Abstract Using photoelectron spectroscopy, we show measurements of energy level alignment of organic semiconducting layers. The main focus is on the properties and the influence of doped layers. The investigations on the pdoping process in organic semiconductors show typ 20−3 ical charge carrier concentrations up to 2∙By a variation of the doping10 cm . concentration, an over proportional influence on the position of the Fermi energy is observed. Comparing the number of charge carriers with the amount of dopants present in the layer, it is found that only 5% of the dopants undergo a full charge transfer. Furthermore, a detailed investigation of the density of states beyond the HOMO onset reveals that an exponentially decaying density of states reaches further into the band gap than commonly assumed. For an increasing amount of doping, the Fermi energy gets pinned on these states which suggests that a significant amount of charge carriers is present there. The investigation of metal top and bottom contacts aims at understanding the asymmetric currentvoltage characteristics found for some symmetrically built device stacks. It can be shown that a reaction between the atoms from the top contact with the molecules of the layer leads to a change in energy level alignment that produces a 1.16 eV lower electron injection barrier from the top. Further detailed investigations on such contacts show that the formation of a silver top contact is dominated by diffusion processes, leading to a broadened interface. However, upon insertion of a thin aluminum interlayer this diffusion can be stopped and an abrupt interface is achieved. Furthermore, in the case of a thick silver top contact, a monolayer of molecules is found to float on top of the metal layer, almost independent on the metal layer thickness. Finally, several device stacks are investigated, regarding interface dipoles, forma tion of depletion regions, energy alignment in mixed layers, and the influence of the builtin voltage. We show schematic energy level alignments ofpnjunctions,pinho mojunctions, more complexpinheterojunctions with Zenerdiode characteristics, as well as a complete OLED stack. The results allow a deeper insight in the working principle of such devices.
Kurzfassung Mit Hilfe der Photoelektronenspektroskopie werden in der vorliegenden Arbeit En ergieniveaus an Grenzflächen von organischen Halbleitern untersucht, wobei ein Haup taugenmerk auf dem Einfluss und den Eigenschaften dotierter Schichten liegt. Bei der Untersuchung grundlegender Eigenschaften eines pdotierten organischen 20−3 Halbleiters können Ladungsträgerkonzentrationen bis zu 2∙nachgewiesen10 cm werden. Eine Variation der Dotierkonzentration zeigt einen überproportionalen Ein fluss der Ladungsträger auf die Position des Ferminiveaus verglichen mit Experi menten an anorganischen Schichten. Durch den Vergleich mit der Anzahl Dotanden in der Schicht kann gezeigt werden, dass dabei nur etwa 5% der Dotanden einen voll ständigen Ladungstransfer eingehen. Eine detaillierte Untersuchungen der Zustands dichte jenseits des HOMOs (Highest Occupied Molecular Orbital) zeigt, dass die ex ponentiell abfallende Flanke der Zustandsdichte weiter in die Bandlücke hineinreicht als üblicherweise angenommen. Das Ferminiveau erfährt bei steigender Dotierung ein Pinning an diesen Zuständen, was für eine signifikante Ladungsträgerkonzentration spricht. Weiterhin wurden Untersuchungen zu Metal Top und Grundkontakten durchge führt. Es kann gezeigt werden, dass die Ursache für die Entstehung unsymmetrischer StromSpannungskurven, trotz eines symmetrischen Probenaufbaus, an einer Reak tion zwischen dem Molekül und den Metallatomen liegt. Dadurch entsteht eine um 1.16 eV reduzierte Injektionsbarriere für Elektronen am Topkontakt. Weitere detail lierte Untersuchungen an diesen Topkontakten zeigen, dass im Falle von Silber als Metall diese Grenzfläche von Diffusionsprozessen dominiert ist. Im Gegensatz dazu zeigt das unedle Metall Aluminium keine Diffusion und führt zu abrupten Grenz flächen. Im ersten Fall kann zudem eine Monolage vom Molekül auf dem Metallkon takt nachgewiesen werden, die unabhängig von der Metalldicke aufschwimmt. Zuletzt werden Bauelemente oder Teile solcher mit Photoelektronenspektroskopie vermessen. Hierbei werden die Grenzflächendipole, die Ausbildung von Verarmungszo nen, die Energieangleichung in Mischschichten und der Einfluss der Eingebauten Spannung untersucht. Es können die Banddiagramme vonpnäggnbÜreniafnee,ench pinHomoübergängen, komplexerenpinHeteroübergänge mit ZenerDioden Verhal ten sowie eine gesamte OLED gezeigt werden. Die Ergebnisse erlauben einen tieferen Einblick in die Arbeitsweise solcher Bauelemente.
Publications
Articles
A1K. Fehse, S. Olthof, K. Walzer, K. Leo, R. L. Johnson, H. Glowatzki, B. Bröker, and N. KochEnergy level alignment of electrically doped hole transport layers with transparent and conductive indium tin oxide and polymer anodes. Journal of Applied Physics102, 073719 (2007).
A2C. Uhrich, D. Wynands, S. Olthof, M. Riede, K. Leo, S. Sonntag, B. Maennig, and M. PfeifferOrigin of open circuit voltage in planar and bulk heterojunction organic thinfilm photovoltaics depending on doped transport layersof. Journal Applied Physics104, 043107 (2008).
A3C. Falkenberg, C. Uhrich, S. Olthof, B. Maennig, M. K. Riede, and K. LeoEf ficient pin type organic solar cells incorporating 1,4,5,8 naphthalenetetracar boxylic dianhydride as transparent electron transport materialof Ap. Journal plied Physics104, 034506 (2008).
A4S. Scholz, Q. Huang, M. Thomschke, S. Olthof, P. Sebastian, K. Walzer, K. Leo, S. Oswald, C. Corten, and D. KucklingSelfdoping and partial oxidation of metalonorganic interfaces for organic semiconductor devices studied by chem ical analysis techniques. Journal of Applied Physics104, 104502 (2008).
A5R. Meerheim, S. Scholz, S. Olthof, G. Schwartz, S. Reineke, K. Walzer, and K. Leo:Influence of charge balance and exciton distribution on efficiency and lifetime of phosphorescent organic lightemitting devicesof Applied. Journal Physics104, 14510 (2008).
A6R. Meerheim, S. Scholz, G. Schwartz, S. Reineke, S. Olthof, K. Walzer, and K. Leo:Efficiency and lifetime enhancement of phosphorescent organic devices. Proc. of SPIE6999, 699917 (2008).
A7S. Olthof, R. Meerheim, M. Schober, and K. Leo:Energy level alignment at the interfaces in a multilayer organic lightemitting diode structure. Physical Review B79, 245308 (2009).
A8S. Olthof, W. Tress, R. Meerheim, B. Lüssem, and K. Leo:Photoelectron spec troscopy study of systematic varied doping concentrations in an organic semi conductor layer using a molecular pdopantof Applied Physics. Journal 106, 03711 (2009).
A9R. Timmreck, S. Olthof, M. K. Riede, and K. Leo:Highly doped layers as effi cient electronhole conversion contacts for tandem organic solar cells. Journal of Applied Physics. Accepted.
A10S. Olthof, J. Meiss, M. K. Riede, B. Lüssem, and K. Leo:Photoelectron spec troscopy investigation of transparent metal top contacts for organic solar cells. Thin Solid Films. Submitted.
A11S. Olthof, H. Kleemann, B. Lüssem, and K. LeoBuiltin potential of a pentacene pin homojunction studied by ultraviolet photoemission spectroscopyMRS. 2010 Spring Meeting Symposium II Proceedings. Accepted.
A12M. Schober, S. Olthof, M. Furno, B. Lüssem, and K. Leo:A novel device concept for the characterization for charge carrier transport in organic semiconductor heterostructuresSubmitted.Physics Letters. . Applied
A13Rosenow, S. Reineke, S. Olthof, M. Furno, B. Lüssem, and K. Leo:Th. C. Highly efficient white organic lightemitting diodes based on fluorescent blue emitters. Nature Photonics. Submitted.
A14P. Freitag, S. Reineke, S. Olthof, M. Furno, B. Lüssem, and K. Leo:White top emitting organic lightemitting diodes with forward directed emission and high color quality. Organic Electronics. Submitted.
A15R. Meerheim, S. Olthof, M. Hermenau, S. Scholz, A. Petrich, B. Lüssem, M. Riede, and K. Leo:Investigation of C60F36 as nonvolatile pdopant for hole transport layers in smallmolecule organic optoelectronic devicesMate. Nature rials. Submitted.
Conference Contributions
9
C1S. Olthof, K. Fehse, K. Walzer, and K. Leo:Photoelectron spectroscopy of or th ganic semiconductor interfacesinternational Summer School, June 18 . OLLA th 25 2007, Krutyn (Poster).
C2S. Olthof, R. Meerheim, K. Walzer, and K. Leo:Measuring the energy level alignment at all interfaces in a complete OLEDJahrestagung der DPG,. 72. th th February 25 29 2008, Berlin (Talk).
C3S. Olthof, R. Meerheim, K. Walzer, and K. Leo:Measuring the energy level th alignment at all interfaces in a complete OLED. 7 International Conference on Electroluminescence of Molecular Materials and Related Phenomena, Septem nd th ber 2 6 2008, Dresden (Talk).
C4S. Olthof, R. Meerheim, and K. Leo:Experimental determination of energy level alignment at all interfaces in a complete OLED structureResearch. Material st th Society Fall Meeting, December 1 5 2008, Boston (Poster).
C5S. Olthof, B. Lüssem, and K. Leo:pdoping organic semiconductors: a study of varying doping concentration. 427. WEHeraeusSeminar on Molecular and th th Organic Electronics: Bridging the Gaps, January 26 29 2009, Physikzentrum Bad Honnef (Poster).
C6S. Olthof, B. Lüssem, and K. Leo:Investigation of the effects of doping concen tration in a pdoped organic semiconductor. 73. Jahrestagung der DPG,March th th 22 27 2009, Dresden (Poster).
C7S. Olthof, H. Kleemann, B. Lüssem, and K. Leo:Builtin potential of a pen tacene pin homojunction studied by ultraviolet photoelectron spectroscopy. Mate th th rial Research Society Spring Meeting, April 5 9 2010, San Fransisco (Poster).
1
Introduction
So far, mainly inorganic semiconducting materials such as silicon and gallium arsenide are employed to produce devices for optoelectronic applications like light emitting diodes or solar cells. However, it was already realized in the beginning of th the 20 century that organic materials like Anthracene can show a semiconducting behavior as well [1]. These first experiments suffered from a lack of material purity and therefore showed little reproducibility. The interest in this field only started to grow in the 50s, when fundamental research had answered the basic questions on inorganic semiconductors and organic material of higher purity became available. By that time, the electroluminescence was first observed by Beranose [2] when applying a high AC voltage to crystalline films of Acridine Orange and Carbazole. It took until the 80s for the first efficient optoelectronic devices to emerge that showed the promising possibilities of this new field. In 1985 the first efficient organic photovoltaic cell was presented at Eastman Kodak by Tang [3]. This device had a power conversion efficiency of 1% which was possible by the application of two different organic materials (Phthalocyanine and a Perylene derivative) in a heterojunction. Two years later, the same group published the first efficient twolayer organic light emitting diode device (OLED) [4] with an external quantum efficiency of about 1%, −2 achieving a luminance of 100 cd/m at a driving voltage of 5.5 V. Today, the research has split into two directions which either employ semicon ducting polymers that are applied by a spin cast process or small molecules deposited by vacuum evaporation. In our group at theInstitut für Angewandte Photophysik we focus on the second approach. These small molecule organic semiconductors are made of conjugated hydrocarbons combined with other low weight atoms and show an extendedπThey are available in form of a powder that is usuallyelectron system. stable under ambient conditions and is evaporated under UHV conditions to produce the semiconducting layers. In comparison to inorganic semiconductors, this new class of material offers promis ing avenues for practical applications due to novel physical properties. For inorganic semiconductors, epitaxially grown single crystals are needed that are complicated and costly to produce and need cleanroom conditions. Defects in the crystal structure or lattice mismatch between adjacent layers lead to dangling bonds that produce traps and optical recombination centers. In contrast, organic semiconductors have a low processing temperature and are produced by the evaporation of amorphous layers. There is no need for lattice match because of the close shelled conformation of mol