Influence of electronic properties on the threshold behaviour of organic laser diode structures

By employing a combined optical/electronic model, we investigate the effect of electronic properties on the performance of three layer organic semiconductor structures, which are a potential candidate for future electrically pumped organic laser diodes. The drift-diffusion equations which describe particle transport are coupled to the spatially inhomogeneous laser rate equations to solve for the dynamics of the excited state and photon population in the laser cavity. Due to the high current densities considered, high particle densities occur, which implies that annihilation processes between the different particle species have to be considered. On the optical side, we take into account the absorption of the metal electrodes required for current injection to obtain the intensity profiles of the guided modes. Our calculations show that the inclusion of annihilation processes leads to a strong dependence of the laser threshold on the charge carrier mobilities, in contrast to the situation when exciton annihilation is neglected. We observe optimum values for the charge carrier mobilities in the emission layer regarding the threshold current and power density. On the other hand, an increase of the mobilities in the transport layers leads to a reduction of these quantities. The threshold voltage decreases for increasing mobilities, regardless of the layer in which the mobility is increased. For optimised values, we obtain a threshold current density of jthr = 267 A/cm2 with annihilation processes taken into account. The presented results can serve as guidelines in the search for material combinations and devices structures suitable for electrically pumped organic semiconductor laser diodes.