At high current densities, the characteristics of organic laser diode structures are strongly influenced by a variety
of loss processes such as bimolecular annihilations, field-induced exciton dissociation and induced absorptions
due to polarons and triplet excitons. Here, we investigate a TE2-mode organic double-heterostructure laser diode
by numerical simulation. The electrical properties are described using a numerical drift-difusion model and the
optical characteristics are modeled using a transfer matrix method. When annihilation processes are included,
a threshold current density of 8.5 kA/cm2 is derived for the considered device. Laser operation is not achieved
when field-induced exciton dissociation is considered. For induced absorptions, maximum relative cross sections
of 9.6 × 10-8 for polarons and 1.4 × 10-4 for triplet excitons have been calculated, which would still allow laser
operation. For higher relative absorption cross sections, laser operation is suppressed for all current densities.
Furthermore, the impact of field quenching is analyzed and the separation of singlet excitons from polarons and
triplet excitons in the time domain is studied.
We examine the influence of various annihilation processes on the laser threshold current density of organic semiconductor laser diode structures. A three-layer laser diode structure is systematically investigated by means of numerical simulations. Our self consistent model treats the dynamics of electrons, holes and singlet as well as triplet excitons in the framework of a drift-diffusion model. The resulting particle distributions enter into the optical model. In our approach, we consider the actual waveguide structure and solve the resulting laser rate equation. The various annihilation processes are included as reactions between the different species in the device. We systematically vary the device dimensions and parameters of our singlet exciton annihilation model to identify the dominating quenching process in order to deduce design rules for potential organic laser diode structures. A standard material with typical material properties and annihilation rate coefficients is investigated. Singlet exciton quenching by polarons is identified as the main loss channel. The laser threshold in three layer devices is found to be very sensitive to the thickness of the emission layer.
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.
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