Liquid Crystalline Ordering and Charge Transport in Semiconducting Materials

Abstract

Organic semiconducting materials offer the advantage of solution processability into flexible films. In most cases, their drawback is based on their low charge carrier mobility, which is directly related to the packing of the molecules both on local (amorphous versus crystalline) and on macroscopic (grain boundaries) length scales. Liquid crystalline ordering offers the possibility of circumventing this problem. An advanced concept comprises: i) the application of materials with different liquid crystalline phases, ii) the orientation of a low viscosity high temperature phase, and, iii) the transfer of the macroscopic orientation during cooling to a highly ordered (at best, crystalline-like) phase at room temperature. At the same time, the desired orientation for the application (OLED or field-effect transistor) can be obtained. This review presents the use of molecules with discotic, calamitic and sanidic phases and discusses the sensitivity of the phases with regard to defects depending on the dimensionality of the ordered structure (columns: 1D, smectic layers and sanidic phases: 2D). It presents ways to systematically improve charge carrier mobility by proper variation of the electronic and steric (packing) structure of the constituting molecules and to reach charge carrier mobilities that are close to and comparable to amorphous silicon, with values of 0.1 to 0.7 cm(2) · V(-1) · s(-1) . In this context, the significance of cross-linking to stabilize the orientation and liquid crystalline behavior of inorganic/organic hybrids is also discussed.

Isocyano-Triphenylene Complexes of Gold, Copper, Silver, and Platinum. Coordination Features and Mesomorphic Behavior

“…V mol is the molecular volume and ρ the density, and h mol = V mol /S is the molecular thickness (N=1). 2 ]BF 4 offer diverse mesomorphism conditioned by the metal fragments. These influence the electronic interactions of the discotic triphenylene moiety, and also the intramolecular availability or restriction to arrange coplanar the triphenylene groups, when more than one is present in the molecule.…”

“…1,2,3,4,5,6,7,8 These devices require high charge-carrier mobility for operation, and columnar liquid-crystalline materials displaying high charge-carrier mobility along their columnar stacks are particularly attractive for this purpose. 1,9,10,11,12 The transfer efficiency depends on the nature and intensity of the intermolecular interactions that maintain the molecular stacks order over long ranges in the columnar structure.…”

“…23 Exploring the possibility to develop columnar materials in which columns of different natures coexist, we reported recently a family of mesomorphic mono and dinuclear ortho-metallated palladium complexes containing a columnar packing of triphenylene moieties that supports an associated stack of metal moieties. 24 We have also prepared some cationic mixed triphenylene-isocyanide ortho-platinated benzoquinolate complexes ([Pt(bzq)(CNC 6 H 4 -O-(CH 2 ) 6 -TriPh) 2 ]A, A -= NO 3 -, BF 4 -, PF 6 -), displaying a columnar mesophase. The supramolecular structure consists of a central column formed by the self-assembly of the organometallic benzoquinolate-platinum fragments, hexagonally surrounded by six columns formed by stacking of the triphenylene groups.…”

Integration of electro-active π-conjugated units in nanosegregated liquid-crystalline phases

“…Liquid-crystalline (LC) semiconductors, including columnar, smectic and nematic compounds, have been studied by European and Japanese researchers. [2][3][4][5] In the LC phases, electronic charge carriers are transported by the hopping mechanism, although band-like conduction has been reported in ordered smectic phases. 6,7 In addition to basic research, the application of these materials to electronic devices, such as light emitting diodes, 8 field-effect transistors 9 and solar cells, 10,11 has been studied.…”

Synthesis and thermotropic behavior of side chain polysiloxane bearing triphenylene moiety

“…There has been an increasing interest in columnar discotics which involve !-conjugated systems [1][2][3]. They have been extensively studied as a result of their electro-optical and semiconducting properties as well as expected applications in organic field effect transistor devices (OFET) and solar cells [4][5][6].…”