Single Pentacene Research Articles

(Invited, Digital Presentation) Epitaxial Organic Molecular Interfaces As Well-Ordered Model Systems for Molecular Semiconductor p-n Junctions for Optoelectronic Applications

Donor-acceptor molecular interfaces are nothing but p-n junctions for organic optoelectronic devices such as organic light emitting diodes and organic solar cells where an exciton forms or dissociates. Understanding about dominating factors determining the intermolecular assembly and charge carrier exchange processes are highly anticipated for the establishment of smart design strategies of practically efficient devices. Molecular heterojunctions built on single crystal organic semiconductors provide well-ordered model systems disentangling complex intermolecular contacts in real devices. Moreover, the p-n junctions of highly ordered molecular assembly lead to the enhancement of the mobility of charge carriers and excitons through the inter-molecular delocalization of the electronic states, which potentially opens a new route for efficient optoelectronic applications of molecular semiconductors. In this contribution, crystallographic and electronic structures of well-ordered molecular semiconductor homo- and hetero-epitaxial junctions are overviewed.One representative system is a “complementary” p-n junction of perfluoropentacene (C22F14) crystalline ad-layers formed on the single crystal pentacene (C22H14) [1]. Grazing-incidence X-ray diffraction (GIXD) analyses revealed hetero-epitaxial growth of perfluoropentacene in a uniform crystallographic orientation with respect to the surface lattice of the single crystal pentacene. The energy-momentum dispersions of inter-molecular electronic bands were successfully demonstrated by angle-resolved photoelectron spectroscopy (ARPES) for the hetero-epitaxial perfluoropentacene as well as for the single crystal pentacene, which strongly suggests realization of the delocalization of electrons and holes across this well-ordered molecular p-n junction.To further improve the crystallinity of the epitaxial molecular heterojunctions, the authors’ group proposed a new concept of “quasi-homoepitaxial organic semiconductor junctions”, at which a molecular species with nearly identical in-plane lattice constants to those of the substrate molecular single crystal surface is stacked [2]. Actually, quasi-homoepitaxial crystallites of bis(trifluoromethyl)dimethylrubrene was revealed to form highly-ordered interface of much improved mean crystallite size on the single crystal surface of (unsubstituted) rubrene by means of high-resolution GIXD. The electronic structures exhibiting spontaneous electron transfer at this inter-molecular contact and possible opto-electronic applications of this quasi-homoepitaxial junction are also discussed in this presentation.This contribution is supported by Grant-in Aids for Transformative Research Areas (A) “Dynamic Exciton: Emerging Science and Innovation” (JSPS-KAKENHI Grant Number JP21H05405).[1] Y. Nakayama, et al., J. Phys. Chem. Lett. 10 (2019) 1312.[2] K. Takahashi, et al., J. Phys. Chem. Lett. 12 (2021) 11430.

Atomically resolved single-molecule triplet quenching.

The nonequilibrium triplet state of molecules plays an important role in photocatalysis, organic photovoltaics, and photodynamic therapy. We report the direct measurement of the triplet lifetime of an individual pentacene molecule on an insulating surface with atomic resolution by introducing an electronic pump-probe method in atomic force microscopy. Strong quenching of the triplet lifetime is observed if oxygen molecules are coadsorbed in close proximity. By means of single-molecule manipulation techniques, different arrangements with oxygen molecules were created and characterized with atomic precision, allowing for the direct correlation of molecular arrangements with the lifetime of the quenched triplet. Such electrical addressing of long-lived triplets of single molecules, combined with atomic-scale manipulation, offers previously unexplored routes to control and study local spin-spin interactions.

Triplet-state lifetime quenched by oxygen

Spectroscopy Little is known about the atomistic mechanism that nature uses to mitigate the destructive interaction of triplet-excited pigment chromophores with omnipresent oxygen. Peng et al. tackled this challenge by developing a technique based on conducting atomic force microscopy to populate and track triplets in a single pentacene molecule, a model ϖ-conjugated system, placed on a sodium chloride surface (see the Perspective by Li and Jiang). The authors show how the triplet-state lifetime can be quenched in controllable manner by atomic-scale manipulations with oxygen co-adsorbed in close vicinity. The presented single-molecule spectroscopy paves the way for further atomically resolved studies of triplet excited states that play an important role in many other fields, such as organic electronics, photocatalysis, and photodynamic therapy. Science , abh1155, this issue p. [452][1]; see also abj5860, p. [392][2] [1]: /lookup/doi/10.1126/science.abh1155 [2]: /lookup/doi/10.1126/science.abj5860

Probing intramolecular vibronic coupling through vibronic-state imaging

Vibronic coupling is a central issue in molecular spectroscopy. Here we investigate vibronic coupling within a single pentacene molecule in real space by imaging the spatial distribution of single-molecule electroluminescence via highly localized excitation of tunneling electrons in a controlled plasmonic junction. The observed two-spot orientation for certain vibronic-state imaging is found to be evidently different from the purely electronic 0–0 transition, rotated by 90°, which reflects the change in the transition dipole orientation from along the molecular short axis to the long axis. Such a change reveals the occurrence of strong vibronic coupling associated with a large Herzberg–Teller contribution, going beyond the conventional Franck–Condon picture. The emergence of large vibration-induced transition charges oscillating along the long axis is found to originate from the strong dynamic perturbation of the anti-symmetric vibration on those carbon atoms with large transition density populations during electronic transitions.

Effect of varying the TD-lc-DFTB range-separation parameter on charge and energy transfer in a model pentacene/buckminsterfullerene heterojunction.

Atomistic modeling of energy and charge transfer at the heterojunction of organic solar cells is an active field with many remaining outstanding questions owing, in part, to the difficulties in performing reliable photodynamics calculations on very large systems. One approach to being able to overcome these difficulties is to design and apply an appropriate simplified method. Density-functional tight binding (DFTB) has become a popular form of approximate density-functional theory based on a minimal valence basis set and neglect of all but two center integrals. We report the results of our tests of a recent long-range correction (lc) [A. Humeniuk and R. Mitrić, J. Chem. Phys. 143, 134120 (2015)] for time-dependent (TD) lc-DFTB by carrying out TD-lc-DFTB fewest switches surface hopping calculations of energy and charge transfer times using the relatively new DFTBABY [A. Humeniuk and R. Mitrić, Comput. Phys. Commun. 221, 174 (2017)] program. An advantage of this method is the ability to run enough trajectories to get meaningful ensemble averages. Our interest in the present work is less in determining exact energy and charge transfer rates than in understanding how the results of these calculations vary with the value of the range-separation parameter (Rlc = 1/μ) for a model organic solar cell heterojunction consisting of a gas-phase van der Waals complex P/F made up of a single pentacene (P) molecule together with a single buckminsterfullerene (F) molecule. The default value of Rlc = 3.03 a0 is found to be much too small as neither energy nor charge transfer is observed until Rlc ≈ 10 a0. Tests at a single geometry show that the best agreement with high-quality ab initio spectra is obtained in the limit of no lc (i.e., very large Rlc). A plot of energy and charge transfer rates as a function of Rlc is provided, which suggests that a value of Rlc ≈ 15 a0 yields the typical literature (condensed-phase) charge transfer time of about 100 fs. However, energy and charge transfer times become as high as ∼300 fs for Rlc ≈ 25 a0. A closer examination of the charge transfer process P*/F → P+/F- shows that the initial electron transfer is accompanied by a partial delocalization of the P hole onto F, which then relocalizes back onto P, consistent with a polaron-like picture in which the nuclei relax to stabilize the resultant redistribution of charges.

Zero drive load inverter with high gain and large noise margin based on organic weak epitaxy growth method

We constructed zero drive load inverters by employing two identical unipolar pentacene thin-film transistors, based on the fabrication technology of organic weak epitaxy growth, which exhibited the features of high gains and large noise margins. Pentacene films were grown on a p-6P inducing layer, which reduced the interface disorder and increased the scale of crystalline domains. The pentacene/p-6P transistors showed high-quality saturation characteristics and the negative shift of threshold voltages, which caused large noise margins and the operation range of full swing. Furthermore, the hole mobility was increased to 1.28 cm2 (Vs)−1. As a result, zero drive load inverters exhibited a large gain of up to 30, compared with the gains of only four based on single pentacene transistors. We also discuss the dependence of large gains and device performance. These results demonstrated the fabrication technology of organic weak epitaxy growth were promising and advantageous for achieving high performance organic inverters and logic circuits.

Evolution of crystallinity at a well-defined molecular interface of epitaxial C60 on the single crystal rubrene

Uniform and well-defined interfaces are required for clarification of fundamental processes at internal interfaces between donor and acceptor molecules constituting organic optoelectronic devices. In this study, evolution of a well-ordered molecular interface, epitaxially grown C60 on the single crystal rubrene (C42H28) surface, was accurately investigated by grazing incidence x-ray diffraction (GIXD) techniques. Contrasting to the case of C60 on the single crystal pentacene forming uniquely aligned epitaxial interfaces, coexistence of two inequivalent crystalline domains of C60 was identified on the single crystal rubrene. Nevertheless, crystallinity of C60/rubrene exhibited even more remarkable improvement to extend its in-plane average crystallite size up to 250 nm as the growth temperature was raised. Probable leading factors determining the structures and crystallinity of the well-defined molecular interfaces are discussed based on close comparison of the present results with the C60/pentacene interfaces. The techniques presented herein for enhancement of the crystallinity in epitaxial molecular interfaces are potentially applicable to development in the photoelectric power conversion efficiency of organic photovoltaics (OPVs) via improved charge carrier mobility in donor-acceptor interfaces.

Anisotropic valence band dispersion of single crystal pentacene as measured by angle-resolved ultraviolet photoelectron spectroscopy

Abstract

Pulse Radiolysis of TIPS-Pentacene and a Fluorene-bridged Bis(pentacene): Evidence for Intramolecular Singlet-Exciton Fission.

Exposing TIPS-pentacene in deaerated benzene to ionizing radiation generates a mixture of singlet- and triplet-excited states of the solute. The singlet undergoes radiative decay without spin conversion whereas the triplet undergoes radiationless decay on the microsecond time scale. The concentration of each species was established by dosimetry. The excited-singlet state is not observed on the nanosecond-time scale for a related fluorene-bridged bis(pentacene), but the triplet is present in high concentration. Failure to detect the excited-singlet state is attributed to fast intramolecular singlet-exciton fission (iSEF) which is found to produce two triplet species. A short-lived intermediate (τT = 145 ns) is identified as the species (T_T) having both pentacene units present as triplet states. The second transient is longer lived (τT = 7.5 μs) and is assigned to the corresponding species (T_G) with a single pentacene promoted to the triplet level. Dosimetry is used to conclude that iSEF partitions overwhelmingly in favor of T_G (70%) relative to T_T (25%). The total triplet yield from iSEF, therefore, is ca. 120% in this system, where the pentacene terminals are weakly coupled.

Crystallinity of the epitaxial heterojunction of C60 on single crystal pentacene

The structure of pn heterojunctions is an important subject in the field of organic semiconductor devices. In this work, the crystallinity of an epitaxial pn heterojunction of C60 on single crystal pentacene is investigated by non-contact mode atomic force microscopy and high-resolution grazing incidence x-ray diffraction. Analysis shows that the C60 molecules assemble into grains consisting of single crystallites on the pentacene single crystal surface. The in-plane mean crystallite size exceeds 0.1µm, which is at least five time larger than the size of crystallites deposited onto polycrystalline pentacene thin films grown on SiO2. The results indicate that improvement in the crystal quality of the underlying molecular substrate leads to drastic promotion of the crystallinity at the organic semiconductor heterojunction.

Local impedance measurement of an electrode/single-pentacene-grain interface by frequency-modulation scanning impedance microscopy

The device performances of organic thin film transistors are often limited by the metal–organic interface because of the disordered molecular layers at the interface and the energy barriers against the carrier injection. It is important to study the local impedance at the interface without being affected by the interface morphology. We combined frequency modulation atomic force microscopy with scanning impedance microscopy (SIM) to sensitively measure the ac responses of the interface to an ac voltage applied across the interface and the dc potential drop at the interface. By using the frequency-modulation SIM (FM-SIM) technique, we characterized the interface impedance of a Pt electrode and a single pentacene grain as a parallel circuit of a contact resistance and a capacitance. We found that the reduction of the contact resistance was caused by the reduction of the energy level mismatch at the interface by the FM-SIM measurements, demonstrating the usefulness of the FM-SIM technique for investigation of the local interface impedance without being affected by its morphology.

Polarization-induced transport in ferroelectric organic field-effect transistors

Ferroelectric dielectrics, permitting access to nearly an order of magnitude range of dielectric constants with temperature as the tuning parameter, offer a great platform to monitor the changes in interfacial transport in organic field-effect transistors (OFETs) as the polarization strength is tuned. Temperature-dependent transport studies have been carried out from pentacene-based OFETs using the ferroelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) as a gate insulating layer. The thickness of the gate dielectric was varied from 20 nm to 500 nm. By fits to an Arrhenius-type dependence of the charge carrier mobility as a function of temperature, the activation energy in the ferroelectric phase is found to increase as the thickness of the PVDF-TrFE layer decreases. The weak temperature-dependence of the charge carrier mobility in the ferroelectric phase of PVDF-TrFE may be attributed to a polarization fluctuation driven transport, which results from a coupling of the charge carriers to the surface phonons of the dielectric. By comparing single layer PVDF-TrFE pentacene OFETs with stacked PVDF-TrFE/inorganic dielectric OFETs, the contribution from Fröhlich polarons is extracted. The temperature-dependent mobility of the polarons increases with the thickness of the PVDF-TrFE layer. Using a strongly coupled polaron model, the hopping lengths were determined to vary between 2 Å and 5 Å.

Electrical transport mechanism of single monolayer pentacene film employing field-effect characterization

We employed a novel electrode-contact architecture to enable operation of single monolayer pentacene-based field-effect transistor with a high electrical performance, whose mobility reaches as high as 0.31cm2V−1s−1, the highest among any pentacene-based monolayer devices ever reported. A temperature dependent charge transport was systematically carried out to elucidate the carrier transport mechanisms within even single layer of molecules. The carrier mobilities are found to exhibit a pure Arrhenius type at the high temperature regime (above 170K), by contrast, a pronounced turning point has been observed when the measurement temperature is below 170K, possible mechanisms were ascribed to the distribution attribute of trapped states among the grain boundary. Furthermore, an electric field dependent mobility characterization shows a unique non-Poole–Frenkel type behavior at room temperature, which shows much different from the case occurred in multiple-layer devices where the quantity of permitted percolation routes among different monolayers guarantees an mobility enhancement as an electric field increasing. But for the case of single monolayer, the electric field-induced potential reduction effect is competing with a drop of percolation path arising from the directional movement of carrier under a strong electric field. The depth understanding of carrier transport within one monolayer may be helpful for optimizing the design of OFETs for better device applications.

Structure of clusters of pentacene molecules and their polarizabilities

We calculate and compare the ground state structures of one, two and three pentacene molecule clusters, using B3LYP and B97-D density functionals, and polarizabilities in the ground state structure to examine the extent of anisotropy for applications in electronic device fabrication. B3LYP and B97-D functionals perform sufficiently well for a single pentacene molecule, but for two and three molecule clusters, the inclusion of dispersive interactions through B97-D becomes necessary. For two and three molecule clusters the energy landscape near ground state is flat in molecular coordinates, implying multiple related structures may be accessible at room temperature. Further, for the ground state structures, the polarizability along the long axis of the molecule is larger than the other two directions. This may make it possible to explain the beneficial effect of applying an electric field during processing of pentacene films in electronic devices, as observed experimentally; specifically, in this context, the structural and polarizability data presented could allow for development of models for nucleation and growth of pentacene films.

Mechanical Conformation Switching of a Single Pentacene Molecule on Si(100)-(2 × 1)

The mechanical switching of a single pentacene molecule chemisorbed in a planar configuration along a dimer row of the Si(100)-(2 × 1) surface was performed experimentally using the tip apex of a scanning tunneling microscope. The mechanical switching reaction path was identified theoretically on the ground state potential energy surface of the pentacene/Si(100)-(2 × 1) system. A low-temperature scanning tunneling microscope as well as semiempirical ASED+ molecular mechanical and elastic scattering quantum chemistry (ESQC) calculations were employed to perform the studies. Pushing with the STM tip apex and at zero bias voltage exactly at the center of the chemisorbed pentacene molecule induces a mechanical conformation change of the pentacene from its metastable to its surface stable conformation on the Si(100)-(2 × 1) surface.

Organic field-effect transistors with a sandwich structure from inserting 2,2′,2″-(1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazole] in the pentacene active layer

Organic field-effect transistors (OFETs) with a pentacene/TPBi/pentacene sandwich structure were realized and characterized. Compared with the single pentacene layer transistors, these devices not only showed an obvious reduction of threshold voltage but also presented a significant enhancement of field-effect mobilities. The performance improvement was attributed to the high conductivity of the sandwich active layers, which were analyzed by applying the transfer line method. Meanwhile, the morphologies of pentacene films with and without the TPBi interlayer were characterized by atomic force microscopy, the smoother surface of the upper pentacene film was observed in pentacene/TPBi/pentacene. Moreover, by applying X-ray diffraction, a higher-order growth phase of the pentacene thin film was observed.

Potassium doping of single crystalline pentacene thin film

The impact of potassium (K) doping on the electronic structure of single crystalline pentacene (Pn) thin film grown on Cu(110) was investigated by high-sensitivity angle-resolved ultraviolet photoemission spectroscopy for K${}_{x}$Pn${}_{1}$ ($0\ensuremath{\le}x\ensuremath{\le}1$). At room temperature (293 K), we observed that (i) Pn electronic states shift toward the high binding side with progressive growth of a gap state with the K-doping level, (ii) the highest-occupied molecular-orbital band dispersion of the pristine Pn SC multilayer survives perfectly even at the K-doping level of $x$ = 1, and (iii) no dispersion occurs for the K-induced gap state. At 50 K, on the other hand, we found a small but clear dispersion (width \ensuremath{\sim}0.08 eV) of the K-induced gap state for the K${}_{1}$Pn${}_{1}$ SC multilayer, which is in good agreement with theoretical prediction on the band structure of K${}_{1}$Pn${}_{1}$ Mott-Hubbard insulator crystal. This finding demonstrates that the K-induced gap state originates from the Mott-Hubbard insulator formed by electron transfer from the K atom to Pn lowest-unoccupied molecular orbital. The Hubbard energy $U$ was estimated to be \ensuremath{\sim}2 eV at 293 and 50 K, which is also in fairly good agreement with the theoretical value of 1.5 eV. Moreover, significant density of states at the Fermi level, which is related to a metallic phase for the metal-insulator transition, could not be detected for the doping level of $0\ensuremath{\le}x\ensuremath{\le}1$. This indicates that the Mott-Hubbard insulator ground state is stable for $0\ensuremath{\le}x\ensuremath{\le}1$ and does not show $x$-dependent metal-insulator transition at least near the surface region. The impact of K doping on the energy-level alignment was discussed in terms of coexistence of K${}_{1}$Pn${}_{1}$ Mott-Hubbard insulator and Pn SC film.

Organic field-effect transistors with nearly non-injection barrier from source/drain electrodes to pentacene

Top contact pentacene field-effect transistors (OFETs) with or without MoO3 source/drain (S/D) electrodes buffer layers were fabricated by utilizing different deposition rates of Au S/D electrodes. The results showed that the characteristics of these OFETs have a strong dependence on the deposition rate of Au S/D electrodes. More importantly, single pentacene layer OFETs with optimized Au deposition process exhibits significantly improved performance, which is almost equal to the OFETs with MoO3 buffer layers. The high performance of OFETs with optimized Au deposition process was attributed to an improved contact of Au and pentacene caused by ordered dipole arrangement at the interface, leading to a nearly non-injection barrier of charge carriers from Au electrodes to pentacene.

Effects of gate dielectric composition on the performance of organic thin-film devices

Four poly(4-vinyl phenol) based gate dielectrics were tested to optimize the performance of pentacene organic field-effect transistors. The dielectrics’ surface tensions, Fourier transform infrared spectra, capacitances and leakage currents were measured. The optimal dielectric allowed the transistor to show negligible hysteresis with high performance even in ambient conditions. A complementary inverter was fabricated by integrating in single substrate pentacene (p-type) and F16CuPc (n-type) OFETs containing the optimized gate dielectric. Its voltage transfer curve showed almost symmetric noise margin; it showed a logic threshold of 22.5V and a maximum voltage gain (δVout/δVin) of 6.2 at Vin=22.5V.

Tailored Single Crystals of Triisopropylsilylethynyl Pentacene by Selective Contact Evaporation Printing

Single crystalline triisopropylsilylethynyl pentacene (TIPS-PEN) arrays are fabricated with both size and shape of each patterned domain precisely tailored by selective contact evaporation printing (SCEP), which exhibit sufficient ON/OFF current ratio as well as high field effect mobility.