Lieven Penninck

Integrated optical model for organic light-emitting devices

One of the most important parameters of organic light-emitting devices (OLEDs) in their application for illumination or displays is their efficiency. In order to maximize the efficiency, one needs to understand all loss mechanisms and effects present in these devices and properly model them. For that purpose, we introduce an integrated model for light emission from OLEDs. The model takes into account the exciton decay time change and light outcoupling. Furthermore, it shows how to calculate the external quantum efficiency, the spectral radiance and the luminous current efficacy of OLEDs. The overall theory is experimentally verified through a range of measurements done on a set of green OLED samples with an Ir-based phosphorescent emitter. From the analysis of simulations and experiments one can estimate the charge balance in the OLED stack and the radiative efficiency of the emitter.

Fast and versatile deposition of aligned semiconductor nanorods by dip-coating on a substrate with interdigitated electrodes

Semiconductor nanorods mainly absorb and emit light with the electric field along the axis of the rods, it is therefore important to align the nanorods along a preferred direction. The homogeneous deposition of aligned nanorods on large substrates is interesting for large size applications such as solar cells and OLEDs. In this work, we present a fast and versatile method for the homogeneous deposition and alignment of nanorods from a colloidal suspension. The method is based on a low-cost dip-coating procedure during which an alternating electric field is applied. The accumulation, orientation, and polarized fluorescence of the nanorods is verified by AFM and polarized fluorescence microscopy. An alignment with order parameter of 0.67 has been obtained with this method.

Dipole radiation within one-dimensional anisotropic microcavities: a simulation method

We present a simulation method for light emitted in uniaxially anisotropic light-emitting thin film devices. The simulation is based on the radiation of dipole antennas inside a one-dimensional microcavity. Any layer in the microcaviy can be uniaxially anisotropic with an arbitrary orientation of the optical axis. A plane wave expansion for the field of an elementary dipole inside an anisotropic medium is derived from Maxwells equations. We employ the scattering matrix method to calculate the emission by dipoles inside an anisotropic microcavity. The simulation method is applied to calculate the emission of dipole antennas in a number of cases: a dipole antenna in an infinite medium, emission into anisotropic slab waveguides and waveguides in liquid crystals. The dependency of the intensity and the polarization on the direction of emission is illustrated for a number of anisotropic microcavities.

RCWA and FDTD modeling of light emission from internally structured OLEDs

We report on the fabrication and simulation of a green OLED with an Internal Light Extraction (ILE) layer. The optical behavior of these devices is simulated using both Rigorous Coupled Wave Analysis (RCWA) and Finite Difference Time-Domain (FDTD) methods. Results obtained using these two different techniques show excellent agreement and predict the experimental results with good precision. By verifying the validity of both simulation methods on the internal light extraction structure we pave the way to optimization of ILE layers using either of these methods.

The effects of planar metallic interfaces on the radiation of nearby electrical dipoles

Light emission by excited species that decay via an electrical dipole transition is modeled as an electrical dipole antenna. We examine the various effects that occur when such a dipole is close to a metallic interface: wide-angle interference, coupling to the surface plasmon mode and absorption of the near-field. Analytical expressions for the power coupled to the surface plasmon and near-field absorption are derived. The case of a non-absorbing metal is compared to that of an absorbing metal.

Numerical simulation of stimulated emission and lasing in dye doped cholesteric liquid crystal films

Dye-doped chiral-nematic liquid crystal lasers have great potential as small size, low-cost, widely tunable lasers. We present a numerical model for stimulated emission and lasing in liquid crystal films based on thin film optics. The gain threshold is modelled and the results are confirmed experimentally. The effect of the orientation of dye molecules and the matching of the photonic bandgap to the dye spectrum on the threshold for lasing is discussed.

Polarized light emission by deposition of aligned semiconductor nanorods

The ability to control the position and orientation of nanorods in a device is interesting both from a scientific and a technological point of view. Because semiconductor nanorods exhibit anisotropic absorption, and spontaneous and stimulated emission, aligning individual NRs to a preferred axis is attractive for many applications in photonics such as solar cells, light-emitting devices, optical sensors, switches, etc. Electric-field-driven deposition from colloidal suspensions has proven to be an efficient method for the controlled positioning and alignment of anisotropic particles. In this work, we present a novel technique for the homogeneous deposition and alignment of CdSe/CdS NRs on a glass substrate patterned with transparent indium tin oxide interdigitated electrodes, with a spacing of a few micrometers. This method is based on applying a strong AC electric field over the electrodes during a dip-coating procedure and subsequent evaporation of the solvent. The reproducible and homogeneous deposition on large substrates is required for large size applications such as solar cells or OLEDs. The accumulation, alignment, and polarized fluorescence of the nanorods as a function of the electrical field during deposition are investigated. A preferential alignment with an order parameter of 0.92 has been achieved.

Detailed analysis of exciton decay time change in organic light-emitting devices caused by optical effects

— The exciton decay time in organic light-emitting devices (OLEDs) depends on the optical environment, i.e., the thicknesses and refractive indices of all layers in a device. The decay of an exciton can occur through a radiative or a non-radiative channel. Each of these channels has a probability, which is expressed by, respectively, the radiative and the non-radiative decay rate. The radiative decay rate is influenced by the optical environment, i.e., the OLEDs thin-film layer structure. In this paper, a model for estimating the change of the exciton decay time (inverse of the decay rate) is presented. In addition, the decay time change in both top- and bottom-emitting OLEDs as a function of the charge-transport layer thicknesses has been investigated. Furthermore, the most important mechanism responsible for the exciton decay time change is outlined.

Optical design for efficient light emission in OLEDs and anisotropic layers

The light emission from an electrical dipole antenna is determined by the orientation of the antenna and the optical properties of the materials that surround it. For the outcoupling efficiency of an OLED it is beneficial if the dipole moment of the luminescent transition is parallel with the substrate. In some OLEDs a preferential orientation of the emitting molecules can indeed be observed. In this paper we discuss how anisotropy of the dipole orientation and optical anisotropy of the materials influence the light emitted from a planar structure. Numerical simulations are verified with measurements of OLEDs (for the dipole orientation) and dye doped liquid crystals (for the optical anisotropy).

Simulating the emission properties of luminescent dyes within one-dimensional uniaxial liquid crystal microcavities

We have developed a simulation method which is capable of simulating the emissive properties of luminescent dyes inside uniaxial anistropic thin film microcavities[1]. The method uses plane wave decomposition of the electric field of a dipole antenna in a uniaxial medium and a scattering matrix formalism to account for interference and reflection from the various interfaces in the device. We apply this method to simulate the excitation of waveguide modes in a slab waveguide formed by reorientation of a liquid crystal. We investigate the emission both outside the device and into waveguided modes inside the liquid crystal device.