Organic Semiconductors (Molecular / Polymeric Materials)

Transcription

1 Organic Semiconductors (Molecular / Polymeric Materials) Van-der-Waals Bonds no dangling bonds Conjugated Materials (extended (delocalized) π-electrons) Bandgap of 1.5 to 3 ev

2 Conducting Polymers Synthetic Metals Conjugated polymers Examples of Conducting Polymers * These materials are insulators or, at best, semiconductors. They become conductive after treatment with oxidizing or reducing agents Doping 2

3 Conducting Polymers 3

4 Conducting Polymers 4

5 Possible Applications of Conductive Polymers Doped conjugated polymers Antistatic materials Electromagnetic shielding Anticorrosion coatings Electrolytic capacitors Batteries Smart windows Sensors Undoped conjugated polymers Light-emitting diodes Photodiodes Solar cells Lasers Field-effect transistors Nonlinear optical material Sensors Processable polymer :PEDOT/PSS - Very stable in the doped state (1000 hrs in air at 100 gives no change in conductivity) Applications: Antistatic coating for photographic film : used in AGFA since 1995 Alternatives to ITO - AGFA s commercial product : Orgacon EL film, PEDOT paste, PEDOT solution Electrode for solid electrolyte capacitor Antistatic coating for CRT Corrosion protection Hole injection layer in LED 5

6 Conducting Polymers Polyacetylene(PA) is a planar molecule. There are three possible structures, i.e., Trans - transoid Trans - PA Cis - transoid Cis - PA Trans - cisoid Cis - cisoid cannot be realized The double bonds are shorter ones, while the single bonds are longer ones. At RT, cis-transoid is first synthesized, and then can be isomerized to trans transoid, a more stable configuration, by heat treatment or doping. The energy of trans cisoid is slightly higher than cis transoid Roughly speaking, the carbon bond angle is 120º the average bond length is 1.4Å 6

7 Conducting Polymers Synthesis of PA ( Polyacetylene )1958 by Mazzanti and Corradini n (CH CH) T 1 ( OBu) 4 / AlEt 3 gray or black semicrystalline powder insoluble in any solvent decompose before melting (CH CH)n 1974 Shirakawa & Coworkers Synthesized and formed strong, free standing film of PA Black, with metallic cluster does not melt until decomposition near 200 o C Micrographs Randomly oriented fibers ( diameter 200Å ) ρ : g/cm 3 can be oriented by stretching the film. 1977, MacDiamid s group & Shirakawa σ : increased via doping by 13 orders of ~ S/cm 7

8 Conjugational Defects In PA, there is strict bond alternation. For a chemist, this is trivial since dimers (pairs) were polymerized For a physicist, dimerization is a phase transition from a metallic to a semiconducting state The phase transition nucleates at several points of the chain with domains growing around these nucleation centers. finally misfits are created.. Domain A Domain B misfit Misfits can be neutral or positively or negatively charged Synonyms : conjugational defects, misfit, dangling bonds, solitons,, 16

9 Solitons : Quasi-particles corresponding to solitary waves *Intrinsic Solitons : as a consequence of synthesis ( 400 radicals per 10 6 carbon atoms ) by ESR results *Extrinsic Solitons : Creation is created in pairs, as soliton / anti-soliton pairs Conservation of particle number (c.f.) breaking of a bond two dangling bonds. Annihilation : closing the bond again Soliton migration A soliton moves by pairing to an adjacent electron and leaving its previous partner unbound. the soliton always occupies odd-numbered sites; the even numbered sites are reserved for anti-solitons. 17

10 Solitons : Quasi-particles corresponding to solitary waves A Soliton is free to move, because the total energy of the system does not depend on the position of the soliton if the chain is long enough. ( In short chains end effects will push the soliton to the center.) A more detailed analysis shows that the soliton has to over come an energy harrier when moving from one site to the next. So called Self-trapping of the soliton on the chain. Free Soliton motion is possible only in polyacetylene. degenerate ground state In the other polymers, motion of a soliton changes the energy. non-degenerate ground state polymers 18

11 Neutral, Positive & Negative Solitons Q = e, S = 0 Positive Q = 0, S = 1/2 Neutral Q = -e, S = 0 Negative +. The neutral soliton :Radical The positive soliton :Carbocation The negative soliton :Carbanion Spin : the neutral soliton : +1/2 or 1/2 ( unpaired electron ) the positive and negative solitons : 0 either no electron or paired electron. Spin charge inversion If the soliton bears a charge it has no spin vice versa... 19

12 Generation of Solitons There are very few solitons created during PA synthesis. ( 400 radicals / 10 6 carbon atoms ) Generation methods: (1) chemical doping (2) photogeneration (3) charge injection (1) chemical doping : a chemical redox reation doping level : up to several %, ppm in classical semi conductor saturation doping leads to a new material & new crystallographic structure. e.g. [(CH) 7 I 3 ]n *Similarity (e.g. Li in Si) 1. The dopants goes to interstitial sites of the crystalline states of the host. 2. charge is transferred from the dopant to the host, thus moving the Fermi level of the host. 20

13 Doping Process : Chemical Doping 1 st step : a double bond is broken 2 nd step : transfer of an electron from the polymer chain to the dopant As a result, two solitons are formed : one neutral and other positively charged. ( p-doping ) The next dopant will then react with the neutral soliton, because in most p-doping reactions. Stoichiometry requires the transfer of two electrons. [PA] + 3I 2 [PA] I 3 In n-doping only one electron will be transferred. [PA] + K [PA] + K + * Very few neutral solitons will survive migrate & annihilate with antisolitons 21

14 Doping Process : Chemical Doping By chemical doping σ : changes by many orders of magintude doping induced insulator-to-metal transition doping generate solitons ( mid gap states ) many solitons many midgap states interact the gap disappear doping reverses the Peierl distortion *Spatial extension of the soliton wave function over 7 lattice site the stoichiometry of 7:1 for the ratio of CH to I 3 in saturation doped PA seems quite plausible Conduction band Valence band Midgap state by chemical doping Large doping Metal:current can flow by hopping 22

15 Doping Process : Photo generation Undisturbed chain hv e h + electron - hole S S + Soliton & anti-soliton 23

16 Doping Process : Charge Injection Non Degenerate Ground State Polymers : Polarons Degenerate means that the energy does not change when single and double bonds are interchanged. Same energy Non - degenerate Polymers : interchange of single and double bond changes the energy. (ex. Poly (para-phenylene)) aromatic lower energy state < quinoidal higher energy state The soliton in polyphenylene seperates a low energy from a high energy region.. driven to the chain end, to lower the energy. 24

17 Polaron To stabilize conjugational defects in a non-degenerate ground state polymer, we have to create bound double defects polaron e.g. a neutral and a positive soliton these defects are pushed toward each other but can not recombine positive polaron Bipolaron : a defect composed of two positive solitons < If two polarons meet, the two neutral solitons can form the bond and only the two charged solitons are left over. > 25

18 Energy States of Polaron two states in the gap + Occupancy of the defect Polaron excitons : There are two possibilities of accomodating two electron (high spin & low spin) Positive =bipolaron Positive =polaron Singlet =polaron =exciton Triplet =polaron =exciton Negative =polaron Negative =bipolaron 26

19 Excitons Exciton: Mobile molecular excited state In some applications it is useful to consider electronic excitation as if a quasiprinciple, capable of migrating, were involved. This is termed as exciton. In organic materials two models are used: - the band or wave model (low temperature, high crystalline order) - the hopping model (higher temperature, low crystalline order or amorphous state) Energy transfer in the hopping limit is identical with energy migration. Excitons are integral to nearly all electronic processes in molecular organic solids - Exciton diffusion is a dominant method of energy transfer - Exciton absorption dominates the absorption spectrum - Exciton formation is necessary for luminescence and photoconductivity. 30

20 Excited States of a Molecule Typical energy levels and energy transfer process. Radiative process Non-radiative process The energy states of molecules correspond to Electronic, vibrational, rotational, and translational degrees of freedom. 31

21 Excitons 32

22 Excitons Depending on the degree of delocalization, the excitons are identified as Frenkel, Chrage-Transfer, or Wannier-Mott. 1. Frenkel Exciton - A correlated electron-hole pair localized on a single molecule - Its radius is comparable to the size of the molecule (typically < 0.5 nm) or is smaller than the intermolecular distance - No internal degrees of freedom - Considered as a neutral particle that can diffuse from site to site 2. Wannier-Mott (WM) Excitons - WM excitons occur in uncorrelated, crystalline materials (e.g., Si, Ge, GaAs), in which overlap between neighboring lattice atoms reduces the Coulombic interaction between the electron and the hole of the exciton - Large exciton radius (4 ~ 10 nm), many times the size of the lattice constant. 3. Charge-Transfer (CT) Excitons - An intermediate between a Frenkel and a WM state, being neither very extended nor tightly bound to a single molecular site. 33

23 Charge-Transfer Excitons Wavefunction density Coulombic potential (V e-h, Electron-hole binding energy) Total potential (V e-h + V pseudo, the periodic crystalline pseudopotential) 34

24 Excitons : PTCDA Electron Energy Transfer Thin-Film Solution (2μM in DMSO) 35

25 Energy Transfer 36

26 Energy Transfer Radiative energy transfer Transfer of excitation energy by radiative deactivation of a donor molecular entity and reabsorption of the emitted light by an acceptor molecular entity. The probability of transfer is given approximately by: P r,t [A]xJ Where J is the spectral overlap integral, [A] is the concentration of the acceptor, and x is the specimen thickness. This type of energy transfer depends on the shape and size of the vessel utilized. Same as trivial energy transfer. 37

27 Förster Transfer 38

28 Förster Transfer : Example 39

29 Förster Transfer Förster excitation transfer (dipole-dipole) excitation transfer A mechanism of excitation transfer which can occur between molecular entities separated by distances considerably exceeding the sum of their van der Waals radii. It is described in terms of an interaction between the transition dipole moments (a dipolar mechanism). The transfer rate constant k D A is given by: K D A = K 2 A( n ϖr 28 mol) Where K is an orientation factor, n the refractive index of the medium, ω the radiative lifetime of the donor, r the distance (cm) between donor (D) and acceptor (A), and J the spectral overlap (in coherent units cm 6 mol -1 ) between the absorption spectrum of the acceptor and the florescence spectrum of the donor. The critical quenching radium r 0, is that distance at which k D A is equal to the inverse of the radiative lifetime. 40

30 Förster Transfer : Rate Equation 41

31 Dexter Transfer 42

32 Dexter Transfer Dexter excitation transfer (electron exchange excitation transfer) Excitation transfer occurring as a result of an electron exchange mechanism. It requires an overlap of the wave functions of the energy donor and the energy acceptor. It is the dominant mechanism in triplet-triplet energy transfer. The transfer rate constant, k ET, is given by: h 2 2r K ET P J exp 2π L where r is the distance between donor (D) and acceptor (A), L and P are constants not easily related to experimentally determinable quantities, and J is the spectral overlap integral. For this mechanism the spin conservation rules are obeyed. 43

33 Non-radiative Energy Transfer 44

34 Diffusion of Excitons 45

35 Single Layer Organic PV Cell 46

36 Photocurrent Generation 47

37 Photocurrent, Photovoltage, Absorption 48