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Organic Electronic, Fall 2005, Dr. Dietmar Knipp Introduction to Organic Electronics (Nanomolecular Science Seminar I) Information: Information: http://www.faculty.iuhttp://www.faculty.iubremen.de/course/c30 bremen.de/course/c42 0331a/ 0411/ (Course Number 420411 ) Fall 2005 Organic materials and electronic Transport Instructor: Dr. Dietmar Knipp Source: Apple VD Source h eh eh eh eh eheheheh h Drain Ref.: Apple Gate Dielectric VG 10-9 10-8 10-7 10-6 10-5 10-4 Neutral substrate 10-3 10-2 10-1 1 101 Critical dimension (m) Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp Introduction to Organic Electronics 2 Structural properties and Electronic Transport 2.1 General properties of organic and polymeric semiconducting material 2.2 Organic and polymeric semiconducting materials 2.3 Organic molecules 2.4 From a single molecule to a solid 2.5 Bandgap in organic solids 2.6 Structural order of materials 2.7 The unit cell 2.8 Structural order in molecular solids 2.9 Electronic Transport 2.9.1 Thermal movement of carriers 2.9.2 Band-like transport 2.9.3 Grain boundaries in polycrystalline material 2.9.4 Trap-controlled transport 2.9.5 Hopping transport References Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.1 General properties of organic and polymeric semiconducting materials Advantages: Tetracene •Low cost processing •Large area compatible •Low temperature processing •Tailoring of electronic and optical properties •Certain properties not easily attainable with conventional materials Structural properties and electronic transport Disadvantages: •Low carrier mobility •Stability •Patterning of films •Novel fabrication technology required Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.2 Organic and polymeric semiconducting materials Two general classes of materials exist: Organic molecules Polymers Poly(3-hexyl thiophene) (P3HT) Antracene Tetracene S Pentacene S C8 N N N N M N C8 poly(9,9-dioctylfluorene-cobithiophene) (F8T2) N N N Phthalocyanine Perylene XPT: regio-regular poly(thiophene) Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.3 Organic Molecules Hydrocarbons, the simplest organic molecules, contain only carbon and hydrogen atoms. They can be subdivided in Alkanes, Alkenes, Alkynes, Arenes. Arenes and Aromaticity Arenes are hydrocarbons based on benzene units. The simplest, yet the most important compound in this class of organic compounds is benzene. "Aromatic" was originally used to describe these compounds since many have pleasent smells. To the chemist, the word aromatic also carries with it stability and reactivity implications. The unusual stability of benzene compared to closely related alkenes is what makes it important and gives benzene its own set of characteristic reactions. Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.3 Organic Molecules Polyaromatic Hydrocarbons Larger systems of benzene rings fused together are known. These are the polyaromatic hydrocarbons. A collection of images of some common systems are shown. Chemical stability of these molecules decreases as the size of the molecule increases. (e.x. pentacene and hexacene oxidize readily in air, while benzene, naphtalene, and anthracene are stable in absence of light). Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.3 Organic Molecules Energy distribution of Benzene pi molecular orbitals for benzene. With 6 C atoms contributing to the p system, we need to create 6 molecular orbitals. Ref.: I. Hunt, University of Calgary π-electron overlap between adjacent carbon atoms: leads to delocalization Within a single molecule there is very good electronic overlap Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.4 From a single molecule to a solid • Orbital overlap – The extent of p-orbital overlap between adjacent molecules – Depends on the direction (in 3-D) – Extent of orbital overlap determines bandgap • Bandgap – The “gap” or distance between the min. and max points of a band. – Typical bandgaps are in the range of 1.5 to 5 eV • Structural order in the material – The structural order of the material is closely related to the electronic properties of the material. (This even applies to polymers.) Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.5 Bandgap in organic solids Electronic states given rise to valence (HOMO level) and conduction bands (LUMO level). The bands are shown for a series of materials from benzene to pentacene. The dashed line corresponds to the Fermi level. The electronic states are given for the gas phase and a solid. Ref.: N. Karl, University Stuttgart Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.5 Structural order of materials Amorphous materials No long-range order Poly crystalline materials Completely ordered in segments (Mono)Crystalline materials Entirely ordered solid The structural properties of organic solid depends on the molecule itself, its electrical structure, the substrate and the growth conditions (temperature, deposition rate, flow of material) or preparation conditions. Ref.: R.F. Pierret, Semiconductor Fundamentals Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.7 The unit cell The periodic arrangement of atoms is called lattice! A unit cell of a material represents the entire lattice. By repeating the unit cell throughout the crystal, one can generate the entire lattice. A unit cell can be characterized by a vector R, where a, b and c are vectors and m, n and p are integers, so that each point of a lattice can be found. Primitive unit cell. R=ma+nb+pc The vectors a, b, and c are called the lattice constants. Ref.: M.S. Sze, Semiconductor Devices Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.7 The unit cell Different unit cells based on cubic unit cells Simple cubic unit cell Body centered cubic unit cell Face centered cubic unit cell (bcc) (fcc) Ref.: M.S. Sze, Semiconductor Devices Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.7 The unit cells Ref.: Joseph R. Smyth, Geology 3010: Introduction to Mineralogy Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.8 Structural order in molecular solids Organic materials can form very highly ordered crystals Van der Waals forces keep these crystals together. These crystals can have a band structure just like any other semiconductor if the crystals are highly order and the concentration of impurities is very low. Structural properties and electronic transport Antracene single crystal (Ref.: University Stuttgart, N. Karl). Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.8 Structural order in molecular solids γ c Pentacene, C22H14 : a α Aromatic hydrocarbons based on linear arranged benzene rings b β Crystal structure: Triclinic: a ≠ b ≠ c, α ≠ β ≠ γ ≠ 90° Electronic transport due to an overlap of π−orbitals Thermal Evaporation: Source temperature: 275300°C Substrate view Structural properties and electronic transport Material: 0, 1 or 2 times sublimation purified Substrate temperature: rt-110°C Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.8 Structural order in molecular solids Substrate temperature Pentacene on thermal oxide 2.5µm Substrate at rt 2.5µm Substrate at 70°C 2.5µm Substrate at 90°C Crystal size Atomic force micrographs of thermally evaporated pentacene films (200nm). Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.8 Structural order in molecular solids X-ray diffraction pattern of pentacene on thermal oxide Diffraction intensity [a.u.] 90 75 60 (002) 11.46° 16° (001) 5.743° (003) 17.22° 45 30 15 0 10 20 2 Θ scan Substrate view The pentacene film was prepared at room temperature. Structural properties and electronic transport 30 Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.8 Structural order in molecular solids Ratio of x-ray diffraction (001)/(001‘) X-ray diffraction pattern of pentacene on thermal oxide 1 room temperature 70°C 90°C Relation between the average crystal size and the ratio of the diffraction peaks. 0.1 0.01 0.1 1 average crystal size [µm] Structural properties and electronic transport 10 Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.9 Electronic Transport 0 Mobility [cm2/Vs] 10 Low mobility materials and its applications - Photoconductors - Organic LEDs Band-like -2 10 -4 10 Hopping transport Grain boundaries or trap-controlled transport transport Disorder - Transistors High mobility materials and its appications -6 10 Structural Order The structural order of the material is closely related to the electronic properties of the material. Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.9 Electronic Transport Electrons in the conduction band and holes in the valence band are able to move upon thermal activation, a gradient or an applied electric field. In the following the concepts of electronic transport in crystalline materials will be described. 2.9.1 Thermal movement of carriers Electrons in the conduction or holes in the valence band can essentially be treated as free carriers or free particles. Even in the absence of an electric field the carriers follow a thermally activated random motion. In thermal equilibrium the average thermal energy of a particle (electron or hole) can be obtained from the theorem for equipartition of thermal Eaverage = 3 kT 2 Average thermal energy of an electron / hole The thermal energy of the particle is equal to the kinetic energy of the electron, so that the velocity of the particle can be calculated. The mass of the electron is equal to the effective mass of the electron. Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.9.1 Thermal movement of carriers Furthermore, the velocity of the electron corresponds to the thermal velocity of the electron, so that the thermal velocity can be determined by: Ekin = 1 meff vth2 2 Kinetic energy of an electron / hole At room temperature the average thermal velocity of an electron is about 105m/s in silicon and GaAs. 3kT vth = meff Thermal velocity of an electron Thermal motion of free carriers can be seen as random collision (scattering) of the free carriers with the crystal lattice. A random motion of an electron or hole leads to zero net displacement of the free carrier over a sufficient long distance / period of time. The average distance between two collisions within the crystal lattice is called mean free path. Associated to the mean free path we can introduce a mean free time τ. A typical mean free path is in the range of 100nm and the mean free time is in the range of 1ps. Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.9.2 Band-like transport When a small electric field is applied to the semiconductor material each free carrier will experience an electro static force Force = − qF So that the carrier is accelerated along the field (in opposite direction of the field). F=0 F Schematic path of an electron in a semiconductor (a) random thermal motion, (b) combined motion due to random thermal motion and an applied electric field. Ref.: M.S. Sze, Semiconductor Devices Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.9.2 Band-like transport An additional velocity component will be superimposed upon the thermal motion of the electron. The additional velocity is caused by an applied electric field F. The additional component is called drift velocity. The drift of the electrons can be described by a steady state motion since the gained momentum is lost due to collisions of the electrons and the lattice. P = mn vn P = − q ⋅ F ⋅τ C Based on momentum conservation the drift velocity can be calculated. The drift velocity is proportional to the applied electric field F. vn = − qτ C F mn Structural properties and electronic transport Electron drift velocity Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.9.2 Band-like transport q ⋅τ C µn ≡ mn q ⋅τV µp ≡ mp Electron and hole mobility The mobility is an important electronic transport parameter. The mobility directly related a the material properties. Rewriting of the expression for the drift velocity leads to v µn = n F µp = vp F Electron and hole mobility The mobility is directly related to the mean free time between two collisions, which is determined by various scattering mechanisms. The most important scattering mechanisms are lattice scattering and impurity scattering. Lattice scattering is caused by thermal vibrations of the lattice atoms at any temperature above 0K. Due to the vibrations energy can be transferred from the carriers and the lattice. Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.9.2 Band-like transport Carriers can move from one molecule to the next molecule. Quantified by mobility Mean free path > intermolecular spacing Temperature dependent behavior: µ (T ) ∝ T − 3 2 Ref.: N. Karl, University Stuttgart Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.9.2 Band-like transport 10 2 Influence of traps on the electronic transport: µ tf N traps N bulk E exp traps kT Exponential Decrease of µ Et ~ 40 - 50 meV Dependence on Trap Density Nt Nt ~ 1016 - 1018 cm-3 Trap-Free Limit : Power Law T-n n ~ 1.6 - 2.3, phonon scattering 10 1 10 0 2 1+ Mobility (cm /Vs) µ 0 (T ) µ eff (T ) = 10 -1 10 -2 10 -3 20 Ref.: Dodabalapur, University Texas, lecture notes EE 396K Structural properties and electronic transport increasing Nt 100 Temperature (K) 300 Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.9.3 Grain boundaries in polycrystalline material 1 EB Pentacene thin film transistor EC Ei EF mobility [cm2/Vs ] NT EV 0.1 L −1 µ −1 = µ 0−1 + µ GB 2-5µm 0.01 0.001 100 100-200nm The mobility decreases with decreasing characterization temperature. Exp. fit Temperature dependent mobility can be explained by a barrier model. 300 400 Smaller crystals leads to higher grain boundary traps density. 200 temperature [K] Structural properties and electronic transport EB kT µ GB = µ GB 0 ⋅ exp − Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.9.4 Trap-controlled transport Mobility edge (E = 0) E =0 DOS, g(E) Energy (Activation) energy Extended states Band-tail states Localized states Deep traps Trapping and release of charges µeff =µc - carrier mobility in extended states Density-of-states distribution Ref.: V. Arkhipov, IMEC Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp 2.9.5 Hoping transport Energy Jumps over-barriers dominate at higher temperatures At lower temperatures tunneling of carriers take over Energy Most hopping models assume: -positions of hopping sites are completely random - positions and energies of hoping sites are uncorrelated Ref.: V. Arkhipov, IMEC Structural properties and electronic transport Organic Electronic, Fall 2005, Dr. Dietmar Knipp References Pope and Swenburg, Electronic Processes in organic crystals and polymers, 2 nd Ed., Oxford Organic molecular crystals, E.A. Sininsh EA and V. Capek. http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/6973Organic-OptoelectronicsSpring2003/CourseHome/ (Organic optoelectronic lecture MIT) Structural properties and electronic transport