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We investigate the impact of induced absorption caused by injected charge carriers and excited states on the threshold current density of an organic laser diode using numerical simulations. The electrical properties of the device are described by a self consistent drift-diffusion model. The optical properties are calculated using a transfer matrix method. Nonradiative annihilation processes are included employing typical rate constants. In our approach, a three-layer double-heterostructure (DH) with typical organic material properties is studied, which exhibits a threshold current density of 564 A/cm2. For this virtual device, upper limits for the charge carrier and triplet-triplet absorption cross section sigmacarrier=1.53times10-3middotsigmaSE and sigmaT 1 TN=4.34times10-3middotsigmaSE have been calculated as a function of the stimulated emission cross section sigmaSE. Additionally, the role of device geometry and material properties concerning induced absorptions is studied. It is shown that the impact of absorption processes is not strongly influenced by the device geometry. By increasing the charge carrier mobilities to mu = 2 cm2/Vmiddots in the transport layers and mu = 0.2 cm2/Vmiddots in the emission layer, the impact of polaron absorption can be greatly reduced. In this case, laser operation might still be possible if sigmacarrier and sigmaSE are within the same order of magnitude. Decreasing the triplet lifetime tau T 1 is a promising way to reduce the impact of triplet-triplet absorption. For sigmaT 1 TN and sigmaSE being within the same order of magnitude, the triplet lifetime tau T 1 has to be reduced to 1 ns for laser operation.
Date of Publication: Nov. 2007