Charge transport in polymeric field effect devices [Elektronische Ressource] / vorgelegt von Silviu-Cosmin Grecu

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• Physik

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• Publié par: universitat_augsburg
• Publié le: 01 janvier 2008
• Nombre de lectures: 23
• Langue: English
• Poids de l'ouvrage: 5 Mo

Extrait

Charge transport in polymeric
eld-e ect devices
Dissertation
zur Erlangung des Doktorgrades
der Mathematisch-Naturwissenschaftlichen Fakult at
der Universit at Augsburg
vorgelegt von
Silviu-Cosmin Grecu
Augsburg, 2008Erstkorrektor: Prof.Dr. Wolfgang Brutting
Zweitkorrektor: Prof.Dr. Christine Kuntscher
Tag der mundlic hen Prufung: 23. Januar 2009Contents
1 Theoretical Background 1
1.1 Charge transport in organic materials . . . . . . . . . . . . . . . . . . . . . 1
1.2 Metal-Insulator-Semiconductor Diode . . . . . . . . . . . . . . . . . . . . . 3
1.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2.2 Operation of MIS diode . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.3 Capacitance of the MIS diode . . . . . . . . . . . . . . . . . . . . . 9
1.2.4 The Real MIS diode . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.3 Organic Field-E ect Transistors . . . . . . . . . . . . . . . . . . . . . . . . 16
1.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.3.2 Principle of Operation . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.3.3 The threshold and switch-on voltage . . . . . . . . . . . . . . . . . 20
1.3.4 Field-E ect Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.3.5 Charge carrier density dependent mobility . . . . . . . . . . . . . . 21
1.4 Space Limited Diodes . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.4.1 Built-in voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.4.2 Current density in a space charge limitted diode . . . . . . . . . . . 24
1.4.3 In uence of traps on SCLC . . . . . . . . . . . . . . . . . . . . . . 25
1.4.4 Field dependent mobility . . . . . . . . . . . . . . . . . . . . . . . . 27
2 Experimental Methods 29
2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.2 Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.3 Experimental techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3 Analysis 43
3.1 The Organic MIS Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.1.2 The capacitance-frequency dependence . . . . . . . . . . . . . . . . 43
3.1.3 The capacitance-voltage dep . . . . . . . . . . . . . . . . . 46
3.2 Field-e ect transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.3 Space Charge Limited Diodes . . . . . . . . . . . . . . . . . . . . . . . . . 49
iii4 In uence of gate dielectrics 51
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.2 Melamin based insulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.3 PMMA as insulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.4 Al O as . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592 3
4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5 In uence of environmental parameters 65
5.1 In uence of air exposure on transport in P3HT . . . . . . . . . . . . . . . 65
5.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.1.2 In uence of air on the performance of MIS diodes . . . . . . . . . . 66
5.1.3 In uence of air on the performance of FETs . . . . . . . . . . . . . 67
5.2 In uence of light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.2.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6 In uence of preparation parameters in SiO eld-e ect devices 732
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.2 Self assembled monolayers on SiO . . . . . . . . . . . . . . . . . . . . . . 742
6.3 MIS diodes with self-assembled monolayers . . . . . . . . . . . . . . . . . . 76
6.4 OFET with self-assembled monolayers . . . . . . . . . . . . . . . . . . . . 79
6.5 In uence of annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
6.6 of solvent choice . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
6.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
7 Charge carrier density dependent mobility 89
7.1 Temperature dependent mobility . . . . . . . . . . . . . . . . . . . . . . . 89
7.2 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Summary 95
Zusammenfassung 98
Bibliography 100
Acknowledgements 104Scope and outline
In the last years organic semiconductor based devices have made the step from the lab-
oratory to the market in mobile telephones displays or RF-ID tags, becoming more and
more present in everyday life devices. The organic semiconductors can be separated in
two classes - small molecules and polymers. From a device point of view the di erence
between the two classes arises from the way the devices are built: the small molecules
have to be evaporated as they are usually insoluble while the polymers are processed from
solution. What makes the semiconducting polymers attractive for applications is the ease
of processability, meaning low costs and the possibility to build complex electronic devices
on exible substrates, which would lead to smaller, more compact and durable devices.
The range of applications is still quite limited as the semiconducting polymers are usually
sensitive to the ambient factors - oxygen humidity, UV-radiation. For this reason a lot of
research has been done in order to nd the right materials for encapsulation and protection
against radiation. This is not a trivial task as the protective layers should not interfere in
any way with the functionality of the device by chemical reactions or degradation in time.
Nonetheless materials satisfying these requirements have been found and organic devices
are reaching the market, as already mentioned, in the RF-ID tags but also in the displays
area.
The scope of this thesis is to investigate the in uence of preparation on the electrical proper-
ties of poly(3-hexyl)thiophene in di erent devices ( eld-e ect transistors, metal-insulator-
semiconductor diodes and hole-only diodes). Firstly the in uence of various organic and
inorganic insulators is analyzed. Another important issue is the modi cation of the sili-
con oxide surface energy through self-assembled monolayers, thus promoting a better self-
organization of the P3HT molecules during the lm formation. The last but not the least
important process which leads to an improvement of the electrical characteristics of FETs
and MIS diodes is the post spin-coating annealing. At the same time special attention
will be given to the in uence of external parameters as temperature, light and environ-
ment upon the operation of these devices. A comparison of our data with existing models
describing the charge transport will be attempted in the end.
vviChapter 1
Theoretical Background
1.1 Charge transport in organic materials
There are two di erent classes of organic materials which are used in the fabrication of
organic eld-e ect devices - the conjugated polymers and the small molecules. The prepa-
ration methods of the devices are di ering from one class to the other. In order to create
thin lms, the polymers are usually spin-coated from solution while the small molecules
are evaporated in high-vacuum. In general the structure of the polymeric thin lms is
amorphous while the small molecules can form highly crystalline lms. In some cases due
to the interaction between the polymeric chains, crystalline regions can form and thus the
morphology to these lms can be regarded as highly crystalline regions embedded in an
amorphous matrix, as shown in gure 1.1(b). The poly(3-hexyl-thiopehene) (P3HT), see
gure 1.1(a), is one of these polymers. In the crystalline regions the chains orientation is so
that they are parallel to the substrate while the side hexyl chains tend to be perpendicular
on it [1], [2]. The size and crystallinity of these regions depends on a number of parameters
which will be analyzed in chapter 6.
Figure 1.1: (a) Poly(3-hexyl-thiophene) chemical structure, (b) thin lm morphology and
(c) crystallite structure
1Charge transport in organic materials
The standard device used in measurements of the eld-e ect mobility of amorphous
materials is the eld-e ect transistor (FET), whose structure is schematically presented in
gure 1.2(a). In such a device the charge transport takes place between the source and
the drain thus in order to obtain a good performance, the semiconductor material has to
have a good conductivity in the direction parallel to the substrate. The P3HT, due to
the self-organization mentioned above, is a good polymeric candidate, as in the crystalline
regions the overlapping of the orbitals is high, thus a high charge carrier mobility being
promoted. Nonetheless, due the the mixed nature of the lms - small crystallites in an
2amorphous matrix, the overall mobility of P3HT lms is in the 0.01-1 cm /Vs, as the regions act like barriers for the charge carriers.
If one computes the charge distribution in the semiconductor material, by solving the
Poisson equation, it can be noticed that this is very high near the insulator surface and
decreases very fast in the bulk of the semiconductor, as it can be seen gure