SPACE-RESOLVED RECOMBINATION ELECTROLUMINESCENCE IN ORGANIC-CRYSTALS

The elucidation of spatial distribution of emitting states in electroluminescent cells is a challenge to both a refined scientific probe of materials and to design of a variety of electroluminescent devices. In molecular organic crystals important aspects of this distribution can be mapped onto a trap-dependent electron-hole recombination process involving exciton-charge carrier interaction. We use this process to describe the space-resolved electroluminescence (EL) in single crystals of anthracene and tetracene as a function of applied electric field. The emphasis of the paper will be on the importance for the EL spatial pattern of the charge injection ability of electrodes and distributions of electron and hole traps. Detailed features of the spatial EL intensity patterns in these crystals will be illustrated as a function of electric field and type of injection contacts. They differ essentially for these two model crystals. In particular, it will be shown that a splitting of the anode EL zone, proceeding regularly with increasing voltage in tetracene, disappears in anthracene crystals. A field-induced shift of the cathode EL zone towards the middle of the EL cell of tetracene is not observed in anthracene. A qualitative interpretation of these results is provided in terms of charge recombination, trapping and release from trap processes. The importance of spatial and energy trap distributions will be pointed out in this context.