ABSTRACT

Photoconductivity was first observed by Smith (1873a, 1873b) and Adams and Day (1876) in Se layers. The first systematic studies of the phenomenon were by Gudden, Pohl, and associates in the 1920s. This work was largely concerned with ZnS, diamond, and alkali halide single crystals. Results of early studies were usually explained by radiation-induced structural effects. With the advent of Hall effect measurements (Hall, 1879; Baedeker, 1909; Königsberger, 1914; Lenz, 1924), however, it became clear that photoconductivity was due to the creation of free electrons, which were normally bound to their constituent atoms, by the absorption of radiation. The electrons could then be displaced by a field for a period of time before they either recombine or become immobilized in states described as traps. For reviews of early work, see Nix (1932), Hughes (1936), Pohl (1937), Mott and Gurney (1940), and Smith (1959). The first observations of photoconductivity in organic solids were by Pochettino (1906) and Volmer (1913) in anthracene. With few exceptions (Zchodro, 1919, 1929; Petrikaln, 1930), there

were no further studies of organic materials until the 1940s (Terenin and Varta­ nyan, 1941; Szent-Gyôrgyi, 1946; Vartanyan, 1947, 1948, 1950; Nelson, 1951).