Tetracene Single 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.

Giant Davydov splitting of the lower polariton branch in a polycrystalline tetracene microcavity

We demonstrate strong exciton-photon coupling of Frenkel molecular excitons at room temperature in an all-metal microcavity containing a thin film of crystalline tetracene, with micron scale domains. In contrast to previous reports of strong coupling in organic microcavities containing localized excitonic states, here we observe large Davydov splitting of the lowest excited state, indicating strong, intermolecular interactions. The angle-resolved reflectivity of the microcavity is measured and understood in terms of the anisotropic dielectric constant of tetracene single crystals. The anisotropy leads to the formation of additional normal modes, and a giant splitting of the lower polariton branch is observed due to birefringence along both Davydov components.

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.

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.

Charge Carrier Mobility Measurements in Tetracene Single Crystals

AbstractWe present data on the transport of charge carriers in the organic semiconductor tetracene. Comparative measurements by time-of-flight (TOF) spectroscopy and measurements in field-effect transistor (FET) geometry reveal hole mobilities of about 1 cm2/Vs. Whereas for FETs only hole transport can be detected, from TOF a strong dispersive transport for negative charge carriers is observed. This observation is mainly caused by deep-level trapping of electrons. By fitting the temperature dependent hole mobility to a model of multiple-trapping and release of charge carriers the trap energy and the relative trap density can be adjusted to 130 meV and 5·10-3, respectively. Comparative chemical and structural analysis of inhomogeneities show that the traps affecting the transport are mainly caused by chemical defects rather than by structural imperfections.

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.

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.

Space charge limited transport and time of flight measurements in tetracene single crystals: A comparative study

We report on a systematic study of electronic transport in tetracene single crystals by means of space charge limited current spectroscopy and time of flight measurements. Both I–V and time of flight measurements show that the room-temperature effective hole mobility reaches values close to μ≃1 cm2/V s and that, within a range of temperatures, the mobility increases with decreasing temperature. The experimental results further allow the characterization of different aspects of the tetracene crystals. In particular, the effects of both deep and shallow traps are clearly visible and can be used to estimate their densities and characteristic energies. The results presented in this article show that the combination of I–V measurements and time of flight spectroscopy is very effective in characterizing several different aspects of electronic transport through organic crystals.

Field-effect transistors on tetracene single crystals

We report on the fabrication and electrical characterization of field-effect transistors at the surface of tetracene single crystals. We find that the mobility of these transistors reaches the room-temperature value of 0.4 cm2/V s. The nonmonotonous temperature dependence of the mobility, its weak gate voltage dependence, as well as the sharpness of the subthreshold slope, confirm the high quality of single-crystal devices. This is due to the fabrication process that does not substantially affect the crystal quality.

An organic solid state injection laser.

We report on electrically driven amplified spontaneous emission and lasing in tetracene single crystals using field-effect electrodes for efficient electron and hole injection. For laser action, feedback is provided by reflections at the cleaved edges of the crystal resulting in a Fabry-Perot resonator. Increasing the injected current density above a certain threshold value results in the decreasing of the spectral width of the emission from 120 millielectron volts to less than 1 millielectron volt because of gain narrowing and eventually laser action. High electron and hole mobilities as well as balanced charge carrier injection lead to improved exciton generation in these gate-controlled devices. Moreover, the effect of charge-induced absorption is substantially reduced in high-quality single crystals compared with amorphous organic materials.

Microfluorescence and microstructure of tetracene aggregates in poly(methyl methacrylate)

Polymer films of tetracene doped into poly(methyl methacrylate) (PMMA) were studied by macroscopic and microscopic spectroscopy. Tetracene forms aggregates in PMMA. The size of the aggregates and the degree of the aggregation increase with concentration. Microscopic measurements demonstrate that different aggregates show different morphologies and luminescence, indicating different structural formations. The global fluorescence spectra and fluorescence excitation spectra for different concentrations of tetracene (doped into poly(methyl methacrylate)) have been monitored in the temperature ranges of 77-298 K. As the temperature increases, the emission peaks shift gradually to longer wavelength. The fluorescence excitation spectra at 77 and 298 K result from the site with the same environment. The experimental results demonstrate that no phase transition is observed in the tetracene/PMMA system with decreasing temperature, in contrast to the case of tetracene single crystals. 34 refs., 9 figs.

Singlet excitation energy transfer in tetracene doped p-terphenyl single crystals

Fluorescence quantum yields of host and guest molecules in tetracene doped p-terphenyl crystals were measured for different guest concentrations c G between 1.2 × 10 −6 and 1.3 × 10 −4 mol/mol at temperatures ranging from 2 to 300 K. In the low-temperature region (2–60 K) the ratio of the quantum yields from guest to host molecules decreases with rising temperature, where the slope of the decrease depends significantly on c G. On the other hand, for temperatures higher than 60 K the ratio of quantum yields increases with rising temperature and this temperature behaviour is independent of c G. These different temperature characteristics of the quantum yield ratio in relation to the guest concentration are interpreted in terms of singlet excitation energy migration processes on the host molecules. The discussion is based on a rationalization which postulates a temperature inhibited exciton motion at temperatures below 60 K and dopant concentrations below 10 −4 mol/mol.

Exciton-phonon interaction in tetracene single crystals under pressure

Absorption and luminescence spectra of tetracene have been investigated under pressure up to 31.7 kbar, using a diamond anvil cell. The exciton-phonon coupling constant increases with increasing pressure and at 22.6 kbar the free-exciton luminescence appears to be replaced by self-trapped-exciton luminescence. The Davydov splitting increases at a rate of 83 cm −1/kbar in the pressure range below 8 kbar, and 46.4 cm −1/kbar in the range above 12 kbar. The intermolecular resonance interaction at high pressures seems to have short-range character. Pressure-induced phase transitions which occur at 3 and 12 kbar are briefly discussed.

Optical Constants of Tetracene Single Crystal within the First Absorption Band

Abstract Optical constants of tetracene single crystal within the first absorption band have been determined for various polarization states of the light incident on its crystallographic (ab) plane at room temperature. A method based on the combination of the reflectance spectra taken from the crystal interface with two different transparent media (TTM method) has been developed in order to avoid approximations needed for the analysis by usual Kramers-Kronig (KK) method. In contrast to the latter, our method does not require the knowledge of the reflectivity throughout the entire energy range and a comparison of the results with independent experimental absorption and refraction data of the crystal shows the method to be more accurate than KK method. Some possible applications of the data are briefly discussed in conjunction with intermolecular structure of crystalline tetracene.

Intermediate Exciton-Phonon Coupling in Tetracene

Polarized absorption spectra of tetracene (naphthacene) single crystals have been investigated between 300 and 13 K, with particular emphasis on the b -polarized spectrum. The low energy tails of the a - and b -polarized first absorption bands are expressed by the Urbach rule with the steepness coefficients σ a ′ =1.37 ±0.12, σ b ′ =1.58 ±0.13, with which possibility of self-trapping of excitons is discussed along with the dimensionality of tetracene crystals. The exciton-phonon coupling constants are found to be g a =1.09 ±0.1 and g b =0.95 ±0.08, consistent with luminescence experiments which show that excitons in tetracene are self-trapped very shallowly. The line halfwidth, the oscillator strength of the first and the second absorption bands, refractive indices, and surface exciton structure are briefly discussed.

Pressure and temperature induced modification of luminescence from tetracene single crystals

Fluorescence spectra of crystalline tetracene have been recorded in the temperature range 120 to 300 K under hydrostatic pressure up to 600 MPa. From discontinuities in both emission spectra and spectral intensities it is concluded that two phase transitions occur. The room temperature phase is transformed to a low temperature phase/high pressure phase I at T I t ( p = 0) = 182 K, the temperature coefficient being d T I t/d p = 0.395 K/MPa. The phase transition is induced by a decrease of the specific volume under pressure and/or upon cooling. Lack of a significant shift of the origin of the fluorescence band near T I t at constant pressure is an artifact resulting from the neglect of reabsorption effects. The Stokes shift is 260 cm -1, independent of temperature and crystal modification. In accord with previous Raman data a second phase transition occurs at T II t ( p) = 143 K, the pressure shift being d T II t/d p = 0.088 K/MPa. In addition, the shift of the triplet energy as a function of pressure as well as the pressure-dependence of the rate constants governing fission of a singlet exciton into a pair of triplets is discussed utilizing their magnetic field dependences.

Thermoreflectance spectrum of tetracene single crystal

The b-polarized thermoreflectance spectra of the (001) face of tetracene single crystals have been measured, at room temperature and 100 K below the structural phase transitions at 182 and 144 K, for the first time. Evidence for the asymmetric broadening of the Frenkel exciton due to the weak exciton (phonon) coupling is found in thermoreflectance spectra. A large exciton red shift at the structural phase transition has been found by decreasing the temperature.