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Showing papers on "Organic semiconductor published in 2012"


Journal ArticleDOI
13 Dec 2012-Nature
TL;DR: A class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates.
Abstract: The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal-organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 10(6) decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.

5,297 citations


Journal ArticleDOI
TL;DR: On the eve of commercialization of organic solar cells, this review provides an overview over efficiencies attained with small molecules/oligomers in OSCs and reflects materials and device concepts developed over the last decade.
Abstract: This article is written from an organic chemist's point of view and provides an up-to-date review about organic solar cells based on small molecules or oligomers as absorbers and in detail deals with devices that incorporate planar-heterojunctions (PHJ) and bulk heterojunctions (BHJ) between a donor (p-type semiconductor) and an acceptor (n-type semiconductor) material. The article pays particular attention to the design and development of molecular materials and their performance in corresponding devices. In recent years, a substantial amount of both, academic and industrial research, has been directed towards organic solar cells, in an effort to develop new materials and to improve their tunability, processability, power conversion efficiency, and stability. On the eve of commercialization of organic solar cells, this review provides an overview over efficiencies attained with small molecules/oligomers in OSCs and reflects materials and device concepts developed over the last decade. Approaches to enhancing the efficiency of organic solar cells are analyzed.

1,649 citations


Journal ArticleDOI
16 Mar 2012-Science
TL;DR: In this paper, the electron-hole pair created via photon absorption in organic photoconversion systems must overcome the Coulomb attraction to achieve long-range charge separation, and this process is facilitated through the formation of excited, delocalized band states.
Abstract: The electron-hole pair created via photon absorption in organic photoconversion systems must overcome the Coulomb attraction to achieve long-range charge separation. We show that this process is facilitated through the formation of excited, delocalized band states. In our experiments on organic photovoltaic cells, these states were accessed for a short time (

1,023 citations


Journal ArticleDOI
TL;DR: A universal energy-alignment trend is observed for a set of transition-metal oxides--representing a broad diversity in electronic properties--with several organic semiconductors, demonstrating that, despite the variance in their electronic properties, oxide energy alignment is governed by one driving force: electron-chemical-potential equilibration.
Abstract: Transition-metal oxides improve power conversion efficiencies in organic photovoltaics and are used as low-resistance contacts in organic light-emitting diodes and organic thin-film transistors. What makes metal oxides useful in these technologies is the fact that their chemical and electronic properties can be tuned to enable charge exchange with a wide variety of organic molecules. Although it is known that charge exchange relies on the alignment of donor and acceptor energy levels, the mechanism for level alignment remains under debate. Here, we conclusively establish the principle of energy alignment between oxides and molecules. We observe a universal energy-alignment trend for a set of transition-metal oxides--representing a broad diversity in electronic properties--with several organic semiconductors. The trend demonstrates that, despite the variance in their electronic properties, oxide energy alignment is governed by one driving force: electron-chemical-potential equilibration. Using a combination of simple thermodynamics, electrostatics and Fermi statistics we derive a mathematical relation that describes the alignment.

865 citations


Journal ArticleDOI
TL;DR: A new small molecule, p-DTS(FBTTh(2))(2)/PC(71)BM solar cells with power conversion efficiencies of up to 7%.
Abstract: A new small molecule, p-DTS(FBTTh(2))(2), is designed for incorporation into solution-fabricated high-efficiency organic solar cells. Of primary importance is the incorporation of electron poor heterocycles that are not prone to protonation and thereby enable the incorporation of commonly used interlayers between the organic semiconductor and the charge collecting electrodes. These features have led to the creation of p-DTS(FBTTh(2))(2)/PC(71)BM solar cells with power conversion efficiencies of up to 7%.

584 citations


Journal ArticleDOI
TL;DR: A solution processing method to grow large arrays of aligned C(60) single crystals that facilitates the fabrication of large amounts of high-quality semiconductor crystals for fundamental studies, and with substantial improvement on the surface coverage of crystals, this method might be suitable for large-area applications based on single crystals of organic semiconductors.
Abstract: Field-effect transistors based on single crystals of organic semiconductors have the highest reported charge carrier mobility among organic materials, demonstrating great potential of organic semiconductors for electronic applications. However, single-crystal devices are difficult to fabricate. One of the biggest challenges is to prepare dense arrays of single crystals over large-area substrates with controlled alignment. Here, we describe a solution processing method to grow large arrays of aligned C60 single crystals. Our well-aligned C60 single-crystal needles and ribbons show electron mobility as high as 11 cm2V–1s–1 (average mobility: 5.2 ± 2.1 cm2V–1s–1 from needles; 3.0 ± 0.87 cm2V–1s–1 from ribbons). This observed mobility is ∼8-fold higher than the maximum reported mobility for solution-grown n-channel organic materials (1.5 cm2V–1s–1) and is ∼2-fold higher than the highest mobility of any n-channel organic material (∼6 cm2V–1s–1). Furthermore, our deposition method is scalable to a 100 mm wafer ...

476 citations


Journal ArticleDOI
TL;DR: Zn2(TTFTB), a new metal-organic framework that contains columnar stacks of tetrathiafulvalene and benzoate-lined infinite one-dimensional channels, is synthesized and represents the first example of a permanently porous MOF with high charge mobility.
Abstract: The tetratopic ligand tetrathiafulvalene-tetrabenzoate (H4TTFTB) is used to synthesize Zn2(TTFTB), a new metal–organic framework that contains columnar stacks of tetrathiafulvalene and benzoate-lined infinite one-dimensional channels. The new MOF remains porous upon desolvation and exhibits charge mobility commensurate with some of the best organic semiconductors, confirmed by flash-photolysis-time-resolved microwave conductivity measurements. Zn2(TTFTB) represents the first example of a permanently porous MOF with high charge mobility and may inspire further exploration of the electronic properties of these materials.

405 citations


Journal ArticleDOI
TL;DR: It is observed for this materials class that electron transport is limited by traps that exhibit a gaussian energy distribution in the bandgap, which indicates that the electron traps have a common origin that, it is suggested, is most likely related to hydrated oxygen complexes.
Abstract: Electron transport in semiconducting polymers is usually inferior to hole transport, which is ascribed to charge trapping on isolated defect sites situated within the energy bandgap. However, a general understanding of the origin of these omnipresent charge traps, as well as their energetic position, distribution and concentration, is lacking. Here we investigate electron transport in a wide range of semiconducting polymers by current-voltage measurements of single-carrier devices. We observe for this materials class that electron transport is limited by traps that exhibit a Gaussian energy distribution in the bandgap. Remarkably, the electron-trap distribution is identical for all polymers considered: the number of traps amounts to 3 × 1023 traps per m3 centred at an energy of ∼3.6 eV below the vacuum level, with a typical distribution width of ∼0.1 eV. This indicates that the electron traps have a common origin that, we suggest, is most likely related to hydrated oxygen complexes. A consequence of this finding is that the trap-limited electron current can be predicted for any polymer. © 2012 Macmillan Publishers Limited. All rights reserved.

401 citations


Journal ArticleDOI
TL;DR: Sulfur-mediated synthesis has been demonstrated as a simple but efficient pathway to control the texture and electronic structure of poly(tris-triazine) based graphitic carbon nitride semiconductors with improved photocatalytic reactivity over the pristine counterpart.
Abstract: Sulfur-mediated synthesis has been demonstrated as a simple but efficient pathway to control the texture and electronic structure of poly(tris-triazine) based graphitic carbon nitride semiconductors with improved photocatalytic reactivity over the pristine counterpart. Here, we advance this strategy by employing cheap and easily available elemental sulfur as the external sulfur species instead of sulfur-containing precursors for the sulfur-mediated synthesis of polymeric carbon nitride photocatalysts. Characterization results revealed that the multiple thermal condensations of carbon nitride precursors in the hot sulfur flux provided a facile means to promote the formation of graphitic-like carbon nitride conjugated systems, altering the traditional route of thermal-induced self-polymerization of melamine. The textural, electronic, and optical properties of the resultants organic semiconductors was therefore strongly modified to endow the materials with improved physical and chemical properties, as demons...

382 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present efficient all-polymer solar cells comprising two different low-bandgap naphthalenediimide (NDI)-based copolymers as acceptors and regioregular P3HT as the donor.
Abstract: The authors present efficient all-polymer solar cells comprising two different low-bandgap naphthalenediimide (NDI)-based copolymers as acceptors and regioregular P3HT as the donor. It is shown that these naphthalene copolymers have a strong tendency to preaggregate in specific organic solvents, and that preaggregation can be completely suppressed when using suitable solvents with large and highly polarizable aromatic cores. Organic solar cells prepared from such nonaggregated polymer solutions show dramatically increased power conversion efficiencies of up to 1.4%, which is mainly due to a large increase of the short circuit current. In addition, optimized solar cells show remarkable high fill factors of up to 70%. The analysis of the blend absorbance spectra reveals a surprising anticorrelation between the degree of polymer aggregation in the solid P3HT:NDI copolymer blends and their photovoltaic performance. Scanning near-field optical microscopy (SNOM) and atomic force microscopy (AFM) measurements reveal important information on the blend morphology. It is shown that films with high degree of aggregation and low photocurrents exhibit large-scale phase-separation into rather pure donor and acceptor domains. It is proposed that, by suppressing the aggregation of NDI copolymers at the early stage of film formation, the intermixing of the donor and acceptor component is improved, thereby allowing efficient harvesting of photogenerated excitons at the donor–acceptor heterojunction.

310 citations


Journal ArticleDOI
TL;DR: The results indicate that incorporation of a DPP moiety to construct quinoidal architecture is an effective approach to enhance the charge-transport capability in TFT devices.
Abstract: We report the synthesis, characterization, and application of a novel series of diketopyrrolopyrrole (DPP)-containing quinoidal small molecules as highly efficient n-type organic semiconductors in thin film transistors (TFTs). The first two representatives of these species exhibit maximum electron mobility up to 0.55 cm2 V–1 s–1 with current on/current off (Ion/Ioff) values of 106 for 1 by vapor evaporation, and 0.35 cm2 V–1 s–1 with Ion/Ioff values of 105–106 for 2 by solution process in air, which is the first demonstration of DPP-based small molecules offering only electron transport characteristics in TFT devices. The results indicate that incorporation of a DPP moiety to construct quinoidal architecture is an effective approach to enhance the charge-transport capability.

Journal ArticleDOI
TL;DR: Flexible thin-film transistors with excellent thermal stability and their viability for biomedical sterilization processes are demonstrated and are stable even after exposure to conditions typically used for medical sterilization.
Abstract: The excellent mechanical flexibility of organic electronic devices is expected to open up a range of new application opportunities in electronics, such as flexible displays, robotic sensors, and biological and medical electronic applications. However, one of the major remaining issues for organic devices is their instability, especially their thermal instability, because low melting temperatures and large thermal expansion coefficients of organic materials cause thermal degradation. Here we demonstrate the fabrication of flexible thin-film transistors with excellent thermal stability and their viability for biomedical sterilization processes. The organic thin-film transistors comprise a high-mobility organic semiconductor, dinaphtho(2,3- b:2',3'-f)thieno (3,2-b)thiophene, and thin gate dielectrics comprising a 2-nm-thick self-assembled monolayer and a 4-nm-thick aluminium oxide layer. The transistors exhibit a mobility of 1.2 cm 2 V − 1 s − 1 within a 2 V operation and are stable even after exposure to conditions typically used for medical sterilization.

Journal ArticleDOI
TL;DR: A model that assumes that due to molecular disorder, only a subset of DPT dimer pairs adopt configurations that promote fission is developed, and it is found that DPT exhibits a surprisingly high singlet fission yield.
Abstract: Singlet exciton fission is a process that occurs in select organic semiconductors and entails the splitting of a singlet excited state into two lower triplet excitons located on adjacent chromophores. Research examining this phenomenon has recently seen a renaissance due to the potential to exploit singlet fission within the context of organic photovoltaics to prepare devices with the ability to circumvent the Shockley–Queisser limit. To date, high singlet fission yields have only been reported for crystalline or polycrystalline materials, suggesting that molecular disorder inhibits singlet fission. Here, we report the results of ultrafast transient absorption and time-resolved emission experiments performed on 5,12-diphenyl tetracene (DPT). Unlike tetracene, which tends to form polycrystalline films when vapor deposited, DPT’s pendant phenyl groups frustrate crystal growth, yielding amorphous films. Despite the high level of disorder in these films, we find that DPT exhibits a surprisingly high singlet f...

Journal ArticleDOI
TL;DR: Recent developments in the area of thiazole-based organic semiconductors, particularly thiazoles, bithiazole, thiazolothiazole and benzobisthiazoles-based small molecules and polymers, for applications in organic field-effect transistors, solar cells and light-emitting diodes are reviewed.
Abstract: Over the past two decades, organic semiconductors have been the subject of intensive academic and commercial interests. Thiazole is a common electron-accepting heterocycle due to electron-withdrawing nitrogen of imine (C=N), several moieties based on thiazole have been widely introduced into organic semiconductors, and yielded high performance in organic electronic devices. This article reviews recent developments in the area of thiazole-based organic semiconductors, particularly thiazole, bithiazole, thiazolothiazole and benzobisthiazole-based small molecules and polymers, for applications in organic field-effect transistors, solar cells and light-emitting diodes. The remaining problems and challenges, and the key research direction in near future are discussed.

Journal ArticleDOI
TL;DR: 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), an aromatic compound containing a naphthalene ring system (fused C6 aromatic rings), is employed, to demonstrate that each carbon in a C6 ring can accept a Li ion to form a Li6/C6 additive complex through a reversible electrochemical lithium addition reaction.
Abstract: A fundamental and persistent problem in the study of carbonbased electrode materials for lithium ion batteries is the question of how many lithium ions can be inserted onto a C6 aromatic ring. Although different empirical models of Lix/C6 (x< 3) have been proposed, the question remains unresolved. Herein we employ 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), an aromatic compound containing a naphthalene ring system (fused C6 aromatic rings), to demonstrate that each carbon in a C6 ring can accept a Li ion to form a Li6/C6 additive complex through a reversible electrochemical lithium addition reaction. This process results in Li ion insertion capacities of up to nearly 2000 mAhg , depending on the exact molecular structure. This value is several times higher than any other organic electrode material previously reported and can be fully released under certain conditions. Our experiments and theoretical calculations indicate that the anhydride groups on the sides of the aromatic system are crucial for this process, which provides a promising strategy for the design of novel high-performance organic electrode materials. Organic molecules are intriguing candidates for electrode materials for use in rechargeable Li ion batteries. The application of such species has aroused much interest recently, owing to the obvious advantages of such a system: no need for rare metals, low safety risks compared to transition metal oxides, and design flexibility at the molecular level. However, organic molecules are usually considered to possess relatively poor specific energies and cycling properties, as compared to those of inorganic materials, and these factors greatly limit their practical application. Recently, studies on aromatic carbonyl derivatives showed that organic materials can possess outstanding electrochemical performance comparable to, or even superior to, inorganic materials. Furthermore, the wide diversity of organic redox systems, as well as the excellent flexibility in their molecular design, suggest even greater prospects for these materials, and this has inspired the exploration of new organic Li ion insertion systems with improved performance. Aromatic C6 rings are the basic structural units of graphite and other carbon-based electrode materials, which are the most commonly used anodes in commercial Li ion batteries owing to their high electric conductivity and low cost. It has traditionally been believed that each C6 ring can accept one Li ion to form an intercalated Li/C6 complex, giving a relatively low theoretical capacity of 372 mAhg . Recently, studies on graphene, nanographene, and their derivatives reveal that, through the reduction of size and dimensionality, these materials exhibit unique electric and electrochemical properties superior to those of conventional graphitic materials; thus, these materials are currently a hot research topic. In studies of electrode materials for Li ion batteries, these derivatives also exhibit high reversible capacities of up to almost twice the theoretical value of graphite, although the detailed mechanism is still unclear. This leads to a fundamental question in the study of carbonbased electrode materials: How many Li ions can actually be inserted onto each C6 aromatic ring? Multi-ring aromatics (for example, naphthalene, NTCDA, perylene, etc.) and their derivatives have planar C6 ring structures similar to graphene or nanographene. NTCDA is a typical example; it has a naphthalene-like ring structure consisting of two C6 rings fused together along with two cyclic anhydride groups (Figure 1a). NTCDA is a well-known organic semiconductor with good crystallinity and has been extensively studied for use in molecular electric devices. It provides an ideal model to study Li ion insertion onto C6 rings owing to the minimal number of C6 rings it possesses, which guarantees the necessary insolubility of the electrode materials in the commonly used electrolyte solution (ethylene carbonate/dimethyl carbonate/LiPF6) for Li ion batteries. NTCDA also possesses the necessary degree of conductivity for electron transport among molecules. We investigated the electrochemical Li ion insertion/deinsertion properties of NTCDA using model test cells with Li metal as the counter electrode. The working electrode consisted of NTCDA, acetylene black (AB), and polytetrafluoroethylene binders in a weight ratio of about 60:35:5. The cells were initially cycled by discharging (Li ion insertion) and charging (Li ion deinsertion) repeatedly in a potential range of 0.001–3.0 V vs. Li/Li at a moderate current rate of 100 mAg . Figure 1b shows selected discharge/charge curves (the 1st, 2nd, 3rd, and 8th cycles) for NTCDA. Figure 1c shows the corresponding discharge and charge capacities of NTCDA versus the cycle number. The first discharge and charge capacities are 1273 and 724 mAhg , respectively, showing a coulombic efficiency [*] X. Han, G. Qing, T. Sun State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology Wuhan, 430070 (China) E-mail: suntaolei@iccas.ac.cn

Journal ArticleDOI
TL;DR: In this work, the thermoelectric properties of the bulk of the conducting polymer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) were controlled electrically by varying the gate voltage.
Abstract: While organic field-effect transistors allow the investigation of interfacial charge transport at the semiconductor-dielectric interface, an electrochemical transistor truly modifies the oxidation level and conductivity throughout the bulk of an organic semiconductor. In this work, the thermoelectric properties of the bulk of the conducting polymer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) were controlled electrically by varying the gate voltage. In light of the growing interest in conducting polymers as thermoelectric generators, this method provides an easy tool to study the physics behind the thermoelectric properties and to optimize polymer thermoelectrics.

Journal ArticleDOI
TL;DR: Using phase-separated organic semiconducting blends containing a small molecule, as the hole transporting material, and a conjugated amorphous polymer as the binder material, the authors demonstrate solution-processed organic thin-film transistors with superior performance characteristics that include; hole mobility >5 cm(2) /Vs, current on/off ratio ≥10(6) and narrow transistor parameter spread.
Abstract: Using phase-separated organic semiconducting blends containing a small molecule, as the hole transporting material, and a conjugated amorphous polymer, as the binder material, we demonstrate solution-processed organic thin-film transistors with superior performance characteristics that include; hole mobility >5 cm(2) /Vs, current on/off ratio ≥10(6) and narrow transistor parameter spread. These exceptional characteristics are attributed to the electronic properties of the binder polymer and the advantageous nanomorphology of the blend film.

Journal ArticleDOI
TL;DR: The engineering of an electronic structure in a semiconducting film is reported on by blending two molecular components, a photochromic diarylethene derivative and a poly(3-hexylthiophene) (P3HT) matrix, to attain phototunable and bistable energy levels for the P3HT's hole transport.
Abstract: Organic semiconductors are suitable candidates for printable, flexible and large-area electronics. Alongside attaining an improved device performance, to confer a multifunctional nature to the employed materials is key for organic-based logic applications. Here we report on the engineering of an electronic structure in a semiconducting film by blending two molecular components, a photochromic diarylethene derivative and a poly(3-hexylthiophene) (P3HT) matrix, to attain phototunable and bistable energy levels for the P3HT's hole transport. As a proof-of-concept we exploited this blend as a semiconducting material in organic thin-film transistors. The device illumination at defined wavelengths enabled reversible tuning of the diarylethene's electronic states in the blend, which resulted in modulation of the output current. The device photoresponse was found to be in the microsecond range, and thus on a technologically relevant timescale. This modular blending approach allows for the convenient incorporation of various molecular components, which opens up perspectives on multifunctional devices and logic circuits.

Journal ArticleDOI
TL;DR: The development of air-stable, high mobility, n-type organic semiconductors for organic electronics is still highly emergent and perylene bisimides (PBIs) will be considered as candidates because of their reasonable electron acceptor ability.
Abstract: Recently, some impressive progress has been made by functionalization of (hetero-)acenes, thiophenes, and arylenes with electron-defi cient constituents. [ 3–5 ] However, the development of air-stable, high mobility, n-type organic semiconductors for organic electronics is still highly emergent. The mobility of organic semiconductors depends on the effi ciency of charge transport from one molecule to another. Hence, some organic semiconductors with dense molecule packing always give high mobility. [ 6 ] As to the stability of organic compounds, it is believed that the highest occupied molecular orbital (HOMO) of p-type organic semiconductors should be more negative than –5.0 eV, e.g., locating at –5.0 to –6.0 eV, and the lowest unoccupied molecular orbitals (LUMO) of n-type organic semiconductors are best located between –4.0 and –4.5 eV, for anti-oxidation in air. [ 2 , 7 ] We have acknowledged these requirements and believe that perylene bisimides (PBIs) will fi t as candidates because of their reasonable electron acceptor ability, [ 8 ] and have been focusing on the expansion of the chemistry of perylene bisimides (PBIs) by a combination of Ullmann coupling and C–H transformation for some time, and have developed a facile strategy to synthesize fully conjugated, triply linked, diperylene bisimides, [ 8 ] conferring the expanded

Journal ArticleDOI
TL;DR: Experimental and theoretical evidence is presented for intermolecular hybridization of organic semiconductor and dopant frontier molecular orbitals and controlling the degree of hybridization emerges as a strategy for overcoming the present limitations in the yield of doping-induced charge carriers.
Abstract: Current models for molecular electrical doping of organic semiconductors are found to be at odds with other well-established concepts in that field, like polaron formation. Addressing these inconsistencies for prototypical systems, we present experimental and theoretical evidence for intermolecular hybridization of organic semiconductor and dopant frontier molecular orbitals. Common doping-related observations are attributed to this phenomenon, and controlling the degree of hybridization emerges as a strategy for overcoming the present limitations in the yield of doping-induced charge carriers.

Journal ArticleDOI
TL;DR: The material properties of chloroboron subphthalocyanine (Cl-BsubPc) and its use as an organic semiconductor and the impact of molecular design on the material properties and the performance are reviewed.
Abstract: Boron subphthalocyanines (BsubPcs) are an emerging class of high performing materials in organic electronics. Since the first use of chloroboron subphthalocyanine in an organic electronic device 6 years ago subphthalocyanines have shown potential as functional materials in organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). Here we review the material properties of chloroboron subphthalocyanine (Cl-BsubPc) and its use as an organic semiconductor. We then highlight our efforts toward derivatives of boron subpthalocyanine beyond Cl-BsubPc and discuss the impact of molecular design on the material properties and the performance of the BsubPc. Finally, we comment on the status of BsubPcs in the field of organic electronics and discuss how we believe future progress can be made.

Journal ArticleDOI
TL;DR: In this article, the authors used density functional theory and many-body perturbation theory to calculate the spectroscopic properties of two prototypical organic semiconductors, pentacene, and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), quantitatively comparing with measured PES, IPES, and optical absorption spectra.
Abstract: The broad use of organic semiconductors for optoelectronic applications relies on quantitative understanding and control of their spectroscopic properties. Of paramount importance are the transport gap---the difference between ionization potential and electron affinity---and the exciton binding energy---inferred from the difference between the transport and optical absorption gaps. Transport gaps are commonly established via photoemission and inverse photoemission spectroscopy (PES/IPES). However, PES and IPES are surface-sensitive, average over a dynamic lattice, and are subject to extrinsic effects, leading to significant uncertainty in gaps. Here, we use density functional theory and many-body perturbation theory to calculate the spectroscopic properties of two prototypical organic semiconductors, pentacene, and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), quantitatively comparing with measured PES, IPES, and optical absorption spectra. For bulk pentacene and PTCDA, the computed transport gaps are 2.4 and 3.0 eV, and optical gaps are 1.7 and 2.1 eV, respectively. Computed bulk quasiparticle spectra are in excellent agreement with surface-sensitive photoemission measurements over several eV only if the measured gap is reduced by 0.6 eV for pentacene and 0.6--0.9 eV for PTCDA. We attribute this redshift to several physical effects, including incomplete charge screening at the surface, static and dynamical disorder, and experimental resolution. Optical gaps are in excellent agreement with experiment with solid-state exciton binding energies of \ensuremath{\sim}0.5 eV for both systems; for pentacene the exciton is delocalized over several molecules and exhibits significant charge transfer character. Our parameter-free calculations provide new interpretation of spectroscopic properties of organic semiconductors critical to optoelectronics.

Journal ArticleDOI
TL;DR: In this article, a facile one-step growth of self-aligning, highly crystalline soluble acene arrays that exhibit excellent field-effect mobilities was reported via an optimized dip-coating process.
Abstract: The preparation of uniform large-area highly crystalline organic semiconductor thin films that show outstanding carrier mobilities remains a challenge in the field of organic electronics, including organic field-effect transistors. Quantitative control over the drying speed during dip-coating permits optimization of the organic semiconductor film formation, although the kinetics of crystallization at the air–solution–substrate contact line are still not well understood. Here, we report the facile one-step growth of self-aligning, highly crystalline soluble acene crystal arrays that exhibit excellent field-effect mobilities (up to 1.5 cm V−1 s−1) via an optimized dip-coating process. We discover that optimized acene crystals grew at a particular substrate lifting-rate in the presence of low boiling point solvents, such as dichloromethane (b.p. of 40.0 °C) or chloroform (b.p. of 60.4 °C). Variable-temperature dip-coating experiments using various solvents and lift rates are performed to elucidate the crystallization behavior. This bottom-up study of soluble acene crystal growth during dip-coating provides conditions under which one may obtain uniform organic semiconductor crystal arrays with high crystallinity and mobilities over large substrate areas, regardless of the substrate geometry (wafer substrates or cylinder-shaped substrates).

Journal ArticleDOI
TL;DR: In this article, the synthesis of a selenophene-diketopyrrolopyrrole monomer and its co-polymerisation with thieno[3,2-b]thiophene monomers by Stille coupling was reported.
Abstract: We report the synthesis of a selenophene–diketopyrrolopyrrole monomer and its co-polymerisation with selenophene and thieno[3,2-b]thiophene monomers by Stille coupling. The resulting low band gap polymers exhibit ambipolar charge transport in organic field effect transistors. High and balanced electron and hole mobilities in excess of 0.1 cm2 V−1 s−1 were observed in bottom-gate, bottom-contact devices, suggesting that selenophene inclusion is a promising strategy for the development of ambipolar organic semiconductors.

Journal ArticleDOI
01 Feb 2012-Synlett
TL;DR: N-Heteropentacenes and their derivatives have been recently discovered as a new family of organic semiconductors exhibiting high performance in organic field effect transistors (OFETs).
Abstract: N-Heteropentacenes and their derivatives have been recently discoveredas a new family of organic semiconductors exhibiting high performancein organic field effect transistors (OFETs). Introducing nitrogenatoms to the pentacene moiety leads to a large number of structurallyrelated π-backbones with tunable electronic structures,stability, solubility, and molecular packing. This gives considerablefreedom when designing organic semiconductors and provides goodopportunities for studying structure-property relationships.In this account, efforts on developing N-heteropentacenes and N-heteropentacenequinonesas organic semiconductors are reviewed, with focus on the recentwork of our own group. 1 Overview 2 Brief Introduction to Organic Semiconductors and Organic FieldEffect Transistors 3 Dihydrodiazapentacenes and Diazapentacenes 4 A Dihydrotetraazapentacene and Its Methylated Derivatives 5 N-Heteropentacenequinones 6 Silylethynylated N-Heteropentacenes 7 Conclusion and Outlook

Journal ArticleDOI
TL;DR: 2-Methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide (o-MeO-DMBI-I) can be stored and handled in air for extended periods without degradation and is thus promising for various organic electronic devices.
Abstract: 2-(2-Methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide (o-MeO-DMBI-I) was synthesized and employed as a strong n-type dopant for fullerene C(60), a well-known n-channel semiconductor. The coevaporated thin films showed a maximum conductivity of 5.5 S/cm at a doping concentration of 8.0 wt% (14 mol%), which is the highest value reported to date for molecular n-type conductors. o-MeO-DMBI-I can be stored and handled in air for extended periods without degradation and is thus promising for various organic electronic devices.

Journal ArticleDOI
TL;DR: It is reported that MoO(x) can strongly and stably dope carbon nanotubes and graphene and make MoO-CNT composites extremely attractive candidates for practical transparent electrodes.
Abstract: MoOx has been used for organic semiconductor doping, but it had been considered an inefficient and/or unstable dopant. We report that MoOx can strongly and stably dope carbon nanotubes and graphene. Thermally annealed MoOx-CNT composites can form durable thin film electrodes with sheet resistances of 100 Ω/sq at 85% transmittance plain and 85 Ω/sq at 83% transmittance with a PEDOT:PSS adlayer. Sheet resistances change less than 10% over 20 days in ambient and less than 2% with overnight heating to 300 °C in air. The MoOx can be easily deposited either by thermal evaporation or from solution-based precursors. Excellent stability coupled with high conductivity makes MoOx-CNT composites extremely attractive candidates for practical transparent electrodes.

Journal ArticleDOI
TL;DR: In this article, the authors discuss the techniques for measuring the charge carrier mobility and not the theoretical underpinnings of the mechanism of charge transport in organic semiconductors, and the relative merits, as well as limitations for each of these techniques are reviewed.
Abstract: The charge transport characteristics of organic semiconductors are one of the key attributes that impacts the performance of organic electronic and optoelectronic devices in which they are utilized. For improved performance in or- ganic photovoltaic cells, light-emitting diodes, and field-effect transistors (FETs), efficient transport of the charge carriers within the organic semiconductor is especially critical. Charac- terization of charge transport in these organic semiconductors is important both from scientific and technological perspec- tives. In this review, we shall mainly discuss the techniques for measuring the charge carrier mobility and not the theoretical underpinnings of the mechanism of charge transport. Mobility measurements in organic semiconductors and particularly in conjugated polymers, using space-charge-limited current, time of flight, carrier extraction by linearly increasing voltage, dou- ble injection, FETs, and impedance spectroscopy are discussed. The relative merits, as well as limitations for each of these techniques are reviewed. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 1130-1144, 2012

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TL;DR: Graphene flexible transistors at gigahertz frequencies are conducted, and it is shown that solution-based single-layer graphene ideally combines the required properties to achieve high speed flexible electronics on plastic substrates.
Abstract: Flexible electronics mostly relies on organic semiconductors but the limited carrier velocity in polymers and molecular films prevents their use at frequencies above a few megahertz. Conversely, the high potential of graphene for high-frequency electronics on rigid substrates was recently demonstrated. We conducted the first study of solution-based graphene transistors at gigahertz frequencies, and we show that solution-based single-layer graphene ideally combines the required properties to achieve high speed flexible electronics on plastic substrates. Our graphene flexible transistors have current gain cutoff frequencies of 2.2 GHz and power gain cutoff frequencies of 550 MHz. Radio frequency measurements directly performed on bent samples show remarkable mechanical stability of these devices and demonstrate the advantages of solution-based graphene field-effect transistors over other types of flexible transistors based on organic materials.

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TL;DR: In this paper, the authors demonstrate that permanent dipole moments and their orientational order also play a significant role in the charge behavior at organic semiconductor interfaces and demonstrate that charge accumulation properties of bilayer devices composed of polar or nonpolar molecules deposited on a 4,4'-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl layer between the anode and cathode were examined by displacement current measurement and impedance spectroscopy.
Abstract: Charge accumulation at the organic heterointerfaces in multilayer organic light-emitting diodes (OLEDs) is an important process for understanding their device operation, efficiency, and degradation properties. Charge accumulation behavior has typically been analyzed in terms of the energy barrier and difference of the charge carrier mobility across heterointerfaces. In this study, we demonstrate that permanent dipole moments and their orientational order also play a significant role in the charge behavior at organic semiconductor interfaces. The charge accumulation properties of bilayer devices composed of polar or nonpolar molecules deposited on a 4,4’-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl layer between the anode and cathode were examined by displacement current measurement and impedance spectroscopy. In addition, Kelvin probe measurements for the corresponding bilayer structures excluding the cathode were performed to analyze the relationship between the potential profile and charge accumulation pr...