Pentacene Research Articles

Near-infrared-II photodetection realized by introducing organic-inorganic charge-transfer-complex photosensitive material into pentacene phototransistor

Abstract Organic phototransistor (OPT) is a promising photodetector with substantial attention due to its advantages such as low cost, light weight, flexibility and ease of large area preparation. Although the second near-infrared (NIR-II) window (1.0–1.7 μm) has important applications in fiber communication, the organic infrared photosensitive materials are scarce in this waveband, which greatly limits the application of organic photodetectors in near-infrared bands. In this work, a charge-transfer-complex (CTC) film with significant hyperchromic effect in the infrared range of 1000–1400 nm was prepared by co-evaporation of metal oxide WO3 and organic molecule m-MTDATA. A high performance near-infrared OPT was fabricated by combining this CTC photosensitive layer and pentacene (PENT) high-mobility channel layer, the photoresponsivity R of which has surpassed the recent reported level of 1.1 μm band inorganic infrared detector. The physical mechanism underlying the operation of CTC-PENT-OPT was clarified and the thickness of the CTC photosensitive film in this device was optimized. This new type of organic-inorganic CTC infrared-sensitive material will offer a new choice for the future study on organic infrared detectors.

Controlling Interface Morphology and Layer Crystallinity in Organic Heterostructures: Microscopic View on C60 Island Formation on Pentacene Buffer Layers.

Controlling the crystallinity of organic thin films is an important aspect in the improvement of organic electronic devices. However, because of high molecular mass, structural anisotropy, and weak intermolecular van der Waals bonding, crystalline ordering is not easily accomplished. While film preparation at elevated substrate temperature often improves the crystalline quality, this approach cannot be applied to temperature-sensitive materials such as plastic foils used as substrates for flexible electronics. Here, we examine in detail a low-temperature approach to improve film crystallinity by using ultrathin pentacene (PEN) buffer layers that allow crystalline growth of buckminsterfullerene (C60) thin films while without such buffer layers, only amorphous fullerene films are formed upon room-temperature deposition on various support substrates. Remarkably, this effect depends critically on the thickness of the PEN buffer and requires a thickness of at least two monolayers to induce crystalline growth, whereas a buffer layer consisting of a monolayer of PEN again yields amorphous C60 films. Combining crystallographic investigations by X-ray diffraction and atomic force microscopy measurements, we determine distinct nucleation sites on buffer layers of different thickness, which are correlated to the amorphous, respectively crystalline C60 islands. Our microscopic analysis reveals distinct differences for the nucleation and diffusivity of fullerenes on the PEN monolayer and on thicker buffer layers, which are attributed to the molecular arrangement in the PEN monolayer. Finally, we show that the crystalline C60 films are exclusively (111)-oriented and the fullerene islands are even heteroepitaxially aligned on the PEN buffer.

Charge transport and transfer phenomena involving conjugated acenes and heteroacenes

Conjugated acenes are very promising for their applicability in different optoelectronic devices. In particular, pentacene (PCn) shows some distinct behaviours, namely high optical absorption and remarkably high carrier mobility which makes it as a landmark semiconductor for use in field-effect transistors or hole transporting materials in organic photovoltaics. During the last few decades, a large number of theoretical and experimental researches has been performed showing the practical applicability of different acenes and heteroatom-doped acenes (heteroacenes). Few reviews are also made in this regard. However, correlating the molecular properties arising from their intrinsic electronic structures with their charge transfer (CT) and transport characteristics is really scattered. Furthermore, very recent understandings on the nanojunctions made of acenes along with suitable electrodes need to be reviewed for further development of their performances in electronic devices. At the same time, the photovoltaic applicability of acenes and heteroacenes has recently been shown as an interplay of the dynamics of CT states as well as the intrinsic charge separation within the molecules or composites. The present review aims to point out those recent observations so as to draw more attention for further development in the area of acene- and heteroacene-based optoelectronic devices. We concentrate more on the structure–property relationships which could guide the device performance. As the subject area is so vast, we put emphasis on the very recent studies on PCn and some lower S,N-heteroacenes.

Template-Free Orientation Selection of Rod-Like Molecular Semiconductors in Polycrystalline Films.

Many organic semiconductors (OSCs) feature strong optical anisotropy that can be exploited to increase the efficiency of optoelectronic devices. We demonstrate that for the technologically relevant, rod-like OSCs diindenoperylene (DIP), pentacene (PEN), and α-sexithiophene (6T) deposited on silicon oxide surfaces it is possible to prepare polycrystalline thin films in which the long molecular axis is oriented parallel to the substrate plane in a template-free fashion. In films grown by organic molecular beam deposition at room temperature or higher, the molecules are oriented upright standing (σ-orientation). Instead, the here-presented growth at low temperatures followed by slow annealing up to a temperature near molecular desorption has the effect of "freezing" the molecules with their long axis oriented parallel to the substrate plane (λ-orientation) while conferring them crystalline long-range order. We discuss the huge impact on the optical anisotropy of the films observed as a consequence of the orientation transition. Finally, we propose a mechanism for explaining the achieved λ-orientation, which is stable under environmental conditions.

Charge-transfer processes and carrier dynamics at the pentacene—C60 interface

Heterostructures of pentacene (PEN) and buckminsterfullerene (C60) are frequently attracting scientific interest as a well-defined small-molecule model system for the study of internal interfaces between two organic semiconductors. They are prototypical representatives forming a donor–acceptor combination for studies of fundamental optoelectronic processes in organic photovoltaics. Despite their importance in exciton dissociation, the energetics of their interfacial charge-transfer (CT) states and their microscopic excitation dynamics are not yet clarified and still being discussed. Here, we present steady-state and time-resolved photoluminescence measurements on stacked heterostructures composed of these two materials. All experiments are performed in the visible and near-infrared spectral regions as CT states are expected at energies below the fundamental electronic transitions of the respective bulk materials. A characteristic, interface-specific emission at around 1.13–1.17 eV is found, which we attribute to an interfacial CT state. Its excitation energy dependence reveals the intricate relaxation dynamics of excitons formed in both constituent materials. Moreover, the analysis of the dynamics of the C60 excitons shows that the lifetime of this state is reduced in the presence of an interface with PEN. This quenching is attributed to a long-range interaction, i.e. the relaxation of excitations into the interfacial CT state.

Impact of molecular orientation on energy level alignment at C60/pentacene interfaces

The molecular orientation and the electronic structure at molecular donor/acceptor interfaces play an important role in the performance of organic optoelectronic devices. Here, we show that graphene substrates can be used as templates for tuning the molecular orientation of pentacene (PEN), selectively driving the formation of either face-on or edge-on arrangements by controlling the temperature of the substrate during deposition. The electronic structure and morphology of the two resulting C60/PEN heterointerfaces were elucidated using ultraviolet photoelectron spectroscopy and atomic force microscopy, respectively. While the C60/PEN (edge-on) interface exhibited a vacuum level alignment, the C60/PEN (face-on) interface exhibited a vacuum level shift of 0.2 eV, which was attributed to the formation of an interface dipole that resulted from polarization at the C60/PEN boundary.

Real-Time Monitoring of Growth and Orientational Alignment of Pentacene on Epitaxial Graphene for Organic Electronics

The interaction between a graphene layer and pentacene (PEN) molecules leads to the formation of a lying-down phase, which can improve charge transport for organic vertical field effect transistors and enhance the optical absorption for increased light harvesting in organic solar cells. Here, we present a comprehensive study of PEN growth on epitaxial graphene on silicon carbide (SiC). Simultaneous grazing-incidence small- and wide-angle X-ray scattering (GISAXS/GIWAXS) were used in situ for real-time monitoring of the PEN crystal growth with millisecond time resolution to identify two distinct anisotropic growth stages after the nucleation of the first monolayer (ML). In the first stage up to 1.5 nm, we observe rapid growth of pentacene domains along the (010) and (001) facets. This growth behavior is saturating after 1.5 nm. In a second stage, this is followed by continuous lateral crystal growth in only one in-plane direction (100) forming needle-shaped domains. In the second stage, an uninterrupted li...

Resolving intramolecular-distortion changes induced by the partial fluorination of pentacene adsorbed on Cu(111)

We experimentally quantify the molecular bending of a partially fluorinated pentacene (PEN) compound, namely 2,3,9,10-tetrafluoropentacene (F4PEN), adsorbed on Cu(111). By means of the x-ray standing wave (XSW) technique, we directly measure the adsorption distance of three inequivalent carbon sites, the fluorine atoms as well as the total and backbone carbon average adsorption distances. The precise positioning of different sites within the carbon core allows us to resolve two adsorption behaviors, namely a PEN-like strong coupling between the backbone and the substrate, and a repulsive interaction involving the fluorinated short molecular edges, which are $0.91\ifmmode\pm\else\textpm\fi{}0.09\phantom{\rule{4pt}{0ex}}\AA{}$ above the central benzene ring. This finding is further supported by additional electronic and in-plane-structure measurements, thus showing that the selective fluorination of a PEN molecule has only a local conformational effect and it is not sufficient to modify its interface properties. Yet, in the multilayer regime, the electronic and growth properties of the film differ completely from those of PEN and its perfluorinated derivative.

Copper Phthalocyanine as Contact Layers for Pentacene Films Grown on Coinage Metals

The metal–semiconductor interface determines the efficiency of charge carrier injection into any organic electronics device. Control of this interface, its structure, and its morphology is therefore essential for device improvement. In this study, we analyze the approach of controlling semiconductor morphology at this interface by insertion of a copper phthalocyanine (CuPc) monolayer as a primer between Ag(111), Au(111), and Cu(100) surfaces and the organic semiconductor pentacene (PEN). Controlled monolayer formation is facilitated by thermal desorption of excess multilayers, monitored via thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), and the growth of PEN on the resultant monolayer primers is investigated by near-edge X-ray absorption spectroscopy (NEXAFS), atomic force microscopy (AFM) and STM. While well-ordered CuPc monolayers with flat-lying molecules are formed on Au(111) and Ag(111), no long-range order is observed on Cu(100)...

Comparison of Layer-Forming and Bromine-Doping Methods of Pentacene Layer in Solid-State Dye-Sensitized Solar Cell

For organic solar cells (OSC) such as dye-sensitized cell (DSSC) to compete with silicon-based cells in terms of stability, there is a need to further improve components which causes degradation. To answer the stability issue with liquid electrolyte, solid state dye-sensitized solar cell (SS-DSSC) was introduced. SS-DSSC promises performance consistency due to less power degradation compared to standard DSSC that uses liquid-based electrolyte. Pentacene (PEN), a semiconductor usually used on field-effect transistors is a material that has a higher hole and electron mobility when compared to amorphous silicon and also has a band gap suitable for solar energy conversion. In this study, PEN used as hole transporting layer in SS-DSSC was fabricated through spin coating and heating using a precursor and through vapour transport using powder. The PEN samples were then doped with bromine through different methods - immersion and vaporization. Characterization of the PEN samples through X-ray Diffraction, Energy Dispersive X-ray Fluorescence, and Atomic Force Microscopy reveal orthorhombic, thin film, and crystalline bulk phases present on different fabricated PEN samples as well as confirmed successful doping. Furthermore, the light harvesting parameters are analysed through SolarTM Light LS1000 Solar Simulator (AM 1.5, 100 mW/cm2) which confirm correlation between the increased efficiency, PEN layer growing methods, and bromine doping methods.

A new instrumentation for in situ characterization of the charge transport and crystallographic properties in co-evaporated organic thin film transistor

ABSTRACTA new instrumentation was developed to study the in situ electrical and crystallographic properties of organic thin film transistor during vacuum deposition. We characterized pentacene (PEN) and perfluoro-pentacene (PFP) co-deposited organic thin film transistor with various mixing ratio using this equipment. Lattice parameters and crystal orientations of PEN and PFP alloyed phases (named σ-phase and λ-phase) were determined using an in situ two-dimensional grazing incidence X-ray diffraction (2D-GIXD) measurement. The observed 2D-GIXD patterns clarified that the σ-phase is in triclinic unit cell with the following lattice parameters: a = 0.67 nm, b = 0.75 nm, c = 1.58 nm, α = 95.7° β = 94.2° and γ = 84.0°. The c plane of σ-phase crystal is parallel to the substrate surface. The λ-phase is also in triclinic with the following lattice parameters: a = 0.66 nm, b = 0.69 nm, c = 1.56 nm, α = 109.5° β = 113.0° and γ = 81.5°. The a plane of λ-phase crystal was parallel to the substrate surface. It was also found the best symmetric hole and electron mobility (μh = 5.5 × 10−4 cm2V−1s−1 and μe = 5.1 × 10−4 cm2V−1s−1) were obtained at PEN: PFP = 1:1.

Electronic and optical properties of hexathiapentacene in the gas and crystal phases

Using density functional theory (DFT) and its time-dependent (TD) extension, the electronic and optical properties of the hexathiapentacene (HTP) molecule, a derivative of pentacene (PNT) obtained by symmetric substitution of the six central H atoms with S atoms, are investigated for its gas and solid phases. For the molecular structure, all-electron calculations are performed using a Gaussian localized orbital basis set in conjunction with the Becke three-parameter Lee-Yang-Parr (B3LYP) hybrid exchange-correlation functional. Electron affinities, ionization energies, quasiparticle energy gaps, optical absorption spectra, and exciton binding energies are calculated and compared with the corresponding results for PNT, as well as with the available experimental data. The DFT and TDDFT results are also validated by performing many-body perturbation theory calculations within the $GW$ and Bethe-Salpeter equation formalisms. The functionalization with S atoms induces an increase of both ionization energies and electron affinities, a sizable reduction of the fundamental electronic gap, and a redshift of the optical absorption onset. Notably, the intensity of the first absorption peak of HTP falling in the visible region is found to be nearly tripled with respect to the pure PNT molecule. For the crystal structures, pseudopotential calculations are adopted using a plane-wave basis set together with the Perdew-Burke-Ernzerhof exchange-correlation functional empirically corrected in order to take dispersive interactions into account. The electronic excitations are also obtained within a perturbative B3LYP scheme. A comparative analysis is carried out between the ground-state and excited-state properties of crystalline HTP and PNT linking to the findings obtained for the isolated molecules.

Molecular-Level Realignment in Donor–Acceptor Bilayer Blends on Metals

Molecular bilayers made up of a donor–acceptor blend in contact with the metal and capped with a single-component layer show a tunable energy level alignment at both metal–organic and organic–organic interfaces. To ensure sharp heteromolecular interfaces, a contact layer is formed by a stable blend of pentacene (PEN) and perfluorinated copper phthalocyanine (F16CuPc) on Au(111) and Ag(111) and of perfluoropentacene (PFP) and copper phthalocyanine (CuPc) on Ag(111). Core-level and valence band photoemission reveal that, upon capping with pure F16CuPc, CuPc, and PEN, the electronic states of both contact and capping layers “realign” with respect to the monolayer and the multilayer references. The sign of the shift depends clearly on whether the capping layer is donor-like (PEN, CuPc) or acceptor-like (F16CuPc). As revealed by NEXAFS, the shift in electronic levels of the contact layer upon capping leads to a spectral density variation across the Fermi edge (EF); i.e., it induces molecule/metal charge transfer.

Observation of electric potential in organic thin-film transistor by bias-applied hard X-ray photoemission spectroscopy

The effect of gate voltage on electric potential in a pentacene (PEN) layer was studied by hard X-ray photoelectron spectroscopy under a bias voltage. It was observed that applying a negative gate voltage substantially increases the width of a C 1s peak. This suggested that injected and accumulated carriers in an organic thin film transistor channel modified the potential depth profile in PEN. It was also observed that the C 1s kinetic energy tends to increase monotonically with threshold voltage.

Two different ground states in K-intercalated polyacenes

The electronic states of potassium- (K-) intercalated zigzag-type polycyclic aromatic (PLA) hydrocarbon [polyacene PLAs] ${\mathrm{K}}_{x}(\mathrm{PLAs})$ are studied for a series of the four smallest molecules: naphthalene (NN), anthracene (AN), tetracene (TN), and pentacene (PN), focusing on their 1:1 stoichiometric phases. Clear experimental differences are identified between the first group [${\mathrm{K}}_{1}(\mathrm{NN})$ and ${\mathrm{K}}_{1}(\mathrm{AN})$] and the second group [${\mathrm{K}}_{1}(\mathrm{TN})$ and ${\mathrm{K}}_{1}(\mathrm{PN})$] by magnetic, vibrational, and optical measurements. The first group is categorized as a Mott insulator with an antiferromagnetic ground state with energy of \ensuremath{\sim}10 meV, whereas the second group is classified as a band insulator via dimer formation due to the spin Peierls instability. In the latter system, the first thermally accessible triplet states are located far apart from the singlet ground states and are not detected by electron spin-resonance spectroscopy until 300 K being very different from what is observed for the hole-doped PN reported earlier. The results give a new systematic understanding on the electronic states of electron-doped PLAs sensitive to the energetic balance among on-site Coulomb repulsion, bandwidth, and the Peierls instability.

Pentacene based Onsager crosses.

A dearomatization/aromatization strategy accesses the tetrahedral (crosslike) pentacene tetramer (tetrakis(4-((6,13-bis(TIPS-ethynyl)pentacen-2-yl)ethynyl)phenyl)silane). This molecular caltrop and one of its derivatives form amorphous thin films, demonstrating the impact of the 3D-tetramerization on the bulk morphology of the otherwise crystalline TIPS-pentacene.

Identification of an organic semiconductor superlattice structure of pentacene and perfluoro-pentacene through resonant and non-resonant X-ray scattering

Highly crystalline and stable molecular superlattices are grown with the smallest possible stacking period using monolayers (MLs) of the organic semiconductors pentacene (PEN) and perfluoro-pentacene (PFP). Superlattice reflections in X-ray reflectivity and their energy dependence in resonant soft X-ray reflectivity measurements show that PFP and PEN MLs indeed alternate even though the coherent ordering is lost after ∼ 4 ML. The observed lattice spacing of 15.9 Å in the superlattice is larger than in pure PEN and PFP films, presumably because of more upright standing molecules and lack of interdigitation between the incommensurate crystalline PEN and PFP layers. The findings are important for the development of novel organic quantum optoelectronic devices.

Spin-Dependent Molecule Symmetry at a Pentacene-Co Spinterface.

Incorporating spin-polarized scanning tunneling microscopy (SP-STM) measurements and first-principles calculations, we resolve spin-polarized states and consequent features in a pentacene(PEN)-Co hybrid system. Symmetry reduction of PEN clarifies the PEN adsorption site and the Co stacking methods. Near the Fermi energy, the molecular symmetry is spin-dependently recovered and an inversion of spin-polarization in PEN with respect to Co is observed. The experimental findings and calculation results are interpreted by a pz-d hybridization model, in which spin-dependent bonding-antibonding splitting of molecular orbitals happens at metal-organic spinterfaces.

Interfacial electronic structures of buffer-modified pentacene/C60-based charge generation layer

Interfacial electronic structures of MoO3 and LiF-modified pentacene (PEN)/fullerene (C60)-based charge generation layer (CGL) have been investigated with photoemission spectroscopy. Important characteristics controlling its functional effectiveness have been analyzed for charge transport properties in tandem organic lighting-emitting diodes (TOLEDs). It is found that a small energy offset at the PEN/C60 heterojunction makes it easy to transfer electrons from PEN to C60 even under a small applied bias, facilitating the occurrence of charge generation. The band bending observed in both PEN and C60 is beneficial to exciton-dissociation and charge transport in opposite directions. At the MoO3/PEN interface, the high work function (WF) of MoO3 brings the highest occupied molecular orbital (HOMO) onset up to the Fermi level (EF) not only for PEN but also for most hole transport layer (HTL) materials of the adjacent electroluminescent (EL) unit as this CGL is connected into TOLED. Therefore, holes can be efficiently injected from PEN into this EL unit. Similarly, at the C60/LiF interface, the low WF of the LiF buffer layer makes the lowest unoccupied molecular orbital (LUMO) to pin close to the EF not only for C60 but also for most electron transport layer (ETL) materials of the other EL unit, which induces the electrons to inject easily from C60 into that EL unit by tunneling through the thin LiF film. The favorable energy level alignment can effectively enhance charge generation, transport, and injection. The advantage of the MoO3/PEN/C60/LiF structure is that thus formed CGL can greatly reduce the voltage drop and thus enhance the power efficiency (PE) of the corresponding TOLED.

Pentacene/K12 solar cells formed by organic vapor phase deposition

This study reports on an organic solar cell structure utilizing K12 as a new low-tem- perature processable small-molecule acceptor material. Pentacene (PEN) and K12 were depos- ited onto indium tin oxide by means of organic vapor phase deposition (OVPD) as bilayer solar cells. The resulting solar cell was characterized electrically by current density-voltage (J − V) measurements and optically by photocurrent and reflectivity measurements. The J − V charac- teristic under AM 1.5 illumination indicates a short-circuit current of 0.45 mA∕cm 2 (Jsc), a fill factor of 38% (FF), and an open-circuit (Voc) voltage of 0.71 V. Current generation is found to predominantly occur in the K12 layer, although strong light absorption in the PEN layer is detected. We suggest that either a dipole shift between the layers or the fission of singlet excitons in the PEN layer leads to this observation. Although the efficiency of the device is low in combination with PEN, our experiment successfully demonstrates the use of K12 as a low- temperature acceptor material in OVPD processes. © 2014 Society of Photo-Optical