In the last 25 years, organic or "plastic" solar cells have gained commercial interest as a light-weight, flexible, colorful, and potentially low-cost technology for direct solar energy conversion into electrical power. Currently, organic solar cells with a maximum power conversion effciency (PCE) of 12% can compete with classical silicon technology under certain conditions. In particular, a variety of strongly absorbing organic molecules is available, enabling custom-built organic solar cells for versatile applications. In order to improve the PCE, the charge carrier mobility in organic thin films must be improved. The transport characterization of the relevant materials is usually done in neat layers for simplicity. However, the active layer of highly efficient organic solar cells comprises a bulk heterojunction (BHJ) of a donor and an acceptor component necessary for effective charge carrier generation from photo-generated excitons.
In the literature, the transport properties of such blend layers are hardly studied. In this work, the transport properties of typical BHJ layers are investigated using space-charge limited currents (SCLC), conductivity, impedance spectroscopy (IS), and thermally stimulated currents (TSC) in order to model the transport with numerical drift-diffusion simulations. Firstly, the influence of an exponential density of trap states on the thickness dependence of SCLCs in devices with Ohmic injection contacts is investigated by simulations. Then, the results are applied to SCLC and conductivity measurements of electron- and hole-only devices of ZnPc:C60 at different mixing ratios. Particularly, the field and charge carrier density dependence of the mobility is evaluated, suggesting that the hole transport is dominated by exponential tail states acting as trapping sites. For comparison, transport in DCV5T-Me33:C60, which shows better PCEs in solar cells, is shown not to be dominated by traps.
Furthermore, a temperature-dependent IS analysis of weakly p-doped ZnPc:C60 (1:1) blend reveals the energy-resolved distribution of occupied states, containing a Gaussian trap state as well as exponential tail states. The obtained results can be considered a basis for the characterization of trap states in organic solar cells. Moreover, the precise knowledge of the transport-relevant trap states is shown to facilitate modeling of complete devices, constituting a basis for predictive simulations of optimized device structures.:1 Introduction
2 Organic Semiconductors and Solar Cells
2.1 Structural, Optical, and Energetic Properties
2.2 Charge Carrier Transport
2.2.1 Classical Transport Models
2.2.2 Hopping and Tunneling Transport
2.2.3 Limitations of Transport Characterization
2.3 Doping
2.4 Single Carrier Devices
2.4.1 Theory of Space-Charge Limited Currents
2.4.2 Electrical Potential Mapping by Thickness Variation
2.4.3 Influence of the Contacts
2.5 Organic Solar Cells
2.5.1 Principles
2.5.2 The p-i-n Concept
2.5.3 Recombination
2.5.4 Electrical Characterization
3 Numerical Drift-Diffusion Simulations
3.1 Modeling Organic Semiconductors
3.2 System of Differential Equations
3.3 Simulation Algorithm and Modules
4 Exploiting Contact Diffusion Currents for Trap Characterization in Organic Semiconductors
4.1 Motivation
4.2 Drift-Diffusion Model
4.3 Results and Discussion
4.4 Conclusion
5 Transport Characterization of Donor-Acceptor Blend Layers
5.1 Motivation
5.2 Device Fabrication
5.3 Hole Transport in ZnPc:C60 Blends with Balanced Mixing Ratios
5.3.1 Current-Voltage Measurements
5.3.2 Drift-Diffusion Model
5.3.3 Modeling Results
5.3.4 Discussion
5.4 Hole Transport in Fullerene-Rich ZnPc:C60 Blends
5.4.1 Results and Discussion
5.5 Electron Transport in ZnPc:C60 (1:1)
5.5.1 Results and Discussion
5.6 Transport in Blend Layers with the High Efficiency Donor DCV2-5T-Me33
5.6.1 Hole Transport in DCV2-5T-Me33:C60
5.6.2 Electron Transport in DCV2-5T-Me33:C60
5.7 Conclusions for Transport in Blend Layers
6 Doping-Enabled Density of States Determination in Donor-Acceptor Blend Layers
6.1 Motivation
6.2 Theory
6.3 Methods
6.4 Results
6.4.1 Impedance Spectroscopy
6.4.2 Fermi level, Mott-Schottky Analysis, and Band Diagram
6.4.3 DOOS Determination
6.4.4 Thermally Stimulated Currents
6.4.5 Solar Cell Characteristics
6.5 Discussion
6.6 Conclusions on the DOS of ZnPc:C60 (1:1)
7 Conclusion and Outlook
Materials, Symbols, Abbreviations
Bibliography / Organische oder "Plastik"-Solarzellen haben in den letzten 25 Jahren eine rasante Entwicklung durchlaufen. Kommerziell sind sie vor allem wegen ihres geringen Gewichts, Biegsamkeit, Farbigkeit und potentiell geringen Herstellungskosten interessant, was zukünftig auf spezielle Anwendungen zugeschnittene Solarzellen ermöglichen wird. Die Leistungseffzienz von 12% ist dabei unter günstigen Bedingungen bereits mit klassischer Siliziumtechnologie konkurrenzfähig. Um die Effzienz weiter zu steigern und damit die Wirtschaftlichkeit zu erhöhen, muss vor allem die Ladungsträgerbeweglichkeit verbessert werden. In organischen Solarzellen werden typischerweise Donator-Akzeptor-Mischschichten verwendet, die für die effziente Generation freier Ladungsträger aus photo-induzierten Exzitonen verantwortlich sind. Obwohl solche Mischschichten typisch für organische Solarzellen sind, werden Transportuntersuchungen der relevanten Materialien der Einfachheit halber meist in ungemischten Schichten durchgeführt.
In der vorliegenden Arbeit wird der Ladungstransport in Donator-Akzeptor-Mischschichten mithilfe raumladungsbegrenzter Ströme (space-charge limited currents, SCLCs), Leitfähigkeit, Impedanzspektroskopie (IS) und thermisch-generierter Ströme (thermally stimulated currents, TSC) untersucht und mit numerischen Drift-Diffusions-Simulationen modelliert. Zunächst wird mittels Simulation der Einfluss exponentiell verteilter Fallenzustände auf das schichtdickenabhängige SCLC-Verhalten unipolarer Bauelemente mit Ohmschen Kontakten untersucht. Die Erkenntnisse werden dann auf Elektronen- und Lochtransport in ZnPc:C60-Mischschichten mit verschiedenen Mischverhältnissen angewendet. Dabei wird die Beweglichkeit als Funktion von elektrischem Feld und Ladungsträgerdichte dargestellt, um SCLC- und Leitfähigkeitsmessungen zu erklären, was mit einer exponentiellen Fallenverteilung gelingt.
Zum Vergleich werden dieselben Untersuchungen in DCV2-5T-Me33:C60, dem effizientesten der bekannten Solarzellenmaterialien dieser Art, wiederholt, ohne Anzeichen für fallendominierten Transport. Des weiteren werden erstmals schwach p-dotierte ZnPc:C60-Mischschichten mit temperaturabhängiger IS untersucht, um direkt die Dichte besetzter Lochfallenzustände zu bestimmen. Dabei werden wiederum exponentielle Fallenzustände sowie eine Gaußförmige Falle beobachtet. Insgesamt tragen die über Fallenzustände in Mischschichten gewonnenen Erkenntnisse zum Verständnis von Transportprozessen bei und bilden damit eine Grundlage für die systematische Identifizierung von Fallenzuständen in Solarzellen. Außerdem wird gezeigt, dass die genaue Beschreibung der transportrelevanten Fallenzustände die Modellierung von Bauelementen ermöglicht, auf deren Grundlage zukünftig optimierte Probenstrukturen vorhergesagt werden können.:1 Introduction
2 Organic Semiconductors and Solar Cells
2.1 Structural, Optical, and Energetic Properties
2.2 Charge Carrier Transport
2.2.1 Classical Transport Models
2.2.2 Hopping and Tunneling Transport
2.2.3 Limitations of Transport Characterization
2.3 Doping
2.4 Single Carrier Devices
2.4.1 Theory of Space-Charge Limited Currents
2.4.2 Electrical Potential Mapping by Thickness Variation
2.4.3 Influence of the Contacts
2.5 Organic Solar Cells
2.5.1 Principles
2.5.2 The p-i-n Concept
2.5.3 Recombination
2.5.4 Electrical Characterization
3 Numerical Drift-Diffusion Simulations
3.1 Modeling Organic Semiconductors
3.2 System of Differential Equations
3.3 Simulation Algorithm and Modules
4 Exploiting Contact Diffusion Currents for Trap Characterization in Organic Semiconductors
4.1 Motivation
4.2 Drift-Diffusion Model
4.3 Results and Discussion
4.4 Conclusion
5 Transport Characterization of Donor-Acceptor Blend Layers
5.1 Motivation
5.2 Device Fabrication
5.3 Hole Transport in ZnPc:C60 Blends with Balanced Mixing Ratios
5.3.1 Current-Voltage Measurements
5.3.2 Drift-Diffusion Model
5.3.3 Modeling Results
5.3.4 Discussion
5.4 Hole Transport in Fullerene-Rich ZnPc:C60 Blends
5.4.1 Results and Discussion
5.5 Electron Transport in ZnPc:C60 (1:1)
5.5.1 Results and Discussion
5.6 Transport in Blend Layers with the High Efficiency Donor DCV2-5T-Me33
5.6.1 Hole Transport in DCV2-5T-Me33:C60
5.6.2 Electron Transport in DCV2-5T-Me33:C60
5.7 Conclusions for Transport in Blend Layers
6 Doping-Enabled Density of States Determination in Donor-Acceptor Blend Layers
6.1 Motivation
6.2 Theory
6.3 Methods
6.4 Results
6.4.1 Impedance Spectroscopy
6.4.2 Fermi level, Mott-Schottky Analysis, and Band Diagram
6.4.3 DOOS Determination
6.4.4 Thermally Stimulated Currents
6.4.5 Solar Cell Characteristics
6.5 Discussion
6.6 Conclusions on the DOS of ZnPc:C60 (1:1)
7 Conclusion and Outlook
Materials, Symbols, Abbreviations
Bibliography
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:29001 |
| Date | 12 June 2015 |
| Creators | Fischer, Janine |
| Contributors | Leo, Karl, Blom, Paul, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | German |
| 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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