Julian Hauss

Large-scale patterning of indium tin oxide electrodes for guided mode extraction from organic light-emitting diodes

We describe a cost-efficient and large area scalable production process of organic light-emitting diodes (OLEDs) with photonic crystals (PCs) as extraction elements for guided modes. Using laser interference lithography and physical plasma etching, we texture the indium tin oxide (ITO) electrode layer of an OLED with one- and two-dimensional PC gratings. By optical transmission measurements, the resonant mode of the grating is shown to have a drift of only 0.4% over the 5mm length of the ITO grating. By changing the lattice constant between 300 and 600nm, the OLED emission angle of enhanced light outcoupling is tailored from −24.25° to 37°. At these angles, the TE emission is enhanced up to a factor of 2.14.

Tailored Highly Transparent Composite Hole‐Injection Layer Consisting of Pedot:PSS and SiO2 Nanoparticles for Efficient Polymer Light‐Emitting Diodes

In recent years organic semiconducting materials made their way from basic research into commercial optoelectronic devices such as organic solar cells (OSCs), organic fi eld effect transistors (OFETs), and organic light emitting diodes (OLEDs). [ 1 ] Conjugated polymers are a favored class of material, since they combine the properties of conventional polymers, such as low weight, mechanical fl exibility, and processability, with semiconducting properties, such as a tunable bandgap and an adjustable conductivity. A commonly used material for hole injection and transport in organic semiconductor devices is poly(3,4-ethylenedioxythiop hene):poly(styrene sulfonate) (PEDOT:PSS). This blend consists of the oxidatively doped, cationic, conducting polythiophene derivative (PEDOT) and the water-soluble poly(styrene sulfonic acid) (PSS) polyanion. [ 2 ] It offers excellent fi lm forming properties, high conductivity, [ 3 ] and low absorption. [ 4 ] Since the properties of the PEDOT:PSS fi lms strongly depend on the processing conditions, the infl uence of different solvents, [ 5 ] water content, [ 6 ]

Enhancing outcoupling efficiency of indium-tin-oxide-free organic light-emitting diodes via nanostructured high index layers

We fabricated organic light-emitting diodes with one-dimensional Bragg gratings as light extraction elements for substrate and waveguide modes. A Ta2O5 layer was introduced to obtain a high refractive index contrast to the subsequent anode layer. As anode we employed a highly conductive polymer. Laser interference lithography and physical plasma etching were used to pattern gratings into the Ta2O5 layer with a lattice constant of 370 nm and various grating depths. Mainly attributed to the outcoupling of the substrate modes, the structured devices exhibit a luminous flux which is up to four times higher compared to the unstructured reference devices.

Improving the outcoupling efficiency of indium-tin-oxide-free organic light-emitting diodes via rough internal interfaces

We present low-cost texturing methods to produce different surface roughnesses on glass substrates. Using sand blasting, abrasion and wet etching we achieve roughnesses of about 50 nm to 250 nm (root mean squared roughness Rq). These textured substrates are used as extraction elements for guided modes and substrate modes in organic light-emitting diodes (OLEDs). We evaporate 50 nm of the high index material Ta₂O₅ on the textured substrate, which acts as waveguide layer, and flatten it with the transparent photoresist SU-8. On top of that, we fabricate indium tin oxide (ITO)-free OLEDs, which are characterized by electroluminescence and photoluminescence measurements. The devices with rough interfaces obtain an up to 37.4% and 15.5% (at 20 mA/cm²) enhanced emission and it is shown that the enhancement is due to an increased outcoupling efficiency.

On the interplay of waveguide modes and leaky modes in corrugated OLEDs

Bragg gratings incorporated into organic light-emitting diodes (OLEDs) establish a coupling between waveguide modes and useful light (leaky modes). Here we demonstrate that the net coupling direction depends on the OLED stack design. We fabricated two different device structures with gold Bragg gratings. Angle resolved electroluminescence spectra were recorded. For the first device peaks of enhanced emission due to the Bragg grating are observed corresponding to a net energy transfer in direction of the leaky modes. The second device, on the other hand, exhibits dips in the emission spectrum. This reversed direction of energy transfer from the leaky modes to the waveguide modes is explained considering transfer matrix simulations of modal intensity distributions and device emission simulations. An OLED efficiency enhancement is only achieved, if the waveguide mode extraction is dominant.

Metallic Bragg-gratings for light management in organic light-emitting devices

We used laser interference lithography to fabricate organic light-emitting diodes (OLEDs) with gold Bragg gratings on top of the indium-tin-oxide layer for efficient light management. We built polymer OLEDs with 15 and 30 nm gold grating thickness as well as reference devices with continuous gold layers and without gold layer. Electrical and optical device characterization was performed and compared to optical simulations. The gratings extract waveguide and substrate modes and change the angular and spectral emission characteristics by cavity effects. The combination of light extraction and cavity effects leads to 25–30% enhancement in power efficiency compared to reference devices.

Nanograting transfer for light extraction in organic light-emitting devices

Damage-free transfer of nanogratings as waveguide mode extraction elements onto the top of top-emission organic light-emitting devices(OLEDs) was realized with the assistance of cyclic olefin copolymer molds. Photoluminescence and electroluminescence measurements together with transfer matrix calculations were used to identify the extracted waveguide modes. The presented one-step, wet-free, and etchless nanotransfer approach can avoid damage on the top-emission OLEDstructure and does not adversely affect the OLED performance. It offers the possibility for the independent optimization of OLEDfabrication and nanofabrication, and has potential applications in organic optoelectronic devices, especially top-emission OLEDs.

Periodic nanostructuring for guided mode extraction in organic light-emitting diodes

We investigated guided mode extraction in organic light-emitting diodes and compare the experimental findings to transfer matrix (T-matrix) and finite difference time domain (FDTD) simulations. To this end, we patterned the indium tin oxide anode with Bragg gratings with lattice constants from 300 to 600 nm and varied the depth of the grating structures. The structuring was done by laser interference lithography and plasma etching. Both techniques allow for a rapid large area processing. We measured angle resolved electroluminescent spectra of the nanostructured devices and reference devices. To obtain the mode distribution in the devices we made use of T-matrix simulations. In addition we performed FDTD simulations of the emission characteristics of the patterned devices. The simulations are in agreement with our experimental findings and give insight into the outcoupling mechanisms.

Polymer light-emitting diodes with inorganic nanocomposite interlayers for efficiency enhancement

We report on polymer light-emitting diodes (PLEDs) with embedded SiO2 nanoparticle interlayers. We fabricated PLEDs with a rather thick (180 nm) SiO2 interlayer between the hole injection layer and a 50-nm thin emission layer. We also made devices, where the interlayer was embedded inside the emission layer. The devices were characterized by electroluminescence and photoluminescence measurements and compared to the respective reference devices. We achieved an enhancement factor of 1.74 for the luminous efficacy and 2.13 for the current efficiency at 4000 cd/m2 for the PLED with the interlayer between the hole injection layer and the emission layer compared to the reference device. For PLEDs with the interlayer between the two emission layers, we obtained an enhancement factor of 1.68 for the luminous efficacy and 1.45 for the current efficiency at 4000 cd/m2 compared to the corresponding reference device. This surprising enhancement can be explained by a superposition of increased internal quantum efficiency and an increase in the extraction efficiency.

Far field calculations and experimental characterization of nanostructured OLEDs

We present far field calculations for nanostructured organic light emitting diodes (OLEDs) and compare these to experimental results. OLEDs with a nanostructured indium tin oxide (ITO) anode were fabricated and characterized. Corresponding two-dimensional far field calculations are carried out via two-dimensional finite difference time-domain code (2-D FDTD) with integrated 2-D far-field projection. Furthermore, spherical three-dimensional (3-D) far field calculations are obtained by an efficient multipole-based near-to-far field transformation from three-dimensional FDTD data.