Rubrene crystal

FIGURE 2.1.23 Pressure dependence of the field-effect mobility (a) and the threshold voltage (b) in single-crystal rubrene OFETs (solid and open symbols correspond to the increasing and decreasing pressure). (From Rang, Z. et ah, Appl. Phys. Lett., 86, 123501, 2005.)... 

It is well known that the carrier mobility is limited in organic solids. In a wide range of molecular crystals, the mobility appears to be limited to around 1-10 cm V s [71]. Recently, single-crystal rubrene OFETs were reported with the mobility of 15 cm s [72]. The reason for the mobility limitation is that molecular materials are not covalently bound and electronic orbital overlap is limited. A robust organic material with the mobility of 1 cm s would still be an interesting competitor to amorphous silicon (a-Si). Some examples of molecular semiconductors are shown in Figure 7.12. 

Reduction of the hydrocarbon 5,6,11,12-tetraphenyltetracene (rubrene) with a sodium mirror in THF gave a dark green solution from which almost black crystals of the tetrakis-sodium salt 83 could be obtained.1353 Two of the four sodium cations (each doubly solvated by THF) are located (Fig. 53) above and below the central tetracene skeleton and the other two are between pendant phenyl groups. The central sodium ions are 8-coordinate with Na-C 260-263 pm to the phenyl-substituted and 272 pm to the other... 

Rubrene crystal structure I. Bulgarovs-kaya, V Vozzhennikov, S. Aleksandrov,... 

Comparing single crystal and vapor-grown devices for these two compounds is difficult, because reports on evaporated tetracene OTFTs are rather scarce [99-101], and despite several (unpublished) attempts, fabrication of an operating thin-film device from rubrene has not yet been successfully achieved. For both compounds the problem seems to arise from an improper deposition mechanism, which, in contrast with experience with pentacene and sexithiophene, does not favor two-dimensional growth. 

There is a very short list of organic semiconductors with reported thin-film field-effect mobilities greater than 1 cm2 V-1 s h These include pentacene, sexithio-phene [5a], and anthradithiophene [17]. If we extend this list to include single crystal and n-type materials, we can add perylene [18], rubrene [19], copper phtha-locyanine (CuPc) [20], tetracyanoquinodimethane (TCNQ) [21], and dithiophene-tetrathiofulvalene (DT-TTF) [22] - still a short list. 

Fig. 6.2. Decrease of mobility with increasing e, as observed in rubrene single crystal FET with different gate insulators. The bars give a measure of spread of mobility values. Inset ...

The laser-excited fluorescence from dibenzofuran in a biphenyl host at 4.2 K and three different sites in the lattice have been identified. The two-photon excitation spectrum of single crystals of carbazole at 4.2 K has been analysed in detail. Very extensive vibrationally unrelaxed fluorescence is observed following dye-laser excitation of single vibronic levels of naphthazanine and the rates and pathways of relaxation are examined by picosecond emission spectroscopy. The photophysics of rubrene peroxide have been measured for the first time by Bayrakceken. Picosecond fluorescence has been used to study the spectra and kinetics for Sj— So and Si transitions for... 

Rubrene forms orange crystals on sublimation at 250-260°/3-4mm [UV Badger Pearce Spectrochim Acta 4 280 1950], It has also been recrystallised from benzene under red light because it is chemiluminescent and light sensitive. [Beilstein 5 IV 2968.]... 

Table 25.2 Representative hole mobilities reported for the bulk (TOP) and the surface (PET) of sublimation grown crystals. The PET data were obtained on samples with on-top deposited polymeric gate insulators. Bulk transport refers to the [001] direction and surface transport to the (001) plane. Except for rubrene, all materials offer comparable or lower PET mobilities at the surface.

Abstract. Structural properties of rubrene thin films on cleaved mica (001) surfaces were investigated by optical microscopy and x-ray diffraction. Optical microscopy shows, that the crystallization of rubrene results in formation of spherulites. X-ray specular diffraction reveals polycrystalline and polymorphic nature of rubrene. The pole figure measurements of films prepared at low deposition rates reveal orthorhombic structure and indicate fiber textures with crystallographic planes (121), (131) and (141) preferentially oriented parallel to the substrate surface. High deposition rate thin films in addition show polymorphism, corroborating the existence of the orthorhombic and the triclinic phase. 

Fig. 3. Pole figures of rubrene films prepared at low deposition rate are measured at qz = 0.86A-1 (a) and qz = 1.19A-1 (b). The calculated pole densities of individual crystal orientations with (141), (131) and (121) are denoted by , and, respectively. High deposition rate films are measured at qz = 1,45A (c) and qz = 1.72A-1 (d). The calculated pole densities due to (001), (010) of the orthorhombic phase and (010) of the triclinic phase are denoted by v, a and o, respectively. The single high intensity spots are due to single crystalline mica substrate.

Rubrene on Mica From the Early Growth Stage to Late Crystallization... 

FIGURE 2.1.2 Single crystals of rubrene and tetracene grown from the vapor phase. 

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