Docent Mindaugas Viliūnas works at the department of Solid State Electronics of Vilnius University. He has graduated from the faculty of Physics of this University in 1992, and seven years later, he has defended doctoral dissertation in physical science. His main area of research is charge carrier transport studies in materials with chaotic structure. In Vilnius University, he has been giving various special courses attended not only by future physics but also by chemists and mathematicians. In the first place, it is a course of Microprocessor technologies that is optional for all the students in the university.
This and other courses taught by Viliūnas – “Digital signal processors”, “Application of digital signal processors” and “Computerized physical and technological measurements” – are directly related with the topic of embedded systems. The significance of these systems as well as the importance of understanding physical processes for engineers working with embedded systems will be discussed in the interview with Mindaugas Viliūnas.
M. Viliūnas. In the Department of Solid State Electronics we work with various semiconducting materials, mainly organic semiconductors, amorphous and microcrystalline structures. An important feature of these materials is significantly lower level of inner order compared to other crystalline semiconductors. This provides these materials with new characteristics. Materials with more chaotic structure are not among the most important ones in semiconductor electronics, however, they are applied in solar energy, image scanning and recovery systems, sensors and some other areas.
How amorphous organic materials you work with can be harmonized with solid-state that appears in the name of the Department?
M. Viliūnas. Organic materials must not necessarily be soft, they can be hard enough. For example, pearls. I doubt that anyone would associate it with a soft object. You see, the purpose of works carried out in our department is not strictly defined: in some cases we work with organics and sometimes with non-organic materials. We simply work with materials, semiconductors, however, with those that are not very popular, at least not like crystalline silicon or germanium… The most common subject of our study is semi-insulating (high resistivity) semiconductors, i.e. materials with significantly lower electrical conductivity than usual.
Is using organic materials is the most efficient way to produce LEDs and solar cells?
M. Viliūnas. LEDs produced from usual semiconducting materials have efficiency coefficient of approx. 15 percent, and those produced of organic materials can only have efficiency coefficient of up to only few percent. You can feel really happy to have achieved at least 5 percent.
Then what pulls you and other researchers towards organic materials?
M. Viliūnas. When LEDs are produced of organic materials, this means that the material may be applied on a wall and it will shine. Similar situation is with solar elements. Using organic materials we could paint our roof with certain paint and instantly have a solar battery with the efficiency coefficient of 3 percent, but even this low efficiency is among our objectives for inexpensive materials, and simplicity of installation will compensate for low efficiency. Currently, the efficiency of massively sold solar batteries is approximately 15 percent. This is battery that is affordable for the user and the most optimum for installation. Another option could be a battery with the efficiency of 30 percent, however it is not affordable for everyone.
We would like to forget that organic materials are tend to age and degrade, which is a significant problem. Organic materials lose their efficiency under the impact of oxygen. Oxygen inhibits carrier transport, which is a significant drawback for solar elements. Contrary to organic materials, crystalline semiconductor retains it efficiency, which is one of its advantages.
Advantage of chaotic organic structures
Maybe you want to say that in your work you are more like hitting head against a brick wall? Should you just say a polite goodbye to chaotic materials and turn to crystalline semiconductors? After all, they are more reliable, long-term, and characterized by greater efficiency.
M. Viliūnas. Some of the properties of these organic semiconductors are very quirky and therefore attractive. Conventional semiconductors illuminated by a strong ionizing radiation stops operating and may even result in permanent damage. For example, after Chernobyl nuclear power plant disaster, an attempt was made to eliminate the consequences of the disaster using robots. They were supposed to push radioactive debris from the roof of the plant, but very soon all the robots went down. Why? Radiation affected semiconductors so strongly that they appeared to be not that enduring at all. Robots were replaced by people to liquidate the consequences …
How would you explain resistance of amorphous semiconductors to radiation?
M. Viliūnas. Amorphous semiconductors are chaotic in principal, i.e. they are of chaotic inner structure. Convenient semiconductors are very orderly thanks to their crystalline structure, however, any damage of these crystals lead to degradation of semiconductor features. Amorphous semiconductor is an example of chaotic structure, and it doesn’t matter how hard you try to corrupt this chaotic structure, it still remains chaotic and the properties virtually remain the same. If the control blocks of the above mentioned robots would have been produced applying amorphous electronics, many lives would have been saved.
Importance of optimum power control
Where in all these things discussed is the point of applying, or maybe designing embedded systems?
M. Viliūnas. I have always been interested in this subject. Engineering activity in electronics is closely related to embedded systems, and lately, it has been especially promising area with a strong growth potential. It would be hard to find an area where these systems would not be applied. The mission of embedded systems is to make every device more functional and efficient. As regards our work, we aim to develop more efficient solar batteries. Let’s say the declared efficiency of sun battery is approx 15 percent. This means that given optimum load we obtain this high efficiency. However, if sun battery charges accumulator, or when it is connected to some other user of energy, then only a part of generated energy is obtained depending on lightness, battery temperature and load characteristics. This is because that the maximum energy is obtained only given certain load impedance. Therefore, we need a device that obtains maximum amount of energy and transmits it to the user.
Is the optimum load required to ensure the transmission?
M. Viliūnas. Yes, it is. As a result, development of optimum power controllers is gaining more and more importance. Certain devices are required to significantly increase energy efficiency though it may seem they will not do anything new.
So this is where the contribution of embedded systems begins?
M. Viliūnas. Correct. An embedded system must calculate all the modes, and “know” how the solar battery works to give it certain load. Based on this information, an optimum operation mode may be selected. Moreover, an embedded system may store data on the power generated and the condition of the solar battery, and given mechanical facilities, it could direct solar battery towards obtaining the maximum power.
In general, embedded systems can be applied almost everywhere where electricity is used. Let’s take a toothbrush as an example. Its individual parts are rotating, vibrate, clean teeth, but this device can also perform other advanced features: measure time and load, and adjust operation of the device based on this information. Embedded systems come in handy in cases where you need to think about how to optimally use various equipment, appliances or devices, and adjust the work accordingly. If we want to use the potential of the device to a maximum we attempt to embed a certain additional system that controls the device to ensure its optimum operation.
Can it also provide some new functions? For example, connecting certain additional tools?
M. Viliūnas. An embedded system is like “brain” for a device. With new “brain”, device can acquire additional features. In such cases, I mostly work with “brains” rather than with tools. Nevertheless, a designer, engineer, or a physicist must have sufficient knowledge about the tools, otherwise the “brain” will do not what they have to do.
To be continued No 12
Interviewer Gediminas Zemlickas