3 - Optical absorption of Alq3 thin films

Abstract

Absorption spectra are reported for 12 Alq3 thin films soon after evaporation and during annealing procedure in different gases, O2, N2, and air, and conditions, wet and dry. After that, samples were kept decaying in the laboratory environment, and absorption was measured again only 4years later, when its values were barely bigger than the minimum measurable amount of about 0.016 optical density, a figure dictated mostly by the S/N ratio of the spectrophotometer. The absorption at the excitation wavelength λp=395nm was at the beginning about 0.25 optical density, so that the dynamic range of the measurements was slightly more than an order of magnitude. No matter how limited these measurements as a function of time are, they still witness the effects of the annealing in various gases and temperatures. In particular, the most influent factor during annealing seems to be temperature, followed by oxygen, water, and nitrogen, the latter one practically not affecting the films at all. However, water terminates the absorption of films at lower annealing temperatures with respect to dry conditions, which indicates the onset of strong water chemical reactions. Afterward, the absorption of both annealed samples and the nonannealed one decays in the atmosphere, and 4years later has decreased by one-two order of magnitude, which was expected but not very useful to describe a well-defined decay trend. However, the remaining tiny absorption is consistent with the ongoing molecular degradation, and the films are still optically active, as shown elsewhere by the emission measurements that continued for another 2years.