Pentacene Single Crystals Research Articles

Interplay of intermolecular and intramolecular vibrations mediates ultrafast singlet fission

Singlet fission (SF) is an exciton multiplication process that splits a singlet exciton in organic semiconductors into two triplet excitons and thus, can overcome the Shockley–Queisser limit to improve the solar energy conversion efficiency in photovoltaics. In this paper, we construct a unified model for the ultrafast primary step of the SF process. To achieve this, we investigate the dynamics of vibrational modes and their interactions with the relevant electronic excited states in prototypical SF materials, pentacene (exothermic SF) and tetracene (endothermic SF) single crystals. Additionally, the functional role of the charge transfer (CT) state is also examined. Using the refined parameters obtained from the reported experimental results, we deduce that the intermolecular vibrations mediate the SF in pentacene with the assistance from strong vibronic couplings to intramolecular modes, which drives the SF process to occur within 100 fs. In this timescale, the CT state has a limiting role towards the SF process in pentacene. However, the CT state plays an important role in a relatively slower SF process in tetracene. Our results disentangle the role of underlying vibrational coherences and clarify the importance of the CT state in tetracene crystal. Hence, with our unified model, we can study the coherent dynamics of the SF process, which can principally be extended to other SF materials as well.

Far- and mid-infrared FT-IR analysis of the single-crystal pentacene using a linearly polarized synchrotron radiation light source

Understanding and control of molecular vibrations are essential aspects for both fundamental and application considerations of organic semiconductor electronics. The reason is that the organic electronic devices are driven by diverse electronic processes in molecular solids, such as charge carrier transport and excitonic progression, that are strongly influenced by coupling with vibrations. In the present study, molecular vibrations of single-crystals of pentacene, a representative organic semiconductor material, were examined in the far- to mid-infrared range by means of Fourier transform infrared (FT-IR) spectroscopy using a linearly polarized synchrotron radiation light source. The IR absorption spectra exhibited significant modulation depending on the crystalline in-plane azimuthasl angle of c*-oriented single-crystal pentacene.

Growth and Characterization of Centimeter-Scale Pentacene Crystals for Optoelectronic Devices

In this work, we present results on the growth of centimeter-scale pentacene crystals using the physical vapor transport method in a dual-temperature zone horizontal furnace. It was established that intensive crystal growth processes occurred in transition regions with sudden temperature changes, while crystal growth was practically not observed in regions with slightly varying temperatures. During crystal growth, co-precipitating golden needle-like crystals reaching lengths of more than 10 mm were obtained. Using the method of single-crystal X-ray diffraction at 85 and 293 K for dark-blue lamellar pentacene crystals, the crystal structure was refined in a triclinic system with sp.gr. P1¯. It was established that the golden needle crystals consisted of molecules of the pentacene derivative—5,14-pentacenedione, the crystal structure of which was solved for the first time in a rhombic system with sp.gr. P212121. The absorption and luminescence spectra of pentacene and 5,14-pentacenedione in toluene solutions were obtained and analyzed. The electrical properties of the prepared pentacene thin films and single crystals grown under physical vapor transport conditions were evaluated by fabricating and characterizing field-effect transistors (FETs). It was shown that the presence of impurities in the commercial pentacene material had a significant effect on the morphological quality of thin polycrystalline films and noticeably reduced the hole mobility.

Molecular single crystals induce chain alignment in a semiconducting polymer

The blending of π-conjugated molecules with polymeric semiconductors into a composite system is an effective strategy to promote charge carrier mobility because of the transmission path by the conductive polymers through electrical bridge connection of the small organic molecule crystalline domain. In this work, pentacene single crystal was prepared to induce the conjugated polymer chain alignment of polymeric semiconductor PDPP2T-TT-OD to form the semiconducting composite, which led to enhanced field-effect mobility of the organic thin-film transistor (OTFT) by improving the crystallinity due to nucleation and growth phase separation. Besides, with the addition of anti-solvents, the crystallization of the blend film was further improved, 27 times higher than that of a pure polymer semiconductor–based OTFT. That was because the pentacene nuclei induced polymer crystallization through π-π interactions and the addition of anti-solvent promoted the aggregation of polymer chains in solution, enabling the molecular chains packed more closely in solid films. Therefore, the chain arrangement of polymers induced via small molecular single crystals provides a new idea to improve mobility in composite semiconductor thin films for the construction of novel organic optoelectronic devices.

Organic photodetectors based on pentacene single crystals with fast response and flexibility

Organic single crystals (OSCs) have attracted increasing interest in advanced optoelectronic devices, in particular for flexible and wearable devices, due to their high carrier mobility and intrinsic mechanical flexibility.

Ultrafast and Sensitive Self-Powered Photodetector Based on Graphene/Pentacene Single Crystal Heterostructure with Weak Light Detection Capacity.

Organic materials exhibit efficient light absorption and low-temperature, large-scale processability, and have stimulated enormous research efforts for next-generation optoelectronics. While, high-performance organic devices with fast speed and high responsivity still face intractable challenges, due to their intrinsic limitationsincluding finite carrier mobility and high exciton binding energy. Here an ultrafast and highly sensitive broadband phototransistor is demonstrated by integrating high-quality pentacene single crystal with monolayer graphene. Encouragingly, the -3dB bandwidth can reach up to 26kHz, which is a record-speed for such sensitized organic phototransistors. Enormous absorption, long exciton diffusion length of pentacene crystal, and efficient interfacial charge transfer enable a high responsivity of >105 AW-1 and specific detectivity of >1011 Jones. Moreover, self-powered weak-light detection is realized using a simple asymmetric configuration, and the obvious zero-bias photoresponses can be displayed even under 750nWcm-2 light intensity. Excellent response speed and photoresponsivity enable high-speed image sensor capability in UV-Vis ranges. The results offer a practical strategy for constructing high-performance self-powered organic hybrid photodetectors, with strong applicability in wireless, weak-light detection, and video-frame-rate imaging applications.

Ambipolar Organic Field‐Effect Transistors and Complementary Circuits Based on Single Crystals with Alcohol Treatment

AbstractAmbipolar organic field‐effect transistors (OFETs) are promising candidates for compact organic complementary circuits. High and balanced ambipolar mobilities that facilitate high operation speed and simplifying layout designs, as well as symmetric electrodes that minimize fabrication complexity are critical integrities for state‐of‐art ambipolar OFETs. Organic single crystals with intrinsic ambipolar characteristics are ideal active layer materials because of the high molecular ordering and low density of defects, whereas the generally low electron mobility hindered their further implantation. In this work, the highly crystallized 6,13‐bis(triisopropylsilylethynyl) pentacene single crystals are employed and successfully unveiled the intrinsic high electron mobility by a facile alcohol treatment approach. The resulting ambipolar OFETs showed averaged hole and electron mobilities both higher than 1 cm2 V−1 s−1 by a single type of metal electrode. The presented ambipolar OFETs not only improved the static and dynamic performance of ambipolar OFET‐based complementary inverters and ring oscillators by showing a high voltage gain of 186 and a short stage delay time of 0.7 ms, but they also allowed one logic gate to show two logic functions and potentially benefit the design and functionalities of future OFET‐based complementary circuits as well.

(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.

Nuclear dynamics of singlet exciton fission in pentacene single crystals.

Singlet exciton fission (SEF) is a key process for developing efficient optoelectronic devices. An aspect rarely probed directly, yet with tremendous impact on SEF properties, is the nuclear structure and dynamics involved in this process. Here, we directly observe the nuclear dynamics accompanying the SEF process in single crystal pentacene using femtosecond electron diffraction. The data reveal coherent atomic motions at 1 THz, incoherent motions, and an anisotropic lattice distortion representing the polaronic character of the triplet excitons. Combining molecular dynamics simulations, time-dependent density-functional theory, and experimental structure factor analysis, the coherent motions are identified as collective sliding motions of the pentacene molecules along their long axis. Such motions modify the excitonic coupling between adjacent molecules. Our findings reveal that long-range motions play a decisive part in the electronic decoupling of the electronically correlated triplet pairs and shed light on why SEF occurs on ultrafast time scales.

Interface Structures and Electronic States of Epitaxial Tetraazanaphthacene on Single-Crystal Pentacene.

The structural and electronic properties of interfaces composed of donor and acceptor molecules play important roles in the development of organic opto-electronic devices. Epitaxial growth of organic semiconductor molecules offers a possibility to control the interfacial structures and to explore precise properties at the intermolecular contacts. 5,6,11,12-tetraazanaphthacene (TANC) is an acceptor molecule with a molecular structure similar to that of pentacene, a representative donor material, and thus, good compatibility with pentacene is expected. In this study, the physicochemical properties of the molecular interface between TANC and pentacene single crystal (PnSC) substrates were analyzed by atomic force microscopy, grazing-incidence X-ray diffraction (GIXD), and photoelectron spectroscopy. GIXD revealed that TANC molecules assemble into epitaxial overlayers of the oriented crystallites by aligning an axis where the side edges of the molecules face each other along the direction of the PnSC. No apparent interface dipole was found, and the energy level offset between the highest occupied molecular orbitals of TANC and the PnSC was determined to be 1.75 eV, which led to a charge transfer gap width of 0.7 eV at the interface.

Emissive spin-0 triplet-pairs are a direct product of triplet-triplet annihilation in pentacene single crystals and anthradithiophene films.

Singlet fission and triplet-triplet annihilation represent two highly promising ways of increasing the efficiency of photovoltaic devices. Both processes are believed to be mediated by a biexcitonic triplet-pair state, 1(TT). Recently however, there has been debate over the role of 1(TT) in triplet-triplet annihilation. Here we use intensity-dependent, low-temperature photoluminescence measurements, combined with kinetic modelling, to show that distinct 1(TT) emission arises directly from triplet-triplet annihilation in high-quality pentacene single crystals and anthradithiophene (diF-TES-ADT) thin films. This work demonstrates that a real, emissive triplet-pair state acts as an intermediate in both singlet fission and triplet-triplet annihilation and that this is true for both endo- and exothermic singlet fission materials.

Anharmonic Lattice Vibrations in Small-Molecule Organic Semiconductors.

The intermolecular lattice vibrations in small-molecule organic semiconductors have a strong impact on their functional properties. Existing models treat the lattice vibrations within the harmonic approximation. In this work, polarization-orientation (PO) Raman measurements are used to monitor the temperature-evolution of the symmetry of lattice vibrations in anthracene and pentacene single crystals. Combined with first-principles calculations, it is shown that at 10 K, the lattice dynamics of the crystals are indeed harmonic. However, as the temperature is increased, specific lattice modes gradually lose their PO dependence and become more liquid-like. This finding is indicative of a dynamic symmetry breaking of the crystal structure and shows clear evidence of the strongly anharmonic nature of these vibrations. Pentacene also shows an apparent phase transition between 80 and 150 K, indicated by a change in the vibrational symmetry of one of the lattice modes. These findings lay the groundwork for accurate predictions of the electronic properties of high-mobility organic semiconductors at room temperature.

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.

Widely Dispersed Intermolecular Valence Bands of Epitaxially Grown Perfluoropentacene on Pentacene Single Crystals.

Strong intermolecular electronic coupling and well-ordered molecular arrangements enable efficient transport of both charge carriers and excitons in semiconducting π-conjugated molecular solids. Thus, molecular heteroepitaxy to form crystallized donor-acceptor molecular interfaces potentially leads to a novel strategy for creating efficient organic optoelectronic devices via the concomitance of these two requirements. In the present study, the crystallographic and electronic structures of a heteroepitaxial molecular interface, perfluoropentacene (PFP, C22F14) grown on pentacene single crystals (Pn-SCs, C22H14), were determined by means of grazing-incidence X-ray diffraction (GIXD) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), respectively. GIXD revealed that PFP uniquely aligned its primary axis along the [11̅0] axis of crystalline pentacene to form well-crystallized overlayers. Valence band dispersion (at least 0.49 eV wide) was successfully resolved by ARUPS. This indicated a significant transfer integral between the frontier molecular orbitals of the nearest-neighbor PFP molecules.

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

Abstract

Efficient and Layer-Dependent Exciton Pumping across Atomically Thin Organic-Inorganic Type-I Heterostructures.

The fundamental light-matter interactions in monolayer transition metal dichalcogenides might be significantly engineered by hybridization with their organic counterparts, enabling intriguing optoelectronic applications. Here, atomically thin organic-inorganic (O-I) heterostructures, comprising monolayer MoSe2 and mono-/few-layer single-crystal pentacene samples, are fabricated. These heterostructures show type-I band alignments, allowing efficient and layer-dependent exciton pumping across the O-I interfaces. The interfacial exciton pumping has much higher efficiency (>86 times) than the photoexcitation process in MoSe2 , although the pentacene layer has much lower optical absorption than MoSe2 . This highly enhanced pumping efficiency is attributed to the high quantum yield in pentacene and the ultrafast energy transfer between the O-I interface. Furthermore, those organic counterparts significantly modulate the bindings of charged excitons in monolayer MoSe2 via their precise dielectric environment engineering. The results open new avenues for exploring fundamental phenomena and novel optoelectronic applications using atomically thin O-I heterostructures.

High-Performance Simulation for Quantum Charge Transport in Large Scale Materials -Application to Carbon Nanotubes and Organic Semiconductors-

Carbon-related materials, such as carbon nanotubes (CNTs), graphenes, and organic semiconductors, have attracted much attention as new functional materials used in electronic devices. For example, the carbon nanotubes (CNTs) and graphenes are expected as high-mobility materials in comparison with silicon semiconductors. On the other hand, the mobility of organic semiconductors is much lower, but organic semiconductors have advantages as low-cost, light-weight, and printable materials. Recently, to improve the low mobility, synthesis of new organic molecules has been actively carried out. Prediction of the charge transport properties of various new materials is very important from a viewpoint of materials development. Furthermore, the systematic understanding of various transport phenomena is a challenging issue in a field of basic science. Therefore, we have developed a new numerical simulation methodology called the time-dependent wave-packet-diffusion (TD-WPD) method [1], in which we evaluate the mobility from the wavepacket dynamics using Kubo’s formula. To reduce the computational cost, we apply Chebyshev polynomial expansion to the time-evolution operator. As the result, we perform the order-N calculations, which enable to treat up to a hundred million atoms. This indicates that we can directly compute the transport property from an atomic point of view and compare the simulation results with experimental observations of samples which have micron-order length. The transport property around room temperature is dominated by the phonons (the molecular vibrations). The effects of phonons on electronic states are included as the time-dependent electron transfer integrals, since the transfer integrals are dynamically modulated by phonons. We obtain the motion of atoms using the classical molecular dynamics, then evaluate the transfer integrals using the density functional theory (DFT). As the result, we can investigate the transport properties of micron-scale material at room temperature from an atomistic viewpoint within an accuracy of DFT. As a demonstration, we apply the TD-WPD method to the metallic (5,5)-CNTs and investigate how the transport property at room temperature changes according to the CNT length. Figure 1(a) shows the calculated length dependence of the resistance. We can confirm the crossover from the ballistic transport region to the diffusive band transport region. When the CNT length becomes small to zero, the resistance becomes independent of the length and converges to the inverse of quantized conductance. This is a typical behavior of the quantum ballistic transport limit. On the other hand, the resistance monotonically increases with the length when the CNT becomes longer than 1 micron. This means that the classical diffusive transport is realized in the long CNTs. From our simulation, the mean free path is estimated as 515 nm. We can specify the quasi-ballistic region for the region where the resistance deviates from the diffusive and ballistic transport limits, which is 300 -- 1000 nm for the (5,5)-CNTs. The calculated results quantitatively agree with experimental data. Then, we investigate the temperature dependence of mobility for representative single-crystal organic semiconductors, pentacene and C8-BTBT. It is well known that the high-quality single-crystal pentacene has the temperature-independent mobility of 1 cm2/Vs around room temperature while the single-crystal C8-BTBT exhibits the diffusive band-like transport with power-law temperature dependence. The mobility of C8-BTBT is about 10 times larger than that of pentacene at room temperature. In Fig. 1(b), circles and squares represent the calculated mobilities as a function of temperature for single-crystal pentacene and C8-BTBT, respectively. The calculated temperature dependences are quantitatively in good agreement with the experimental values for the single crystal devices. The simulations give the theoretically predicted upper limit of intrinsic mobility. Even if the same organic semiconductors are used for devices, the measured mobilities have distributions to some extent depending on the employed device architecture and the crystal quality. Low-quality devices show the low mobility with thermally activated behavior. To take the difference of device quality into account, we introduce the carrier-trap potentials in the simulation. The calculated mobility for pentacene are plotted by triangles. We can confirm the magnitude of mobility dramatically decreases and the temperature dependence changes to thermally activated behavior characterized by the hopping transport nature. In our presentation, we will present our TD-WPD methodology and discussions about the above-mentioned results for CNTs and organic semiconductors in detail. We also show you how our simulation can provide a systematic understanding for different transport mechanisms, such as the ballistic transport, diffusive-band transport, and hopping transport. [1] H. Ishii, et al., Phys. Rev. B 82, 085435 (2010); Phys. Rev. B 85, 245206 (2012); Phys. Rev. B 90, 155458 (2014); Phys. Rev. B 95, 035433 (2017). Figure 1

Polymer single crystal dielectrics for organic field-effect transistors

Single crystal dielectrics are ideal candidates for high-performance organic field-effect transistors (OFETs) due to the ordered molecular packing and smooth surface. However, it doesn't obtain much attention due to the lack of facile preparation methods. Here, solution-grown smooth poly(ε-caprolactone) (PCL) single crystals with width in tens of microns are obtained and used as the dielectric for the OFETs. The interface between the semiconductor and the PCL single crystal dielectrics are more favorable to the charge carrier transport than spin-coated PCL thin-film dielectrics, exhibiting much higher charge mobility. With 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) single crystal as semiconductor layer, the average OFET mobility reaches up to 1.70 ± 0.97 cm2V−1s−1 with the highest value of 4.95 cm2V−1s−1. This result demonstrated that structural ordering of the polymer dielectrics is critical to further advance the performance of OFET devices.

Effects of the ambient exposure on the electronic states of the clean surface of the pentacene single crystal

ABSTRACTUnderstanding of the electronic structures is indispensable for complete elucidation of the charge carrier behaviors in organic semiconductors. Although recent progress enabling accurate photoemission demonstrations of organic single crystals has greatly promoted such understanding, it had been achieved merely on partially oxidized surfaces by exposure to ambient conditions. In this study, we successfully prepared an oxide-free surface of the pentacene single crystal (PnSC) by cleavage in vacuum. X-ray and ultraviolet photoelectron spectroscopy measurements on the PnSC clean surface revealed improved energetic homogeneity of the C1s level and highest-occupied state in comparison to those of the partially oxidized surface.

Single-Crystal Pentacene Valence-Band Dispersion and Its Temperature Dependence.

The electronic structures of the highest occupied molecular orbital (HOMO) or the HOMO-derived valence bands dominate the transport nature of positive charge carriers (holes) in organic semiconductors. In the present study, the valence-band structures of single-crystal pentacene and the temperature dependence of their energy-momentum dispersion relations are successfully demonstrated using angle-resolved ultraviolet photoelectron spectroscopy (ARUPS). For the shallowest valence band, the intermolecular transfer integral and effective mass of the holes are evaluated as 43.1 meV and 3.43 times the electron rest mass, respectively, at room temperature along the crystallographic direction for which the widest energy dispersion is expected. The temperature dependence of the ARUPS results reveals that the transfer integral values (hole effective mass) are enhanced (reduced) by ∼20% on cooling the sample to 110 K.