Carsten Rothe

Exceptionally efficient organic light emitting devices using high refractive index substrates.

Organic light emitting devices (OLEDs) are now used in commercial cell phones and flat screen displays, but may become even more successful in lighting applications, in which large area, high efficiency, long lifetime and low cost are essential. Due to the relatively high refractive index of the organic layers, conventional planar bottom emitting OLEDs have a low outcoupling efficiency. Various approaches for enhancing the optical outcoupling efficiency of bottom emitting OLEDs have been introduced in the literature. In this paper we demonstrate a green bottom emitting OLED with a record external quantum efficiency (42%) and luminous efficacy (183 lm/W). This OLED is based on a high index substrate and a thick electron transport layer (ETL) which uses electrical doping. The efficient light outcoupling is modeled by optical simulations.

White stacked OLED with 38 lm/W and 100,000‐hour lifetime at 1000 cd/m2 for display and lighting applications

— The three critical parameters in determining the commercial success of organic light-emitting diodes (OLEDs), both in display and lighting applications, are power efficiency, lifetime, and price competitiveness. PIN technology is widely considered as the preferred way to maximize power efficiency and lifetime. Here, a high-efficiency and long-lifetime white-light-emitting diode, which has been realized by stacking a blue-fluorescent emission unit together with green- and red-phosphorescent emission units, is reported. Proprietary materials have been used in transport layers of each emission unit, which significantly improves the power efficiency and stability. The power efficiency at 1000 cd/m2 is 38 lm/W with CIE color coordinates of (0.43, 0.44) and a color-rendering index (CRI) of 90. An extrapolated lifetime at an initial luminance of 1000 cd/m2 is above 100,000 hours, which fulfils the specifications for most applications. The emission color can also be easily tuned towards the equal-energy white for display applications by selecting emitting materials and varying the transport-layer cavities.

66.2: Efficiency Enhancement in White PIN OLEDs by Simple Internal Outcoupling Methods

Highly efficient white PIN OLED structures based on internal outcoupling approaches using vacuum processable materials have been developed. In combination with external MLA outcoupling films an efficiency enhancement of 1.8 can be achieved resulting in a power efficiency of 50 lm/W at color coordinates x,y = 0.46/0.41 for a tandem OLED using fluorescent blue and phosphorescent green and red emitter systems. In a single unit 2-color fluorescent white system with the same combination of internal and external outcoupling methods an efficiency of 37 lm/W at a color x,y = 0.48/0.42 is reached. This is to our knowledge the highest power efficiency reported for a fluorescent white OLED with a flat light extraction method. A similar approach using a vacuum processable scattering material in top emission OLED architectures on metal substrates allows to reach efficiencies of 36.5 lm/W with a CRI of 75 at a color x,y = 0.45/0.41.

Improvement of device efficiency in PIN-OLEDs by controlling the charge carrier balance and intrinsic outcoupling methods

Organic light-emitting diodes (OLEDs) with high power efficiency are desirable for lighting applications. A prerequisite for high power efficiency is the achievement of low driving voltages, which can be done via charge carrier doping of the transport layers in PIN organic light-emitting diodes (PIN- OLEDs). We have looked at how to combine low voltage with high current efficiency and long lifetime and have therefore investigated different ways of changing the charge carrier balance in PIN-OLEDs. The carrier supply to the emitting layer was adjusted by electron and hole-blocking layers to allow for an undisturbed exciton formation and radiative decay while keeping the total voltage low. In further investigations, we developed highly efficient and stable white PIN bottom emission OLED devices using novel evaporation processable outcoupling enhancement materials inside the PIN OLED stack. In white bottom emission OLEDs the use of this outcoupling enhancement material in combination with a standard micro-lens arrays (MLA) outcoupling film can yield an efficiency enhancement of up to 1.8. By combining the well-balanced stack with the internal outcoupling approach we achieved a power efficiency of 51.9  lm/W. With additional flat external outcoupling, the power efficiency has been further increased to 60  lm/W. The color coordinates are 0.470/0.429 with color rendering index (CRI) of 87. The 50% lifetime of the OLED could be estimated to 90,000 h.

An overview about the use of electrical doping of charge carrier transport layers in OLEDs and further organic electronic applications

Electrical doping of organic layers is now a well established method for building highly efficient and long living OLEDs. A unique class of OLED devices called PIN-OLEDs based on redox doping technology is emerging as one key technology for OLED applications. These devices exhibit high power efficiency and long life time, which are critical parameters for commercial success. Moreover, PIN OLEDs offer high degree of freedom in choosing layer structures for optimizing the device performance for specific lighting and display applications. For example, optimizing color and power efficiency of OLEDs can be easily achieved without compromising the device operating voltage. It is worth to mention that PIN OLEDS, especially the red emitting PIN OLEDs, exhibit record breaking half life time of more than one million hours with the starting device brightness of 1000 cd/m2. The doping technology also offers benefits to other organic electronic devices such as OTFTs and photovoltaic devices. This paper briefly discusses the improvements made on the OLED device performance such as power efficiency and lifetime using doped transport layers. Few examples of device optimization using doped layers are presented in detail. In addition, a brief discussion on performance of doped transport layers in photovoltaics is also presented.

35.2: Improved Efficiency and Lifetime by Tailoring the Charge Carrier Supply in OLEDs

The electron and hole supply into the recombination zone of an OLED is of crucial importance to achieve optimum performance. By adjusting the energetic levels of the transport and the blocking layers we optimize lifetime and power efficiency of blue and red fluorescent PIN OLEDs. for a red OLED, we achieve 100 cd/m2 below 2 V. By deliberately introducing barriers in the electron supply of red fluorescent OLEDs, we can “switch on” triplet-triplet annihilation, leading to a significant boost in efficiency — up to 12 % external quantum efficiency were measured in an integrating sphere.

Application of redox doping in OTFTs

Molecular redox dopants are tested concerning their application to organic thin-film transistors (OTFT). Here we report on the feasibility of solution processing of molecular-doped transport layers, showing high air-stability of solutions and layers. We apply capacitance spectroscopy to investigate the interface of intrinsic and electrically doped layers. We also show that there is virtually no dopant migration in real devices, even when high electric fields up to 300 kV/cm2 are applied for 1000 h. We report on p- and n-type on OTFTs with silver contacts. The application of injection layers based on redox dopants improves the measured field-effect mobility by about 2 orders of magnitude.

36.5: Late‐News Paper: The Light Distribution in OLEDs and Ways to Increase the Light Outcoupling Efficiency

Power efficiency of organic light emitting devices (OLEDs) is in the focus of worldwide research in order to reduce power consumption of displays and to improve the competitive situation of OLEDs for lighting devices. The optical properties of OLEDs are studied with the aim to enhance the fraction of light coupled out of these devices. Simulation software has been used to analyze the transfer of internally generated emission into usable external light modes. Results from modeling have been compared to experimental data to build trust into the validity of the results. Critical parameters influencing outcoupling properties have been identified. Thus, 185lm/W at 1000cd/m2 have been achieved for a green PIN-OLED using a high-n substrate & half ball lens. Finally, some conclusions for the design of outcoupling enhancing means are drawn.