Absorption Spectra and Conductivity of Vacuum-Deposited Thin Films of Dyes

Vacume-deposited thin films of various dyes were prepared to evaluate their solid state absorption spectra and conductivity. Typical spectral changes in solution and in solid state were classified into 5 types which depended on the dye chromophores. The X-ray powder diffraction method revealed that the thin films generally have as good crystallinity as crystals. The solid state absorption spectra of dye films were affected by aggregation, crystallization, tautomerism of dyes, and by an electron-transfer in cases of charge-transfer (CT) complexes. The absorption spectra and conductivity of tetraaminoanthraquinone (TAAQ) -7, 7, 8, 8-tetracyanoquinodimethane (TCNQ) CT complexes were examined. The absorption peak attributed to the radical species of TAAQ, which was observed in solution, was not observed in solid state. The resistivity was measured on various devices by two probe or four probe methods. The resistivity of the film along the parallel direction showed semiconductive behavior which depended on temperature. The hetero laminating device, on which were deposited alternately a donor and an acceptor, and which has a structure analogous to that of crystals of CT complex, also showed conductivity but transformed into an insulator below 50 K. The resistivity of the film was affected by thickness of film, compositions, laminating structures, and device structures.