28.2: The Molecular Orientation of Polyimide Alignment Layer used in TFT‐LCDs with NEXAFS Study

Abstract

Aromatic polyimide thin films are used as the alignment film in thin film transistor‐liquid crystal displays (TFT‐LCD's). The films play an important role in the initial alignment of the liquid crystal materials in LCD panels. Therefore, any structural or morphological deformation may lead to defect in the physical interactions thus malfunction of the display eventually. for example, an LCD is known to bear image sticking problem when it is assembled following 3 days or longer stagnancy in the process after main curing of the layer. In this paper, the molecular orientation of polyimide surfaces with respect to the stagnancy is investigated by means of surface sensitive Near Edge X‐ray Absorption Fine Structure (NEXAFS) spectroscopy. NEXAFS measurements clearly reveal preferred in‐plane and out‐of‐plane orientation of phenyl and C=O groups at the surface of the polyimide thin film in regard to the stagnancy after main curing. with longer stagnancy phenyl and C=O groups preferred in‐plane orientation. The preferred in‐plane orientation within the film can accelerate the image sticking problem in LCD's operating in PVA mode. Based on the result, vertical alignment of polyimide pendant group is found helpful for the better alignment of LC molecules in LCD's operating in the patterned vertical alignment (PVA) mode.