Tetracene Crystals Research Articles

Morphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors

We present a comparative study of ultrafast photoconversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photoinduced absorption in Tc and Pc crystals is activated and temperature-independent, respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C60-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.

Rubrene Heteroepitaxial Nanostructures With Unique Orientation

The development of a thin film technology based on the outstanding organic semiconductor rubrene is seriously prevented by its tendency to solidify in an amorphous state when deposited on foreign substrates. Here, a crystalline thin film of rubrene was successfully grown by vapor deposition on the surface of a tetracene single crystal. Atomic resolution imaging performed with scanning force microscopy revealed the high crystallinity of the film and its unique orientation achieved through a line-on-line epitaxial relation.

Dielectric tensor of tetracene single crystals: The effect of anisotropy on polarized absorption and emission spectra

The full UV-visible dielectric tensor and the corresponding directions of the principal axes of triclinic tetracene crystals are reported as deduced either by polarized absorption and ellipsometry measurements or by calculations based on the molecular and crystallographic data. The results allow the attribution of the polarized bands observed in both absorption and photoluminescence emission spectra. In particular, the spectral line shape and polarization of the emission are found to depend on the sample thickness, and the effect is attributed to the modification of the state of polarization of the emitted light during its propagation inside the crystal. Indeed, the directions of polarization of the lowest optical transitions and the directions of the principal axes of the dielectric tensor are demonstrated not to coincide, in contrast to the assumptions typically made in the literature, thus causing the mixed transverse/longitudinal character of light propagation.

Monolithic vertical microcavities based on tetracene single crystals

The authors report on monolithic, light-emitting vertical microcavities based on an organic semiconductor single crystal. The devices are realized by reactive electron-beam deposition of dielectric mirrors and growth of tetracene crystals by physical vapor transport. The microcavities exhibit optical cavity modes in the visible range (550–580nm) with full width at half maximum down to 2–3nm, corresponding to a Q factor of about 200, and polarization-induced modal splitting up to 20meV. These results open perspectives for the realization of polarized-emitting optoelectronic devices based on organic crystals.

Anisotropy in Organic Single‐Crystal Photovoltaic Characteristics

Organic single crystals have been well researched for many years. Typical vapor-phase growth of organic crystals developed from vertical to horizontal growth in order to achieve improved crystalline quality. Recently, a field-effect study on these single-crystal semiconductors demonstrated high carrier mobilities, up to ca. 15 cm Vs, along with anisotropic charge-transport properties. These single-crystal transistors were usually fabricated with rigid and thick crystals (tens of micrometers to millimeters), which were fragile and difficult to process because of poor mechanical properties. Recently, a growth method affording thin, 150 nm thick, organic crystals demonstrated the processability of single-crystal transistors on flexible substrates, and these organic crystals could be patterned on individual channels for transistors. However, in addition to transistor studies of these organic crystals, other types of electronic applications, such as two-terminal devices, have not yet been realized, mainly because of the difficultly processing thick crystals. In this Communication, we report organic single-crystal photovoltaics fabricated from single pieces of thin tetracene crystals on bilayer heterojunctions with fullerene (C60) thin films. These organic singlecrystal devices exhibited excellent diode behavior with rectifying ratios of 10 and an external power conversion efficiency (PCE) of ca. 0.34 %. By employing these high-quality single crystals in two-terminal devices, high-performance optoelectronic devices, such as organic diodes, photovoltaics, and photodetectors, become possible alternatives for large-area, low-cost flexible electronics. The quality of the single crystals was examined by cross-polarized microscopy, X-ray powder diffraction (XRD), and single-crystal X-ray diffraction. The optical microscopy images (Fig. 1a and b) recorded at 0° and 90° from the entrance polarizer and exit analyzer, respectively, show large birefringence, confirming the anisotropic crystalline nature of the tetracene crystals. The XRD data exhibit strong and narrow first (001) and second order (002) reflections (Fig. 1c), indicating the tetracene crystal is c-oriented, with molecular-plane growth along the vertical direction. The crystal data obtained for tetracene confirm a C18H6 molecular formula with a molecule weight of 222.23 g mol. The lattice constants are a = 6.02 ± 0.025 A, b = 7.77 ± 0.032 A, c = 12.46 ± 0.054 A, a =101.11 ± 0.078°, b = 99.41 ± 0.092°, and c = 94.40 ± 0.088°, and tetracene crystallizes with a triclinic crystal structure in space group P 1. The coordinates for tetracene can be obtained in the crystallographic information file (CIF) format from the Cambridge Crystallographic Data Center (CCDC), and the above crystallographic data are consistent with the data in the CCDC. Both the strong birefringence and the XRD results indicate these organic single crystals are of high quality. A schematic structure of a single-crystal solar cell is shown in Figure 2a. The device structure comprised a poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS)coated indium tin oxide (ITO) substrate, a tetracene crystal, evaporated thin films of C60 and bathocuproine (BCP), and an aluminum thin-film electrode. We note that thin crystals (ca. 200 nm) used in this study conformed better on substrates C O M M U N IC A IO N

Heteroepitaxy of α-Quaterthiophene on Tetracene Single Crystals

α-Quaterthiophene thin films were grown on the {001} face of single crystals of tetracene by organic molecular beam epitaxy. The peculiar corrugation of the {001} surface of tetracene, its chirality, and its low symmetry give rise to two phases of the overlayer. The morphology−structure and epitaxial relations of these two phases are investigated on a local scale by atomic force microscopy and on a large scale by optical spectroscopy, revealing the orientation mechanisms driving the formation of this organic−organic heteroepitaxial system of semiconductors.

Evidence of low intermolecular coupling in rubrene single crystals by Ramanscattering

The observed Raman spectra for single crystals of rubrene and tetracene are compared withthe calculated spectra for the isolated molecules. The Raman measurements presentedare of the bulk properties of the material, and they confirmed that the vapourgrowth process yields very pure, unstrained rubrene crystals. Finally, Ramanmeasurements indicate that rubrene, unlike many other oligoacenes, has very weakintermolecular coupling and no observable intermolecular Raman vibrationalmodes. We discuss the apparent conflict between the high mobility and the weakπ-electron overlap in this material.

TEM characterization of organic nanocrystals grown in sol–gel thin films

The tetracene molecule (2,3-benzanthracene, C8H12) was used to synthesize nanocrystals grown in sol–gel thin films, ranging from 10 to 100 nm of diameter. This confined nucleation and growth was compared to microcrystallizations of the same molecule in free solution. Transmission electron microscopy (TEM) was used to characterize these two kinds of tetracene crystals. The observation was performed under low-dose illumination to avoid amorphization of the samples during electron irradiation. Spatial confinement and size distribution of micro- and nanocrystals were compared. Using electron microdiffraction and diffraction patterns simulations, we showed that free microcrystals and nanocrystals confined in gel glasses exhibit the same triclinic $$ P\overline 1 $$ structure. In addition, time-resolved spectroscopy was used to record fluorescence decays, showing a monoexponential fluorescence decay for nanocrystals while microcrystals exhibit a multiexponential decay. The simple signature of nanocrystals luminescence is promising for the future development of chemical or biological sensors.

Post-processing of spin-coated organic thin films in solvent vapors: Vapor pressure monitoring by infrared absorption and the effect of electric fields

We have examined the effect of the post-processing of spin-coated tetracene and DAST (4′-dimethylamino-N-methyl-4-stilbazolium tosylate) films in their solvent vapor to examine the possibility of precisely controlling the vapor pressure and studying the effect of applying electric fields to their structural properties. It was demonstrated that the solvent concentration in the vapor phase can be monitored by infrared spectroscopy. The thickness uniformity of the tetracene films was improved by the formation of a mixture of the tetracene crystals with the saturated solution (“solution ultrathin film”) on the surface. Crystals with an ordered orientation were obtained by additionally applying an electric field during the vapor exposure of the DAST. The present technique for monitoring the solvent vapor pressure is also applicable for controlling the drying speed during the inkjet printing of functional organic materials.

Contact Pressure Dependence of Carrier Mobility in Cleaved Tetracene Single-Crystal Field-Effect Transistors

Tetracene single-crystal field-effect transistors (FETs) were fabricated using fresh cleaved tetracene single crystals. In order to obtain a fine contact between the tetracene crystals and the substrate, a stress was applied using an Instron-type machine. Carrier mobility was measured as a function of applied stress and it was observed that 0.5 cm2/(V s) was the highest mobility under the optimum applied stress.

Ultrafast carrier dynamics in pentacene, functionalized pentacene, tetracene, and rubrene single crystals

We measure the transient photoconductivity in pentacene, functionalized pentacene, tetracene, and rubrene single crystals using optical pump-terahertz probe techniques. In all of the samples studied, we observe subpicosecond charge photogeneration and a peak photoconductive response that increases as the temperature decreases from 297 down to 20K, indicative of bandlike transport. Similar decay dynamics are observed at room temperature, but at low temperatures the decay dynamics measured in pentacene, rubrene, and tetracene crystals are much faster than those observed in functionalized pentacene crystals, revealing different charge trapping properties.

Chemical and structural effects on the electronic transport in organic single crystals

Effects induced by chemical and structural inhomogeneities on the electronic transport in organic crystals with the focus on highly-ordered polyacenes are discussed in this paper. The material class chosen is of great importance as polyacenes are widely studied with perspectives on applications in thin film devices. As will be shown, the charge carrier mobility in these long-range ordered molecular systems is limited by chemical impurities rather than by structural inhomogeneities. Time-of-flight spectroscopy on polyacenes purified by zone-refinement does not only show coherent carrier movement for holes but also for electrons, which is rarely observed for compounds that can be purified by sublimation only. Finally, we will present a direct comparison between the chemical species and their amount on the surface and in the volume of tetracene crystals. The significantly higher impurity concentration at the surfaces is relevant for the mobility estimated by field-effect transistor (FET) studies on single crystals in planar-geometry.

Correlation Field Splitting of Intramolecular Vibrations of Crystalline Tetracene in Terahertz Region

The temperature dependence of terahertz (THz) absorption was observed for molecular crystals of tetracene using the Fourier transform infrared (FT-IR) spectroscopy. Over the frequency range of 1 to 6 THz (33 to 200 cm-1), THz absorptions corresponding to phonon and intramolecular vibrational modes were confirmed. Furthermore, the intramolecular vibrational modes exhibited correlation field splitting that originated from crystallization. A temperature-induced phase transition was successfully detected for the intramolecular vibrational modes of tetracene, even in powder form. Our results showed that these THz absorptions are highly sensitive to the molecular orientations within the crystal.

Comparison of the electronic properties of sublimation- and vapor-Bridgman-grown crystals of tetracene

The hole mobilities in a vapor-Bridgman-grown crystal and in sublimation-grown crystals of tetracene were measured along the c*-direction in a temperature range from 300to450K by the time-of-flight method. The mobility was found to increase as a function of temperature in the vapor-Bridgman-grown crystal, indicating that the transport was thermally activated. After reaching its maximum of about 1cm2∕Vs at 320K, the mobility in the sublimation-grown crystals decreased with temperature in accordance with band transport models. Gas chromatography measurements indicated that the difference in the electronic transport properties could be related to a higher concentration of chemical impurities in the vapor-Bridgman crystal.

Numerical simulation of tetracene light-emitting transistors: A detailed balance of exciton processes

We assess the possibility to use an ambipolar organic light-emitting transistor structure as gain medium for an electrically pumped laser. Singlet and triplet continuity equations are solved together with Poissons and drift-diffusion equations in two dimensions. The solution allows for a detailed balance between the exciton decay, quenching and generation mechanisms. Simulations of a tetracene light-emitting transistor show that triplets are most dominant in quenching singlets. Singlet–triplet quenching can ultimately prevent pure tetracene crystals or films—when provided with a realistic optical feedback structure, to reach the threshold for stimulated emission.

Phonons and structures of tetracene polymorphs at low temperature and high pressure

Crystals of tetracene have been studied by means of lattice phonon Raman spectroscopy as a function of temperature and pressure. Two different phases (polymorphs I and II) have been obtained, depending on sample preparation and history. Polymorph I is the most frequently grown phase, stable at ambient conditions. A pressure induced phase transition, observed above 1 GPa, leads to polymorph II, which is also obtained at temperatures below 140 K. Polymorph II can also be maintained at ambient conditions. We have calculated the crystallographic structures and phonon frequencies as a function of temperature, starting from the configurations of the energy minima found by exploring the potential energy surface of crystalline tetracene. The spectra calculated for the first and second deepest minima match satisfactorily those measured for polymorphs I and II, respectively. All published x-ray structures, once assigned to the appropriate polymorph, are also reproduced.

Hole mobility in organic single crystals measured by a “flip-crystal” field-effect technique

We report on single crystal high mobility organic field-effect transistors prepared on prefabricated substrates using a “flip-crystal” approach. This method minimizes crystal handling and avoids direct processing of the crystal that may degrade the field-effect transistors' electrical characteristics. A chemical treatment process for the substrate ensures a reproducible device quality. With limited purification of the starting materials, hole mobilities of 10.7, 1.3, and 1.4cm2∕Vs have been measured on rubrene, tetracene, and pentacene single crystals, respectively. Four-terminal measurements allow for the extraction of the “intrinsic” transistor channel resistance and the parasitic series contact resistances. The technique employed in this study shows potential as a general method for studying charge transport in field-accumulated carrier channels near the surface of organic single crystals.

Current–voltage characteristics of a tetracene crystal:Space charge or injection limited conductivity?

We show that current–voltage characteristics, measured on a tetracene crystal sandwiched between two gold electrodes, are in quantitative agreement with the concept of dopant-assisted charge injection across a metal–organic interface. This notion explains the wild range of carrier mobilities recently found by space-charge limited current measurements in apparently identical tetracene crystals [de Boer et al., J. Appl. Phys. 95, 1196 (2004)] and the difference between these data and the mobility measured in the time-of-flight experiments.

Transport properties of single-crystal tetracene field-effect transistors with silicon dioxide gate dielectric

Single-crystal organic field-effect transistors (SX-OFETs) with channel lengths of 1 and 100μm have been fabricated by adhering thin crystals of tetracene to freshly ashed SiO2 substrates containing countersunk gold contacts. The intrinsic transport properties of the tetracene single crystals, corrected for potential contact effects by using a standard four-probe configuration, have been measured from room temperature down to 4.2K. These OFETs exhibit mobilities as high as 0.1cm2V−1s−1, subthreshold swings of <500mV∕decade, and Ion∕Ioff ratios in excess of 109. The larger devices (L=100μm,W=1000μm) show thermally activated mobilities over the temperature range 200K<T<300K, but thermally induced cracks in the crystal prevent this analysis from being extended to lower temperatures. The smaller devices have a greater probability of surviving to low temperatures without a crack permeating the active channel, and representative devices have been investigated over the full range 4.2K<T<300K. The transport mechanism in these smaller devices can be varied from thermally activated to nearly temperature invariant by the application of drain fields >105V∕cm.

Field-effect transistors made from macroscopic single crystals of tetracene and related semiconductors on polymer dielectrics

We report properties of devices made by the adhesion of semiconductor crystals, including several tetracene specimens, to polymer gate dielectrics along with measurements of tetracene crystals on conventional Si/SiO2 dielectric surfaces. For the best tetracene, pentacene, and alpha-6T devices, mobilities exceeding 0.1 cm2/V were measured, correlating well with expectations based on the literature, and in the case of tetracene and alpha-6T, exceeding the thin film mobility value. The devices were prepared in the open laboratory using simpler crystal handling techniques than had been thought necessary.