Absorption Of Frenkel Excitons In Ptcda Thin Films, In Ptcda/Alq3 Multilayers And In Co-Deposited Films

We investigate the optical densities of polycrystalline PTCDA thin films, PTCDA/Alq(3) multilayers and of co-deposited films in the temperature range from 10 to 300 K. The measurements provide information about microscopic interactions between molecules and the effect of strain within the organic films. Compared to pure PTCDA the absorption spectra of multilayers and co-deposited films are shifted to higher energy. With decreasing temperature the spectra shift to lower energy, mostly pronounced in pure PTCDA films. The observed differences in the OD spectra are qualitatively explained by comparison with a recent Frenkel exciton model.