Discotic Liquid Crystals with Highly Ordered Columnar Hexagonal Structure for Ultraviolet Light-Sensitive Phototransistor Memory

Organic phototransistors have been vigorously investigated for their superior charge-transport performance, photosensitivity, and compatibility with integrated circuits. Accordingly, liquid crystal molecules have been proven to be a high-performance organic electret, and discotic liquid crystal with a face-on orientation is able to show ultrafast photoresponse. Thus far, there is still no application of discotic liquid crystal as a charge-trapping electret in phototransistors. In this study, a series of discotic liquid crystalline molecules consisting of alkyl-triphenylene (Cx-TP, where x = 1, 6, 8, and 10) were designed and applied as an electret in the ultraviolet light-sensitive photomemory. The discotic liquid crystal with optimized side-chain length produced a homeotropically aligned nanoarray structure and endowed the best performance and photoresponse in phototransistor memory. Therefore, C6-TP- and C8-TP-based devices produced high memory ratios of approximately 10³ and 10⁴, respectively, which outperformed their analogues of C1-TP and C10-TP with a memory ratio of approximately 10². This achievement indicates the columnar hexagonal structure induced by the homeotropic alignment during liquid crystalline transition produces a huge impact on the photoresponse and device performance. The result of this study underlines the potential of discotic liquid crystals as a photoactive electret in phototransistor applications.