HOMO Stabilisation in π‐Extended Dibenzotetrathiafulvalene Derivatives for Their Application in Organic Field‐Effect Transistors

Three new organic semiconductors, in which either two methoxy units are directly linked to a dibenzotetrathiafulvalene (DB‐TTF) central core and a 2,1,3‐chalcogendiazole is fused on the one side, or four methoxy groups are linked to the DB‐TTF, have been synthesised as active materials for organic field‐effect transistors (OFETs). Their electrochemical behaviour, electronic absorption and fluorescence emission as well as photoinduced intramolecular charge transfer were studied. The electron‐withdrawing 2,1,3‐chalcogendiazole unit significantly affects the electronic properties of these semiconductors, lowering both the HOMO and LUMO energy levels and hence increasing the stability of the semiconducting material. The solution‐processed single‐crystal transistors exhibit high performance with a hole mobility up to 0.04 cm² V^?1 s^?1 as well as good ambient stability.     

Impact of replacement of the central benzene ring in anthracene by a heterocyclic ring on electronic excitations and reorganization energies in anthratetrathiophene molecules

The impact of changing the central benzene ring on the electronic excitations and reorganization energies (λ) of the anthratetrathiophene (ATT) molecules is studied by density functional theory (DFT) and time‐dependent DFT (TD‐DFT) quantum chemical calculations. The effect of changing the position of the sulfur atom at the periphery of anthracene on the optical and charge transfer properties is also studied. The calculated results suggest that the HOMO, LUMO, HOMO–LUMO energy gap, ionization potential (IP), electron affinity (EA), hole extraction potential (HEP), electron extraction potential (EEP), and reorganization energies (λ) are affected by replacing the central ring with different heterocyclic rings and the position of the sulfur atom. In addition, all molecules show good hole‐ and electron‐transport properties. This work may be helpful for future design and preparation of high‐performance charge‐transport materials.     

Electronic structure and microscopic charge‐transport properties of a new‐type diketopyrrolopyrrole‐based material

Recently, diketopyrrolopyrrole (DPP)‐based materials have attracted much interest due to their promising performance as a subunit in organic field effect transistors. Using density functional theory and charge‐transport models, we investigated the electronic structure and microscopic charge transport properties of the cyanated bithiophene‐functionalized DPP molecule (compound 1 ). First, we analyzed in detail the partition of the total relaxation (polaron) energy into the contributions from each vibrational mode and the influence of bond‐parameter variations on the local electron–vibration coupling of compound 1 , which well explains the effects of different functional groups on internal reorganization energy (λ). Then, we investigated the structural and electronic properties of compound 1 in its isolated molecular state and in the solid state form, and further simulated the angular resolution anisotropic mobility for both electron‐ and hole‐transport using two different simulation methods: (i) the mobility orientation function proposed in our previous studies (method 1); and (ii) the master equation approach (method 2). The calculated electron‐transfer mobility (0.00003–0.784 cm² V^?1 s^?1 from method 1 and 0.02–2.26 cm² V^?1 s^?1 from method 2) matched reasonably with the experimentally reported value (0.07–0.55 cm² V^?1 s^?1). To the best of our knowledge, this is the first time that the transport parameters of compound 1 were calculated in the context of band model and hopping models, and both calculation results suggest that the intrinsic hole mobility is higher than the corresponding intrinsic electron mobility. Our calculation results here will be instructive to further explore the potential of other higher DPP‐containing quinoidal small molecules. © 2015 Wiley Periodicals, Inc.     

Theoretically Rational Designs of Transport Organic Semiconductors Based on Heteroacenes

A Marcus electron transfer theory coupled with an incoherent polaron hopping and charge diffusion model in combining with first‐principle quantum chemistry calculation was applied to investigating the effects of heteroatom on the intermolecular charge transfer rate for a series of heteroacene molecules. The influences of intermolecular packing and charge reorganization energy were discussed. It was found that the sulphur and nitrogen substituted heteroacenes were intrinsically hole‐transporting materials due to the reduced hole reorganization energy and the enhanced overlap between HOMOs. For the oxygen‐substituted heteroacene, it was found that both the electronic couplings and the reorganization energies for holes and electrons were comparative, indicating the application potential of ambipolar devices. Most interestingly, for the boron‐substituted heteroacenes, theoretical calculations predicted a promising electron‐transport material, which is rare for organic materials. These findings provide insights into rationally designing organic semiconductors with specific properties.     

Structural Polymorphism and Thin Film Transistor Behavior in the Fullerene Framework Molecule 5,6;11,12‐di‐o‐Phenylenetetracene

The molecular structure of the hydrocarbon 5,6;11,12‐di‐o‐phenylenetetracene (DOPT), its material characterization and evaluation of electronic properties is reported for the first time. A single‐crystal X‐ray study reveals two different motifs of intramolecular overlap with herringbone‐type arrangement displaying either face‐to‐edge or co‐facial face‐to‐face packing depicting intensive π–π interactions. Density functional theory (DFT) calculations underpin that a favorable electronic transport mechanism occurs by a charge hopping process due to a π‐bond overlap in the DOPT polymorph with co‐facial arene orientation. The performance of polycrystalline DOPT films as active organic semiconducting layer in a state‐of‐the‐art organic field effect transistor (OFET) device was evaluated and proves to be film thickness dependent. For 40 nm layer thickness it displays a saturation hole mobility (μ_hole) of up to 0.01 cm² V^?1 s^?1 and an on/off‐ratio (I_on/I_off) of 1.5×10³.     

Theoretical comparative studies on transport properties of pentacene,pentathienoacene, and 6,13‐dichloropentacene

Pentacene derivative 6,13‐dichloropentacene (DCP) is one of the latest additions to the family of organic semiconductors with a great potential for use in transistors. We carry out a detailed theoretical calculation for DCP, with systematical comparison to pentacene, pentathienoacene (PTA, the thiophene equivalent of pentacene), to gain insights in the theoretical design of organic transport materials. The charge transport parameters and carrier mobilities are investigated from the first‐principles calculations, based on the widely used Marcus electron transfer theory and quantum nuclear tunneling model, coupled with random walk simulation. Molecular structure and the crystal packing type are essential to understand the differences in their transport behaviors. With the effect of molecule modification, significant one‐dimensional π‐stacks are found within the molecular layer in PTA and DCP crystals. The charge transport along the a‐axis plays a dominant role for the carrier mobilities in the DCP crystal due to the strong transfer integrals within the a‐axis. Pentacene shows a relatively large 3D mobility. This is attributed to the relatively uniform electronic couplings, which thus provides more transport pathways. PTA has a much smaller 3D mobility than pentacene and DCP for the obvious increase of the reorganization energy with the introduction of thiophene. It is found that PTA and DCP exhibit lower HOMO (highest occupied molecular orbital) levels and better environmental stability, indicating the potential applications in organic electronics. © 2015 Wiley Periodicals, Inc.     

Non-symmetric Half-Fused B←N Coordinated Diketopyrrolopyrrole Building Block for n-type Semiconducting Polymers

The development of conjugated polymers with high semiconducting performance and high reliability is of great significance for flexible electronics. Herein, we developed a new type of electron-accepting building block; i.e., non-symmetric half-fused B←N coordinated diketopyrrolopyrrole (DPP) ( HBNDPP ), for amorphous conjugated polymers toward flexible electronics. The rigid B←N fusion part of HBNDPP endows the resulting polymers with decent electron transport, while its non-symmetric structure causes the polymer to exhibit multiple conformation isomers with flat torsional potential energies. Thus, it gets packed in an amorphous manner in solid state, ensuring good resistance to bending strain. Combined with hardness and softness, the flexible organic field-effect transistor devices exhibit n-type charge properties with decent mobility, good bending resistance, and good ambient stability. The preliminary study makes this building block a potential candidate for future design of conjugated materials for flexible electronic devices.     

Extended Conjugated Donor–Acceptor Molecules with E‐(1,2‐Difluorovinyl) and Diketopyrrolopyrrole (DPP) Moieties toward High‐Performance Ambipolar Organic Semiconductors

Two diketopyrrolopyrrole (DPP)‐based donor–acceptor (D–A) conjugated molecules, DPP‐F and DPP‐2F, which contain E‐(1,2‐difluorovinyl) moieties, are reported. The LUMO energies of DPP‐F and DPP‐2F were estimated to be ?3.49 and ?3.70 eV, respectively, based on their redox potentials and absorption spectral data; these values were clearly lowered because of the incorporation of electron‐withdrawing E‐(1,2‐difluorovinyl) moieties. Organic field‐effect transistors (OFETs) with thin films of DPP‐F and DPP‐2F were successfully fabricated with conventional techniques. Based on the respective transfer and output characteristics measured in an inert atmosphere, thin films of DPP‐2F display ambipolar semiconducting behavior with hole and electron mobilities reaching 0.42 and 0.80 cm² V^?1 s^?1, respectively. The as‐prepared OFET of DPP‐2F already shows high hole and electron mobilities that are not influenced remarkably by thermal annealing. For thin films of DPP‐F, only p‐type semiconducting behavior was observed in both an inert atmosphere and air, and the hole mobility increased to 0.1 cm² V^?1 s^?1 after thermal annealing. XRD and AFM studies were performed with thin films of DPP‐F and DPP‐2F after annealing at different temperatures.     

Theoretical Prediction on Photovoltaic Properties of 4Cl‐BPPQ/PC61BM System via Density Functional Theory Calculations

Designing and synthesizing high‐performable electron donor materials are very important for fabricating organic solar cell devices with high power conversion efficiency (PCE). In this work, quantum chemical and molecular dynamics calculations coupled with the Marcus‐Hush charge transfer model were used to investigate the photovoltaic properties of 4Cl‐BPPQ/PC₆₁BM. Results reveal that 4Cl‐BPPQ/PC₆₁BM system theoretically possesses a large open‐circuit voltage (1.29 V), high fill factor (0.90), and over 9% PCE. Moreover, calculations also reveal that the 4Cl‐BPPQ/PC₆₁BM system has a middle‐sized exciton binding energy (0.492 eV), but relatively small charge‐dissociation and charge‐recombination reorganization energies (0.345 eV and 0.355 eV). Based on the 4Cl‐BPPQ/PC₆₁BM complex, the charge‐dissociation rate constant, k_dis, is estimated to be as large as 6.575×10¹² s^?1, while the charge‐recombination one, k_rec, is very small (<1.0 s^?1) under the same condition due to the very small driving force (ΔG_rec=?1.900 eV). In addition, by means of an amorphous cell containing one hundred 4Cl‐BPPQ molecules, the hole carrier mobility of 4Cl‐BPPQ solid is estimated as high as 3.191×10^?3 cm²·V^?1·s^?1. In brief, our calculation shows that 4Cl‐BPPQ/PC₆₁BM system is a very promising organic solar cell system, and is worth of making further device research by experiments.     

Optoelectronic properties of benzotrithiophene isomers: A density functional theory study

Benzotrithiophene (BTT) isomers were investigated using density functional theory (DFT) and time‐dependent DFT (TD‐DFT) with the aim to explore their structures, linear optical properties, vertical and adiabatic ionization potentials (IP_v and IP_a), electron affinities (EA_v and EA_a), and reorganization energies (λ). The computed bond lengths and bond angles at the B3LYP/6–311+G (d, p) level of theory are in good agreement with experimental crystal structures of the known BTTs. These molecules are planar with zero dihedral angle, making them an ideal backbone for high charge mobility. The UV–visible spectra of BTT isomers are in the range 280–360 nm. All BTT isomers have low hole/electron reorganization energies, which is the main characteristic of good hole/electron transporting materials, and these isomers in turn have potential applications in the field of organic materials.     

Synthesis and Properties of Bisphosphole‐Bridged Ladder Oligophenylenes

Ladder‐type oligophenylenes (LOPP) with bridging heteroatoms are interesting systems as they offer novel electronic and photophysical properties on account of the rigid structural features, more efficient electron delocalization on the coplanar aromatic framework, and strong intermolecular interactions. LOPPs incorporating multiple phosphorous centers combine the excellent electronic properties of phospholes and rigidified conjugated framework of LOPPs, thus positioning themselves as an attractive class of organic semiconductors. To date, there still lacks an effective synthetic methodology towards LOPPs with multiple phosphorous bridges. Herein, we describe the synthesis and properties of a new class of bisphosphole‐bridged ladder oligo(p‐phenylene)s and the related phosphoxides. The synthesis of phospholes was achieved by a four‐fold free‐radical phosphanylation reaction of a tetrabromo p‐terphenylene or biphenyl‐thiophene. Sequential trapping of four highly reactive aryl radicals occurred effectively to give the desired phosphorous‐containing ladder compound. The oxides of the phospholes are shown to be strong fluorophores that can be used as potential n‐type building blocks for organic semiconducting materials.     

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     

ipso‐Arylative Ring‐Opening Polymerization as a Route to Electron‐Deficient Conjugated Polymers

ipso‐Arylative ring‐opening polymerization of 2‐bromo‐8‐aryl‐8H‐indeno[2,1‐b]thiophen‐8‐ol monomers proceeds to M_n up to 9 kg mol^?1 with conversion of the monomer diarylcarbinol groups to pendent conjugated aroylphenyl side chains (2‐benzoylphenyl or 2‐(4‐hexylbenzoyl)phenyl), which influence the optical and electronic properties of the resulting polythiophenes. Poly(3‐(2‐(4‐hexylbenzoyl)phenyl)thiophene) was found to have lower frontier orbital energy levels (HOMO/LUMO=?5.9/?4.0 eV) than poly(3‐hexylthiophene) owing to the electron‐withdrawing ability of the aryl ketone side chains. The electron mobility (ca. 2×10^?3 cm² V^?1 s^?1) for poly(3‐(2‐(4‐hexylbenzoyl)phenyl)thiophene) was found to be significantly higher than the hole mobility (ca. 8×10^?6 cm² V^?1 s^?1), which suggests such polymers are candidates for n‐type organic semiconductors. Density functional theory calculations suggest that backbone distortion resulting from side‐chain steric interactions could be a key factor influencing charge mobilities.     

Siloxane‐Based Hybrid Semiconducting Polymers Prepared by Fluoride‐Mediated Suzuki Polymerization

Siloxane‐containing materials are a large and important class of organic‐inorganic hybrids. In this report, a practical variation of the Suzuki polymerization to generate semiconducting polymeric hybrids based on siloxane units, which proceeds under essentially nonbasic conditions, is presented. This method generates solution‐processable poly(diketopyrrolopyrrole‐alt‐benzothiadiazole) (PDPPBT‐Si) consisting of the hybrid siloxane substituents, which could not be made using conventional methods. PDPPBT‐Si exhibits excellent ambipolar transistor performance with well‐balanced hole and electron FET mobilities. The siloxane‐containing DPP‐thiophene polymer classes (PDPP3T‐Si and PDPP4T‐Si), synthesized by this method, exhibit high hole mobility of up to 1.29 cm² V^?1 s^?1. This synthetic approach should provide access to a variety of novel siloxane‐containing conjugated semiconductor classes by using a variety of aryldihalides and aryldiboronic acids/esters.     

Molecular dynamics simulations of cation‐controlled aggregation in fluorene–triarylamine copolymers

Systematic molecular dynamics (MD) simulations are applied, combining the sulfonated fluorene–triarylamine copolymer anion poly((N‐(4‐butylphenyl)‐diphenylamine)‐alt‐fluorene 9,9'di‐n‐propane sulfonate) with a wide range of monovalent cations of different cation field strengths. The resulting MD trajectories are analyzed for each of the conjugated polyelectrolyte focusing on ion aggregation, and its influence on the static and dynamic polymer morphology as well as on the charge carrier mobilities that are relevant for the use of such hole transporting materials in organic solar cells and organic light emitting devices. A pronounced variation of the degree of cation clustering with the cation field strength is found to control the polymer morphology, cation mobility and thereby the time evolution of the Coulomb energy landscape for hole transport. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 965–974     

Design of donor–acceptor–donor (D–A–D) type small molecule donor materials with efficient photovoltaic parameters

Four Donor–Acceptor–Donor (D–A–D) type of donor molecules (M1‐M4) with triphenylamine (TPA) as donor moiety, thiophene as bridge, and thiazolothiazole as acceptor unit were designed and its photovoltaic parameters were equated with reference molecule “R.” DFT functional CAM‐B3LYP/6‐31G (d,p) was found best for geometry optimization and TD‐CAM‐B3LYP/6‐31G (d,p) was found suitable for excited state calculations. Among designed donor molecules, M4 manifests suitable lowest band gap of 4.73 eV, frontier molecular orbital energy levels as well as distinctive broad absorption of 455.3 nm due to the stronger electron withdrawing group. The electron‐withdrawing substituents contribute to red shifts of absorption spectra and better stabilities for designed molecules. The theoretically determined reorganization energies of designed donor molecules suggested excellent charge mobility property. The lower λ_e values in comparison with λ_h illustrated that these four donor materials would be ideal for electron transfer and M4 would be best amongst the investigated molecules with lowest λ_e of 0.0177. Furthermore, the calculated Voc of M4 is 2.04 V with respect to PC₆₀BM (phenyl‐C61‐butyric acid methyl ester). This study revealed that the designed donor materials are suitable and recommended for high performance organic solar cell devices.     

Investigation of spacer influences in phosphorescent‐emitting nonconjugated PLED systems

An assay was introduced to clarify influences on electroluminescent behavior for RGB‐colored phosphorescent terpolymers with N,N‐Di‐p‐tolyl‐aniline as hole‐transporting unit, 2‐(4‐biphenyl)‐5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole (tert‐BuPBD) as electron‐transporting unit, and different iridium complexes in RGB‐colors as triplet emitting materials. All monomers were attached with spacer moieties to the “para” position of a polystyrene. Polymer light emitting diodes (PLEDs) were built to study the electro‐optical behavior of these materials. The gist was a remarkable influence of hexyl‐spacer units to the PLED performance. For all three colors only very restricted PLED performances were found. In comparison RGB‐terpolymers were synthesized with directly attached charge transport materials to the polymer backbone. For this directly linked systems efficiencies were 28 cd A^?1 @ 6 V (green), 4.9 cd A^?1 @ 5 V (red) and 4.3 cd A^?1 @ 6 V (bluish). In summary we assume that an improved charge percolation pathways regarding to the higher content of semiconducting molecules and an improved charge transfer to the phosphorescent dopand in the case of the copolymers without spacers are responsible for the better device performance comparing the copolymers with hexyl spacers. The approach of the directly connected charge transport materials at the nonconjugated styrene polymer backbone should be favored for further investigations, therefore. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 389–402, 2010     

Crystal Engineering of Dual Channel p/n Organic Semiconductors by Complementary Hydrogen Bonding

The supramolecular arrangement of organic semiconductors in the solid state is as critical for their device properties as the molecular structure, but is much more difficult to control. To enable supramolecular design of semiconducting materials, we introduced dipyrrolopyridine as a new donor semiconductor capable of complementary hydogen bonding with naphthalenediimide acceptors. Through a combination of solution, crystallographic, and device studies, we show that the self‐assembly driven by H bonding a) modulates the charge‐transfer interactions between the donor and acceptor, b) allows for precise control over the solid‐state packing, and c) leads to a combination of the charge‐transport properties of the individual components. The predictive power of this approach was demonstrated in the synthesis of three new coassembled materials which show both hole and electron transport in single‐crystal field‐effect transistors. These studies provide a foundation for advanced solid‐state engineering in organic electronics, capitalizing on the complementary H bonding.     

New quinoxaline derivatives as accepting units in donor–acceptor type low‐band gap polymers for organic photovoltaic cells

A series of new donor–acceptor‐type low‐band‐gap semiconducting polymers were synthesized as electron donors for organic photovoltaic cells. The polymers comprised quinoxaline derivatives as the acceptors and a benzodithiophene (BDT) derivative as the donors. 5,8‐Dibromoquinoxaline (Qx), 8,11‐dibromobenzo[a]phenazine (BPz), 10,13‐dibromodibenzo[a,c]phenazine (DBPz), and 8,11‐dibromo‐5‐(9H‐carbazol‐9‐yl)benzo[a]phenazine) (CBPz) were synthesized and polymerized with 2,6‐bis(trimethyltin)?4,8‐diethylhexyloxybenzo‐[1,2‐b;3,4‐b]dithiophene (BDT) through Stille cross‐coupling to produce four types of fully conjugated semiconducting polymers: PBDT‐Qx, PBDT‐BPz, PBDT‐DBPz, and PBDT‐CBPz , respectively. Intramolecular charge transfer between the electron donating and accepting units in the polymeric backbone induced a broad absorption from 300 to 800 nm. The optical band gap energies of the polymers were measured from their absorption onsets to be 1.54–1.80 eV depending on the polymer structure. Solution‐processed field‐effect transistors were fabricated to measure the hole mobilities of the polymers, and bulk hetero‐junction photovoltaic devices were fabricated using the synthesized polymers as electron donors and fullerene derivatives as electron acceptors. One of these devices showed a high power conversion efficiency of 3.87% with an open‐circuit voltage of 0.78 V, a short‐circuit current of 9.68 mA/cm², and a fill factor of 0.51 under air mass 1.5 global (AM 1.5 G) illumination conditions (100 mW/cm²). © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4136–4149     

Light‐emitting diodes and lasers based on polymer films doped with small organic molecules and rare‐earth complexes

We investigated the lasing properties of optically pumped polymer films. Amplified spontaneous emission (ASE) around 400 nm was observed in polymer films of polystyrene (PS) and poly(N‐vinylcarbazole) (PVK) doped up to 20% with the hole‐transporting organic molecule N,N′‐bis(3‐methylphenyl)‐N,N′‐diphenylbenzidine (TPD). Thus, TPD‐based films are candidates for blue‐emitting organic diode lasers. Films containing several semiconducting organic molecules and polymers and rare‐earth complexes were also investigated. Energy transfer was observed in PVK films doped with various europium and samarium complexes. PS films containing the electron‐transporting organic molecule 2‐(4‐biphenylyl)‐5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole and small amounts of TPD also showed energy transfer to the europium complexes, but not to the samarium ones. None of these films demonstrated ASE; therefore, they are not appropriate for lasing purposes. However, because rare‐earth ions have very sharp emission spectra, these materials are candidates for very monochromatic light‐emitting diodes. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2706–2714, 2003