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Liquid Crystalline Ordering and Charge Transport in Semiconducting Materials
Author(s) -
Pisula Wojciech,
Zorn Matthias,
Chang Ji Young,
Müllen Klaus,
Zentel Rudolf
Publication year - 2009
Publication title -
macromolecular rapid communications
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.348
H-Index - 154
eISSN - 1521-3927
pISSN - 1022-1336
DOI - 10.1002/marc.200900251
Subject(s) - materials science , amorphous solid , discotic liquid crystal , chemical physics , liquid crystal , phase (matter) , context (archaeology) , charge carrier , steric effects , electron mobility , organic semiconductor , crystalline silicon , crystallography , silicon , organic chemistry , optoelectronics , chemistry , paleontology , biology
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.