Photoconductivity in Rigid Organic Solutions. I. Phenomenology of the Photocurrents

Abstract

Dilute solutions of several benzene derivatives in 3-methylpentane solvent display three types of photocurrent when examined in their rigid, amorphous state at 77°K. There is the primary steady-state photocurrent seen only when illuminating in the absorption region of the solute molecule. Following ionization in this ``primary'' spectral region (and never before) two new induced signals are seen. One is characterized by a photocurrent spike which is excited in the near infrared or higher energies. It is a transient signal which, once destroyed at any wavelength, can be seen again only after further primary irradiation. The second induced signal appears to have a threshold in the near-ultraviolet region but at energies definitely below the onset of solute absorption. This signal is also basically transient. However, its decay is so slow compared to the decay of the spike signal, that it appears rather as a pseudo steady-state photocurrent. It is found that oxygen exerts its influence primarily on the spike signal with a strong quenching effect. A volt—ampere study is reported. At the higher fields Poole's law is seen to hold. Ohm's law holds at lower fields. The parameter characterizing Poole's law is shown to be independent of solute and wavelength of primary excitation. It rather reflects any given preparation of the rigid solid. The Poole's-law behavior can be removed by bathing the sample with near-infrared light during the volt—ampere studies. The Poole's-law behavior is also destroyed when the viscosity of the solid, at 77°K, is reduced by use of a two-component solvent. A model is discussed which succeeds in rationalizing these observations. Basic to the model is that (1) the charge carrier is the electron, (2) the solid matrix contains sites (neutral) which trap electrons so firmly that their thermal mobilization is negligible, and (3) the photoionized system contains, homogeneously dispersed, the long-range attractive Coulomb wells of the immobile cations—products of the photoionization, and (4) there is a very short-lived mobile state of the electron. To explain the volt—ampere behavior and viscosity effects, a substructure is assigned to the mobile state of the electrons. It is proposed that this state consists of electrons trapped somewhat deeper than kT which are thermally activated into a condition of ``maximum mobility'' — the closest condition to a conduction band in this amorphous solid. Sufficiently intense fields aid in the activation of these electrons to the state of maximum mobility, while reduced viscosity permits thermal saturation of this state by diminishing the shallow trap depths.