Conducting polymer ionization potential

As mentioned above, the operation of the device requires that electrons and holes be injected from opposite electrodes. Electrons are injected into the conduction band states of the polymer, and holes into the valence band states, and for a diode formed with a polymer such as PPV, a schematic energy level diagram as shown in Fig. 29.9 is considered appropriate. Note that there are barriers at the electrodes for injection of both electrons and holes from the aluminum and indium-tin oxide electrodes, respectively. It is difficult to make accurate predictions about the barriers to electron and hole injection (J e and respectively, in Fig. 29.9). Making reasonable assumptions about the polymer ionization potential and electrode work functions, it is clear that the barrier to injection of electrons from aluminum must be significantly larger than the barrier to injection of holes from ITO [90]. The majority of the current is therefore expected to be due to holes. Electroluminescence, however, requires the simultaneous injection of electrons, and the quantum efficiency will therefore depend strongly on the barrier to electron injection. 

Fig. 9. Sketch of potential energy curves of a segment of conducting polymers in the ground state and in the ionized state Eip, is the vertical ionization energy, E ] the relaxation energy gained in the ionized state, Eip d the ionization energy of the distorted molecule, and Ej, the geometrical distortion energy in the ground state...

Fig. 15 Simplified schematic representation of the electronic energy levels in a single-layer PLED. CB and VB are the conduction hand and valence hand, respectively, of the semiconducting polymer, which correspond to the ionization potential (IP) and electron affinity (EA) relative to vacuum level (EV). The work functions for anode (and cathode (

The ionization potential of polyphenylene is around 8 eV and it is not surprising that oxidative degradation is not a problem the undoped polymer can withstand long periods at high temperatures in air with no change in its conductivity or its ability to dope 386). However, the high oxidation potential creates two problems. Firstly, the range of dopants with sufficient electron affinity to oxidize the polymer is limited, and there are few solvents in which the oxidation can take place without destruction of the solvent. Secondly, the doped polymer is expected to be reactive towards water and this is indeed the case 386). 

Doping with iodine, on the other hand, gave lower conductivities (ca 10-6 to 10-8 Scm-1, see Table 1), but which could be correlated with the ionization potentials of the polymers (lower ionization potentials giving higher conductivity). This is consistent with simple electron transfer from the polysilane to the iodine, with formation of delocalized holes along the polysilane chain. 

Para-phcnylcncs are comparably stable materials under environmental conditions for the degradation and stability of conducting polymers, see Ref. 2. Both theoretical and experimental studies find their oxidation and ionization potentials about 1 eV higher than for conjugated molecules based on pyrrole or thiophene.3... 

The ionization potential of PTH has been estimated to be above 5.0 eV [347] whereas that of (CH), and undoped PPY are 4.7 eV and 4.0 eV respectively [348], The undoped PPY and (CH) readily reacts with oxygen, whereas PTH should be resistant to oxygen since its Fermi energy is sufficiently low that there is no tendency for electron transfer from the polymer to oxygen [348]. The doped polymer would also be stable to oxygen but vulnerable to degradative reactions with the counter-ions. The conductivities of PTH-BF and PTH/CIO4 decreased markedly when heated in air to above 70°C [348], while electrochemically syntehsized... 

Figure 16.4. Relationship of polymer n-eleetron band structure to vacuum and various energetic parameters. g is the optical band gap, BW is the band width of the fully occupied valence band, EA is the electron affinity (measured from the bottom of the conduction band to the vacuum) and IP is the ionization potential (measured from the top of the valence band to the vacuum).

The polymer is very stable and can withstand temperatures up to 450°C in air without degrading. It is an insulator in the pure state but can be both n- and p-doped using methods similar to those for polyacetylene. However, as PPP has a higher ionization potential it is more stable to oxidation and requires strong p-dopants. It responds well to AsFs, with which it can achieve conductivity levels of... 

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