Vacuum electronics (VE) have dominated development and industrial growth in their application areas from the end of the 19th century to the end of 20th century. VE have contributed to basic concepts of physics and have enabled important basic inventions. Despite this bright past, in the meantime also a complete or partial replacement by new technologies such as solid-state electronics (SSE) occurred in several applications areas, triggered by the demand for new features and leading to new applications. Based on a review of the historical development of vacuum electronics from the basic inventions to the modern state of the art, the aim of this paper is to identify future trends and prospects of this field. The appearance of generic technology cycles, as in the case of radio-receiving tubes and cathode-ray display tubes, is discussed. Microwave tubes did experience only a partial replacement by solid-state devices and defended the high-power, high-frequency domain. The reason for their superiority in this domain is discussed. The development of the base technologies for VE, namely vacuum technology and electron source technology, is outlined, enabling further improvements. Besides the high-power, high-frequency domain of microwave tubes, VE technology applications with positive future prospects are addressed, e.g., space applications (long-lived microwave tubes, ion thrusters); thermionic energy converters; e-beam lithography; x-ray tubes; vacuum-based high-resolution characterization, and high-brightness beams for free electron lasers or particle accelerators. The continuous growth and increase in performance of solid-state electronics is shortly reviewed, SSE taking the lead with respect to total sales in the 1980s. Now, despite inherent advantages, solid-state electronics also seem to approach technical limitations. These include increasing energy consumption in conjunction with reduced long-term reliability when further scaling down. It is envisioned that vacuum nanoelectronics can help to overcome these limitations when scaling down feature sizes of integrated circuits below 22 nm.

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In a conventional bottom emitting organic light emitting diode only about half of the generated photons are emitted into the glass substrate (out of which 25% are extracted into air), the other half being wave-guided and dissipated in the OLED stack. This is due to the refractive index mismatch between the organic layers (n=1.7-1.9) and the glass substrate (n=1.5). By matching the refractive index of the substrate (n=1.8) and organic layers and augmenting the distance of the emission zone to the cathode to suppress plasmonic losses light extraction into the substrate can be increased to 80-90%. This is shown by simulation and experiment. Furthermore the effect of pyramidal structures on the light extraction from the substrate into air is studied by experiment and simulation. Ultimately it is limited by the reflectance of the OLED stack. The experimental results for monochromatic light are well corroborated by simulations. The main conclusion is that most photons can be out-coupled from the organic stack into an index matched substrate. The OLED light extraction problem is thus reduced to an effective extraction from the substrate into air.

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Organic light emitting diodes (OLEDs) provide potential for power-efficient large area light sources that combine revolutionary properties. They are thin and flat and in addition they can be transparent, colour-tuneable, or flexible. We review the state of the art in white OLEDs and present performance data for three-colour hybrid white OLEDs on indexmatched substrates. With improved optical outcoupling 45 lm/W are achieved. Using a half-sphere to collect all the light that is in the substrate results in 80 lm/W. Optical modelling supports the experimental work. For decorative applications features like transparency and colour tuning are very appealing. We show results on transparent white OLEDs and two ways to come to a colour-variable OLED. These are lateral separation of different colours in a striped design and direct vertical stacking of the different emitting layers. For a striped colour tuneable OLED 36 lm/W are achieved in white with improved optical outcoupling.

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