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Effect of Side Chain Length on Film Structure and Electron Mobility of Core-Unsubstituted Pyromellitic Diimides and Enhanced Mobility of the Dibrominated Core Using the Optimized Side Chain
Abstract
Pyromellitic diimides (PyDIs) are π-conjugated electron-transport materials based on an unusually small aromatic core (benzene), which provides low temperature processing and transparency in much of the visible range. We synthesized PyDI derivatives with a systematic series of fluoroalkyl side chains and investigated their film structures and electrical performances in thin-film transistors. The effect of the length of the fluorinated segment in fluoroalkylmethylene side chains was examined. Shorter side chains within this series induce higher electron mobilities, with a maximum of 0.026 cm2 V−1 s−1 achieved with the perfluorobutylmethyl side chain. Atomic force microscopy images and X-ray diffraction peak widths were used as indications of crystallinity correlating with the mobility trend. The perfluorobutylmethyl side chain, when attached to 3,6-dibromo PyDI using a total of three synthetic steps, allowed nearly parallel PyDI cores and an exceptional mobility of 0.2 cm2 V−1 s−1, accompanied by a correspondingly excellent morphology and effective intermolecular packing illustrated by a single crystal X-ray structure. This is the highest PyDI mobility yet reported, and is an unusually high mobility for a compound with such a small core, having such low visible range absorbance, and requiring so few synthetic steps.
... Molecules 2023, 28, 7098 2 of 17 can be controlled with the length of the fluorinated chain at the nitrogen atoms [16]. Bipolar redoxactive organic materials (BROMs) based on N-substituted tetramethylpiperidine 1-oxyl-pyromellitic diimide were developed, which are electroactive compounds having several oxidation states. ...
... It was also found that the electron mobility in thin-film transistors based on pyromellitic diimides tors with particularly wide energy gaps of 3.56 eV and 3.49 eV in the solid state, respectively, were created [15]. It was also found that the electron mobility in thin-film transistors based on pyromellitic diimides can be controlled with the length of the fluorinated chain at the nitrogen atoms [16]. Bipolar redoxactive organic materials (BROMs) based on N-substituted tetramethylpiperidine 1-oxyl-pyromellitic diimide were developed, which are electroactive compounds having several oxidation states. ...
Photochemical properties of symmetrical pyromellitic diimide containing two cymantrenyl fragments at two nitrogen atoms were studied with IR, NMR, UV-vis, ESI-MS, and cyclic voltammetry. It was found that new unstable chelates are formed during photolysis. At the same time, the CO ligand dissociates from two Mn(CO)3 fragments during photoexcitation, which dramatically changes the electronic and redox properties of the molecule compared to the cymantrene derivative containing one imide fragment. Photolysis leads to a color change from light yellow to green. DFT calculations confirmed the possibility of the formation of complexes due to the loss of one or two CO ligands from manganese atoms. The results obtained with variation of photolysis conditions demonstrated the hemilabile character of the Mn-O=C(imide) bond. On addition of external ligands, the color and electrochemical properties changed, which is promising for the use of this complex as a sensor for small molecules.
... Electron transport is highly influenced with the side chain type and length as observed earlier [41]. The J Mater Sci charge transfer is dependent on the crystalline morphology and molecular packing in film [42]. ...
Previous studies have shown influence of aliphatic side chain length and type on the transport properties of naphthalenediimide (NDI) materials by affecting molecular arrangement. There is lack of comparative study on the presence or absence of unsaturation in side chain and its effect on optical and electronic properties of NDI. The present work focuses on the structure–property relationship of four NDI derivatives bearing octyl (C8, OctA-NDI), hexadecyl (C16, HD-NDI), octadecyl (C18, ODA-NDI) and oleyl (C18-un, unsaturated, OLA-NDI) chain on imide-nitrogen. The self-assembling behaviour of the molecules is studied in concentrated solutions as fresh and aged samples in four different solvents by absorbance and emission spectroscopy. With increase in alkyl chain length, the aggregation behaviour is observed to increase. Very interestingly introduction of unsaturation in side chain reduces aggregation and restores the monomeric properties. Self-assembled microstructures formation was studied by scanning electron microscopy where all the four materials show different types of self-assembly formation. Finally, we compared the thermally activated electron conductivity and electron mobility of NDI derivatives, where also the side chain structure clearly influences the electron transport. Electron mobility decreases on increasing chain length from C8 to C18 and again increases in C18-un. A rationale for the structure–property relationship has been given based on the molecular packing and intermolecular π–π interactions. This study contributes significantly towards designing new NDI derivatives bearing long side chains with hampered aggregation for niche applications.
... As we know, the representative air-stable n-type candidates for OFETs are naphthalene diimides (NDIs), perylene diimides (PDIs) and pyromellitic diimides (PyDIs), which have the deep lowest unoccupied molecular orbital (LUMO) levels and high electron mobilities. [38][39][40] Among them, PyDIs exhibited a larger energy gap (E g 3.5 eV) with higher transparency than that of NDIs and PDIs with the same side introductions, 41,42 indicating their great potential in TOFET applications. ...
This study reports two novel n-channel pyromellitic diimide (PyDI) derivatives, PyDI-BOCF3 and PyDI-BSCF3, with particularly wide energy gap of 3.56 eV and 3.49 eV in solid state, which induce the investigations of their charge transport properties, photo stabilities and thin-film transparency of organic field-effect transistors (OFETs) in this work. Although PyDI-BOCF3 and PyDI-BSCF3 exhibit similar two-dimensional (2D) lamellar packing motifs, PyDI-BSCF3 demonstrates stronger electronic couplings than that of PyDI-BSCF3, suggesting it may show better electrical performance in OFETs. As predicted, PyDI-BSCF3 shows electron mobility of 0.09 cm2V-1s-1 at deposition temperature of 70 °C in ambient air, which is higher than PyDI-BOCF3 obtained 0.058 cm2V-1s-1. In contrast, PyDI-BOCF3 exhibits better thermal stability in mobility, which maintained at ~0.056 cm2V-1s-1 after increasing the deposition temperature from room temperature (RT) to 70 °C. More importantly, it is worth mentioning that the wide energy gap of PyDI-BOCF3 and PyDI-BSCF3 leads to excellent photostability in OFETs at illumination condition and optical transparency in the visible range even better than DPh-BTBT thin films on transparent glass and flexible PET substrate.
... Introduction π-conjugated polymers have gained great significance over the past two decades as solution-processable semiconductors in the next generation optoelectronics. [1][2][3][4][5][6][7] Unlike inorganic-based materials currently used in mainstream optoelectronic devices, polymeric semiconductors offer advantageous rheological properties, which allows the fabrication of electro-active ink formulations for high-throughput manufacturing process by printing on light-weight, large-area, and flexible plastic substrates. [8][9][10][11][12][13][14][15][16] In addition, the chemical structure of the polymer π-backbone and substituents can be easily tuned via synthetic organic chemistry. ...
We demonstrate here the design, synthesis and characterization of two new chlorinated polymers, P(NDI2HD–T2Cl2) and P(NDI2OD–T2Cl2) based on N,N′-difunctionalized naphthalene diimide (NDI) and 3,3′-dichloro-2,2′-bithiophene (T2Cl2) moieties. Our results indicate that organic thin-film transistors (OTFTs) based on these new chlorinated polymers exhibit electron mobilities approaching 0.1 cm2V−1s−1 (Ion:Ioff ~ 106–107), with far less ambipolarity due to their lower highest occupied molecular orbital energies, and they are more stable under deleterious high-humidity conditions (RH ~ 60%) and upon submersion in water, compared with those fabricated with the parent non-chlorinated polymers. In addition, OTFTs fabricated with the new chlorinated polymers exhibit excellent operational stabilities with <3% degradations upon bias-stress test.
A series of highly soluble copolymers (EH4P-Th, EH4P-Se, EH4P-TT, and EH4P-BT) based on phosphonate chain-end functionalized diketopyrrolopyrrole monomer and four different counterpart comonomers with varied electron-donating strength and conjugation length have been synthesized, characterized, and used in p-channel organic field-effect transistors (OFETs). It was found that introducing different counterpart comonomers into the main backbone alters the copolymers' intrinsic properties, including absorption, frontier energy levels, molecular microstructure, and charge transport in OFETs. In OFETs fabricated on n-octadecyltrimethoxysilane (OTS)-treated silicon (Si)/silicon dioxide (SiO2) surfaces, the copolymers exhibit good hole transport with maximum hole mobility (μh) of 1.46 × 10-1 cm2 V-1 s-1 in EH4P-TT, which is attributed to edge-on packing, fibrillar intercalating networks, and large crystalline ?-stacking. More intriguing is the fact that high solubility and polarity of the resulting copolymers are induced via polar and bulky phosphonate chain-end groups, allowing for proper OFET operation using not only OTS-untreated Si/SiO2 substrates but also an eco-friendly 2-methyltetrahydrofuran solution process. These results demonstrate promising applications of phosphonate chain-end groups in the design of conjugated polymers for various purposes.
New N, N′-bis(4,6-dimethylpyrimidin-2-yl)- and N, N′-bis(2,3,5,6-tetrafluorophenyl)-substituted pyromellitic diimides were synthesized. Their properties were studied in comparison with the previously synthesized N, N′-bis(4-fluorophenyl)pyromellitic diimide. Thermogravimetry, UV spectroscopy, cyclic voltammetry, and quantum chemical calculations in the framework of the density functional theory were used to characterize the synthesized compounds. The introduction of the pyrimidine cycle significantly decreases the energy of the lowest unoccupied molecular orbital. The highest occupied molecular orbitals in all compounds synthesized are deep-lying (about −7 eV).
The subnaphthalocyanine triimides (SubNcTIs) as solution processable electron acceptors were designed and synthesized by introducing three electron-withdrawing imide groups to subnaphthal- ocyanines. Their solubility and crystallinity could be adjusted conveniently by substituents at imide terminals or boron atom. Their absorption, electrochemistry, thermal properties, and applications as electron acceptors in bulk heterojunction organic solar cells (BHJOSCs) were investigated. SubNcTIs with strong absorption in 300 ~ 750 nm, maximum extinction coefficient of up to 16.8 × 104 M−1 cm−1, and deep lowest unoccupied molecular orbital energy levels (-3.79 ~ -3.90 eV) are expected to be excellent electron acceptors. The four SubNcTIs exhibit good thermal stability, with 5% weight loss temperature higher than 350 °C. Blending with donor polymer of PTQ10, BHJOSCs based on acceptor 9b gave the highest power conversion efficiency (PCE) of 6.25%, which is the highest value among solution processable cyanine family. Space-charge-limited current (SCLC), charge recombination, and charge collection ability measurements showed that PTQ10:9b devices have high and balanced carrier mobility, less charge recombination, and better charge transport, which lead high photovoltaic performance. Grazing incidence wide-angle X-ray scattering (GIWAXS) measurement revealed that relatively strong π–π stacking and large correlation lengths of PTQ10:9b film are favorable for charge transfer, which caused high Jsc of corresponding solar cells. This study demonstrates that SubNcTIs as a promising chromophore could be used to construct potential electron acceptors.
Rylene NIR absorption dyes APMI-N (λmax = 796 nm) and (APMI-K, λmax = 787 nm) with compact conjugation, and excellent solubility, were designed and synthesized. Acid/base sensitive dyes 3, 4, 6, and 7 were obtained as well. Their maximum absorption could be adjusted conveniently from 532 nm to 715 nm by adding acid or base in the solvent. Theoretical simulations indicate both protonation of carbonyl O and electron donating O⁻ in perylene monoimide play a critical role in absorption changing of those dyes.
The development of top-performing π-conjugated polymers that can provide good solubility and processability in non-toxic solvents is imperative for the advancement of organic electronic devices. Herein, we report eco-friendly solution-processable semiconducting copolymers prepared by using two dithienylvinylene (TVT) and selenophene (Se) donor units in conjugation with diketopyrrolopyrrole (DPP) as an acceptor moiety. A series of the copolymers are fabricated with different TVT to Se composition ratios present in the DPP backbone that are represented by [10-0], [7-3], [5-5], [3-7], [2-8], [1-9], and [0-10]. Detailed structure–property investigations covering optical, electrochemical, morphological, and charge-transport properties with respect to the TVT/Se ratio in the copolymers are performed by a series of structural characterization techniques. The best ambipolar charge transport is obtained from [3-7] for which the hole mobility (μh) is 6.31 cm² V⁻¹ s⁻¹ and the electron mobility (μe) is 0.78 cm² V⁻¹ s⁻¹. Moreover, high μh and μe of 4.15 and 0.34 cm² V⁻¹ s⁻¹, respectively, are achieved for [3-7] devices processed from bio-derived, eco-friendly, 2-methyltetrahydrofuran solvent. To the best of our knowledge, these are the highest recorded hole and electron mobilities for ambipolar organic field-effect transistors fabricated from a non-chlorinated solvent to date. Thus, this work is an important scientific step toward developing highly efficient green plastic transistors.
A relatively unexplored, yet important, research focus has been to develop optically transparent organic semiconductor thin films with efficient charge‐transport characteristics. This chapter focuses on transparent organic semiconductors. Organic thin‐film transistors (TFTs) are envisioned as key building blocks of next generation electronic technologies such as low‐power‐consumption flexible displays, electronic papers, plastic RFID tags, and sensors. The historic development of oligothiophene derivatives has significantly advanced the field of organic semiconductors for organic TFT and achieved record charge carrier mobilities for both hole and electron. In the past decade, organic semiconductor research has extensively focused on fused heteroacenes as a new promising class of p‐type semiconductors. Rylene dicarboximides belong to one of the most popular and deeply studied classes of n‐type semiconductors. Among the rylene dicarboximide semiconductors developed in the past two decades for organic electronics, only pyromelliticdicarboximides (PyDI) and naphthalenedicarboximides (NDI) have the potential for use in transparent electronics.
Naphthalene tetracarboxylic diimides (NDIs) derivatives, functionalized with p-fluorophenyl (NDI-FAN), p-chlorophenyl (NDI-ClAN), p-fluorobenzyl (NDI-FBN), were synthesized and used to fabricated organic field effect transistors (OFETs) through vacuum evaporation. All materials display high decomposition temperature and low LUMO energy level facilitating air stable electron transport. OFETs devices based on N-octadecylphosphonic acid (ODPA) treated SiO2/Si substrate affords electron mobility up to 1.8 × 10⁻¹ cm²v⁻¹s⁻¹ with high on/off ratio of 6.7 × 10³ in air. Replaced the p-fluorophenyl by p-fluorobenzyl also leads to a comparable mobility reaching 1.1 × 10⁻¹ cm²v⁻¹s⁻¹ with a low threshold voltage of 1.8 V. Microstructure and morphology of thin films were investigated to shed light on the reason for distinct electrical performance. These results demonstrate that fluorinated N-substituent is an alternative method to optimize molecular arrangement and device performance.
The nitrogenization of phenyl rings on DIM derivatives not only enhances molecular coplanarity but also stabilizes molecular LUMO levels, favoring charge transfer and improving air stability. Therefore, n-type organic field-effect transistors (OFETs) that are based on DIM-N2C8 with nitrogen atoms on both sides of the phenyl rings exhibit a moderate electron mobility of 0.059 cm² V–1 s–1 under ambient conditions.
We report a novel reductive desulfurization reaction involving π-acidic naphthalene diimides (NDI) 1 using thionating agents such as Lawesson’s reagent. Along with the expected thionated NDI derivatives 2–6, new heterocyclic naphtho-p-quinodimethane compounds 7 depicting broken/reduced symmetry were successfully isolated and fully characterized. Empirical results and theoretical modeling suggest that 7 was formed via a six-membered ring oxathiaphosphenine intermediate rather than the usual four-membered ring oxathiaphosphetane of 2–6. Aside from the reduced symmetry in 7 as confirmed by single crystal XRD analysis, we established that the ground state UV-Vis absorption of 7 is red-shifted in comparison to the parent NDI 1. This result was expected in the case of thionated polycyclic diimides. However, unusual low energy transitions originate from Baird 4n-π aromaticity of compounds 7 in lieu of the intrinsic Hückel (4n + 2)π aromaticity as encountered in NDI 1. Moreover, complementary theoretical modeling results also corroborate this change in aromaticity of 7. Consequently, photophysical investigations show that compared to parent NDI 1, 7 can easily access and emits from its T1 state with a phosphorescence 3(7a)* lifetime of τP = 395 µs at 77 K indicative of the formation of the corresponding aromatic triplet species according to the Baird’s rule of aromaticity.
Three kind of 1,2,4,5-benzenetetracarboxylic diimides (BTDs) with halogenated phenyl groups were synthesized through one-step reaction with high yields. Top-contact organic field-effect transistors (OFETs) were fabricated via vacuum deposition of BTDs as the semiconducting channel materials on N-octadecylphosphonic acid (ODPA) treated SiO2/Si substrates. The electronic characterization was measured in ambient condition. All these derivatives exhibit excellent n-channel OFET transport with the highest mobility up to 9.2 × 10⁻² cm²v⁻¹s⁻¹ for BTD-ClAN deposited at room temperature. The crystallinity and morphology of film were improved by modified the substrate and demonstrated by X-ray diffraction (XRD) and atomic force microscopy (AFM). Our results indicates that introducing electron-withdrawing substituents into 1,2,4,5-benzenetetracarboxylic diimides is an effective way to achieve high OFETs performance.
A series of five thionated naphthalene diimides (NDIs) with linear alkyl chains was synthesized and the optoelectronic, self-assembly, and device properties were studied. When tested in organic thin-film transistors, the electron mobilities of the thionated derivatives are three orders of magnitude higher than the non-thionated parent analogue, with the highest mobility measured for cis-S2 (μmax = 7.5 × 10-2 cm2 V-1 s-1). In contrast to branched chain PDIs and NDIs, the electron mobility does not increase appreciably with degree of thionation, and the average mobilities are quite consistent ranging from 3.9 × 10-2 to 7.5 × 10-2 cm2 V-1 s-1 for one to three sulfur for oxygen substitutions. This shows that a high degree of thionation may not be necessary to improve the performance of NDIs and related materials.
The electronic properties of materials targeted for N-type organic field-effect transistors (OFETS), such as pyromellitic diimides (PyDIs), degrade under ambient conditions because of low susceptibility of radical anions. To improve the electronic properties of PyDIs, we synthesized two PyDI analogs by imidization of pyromellitic dianhydride with different amines and thionated them with Lawesson’s reagent in toluene. Thin films of the parent and derivative compounds were then deposited onto Si/SiO2 substrates as prototype OFETs. Thionation and fluorination increased the electron mobility and on/off ratio of the original diimides by two orders of magnitude and improved the threshold voltage and air-stability of diimide compounds. Our derived compounds are expected to realize air-stable N-type OFETs for large-area and flexible electronics
The structure of the title compound, C38H60N2O4, has been determined and is similar to other compounds of this type, being essentially rod-shaped with the packing dominated by the lamellar arrangement of the molecules. The molecule lies on an inversion centre; thus only one alkyl chain, one imide ring and one of the non-bridgehead C atoms in the benzene ring are unique. The diimide moieties are arranged in a classic herring-bone structure, with two close non-hydrogen-atom contacts of 2.874 (5) and 2.946 (5) Å.
The functioning principles of electronic sensors based on organic semiconductor field-effect transistors (OFETs) are presented. The focus is on biological sensors but also chemical ones are reviewed to address general features. The field-induced electronic transport and the chemical and biological interactions for the sensing, each occurring at the relevant functional interface, are separately introduced. Once these key learning points have been acquired, the combined picture for the FET electronic sensing is proposed. The perspective use of such devices in point-of-care is introduced, after some basics on analytical biosensing systems are provided as well. This tutorial review includes also a necessary overview of the OFET sensing structures, but the focus will be on electronic rather than electrochemical detection. The differences among the structures are highlighted along with the implications on the performance level in terms of key analytical figures of merit such as: repeatability, sensitivity and selectivity.
Organic optoelectronics is an emerging field that exploits the unique properties of conjugated organic materials to develop new applications that require a combination of performance, low cost, light weight, and processability. For instance, disposable or wearable electronics, light-emitting diodes, smart tags, sensors, and solar cells all fall into this active area of research. Single crystals of conjugated organic molecules are, undoubtedly, the materials with the highest degree of order and purity among the variety of different forms of organic semiconductors. Electronic devices comprising these materials, such as single-crystal transistors and photo-conductors developed during the last decade, are by far the best performers in terms of the fundamental parameters such as charge-carrier mobility, exciton diffusivity, concentration of defects, and operational stability. Extremely low density of defects and the resultant remarkable electrical characteristics of some of the organic single crystal devices allow experimental access to the intrinsic charge transport properties not dominated by charge scattering and trapping. This enables basic studies of the physics of organic semiconductors, including examining the intrinsic structure-property relationship, thus providing a test bed for charge and energy transport theories. The goal of this issue of MRS Bulletin is to provide a broad overview of the state of the art of the field of organic semiconductor single-crystal materials, devices, and theory.
This critical review discusses specific chemical and physicochemical requirements which must be met for organic compounds to be considered as promising materials for applications in organic electronics. Although emphasis is put on molecules and macromolecules suitable for fabrication of field effect transistors (FETs), a large fraction of the discussed compounds can also be applied in other organic or hybrid (organic-inorganic) electronic devices such as photodiodes, light emitting diodes, photovoltaic cells, etc. It should be of interest to chemists, physicists, material scientists and electrical engineers working in the domain of organic electronics (423 references).
Naphthalene diimide (NDI) main chain conjugated polymers have seen a high and steadily increasing level of activity during the last five years. It is mainly the intriguing properties of high electron mobilities and tunable absorption up to the near IR region that have driven researchers to design new polymeric structures having main chain NDIs in the backbone. While the field is still in its infancy, many important results have been obtained and the first structure–function relationships can be drawn. By reviewing synthetic aspects (step growth and chain growth polycondensation techniques), polymeric architectures made, structure formation, and applications in OFET devices and organic photovoltaics, the reader is equipped with some of the key aspects of this important class of materials.
We report comprehensive characterization of electrolyte-gated polymer thin-film transistors (TFTs) incorporating solution processable polymer semiconductors and high capacitance “ion gel” gate dielectrics. The ion gel dielectrics comprise self-assembled networks of triblock copolymers such as poly(styrene-b-methylmethacrylate-b-styrene) [PS-PMMA-PS] that are swollen with ionic liquids, e.g., (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]). The capacitance of the gels is exceptionally large (>10 μF/cm2 at 10 Hz), which is derived from the high concentration of mobile ions and facilitates operation of ion gel-gated organic TFTs (GEL-OTFTs) at very low voltages (< 2.5 V). Gate-induced hole densities in GEL-OTFTs employing different polythiophene semiconductors in the channel are on the order of 1014 carriers/cm2, with associated saturation hole mobilities that are also remarkably large, 1 cm2/(V s), likely because of the large gate-induced carrier densities. Examination of the frequency response of GEL-OTFTs indicates that increases in the OFF current with frequency ultimately limit switching speed; the cutoff frequency correlates with the ionic conductivity versus frequency response of the gel dielectric. Further, attenuated total internal reflection infrared (ATR-IR) spectroscopy of the ion gel/polymer semiconductor gate stack reveals that the conductance switching mechanism in GEL-OTFTs spans both electrochemical and electrostatic (field effect) regimes. Specifically, modeling of the time dependence of the near-infrared polaron absorption in gated GEL-OTFTs indicates that the [TFSI]− anion diffusivity in regioregular poly(3-hexylthiophene) is on the order of 10−12 cm2/s at room temperature. This diffusivity implies that, for time scales greater than 1 ms, there is significant penetration (>1 nm) of [TFSI]− anion into the polymer semiconductor at the gel/polymer semiconductor interface, corresponding to an electrochemical doping process. On the other hand, for time scales shorter than 1 ms (i.e., for GEL-OTFT switching frequencies >1 kHz), the device switching mechanism can be viewed as primarily electrostatic as average ion penetration depths are less than 1 nm.
The synthesis and field-effect transistor (FET) electron mobility of ten N-substituted naphthalene 1,4,5,8-tetracarboxylic diimide (NTCDI) derivatives deposited at ambient and elevated temperatures are reported. Mobilities >0.01 cm2/(V s) were measured in air for three NTCDIs with partially fluorinated substituents, and >0.001 cm2/(V s) for a hydroxy-terminated compound. Mobilities 0.001−0.1 cm2/(V s) were also found for three n-alkyl NTCDIs, but only under vacuum; FET operation with gold bottom contacts was enabled by specific thiol coatings of the contacts. The highest mobility in air, >0.1 cm2/(V s), was conferred by 4-trifluoromethylbenzyl substitution, while 1H,1H-perfluorooctyl substitution resulted in an on/off ratio in air >105. Solution electrochemistry and solid-state X-ray and electron diffraction were employed to partially explain the results, and applications of the materials to complementary circuits are considered.
Electrochemistry is an increasingly popular technique for the characterization of new compounds. The basic thermodynamic quantity that is assigned to an electrode process is the standard or formal reduction potential (E^o or E^f). In aqueous solution the measurement of reduction potentials is facilitated by the use of reliable and universally accepted reference electrodes such as the normal hydrogen electrode (NHE) or the saturated calomel electrode (SCE). In many instances electrochemical measurements in water are impossible due to insolubility or instability of the compound. Unfortunately, no universal reference electrode exists for nonaqueous solvents.
This review covers the various classes of molecular structures that may be used as the basis for the synthesis of organic semiconductors that favor electron transport in field-effect transistors and related electronic and optoelectronic devices. The types of compounds include tetracarboxylic diimides, heterocyclic oligomers, fullerenes, and metal complexes. Approaches to polymers are also mentioned. Although brief discussions of transistor operation and applications are included, the emphasis is on the rationale for choosing these structures, and synthetic routes to them. Performance of exemplary compounds in transistors is also discussed.
Organic semiconductors have great potential as the active material in low-cost, large area plastic electronics, whether as light-emitting diodes (LEDs), field-effect transistors (FETs) or solar cells. Organic semiconducting materials retain the processability associated with polymers while maintaining good optoelectronic properties, for example, high absorption coefficients for photons in the visible, and field-effect mobilities comparable with that of amorphous silicon. The elucidation of important structure−property relationships is vital for the design of functional, high-performance organic semiconductors. In this short review, we summarize such relationships stemming from the halogenation of organic semiconductors. While it has been known in the past decade that fluorination lowers the energy levels in carbon based systems, induces stability and electron transport, less is known about the effect of the other halogens. Chlorination has recently been shown to be a viable route to n-type materials. The bandgap of conjugated compounds can also be decreased slightly by the addition of Cl, Br, and I to the aromatic core. The effect of the halogenated moieties on the packing of molecules is discussed.
Dicyanoperylene diimides (PDI-RCN2) is an effective organic semiconductor used for the fabrication air-stable, flexible, and optically transparent n-channel field effect transistors (FET). These organic semiconductor materials feature a unique combination of high electron mobility, environmental stability, and solution processability. These materials yield complementary organic logic and frequency-generating devices with unprecedented performance. NDI-8CN and NDI-8CN2 are two core-cyanated naphthalene diimide (NDI) semiconductors, which represent the first air-stable, high-mobility, and transparent organic n-type semiconductors. All thin films were characterized by OFET measurements, X-ray diffraction (XRD), and tapping-mode atomic force microscopy (AFM). This air-stable, flexible, transparent OFET, fabricated with NDI-8CN2 to demonstrate the unique materials properties, exhibits a mobility of 0.03 cm2.
N,N′-bis(3-(perfluoroctyl)propyl)-1,4,5,8-naphthalenetetracarboxylic acid diimide (8–3-NTCDI) was newly synthesized, as were related fluorooctylalkyl-NTCDIs and alkyl-NTCDIs. The 8–3-NTCDI-based organic thin-film transistor (OTFT) on an octadecyltrimethoxysilane (OTS)-treated Si/SiO2 substrate shows apparent electron mobility approaching 0.7 cm2 V-1s-1 in air. The fluorooctylethyl-NTCDI (8–2-NTCDI) and fluorooctylbutyl-NTCDI (8–4-NTCDI) had significantly inferior properties even though their chemical structures are only slightly different, and nonfluorinated decyl and undecyl NTCDIs did not operate predictably in air. From atomic force microscopy, the 8–3-NTCDI active layer deposited with the substrate at 120 °C forms a polycrystalline film with grain sizes >4μm. Mobilities were stable in air for one week. After 100 days in air, the average mobility of three OTFTs decreased from 0.62 to 0.12 cm2 V-1s-1, but stabilized thereafter. The threshold voltage (VT) increased by 15 V in air, but only by 3 V under nitrogen, after one week. On/off ratios were stable in air throughout. We also investigated transistor stability to gate bias stress. The transistor on hexamethlydisilazane (HMDS) is more stable than that on OTS with mobility comparable to amorphous Si TFTs. VT shifts caused by ON (30 V) and OFF (–20 V) gate bias stress for the HMDS samples for 1 hour were 1.79 V and 1.27 V under N2, respectively, and relaxation times of 106 and 107 s were obtained using the stretched exponential model. These performances are promising for use in transparent display backplanes.
We report a class of highly efficient electroluminescent materials based on fluorinated iridium compounds. Using aluminum as the cathode, a device, using fac-tris[5-fluoro-2(5-trifluoromethyl-2-pyridinyl)phenyl-C,N]iridium (Ir-2h) as the luminescent layer, displayed intense electroluminescence at 525 nm with an efficiency of 20 cd/A and a maximum radiance of 4800 cd/m<sup>2</sup>. Differing from the previously reported Ir(ppy) <sub> 3 </sub>, Ir-2h can be used in the undiluted form without the use of a charge-transporting host. This indicates that Ir-2h by itself has good enough charge-transporting properties. Photoluminescence studies at room temperature and 77 K revealed that electroluminescence originates from the metal-to-ligand charge transfer state with a quantum yield of 0.56 for Ir-2h and 0.5 for Ir(ppy) <sub> 3 </sub> in toluene at room temperature. In the thin-film form, photoluminescence quantum yield of Ir-2h is a factor of 10 greater than that of Ir(ppy) <sub> 3 </sub> due to the larger self-quenching effect of Ir(ppy) <sub> 3 </sub>. © 2001 American Institute of Physics.
A high‐mobility organic semiconductor employed as the active material in a field‐effect transistor does not guarantee per se that expectations of high performance are fulfilled. This is even truer if a downscaled, short channel is adopted. Only if contacts are able to provide the device with as much charge as it needs, with a negligible voltage drop across them, then high expectations can turn into high performances. It is a fact that this is not always the case in the field of organic electronics.
In this review, we aim to offer a comprehensive overview on the subject of current injection in organic thin film transistors: physical principles concerning energy level (mis)alignment at interfaces, models describing charge injection, technologies for interface tuning, and techniques for characterizing devices. Finally, a survey of the most recent accomplishments in the field is given. Principles are described in general, but the technologies and survey emphasis is on solution processed transistors, because it is our opinion that scalable, roll‐to‐roll printing processing is one, if not the brightest, possible scenario for the future of organic electronics.
With the exception of electrolyte‐gated organic transistors, where impressively low width normalized resistances were reported (in the range of 10 Ω·cm), to date the lowest values reported for devices where the semiconductor is solution‐processed and where the most common architectures are adopted, are ∼10 kΩ·cm for transistors with a field effect mobility in the 0.1–1 cm ² /Vs range. Although these values represent the best case, they still pose a severe limitation for downscaling the channel lengths below a few micrometers, necessary for increasing the device switching speed. Moreover, techniques to lower contact resistances have been often developed on a case‐by‐case basis, depending on the materials, architecture and processing techniques. The lack of a standard strategy has hampered the progress of the field for a long time. Only recently, as the understanding of the rather complex physical processes at the metal/semiconductor interfaces has improved, more general approaches, with a validity that extends to several materials, are being proposed and successfully tested in the literature. Only a combined scientific and technological effort, on the one side to fully understand contact phenomena and on the other to completely master the tailoring of interfaces, will enable the development of advanced organic electronics applications and their widespread adoption in low‐cost, large‐area printed circuits.
We investigated substituent-induced variations in microstructure and physical properties of a family of functionalized pentacenes, materials currently of intensive interest for making organic electronic devices such as thin film transistors, to shed light on the complex relationships between functionalization, film formation, stability, and microstructure. In this study, the pentacenes were modified with alkyl acetylene or alkylsilylethynyl groups with systematic variations in the alkyl chain length. With a proper side chain, this modification can effectively disrupt the herringbone packing seen in neat pentacene, promoting face-to-face arrangements between the acene rings and providing solubility in a variety of convenient solvents. Thin films can be readily formed by solution casting from THF, bromobenzene, toluene and other organic solvents. We have investigated the structure and properties of the functionalized pentacenes using UV-vis spectroscopy, hot stage optical microscopy, differential scanning calorimetry, transmission electron microscopy, X-ray and electron diffraction. The materials show regular variations in their thermal behavior, crystal packing and macroscopic properties as the chemistry of the side-group substituent changes.
Existing knowledge about Scherrer constants is reviewed and a summary is given of the interpretation of the broadening arising from small crystallites. Early work involving the half-width as a measure of breadth has been completed and Scherrer constants of simple regular shapes have been determined for all low-angle reflections (h2 + k2 + l2 ≤ 100) for four measures of breadth. The systematic variation of Scherrer constant with hkl is discussed and a convenient representation in the form of contour maps is applied to simple shapes. The relation between the `apparent' crystallite size, as determined by X-ray methods, and the `true' size is considered for crystallites having the same shape. If they are of the same size, then the normal Scherrer constant applies, but if there is a distribution of sizes, a modified Scherrer constant must be used.
The first calculations on polyenes and the attention given to the issues of bond length alternation and ordering of the lowest singlet excited state served as impetus for the description of the electronic structure of π-conjugated materials. Initially, the goal of most calculations was to determine the nature of the (unrelaxed) excited states playing a role in the second-order and third-order molecular polarizabilities. Later on, the relaxation effects in the excited states and impact of intermolecular interactions drew significant interest. These and other related works point to the increased significance of the dynamic processes taking place in π-conjugated materials, such as charge transport, charge recombination, exciton formation, exciton diffusion, or exciton dissociation.
A study was conducted to investigate the role of molecular order and solid-state structure in organic field-effect transistors (OFET). Investigations revealed that the operation principle of an OFET relied on the application of an electric field that led to the formation of a conducting channel in the dielectric or semiconductor interface. The performance of such an OFET was mainly determined by the charge carrier mobility, which had been improved significantly after the fabrication of the first OFET. The investigations also revealed that there were large number of factors that affect the device performance and the comparison of measurements needed to be carried out with caution. There are two main categories of organic semiconductors that were used in OFETs, such as conjugated polymers and small conjugated molecules with low molecular weight. It was also revealed that the importance of the molecular ordering for fabricating high performance OFETs was more important in small-molecule-based transistors.
We designed a new naphthalenetetracarboxylic diimide (NTCDI) semiconductor molecule with long fluoroalkylbenzyl side chains. The side chains, 1.2 nm long, not only aid in self-assembly and kinetically stabilize injected electrons but also act as part of the gate dielectric in field-effect transistors. On Si substrates coated only with the 2 nm thick native oxide, NTCDI semiconductor films were deposited with thicknesses from 17 to 120 nm. Top contact Au electrodes were deposited as sources and drains. The devices showed good transistor characteristics in air with 0.1-1 μA of drain current at 0.5 V of V(G) and V(DS) and W/L of 10-20, even though channel width (250 μm) is over 1000 times the distance (20 nm) between gate and drain electrodes. The extracted capacitance-times-mobility product, an expression of the sheet transconductance, can exceed 100 nS V(-1), 2 orders of magnitude higher than typical organic transistors. The vertical low-frequency capacitance with gate voltage applied in the accumulation regime reached as high as 650 nF/cm(2), matching the harmonic sum of capacitances of the native oxide and one side chain and indicating that some gate-induced carriers in such devices are distributed among all of the NTCDI core layers, although the preponderance of the carriers are still near the gate electrode. Besides demonstrating and analyzing thickness-dependent NTCDI-based transistor behavior, we also showed <1 V detection of dinitrotoluene vapor by such transistors.
Functional organic field-effect transistors (OFETs) have attracted increasing attention in the past few years due to their wide variety of potential applications. Research on functional OFETs underpins future advances in organic electronics. In this review, different types of functional OFETs including organic phototransistors, organic memory FETs, organic light emitting FETs, sensors based on OFETs and other functional OFETs are introduced. In order to provide a comprehensive overview of this field, the history, current status of research, main challenges and prospects for functional OFETs are all discussed.
Analogous to conventional inorganic semiconductors, the performance of organic semiconductors is directly related to their molecular packing, crystallinity, growth mode, and purity. In order to achieve the best possible performance, it is critical to understand how organic semiconductors nucleate and grow. Clever use of surface and dielectric modification chemistry can allow one to control the growth and morphology, which greatly influence the electrical properties of the organic transistor. In this Review, the nucleation and growth of organic semiconductors on dielectric surfaces is addressed. The first part of the Review concentrates on small-molecule organic semiconductors. The role of deposition conditions on film formation is described. The modification of the dielectric interface using polymers or self-assembled mono-layers and their effect on organic-semiconductor growth and performance is also discussed. The goal of this Review is primarily to discuss the thin-film formation of organic semiconducting species. The patterning of single crystals is discussed, while their nucleation and growth has been described elsewhere (see the Review by Liu et. al).([¹]) The second part of the Review focuses on polymeric semiconductors. The dependence of physico-chemical properties, such as chain length (i.e., molecular weight) of the constituting macromolecule, and the influence of small molecular species on, e.g., melting temperature, as well as routes to induce order in such macromolecules, are described.
A new class of n-type semiconductors for organic thin film transistors (OTFTs), based on core-expanded naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)malonitrile groups, is reported. The first two representatives of these species, derived from long branched N-alkyl chains, have been successfully used as active layers for high-performance, ambient-stable, solution-processed n-channel OTFTs. Their bottom-gate top-contact devices fabricated by spin-coating methods exhibit high electron mobilities of up to 0.51 cm(2) V(-1) s(-1) with current on/off ratios of 10(5)-10(7), and small threshold voltages below 10 V under ambient conditions. As this class of n-type organic semiconductors has relatively low-lying LUMO levels and good film-formation ability, they also displayed good environmental stability even with prolonged exposure to ambient air. Both the device performance and the ambient stability are among the best for n-channel OTFTs reported to date.
Charge carrier mobility is at the center of organic electronic devices. The strong couplings between electrons and nuclear motions lead to complexities in theoretical description of charge transport, which pose a major challenge for the fundamental understanding and computational design of transport organic materials. This tutorial review describes recent progresses in developing computational tools to assess the carrier mobility in organic molecular semiconductors at the first-principles level. Some rational molecular design strategies for high mobility organic materials are outlined.
The synthesis, processing, and device performance of polymeric semiconductors has been reported. The polysilicon TFT technology is used for active matrix organic light-emitting diode (AMOLED) as the higher carrier mobilities of polysilicon as compared to a-Si, and increased stability of polysilicon-based devices under bias stress, are more effective in AMOLED. The radio frequency (RF) wireless applications are required in large area, self-powered, or maximized range device is partially driven by the fundamental physics of the frequency regimes in which they operate. Large area, high throughput manufacturing of organic electronic roducts is most efficiently enabled by solution based, additive printing techniques. Regioregular (RR) poly(3-hexylthiophene) (P3HT) is an exemplary semiconducting polymer due to its ready availability, ease of processing from solution, and its promising electrical properties arising from a highly crystalline microstructure.
The syntheses and comprehensive characterization of 14 organic semiconductors based on perylene bisimide (PBI) dyes that are equipped with up to four halogen substituents in the bay area of the perylene core and five different highly fluorinated imide substituents are described. The influence of the substituents on the LUMO level and the solid state packing of PBIs was examined by cyclic voltammetry and single crystal structure analyses of seven PBI derivatives, respectively. Top-contact/bottom-gate organic thin film transistor (OTFT) devices were constructed by vacuum deposition of these PBIs on SiO(2) gate dielectrics that had been pretreated with n-octadecyl triethoxysilane in vapor phase (OTS-V) or solution phase (OTS-S). The electrical characterization of all devices was accomplished in a nitrogen atmosphere as well as in air, and the structural features of thin films were explored by grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM). Several of those PBIs that bear only hydrogen or up to two fluorine substitutents at the concomitantly flat PBI core afforded excellent n-channel transistors, in particular, on OTS-S substrate and even in air (mu > 0.5 cm(2) V(-1) s(-1); I(on)/I(off) > 10(6)). The best OTFTs were obtained for 2,2,3,3,4,4,4-heptafluorobutyl-substituted PBI 1a ("PTCDI-C4F7") on OTS-S with n-channel field effect mobilities consistently >1 cm(2) V(-1) s(-1) and on-to-off current rations of 10(6) in a nitrogen atmosphere and in air. For distorted core-tetrahalogenated (fluorine, chlorine, or bromine) PBIs, less advantageous solid state packing properties were found and high performance OTFTs were obtained from only one tetrachlorinated derivative (2d on OTS-S). The excellent on-to-off current modulation combined with high mobility in air makes these PBIs suitable for a wide range of practical applications.
Three pyromellitic diimides were synthesized in high yields by one conventional reaction between pyromellitic dianhydride and various amines. The films made from these pyromellitic diimides derivatives exhibit a mobility up to 0.079 cm2/(V.s). In addition, the on/off ratios of n-channel devices are as high as 1 000 000.
The charge carrier dynamics in organic semiconductors has been traditionally discussed with the models used in inorganic crystalline and amorphous solids but this analogy has severe limitations because of the more complicated role of nuclear motions in organic materials. In this perspective, we discuss how a new approach to the modelling of charge transport is emerging from the alliance between the conventional quantum chemical methods and the methods more traditionally used in soft-matter modelling. After describing the conventional limit cases of charge transport we discuss the problems arising from the comparison of the theory with the experimental and computational results. Several recent applications of numerical methods based on the propagation of the wavefunction or kinetic Monte Carlo methods on soft semiconducting materials are reviewed.
Highly colored and photoluminescent naphthalene bisimide dyes have been synthesized from 2,6-dichloronaphthalene bisanhydride 1 by means of a stepwise nucleophilic displacement of the two chlorine atoms by alkoxides and/or alkyl amines. The alkoxy-substituted derivatives are yellow dyes with green emission and low photoluminescence quantum yields, whereas the amine-substituted derivatives exhibit a color range from red to blue with strong photoluminescence up to 76%. Structure-property relationships for this class of two-dimensional chromophores were evaluated based on a single-crystal X-ray analysis for dye 5a, the observed solvatochromism, and quantum-chemical calculations. Owing to the simple tuning of the absorption properties over the whole visible range by the respective substituents, the pronounced brilliancy, and the intense photoluminescence, this class of dyes is considered to be highly suited for numerous applications such as fluorescent labeling of biomacromolecules and light-harvesting in supramolecular assemblies. As an important step towards such applications efficient FRET (fluorescence resonance energy transfer) has been demonstrated for a covalently tethered bichromophoric compound that contains a red and a blue naphthalene bisimide dye.
For electron or hole transfer between neighboring conducting polymer strands or oligomers, the intrinsic charge-transfer rate is dictated by the charge-resonance integral and by the reorganization energy due to geometric relaxation. To explain conduction anisotropy and other solid-state effects, a multivariate, systematic analysis of bandwidth as a function of intermolecular orientations is undertaken for a series of oligoheterocycles, using first-principles methods. While cofacial oligomers show the greatest bandwidths at a given intermolecular C-C contact distance, for a fixed center-to-center intermolecular distance, tilted pi-stacking increases pi-overlap (particularly for LUMO orbitals) and decreases electrostatic repulsion, yielding optimum tilt angles for packing of approximately 40-60 degrees at small intermolecular separations. The calculations also reveal that bandwidths and intrinsic mobilities of holes and electrons in conjugated oligoheterocycles can be quite comparable.
Structural and electronic criteria for ambient stability in n-type organic materials for organic field-effect transistors (OFETs) are investigated by systematically varying LUMO energetics and molecular substituents of arylene diimide-based materials. Six OFETs on n+-Si/SiO2 substrates exhibit OFET response parameters as follows: N,N'-bis(n-octyl)perylene-3,4:9,10-bis(dicarboximide) (PDI-8): mu = 0.32 cm2 V(-1) s(-1), Vth = 55 V, I(on)/I(off) = 10(5); N,N'-bis(n-octyl)-1,7- and N,N'-bis(n-octyl)-1,6-dibromoperylene-3,4:9,10-bis(dicarboximide) (PDI-8Br2): mu = 3 x 10(-5) cm2 V(-1) s(-1), Vth = 62 V, I(on)/I(off) = 10(3); N,N'-bis(n-octyl)-1,6,7,12-tetrachloroperylene-3,4:9,10-bis(dicarboximide) (PDI-8Cl4): mu = 4 x 10(-3) cm2 V(-1) (s-1), Vth = 37 V, I(on)/I(off) = 10(4); N,N'-bis(n-octyl)-2-cyanonaphthalene-1,4,5,8-bis(dicarboximide) (NDI-8CN): mu = 4.7 x 10(-3) cm2 V(-1) s(-1), Vth = 28, I(on)/I(off) = 10(5); N,N'-bis(n-octyl)-1,7- and N,N'-bis(n-octyl)-1,6-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2): mu = 0.13 cm2 V(-1) s(-1), Vth = -14 V, I(on)/I(off) = 10(3); and N,N'-bis(n-octyl)-2,6-dicyanonaphthalene-1,4,5,8-bis(dicarboximide) (NDI-8CN2): mu = 0.15 cm2 V(-1) s(-1), Vth = -37 V, I(on)/I(off) = 10(2). Analysis of the molecular geometries and energetics in these materials reveals a correlation between electron mobility and substituent-induced arylene core distortion, while Vth and I(off) are generally affected by LUMO energetics. Our findings also indicate that resistance to ambient charge carrier trapping observed in films of N-(n-octyl)arylene diimides occurs at a molecular reduction potential more positive than approximately -0.1 V (vs SCE). OFET threshold voltage shifts between vacuum and ambient atmosphere operation suggest that, at E(red1) < -0.1 V, the interfacial trap density increases by greater than approximately 1 x 10(13) cm(-2), while, for semiconductors with E(red1) > -0.1 V, the trap density increase is negligible. OFETs fabricated with the present n-type materials having E(red1) > -0.1 V operate at conventional gate biases with minimal hysteresis in air. This reduction potential corresponds to an overpotential for the reaction of the charge carriers with O2 of approximately 0.6 V. N,N'-1H,1H-Perfluorobutyl derivatives of the perylene-based semiconductors were also synthesized and used to fabricate OFETs, resulting in air-stable devices for all fluorocarbon-substituted materials, despite generally having E(red1) < -0.1 V. This behavior is consistent with a fluorocarbon-based O2 barrier mechanism. OFET cycling measurements in air for dicyanated vs fluorinated materials demonstrate that energetic stabilization of the charge carriers results in greater device longevity in comparison to the OFET degradation observed in air-stable semiconductors with fluorocarbon barriers.
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