In this thesis, methods for improving charge carrier mobility and deposition conditions for the solution shearing of organic semiconductors for organic field-effect transistors (OFETs) are investigated. Electrical performance for OFETs is currently still limited by the charge carrier mobility, especially when high fabrication speeds are required. In this work, adaptations are made to the solution shearing method to enhance charge carrier mobility values and to increase the deposition speed and film uniformity of semiconductor films. The solution shearing method can be easily adapted to large-scale roll-to-roll fabrication, a low-cost and high throughput fabrication process. In this work, the fabrication of OFETs with both crystalline small-molecule and donor-acceptor polymer semiconductors as the active layer is performed, and significant improvements in charge carrier mobility and film formation are achieved.
Specifically, the crystalline small-molecule semiconductor TIPS-pentacene is blended with the inert dielectric polystyrene, and solution shearing parameters are optimized to obtain highly-aligned crystalline films. The thin film with optimized morphology is deposited on a very thin polymer dielectric film, demonstrating the feasibility of high-performance OFETs (effective mobility of ~1.2 cm2 V-1s-1) and an ultra-low operating voltage (~1 V) – at the time a record value.
To improve crystal growth, the solution shearing method is modified to add vibrations to the liquid during the coating process. The new coating method, named “piezoshearing”, allows the application of vibrations to the liquid during deposition through the attachment of a piezo actuator to the shearing blade. The piezoshearing is implemented to enhance crystal growth during the solution shearing of crystalline materials, and tests of piezoshearing for the material 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) demonstrate that substrate coverage can be increased due to induced stick-and-slip caused by the piezoshearing.
Due to the unfavorable wetting conditions of semiconducting donor-acceptor polymer solutions on the commonly used low surface energy OFET substrates, conventional solution shearing is problematic. With piezoshearing, film deposition can be significantly improved. In particular, through piezoshearing the so-called stick-and-slip instabilities are mitigated, allowing the doubling of the shearing speed, and the deposition of smooth and ultrathin films (~7 nm). In addition to enabling higher coating speeds, piezoshearing also lowers the polymer material consumption by up to ~ 70% in comparison to the conventional solution shearing method. For some materials, piezoshearing is also found to increase the charge carrier mobility in OFET devices by up to two orders of magnitude.
The piezoshearing is utilized for viscous polymer solutions, which are challenging to coat, and usually, result in non-uniform films. Three donor-acceptor polymer systems were tested, and morphology changes are observed for all materials when piezoshearing is applied. For one of the polymeric solutions, an increase in crystallinity is achieved, possibly accompanied by a change in the degree of alignment of the polymer chains. For two other polymer solutions with higher molecular weight chains, very smooth films were obtained with the piezoshearing – saving 30% of material. Without the application of vibrations, such materials yield very non-uniform films, with significant thickness variations, which is unsuitable for OFET devices.
In summary, this work leads to significant improvements in the solution shearing of organic semiconductor materials by adding vibrations in the kHz range to the deposition process. The effects and benefits of utilizing the piezoshearing are demonstrated, and suggestions for further improvement and studies are made.:Contents 7
1.Introduction 11
Motivation 11
Outline 12
2.Theoretical Principles of Organic Electronic Materials and Devices 13
Organic Electronics 13
Organic Semiconductors 14
Charge Transport Mechanisms in Organic Semiconductors 16
Organic Field-effect Transistors 19
Operation 19
The Metal-Semiconductor Interface 22
The Dielectric 25
Film Morphology and Charge Transport in OFETs 27
Small Molecules 27
Semicrystalline Polymers 29
3.Solution Shearing and Control of Film Morphology 33
The Solution Shearing Method 34
Capillary Flow and the Pinned Contact Line. 36
Marangoni Flow 36
Shear Flow 37
Film Formation in Solution Shearing 38
Small Molecules 38
Polymers 43
Stick-and-slip Instabilities 50
Contact Angle Hysteresis and Stick-and-slip 52
Vibration-assisted Thin-film Solution Fabrication Methods 53
Effects on a Liquid stemming from Vibration 53
Relevant Characteristics 57
Vibrations and Thin-film Formation 58
Combining the Solution Shearing and Vibrations 61
4.Experimental Methods 63
Device Fabrication 63
Substrate Preparation 63
Electrode Evaporation . 65
Piezoshearing Setup 65
Thin-film Characterization 68
Cross-Polarized Optical Microscopy 68
Grazing Incidence Wide-Angle X-ray Scattering 71
Electrical characterization 77
Characterization 77
Mobility estimation and overestimation discussion 77
5.Alignment Improvement from Blending the Small molecule TIPS- pentacene with an inert Polymer 81
Introduction 81
Optimization of film morphology for TIPS-pentacene . 82
Device Fabrication 82
Electrical Characterization .. 83
Film morphology characterization 86
Fabrication of Ultra-low-voltage Operation Devices 96
Figure of Merit of this Study 97
6.Piezoshearing of Crystalline Materials 101
Introduction 101
Piezoshearing of Pristine TIPS-pentacene 102
Film Fabrication 102
Thin-film Characterization 102
TIPS-pentacene blended with PS in Toluene: Better Performing Devices 104
Piezoshearing of C8-BTBT 105
7.Addressing Stick-and-Slip Instabilities in solution-sheared films for Introduction 109
Device Fabrication 110
The Effect of Piezoshearing on Stick-and-Slip Instabilities 111
Increasing Shearing Speed 111
Thin-film Characterization 114
Electrical Characterization 116
Energy Barriers and Overcoming them with Vibration 119
Acceleration Threshold for Mitigating Stick-and-slip 122
8.Piezoshearing of Viscous Polymer Solutions 127
Introduction 127
Device Fabrication 128
DPP4DE-TT and Film Morphology 129
DPP6DO-TT, DPP6DO-T, and Faraday Instabilities 137
Thin-film Characterization 141
Piezoshearing as a Parametric Oscillator System 145
Solid Friction 146
Viscosity 146
Transition Between Regimes 147
9.Conclusion and Outlook 149
Conclusion 149
Outlook 150
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:71828 |
| Date | 02 September 2020 |
| Creators | Rocha, Cecilia Teixeira da |
| Contributors | Mannsfeld, Stefan C. B., Molina-Lopez, Francisco, Ellinger, Frank, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | 10.1021/acsami.9b07832, 10.1002/aelm.201800141, 10.1002/aelm.201800076 |
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