Showing papers on "Pentacene published in 2012"

The synthesis, crystal structure and charge-transport properties of hexacene

Abstract: Acenes can be thought of as one-dimensional strips of graphene and they have the potential to be used in the next generation of electronic devices. However, because acenes larger than pentacene have been found to be unstable, it was generally accepted that they would not be particularly useful materials under normal conditions. Here, we show that, by using a physical vapour-transport method, platelet-shaped crystals of hexacene can be prepared from a monoketone precursor. These crystals are stable in the dark for a long period of time under ambient conditions. In the crystal, the molecules are arranged in herringbone arrays, quite similar to that observed for pentacene. A field-effect transistor made using a single crystal of hexacene displayed a hole mobility significantly higher than that of pentacene. This result suggests that it might be instructive to further explore the potential of other higher acenes.

Competition between singlet fission and charge separation in solution-processed blend films of 6,13-bis(triisopropylsilylethynyl)pentacene with sterically-encumbered perylene-3,4:9,10-bis(dicarboximide)s

Abstract: The photophysics and morphology of thin films of N,N-bis(2,6-diisopropylphenyl)perylene-3,4:9,10-bis(dicarboximide) (1) and the 1,7-diphenyl (2) and 1,7-bis(3,5-di-tert-butylphenyl) (3) derivatives blended with 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) were studied for their potential use as photoactive layers in organic photovoltaic (OPV) devices. Increasing the steric bulk of the 1,7-substituents of the perylene-3,4:9,10-bis(dicarboximide) (PDI) impedes aggregation in the solid state. Film characterization data using both atomic force microscopy and X-ray diffraction showed that decreasing the PDI aggregation by increasing the steric bulk in the order 1 < 2 < 3 correlates with a decrease in the density/size of crystalline TIPS-Pn domains. Transient absorption spectroscopy was performed on ∼100 nm solution-processed TIPS-Pn:PDI blend films to characterize the charge separation dynamics. These results showed that selective excitation of the TIPS-Pn results in competition between ultrafast sing...

Quasiparticle and optical spectroscopy of the organic semiconductors pentacene and PTCDA from first principles

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.

Highly Crystalline Soluble Acene Crystal Arrays for Organic Transistors: Mechanism of Crystal Growth During Dip‐Coating

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).

N-Heteropentacenes and N-Heteropentacenequinones:From Molecules to Semiconductors

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

High‐Performance Transistors and Complementary Inverters Based on Solution‐Grown Aligned Organic Single‐Crystals

Abstract: Constructing a complementary inverter is technically more complex because both pand n-channel transistors are required to be patterned onto a common substrate. Here, we report a simple solution processing method to fabricate complementary inverters based on n-channel C 60 single crystals and p-channel 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene) single crystals. We achieved a signal gain as high as 155. Hence, this work provides a platform to study high-performance complementary circuits based on organic single-crystals. Organic FETs have been widely used for electronic applications such as displays [ 5 , 6 ] and sensors. [ 7–9 ] Organic single-crystals show the best FET performance with the highest charge mobility among organic materials. The p-channel organic single-crystal FETs have exhibited hole mobility as high as 40 cm 2 V − 1 s − 1 , [ 10–14 ]

High-Quality Large-Area Graphene from Dehydrogenated Polycyclic Aromatic Hydrocarbons

Abstract: Recent studies show that, at the initial stage of chemical vapor deposition (CVD) of graphene, the isolated carbon monomers will form defective carbon clusters with pentagons that degrade the quality of synthesized graphene. To circumvent this problem, we demonstrate that high-quality centimeter-sized graphene sheets can be synthesized on Cu foils by a self-assembled approach from defect-free polycyclic aromatic hydrocarbons (PAHs) in a high vacuum (HV) chamber without hydrogen. Different molecular motifs, namely coronene, pentacene, and rubrene, can lead to significant difference in the quality of resulting graphene. For coronene, monolayer graphene flakes with an adequate quality can be achieved at a growth temperature as low as 550 °C. For the graphene obtained at 1000 °C, transport measurements performed on back-gated field-effect transistors (FETs) with large channel lengths (∼30 μm) exhibit a carrier mobility up to ∼5300 cm2 V–1 s–1at room temperature. The underlying growth mechanism, which mainly i...

Solvent and polymer matrix effects on TIPS-pentacene/polymer blend organic field-effect transistors

Abstract: We report on a systematic study of solvent and polymer matrix effects on the phase segregation behavior of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) blends incorporated into two different amorphous polymer matrices, poly (α-methyl styrene) and poly (triarylamine), and using two solvents, chlorobenzene and tetralin. Optical microscopy, X-ray diffraction analyses, and optical absorption measurements are used to evaluate the film morphology, crystallinity, and optical density, respectively. These analyses are correlated with the extent of vertical segregation of TIPS-pentacene, as observed for the blended films by depth-profile XPS analyses. The microstructure and vertical phase segregation of TIPS-pentacene in blend films are found to be strongly influenced by the choice of solvent. Tetralin, a solvent with a high boiling temperature, was found to be more desirable for achieving distinct phase segregation/crystallization of TIPS-pentacene in blend films and best performance in OFETs with a dual-gate geometry. The electrical properties of top and bottom channels were consistent with the morphological characterization and OFETs processed from tetralin showed higher mobility values than those from chlorobenzene. Further modification of the annealing conditions in the TIPS-pentacene/PTAA/tetralin ternary system led to top-gate OFETs with mobility values up to 2.82 cm2/Vs.

Acene‐Modified Triphenylamine Dyes for Dye‐Sensitized Solar Cells: A Computational Study

Abstract: A series of metal-free acene-modified triphenylamine dyes (benzene to pentacene, denoted as TPA-AC1 to TPA-AC5) are investigated as organic sensitizers for application in dye-sensitized solar cells (DSSCs). A combination of density functional theory (DFT), density functional tight-binding (DFTB), and time-dependent DFT (TDDFT) approaches is employed. The effects of acene units on the spectra and electrochemical properties of the acene-modified TPA organic dyes are demonstrated. The dye/(TiO2)46 anatase nanoparticle systems are also simulated to show the electronic structures at the interface. The results show that from TPA-AC1 to TPA-AC5 with increasing sizes of the acenes, the absorption and fluorescence spectra are systematically broadened and red-shifted, but the oscillator strength and electron injection properties are reduced. The molecular orbital contributions show increasing localization on the bridging acene units from TPA-AC1 to TPA-AC5. From the theoretical examination of some key parameters including free enthalpy related to the electron injection, light-harvesting efficiency, and the shift of semiconductor conduction band, TPA-AC3 with an anthracene moiety demonstrates a balance of the above crucial factors. TPA-AC3 is expected to be a promising dye with desirable energetic and spectroscopic parameters in the DSSC field, which is consistent with recent experimental work. This study is expected to deepen our understanding of TPA-based organic dyes and assist the molecular design of new metal-free dyes for the further optimization of DSSCs.

Excitons in molecular crystals from first-principles many-body perturbation theory: Picene versus pentacene

Abstract: By solving the first-principles many-body Bethe-Salpeter equation, we compare the optical properties of two prototype and technological relevant organic molecular crystals: picene and pentacene. Albeit very similar for the structural and electronic properties, picene and pentacene show remarkable differences in their optical spectra. While for pentacene the absorption onset is due to a charge-transfer exciton, in picene it is related to a strongly localized Frenkel exciton. The detailed comparison between the two materials allows us to discuss, on general grounds, how the interplay between the electronic band dispersion and the exchange electron-hole interaction plays a fundamental role in setting the nature of the exciton. It represents a clear example of the relevance of the competition between localization and delocalization in the description of two-particle electronic correlation.

Thermionic Emission and Tunneling at Carbon Nanotube–Organic Semiconductor Interface

Abstract: We study the charge carrier injection mechanism across the carbon nanotube (CNT)–organic semiconductor interface using a densely aligned carbon nanotube array as electrode and pentacene as organic semiconductor. The current density–voltage (J–V) characteristics measured at different temperatures show a transition from a thermal emission mechanism at high temperature (above 200 K) to a tunneling mechanism at low temperature (below 200 K). A barrier height of ∼0.16 eV is calculated from the thermal emission regime, which is much lower compared to the metal/pentacene devices. At low temperatures, the J–V curves exhibit a direct tunneling mechanism at low bias, corresponding to a trapezoidal barrier, while at high bias the mechanism is well described by Fowler–Nordheim tunneling, which corresponds to a triangular barrier. A transition from direct tunneling to Fowler–Nordheim tunneling further signifies a small injection barrier at the CNT/pentacene interface. Our results presented here are the first direct ex...

Epitaxial Growth of π-Stacked Perfluoropentacene on Graphene-Coated Quartz

Abstract: Chemical-vapor-deposited large-area graphene is employed as the coating of transparent substrates for the growth of the prototypical organic n-type semiconductor perfluoropentacene (PFP). The graphene coating is found to cause face-on growth of PFP in a yet unknown substrate-mediated polymorph, which is solved by combining grazing-incidence X-ray diffraction with theoretical structure modeling. In contrast to the otherwise common herringbone arrangement of PFP in single crystals and “standing” films, we report a π-stacked arrangement of coplanar molecules in “flat-lying” films, which exhibit an exceedingly low π-stacking distance of only 3.07 A, giving rise to significant electronic band dispersion along the π-stacking direction, as evidenced by ultraviolet photoelectron spectroscopy. Our study underlines the high potential of graphene for use as a transparent electrode in (opto-)electronic applications, where optimized vertical transport through flat-lying conjugated organic molecules is desired.

On the Origin of Contact Resistances of Organic Thin Film Transistors

Abstract: A model is presented that describes the gate-voltage-dependent contact resistance and channel-length-dependent charge carrier mobility of small-molecule-based organic thin-film transistors in top and bottom drain/source contact configuration.

Intermolecular hydrogen-bonded organic semiconductors—Quinacridone versus pentacene

Abstract: Quinacridone is a five-ring hydrogen-bonded molecule analogous in structure and size to the well-known organic semiconductor pentacene. Unlike pentacene, quinacridone has limited intramolecular π-conjugation and becomes highly colored in the solid state due to strong intermolecular electronic coupling. We found that quinacridone shows a field-effect mobility of 0.1 cm2/V·s, comparable to mobilities of pentacene in similarly prepared devices. Photoinduced charge generation in single-layer quinacridone metal-insulator-metal diodes is more than a hundred times more efficient than in pentacene devices. Photoinduced charge transfer from quinacridone to C60 is not effective, as evidenced by measurements in heterojunctions with C60. Hydrogen-bonded organic solids may provide new avenues for organic semiconductor design.

Inkjet-Printed Reduced Graphene Oxide/Poly(Vinyl Alcohol) Composite Electrodes for Flexible Transparent Organic Field-Effect Transistors

Abstract: The characteristics of organic field-effect transistors (OFETs) prepared with inkjet-patterned reduced graphene oxide (RGO)/poly(vinylalcohol) (PVA) composite electrodes were studied. PVA was blended with graphite oxide to enhance exfoliation of the graphite sheets and to provide for stable inkjet printing. Multistep reduction based on a combination of chemical and thermal reduction was conducted to increase the conductivity. Use of the inkjet-patterned RGO/PVA electrodes increased field-effect mobility of the bottom-contact pentacene FETs to 0.23 cm2/(V·s), which was significantly enhanced relative to that of FETs with Au or PEDOT:PSS electrodes. Moreover, we successfully prepared all-organic flexible transparent OFETs using inkjet-patterned RGO/PVA electrodes on plastic substrates.

Ammonia gas sensor based on pentacene organic field-effect transistor

Abstract: Ammonia (NH3) sensors based on bottom contact organic field-effect transistors (OFETs) were created using pentacene as an active layer and polymerthylmethacrylate (PMMA) as an insulator. The OFET sensors exhibited a change in the electric characteristic such as the threshold voltage, the saturation current, and the field-effect charge carrier mobility when the sensors were exposed to different NH3 concentrations. Meanwhile, the favorable and rapid NH3 response characteristics of the OFET sensors were observed from the change in the drain-source current as a function of time, when the sensors were exposed to various cycles of exposure/evacuation of different NH3 concentrations ranging from 10 to 100 ppm. Moreover, the environmental stability of the OFET sensors to constantly detect the NH3 gas in air was studied by storing the sensors in air for 30 days.

In-situ probe of gate dielectric-semiconductor interfacial order in organic transistors: origin and control of large performance sensitivities.

Abstract: Organic thin film transistor (OTFT) performance is highly materials interface-dependent, and dramatic performance enhancements can be achieved by properly modifying the semiconductor/gate dielectric interface. However, the origin of these effects is not well understood, as this is a classic “buried interface” problem that has traditionally been difficult to address. Here we address the question of how n-octadecylsilane (OTS)–derived self-assembled monolayers (SAMs) on Si/SiO2 gate dielectrics affect the OTFT performance of the archetypical small-molecule p-type semiconductors P-BTDT (phenylbenzo[d,d]thieno[3,2-b;4,5-b]dithiophene) and pentacene using combined in situ sum frequency generation spectroscopy, atomic force microscopy, and grazing incidence and reflectance X-ray scattering. The molecular order and orientation of the OTFT components at the dielectric/semiconductor interface is probed as a function of SAM growth mode in order to understand how this impacts the overlying semiconductor growth mode,...

The relationship between threshold voltage and dipolar character of self-assembled monolayers in organic thin-film transistors.

Abstract: We report a quantitative study that describes and correlates the threshold voltage of low-voltage organic field-effect transistors with the molecular structure of self-assembled monolayer dielectrics. We have observed that the component of the dipole moment of such self-assembled molecules perpendicular to the surface correlates linearly with the threshold voltage shift in devices. The model was validated using three different organic semiconductors (pentacene, α,α′-dihexylsexithiophene, and fullerene–C60) on six different self-assembled monolayers. The correlation found can help optimize future devices, by tuning the dipole moments of the molecules that constitute the self-assembled monolayer.

Unusual thermoelectric behavior indicating a hopping to bandlike transport transition in pentacene.

Abstract: An unusual increase in the Seebeck coefficient with increasing charge carrier density is observed in pentacene thin film transistors. This behavior is interpreted as being due to a transition from hopping transport in static localized states to bandlike transport, occurring at temperatures below $\ensuremath{\sim}250\text{ }\text{ }\mathrm{K}$. Such a transition can be expected for organic materials in which both static energetic disorder and dynamic positional disorder are important. While clearly visible in the temperature and density dependent Seebeck coefficient, the transition hardly shows up in the charge carrier mobility.

Nonvolatile memory based on pentacene organic field-effect transistors with polystyrenepara-substituted oligofluorene pendent moieties as polymer electrets

Abstract: We report the nonvolatile memory characteristics of pentacene-based organic field-effect transistors (OFET) using polystyrenepara-substituted with π-conjugated oligofluorenes (P(St-Fl)n (n = 1–3)) as chargeable polymer electrets. Effects of fluorene conjugated length on the surface structure and memory characteristics of pentacene OFET were investigated. Among these polymer electrets, the device with the P(St-Fl) exhibited the highest field-effect mobility of 0.47 cm2 V−1 s−1 due to the largest grain size of pentacene growth. The device with P(St-Fl)3 revealed the largest hysteresis window of 76 V due to it having the longest fluorene conjugation length among the studied electrets. The smallest difference of the HOMO energy level between pentacene and P(St-Fl)3 facilitated the charge transfer from pentacene to the polymer electret. The shifts on the transfer curves in both positive and negative directions could be reversibly controlled when applied an external gate bias of ±100 V for a short time (1 μs), indicating the fast trapping-detrapping ability of the polymer electrets. The devices showed excellent nonvolatile behaviors for bistable switching. The ON and OFF states were maintained over 104 s with the Ion/Ioff current ratio of 105–106. The write-read-erase-read (WRER) cycles could be operated over 100 cycles. This study suggested that surface characteristics, charge transport, and memory characteristics of pentacene-based OFET can be manipulated by polymer electrets with different pendent conjugation length.

Pentacene-Based Organic Thin Film Transistors for Ammonia Sensing

Abstract: Non-invasive ammonia sensors are attractive alternatives for the diagnoses of a variety of chronic diseases such as liver cirrhosis and renal failure. A low cost pentacene-based organic thin film transistor (OTFT) fabricated by a novel and simple process was demonstrated to be highly sensitive and specific for ammonia gas. Various measurement parameters that reflected OTFT device characteristics for ammonia detection were investigated. Significant variations of the turn-on current, intrinsic mobility, and threshold voltage (Vth) were observed while subthreshold swing (S.S.) was almost unchanged to the alteration of ammonia concentration. The OTFT device detected 0.5 ~ 5 ppm concentration ammonia gas at room temperature, which is in the critical range that can distinguish between healthy person and paticents with liver cirrhosis and renal failure. The sensitivity of the device was further enhanced following a simple UV irradiation treatment to modify the functional groups on poly(methyl methacrylate) (PMMA) dielectric layer. Possible interference for ammonia detection such as humidity effect and selectivity among nitrogen, alcohol, carbon dioxide, acetone, methane and ammonia were also examined. We concluded that the proposed pentacene-based OTFT is a promising device for the future application in non-invasive medical diagnoses.

Anisotropic charge transport and contact resistance of 6,13-bis(triisopropylsilylethynyl) pentacene field-effect transistors fabricated by a modified flow-coating method

Abstract: Using a modified flow-coating method, bottom-gate/bottom-contact type organic field-effect transistors (OFETs) with a highly oriented active layer of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) were fabricated. The flow-coated TIPS-pentacene films were fairly uniform and consisted of arrays of needle-shaped crystals along the flow-coating direction. The uniformity allowed us to determine the contact resistance by a transfer line method. The usefulness of the modified flow-coating method for fabricating high performance OFETs has been demonstrated, and we found that not only the field-effect mobility but also the contact resistance significantly depends on the channel current direction with respect to the flow-coating direction.

Fabrication and characterization of controllable grain boundary arrays in solution-processed small molecule organic semiconductor films

Abstract: We have produced solution-processed thin films of 6,13-bis(tri-isopropyl-silylethynyl) pentacene with grain sizes from a few micrometers up to millimeter scale by lateral crystallization from a rectangular stylus. Grains are oriented along the crystallization direction, and the grain size transverse to the crystallization direction depends inversely on the writing speed, hence forming a regular array of oriented grain boundaries with controllable spacing. We utilize these controllable arrays to systematically study the role of large-angle grain boundaries in carrier transport and charge trapping in thin film transistors. The effective mobility scales with the grain size, leading to an estimate of the potential drop at individual large-angle grain boundaries of more than 1 volt. This result indicates that the structure of grain boundaries is not molecularly abrupt, which may be a general feature of solution-processed small molecule organic semiconductor thin films, where relatively high energy grain bounda...

The application of a high-k polymer in flexible low-voltage organic thin-film transistors

Abstract: Although much progress has been made in the study of high-mobility organic semiconductors in recent years, one major challenge for organic thin film transistors (OTFTs) in real applications is the relatively high operating voltage, which is mainly related to the gate dielectric of the OTFT. Here we show the application of a high-k poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) (P(VDF-TrFE-CFE)) as the gate dielectric of flexible low-voltage pentacene OTFTs for the first time. The performance of the OTFTs is optimized by modifying the surface of P(VDF-TrFE-CFE) films with various thin polymer films by a solution process. The flexible OTFTs show excellent performance at the operating voltage of 4 V and good stability after 1000 bending tests. It is expected that P(VDF-TrFE-CFE) is a suitable gate dielectric for low-voltage OTFTs based on various small-molecule organic semiconductors because P(VDF-TrFE-CFE) terpolymer films can be easily modified with different thin polymer films by a solution process.

Hybrid pentacene/a-silicon solar cells utilizing multiple carrier generation via singlet exciton fission

Abstract: Silicon dominates the solar cell market because of its abundance, mature production processes, and high efficiencies, with the best solar cells approaching the Shockley-Queisser limit. Multiple exciton photogeneration provides a route to solar cells that surpass the Shockley-Queisser limit, and we report the use of pentacene, for which photogenerated singlet excitons rapidly convert into two lower-energy spin-triplet excitons. We report solar cells that couple amorphous silicon to pentacene. We show that a thin layer of nanocrystals between silicon and pentacene allows simultaneously harnessing low-energy photons absorbed in silicon and high-energy photons absorbed in pentacene, generating two excitons via singlet fission.

Thermal Narrowing of the Electronic Bandwidths in Organic Molecular Semiconductors: Impact of the Crystal Thermal Expansion

Abstract: We report on electronic-structure calculations for the pentacene and rubrene crystals, based on experimental crystal geometries measured at different temperatures. The results are in very good agreement with angle-resolved photoelectron spectroscopy data that indicate that the widths of the valence and conduction bands in both materials become narrower at higher temperatures. Our findings strongly suggest that the thermal bandwidth narrowing in the pentacene and rubrene crystals is primarily caused by the thermal expansion of the lattice rather than by a renormalization of the transfer integrals induced by a polaron effect. The effect of thermal expansion on the charge-transport properties is also discussed.