Top > Research > Developments of Liquid Crystalline Organic Semiconductors

Developments of Liquid Crystalline Organic Semiconductors

There are two types of liquid crystal materials: rod-like liquid crystals and disk-like liquid crystals, depending on their molecular shapes. In rod-like liquid crystals, it is easy to control the molecular orientation of the whole film by utilizing the property of substrate surface. In addition, rod-like liquid crystal molecules form a layered structure (smectic layer) and a two-dimensional carrier transport path within the layer, which is good structure exhibiting high mobility for organic semiconductor material. Among the smectic liquid crystal phases that form a layered structure, the smetic E liquid crystal phase (SmE phase), that aggregates densely within the layer and has a highly ordered structure like crystal, is very useful as liquid crystalline organic semiconductor materials.
As a material that exhibits this liquid crystalline phase (SmE phase), we developed a liquid crystalline phenyl-benzothienobenzothiophene derivative (Ph-BTBT-10) in 2011. This material exhibits the SmE phase at temperatures from 142 °C to 210 °C, and because of its asymmetric structure, the supercooled liquid crystal phase until near room temperature. The asymmetric structure of the molecule allows the supercooled liquid crystal phase to continue until near room temperature, which makes it easy to fabricate thin films using the liquid crystal properties, and once crystallized, the crystal thin films are stable up to 142°C. Ph-BTBT-10 has high solubility in common organic solvents such as toluene, and has good thin film fabrication processability. Furthermore, polycrystalline thin films prepared by conventional solution processes show high mobility (5 cm²/Vs) and are attracting attention as a new organic semiconductor material.
H. Iino et al. Nature Communications (2015).

In disk-like liquid crystals, we are focusing on liquid crystalline phthalocyanine derivatives. We have found that molecules having alkyl chains on inner position of phthalocyanine structure as shown below have high solubility in common organic solvents, and that the aggregated structure of columns is easy to control the vertical orientation on substrate and exhibits high mobility (0.3 cm²/Vs) in the liquid crystal phase. Since the phthalocyanine chemical structure has strong absorption in the near-infrared (600-800nm) region, we are considering using it in organic photodiodes for image sensors in the near-infrared.
H. Iino et al. Appl. Phys. Lett. (2005).

Currently, we are also developing liquid crystalline organic semiconductor materials that are easy to inject electrons from electrodes by selecting rod-like molecular stucture having deep LUMO levels.
T. -Z. Yang et al. Chem. Lett. (2016).
M.-C. Yang et al. J. Mater. Chem. C (2019).

Research