The rubrene molecule is basically the tetracene molecule with four wings. Its family are the polycyclic aromatic hydrocarbons. When rubrene molecules combine to build orthorhombic crystals, the molecules have a centrosymmetric structure with 2/m symmetry. This is a very important point when it comes to understanding the most common rubrene crystals, which have an orthorhombic structure with all molecules (all of them) aligned with their 2-fold symmetry axis M parallel to each other, and generally perpendicular to the natural large facet of rubrene crystals grown by vapor transport.

Rubrene Thin Films with Viably Enhanced Charge Transport

The organic semiconductor (OS) rubrene (Rubrene; 5,6,11,12—tetraphenyltetracene), is an efficient laser dye, which has been widely studied due to its long spin diffusion length and charge-carrier mobility of up to 20 cm2V?1s?1. It is noted for its single crystals at room temperature; the highest reported value for acenes. Organic components with hydrocarbons of low atomic numbers (low-Z) are preferred in hybrid spintronic and optoelectronic device applications because of the ~Z4 dependence of the spin-orbit and hyperfine interactions, which are the main contributors to the loss of spin information. The key requirement of their design is the controlled fabrication of chemically clean OS layers. The structural forms of rubrene are either amorphous, single crystal, or polycrystalline, and the three most common polymorphs are monoclinic plates, triclinic ribbons, and orthorhombic crystals. In the monoclinic structure, the molecular planes of two adjacent molecules form an angle of approximately 90°, thus, the π–π interaction that favors band transport does not occur. In the case of the triclinic polymorph, the molecular planes of adjacent molecules are parallel, but shifted relative to each other, which leads to only a partial superposition of the π orbitals towards the interaction of the molecules.[1]

Film growth in Rubrene is associated with the appearance of the amorphous phase and the presence of spherulitic structures. This undesirable effect originates in the Rubrene molecular conformation, characterized by the phenyl rings twisted in respect to the tetracene backbone, which causes non-planarity of the molecule, and is the main obstacle in growing Rubrene films with controlled crystallinity. Furthermore, in the growth of thin Rubrene layers with one orientation, precise control is necessary due to the high anisotropy of the optical and electrical properties of the material. Notably, it has been predicted that when going from the Rubrene amorphous to the crystalline form, the spin diffusion length can increase by several orders of magnitude due to minimal dangling bond-related defects and a reduced grain boundary.

In summary, thin films of organic semiconductor rubrene, fabricated by an original extension of the MAPLE method, were studied from the point of view of application capacity. Observations of films fabricated on Si or ITO/glass substrates revealed the homogeneous distribution of irregular Rubrene crystallites, in a ~100 nm-thick layer with surface roughness of 7.5 nm (RMS). The crystallinity and molecular structure of the deposited Rubrene confirmed the dominant presence of orthorhombic crystallites in the films. Furthermore, one-dimensional nanowires were observed vertically growing on the edges of plate-like crystals. The high chemical purity was determined and only negligible Rubrene decomposition, associated with presence of residual oxygen and water originating from the post-fabrication storage conditions of the films, was found. The charge-carrier mobility was significantly improved, compared to amorphous Rubrene and data from other evaporation methods, including PLD. The observed mobility values of up to 0.13 cm2V?1s?1, are close to the technologically accepted values. These results demonstrate the potential of the applied cryo-matrix supported evaporation method in the controlled, scalable, and non-destructive fabrication of Rubrene thin films for optoelectronic components. Further application-oriented studies on the MOSFET structure containing Rubrene layer, including control of the interface of the charge and spin transporting OS and the carrier injecting inorganic ferromagnetic layer are in progress.

The actual electronic band structure of a rubrene single crystal

Among the organic SCs, rubrene (5,6,11,12-tetraphenyltetracene) SC is the one whose electronic band structure was most intensively studied due its high hole mobility as reported by transport measurement (40–45?cm2/Vs). Nevertheless, despite of the great experimental and theoretical efforts, there is still no complete consensus between the band dispersion and hole mobility, and not even between the experimentally and theoretical dispersions. This means that the carrier transport mechanism in rubrene SC, which is an essential information for designing organic devices with high carrier mobility, is still an unresolved issue. To confirm the quality of the rubrene SC used in the present study, we made a rubrene SC field-effect transistor (FET) and measured its carrier transport characteristic, i.e., the drain current as a function of gate voltage. The hole mobility was μh?>?30?cm2/Vs with an average value of 20?cm2/Vs, which is comparable to the highest mobility measured with the same method. Both the hole mobility and the negligible hysteresis indicating the high stability of the FET device confirm the high quality of the rubrene SC used in the present study.[2]

A proper understanding on the charge mobility in organic materials is one of the key factors to realize highly functionalized organic semiconductor devices. So far, however, although a number of studies have proposed the carrier transport mechanism of rubrene single crystal to be band-like, there are disagreements between the results reported in these papers. Here, we show that the actual dispersion widths of the electronic bands formed by the highest occupied molecular orbital are much smaller than those reported in the literature, and that the disagreements originate from the diffraction effect of photoelectron and the vibrations of molecules. The present result indicates that the electronic bands would not be the main channel for hole mobility in case of rubrene single crystal and the necessity to consider a more complex picture like molecular vibrations mediated carrier transport. These findings open an avenue for a thorough insight on how to realize organic semiconductor devices with high carrier mobility.

Rubrene: The Interplay between Intramolecular and Intermolecular Interactions

A classic example is tetracene functionalized with phenyl rings at the 5-, 6-, 11-, and 12-positions, a compound referred to as rubrene. The presence of these phenyl rings converts the typical herringbone structure found in oligoacenes to a slipped-cofacial packing of the π-conjugated tetracene backbones. For rubrene, large hole mobilities (as high as 40 cm2 V–1 s–1) arise from the strong intermolecular electronic couplings (on the order of 100 meV, as calculated with density functional theory methods) that result from the good wave function overlap among the frontier molecular orbitals of the stacked molecular neighbors in the crystallographic ab-plane of the orthorhombic crystal. However, electronic structure calculations on isolated rubrene molecules show that the presence of the side phenyl rings makes the tetracene backbone of rubrene preferentially twist (～40°),  which is confirmed by experimental evidence of twisted conformations both in solution and thin films. Interestingly, examination of rubrene thin films grown on Au(111) surfaces reveals that the twisted conformation is present in the initial layers and then transitions solely to the planar conformation, an indication of the decisive influence that surrounding molecules play in leading to a planar conformation in the bulk molecular structure.[3]

The comprehensive analysis of these noncovalent interactions allows us to identify (at least some of) the key chemical aspects that can stabilize the planar rubrene conformation and replicate the advantageous packing demonstrated in unsubstituted rubrene. The quantum chemistry-based understanding presented here underlines that improved synthetic derivatization schemes to increase favorable noncovalent interactions have the potential to improve the materials performance of this benchmark molecular material, in particular, with regard to charge transport.

References

[1]Jendrzejewski R, Majewska N, Majumdar S, Sawczak M, Ryl J, ?liwiński G. Rubrene Thin Films with Viably Enhanced Charge Transport Fabricated by Cryo-Matrix-Assisted Laser Evaporation. Materials (Basel). 2021 Aug 6;14(16):4413.

[2]Nitta J, Miwa K, Komiya N, Annese E, Fujii J, Ono S, Sakamoto K. The actual electronic band structure of a rubrene single crystal. Sci Rep. 2019 Jul 4;9(1):9645.

[3]Sutton C, Marshall MS, Sherrill CD, Risko C, Brédas JL. Rubrene: The Interplay between Intramolecular and Intermolecular Interactions Determines the Planarization of Its Tetracene Core in the Solid State. J Am Chem Soc. 2015 Jul 15;137(27):8775-82.