Chapter 1: Introduction
1.1.2. Different types of organic semiconductors
A semiconducting material is an insulator at very low temperature, but which has a sizeable electrical conductivity at room temperature.20 In band theory, the valence and conduction bands describe the state of electron mobility in an inorganic semiconductor material. The electronic properties of a semiconductor may be easily modified by introducing tiny amounts of impurity atoms. The process of adding impurities is called “doping”.21 Semiconductors doped with donor impurities are called as n-type, while those doped with acceptor impurities are known as p-type. On the other hand, organic semiconductor (OSC) is an organic material with semiconducting properties. They can be classified as monomers and polymers. The monomers are the small molecules whereas polymers are the long chain molecules with high molecular weight. The classification of organic semiconductors into n- and p-type is different from inorganic semiconductors. In organic semiconductors, HOMO and LUMO energy levels corresponds to the charge transport bands. The location of the HOMO and LUMO levels corresponding to the fermi level of the metal electrode determines, whether the material can easily transports electrons or holes. The maximum light absorption of the organic material is determined by energetic transitions from the LUMO to the HOMO level. The energy difference between the metal work function and HOMO/LUMO levels create barriers to charge injection/extraction at the metal-semiconductor junctions. The contact resistance of the device is the measure of the barrier height. When the LUMO level of the organic semiconductor close
Chapter 1 6
electrons as the majority carriers. When the HOMO level of the organic semiconductor close to the fermi level of the metal contact, then the material acts as p-type material with holes as the majority carriers are shown in Figure 1.3. Commonly used electrode materials are listed with their work functions in Table 1.1. Au is a popular metal for accessing the HOMO level of organic semiconductors while Al and Ca are used to access the LUMO levels. Lower work function metals such as Al or Ca, would better facilitate electron injection. It is difficult to work with due to oxidative instability under ambient.22 The stability of low work function metals can be increased by making them very thin (~ 10 nm) followed by higher work function metals.
Figure 1.3. In metals, the valance band and conduction band come very closer to each other and may even overlap.
Insulators have large band gap, ideally electrical conductivity of an electrical insulator is nil. In semiconductor the valance band and conduction band are separated by a forbidden gap of sufficient width. Representative energy level diagrams are shown to distinguish p- and n-type organic semiconductors through source – drain electrode band offsets with HOMO and LUMO levels.
Table 1.1: Common metal electrodes with their work functions.23 Metal Ca Al Cu Au Pd Ni Pt Work function (eV) 2.87 4.28 4.65 5.05 5.12 5.15 5.65
1.1.2.1. p- type Organic Semiconductors
The first study of an organic transistor was reported in 1987 by Koezuka and coworkers,24 and since then remarkable progress has been made in the design and synthesis of high-performance p-type semiconducting small molecules and polymers. Polycyclic aromatic hydrocarbons are the p-type organic π-functional materials. Particular attention has been paid to linear aromatic hydrocarbons composed of laterally fused benzene rings, called linear acenes or oligoacenes, because of their high stability and mobility.25 The HOMO energy level in linearly condensed n-acenes significantly increases with n, which facilitates the injection of holes at the interface between the source and semiconductor layer under an applied gate voltage (VGS). Furthermore, their planar shape facilitates crystal packing and enhances the intermolecular overlap of π- systems. Because of these features, anthracene, tetracene and pentacene are the most promising molecular semiconductors for OFETs. Anthracene is the smallest member of the acene series with reported transistor characteristics26 and pentacene has been shown to exhibit highest charge carrier hole mobility.27 The molecule structure of the pentacene and other molecules were shown in Figure 1.4.
Figure 1.4. Molecular structures of the some of the p-type organic semiconducting molecules used in the literature for the fabrication of p-type OFETs.
Phthalocyanines are porphyrin derivatives, which have attracted enormous interest recently
Chapter 1 8
they have also been employed as building blocks for the construction of new molecular materials for electronic and optoelectronic applications.28 These phthalocyanine are planar molecules consisting of four isoindole subunits linked together through nitrogen atoms.29 Rubrene has been well-studied material with high mobility because of its single crystalline nature. P3HT is the p-type polymer organic semiconductor used in the application of solar cells and OFETs because of its high stability and solution processability. The molecular structures of the some of the p-type organic semiconducting molecules are shown in Figure 1.4.
1.1.2.2. n-type Organic Semiconductors
Figure 1.5. Examples of the some of the n-type organic semiconducting molecules used in the literature for the fabrication of n-channel OFETs.
In n-type organic semiconductors electrons are the majority carriers. These are an important class of materials, due to their necessity in the fabrication of bipolar transistors and complementary logic circuits. However, most of the literature to-date has focused on the design and synthesis of p-type semiconductors. In order to lower the electron injection barrier between the LUMO level of the organic semiconductor with respect to the Fermi level of the metal electrodes, strong electron-withdrawing groups are often added to the outer rings of molecules.
This has been done successfully with several semiconductor core systems. These groups increase the electron affinity and stability of the molecule, allowing for the possibility of efficient electron injection and transport. Fullerenes (C60 and C60/C70) and fullerene derivatives (shown in Figure 1.5) are some of the first n-type organic materials studied.25
Naphthalene tetracarboxylic dianhydride (NTCDA), Perylene tetracarboxylic dianhydride (PTCDA) and their derivatives are easily synthesized from commercially available starting materials. These materials are unstable at ambient environments. Recently, perylene diimide derivatives were successfully synthesized, showing a reasonable charge carrier mobilities in ambient conditions.30-31 The energy levels of alkyl substituted diimides are similar to unsubstituted versions except an increased solubility. N,N'-diphenyl-3,4,9,10-perylene tetra- carboxylic diimide (PTCDI-Ph) belongs to the family of perylene bisimide, which were initially applied for industrial purposes as red dyes.32-36 TCNQ also well studied material for the fabrication of n-type OFETs and solar cell applications. Some of the n-type molecules are displayed in the Figure 1.5.
1.2. Charge Transport Mechanisms in Organic Semiconductors