Showing papers on "Pentacene published in 2010"

Exciton fission and charge generation via triplet excitons in pentacene/C60 bilayers.

Abstract: Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs.

Flexible low-voltage organic transistors and circuits based on a high-mobility organic semiconductor with good air stability.

Abstract: Flexible transistors and circuits based on dinaphtho-[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT), a conjugated semiconductor with a large ionization potential (5.4 eV), are reported. The transistors have a mobility of 0.6 cm(2) V-1 s(-1) and the ring oscillators have a stage delay of 18 mu s. Due to the excellent stability of the semiconductor, the devices and circuits maintain 50% of their initial performance for a period of 8 months in ambient air.

Trap density of states in small-molecule organic semiconductors: A quantitative comparison of thin-film transistors with single crystals

Abstract: We show that it is possible to reach one of the ultimate goals of organic electronics: producing organic field-effect transistors with trap densities as low as in the bulk of single crystals. We studied the spectral density of localized states in the band gap [trap density of states (trap DOS)] of small-molecule organic semiconductors as derived from electrical characteristics of organic field-effect transistors or from space-charge-limited current measurements. This was done by comparing data from a large number of samples including thin-film transistors (TFT's), single crystal field-effect transistors (SC-FET's) and bulk samples. The compilation of all data strongly suggests that structural defects associated with grain boundaries are the main cause of ``fast'' hole traps in TFT's made with vacuum-evaporated pentacene. For high-performance transistors made with small-molecule semiconductors such as rubrene it is essential to reduce the dipolar disorder caused by water adsorbed on the gate dielectric surface. In samples with very low trap densities, we sometimes observe a steep increase in the trap DOS very close $(l0.15\text{ }\text{eV})$ to the mobility edge with a characteristic slope of 10--20 meV. It is discussed to what degree band broadening due to the thermal fluctuation of the intermolecular transfer integral is reflected in this steep increase in the trap DOS. Moreover, we show that the trap DOS in TFT's with small-molecule semiconductors is very similar to the trap DOS in hydrogenated amorphous silicon even though polycrystalline films of small-molecules with van der Waals-type interaction on the one hand are compared with covalently bound amorphous silicon on the other hand.

Direct evaluation of low-field mobility and access resistance in pentacene field-effect transistors

Abstract: Organic field-effect transistors (OFETs) suffer from limitations such as low mobility of charge carriers and high access resistance. Direct and accurate evaluation of these quantities becomes crucial for understanding the OFETs properties. We introduce the Y function method (YFM) to pentacene OFETs. This method allows us to evaluate the low-field mobility without the access or contact resistance influence. The low-field mobility is shown to be more appropriate than the currently applied field-effect mobility for the OFETs’ performance evaluation. Its unique advantage is to directly suppress the contact resistance influence in individual transistors, although such contact resistance is a constant as compared to the widely accepted variable one with respect to the gate voltage. After a comparison in detail with the transmission-line method, the YFM proved to be a fast and precise alternative method for the contact resistance evaluation. At the same time, how the contact resistance affects the effective mobi...

Interaction of charge carriers with lattice vibrations in oligoacene crystals from naphthalene to pentacene.

Abstract: A key feature of organic π-conjugated materials is the strong connection between their electronic and geometric structures. In particular, it has been recently demonstrated that nonlocal electron-vibration (electron-phonon) interactions, which are related to the modulation of the electronic couplings (transfer integrals) between adjacent molecules by lattice vibrations, play an important role in the charge-transport properties of organic semiconductors. Here, we use density functional theory calculations and molecular mechanics simulations to estimate the strength of these nonlocal electron-vibration couplings in oligoacene crystals as a function of molecular size from naphthalene through pentacene. The effect of each optical vibrational mode on the electronic couplings is evaluated quantitatively. The results point to a very strong coupling to both intermolecular vibrational modes and intramolecular (including high-frequency) modes in all studied systems. Importantly, our results underline that the amount of relaxation energy associated with nonlocal electron-phonon coupling decreases as the size of the molecule increases. This work establishes an original relationship between chemical structure and nonlocal vibrational coupling in the description of charge transport in organic semiconductor crystals.

The mechanisms underlying the enhanced resolution of atomic force microscopy with functionalized tips

Abstract: By functionalizing the tip of an atomic force microscope (AFM) with a molecule or an atom that significantly contributes to the tip–sample interaction, the resolution can be dramatically enhanced. The interaction and therefore the resolution crucially depend on the chemical nature of the tip termination. Employing a tip functionalized with a CO molecule, atomic resolution of a pentacene molecule was recently demonstrated. In this work, the interaction between the CO tip and the pentacene imaged are studied with first principles calculations. The calculated frequency shifts compare very well with the experiment. The different energy contributions are analyzed and the Pauli energy is computed. We demonstrate that the source of the high resolution is Pauli repulsion, whereas van der Waals and electrostatic interactions only add a diffuse attractive background.

On the interface dipole at the pentacene-fullerene heterojunction : A theoretical study

Abstract: The electronic structure at organic/organic interfaces plays a key role, among others, in defining the quantum efficiency of organics-based photovoltaic cells. Here, we perform quantum-chemical and microelectrostatic calculations on molecular aggregates of various sizes and shapes to characterize the interfacial dipole moment at pentacene/C60 heterojunctions. The results show that the interfacial dipole mostly originates in polarization effects due to the asymmetry in the multipolar expansion of the electronic density distribution between the interacting molecules, rather than in a charge transfer from donor to acceptor. The local dipole is found to fluctuate in sign and magnitude over the interface and appears as a sensitive probe of the relative arrangements of the pentacene and C60 molecules (and of the resulting local electrical fields sensed by the molecular units).

Layered distribution of charge carriers in organic thin film transistors.

Abstract: Drain-source current in organic thin-film transistors has been monitored in situ and in real time during the deposition of pentacene. The current starts to flow when percolation of the first monolayer (ML) occurs and, depending on the deposition rate, saturates at a coverage in the range 2-7 MLs. The number of active layers contributing to the current and the spatial distribution of charge carriers are modulated by the growth mode. The thickness of the accumulation layer, represented by an effective Debye length, scales as the morphological correlation length. These results show that the effective Debye length is not just a material parameter, but depends on the multiscale morphology. Earlier controversial results can be unified within this framework.

Density functional theoretical study of pentacene/noble metal interfaces with van der Waals corrections: Vacuum level shifts and electronic structures

Abstract: In order to clarify factors determining the interface dipole, we have studied the electronic structures of pentacene adsorbed on Cu(111), Ag(111), and Au(111) by using first-principles density functional theoretical calculations. In the structural optimization, a semiempirical van der Waals (vdW) approach [S. Grimme, J. Comput. Chem. 27, 1787 (2006)] is employed to include long-range vdW interactions and is shown to reproduce pentacene-metal distances quite accurately. The pentacene-metal distances for Cu, Ag, and Au are evaluated to be 0.24, 0.29, and 0.32 nm, respectively, and work function changes calculated by using the theoretically optimized adsorption geometries are in good agreement with the experimental values, indicating the validity of the present approach in the prediction of the interface dipole at metal/organic interfaces. We examined systematically how the geometric factors, especially the pentacene-substrate distance (ZC), and the electronic properties of the metal substrates contribute to...

Growth and structure of pentacene films on graphite: Weak adhesion as a key for epitaxial film growth

Abstract: The microstructure of pentacene films grown on the basal plane of graphite has been investigated. By combining various complementary techniques including scanning tunneling microscopy, atomic force microscopy, x-ray diffraction, thermal desorption spectroscopy, and x-ray absorption spectroscopy the molecular orientation, crystalline structure, and morphology of the films as well as their thermal stability have been characterized in detail as a function of the film thickness. Initial film growth leads to the formation of a commensurate monolayer consisting of flat-lying molecules while upon subsequent deposition epitaxially ordered (022)-oriented pentacene films are formed which adopt the Siegrist phase. The detailed analysis shows that this epitaxial growth of films with an essentially recumbent molecular orientation is brought about by a slight rotation of the molecules in the first layer around their long molecular axis upon deposition of overlying molecular layers. Such a structural modification is unusual and becomes possible by the rather weak adsorption energy on graphite. In contrast, a very different film structure including an upright orientation of molecules even in the first layer is found on nonperfect but rough graphite surfaces leading to the formation of (001)-oriented films which initially reveal the thin-film phase and continue to grow in the Campbell phase of pentacene.

Synthesis, structure, and optoelectronic properties of a new twistacene 1,2,3,4,6,13-hexaphenyl-7 : 8,11 : 12-bisbenzo-pentacene

Abstract: We report the synthesis, optical and electrochemical properties, as well as the fabrication of light-emitting devices for a new twistacene 1,2,3,4,6,13-hexaphenyl-7 : 8,11 : 12-bisbenzo-pentacene (HBP 1). Its structure, determined by X-ray crystallography, confirmed that this material has a twisted topology with the torsion angle as high as 23.0°. HBP 1 showed bright green emission both in solution and solid state. The HOMO–LUMO gap of HBP 1 calculated from the difference between the half-wave redox potentials (E1/2ox = +0.74 eV and E1/2red = −1.93 eV) is 2.67 eV, which is in good agreement with the band gap, 2.64 eV, derived from the UV-Vis absorption data. Organic light emitting devices using HBP 1 as the emitters have been fabricated. The results revealed that twistacenes are promising materials to enhance the efficiency of OLEDs.

Flexible Low-Voltage Organic Thin-Film Transistors Enabled by Low-Temperature, Ambient Solution-Processable Inorganic/Organic Hybrid Gate Dielectrics

Abstract: We report here on the design, synthesis, processing, and dielectric properties of novel cross-linked inorganic/organic hybrid blend (CHB) dielectric films which enable low-voltage organic thin-film transistor (OTFT) operation. CHB thin films (20−43 nm thick) are readily fabricated by spin-coating a zirconium chloride precursor plus an α,ω-disilylalkane cross-linker solution in ambient conditions, followed by curing at low temperatures (∼150 °C). The very smooth CHB dielectrics exhibit excellent insulating properties (leakage current densities ∼10−7 A/cm2), tunable capacitance (95−365 nF/cm2), and high dielectric constants (5.0−10.2). OTFTs fabricated with pentacene as the organic semiconductor function well at low voltages (<−4.0 V). The morphologies and microstructures of representative semiconductor films grown on CHB dielectrics prepared with incrementally varied compositions and processing conditions are investigated and shown to correlate closely with the OTFT response.

Dielectric Surface-Controlled Low-Voltage Organic Transistors via n-Alkyl Phosphonic Acid Self-Assembled Monolayers on High-k Metal Oxide

Abstract: In this paper, we report on n-alkyl phosphonic acid (PA) self-assembled monolayer (SAM)/hafnium oxide (HfO(2)) hybrid dielectrics utilizing the advantages of SAMs for control over the dielectric/semiconductor interface with those of high-k metal oxides for low-voltage organic thin film transistors (OTFTs). By systematically varying the number of carbon atoms of the n-alkyl PA SAM from six to eighteen on HfO(2) with stable and low leakage current density, we observe how the structural nature of the SAM affects the thin-film crystal structure and morphology, and subsequent device performance of low-voltage pentacene based OTFTs. We find that two primary structural factors of the SAM play a critical role in optimizing the device electrical characteristics, namely, the order/disorder of the SAM and its physical thickness. High saturation-field-effect mobilities result at a balance between disordered SAMs to promote large pentacene grains and thick SAMs to aid in physically buffering the charge carriers in pentacene from the adverse effects of the underlying high-k oxide. Employing the appropriate n-alkyl PA SAM/HfO(2) hybrid dielectrics, pentacene-based OTFTs operate under -2.0 V with low hysteresis, on-off current ratios above 1 x 10(6), threshold voltages below -0.6 V, subthreshold slopes as low as 100 mV dec(-1), and field-effect mobilities as high as 1.8 cm(2) V(-1) s(-1).

Nonvolatile nano-floating gate memory devices based on pentacene semiconductors and organic tunneling insulator layers

Abstract: Controlled gold nanoparticle (AuNP)-based nonvolatile memory devices were developed based on pentacene organic transistors and polymethylmethacrylate (PMMA) insulator layers. The memory device had the following configuration: n+Si gate/SiO2 blocking oxide/polyelectrolytes/AuNP/PMMA tunneling dielectric layer/Au source-drain. According to the programming/erasing operations, the memory device showed good programmable memory characteristics with a large memory window. In addition, good reliability was confirmed by the data retention characteristics. The fabrication procedures for the charge trapping and tunneling layers were based on simple solution processes (by dipping and spin-coating) and the maximum processing temperature was <100 °C, so this method has potential applications in plastic/flexible electronics.

Electronic Properties of Pentacene versus Triisopropylsilylethynyl-Substituted Pentacene: Environment-Dependent Effects of the Silyl Substituent

Abstract: Energy measures of the intra- and intermolecular electronic effects of triisopropylsilylethynyl substitution on pentacene have been obtained from the combination of closely related gas phase and solid phase ultraviolet photoelectron spectroscopy (UPS) measurements along with solution electrochemical measurements. The results show that the shift to lower ionization energy that is expected with this substitution and observed in the gas phase measurements becomes negligible in solution and is even reversed in the solid phase. The principles that emerge from this analysis are supported by electronic structure calculations at the density functional theory level. The relation between the gas phase and solid phase UPS measurements illustrated here provides a general approach to investigating the electronic effects acting on molecules in the condensed phase, which in this case are greater than the direct substituent electronic effects within the molecule. Electronic properties such as lower ionization energies bu...

Improved thermoelectric performance of organic thin-film elements utilizing a bilayer structure of pentacene and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ)

Abstract: We demonstrate an improved thermoelectric performance of small molecular thin films fabricated by thermal deposition of pentacene as a p-type conduction layer. To enhance the performance, a bilayer structure composed of an intrinsic pentacene layer and an acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane layer is utilized as the prototype thermoelectric element. With the bilayer structure, the electrical conductivity reaches 0.43 S/cm, exhibiting a positive Seebeck coefficient of about 200 μV/K. We thus obtain a high power factor of 2.0 μW/mK2 with an optimized layer thickness.

Interfacial trap density-of-states in pentacene- and ZnO-based thin-film transistors measured via novel photo-excited charge-collection spectroscopy.

Abstract: Organic and oxide-based thin-fi lm transistors (TFTs) have recently been expected to promote advances in display electronics based on low cost, high transparency, and fl exibility. [ 1 , 2 ] The operational stabilities of such TFTs are thus important, strongly depending on the nature and density of charge traps present at the channel/dielectric interface or in the thinfi lm channel itself. [ 3–5 ] In particular, the illuminated display back panel is susceptible to the charge-trap-induced instability. Therefore the characterization of these traps is critical. Despite such immediate demands, appropriate methodologies to directly analyze the interfacial properties are quite rare, to the best of our knowledge, and conventional analysis techniques are rather unsatisfactory. [ 5 ] Photoluminescence (PL) is a direct method to observe deep-level defects in semiconductors but can not be used on a working device with interfaces. [ 6 , 7 ] Deep-level transient spectroscopy (DLTS) and gate-bias stress techniques may be used with working TFT devices to characterize the interface trap states, utilizing thermal and electrical energies, respectively. [ 7–11 ] However, DLTS is not adequate for organic based devices due to their thermal instability and gatebias stress technique can never properly examine the electronic states of the deep-level traps. For instance Lang et al. [ 10 ] and Goldmann et al. [ 11 ] extracted the density-of-gap states from organic fi eld-effect transistors (OFETs), obtaining the activation energy for trap charge release as a function of the gate bias. They quantitatively determined the total value of interfacial trap

Real-Time Changes in the Optical Spectrum of Organic Semiconducting Films and Their Thickness Regimes during Growth

Abstract: We present real-time in situ studies of optical spectra during thin film growth of several prototype organic semiconductors (pentacene, perfluoropentacene, and diindenoperylene) on SiO2. These data provide insight into surface and interface effects that are of fundamental importance and of relevance for applications in organic electronics. With respect to the bulk, the different molecular environment and structural changes within the first few monolayers can give rise to significant optical changes. Similar to interface-driven phenomena in, e.g., magnetism, spectral changes as a function of thickness d are a very general effect, decaying as 1/d in the simplest approximation. We observe energy shifts of 50-100 meV, rather small changes of the exciton-phonon coupling, and new transitions in specific systems, which should be considered as general features of the growth of organics.

Bias-Stress Effect in Pentacene Organic Thin-Film Transistors

Abstract: The effects of bias stress in integrated pentacene organic transistors are studied and modeled for different stress conditions. It is found that the effects of bias stress can be expressed in terms of the shift in applied gate voltage ?V for a given current. An empirical equation describing ?V in terms of different gate and drain bias stress measurements and stress times is presented and verified. In the measured devices, ?V saturates at 14 V, independent of the gate bias-stress condition. A model based on carrier trapping rate equation that accounts for this ?V saturation is developed. The model suggests that the ?V saturation is due to the small density of traps compared to the channel carrier density.

High Tg Cyclic Olefin Copolymer Gate Dielectrics for N,N'-Ditridecyl Perylene Diimide Based Field-Effect Transistors: Improving Performance and Stability with Thermal Treatment

Abstract: A novel application of ethylene-norbornene cyclic olefin copolymers (COC) as gate dielectric layers in organic field-effect transistors (OFETs) that require thermal annealing as a strategy for improving the OFET performance and stability is reported. The thermally-treated N,N'-ditridecyl perylene diimide (PTCDI-C13)-based n-type FETs using a COC/SiO 2 gate dielectric show remarkably enhanced atmospheric performance and stability. The COC gate dielectric layer displays a hydrophobic surface (water contact angle = 95° ± 1°) and high thermal stability (glass transition temperature =181 °C) without producing crosslinking. After thermal annealing, the crystallinity improves and the grain size of PTCDI-C13 domains grown on the COC/SiO 2 gate dielectric increases significantly. The resulting n-type FETs exhibit high atmospheric field-effect mobilities, up to 0.90 cm 2 V -1 s -1 in the 20 V saturation regime and long-term stability with respect to H 2 O/O 2 degradation, hysteresis, or sweep-stress over 110 days. By integrating the n-type FETs with p-type pentacene-based FETs in a single device, high performance organic complementary inverters that exhibit high gain (exceeding 45 in ambient air) are realized.

A modular synthetic approach to conjugated pentacene di-, tri-, and tetramers.

Abstract: Pentacene and functionalized pentacenes are leading candidates for small-molecule semiconductor applications and have been widely explored over the past decade. On the other hand, examples of pentacene polymers 3a–c] and oligomers remain scarce, even though they present obvious opportunities for discovery. In comparison to polymers, monodisperse oligomers provide the added benefit of structural homogeneity. Furthermore, oligomers allow for the systematic study of structure–property relationships, in which a property of interest (e.g., HOMO–LUMO gap (HOMO = highest occupied molecular orbital, LUMO = lowest unoccupied molecular orbital)) is examined as a function of oligomer length. If oligomers of sufficient length are accessible, the effective conjugation length can be determined by observing saturation of a property; that is, when the oligomer starts to behave like the polymer. To date, the synthesis of functionalized pentacenes has relied, to a large extent, on the addition of nucleophiles to acenequinones such as 6,13-pentacenequinone, followed by reduction to form the aromatic pentacene framework. This methodology cannot, however, easily be adapted to the synthesis of conjugated oligomers. Such synthetic limitations are due, in part, to the reactive nature of the pentacene core and to the limits of current methodology for desymmetrizing the pentacene chromophore. It is unlikely that this issue can be effectively addressed by condensation and cycloaddition reactions to form acenes, since such methods often produce isomeric mixtures. Oxidative acetylenic homoand cross-coupling reactions of terminal and/or haloacetylenes (e.g., Hay and Cadiot–Chodkiewicz reactions) are a well-established means of assembling carbon-rich materials. These reactions, when paired with recently developed methods to form unsymmetrical pentacenes, would serve as the foundation for the synthesis of a homologous series of pentacene oligomers. Pentacenes that bear a terminal acetylene functionality, such as 1 or 2 (Scheme 1), are quite reactive, and have been ineffective to date when isolated and used as precursors for

Improvement in open circuit voltage of organic solar cells by inserting a thin phosphorescent iridium complex layer

Abstract: The authors develop organic solar cells (OSCs) with a multicharge separation (MCS)interface by inserting ∼ 4 nm thin layer of phosphorescent bis(2-(4-tertbutylphenyl) benzothiazolato- N , C 2 , ) iridium (acetylacetonate) ( t-bt ) 2 Ir ( acac ) comparing with copper phthalocyanine (CuPc) between pentacene/ C 60 heterojunction. The result showed that open circuit voltage is remarkably enhanced without obviously changing external quantum efficiency η EQE and short-circuit current density. As a result, power conversion efficiency is improved from 1.53% for pentacene/ C 60 device to 1.83% with sandwich layer ( t-bt ) 2 Ir ( acac ) and 1.98% with CuPc. Moreover, a suitable equivalent circuit model is proposed to intuitively reveal the inner photogeneration process of OSC with MCSinterface.

Hysteresis behaviour of low-voltage organic field-effect transistors employing high dielectric constant polymer gate dielectrics

Abstract: Here, we report on the fabrication of low-voltage-operating pentacene-based organic field-effect transistors (OFETs) that utilize crosslinked cyanoethylated poly(vinyl alcohol) (CR-V) gate dielectrics. The crosslinked CR-V-based OFET could be operated successfully at low voltages (below 4 V), but abnormal behaviour during device operation, such as uncertainty in the field-effect mobility (μ) and hysteresis, was induced by the slow polarization of moieties embedded in the gate dielectric (e.g. polar functionalities, ionic impurities, water and solvent molecules). In an effort to improve the stability of OFET operation, we measured the dependence of μ and hysteresis on dielectric thickness, CR-V crosslinking conditions and sweep rate of the gate bias. The influence of the CR-V surface properties on μ, hysteresis, and the structural and morphological features of the pentacene layer grown on the gate dielectric was characterized and compared with the properties of pentacene grown on a polystyrene surface.

Effect of pentacene–dielectric affinity on pentacene thin film growth morphology in organic field-effect transistors

Abstract: Organic field-effect transistors (OFETs) are fabricated by depositing a thin film of semiconductor on the functionalized surface of a SiO2 dielectric The chemical and morphological structures of the interface between the semiconductor and the functionalized dielectric are critical for OFET performance We have characterized the effect of the affinity between semiconductor and functionalized dielectric on the properties of the semiconductor–dielectric interface The crystalline microstructure/nanostructure of the pentacene semiconductor layers, grown on a dielectric substrate that had been functionalized with either poly(4-vinyl pyridine) or polystyrene (to control hydrophobicity), and grown under a series of substrate temperatures and deposition rates, were characterized by X-ray diffraction, photoemission spectroscopy, and atomic force microscopy By comparing the morphological features of the semiconductor thin films with the device characteristics (field-effect mobility, threshold voltage, and hysteresis) of the OFET devices, the effect of affinity-driven properties on charge modulation, charge trapping, and charge carrier transport could be described