Spatial behaviour of the charge created at the illuminated interface molecular crystal/electrolyte

Abstract

Some theoretical and experimental aspects of space charge formation in real organic molecular crystals near dark and illuminated electrolytic contacts are studied. The results indicate that the spatial distributions of the trapped as well as of the free charge carriers differ, in general, from that given by Mott and Gurney for an ideal insulator at a dark metallic contact. The difference is attributed to the influence of charge - trapping states, to the action of image forces and to optical detrapping of the charge. The quenching of a spatially inhomogencous distribution of excitons by trapped carriers can be utilized for determining the spatial charge distribution in anthracene and tetracene single crystals. The measurements, in agreement with the theoretical predictions, showed that in the distance range from ≈0.1 to ≈0.6 μm the trapped charge concentration decreases according to the Mott—Gurney distribution, which is here a good approximation of a more complicated function; this does not hold true for distances x ≳ 0.6 μm and x ≲ 0.1 μm. The fitting procedure employed in the comparison of the theoretical distribution functions with the experimental results allowed the determination of the exciton-trapped hole interaction rate constant. For tetracene triplets a value K Tt = (2 ± 1.5) × 10 −9 cm 3 S −1 is obtained and briefly discussed in terms of the exciton diffusivity.