CHARGE TRANSPORT IN NANOSCALE LATERAL AND VERTICAL ORGANIC SEMICONDUCTOR DEVICES Bojian Xu The research described within this thesis was carried out in the NanoElectronics Group at the MESA+ Institute for Nanotechnology at the University of Twente, Enschede, The Netherlands. The NWO-nano (STW) program, grant no. 11470, and the China Scholarship Council program, grant no. 201206090154 financially supported this research. Thesis committee members Chairman & secretary: Prof.dr. P.M.G. Apers University of Twente Promotor: Prof.dr.ir. W.G. van der Wiel University of Twente Other members: Prof.dr. P.A. Bobbert University of Twente Prof.dr.ir. G. Koster University of Twente Dr.ir. M.P. de Jong University of Twente Prof.dr. P.W.M. Blom Max Planck Institute for Polymer Research, Mainz Prof.dr. B.J. Ravoo University of Münster Title: Charge transport in nanoscale lateral and vertical organic semiconductor devices Author: Bojian Xu Cover design: Ximing Fu Copyright © 2017 by Bojian Xu, Enschede, The Netherlands. Printed by Gildeprint, Enschede, The Netherlands, 2017. ISBN: 978-90-365-4286-9 DOI: 10.3990/1.9789036542869 CHARGE TRANSPORT IN NANOSCALE LATERAL AND VERTICAL ORGANIC SEMICONDUCTOR DEVICES DISSERTATION to obtain the degree of doctor at the University of Twente, on the authority of the rector magnificus, prof.dr. T.T.M. Palstra, on account of the decision of the graduation committee, to be publicly defended on Friday March 10th 2017 at 14.45 by Bojian Xu born on March 14th, 1987 in Jiangsu, China This dissertation has been approved by: Promotor: Prof.dr.ir. W.G. van der Wiel This thesis is dedicated to my family. Contents 1 Introduction and motivation ......................................................................................... 1 1.1 Motivation ............................................................................................................ 1 1.2 Thesis outline ....................................................................................................... 2 References ......................................................................................................................... 4 2 Theoretical background ................................................................................................. 6 2.1 Basic concepts of electrical conduction ............................................................... 6 2.2 Introduction to organic electronics ...................................................................... 9 2.2.1 Molecular orbitals, energy bands/levels, band conduction and hopping conduction ..................................................................................................................... 9 2.2.2 Why organic materials? ................................................................................. 13 2.2.3 Subfields of organic electronics ..................................................................... 14 2.3 Organic field-effect transistors ........................................................................... 16 References ....................................................................................................................... 25 3 Nanoindentation with accurate positioning........................................................... 32 3.1 Introduction and motivation .............................................................................. 32 3.2 Nanoindentation ................................................................................................ 35 3.3 Device fabrication .............................................................................................. 37 3.4 Nanoindentation results and discussion ............................................................ 38 3.4.1 Nanoindentation using conventional working modes of the Veeco Dimension 3100 AFM ..................................................................................................................... 38 3.4.2 Nanoindentation with accurate positioning using the “point-and-shoot” function …………………………………………………………………………………………………………………..46 3.5 Summary and outlook ........................................................................................ 52 References ....................................................................................................................... 54 4 DXP lateral field-effect transistors ............................................................................ 56 4.1 Introduction and motivation .............................................................................. 56 4.2 Experiments and results ..................................................................................... 56 4.3 Summary and outlook ........................................................................................ 66 i References ....................................................................................................................... 67 5 Charge transport in nanoscale vertical P3HT pillar devices ............................ 69 5.1 Introduction and motivation .............................................................................. 69 5.2 Device fabrication .............................................................................................. 71 5.3 Electrical transport measurements .................................................................... 74 5.4 Simulation of the temperature dependence ..................................................... 81 5.5 Summary and outlook ........................................................................................ 86 References ....................................................................................................................... 87 6 Vertical P3HT field-effect transistors ....................................................................... 91 6.1 Introduction and motivation .............................................................................. 91 6.2 Device fabrication .............................................................................................. 91 6.3 Electrical transport measurements .................................................................... 94 6.4 ATLAS device simulations ................................................................................... 96 6.5 Summary and outlook ...................................................................................... 111 References ..................................................................................................................... 112 7 Conclusions and outlook ............................................................................................ 113 References ..................................................................................................................... 115 Summary ................................................................................................................................. 116 Samenvatting ......................................................................................................................... 119 Acknowledgements………………………………………………………………………………………..123 ii Chapter 1: Introduction and motivation Chapter 1 Introduction and motivation 1.1 Motivation Organic semiconductor materials are being intensely studied because of their extensive application in low-cost [1], flexible [2], biocompatible [3] electronic and spintronics [4] devices. Examples include organic field-effect transistors (OFETs) [5], organic light-emitting diodes (OLEDs) [6], and organic spin valves (OSVs) [7]. In this thesis research, we have investigated charge transport in organic semiconductor devices. Specifically, we focused on two organic materials, N, N’-bis(2,6-dimethylphenyl)-perylene-3,4,9,10-tetracarboxylic diimide (DXP) and poly(3-hexylthiophene) (P3HT), in different device configurations. DXP is a molecular organic semiconductor. It can be assembled into molecular wires when the molecules are inserted in the nanopores of zeolite L crystals [8, 9]. Electrical transport in the molecular wires is expected to have distinctive properties due to the spatial constriction of the molecular wires [10]. To investigate the electrical transport of the DXP- loaded zeolite L crystals in multiple experimental instruments, we planned to embed the crystals into devices. Hence, we designed a device fabrication process based on the nanoindentation technique using atomic force microscope (AFM). In the fabrication process, a critical step was to execute the nanoindentation precisely and only on top of the crystals with ~150 - 300 nm diameter. Therefore, it was necessary to carefully verify the positioning properties of the nanoindentation technique. Lateral field-effect transistors are suitable for investigation of the electrical transport properties in organic semiconductors [11]. In addition, one can integrate on-chip coplanar waveguides so that magnetic resonance experiments can be performed to study spin dynamics [12, 13]. By investigating lateral FETs with channel lengths similar to the lengths (sub-100 to hundreds of nm) of the molecular wires in the zeolites mentioned above, we could compare results between the lateral devices and the DXP-loaded crystals. Hence, apart from the DXP-molecular-wire devices, we also investigated the charge transport properties of the DXP lateral field-effect transistors with 100 nm channel length. There are more reasons to investigate OFETs with a short channel length. It has been reported that decreasing the channel length can increase the cut-off frequency of devices