In this work, the capacitive response of organic electronic devices is analysed. Particular attention is given to small-molecule organic solar cells, with the purpose of deriving an equivalent circuit for the small-signal response of these devices. The different components characterising the solar cells electrical response are individuated and discussed and a specific physical meaning is associated with each element of the equivalent circuit.
In the experimental section, the capacitive elements of the equivalent circuit are characterised by analysing organic diodes and solar cells. It is found that the capacitance of an organic solar cell is a combination of four components: the dielectric response of the materials, the depletion regions formed at the interfaces, the accumulation of free and trapped charge carriers.
The depletion regions formed in organic doped semiconductors are characterised by analysing organic p/n homojunction diodes composed of Zinc-Phtalocyanine (ZnPc). The results demonstrate that the mechanisms involved in the formation of depletion zones in organic semiconductors can be described by the classical Mott-Schottky theory. This allows to estimate the free charge carrier density of doped layers with capacitance measurements. In addition, the current-voltage characteristics of organic p/n homojunctions are found not to obey the classical Shockley theory.
It is demonstrated that charge carrier tunnelling is the cause of this discrepancy and an analytic model is used to describe the current-voltage characteristics. The accumulation of free charge carriers is found to induce capacitance effects typical of relaxation semiconductors. In presence of unbalanced charge carriers injection, negative capacitance values are observed.
It is shown that in different organic semiconductor devices, the injection of minority charge carriers induces a depletion in the majority concentration, resulting in a negative value of the accumulation capacitance.
Finally, the capacitance associated to trap states in ZnPc:C60 organic solar cells is analysed. The spatial position and occupation mechanisms of the traps are estimated. The trapping mechanism in small-molecule organic solar cells is clarified and the energetic distribution of these trap states is estimated being a Gaussian function with 55 meV width, a density of 3.5 × 1016 cm−3 and centred 0.458 eV below the electron transport level. Trap states are also found to act as recombination centres, limiting the efficiency of organic solar cells.:1 Introduction
1.1 The energetic problem
1.2 The role of renewables
1.3 Photovoltaic energy
2 Basics
2.1 Organic Semiconductors
2.1.1 Molecular Orbital Theory
2.1.2 Conjugated Molecules
2.1.3 From Molecules to Solids
2.1.4 Charge Transport in Organic Semiconductors
2.1.5 Molecular Doping
2.2 Conventional Solar Cells
2.2.1 The Shockley Diode Equation
2.2.2 Solar Cells Characteristic
2.2.3 Non-idealities in Real Devices
2.3 Organic Solar Cells
2.3.1 Working principle of Organic Solar Cells
2.3.2 Exciton Dissociation and Charge Carriers Separation
2.3.3 Recombination in Organic Solar Cells
2.3.4 The p-i-n Concept
3 Impedance Spectroscopy
3.1 Introduction to Impedance Spectroscopy
3.1.1 Impedance Measurement Techniques
3.1.2 Impedance-Derived Functions
3.1.3 Graphical Representation of the Impedance
3.1.4 Fitting techniques for EIS Data
3.2 Impedance of Fundamental Circuits
3.2.1 Series RC circuit
3.2.2 Parallel RC circuit
3.2.3 The R(RC) circuit
3.3 Equivalent circuits for organic solar cells
3.3.1 The equivalent circuit of solar cells without defects
3.3.2 Equivalent circuits in presence of non-idealities
4 Organic p/n Homojunction Diodes
4.1 Introduction
4.2 Experimental
4.3 Asymmetrical Junction Diodes
4.3.1 n-doping of ZnPc
4.3.2 Capacitance Measurements
4.4 p/n Organic Diodes
4.4.1 Depletion Region Analysis
4.4.2 Current-Voltage Characteristic of Organic p/n Diodes
4.4.3 Tunnelling Current in Organic p/n Diodes
4.5 Conclusions
5 Negative Capacitance
5.1 Introduction
5.2 Experimental
5.3 Results and Discussion
5.3.1 Organic p/n homojunction diodes
5.3.2 Metal/ZnPc/Metal devices
5.3.3 Small-molecule organic solar cells
5.4 Conclusions
6 Trap States
6.1 Introduction
6.2 Experimental
6.3 Trap states impedance
6.3.2 Distributed element equivalent circuit
6.3.1 Physical model of the traps impedance
6.3.3 Steady-state simulations
6.4 Experimental Results
6.4.1 Variation of Active Layer Thickness
6.4.2 The Effect of Doped Transport Layers
6.4.3 Response to the Bias Voltage
6.4.4 Trap Response under Illumination
6.4.5 Impedance Spectra Fitting
6.5 Conclusions
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:28142 |
| Date | 19 December 2013 |
| Creators | Burtone, Lorenzo |
| Contributors | Ellinger, Frank, Leo, Karl, 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 |
Page generated in 0.0024 seconds