Anisotropic photoconductivity of aromatic and semi-aliphatic polyimide films: Effects of charge transfer, molecular orientation, and polymer chain packing

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The anisotropic photoconductive properties of polyimide (PI) films prepared using seven types of dianhydrides and a diamine containing a diphenyl-benzidine structure featuring rigid linear structure and strong electron-donating ability were analyzed. Photoirradiation wavelength dependences of the in-plane and out-of-plane photocurrents, which are known as anisotropic photocurrent spectra, were investigated in comparison with optical absorption spectra, the degrees of molecular aggregation, in-plane/out-of-plane orientation of PI chains, and ordered structures formed in film state. We have previously reported that intermolecular charge transfer (CT) interaction is a key factor for increasing the out-of-plane photoconductivity of PI films because they determine the efficiency of charge separation. However, when sufficient charge carriers are generated in thin PI films via photo irradiation, in which penetration depth of irradiated light is sufficiently larger than the film thickness, suppression of charge recombination by reducing molecular chain packing is rather effective at enhancing both the in-plane and out-of-plane photoconductivity. This is because the mobility of carriers is significantly increased in the dense aggregation structures, and thus collisions between carriers becomes more frequent, which led to higher recombination efficiency. Furthermore, the photocurrent anisotropy was increased as the degree of in-plane orientation of the main PI chains increased, which agrees with the fact that charged carriers are preferentially transported between PI chains. Accordingly, promoting charge transport efficiency by reducing PI chain packing and increasing the out-of-plane orientation of PI chains are essential to increase the in-plane photoconductivity of PI films.