Gain and polaron absorption in electrically pumped single-layer organic laser diodes

In this article, a model to calculate the modal gain in organic laser diode structures is presented. A single layer design is considered, to investigate the dependence of the gain on electron mobility and the thickness of the active layer. We show that unequal charge carrier mobilities are detrimental and that there is an optimum active layer thickness of d≈200 nm, when different devices are compared on the basis of power density. The calculated gain is 0.7/cm for a power density of P=50 kW/cm², neglecting all losses. Furthermore, the influence of absorption by polarons is quantified. We show that the cross section for this process has to be less than 4% of the cross section for stimulated emission in order to achieve net gain.