Organic devices for solid state lighting : technology and processing (2008)

The invention of the electrical generation of light 150 years ago by Thomas Swan changed the world fundamentally. Before, the circadian rhythm was determined by the natural light of the sun. All of a sudden, it became possible to bring light into the darkness without being dependent on the weak light of candles or paraffin lamps. Ever since, electrical light sources play a crucial role in modern society. In the early 20th century, the technology of fluorescent tubes was introduced in addition to incandescent bulbs. However, until today both these light sources comprise several disadvantages. The most fundamental deficiency of incandescent bulbs surely is their very low efficiency of about 15 lm/W, which results in a high amount of lost heat. Furthermore, their lifetime is limited to 1000-2000 hours. Due to this, they have to be replaced frequently and also can pose a safety risk if they are employed in traffic lights, for example. Fluorescent lights, however, offer a high efficiency of about 90 lm/W. Nevertheless, the unfavorable design limits their popularity especially in the private environment. Another disadvantage is the fact that fluorescent tubes mostly contain mercury, which leads to environmental problems if they brake or are improperly disposed. A universal light source has to fulfill a couple of requirements for different applications. The brightness and chromaticity should be tunable by the manufacturer and in addition to this, it should offer maximum flexibility in regard to the design. Furthermore, a high efficiency in order to save energy and minimize the heat loss is required for almost every application. Obviously, all these requirements together cannot be fulfilled by fluorescent tubes, energy saving lamps and even less by incandescent bulbs. A solution to this problem is the physical effect of electroluminescence (EL), which is the direct conversion of current to light. This concept is applied in light emitting diodes (LED). Especially organic LED (OLED) offer the potential to achieve all requirements of a universal light source as described above. In regard to lighting, OLED have tremendous advantages: They are thin, lightweight and generate a diffusive light which does not create sharp transitions from bright to dark areas. These properties do not only render OLED as promising candidates for general lighting, but also for applications such as signage, decorative lighting or automotive interior lighting. In theory, OLED can be manufactured in arbitrary shapes. In combination with a multitude of possible colors or even the dynamic tuning of the emission color, they are ideal solutions for advertising applications. Last but not least, OLED possess an enormous energy saving potential. It is generally acknowledged that OLED, in principle, have the capability to exceed the efficiencies of modern fluorescent tubes. This would result in a more economic consumption of energy and thus, direct benefit for the environment. During the manufacturing process, no heavy metals are required and the glass or plastic foil which is used as a substrate can be fully recycled. Therefore, OLED are promising candidates to substitute conventional light sources in many areas of application. Nevertheless, some challenges have to be mastered before OLED are ready for the introduction into the market. Many research groups worldwide are working on the improvement of OLED efficiencies, especially for white emitting devices. Other important aspects for OLED are their reliability and lifetime. These can be advanced by optimizing the charge carrier balance in the devices, the development of novel organic materials or the improvement of encapsulation technologies. In regard to the conception of pre-pilot systems for the manufacturing of devices on an industrial scale, the evaluation of different deposition technologies is a fundamental matter. In this work, the technology of organic vapor phase deposition (OVPD), which was invented by Prof. S. Forrest at Princeton University, is investigated. Monochrome and white emitting devices as well as organic single layers were fabricated. Due to the fact that OVPD is a relatively young technology compared to the established vacuum thermal evaporation (VTE), a primary goal of this work was to generate an understanding in regard to the impact of various process parameters on the performance of OLED. While for VTE systems, the tunable process parameters are mostly limited to the deposition rate, OVPD offers several precisely adjustable parameters such as deposition chamber pressure, substrate temperature and carrier gas flow. As of today, the influence of these variables on the organic thin films and devices is vastly unknown. The impact of various parameters during the gas phase deposition process was investigated in this work and in addition to this, crucial technological aspects for OVPD systems were identified.