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Charge Carrier Transport in Organic Field-Effect Devices Based On Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften (Dr. rer. nat.) Charge carrier transport in organic field-effect devices based on copper-phthalocyanine Michael Kraus M¨arz 2011 Arbeitsgruppe Organische Halbleiter Lehrstuhl f¨ur Experimentalphysik IV Institut f¨ur Physik Mathematisch-Naturwissenschaftliche Fakult¨at Universit¨at Augsburg Erstgutachter: Prof. Dr. WolfgangBr¨utting Zweitgutachter: Prof. Dr. AchimWixforth Tag der m¨undlichen Pr¨ufung: 06.06.2011 Contents 1 Motivation 1 2 Background 3 2.1 Introduction toorganic semiconductors . ... 3 2.2 Organicfield-effectdevices . 7 2.2.1 Metalinsulatorsemiconductordiodes . 7 2.2.2 Organicfield-effecttransistors . 9 2.3 Density functional theory calculations of molecular orbitals in CuPc 21 3 Data analysis 25 3.1 Determination of mobility and contact resistance in OFET devices . 25 3.1.1 Dataevaluationinthelinearregime. 25 3.1.2 Transmissionlinemethod . 27 3.1.3 Single-curveanalysis . 29 3.2 Determination of doping and mobility in MIS diodes . ... 30 3.2.1 ImpedanceSpectroscopy . 30 3.2.2 Gate voltage dependence of the capacitance . 31 3.2.3 Frequency dependence of the capacitance . 34 4 Materialsandexperimentaltechniques 39 4.1 Materials for organic field-effect devices . .... 39 4.1.1 Dielectricpassivationlayers . 39 4.1.2 Organicsemiconductors . 41 4.1.3 Organicmetals ......................... 44 4.2 Samplepreparation........................... 46 4.2.1 Fabricationoftransistors. 46 4.2.2 FabricationofMISdiodes . 50 4.3 Measurementtechniques . 51 i ii Contents 5 Unipolar p-typetransistorsbasedonCuPc 57 5.1 Morphology ............................... 57 5.1.1 AFMmeasurements ...................... 57 5.1.2 XRDmeasurements. 59 5.2 Transistorcharacteristics . 60 5.2.1 Measurements.......................... 60 5.2.2 Discussion............................ 62 5.3 Summary ................................ 65 6 Ambipolar transistors with polymeric passivation layer 67 6.1 Ambipolarfield-effecttransistors . 67 6.1.1 Conditionsforambipolartransport . 67 6.1.2 RealizationofambipolarOFETs . 69 6.1.3 Preliminaryworkofourgroup. 70 6.2 Single-component CuPc transistors with PMMA . .. 72 6.2.1 Morphology ........................... 73 6.2.2 Ambipolarcharacteristics . 74 6.2.3 Control of charge carrier type by contact modification .... 77 6.2.4 Influenceofoxygendoping . 83 6.3 Summary ................................ 85 7 Ambipolar transistors with aliphatic TTC passivation layer 87 7.1 Morphology ............................... 88 7.1.1 MorphologyofTTCpassivationlayers . 88 7.1.2 GrowthofCuPconTTC. 92 7.2 Transistorcharacteristics . 95 7.2.1 ComparisontoPMMAaspassivationlayer . 95 7.2.2 Influence of the TTC thickness on the device performance . 97 7.3 Reductionofcontactresistance . 100 7.3.1 Measurements. 100 7.3.2 Discussion............................ 103 7.4 Summary ................................ 105 8 Balanced charge carrier mobilities with annealed TTC layers 107 8.1 Morphology ............................... 107 8.1.1 Morphology of annealed TTC passivation layers . 107 8.1.2 GrowthofCuPconannealedTTC . 111 8.2 Transistorcharacteristics . 115 8.2.1 Measurements. 115 8.2.2 Discussion............................ 116 Contents iii 8.3 Summary ................................ 119 9 Ambipolar MIS diodes 121 9.1 MISdiodeswithPMMApassivationlayer . 121 9.1.1 C (V )measurements . .. .. ... .. .. .. .. .. ... 121 9.1.2 C (f )measurements. 124 9.2 MISdiodeswithTTCpassivationlayer . 128 9.3 Summary ................................ 130 10Modelingofchargecarriertransport 131 10.1 Models for charge carrier transport in polycrystalline organic films . 131 10.1.1 Grain boundary trap density model by Levinson . 131 10.1.2 PolycrystallinemodelbyHorowitz. 133 10.1.3 AnalysisbyLang . 133 10.2 Simulationoftransistorcurves . 134 10.3 Fitsofexperimentaldata. 136 10.3.1 FitswithLevinsonmodel . 136 10.3.2 FitswithHorowitzmodel . 139 10.3.3 Determination of the density of trap states . 140 10.4Summary ................................ 143 11 Organic Inverters 145 11.1 Workingprinciples . 145 11.2Experiments............................... 148 11.2.1 Ambipolarinverters. 148 11.2.2 Complementaryinverters. 150 11.3Summary ................................ 151 12 Unipolar n-type transistors based on F16CuPc 153 12.1Morphology ............................... 153 12.2 Transistormeasurements . 154 12.2.1 Unipolar n-typeOFETs .................... 154 12.2.2 Dopingexperiments. 157 12.3Summary ................................ 159 13 Conclusion and outlook 161 Bibliography 165 List of publications 179 Curriculum vitae 181 Chapter 1 Motivation Every day’s presence of electronic devices has been increasing drastically for the last two or three decades. Fighting against saturation of markets, one of today’s major challenges for industry is the development of new fields of application—the so-called “ubiquitous electronics”. Examples are large-area devices or flexible con- structions for displays, photovoltaic cells or light sources. This cannot be achieved easily with classical inorganic semiconductors like silicon, germanium or compound semiconductors like gallium arsenide or gallium nitride as active materials although some success has been reported on flexible thin-film transistors based on amorphous oxides [1]. Thus, a new class of electronics, which is very promising for these kinds of applications, is under intensive research: organic electronics. Herein, organic materials that exhibit semiconducting properties are used instead of inorganic semi- conductors. For the last couple of years tremendous progress has been achieved in this field of research. Three types of devices are especially promising: organic light-emitting diodes (OLEDs), organic photovoltaic cells (OPVCs) and organic field-effect transistors (OFETs). Out of these three classes, OLEDs and OPVCs are already marketable. One of the outstanding features of OLEDs is the possibility to fabricate large area light sources in contrast to the point-shaped inorganic light- emitting diodes [2]. OPVCs play an important role in research at the moment and efficiencies up to 8.3% have been achieved recently [3,4]. Field-effect transistors (FETs) are crucial for the realization of logic circuits, e.g. in computers or displays. The first OFET was reported in 1984 [5]. Since this publication the performance of OFETs has increased dramatically and nowadays performance data comparable to amorphous silicon FETs are reported [6]. OFETs are considered to be promising 1 2 1 Motivation candidates for the realization of flexible displays (E-paper) [7] or radio-frequency identification (RFID) tags used as passive electronic devices for the contact-free identification of products or objects [8,9]. Generally, there are two classes of materials used for organic electronics: polymeric and molecular materials (often referred to as “small molecule semiconductors”) [6]. Although these materials are semiconductors, the physical properties are different to their inorganic counterparts and not yet understood in all details. Thus, research concerning investigation of physical processes leading to charge carrier transport in these materials is still carried out. OFETs are known to be a powerful tool for fundamental analysis [10]. This work concentrates on a typical feature of many molecular organic semiconductors: am- bipolar charge carrier transport. Ambipolar transport is the transport of positive and negative charge carriers (i.e. holes and electrons, respectively) in the active material at the same time. This effect has also been observed in inorganic semi- conductors, but has not yet been applied technologically. Possible applications for ambipolar OFETs are light-emitting OFETs (LEOFETs) [11,12] or complemen- tary logic circuits where both the p- and the n- channel are based on the same substrate [13]. In the 1970s and 1980s, it has been demonstrated by time-of-flight measurements on highly pure organic semiconductor crystals that ambipolar charge carrier transport in organic semiconductors is possible and that there is no fun- damental difference between the transport of electrons and holes [14]. However, when organic semiconductors have been applied to thin-film devices in the late 1980s and the 1990s, they have usually been classified as either p-type or n-type. Nowadays it is known that charge carrier traps or injection barriers are possible reasons for the suppression of one charge carrier type. Today, highly pure materials are available and ambipolar charge carrier transport can be observed in numerous organic semiconductors. Nevertheless, there is a pronounced asymmetry between both charge carrier types in most cases, which is counterproductive for the real- ization of high-performance devices. In the scope of this thesis it can be seen that this asymmetry is affected strongly by various parameters, e.g. substrate material, substrate treatment or morphology of the semiconductor. This thesis gives a systematic study of ambipolar charge carrier transport in field- effect devices based on the organic semiconductor copper-phthalocyanine. Chapter 2 Background The following chapter provides a short introduction into organic semiconductors. Thereafter, the fundamental basics of field-effect devices will be explained by means of a metal-insulator-semiconductor diode. Finally, the working principles of organic field-effect transistors will be discussed with the help of a model for ambipolar charge carrier transport. 2.1 Introduction to organic semiconductors As already mentioned in the introduction there are
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