Professor Karl Leo, from the Institute for Applied Photophysics at the Technische Universität Dresden, Germany will visit the Institute for Optical Sciences from 12 to 16 April 2010, as part of the Institute’s Distinguished Visiting Scientists program.

Karl Leo obtained the Diplomphysiker degree from the University of Freiburg in 1985, working with Adolf Goetzberger at the Fraunhofer-Institut für Solare Energiesysteme. In 1988, he obtained the PhD degree from the University of Stuttgart for a PhD thesis performed at the Max-Planck-Institut für Festkörperforschung in Stuttgart under supervision of Hans Queisser. From 1989 to 1991, he was postdoc at AT&T Bell Laboratories in Holmdel, NJ, U.S.A. From 1991 to 1993, he was with the Rheinisch-Westfälische Technische Hochschule (RWTH) in Aachen, Germany. Since 1993, he is full professor of optoelectronics at the Technische Universität Dresden; since 2002, he is also at the Fraunhofer-Institute for Photonic Microsystems, presently as director. His main current interests are novel semiconductor systems like semiconducting organic thin films; with special emphasis to understand growth, basic device principles and the optical response. Recently, he has also worked on device development, such as highly efficient organic LED and solar cells. His work was recognized by several awards, including the Leibniz-Award in 2002. He was involved in the founding of several companies including Novaled AG and Heliatek GmbH.

Professor Leo will give 3 lectures during his visit, as described below:

Lecture 1: Basic Studies on Organic Materials

Monday, April 12 at 2 p.m.
Physics Building, Room MP134
60 St. George Street

Organic semiconductors are currently investigated intensively, both because the basic physics are little understood and because of attractive application possibilities, such as flat-panel displays based on organic light emitting diodes (OLED). Despite the fact that organic semiconductors have, in particular as thin films, rather low mobilities, they perform surprisingly well in optoelectronic devices.

In this talk, I will briefly introduce some basic concepts of organic semiconductors. Then, I will discuss recent work on some basic aspects of electrical doping of these materials. In contrast to classical semiconductor technology, where device applications started when it was possible to control the carrier concentration, doping in organics has been less considered. I will discuss results of a comprehensive study of controlled n- and p-type doping of various molecular organic materials. In particular, I will also discuss how the archetype of the semiconductor devices, the pn-homojunction, can be realized in organics.

For an overview, see the review article K. Walzer et al., Chem. Rev. 107, 1233 (2007).

Lecture 2: Materials and Structures for Highly Efficient Organic LED

Tuesday, April 13 at 2 p.m.
Bahen Building, Room BA1160
40 St. George Street

Organic LED have developed in the past 20 years from a lab curiosity to a successful product. Despite large advances in performance and lifetime, future applications in OLED TV and lighting require further progress.

In this talk, I will first review some of the basic operating principles of OLED. Then, I will discuss our own work on improving efficiency and lifetime of OLED. We have recently shown that the electrical doping concepts can be successfully applied in OLED devices: the concept of molecular doping allows to realise green OLED devices with the highest efficiencies reported so far, well exceeding the efficiency of current inorganic GaN devices! I will also discuss our recent results on white OLED which have recently achieved efficiencies comparable to fluorescent tubes, opening the path to a new form of high-efficiency area lighting devices [1]. Surprisingly, the field of OLED is currently dominated by evaporated small-molecule devices, despite the fact that initially, polymer OLED which allow loquid processing were seen as the more direct and cost-effective approach to devices.

[1] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K. Leo, Nature 459, 234 (2009).

Lecture 3: Organic Solar Cells and Organic Lasers

Thursday, April 15 at 2 p.m.
Physics Building, Room MP134
60 St. George Street

Due to their high absorption and other attractive optical properties, organic materials are suitable for applications in solar cells and laser.

First, I will briefly discuss some of the basics of organic solar cells. Then, I will discuss some recent work investigating new absorber compounds for small-molecule organic solar cells. These materials are also very useful to better understand the origin of the open-circuit voltage of organic solar cells.

Also, I will discuss how the doping concepts can be applied in organic solar cells. Here, the use of electrically doped transport layers is helpful for an optimized optical design since it yields large freedom in the choice of window layer thickness. Also, doped layers are a key point in efficient charge recombination junctions for tandem solar cells. Recently, together with our spin-off company Heliatek, we have achieved efficiencies of around 7% on areas >1cm2. These cells show also very promising stability.

Finally, I will briefly discuss some recent work on optically pumped organic vertically emitting lasers, which have many interesting properties.

For more information on this lecture series please contact Emanuel Istrate.