted pentacenes act as ditopic linkers and assemble into highly ordered π-stacks with long-range order. Layer-by-layer fabrication of the MOF yields arrays of electronically coupled pentacene chains, running parallel to the substrate surface. Detailed photophysical studies reveal strong, anisotropic inter-pentacene electronic coupling, leading to efficient charge delocalization. Despite a high degree of structural order and pronounced dispersion of the 1D-bands for the static arrangement, our experimental results demonstrate hopping-like charge transport with an activation energy of 64 meV dominating the band transport over a wide range of temperatures. A thorough combined quantum mechanical and molecular dynamics investigation identifies frustrated localized rotations of the pentacene cores as the reason for the breakdown of band transport and paves the way for a crystal engineering strategy of molecular OSCs that independently varies the arrangement of the molecular cores and their vibrational degrees of freedom.

Pentacene assembled into 1D arrays using a metal–organic framework (MOF) approach. This cofacial packing motif, which is not present in pentacene bulk, shows an interesting interplay of band-like and hopping-type transport.     

Formation of intra-island grain boundaries in pentacene monolayers

To assess the formation of intra-island grain boundaries during the early stages of pentacene film growth, we studied sub-monolayers of pentacene on pristine silicon oxide and silicon oxide with high pinning centre density (induced by UV/O(3) treatment). We investigated the influence of the kinetic energy of the impinging molecules on the sub-monolayer growth by comparing organic molecular beam deposition (OMBD) and supersonic molecular beam deposition (SuMBD). For pentacene films fabricated by OMBD, higher pentacene island-density and higher polycrystalline island density were observed on UV/O(3)-treated silicon oxide as compared to pristine silicon oxide. Pentacene films deposited by SuMBD exhibited about one order of magnitude lower island- and polycrystalline island densities compared to OMBD, on both types of substrates. Our results suggest that polycrystalline growth of single islands on amorphous silicon oxide is facilitated by structural/chemical surface pinning centres, which act as nucleation centres for multiple grain formation in a single island. Furthermore, the overall lower intra-island grain boundary density in pentacene films fabricated by SuMBD reduces the number of charge carrier trapping sites specific to grain boundaries and should thus help achieving higher charge carrier mobilities, which are advantageous for their use in organic thin-film transistors.     

Pendant perylene polymers with high electron mobility

Four different perylene side‐chain semiconductor polymers, synthesized by a combination of “click” chemistry and nitroxide‐mediated radical polymerization, are compared in terms of their optical, electrochemical, and charge transport properties. The nature of the solubilizing side chains and the chromophoric π‐conjugation system of the pendant perylene moieties are systematically changed. Two poly(perylene bisimide)s with hydrophobic (PPBI 1) and hydrophilic substituents (PPBI 2) are compared with poly(perylene diester benzimidazole) (PPDEB) and poly(perylene diester imide) (PPDEI). Optical properties are investigated by UV/vis and photoluminescence spectroscopy, and charge transport is studied by organic field effect transistor and space‐charge‐limited current measurements. Cyclic voltammetry is used to estimate highest occupied molecular orbital and lowest unoccupied molecular orbital levels. The extended π‐conjugation system of PPDEB leads to a broader absorption in the visible region when compared with PPDEI and the PPBIs. Although absorption properties of PPDEB could be considerably improved by varying the perylene core, the charge carrier mobility could be drastically improved by tuning the substituents. Very high electron mobilities of 1 × 10^?2 cm² V^?1 s^?1 were achieved for PPBI 2 carrying oligoethyleneglycol substituents. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1480–1486     

N‐Heterocyclic‐Carbene‐Treated Gold Surfaces in Pentacene Organic Field‐Effect Transistors: Improved Stability and Contact at the Interface

N‐Heterocyclic carbenes (NHCs), which react with the surface of Au electrodes, have been successfully applied in pentacene transistors. With the application of NHCs, the charge‐carrier mobility of pentacene transistors increased by five times, while the contact resistance at the pentacene–Au interface was reduced by 85 %. Even after annealing the NHC–Au electrodes at 200 °C for 2 h before pentacene deposition, the charge‐carrier mobility of the pentacene transistors did not decrease. The distinguished performance makes NHCs as excellent alternatives to thiols as metal modifiers for the application in organic field‐effect transistors (OFETs).     

Synthesis and characterization of fused‐thiophene containing naphthalene diimide n‐type copolymers for organic thin film transistor and all‐polymer solar cell applications

Naphthalene diimide copolymers are attractive n‐type materials due to their high electron affinities, high electron mobilities, and exceptional stability. Herein, we report a series of NDI‐fused‐thiophene based copolymers with each copolymer differing in the number of fused thiophenes in the donor monomer. Increasing the number of fused‐thiophene moieties within an NDI‐copolymer backbone is shown to not only enable tuning of the electronic structure but also improve charge mobilities within the active layer of organic field‐effect transistors. Electron mobilities and on/off ratios as high as 0.012 cm² V^?1 s^‐1 and I_on/I_off > 10⁵ were measured from n‐channel thin‐film transistors fabricated using NDI‐xfTh copolymers. Bulk heterojunction solar cell devices were also fabricated from the NDI‐xfTh copolymer series in blends with poly(3‐hexylthiophene) (P3HT) with PNDI‐4fTh ‐ based devices yielding the largest J_sc (0.57 mA cm^?2) and fill factor (55%) in addition to the highest measured PCE for this series (0.13%). © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4061–4069     

Perylene diimide copolymers with dithienothiophene and dithienopyrrole: Use in n‐channel and ambipolar field‐effect transistors

Solution‐processable polymers consisting of perylene diimide (PDI) acceptor moieties alternating with dithienothiophene (DTT), N‐dodecyl‐dithienopyrrole (DTP), or oligomers of these donor groups have been synthesized. We have, in addition to varying the donor, varied the N,N′ substituents of the PDIs. The thermal, optical, electrochemical, and charge‐transport properties of the polymers have been investigated. The polymers show broad absorption extending from 300 to 1000 nm with optical band gaps as low as 1.2 eV; the band gap decreases with increasing the conjugation length of donor block, or by replacement of DTT by DTP. The electron affinities of the polymers, estimated from electrochemical data, range from ?3.87 to ?4.01 eV and are slightly affected by the specific choice of donor moiety, while the estimated ionization potentials (?5.31 to ?5.92 eV) are more sensitive to the choice of donor. Bottom‐gate top‐contact organic field‐effect transistors based on the polymers generally exhibit n‐channel behavior with electron mobilities as high as 1.7 × 10^–2 cm²/V/s and on/off ratios as high as 10⁶; one PDI‐DTP polymer is an ambipolar transport material with electron mobility of 4 × 10^–4 cm²/V/s and hole mobility of 4 × 10^–5 cm²/V/s in air. There is considerable variation in the charge transport properties of the polymers with the chemical structures. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

A highly pi-stacked organic semiconductor for field-effect transistors based on linearly condensed pentathienoacene

We present the synthesis and characterization of a fused-ring compound, dithieno[2,3-d:2',3'-d']thieno[3,2-b:4,5-b']dithiophene (pentathienoacene, PTA). In contrast to pentacene, PTA has a larger band gap than most semiconductors used in organic field-effect transistors (OFETs) and therefore is expected to be stable in air. The large pi-conjugated and planar molecular structure of PTA would also form higher molecular orders that are conductive for carrier transport. X-ray diffraction and atomic force microscopy experiments on its films show that the molecules stack in layers with their long axis upright from the surface. X-ray photoelectron spectroscopy suggests that there are no chemical bonds at the PTA/Au interface. OFETs based on the PTA have been constructed, and their performances as p-type semiconductors are also presented. A high mobility of 0.045 cm(2)/V s and an on/off ratio of 10(3) for a PTA OFET have been achieved, demonstrating the potential of PTA for application in future organic electronics.     

Synthesis of Poly(benzothiadiazole‐co‐dithienobenzodithiophenes) and Effect of Thiophene Insertion for High‐Performance Polymer Solar Cells

We describe herein the synthesis of novel donor–acceptor conjugated polymers with dithienobenzodithiophenes (DTBDT) as the electron donor and 2,1,3‐benzothiadiazole as the electron acceptor for high‐performance organic photovoltaics (OPVs). We studied the effects of strategically inserting thiophene into the DTBDT as a substituent on the skeletal structure on the opto‐electronic performances of fabricated devices. From UV/Vis absorption, electrochemical, and field‐effect transistor analyses, we found that the thiophene‐containing DTBDT derivative can substantially increase the orbital overlap area between adjacent conjugated chains and thus dramatically enhance charge‐carrier mobility up to 0.55 cm² V^?1 s^?1. The outstanding charge‐transport characteristics of this polymer allowed the realization of high‐performance organic solar cells with a power conversion efficiency (PCE) of 5.1 %. Detailed studies on the morphological factors that enable the maximum PCE of the polymer solar cells are discussed along with a hole/electron mobility analysis based on the space‐charge‐limited current model.     

Heptagon‐Embedded Pentacene: Synthesis,Structures, and Thin‐Film Transistors of Dibenzo[d,d′]benzo[1,2‐a:4,5‐a′]dicycloheptenes

This study presents a new class of conjugated polycyclic molecules that contain seven‐membered rings, detailing their synthesis, crystal structures and semiconductor properties. These molecules have a nearly flat C₆‐C₇‐C₆‐C₇‐C₆ polycyclic framework with a p‐quinodimethane core. With field‐effect mobilities of up to 0.76 cm² V^?1 s^?1 as measured from solution‐processed thin‐film transistors, these molecules are alternatives to the well‐studied pentacene analogues for applications in organic electronic devices.     

The Impact of Interlayer Electronic Coupling on Charge Transport in Organic Semiconductors: A Case Study on Titanylphthalocyanine Single Crystals

Traditionally, it is believed that three‐dimensional transport networks are preferable to those of lower dimensions. We demonstrate that inter‐layer electronic couplings may result in a drastic decrease of charge mobilities by utilizing field‐effect transistors (FET) based on two phases of titanyl phthalocyanine (TiOPc) crystals. The α‐phase crystals with electronic couplings along two dimensions show a maximum mobility up to 26.8 cm² V^?1 s^?1. In sharp contrast, the β‐phase crystals with extra significant inter‐layer electronic couplings show a maximum mobility of only 0.1 cm² V^?1 s^?1. Theoretical calculations on the bulk crystals and model slabs reveal that the inter‐layer electronic couplings for the β‐phase devices will diminish remarkably the device charge transport abilities owing to the coupling direction perpendicular to the current direction. This work provides new insights into the impact of the dimensionality and directionality of the packing arrangements on charge transport in organic semiconductors.     

Influence of Intermolecular N?H⋅⋅⋅π Interactions on Molecular Packing and Field‐Effect Performance of Organic Semiconductors

Charge transport in organic semiconductors is strongly dependent on their molecular packing modes in the solid state. Therefore, understanding the relationship between molecular packing and charge transport is imperative, both experimentally and theoretically. However, so far, the fundamental effects of solid‐state packing and molecular interactions (e.g. N? H ??? π) on charge transport need further elucidation. Herein, indolo[3,2‐b]carbazole (ICZ) and a derivative thereof are used as examples to approach this scientific target. An interesting insight obtained thereby is that N? H ??? π interactions among ICZ molecules facilitate charge transport for higher mobility. Subtle changes in the of N? H ??? π interactions can significantly influence both the molecular packing and the charge‐transport properties. Therefore, a method for exploiting intermolecular N? H ??? π interactions would yield novel molecular systems with designable characteristics.