Due to the relevance of phosphor-converted semiconductor emitters for general lighting, the research focus of the past decades has been on the optimization of white LED's components. Especially the emitters of the blue spectral range, which are predestined for the excitation of phosphors, have been optimized through technological development. By contrast, the emitters of the ultraviolet and green spectral ranges, which are based on the same material system, serve niche applications and have technological hurdles due to their material composition. The technological challenges offer a corresponding optimization potential, which can be related to both the increase in efficiency and the long-term stability of the devices. For an optimization of the structures regarding their lifetime, physical degradation mechanisms and their dependencies on operating parameters have to be analyzed. Consequently, the dynamics and mechanisms of current and temperature induced degradation processes are analyzed within the scope of the present work. The indium-rich emitters of the green spectral range indicate a significant reduction of optical power, which can be attributed to the formation of point defects. The results indicate a reactivation of passivated point defects, whose activation is enhanced by interactions with Auger processes and absorbed photons. Due to the dependencies described above the degradation is significantly influenced by the selected aging current. The analysis of the temperature dependence indicates an additional degradation mechanism that counteracts the formation of point defects. Due to the superposition of the mechanisms, the temperature-dependent behavior cannot be described with the common approach of an Arrhenius equation. Emitters of the ultraviolet spectral range indicate a comparable formation of point defects, which is significantly reflected in the electrical characteristics of the devices. As a consequence of the point defects, the reduction of the optical power differs between various operating points. In addition, the gradual degradation of the semiconductor structures is strongly affected by the aging current. The degradation behavior of silicone encapsulated packages is dominated by the aging processes within the primary optics. The crack formation associated with the embrittlement of the primary lenses promotes processes of electromigration, which contribute to shorting the active region. The development of crack formation is accelerated by temperature and irradiation and can be described on the basis of the developed model. The gradual degradation of the semiconductor structures, which is independent of the damaged primary lenses, is modelled additionally. The time-dependent model equations of the optical degradation allow the consideration of different measurement and degradation currents and can be used for the lifetime calculation of the devices. Furthermore, the results of the degradation tests are evaluated on the basis of simulations of multi-channel light systems. Based on the simulation results, the degradation limits of the individual color channels can be defined as a function of the spectral compositions. Due to the colorimetric irrelevance of ultraviolet radiation sources, the aging behavior of UV emitters is discussed regarding photocatalytic disinfection applications. Despite the previously developed model equations, discrepancies can arise between laboratory aging tests and real applications, which makes it difficult to calculate the actual in-situ degradation behavior.
In order to be able to determine the degradation state of the components in field use, correlations between the changes in electrical characteristics and optical degradation are identified on the basis of the aging data collected. The correlations allow conclusions to be drawn about the actual in-situ aging of the device. Using the developed correlation model, the degradation of the optical power can be predicted from changes in the electrical characteristics. | English |