Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Polymer electronics Polymer Electronics (A tutorial) Polymer: A chemical compound or mixture of compounds formed by polymerization and consisting essentially of repeating structural units Electronics: A branch of physics that deals with the emission, behavior, and effects of electrons (as in electron tubes and transistors) and with electronic devices In constructing a polymer electronic device we are fundamentally interested controlling the flow of energy Where and how it is absorbed Where and how it flows Where and how it is emitted Also: How the device interacts with the environment around it There are now many different types of polymer based electronics: Transistors, Light Emitting Diodes, Photovoltaic, Sensors are just a few examples. To fully understand and appreciate the difficulties with polymer based electrons we first need to review more conventional device technologies (i.e., metals, oxides and inorganic semiconductors). General electrical properties Polyacetylene (PA) or (CH)x is chemically the simplest (as synthesized) (after thermal conversion) A semiconductor in which chain conformation (structure) impacts band gap Conducting polymers behave as semiconductors Conductivity (Siemens/meter) Poly(p-phenylene vinylene) (A “highly” crystalline polymer host) a Silver Polyacetylene (After doping!) Temperature (K) Even when doped to a highly conductive state most p-conjugated polymers behave as classic semiconductors (VRH-variable range hoping is the standard proposed mechanism) + Applications: FET transistors (no doping) C. D. Dimitrakopoulos et al., Science 283, p. 822 (1999) Copyright © 1999 Lucent Technologies. Mobility (cm2/V-s) 10 Plots of ID (drain) versus VG (gate) and (ID)1/2 versus VG (gate) (Inset) Schematic diagram of organic IGFET. Annual gains in room temperature mobilities 1 10-1 -2 10 10-3 10-4 10-5 '86 '88 '90 '92 Year '94 '96 Conventional Semiconductors: The top down approach A FET (field effect transistor) •Single crystal substrates (e.g., Si, GaAs) •Expensive processing facilities (Billion dollar fabrication plants) •Generally invokes a series of process steps in which a substrate is coated with a photoresist, masked off, developed, etched, vapor doped, and so on ad infinitum. •There are a very limited number of device architectures which have been highly optimized •Despite the complexity silicon based devices are good and aren’t going away anytime soon. a) Top-contact device, with source and drain electrodes evaporated onto organic layer. •Conventional devices configurations are applicable to p-conjugated polymers b) Bottom-contact device with organic deposited onto source and drain. Conventional Semiconductors at the atomic level n-type doping p-type Si doping Phosphorous has 5 valence electrons Si Si Si An unbonded electron Al Si Si eP+ CB Egap + Si Si Si Edonor + 0 Al - CB VB Si CB CB ef Edonor Si hole in valence band VB At room temperature hole is delocalized in valence band (VB) Band structure is essentially Eacceptor rigid Mobility is everything ef VB + (hole in valence band) E acceptor 0 + At room temperature electron is delocalized in conduction band (CB) Si VB When charge is moving the key word is mobility, mobility, mobility (cm2 V–1 s -1) Device characteristics for a DH6T OTFT 1986-2000 having an aluminum gate electrode, a 3700 Å vapor-deposited parylene-C gate insulating layer, and gold source and drain electrodes with L=137 mm and W=1.5 mm:(a) and (b). Drain current ID versus drain voltage VD for a range of gate voltage values VG plotted (a) linearly and (b) semi-logarithmically; (c) ID versus VG at VD= 2 V. Fitting this data yielded a linear regime Current mobilities are good enough for many device mobility of 0.122 cm2 Vapplications but critical technology related 1issues s-1. still remain. Device costs are potentially very,very low ($0.05). From C.D. Dimitrakopoulos and D.J. Mascaro, IBM J. Res. & Dev. (2001) p-conjugated “polymers” (pentacene) at the molecular level Mobility X-ray diffraction Microstructure Structure matters Consequences of molecular morphology in poly(3-alkylthiophenes) a) Solution cast Processing can impact crystal orientation and, thereby, the mobility. S S S S S Mobilities Solution cast Thin-film X-ray diffraction Spin coated H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P. Herwig & D. M. de Leeuw, Nature 401 (1999). S The bigger picture From J.M. Shaw and P.F. Seidler, IBM J. Res. & Dev. (2001) Coming to a production facility near you … Litrex 80L 200mm PLED System shipping May 2001 Piezo multi-nozzle selected technology Orifice plate Ink droplet Ink Channel Ink Manifold • Piezo crystal deforms when electrical pulse applied, moving diaphragm into ink channel • Shear and Length mode piezoelectric response available • Drop on Demand; Very little waste / complexity • frequency response - 0.1 to 20kHz typical • drop sizes 25 - 70 um diameter Accoustic Pulse Diaphragm “Enabling Technology for PLED “ Rigid Shell Piezoelectric transducer • Capable of jetting high molecular weight polymers (Shear Mode Piezo) • Long Life - 10’s of liters + Poled PZT GND Electrodes “E” field applied GND E GND GND + • Multi nozzle / low frequency allows high substrate throughput with low mechanical stress and vibration E GND PZT Response Shearing of PZT occurs • Ink conductivity and solvent system choice are virtually unrestricted with several suppliers of PZT heads. Polymer LEDs Other applications: LEDs O O S n PEDOT PPV Photoluminescence 15 K Absorbance T=77 K n 4.8 eV PSS SO3H 5.0 eV Friend, Burroughes and Tatsuya, Physics World (Vol. 12) p35-40 (1999) Construction of Polymer LEDs Burroughes et al., Nature, 347, 539 (1990), US patent 5,247,190 LED design strategy Poly(2-methoxy-5-(2'-ethylhexoxy)-phenylene) Side chain structure reduces side ch poly(p -phenylene vinylene) crystallization and frustrates packing O glass substrate Electron and hole mobilities are now an issue O n O indium tin oxide semiconducting polymer 40 nm 100 nm The calcium injects electrons into the polymer film, while the anode injects holes. When an electron and hole capture one another within the PPV, they form neutral "excitons" (bound excited states) that decay by emitting a photon of light. 3.5 aluminum, magnesium, or low-workfunction metal External Circuit calcium cathode O 2.5 Energy (eV) n PPV 5.2 eV indium-tinoxide anode 1.5 2.8 eV 2.7 eV Poor interchain p overlap enhances photo- and electro- luminescence Evolution of LED/OLED performance From J.R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon, D. Roitman, and A. Stocking, Science 273, 884 (1996). A litany of new materials 1: The prototypical (green) fluorescent polymer is poly(p-phenylene vinylene) 2 & 3: Two best known (orange-red) solution processible conjugated polymers MEH-PPV (2) and ``OC1C10'' PPV (3). 4: Copolymers have been widely developed because they allow color tuning and can show improved luminescence. 5 & 6: Cyano-derivatives of PPV 5 and 6 show increased electron affinities and are used as electron transport materials. 7: Blue emitters include high-purity polymers such as poly(dialkylfluorene)s. 8 & 9: `Doped' polymers such as poly(dioxyethylene thienylene), PEDOT (8), doped with polystyrenesulphonic acid, PSS (9), are widely used as hole-injection layers. A simple picture of photophysics in isolated molecules Conjugated polymers display Absorption 0-3 0-2 0-1 0-0 3 2 1 0 p* inhomogeneous broadening Emission Ef 3 2 1 0 0-3 0-2 0-1 0-0 PPV p E 0-0 = E 0-0 (gas phase) Photoluminescence 15 K Absorbance T=77 K Dilute solutions in solvent or the solid-state: Stokes shift Emission Intensity 0-0 0-1 Absorption 0-0 0-1 0-2 1.5 2.5 3.5 Energy (eV) 0-2 (from R.H. Friend et al.) 0-3 0-3 Photon Energy Absorption occurs at all sites but emission dominated by longest conjugated segments. A simple picture of intrachain photophysics for a conjugated polymer Imagine a series of one-dimensional potential wells which Particle in a box represents a distribution of effective conjugation lengths. n=2 (p*) Different crystalline domains n=1 (p) 0 L n=2 Emission 0 n=1 L' a n=2 0 n=1 L'' Energy eigenvalues are -2 proportional to L 1 Absorption can potentially occur at all sites (if E>Egap) 2 Exciton formation (bound "electron-hole“ pair) is rapid (subpicosecond) 3 Energy migration along chain to segment with lowest energy band (tens of picoseconds) 4 Photoluminescence is dominated by emission at longest conjugated segments. Absorption (Abs) averages over all “chromophores” while photoluminescence (PL) identifies a small subset Engineering where the energy goes in and where it comes out MEH-PPV loaded into a mesoporous silica composite 1: A single polymer chain can be loaded into each micropore 2: Part of the chain extends beyond the micropore 3: Energy (light) is captured everywhere but preferentially transfers into extended chain conformations in the pore 4: Emission is preferentially polarized From: Thuc-Quyen Nguyen, Junjun Wu, Vinh Doan, Benjamin J. Schwartz, Sarah H. Tolbert, Science 288 (2001) Keys to conducting polymer applications: Synthetic control and processibility Addition of solubilizing side chains to “conducting” polymers has created a myriad of new, processible polymers. Examples: b) a) S S S O S S S c) a) Regioregular poly(3-hexyl thiophene) or r-P3HT O H N H N N H N H O O b) Poly(2-methoxy-5-(2'-ethylhexoxy)phenylene vinylene) or 2MehPPV c) Polyaniline dodecylbenzene sulfonate or DBSA-PANI Competing interactions and chemical incompatibilities (i.e. polar vs. nonpolar) has given rise to new structure/property relationships. Self-Assembly (or … you I like, but you I hate) X-ray Intensity (arb. units) For example: Self-assembly leads to ordered (lamellar) phases (200) l=0 x1 l=2 x2 (002) (102) (202) l=1 x2 (311) (111) (411)(611) X-Ray Diffraction of Poly(3-n-octylthiophene) 0.0 (400) (600) l=0 x2 (220) (120) (320) (420) (020) 6.0 12.0 18.0 24.0 30.0 New PHYSICS: Order-disorder transitions (ODTs) of the alkyl side chains leads to conformational changes along conjugated polymer backbone 2q (deg.) • This gives rise to new properties such as thermochromism • Details are sensitive to the chemical architecture and physiochemical processing Side chain behavior couples to main chain conformation: Thermochromism, solvatochromism, ionochromism,… This is the basis for sensor technologies 250 In the ordered state isosbestic point X-ray powder diffraction 300 350 Wavelength (nm) 400 A Simple Free Energy Diagram Free Energy Order-Disorder Transition Temperature B Ordered state A B is the path on cooling Thermotropic columnar A mesophase Intensity (arb units.) 50 C 45 C 40 C 35 C 30 C 20 C 15 C 5C 0C -20 C -35 C On warming Absorbance (Arb. Units) UV absorption 25 C (ordered) 45 C (disordered) 10 20 30 2q (deg.) Photoluminescence is also strongly impacted! “Self-assembly” leads to formation of helical phases Repeat unit: - (SiR2 ) – and R= (CH2)3CH3 Conformational energy surface of an all transoid (15/7 helix) in poly(di-n-butylsilane) oligomer Energy (kcal/mol per dimer) Si-Si-Si-Si dihedral angle is ~170° 15/7 helix C-Si-C-C dihedral (deg.) C-C-Si-C dihedral (deg.) Topology of the surface is well defined with one clear minimum Side chain ordering does not always work in your favor! Two different atactic silicon backboned (s-conjugation) polymers in dilute solution The polymer on right: Minimal Stokes shift Both absorption and emission are extremely sharp From M. Fujiki, JACS 122, 3336 (2001) Morphology impacts energy flow in subtle ways: In the case of a bimodel distribution of conformations Monitoring isothermally the Order-Disorder Transition of a thermochromic polymer Sample: 0.2% poly(di-n-hexlysilane) in toluene spin coated (~300 Å) on quartz substrate 0.30 200 Full Set 0.20 0.25 0.10 0.00 300 350 0.20 40 0 0 sec 0 sec 10 sec 35 sec 77 sec 121 sec 84 sec 127 sec 164 sec 622 sec 172 sec 617 sec 5 sec 41 sec lEx=305 nm 150 100 0.15 0.10 50 0.05 0 0.00 300 350 400 325 350 375 Energy migrates between the two structural phases 400 (nm) Photoluminescence (a.u.) Absorbance (absolute) Combined PL and Abs 0.30 Model depicting energy transfer from disordered to ordered phase Formation of the ordered phase depends on nucleation and growth For athermal nucleation (all nucleation occurs at t=0) n=1 Polymer thin film Substrate • 1st: Low absorption in ordered phase but efficient PL • 2nd: High absorption in disordered phase and exciton migration to the ordered region (I.e., red arrows) • One-dimensional growth increases relative PL from 1st process Nucleation then growth Back to electronic properties…. Photoluminescence or electroluminescence is a complex process involving the formation of excitons, excimers, exiplexes and other exotic charge states. p-conjugated polymers have unusual charge excitations Minding the gap Electronic states are split off from the valence and conduction bands All charge excitations involve local self-consistent structural distortions of the lattice Schematic representations of recombination pathways Excitons are bound “electron-hole” pairs which includes structural relaxation of a single polymer chain (i.e., electron-phonon coupling) Decay pathways for singlet and triplet decay Radiative processes, corresponding to absorption or emission of light; Non-radiative processes. a: Fluorescence b: Intersystem crossing (ISC) c: Photoinduced triplet-triplet absorption d: Phosphorescence S0 is the ground (singlet) state; S1 is the first excited singlet; T1 the first excited triplet; and Ti and Tn are higher lying triplet states. Recombination is spin dependant LED’s require injection of holes at one electrode and electrons at the other. If no spin polarization An eigenstate requires: triplet triplet triplet mixed mixed triplet triplet triplet singlet singlet Three states with s = 1 (triplet) and one state with s = 0 (singlet) or, in the absence of other effects, 75% triplet and 25% singlet recombination There is more to the singlet-triplet story Singlet-Triplet Cross Section Ratio Formation of singlet excitons exceeds that of the triplets and it is dependent on the band gap! From: M. Wohlgenannt, Kunj Tandon, S. Mazumdar, S. Ramasesha, Z. V. Vardeny, Nature 409 (2001) Electronic band structure in one-dimension: A primer Everything begins with directional wavefunction overlap in the context of tight-binding (Hückel theory) invoking the Born-Oppenheimer approximation. Schrödinger Equation: Of course y is the wavefunction, m is the electron mass and U is the potential The calculation is exact and analytic if we use a H2+ion. Ebonding < Eantibonding Bonding orbitals occur when symmetric solution is used yH(r) + yH(r + R) Anti-bonding orbitals occur when asymmetric solution is used yH(r) - yH(r + R) + + Tight-binding for p-electrons and semiconducting polymers For carbon in a diamond lattice: C C C C sp3 hybridization In conjugated polymers the carbon has sp2 + p bonding Now the mix is C Extending the chain p* LUMO band p HOMO band H C C C H H sp2 hybridization pz pz Isolated orbitals Paired Linear electrons chain (1-D band) Interchain stacking and overlap (3-D band) A simple picture of the one-dimensional band as the chain lengthens: A one-dimensional chain (trans-polyacetylene) From a tight-binding perspective: EA is the energy of a single atomic orbital A(R) is an overlap integral R After a simple approximation: [B(0) is approx. 1] Overlap intergral For a linear chain only R = +a and –a are important This gives E(k) = EA – a + 2 A cos ka and a half-filled band t = A(R) The Peierls instability (1939) Competition: Electronic energy vs. Lattice (elastic) energy (recall that band electrons move in a weak, periodic potential) Electronic states moves lower in energy ~ D2 ln(t/D) Elastic energy is increased ~ D2 At low temperature dimerization always favorable kFermi is still p/2a and since d = 2p/k , d = 4a (charge density wave) Conformational structure impacts electronic properties 1. Cis and trans polyacetylene, according to the tight binding picture just presented, should have the same band gap but they don’t! (as synthesized) Question: Why not? (after thermal conversion) 2. Conjugation yields an energy/unit length which is minimized with increasing backbone planarity p C C C s bonding C C C C Si C C C bonding C Si Si Si Si C Si Si Si geminal vicinal 3. Rotation breaks conjugation Structural instabilities and disorder are extremely important (at all length scales) Simple picture of polyacenes and poly-p-phenylenes a Benzene a a a a a Free electron perspective: (works best for polyacenes) Energy levels +2 -2 +1 -1 0 A circular ring of length L = 6a 6 unpaired pz electrons (1 per C) Now from the tight binding perspective E(k) = EA - a – 2 t cos ka , y(f) = c eimf m = 0, 1, 2, 3 (only six states) k = mp/3a k = p/a k = 2p/3a k = p/3a Now consider: k= 0 Poly(p-phenylene) Polarization direction Poly-p-phenylene from tight binding Weak interactions Strong interactions Egap Strong interactions give bands and tend to delocalize charge Impact of band structure on photocell device physics Experiment Theory thick I Absorption (not perfect) II III thin IV I II III From: A. Kohler, D. A. dos Santos, D. Beljonne, Z. Shuai, J.-L. Bredas, A. B. Holmes, A. Kraus, K.Mullen, R. H. Friend, Nature 392 (1998) A closer look at the calculation Efficiency Exciton wavefunction 3.1 ev Poor 4.8 ev Better Best In a photocell it is advantageous to efficiently separate charge Molecular wave function is important 6.5 ev Separating electrons and holes: A prerequisite for photovoltaic applications Blending C60 and p-conjugated polymer Recombination of P+ and Preduces device efficiency From: Christoph J. Brabec, N. Serdar Sariciftci, and Jan C. Hummelen, Advanced Functional Materials 11 (2001). Nearly 100% electron transfer to the C60 Because of bipolaron BP2+ formation minimal back transfer Polymer has good hole mobilities Design of interface and electron transport are bottle necks Blended device behavior 2.8 eV 3.7 eV p* MEH-PPV LUMO C60 Carbon-60 LUMO is well positioned to accept electrons Egap 4.3 eV 4.7 eV 5.0 eV p MEH-PPV Al ITO HOMO C60 6.1 eV p* MEH-PPV LUMO C60 ITO p MEH-PPV HOMO C60 Al Simple design strategy The future (assuming one has a good crystal ball) Better control of interface through intelligent design Purer materials and more robust synthetic procedures Control of both spin and charge Top down meets bottom up (passive and active types of molecular self-assembly)