Spontaneously aligning emitter molecules can improve the external efficiency of OLED devices and have been reported for different types of emitters doped into suitable hosts. Similar to the morphological alignment of the active molecules, a birefringence of the host affects the apparent emitter orientation. In order to investigate this effect experimentally, Ir(ppy)<sub>3</sub> is co-evaporated into the emitting layer of OLED using hosts with different birefringence properties. Analyzing the electroluminescence emission patterns quantitatively reveals the apparent orientation distribution of the emitters. The results emphasize that host birefringence can be used to suppress perpendicular emitter contributions while amplifying the impact of the horizontal dipoles, thus leading to an enhanced forward emission.
We present single mode optically pumped lasing from a new resonator structure for polymer lasers employing nonperiodic
circular Bragg gratings. Our devices, using a polyfluorene derivative (BN-PFO) as gain medium, are the first
blue-emitting circular grating semiconductor lasers (either organic or inorganic). They exhibit feature sizes as small as
47 nm and emit azimuthally polarized beams with a spectral linewidth ≈ 0.2 nm. We find a minimum lasing threshold
energy density of 1.2 μJ/cm<sup>2</sup> (10 Hz, 8 ns, 355 nm Nd:YAG laser excitation). The quality factor of the resonator modal
fields is found to be at least 2200 for these devices.
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 j<sub>thr</sub> = 267 A/cm<sup>2</sup> 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.
The properties of electrically pumped organic laser devices are investigated by the self consistent numerical solution of the spatially inhomogeneous laser rate equations coupled to a drift-diffusion model for the electrons, holes and singlet excitons. By fully taking into account the effect of stimulated emission on the exciton population, we determine the spatial and temporal evolution of the photon density in organic multilayer structures. We apply the model to calculate laser threshold current densities and investigate transient phenomena like the delay of radiation onset. By performing systematic parameter variations, we derive design rules for potential organic laser diode structures.
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 power
density, charge carrier mobility and thickness of the active layer.
We show that unequal charge carrier mobilities are detrimental and
that there is an optimum active layer thickness of approximately
200 nm, if different devices are compared on the basis of equal
power density. Neglecting all losses, the highest calculated gain
is 0.7/cm for a power density of P=50 kW/cm<sup>2</sup> in our MEH-PPV
like model material. Furthermore, the influence of absorption by
polarons is quantified. We show that the cross section for this
process has to be at least 20 times smaller than the cross section
for stimulated emission in order to achieve net gain in the most
favourable case that was considered.