This work aims at improving the understanding of the fundamental physics behind molecular doping of organic semiconductors, being a requirement for efficient devices like organic light-emitting diodes (OLED) and organic photovoltaic cells (OPV). The underlying physics is studied by electrical conductivity and thermoelectrical Seebeck measurements and the influences of doping concentration and temperature are investigated. Thin doped layers are prepared in vacuum by thermal co-evaporation of host and dopant molecules and measured in-situ.
The fullerene C60, known for its high electron mobility, is chosen as host for five different n-dopants. Two strongly ionizing air-sensitive molecules (Cr2(hpp)4 and W2(hpp)4) and three air-stable precursor compounds (AOB, DMBI-POH and o-MeO-DMBI-I) which form the active dopants upon deposition are studied to compare their doping mechanism. High conductivities are achieved, with a maximum of 10.9 S/cm. Investigating the sample degradation by air-exposure, a method for regeneration is proposed, which allows for device processing steps under ambient conditions, greatly enhancing device fabrication possibilities.
Various material combinations for p-doping are compared to study the influence of the molecular energy levels of host (MeO-TPD and BF-DPB) and dopant (F6-TCNNQ and C60F36). Corrections for the only estimated literature values for the dopant levels are proposed. Furthermore, the model system of similar-sized host pentacene and dopant F4-TCNQ is studied and compared to theoretical predictions.
Finally, a model is developed that allows for estimating charge carrier mobility, density of free charge carriers, doping efficiency, as well as the transport level position from combining conductivity and Seebeck data.:1 Introduction
2 Fundamentals of Organic Semiconductors
2.1 Conventional Semiconductors
2.2 Organic Semiconductors
2.3 Seebeck Effect
2.4 Correlation of Seebeck Coefficient and Charge Carrier Density
3 Experimental
3.1 Seebeck Setup
3.2 Materials
4 Air-Sensitive n-Dopants in C60
4.1 Conductivity
4.2 Thermoelectric Measurements
4.3 Morphology
4.4 Degradation
4.5 Conclusion
5 Air-Stable n-Dopants in C60
5.1 Conductivity
5.2 Thermoelectric Measurements
5.3 Morphology
5.4 Conclusion for AOB and DMBI-POH
5.5 o-MeO-DMBI-I
6 p-Dopants in Amorphous Hosts
6.1 Conductivity
6.2 Thermoelectric Measurements
6.3 Degradation
6.4 Conclusion
7 Pentacene p-Doped by F4-TCNQ
7.1 Conductivity Changes after Preparation
7.2 Relation of Conductivity to Doping Concentration
7.3 Comparison of Seebeck Energy and Activation Energy
7.4 Conclusion
8 Estimating the Doping Efficiency and the Mobility
8.1 Lower Limit of the Mobility
8.2 Lower Limit of the Doping Efficiency
8.3 Conclusions from Seebeck Measurements
8.4 Assuming a Constant Transport Level
8.5 Applying the Models to p-Doped Data
8.6 Conclusion
9 Summary and Outlook
9.1 Summary
9.2 Outlook / Diese Arbeit untersucht organische Halbleiter und den Einfluss von molekularer Dotierung auf deren elektrische Eigenschaften, mit dem Ziel effizientere Bauelemente wie organische Leuchtdioden oder Solarzellen zu ermöglichen. Mittels Leitfähigkeitsuntersuchungen sowie thermoelektrischen Seebeck-Messungen werden die Einflüsse der Dotierkonzentration sowie der Temperatur auf die elektrischen Eigenschaften dünner dotierter Schichten analysiert. Das Abscheiden der Schichten durch Koverdampfen im Vakuum ermöglicht eine in-situ Analyse.
Das Fulleren C60, bekannt für besonders hohe Elektronenbeweglichkeit, wird als Wirt für fünf verschieden n-Dotanden, zwei extrem stark ionisierende luftreaktive (Cr2(hpp)4 und W2(hpp)4) sowie drei luftstabile (AOB, DMBI-POH und o-MeO-DMBI-I), verwendet. Dies ermöglicht Schlüsse auf die unterschiedlichen zugrunde liegenden Dotiermechanismen und das Erreichen von Leitfähigkeiten von bis zu 10.9 S/cm. Für einen der luftreaktiven Dotanden wird die Probendegradation an Luft untersucht und eine Regenerationsmethode aufgezeigt, die Prozessierungsschritte in Luft erlaubt und somit entscheidend für zukünftige Bauelementfertigung sein könnte.
Verschiedene p-dotierte Materialkombinationen werden untersucht, um den Einfluss der molekularen Energieniveaus von Wirt (MeO-TPD und BF-DPB) und Dotand (F6-TCNNQ und C60F36) auf die Dotierung zu studieren. Dies ermöglicht Schlussfolgerungen auf die in der Literatur bisher nur abgeschätzten Energieniveaus dieser Dotanden. Ferner werden die Eigenschaften des bereits theoretisch modellierten Paares Pentacen und F4-TCNQ mit den Vorhersagen verglichen und die Abweichungen diskutiert.
Abschießend wird ein Modell entwickelt, das die Abschätzung von Dotiereffizienz, Ladungsträgerkonzentration, Ladungsträgerbeweglichkeit sowie der Position des Transportniveaus aus Leitfähigkeits- und Seebeck-Messungen erlaubt.:1 Introduction
2 Fundamentals of Organic Semiconductors
2.1 Conventional Semiconductors
2.2 Organic Semiconductors
2.3 Seebeck Effect
2.4 Correlation of Seebeck Coefficient and Charge Carrier Density
3 Experimental
3.1 Seebeck Setup
3.2 Materials
4 Air-Sensitive n-Dopants in C60
4.1 Conductivity
4.2 Thermoelectric Measurements
4.3 Morphology
4.4 Degradation
4.5 Conclusion
5 Air-Stable n-Dopants in C60
5.1 Conductivity
5.2 Thermoelectric Measurements
5.3 Morphology
5.4 Conclusion for AOB and DMBI-POH
5.5 o-MeO-DMBI-I
6 p-Dopants in Amorphous Hosts
6.1 Conductivity
6.2 Thermoelectric Measurements
6.3 Degradation
6.4 Conclusion
7 Pentacene p-Doped by F4-TCNQ
7.1 Conductivity Changes after Preparation
7.2 Relation of Conductivity to Doping Concentration
7.3 Comparison of Seebeck Energy and Activation Energy
7.4 Conclusion
8 Estimating the Doping Efficiency and the Mobility
8.1 Lower Limit of the Mobility
8.2 Lower Limit of the Doping Efficiency
8.3 Conclusions from Seebeck Measurements
8.4 Assuming a Constant Transport Level
8.5 Applying the Models to p-Doped Data
8.6 Conclusion
9 Summary and Outlook
9.1 Summary
9.2 Outlook
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:27130 |
| Date | 19 July 2013 |
| Creators | Menke, Torben |
| Contributors | Leo, Karl, von Hauff, Elizabeth, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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