Enhancing photoconductivity of aromatic polyimide films by incorporating fluorinated dianhydrides and main chain triphenylamine structure

Abstract

The photoconductive properties of seven types of fully aromatic polyimide (PI) films with a main chain triphenylamine structure were investigated by examining the wavelength dependence of photo-irradiation in the visible region under a direct current (DC) electric field. It was clarified that the out-of-plane photoconductivity of the PIs synthesized from seven types of dianhydrides and 4,4′-diamino-4″-methyltriphenylamine (DATPA) tends to increase as the intramolecular charge transfer (CT) interactions increase, which is essentially dominated by the electron accepting properties of the dianhydrides. In particular, the PIs derived from the pyromellitic dianhydrides substituted by two fluoro (−F) or one trifluoromethyl (−CF3) substitutes with strong electron accepting properties (P2FDA/DATPA and P3FDA/DATPA) exhibited significantly enhanced photoconductivity with relatively strong optical absorbance in the visible region. Moreover, the P6FDA/DATPA PI prepared from pyromellitic dianhydrides and substituted by two bulky CF3 substitutes exhibited intense optical absorbance but lower photoconductivity, owing to the enlarged interchain distance, as evaluated by the out-of-plane GI-WAXS pattern. This suggests that the charged carriers are preferentially transported along the interchain directions, and the photoconductivity of the PI films is significantly influenced, not only by the degree of intramolecular CT interactions, but also by the polymer aggregation structures facilitating the interchain charge transportation.