Doping of Organic Semiconductors

Abstract

Björn Lü ssem studied electrical engineering at the Rheinisch-Westfälische Technische Hochschule (RWTH) in Aachen and the University of Bath and obtained his degree as Diplomingenieur in 2003. He prepared his PhD thesis at the Research Center in Jülich, Germany, in the field of molecular electronics. His thesis concentrates on scanning tunneling microscopy of pure and mixed selfassembled monolayers and has been awarded the VDE-Promotionspreis and the Günther-Leibfried-Preis. After staying at the Materials Science Laboratory of Sony in Stuttgart from 2006 to 2008, he joined Prof. Leo's group at the Technische Universität Dresden where he is now the head of the New Devices Group. His main interests are new semiconducting devices based on organic materials and their differences or similarities to inorganic semiconductors. Moritz Riede studied physics at the University of Cambridge, UK, where he received his MSc degree in 2002. He obtained his PhD degree from the University of Konstanz in 2006 for research carried out at the Fraunhofer-Institut für Solare Energiesysteme and the Freiburger Materialforschungszentrum FMF. The focus of his thesis was on solution-processed organic solar cells. After his PhD he joined the research group of Prof. Leo at the Institut für Angewandte Photophysik (IAPP) of the Technische Universität Dresden in 2007, where he is now head of the Organic Solar Cell Group. His research concentrates on small-molecule-based organic solar cells, their fundamental working principles, and strategies for device optimization. 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 the supervision of Hans Queisser. From 1989 to 1991, he was a postdoctoral researcher at AT&T Bell Laboratories in Holmdel, NJ, USA. From 1991 to 1993, he was with the Rheinisch-Westfälische Technische Hochschule (RWTH) in Aachen, Germany. Since 1993, he has been full professor of optoelectronics at the Technische Universität Dresden. Since 2002, he is also with the Fraunhofer society, currently as director of Fraunhofer COMEDD. His main current interests are novel semiconductor systems such as 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 such as Novaled AG and Heliatek GmbH. Figure 1 Operation principles of an OLED (left) and an OSC (right). Two-layer devices are shown. Reprinted with permission from Walzer et al., Chem. Rev. 107, 1233 (2007).

Review—Organic Materials for Thermoelectric Energy Generation

“…An n-type dopant on the other hand, is a reducing agent which donates an electron to the lowest unoccupied molecular orbital (LUMO) of the organic semiconductor introducing a negative charge carrier. 12 The integer charge transfer model and the molecular orbital levels involved in the doping processes (p-type and n-type) are schematically represented in Figure 2.…”

“…Unlike inorganic semiconductors, in which an impurity is introduced to a crystalline semiconductor to reduce the gap between the valence and conduction bands, doping in organic semiconductors requires the addition of strongly electron donating (n-type) or strongly electron withdrawing (p-type) molecular species which induce mobile charge carriers along the polymer backbone. 10,12 A p-type dopant can be thought of as an oxidizing agent which removes an electron from the highest occupied molecular orbital (HOMO) of the organic semiconductor, introducing a positive hole along the backbone. An n-type dopant on the other hand, is a reducing agent which donates an electron to the lowest unoccupied molecular orbital (LUMO) of the organic semiconductor introducing a negative charge carrier.…”

Measurement of Small Molecular Dopant F4TCNQ and C60F36 Diffusion in Organic Bilayer Architectures

“…11−16 The doping mechanism occurs by an electron transfer process between the dopant molecule and the host material. 11,17 For p-type doping, the electron is transferred from the host material to the dopant, and for n-type doping the electron transfers from the dopant molecule to the host material. An important difference between doping inorganic semiconductors and doping organic electronic materials is that, for organic doping, the dopant molecules are not bound to their host material by strong covalent bonds.…”

“…The diffusion of dopant molecules within and between layers of a device can greatly hinder the performance by causing unwanted interactions with other materials, but there have been very few studies directly investigating this process. 11,18,19 The first doping experiments were performed using atomic dopants such as iodine and bromine 20,21 and later using atoms such as lithium, cesium, and strontium. 22,23 However, those dopants are all too small.…”

Influence of doping on charge carrier collection in normal and inverted geometry polymer:fullerene solar cells

“…Additionally, polymer:fullerene solar cells are not intentionally doped like their inorganic counterparts or like many small molecule solar cells 11 and therefore rely on selective contacts and the difference in work function between electrodes for efficient charge collection. However, several studies have found evidence for unintentional doping [12][13][14][15][16][17][18][19] and discussed the consequences for device behaviour 6,[20][21][22][23][24][25][26][27][28][29][30] . Whilst the origin of this doping is unclear 15 , its effects on photovoltaic performance can be substantial; however many recent analyses of device performance neglect doping 8,[31][32][33] despite the fact that the influence of doping and the electric field on charge carrier collection is well known for a long time 34 and wellstudied for instance in the field of quantum dot photovoltaics 35,36 .…”