Parametric study of modal gain and threshold power density in electrically pumped single-layer organic optical amplifier and laser diode structures

In this paper, a model to calculate the modal gain in organic optical amplifiers and the laser threshold power density in organic laser diode structures is presented. We consider a single-layer design to investigate the dependence of the modal gain and threshold power density on electron and hole mobility, injection barriers, the thickness of the active layer, as well as exciton dissociation at the injecting contacts. A figure of merit is introduced to quantify the influence of absorption by polarons in optical amplifiers. We show that equal charge carrier mobilities are of crucial importance to achieve appreciable gain on the order of 1/cm at a power density of P= 50 kW/cm² for the considered poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV)-like model material. Increasing the injection barriers to φ_b≈ 0.3 eV decreases the gain marginally but is beneficial in terms of polaron absorption. Regarding modal gain, there is an optimum thickness for the active layer of d≈ 200 nm, if different devices are compared on the basis of equal power density. We derive power laws for the dependence of modal gain on mobility and power density, which can serve as guidelines for future device design considerations. We determine the maximum allowed polaron absorption cross section σ_abs relative to the cross section σ_stim for stimulated emission that may not be exceeded to achieve positive net gain necessary for optical amplification. For the most favorable parameters, σ_abs has to be at least 20 times smaller than σ_stim. The dependence of the laser threshold power density on all of the above-mentioned parameters is investigated. We show that, in the optimum case considered, the power density necessary for lasing is 40 times higher than the highest value reported in the literature.