Prospects for electrically pumped organic lasers

We examine the effects of nonradiative losses on lasing in crystalline and amorphous organic thin films. In crystalline films, the dominant loss mechanism is singlet-singlet annihilation, and this must be avoided if lasing is to be achieved at practical current densities. The electrically pumped crystalline-tetracene laser structure of Schön et al. [Science 289, 599 (2000)] is studied in detail. Optical and electrical confinement in the bulk structure appears unable to explain the spectral narrowing reported; consequently, we consider electron-hole plasmas, self-focusing at interfaces, and crystal defects as possible sources of the reported phenomena. In amorphous films, lasers are likely to have to operate at current densities J < 1000 A/cm² due to a combination of nonradiative losses. The performance of potential lasing materials is quantified by the external quantum efficiency-current-density product, η_EXTJ. Electrically pumped lasers require η_EXTJ∼5 A/cm²; the best amorphous devices currently posses η_EXTJ∼0.3 A/cm². However, we demonstrate that electrically pumped lasing in amorphous materials should be possible using indirect pumping techniques.