Introduction

More than 20% of the world’s total electricity consumption is used for lighting applications. This number reaches 25% when consumption from display and TV applications is included [1]. Therefore innovative, cost and energy effective solutions for display and lighting applications are the focus of an ongoing intense world-wide effort in the field of photonics research and development activities. In particular, organic light-emitting diodes (OLEDs), alongside with inorganic solid-state lighting technologies, are the most promising candidates for future display and lighting technologies of the 21st century. The realization of lightweight, potentially flexible and cheap-to-fabricate, highly energy efficient lighting and display applications would save hundreds of GWh or millions tons of coal per year if deployed ubiquitously.

While the first report on organic electroluminescence from an organic semiconductor - an anthracene crystal - dates back to the 1960s [2], only the discovery of conducting polymers by Shirakawa, MacDiarmid and Heeger in the 1970s [3] has resulted in a tremendous boost of this field of research. From the beginning the field was characterized by a very interdisciplinary approach, at the intersection of chemistry, physics, and engineering, as also reflected in the contributions from these different research areas to this Focus Issue on OLEDs. Yet, the initial enlightening observations for the field of organic light-emitting devices were made in the 1980s, with the first report on thin film OLEDs based on small molecules at Eastman Kodak by Ching W. Tang and Steven Van Slyke [4], and three years later, a report of the first polymer light-emitting diode (PLED) by R. Friend and associates [5]. These two reports triggered enormous efforts, both within academic research as well as industrial development, since it was instantaneously recognized that light-emitting small molecules and light-emitting conjugated polymers have enormous potential as the active materials for large area flat-panel displays [6], lighting [7] as well as laser applications [8].

In particular, the technological potential to realize displays with areas as large as several square meters, and be fabricated by cheap printing techniques while offering a flexible or conformable lightweight design, has been inspiring these aspects since OLED research first began. While some of the above mentioned features still need to be demonstrated beyond lab scale, the observed fast temporal response (typically under one millisecond), the high contrast ratios of up to 1,000,000:1, the typically achieved device efficiencies (10 cd/A for blue light, 70 cd/A for green light, and 30 cd/A for red light) in display applications, and of ca. 50 lm/W in OLED lighting panels, have led to the transfer of OLEDs into commercial applications.

From the very beginning of OLED research, chemical stability of the active materials and reliability of the devices has been a pivotal point [9]. While intense research has led to well established active fluorescent and phosphorescent emitters (both small molecules and conjugated polymers) for the red and green spectral range [10], there is still a quest for stable, highly efficient and easy to synthesize molecules that emit light in the deep blue wavelength range. Yet, despite considerable skepticism that these organic semiconductors would ever reach the levels of purity and stability required for commercial devices, the remarkable efforts of the research community engaged alongside a resilient industrial involvement has - in the meantime - lead to a number of commercial OLED-based products beyond the niche markets in which this technology has been first introduced. Beginning with the first commercial OLED-TV XEL produced by Sony in 2008, the great success of OLEDs and active matrix (AMOLED) panels as found in different cell- and smart-phones, with a reported production of 30 million panels/month by Samsung, and the introduction of the first white OLED lighting panels produced by Philips and OSRAM, may be regarded as only the first step. The ongoing press announcements of flexible and transparent end-user applications for the years to come will most likely establish OLED as the leading technology for displays and lighting applications.

Yet, despite this first promising commercial success of OLED technologies, a number of challenges need to be further addressed to allow full exploitation of its potential. Along with the ever-ongoing quest for emitter materials with improved stability and enhanced processability through printing techniques or easy to realize multilayer structures, the challenges of light outcoupling and light management in OLEDs are the focus of current attention. From this point of view, this Focus Issue on OLEDs comprises a number of articles addressing these important questions.

The outcoupling of light from an OLED may be tackled by different means, including optical feedback structures in the active layer, by high index-media in top emitting OLED, by lens or microlens-like arrays, or by using microcavity effects in the active device. As reviewed and discussed by Simone Hofmann, Karl Leo and their colleagues from the Institute for Applied for Photophysics, TU Dresden [11], in particular top-emitting OLEDs seem to be beneficial for lighting and display applications. Here, non-transparent substrates are used. The authors review and discuss different optical effects of the microcavity structure and indentify important loss mechanisms due to waveguiding and surface plasmons, and show that further improvement in light extraction is required to reach the targeted high outcoupling efficiencies.

This very same topic of light outcoupling from a top emitting OLED is also discussed in the paper by Bert J. Scholz, Wolfgang Brütting and their colleges from the Institute of Physics, University of Augsburg and their colleagues from OSRAM Opto Semiconductors GmbH [12]. Based on a combination of optical simulations and experiments on simplified test-structures they demonstrate that it is also possible to outcouple light lost to wave-guided modes and surface plasmons in a top-emitting white OLED using a high-index prism to at least overcome the as-mentioned limitations in a laboratory setup.

A practical approach to improve the outcoupling efficiency in OLEDs up to 60% is demonstrated by Ruth Shinar and Joseph Shinar from the Ames Laboratory - USDOE and Department of Physics and Astronomy, Microelectronics Research Center and Department of Electrical and Computer Engineering at Iowa State University, and associates [13], using index-matching microporous phase-separated films of polymer blends acting as a random microlens-like arrays. They demonstrate that the use of such blended thin films provides an economical method, independent of the OLED fabrication technique, for improving outcoupling.

To overcome the losses at the organic layer/cathode interface and to optimize the optical path in the devices Lian Duan, Yong Qiu and their colleagues from the Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, and the R&D Center, Visionox Tech. Ltd, Beijing [14], introduce an approach by using a novel rather thick n-doped layer. Using a combination of a low-temperature-evaporable n-dopant KBH4 and a high charge carrier mobility electron transport material they show excellent performance of their devices due to reduced losses at the organic layer/cathode interface.

The fabrication of multilayer structure OLEDs from an all-solution process is a key prerequisite to also pave the way for solution processed and possibly printed devices. Here, Jianhong Lü, Lixiang Wang and associates from the State Key Laboratory of Polymer Physics and Chemistry, and the Graduate School of the Chinese Academy of Sciences [15], report on an alcohol soluble efficient electron transporting material for all-solution processed blue electrophosphorescent devices. The authors demonstrate that this approach may be used for solution processed highly efficient light-emitting multilayer devices.

Addressing the demand for stable blue emitting polymers is the focus of the article by K. Müllen from the Max-Planck-Institut for Polymer Research, and his associates from NanoTecCenter Weiz GmbH, Helmholtz Zentrum Berlin GmbH, Humboldt-Universität zu Berlin, University of Groningen, and TU Graz [16]. The authors show that polypyrene may be regarded as a novel and very promising light-emitting polymer for deep blue light-emitting PLEDs. The work not only demonstrates that the polymer presented does not show the unwanted excimer emission typical for pyrene, but also may be synthesized in a very easy to perform low-cost three-step synthesis.

In conclusion, the contributions in this Focus Issue offer a comprehensive compilation of topics currently of interest to the OLED research and development community with an emphasis on light outcoupling issues in OLEDs. We would like to thank Bernard Kippelen, Editor of the Energy Express supplement of Optics Express, for his kind invitation to realize this Focus Issue; all Associate Editors and the staff coordinating OSA’s publications for their kind support; and we are grateful to all contributors for publishing articles tremendously relevant to the topic.

November 7, 2011

Emil J.W. List and Norbert Koch

Guest Editors of the Optics Express Energy Express supplement Focus Issue: OLEDs.