Photoconduction in organic dyes is caused by the formation and migration of electronic charge carriers, and is not due to impurities or photochemical decomposition. The phenomena of dark and light-induced conduction in dyes can therefore be interpreted in the same way as in inorganic semiconductors.
A. lntroductionIn spite of Zcodro's discovery in 1919 [l] that organic dyes undergo slight changes in conductivity on illumination, and the observation of similar phenomena in anthracene several years earlier [2, 31, systematic investigation of this effect did not commence until thirty years later. This delay might well have been due to the predominant application of inorganic semiconductors, brought about by the rapidly developing science of semiconductor physics after 1920 and initiated by the work of Gudden and Poh2 [4-81. The slow response, irreversibility, dependence on the surrounding gas, and small magnitude of the then known photoelectric effects of organic substances [l -3,9,10] suggested that changes in the resistivity on illumination were due to bleaching rather than to internal photoelectric effects analogous to those occurring in inorganic photoconductors. Growing interest in the elucidation of the spectral sensitization of photographic materials and in photochemical aspects of biology led to a systematic investigation of the photoelectric behavior of dyes. Noddack's group demonstrated that acidic and nonionic dyes also exhibit this effect. From the very beginning, the laws established from a study of more than 50 dyes of various types indicated an electronic mechanism in accord with the phenomena observed in inorganic photoconduction effects [14-161. Thermoelectric power measurements and the Hall effect have since established clearly that electronic charge carriers formed in the solid dyes, rather than impurities or decomposition products, are responsible for photoconduction.
B. FundamentalsThe agreement between photoconduction phenomena of organic and inorganic photoconductors leads to an approach to the fundamental semiconductor properties of organic dyes according to the theory of photoconductors in general 17, 17-20].[12] R. C. Nelson, J. chem. Physics 19,798 (1951) ; 20, 1327 (1952
I. Thermal and Optical Excitation EnergyPhotoconduction is based on the formation of negative and positive electronic charge carriers by optical means, whereas dark conduction depends on the thermal excitation of electrons and positive holes (dissociation or auto-excitation).
OD = O D 0 exp (-AEthe,,/2kT) (1)The thermal activation energy, AEtherm, is obtained from the temperature coefficient of the dark conductivity aD and is in substantial agreement with the energy of the light quanta, A E~~~, which must be absorbed to produce photoconduction. AEopt can be determined from the limiting wavelength Ag corresponding to the absorption edge or from the long wavelength The agreement of the excitation energies AEtherm and AEopt of intrinsic semiconductoi s indicates that dark and light-induced conduction are distinguished mer...
