MSE 542 – Final Term Paper Title: Organic Semiconductor for Flexible Electronics Name: Chunhung Huang Introduction: An organic semiconductor is an organic compound that possesses similar properties to inorganic semiconductors with hole and electron conduction layer and a band gap.4 Organic semiconductors differ from other organic material in that the molecules that they are made of have π conjugate bonds which allow electrons to move via π-electron cloud overlaps. Conduction mechanisms for organic semiconductor are mainly through tunneling, hopping between localized states, mobility gaps, and phonon-assisted hopping.5 Like inorganic semiconductors, organic semiconductors can be doped in order to change its conductivity. Although inorganic semiconductors such as silicon, germanium and gallium arsenide have been the backbone of semiconductor industry, for the past decade, demands for pervasive computing have led to a dramatic improvement in the performance of organic semiconductor. Recently, organic semiconductors have been used as active elements in optoelectronic devices such as organic light emitting diodes (OLED), organic solar cells, and organic field effect transistors (OFET). There are many advantages of using organic semiconductors, such as easy fabrication, mechanical flexibility, and low cost. Organic semiconductors offer the ability to fabricate electronic device at lower temperature and over large areas on various flexible substrate such as plastic and paper. They can be processed using existing techniques used in semiconducting industry as well as in printing industries such as roll-to-rollmanufacturing.3 These manufacturing advantages can create low-cost, pervasive electronic applications such as flexible displays and RFID tags. However, the mobility of organic semiconductor cannot match the performance of field-effect transistors based on single-crystalline inorganic semiconductor such as silicon or germanium. These inorganic semiconductors have charge carrier mobilities nearly three order of magnitude higher than typical organic semiconductor. 3 As a result of this limitation, organic semiconductors are not suitable for use in electronic applications that require very high switching speeds. However, the performance of some organic semiconductors, coupled with their ease of processing makes it competitive in electronic applications that do not require high switching speed such as amorphous silicon used for TFT-LCD. Main Body: Organics semiconductors are possible because carbon atoms can form sp2-hybridisations where the sp2-orbitals form within a plane and the pz orbitals are in the plane perpendicular to it. 4 For organic molecules, a σ -bond between two carbons are formed by creating an orbital overlap of two sp2-orbitals (Figure 1). This creates a large energy difference between the occupied binding orbitals and the unoccupied anti-binding orbitals. This large energy difference leads to insulating properties, and thus longer chains of carbon atoms would have a larger gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). However, in sp2-hybridisation, the pz orbitals form additional π -bonds. These bonds have much smaller energetic difference between the HOMO and LUMO, leading to semiconducting properties.Organic semiconductors can be divided into two types, short chain (oligomers) and long chain (polymers).5Typical examples for semiconducting oligomers are pentacene, anthracene and rubrene. Some semiconducting polymers are Poly(3-hexylthiophene) and poly(p-phenylene vinylene). Short chain organic semiconductors are usually formed by a series of benzene rings in which the π -bonds become delocalized to form a π -system. The gap between occupied and empty states in these π systems becomes smaller with increasing delocalization, leading to smaller bandgap (Figure 2). Figure 1 Figure 2Organic semiconductors can be prepared as molecular single crystals by vacuum evaporation. This brings closer the coupling of the π systems and increases the band transport mobility. Long chain organic semiconductors are usually polymers that have π bonds that are delocalized along the chain to form a one-dimensional system resulting in a 1D-band structure that has considerable band width. The transport properties of such polymers are usually determined by defects in the 1D-chains or by hopping from chain to chain. Polymer organic semiconductors are usually deposited in wet processes, like spin coating or doctor blading. Delocalization in polymer semiconductor is accomplished by forming a conjugated backbone of continuous overlapping orbitals. This includes alternating single and double carbon-carbon bonds, which leaves a continuous path of overlapping p orbitals (figure 4). Figure 3In organic semiconducting polymer, this continuous string of orbitals creates degeneracy in the highest occupied and unoccupied orbitals and leads to the filled and unfilled bands that define a semiconductor. However, conductive polymers generally exhibit very low conductivities. This is because conduction in such relatively disordered materials are due to mobility gaps with phonon-assisted hopping, tunneling between localized states but not band gaps as in crystalline semiconductors.8 As mentioned before, organic semiconductor can be doped either by removing an electron from valence band or adding an electron to the conduction band to increase its conductivity. Doping organic semiconductor creates more charge carriers which move in an electric field. This movement of charge is responsible for electrical conductivity in organic semiconductor. Doping a polymer is different from that of inorganic semiconductor in which elements with excess and shortage of electrons are introduced. In polymer, both doping process involves an oxidation and reduction process. The first method involves exposing a polymer to an oxidant such as iodine or bromine or a reductant such as alkali metals.2 The second is electrochemical doping in which a polymer-coated electrode is suspended in an electrolyte solution. The polymer is Figure 4insoluble in the solution that contains separate counter and reference electrodes. By applying an electric potential difference between the electrodes, counter ion from the electrolyte diffuses into the polymer in the form of electron addition (n doping) or removal (p doping). 2 One of the problem with organic