A STUDY OF THE ELECTRICAL PROPERTIES OF POLYCRYSTALLINE BASED ORGANIC DEVICES Thesis submitted in accordance with the requirements of the University of Liverpool for the degree of Doctor in Philosophy by SIDRA AFZAL JUNE 2012 ii ABSTRACT A study of the Electrical Properties of Polycrystalline based Organic Devices Sidra Afzal In this thesis, examination of polycrystalline organic based devices such as the Schottky diode and MOS capacitor is carried out. The data is interpreted in terms of a polycrystalline model based mainly on a conventional polysilicon model with slight modifications to fit the organic properties. A brief introduction to the existing charge transport models for organic materials is presented. The most dominant being the variable range hopping model for disordered materials. The disorder analysis is appropriate in the grain boundaries of a polycrystalline material. The distribution of the traps of density of states (DOS) is commonly described by the Gaussian distribution and the associated exponential approximate at low energies. This is a valid assumption for organic semiconductors with low carrier mobility values [N. Sedghi et al., J. Non Crys. Solids 352, 1641, 2006]. Detailed investigation on the temperature effects of the polycrystalline Schottky diode leads to determination of important electrical parameters. Such studies are essential in understanding the conduction processes of the organic device, particularly in terms of trapping effects, which is essential in the development of device models for organic circuitry. Several parameters such as dopant (ND) and carrier concentrations (p), effective mobility (μeff), depletion width (Wdep), effective Debye length (LDe), Meyer Neldel energy (MNE) and the characteristic temperature of the carriers (T0) are extracted from the current-voltage characteristics of the diode. For a soluble derivative of pentacene, 6, 13- triisopropylsilyethynyl pentacene (TIPS) blended with Polytriarylamine (PTAA), the respective values extracted at room temperature are found to be approximately 1017 cm-3, 1.8x10-2 cm2V-1s-1, 185 nm, 11 nm, 31.5 meV and 780 K, respectively. As the temperature falls, the values of most parameters remain constant until a critical temperature. The activation energy also remains constant at approximately 0.3 eV for various applied voltages in saturation. Below this critical temperature, Wdep, LDe and T0 increase whilst μeff, ND/p and characteristic iii temperature of the states (TC) decrease. Similar analysis is carried out on doped layers of TIPS with a different insulating binder Poly-alpha methylstyrene (PAMS). The value of Wdep, LDe, T0, μeff, ND/p, MNE and TC obtained from such doped Schottky diode are approximately 100 nm, 5 nm, 1200 K, 1x10-2 cm2V-1s- 1, 2x1017 cm-3, 35 meV and 400 K, respectively. The Capacitance-Voltage (C-V) analysis on polycrystalline Schottky diodes provides ND of approximately 7.6 x 1016 cm-3, 5.2 x 1014 cm-3 and 2.98 x 1014cm-3 at 500 Hz, 1 kHz and 2 kHz respectively. These ND values are lower than those extracted from current- voltage characteristics and decrease with increasing frequency. This is thought to be due to the low mobility of holes, unable to respond to the signal at higher frequencies. The conduction in polycrystalline organic Schottky diode is proposed using a 2- dimensional (2D) model, which focuses on both the lateral and vertical conduction paths. The organic semiconductor layer is assumed to be relatively thin so that only a single layer of the grain exists between adjacent grain boundaries for the vertical conduction. A two dimensional situation is treated as two separate one dimensional problems that are positioned at right angles to each other. The grain and grain boundaries in the polycrystalline material are explained in terms of two boundary conditions. The variation of potential in grain boundary is the basis in defining the variation of potential in the grains. Conduction under forward bias in the grain and grain boundary is thus established assuming two distributions for the DOS, namely the Gaussian and Laplace. In comparison, Laplace DOS is believed to be a better representation of the distribution of states, where a large number of energy levels are being scanned with applied voltage. The ac properties of a polycrystalline based MOS capacitor are investigated. The frequency and temperature effects on the C-V characteristics of MOS capacitor based on another soluble derivative of pentacene, refered here as S1150, are studied. Equivalent circuits which include the effects of bulk and series resistance due to contact effects are analysed. The bulk resistance (Rb), bulk capacitance (Cb) and series resistance (RS) are found to be approximately iv 13 kΩ, 760 pF and less than 309 Ω respectively, for an organic film thickness 17 -3 (tOSC) of 27nm. For ND ≈ 3.6 x 10 cm at 1 kHz, the hole mobility is found to be approximately 4.6x10-7cm2V-1s-1. As expected the mobility decreases with increase in frequency. Furthermore, the temperature study of inverse square 2 space charge capacitance (1/CS ) against absolute temperature (T) provides an intercept close to T (~ 330K) instead of TC. A low intercept value indicates a decrease in disorder which suggests that the large grains may be dominating the capacitance. v CONTENTS ABSTRACT .......................................................................................... III CONTENTS .......................................................................................... VI ACKNOWLEDGEMENTS ..................................................................... XI ABBREVIATIONS ................................................................................ XII SYMBOLS .......................................................................................... XIII CHAPTER 1- INTRODUCTION .............................................................. 1 1.1 INTRODUCTION TO ORGANIC MATERIALS AND TECHNOLOGY ........ 2 1.2 THESIS ORGANISATION ................................................................................. 4 1.3 CONTRIBUTIONS ............................................................................................. 6 1.4 REFERENCES .................................................................................................... 8 CHAPTER 2- CHARGE TRANSPORT MECHANISMS IN ORGANIC MATERIALS .......................................................................................... 9 2.1 INTRODUCTION ............................................................................................. 10 2.2 ORGANIC MATERIALS ................................................................................. 10 2.2.1 Disordered Organic Semiconductor ............................................................... 11 2.2.2 Polycrystalline Organic Semiconductor ......................................................... 12 2.3 DISTRIBUTION FUNCTIONS FOR OCCUPANCY OF THE CARRIERS .. 13 2.4 MODELS FOR DENSITY OF STATES (DOS)................................................ 15 2.4.1 Gaussian Distribution ................................................................................... 15 2.4.2 Exponential Distribution of States ................................................................. 16 2.4.3 Laplace Distribution ..................................................................................... 18 2.4.4 Significance of Meyer Neldel Energy in DOS ............................................... 19 2.5 CHARGE TRANSPORT MECHANISMS IN ORGANICS ............................. 21 2.5.1 Miller Abraham Hopping Model ................................................................... 22 vi 2.5.2 Percolation Model ......................................................................................... 24 2.5.3 Polaron Model ............................................................................................... 26 2.5.4 Variable Range Hopping Model ..................................................................... 28 2.5.5 Multiple Trap and Release Model .................................................................. 30 2.5.6 Charge Transport in Polycrystalline Organic Semiconductors ......................... 31 2.6 SUMMARY ........................................................................................................ 36 2.7 REFERENCES ................................................................................................... 37 CHAPTER 3- ANALYSIS OF POLYCRYSTALLINE ORGANIC SCHOTTKY DIODES............................................................................41 3.1 INTRODUCTION .............................................................................................. 42 3.2 FUNDAMENTAL THEORY OF SCHOTTKY DIODES ................................. 43 3.2.1 Contact Barrier Height in Ordered Inorganic Semiconductors......................... 43 3.2.2 Schottky Effect .............................................................................................. 46 3.3 TIPS-PENTACENE BASED SCHOTTKY DIODES ........................................ 50 3.3.1 Fabrication of TIPS-Pentacene Schottky Diodes............................................. 50 3.3.2 Electrical Characteristics of TIPS-Pentacene Schottky ................................... 54 3.4 TEMPERATURE EFFECTS ON THE ELECTRICAL CHARACTERISTICS OF TIPS-PENTACENE SCHOTTKY DIODES ..................................................... 75 3.4.1 Experimental Details ..................................................................................... 75 3.4.2 Temperature Effects