Coupling of Matrix-Trapped Electrons with an External Electric Field: Electrophotoluminescence in a Rigid Organic Solution at 77°K

Abstract

Electrophotoluminescence (EPL) signals obtained when a photoionized rigid organic solution at 77°K is subjected to an intense electric field are studied from a quantitative point of view. This luminescence appears as a strong enhancement of the isothermal luminescence (ITL) observed under normal conditions. Both EPL and ITL are apparently due to the slow recombination of matrix trapped electrons with those parent positive centers (solute cations) to which they are correlated. Previously, a one-parameter equation has been found which quantitatively relates any EPL decay at fields up to 1×106 V/cm to the ITL decay seen in the same sample. The theoretical approach is in terms of ordinary diffusion theory modified to take explicit account of electron trapping in the solid and how that trapping is altered when an applied perturbation, such as a temperature jump, or an electric field is coupled to the trapped electron. The single parameter is expressed as an exponential in the perturbation (temperature jump or applied field). However, only if the central field of the positive center (solute cation) is considered to assert the dominant role upon the activation of the trapped electrons can the successful EPL–ITL equation be even qualitatively justified. The EPL–ITL equation is successfully extended to a quantitative description of any signal seen after a first field-on period corresponding to either a new ITL, or a new ITL followed by a second EPL obtained when the field is reapplied in the same or in the opposite direction. In addition, it is shown that, after a sufficiently long application of the unidirectional electric field, a situation is reached where removal of the field gives an increase of the light level and subsequent reapplication of the field leads to a field quenching of the recombination luminescence (EPL<ITL). In view of the extended successful testing of the EPL–ITL equation, it is found that a rigorous interpretation of the particular model being employed leads to certain unique statements regarding the distribution and diffusion of electrons in the r, θ, and φ space about the cation. These statements apply only to the 3-methylpentane solid exclusively employed in this work. First it must be concluded that the electrons be radially distributed in the range ∼15 Å < r < 40 Å. Secondly, the electrons must be anisotropically (or sharply) distributed in θ space, and furthermore over periods as long as 15 min, and even in the presence of high fields, they exhibit no diffusion in θ space but only inward diffusion in the radial direction controlled by the central field of the cation. The theory also predicts that photoconductivity in these systems should exhibit Poole's law behavior (deviation from Ohm's law exponential in the field) but that the electron-field coupling parameter must be smaller than that found from the EPL studies. A simultaneous study of Poole's law behavior and the EPL–ITL relationship in one sample fully confirms this. In general it is seen that from sample to sample, in 3-methylpentane at 77°K, the electron-field coupling parameter varies from 0.4–0.8 Å in photoconductivity, while in EPL it varies from 0.9–1.9 Å being highly reproducible for any one preparation.