A highly photoconductive polyimide film.

We developed a new type of polymer having a high photoconductivity in the visible light region. The polymer is composed of a series of thiophenylene moieties, -C6H4S-, and an imide group. The polymer with this type of chemical structure is called polyimide [PI].The casted films of polyamic acid precursors were imidized to PI films and crystallized by annealing at suitable temperatures near the melting points. The thickness of crystallites in the PI film is estimated to be about 100ÅÅ, which corresponds to the 20 polymer chains stacked perpendicular to the axis. In the crystallizing process, both the absorption and the photoconductivity of this film in the visible light region increase extremely. From the UV-visible absorption spectra, we evaluated the optical energy gap, Eg. In the most sensitive polymer, BPDA-Ph3, Eg changed from about 2.5eV in the amorphous phase to about 1.85eV in the crystalline phase. Aromatic rings of thiophenylene moieties and imide rings of the PI films are so stacked that the π-orbitals of both rings are more overlapped, which increases absorption in the visible region. Our highly crystallized PI films with extensively overlapped π-orbitals between the adjacent chains show high photoconductivities. In the crystallizing process the photoconductive property, ημτ, increases in magnitude three orders or more. The most sensitive polymers, BPDA-Ph3 and -Ph5, have ημτ values more than 10-8cm2/v in the visible region 350-550nm.