Hole injection conducting polymers

Lee, T.W. et al.. Hole-injecting conducting-polymer compositions for highly efficient and stable organic light-emitting diodes, Appl. Phys. Lett. 87, 231106, 2005. 

Several typical properties of PEDT PSS polymers that depend on the PEDT PSS ratio are summarized in Table 10.1. To meet requirements for conductivity, antistatic grades of PEDT PSS have relatively low PSS-contents, and therefore higher conductivity values. In contrast, PEDT PSS grades designed for hole-injection in polymer OLEDs have larger PSS contents, smaller particles (Figure 10.6), and lower conductivities. Specifically, PEDT PSS grades useful for passive matrix OLED displays have the lowest... 

The group in the Swiss Federal Institute of Technology [55] has fabricated a macroscale device by depositing the conducting polymer (poly(/j-phenylenevinylene)) on the MWCNT film (Fig. 16). They have observed the characteristic rectifying effect from the l-V curve, which suggests the CNTs inject holes efficiently into the polymer layer. However, due to the difficulty in... 

Baytron , a conducting polymer [154] derived from 3,4-ethylenedioxythiophene, is a commercially available product that can be used as an antistatic or electrostatic coating of plastics and glass. Moreover, it has successfully been applied as counterelectrode in capacitors and as a hole-injection layer in organic light-emitting diodes [155]. 

Many other polymeric systems are of interest in polymer LEDs. Polythiophenes have been known for some time but it was not until improved synthetic methods were developed that their potential was realised. The process involves the reaction of the substituted monomer with FeClj in chloroform solution. After polymerisation has occurred the product precipitates and is isolated and washed. Further special purification methods are required to obtain satisfactorily pure materials. One product, of commercial interest, developed by Bayer is poly(ethylenedioxy)thiophene, known as PEDOT (3.110). This product when doped with polystyrene sulfonate, sold as Baytron P, has been found to be effective as a conducting, hole-injecting layer on the ITO electrode. ... 

Transport in DNA samples with all bases the same could be either by free carriers, i.e., band transport, or by polarons. As will be further discussed in the next section, the polarons are expected to be large polarons, not small. In the conducting polymers there is overwhelming evidence that electrons (holes) from a metal contact are injected directly into polaron states in the polymer, because the polaron states have lower energies than the LUMO (HOMO) or conduction (valence) band edge. As has recently been shown theoretically [30], the injection takes place preferably into a polaron state made available when a polaron-like fluctuation occurs on the polymer chain close to the interface, rather than into a LUMO state, with subsequent deformation to form the polaron. It could also be expected for DNA that injection... 

Electrical conduction will occur by the hopping of either electrons or holes within these distributions of energy levels. Charge transport can be either of holes by transfer between the LUMO states or of electrons between the HOMO states. These correspond to the formation of either a radical cation by the removal of an electron to an adjacent electrode or an anion by the injection of an electron. The nature of the majority carriers will, therefore, be determined by the ionisation potentials and electron affinities of the conjugated moieties. A low ionisation potential will favour hole transport while a high electron affinity will favour electron transport. Most of the conductive polymers reported in the literature have low ionisation potentials and are hole, conductors. ... 

Optical quality thin films of metallic polymers are useful, therefore, as transparent electrodes [68]. For example, polyaniline [69], polypyrrole [70] and PEDOT [71] have been used as transparent hole-injecting electrodes in polymer LEDs (the initial demonstration of mechanically flexible polymer LEDs utilized PANl as the anode [69]). Transparent conducting films can be used for a variety of purposes for example, as antistatic coatings on CRT screens, as electrodes in liquid crystal display cells, or for fabricating electrochromic windows. 

The devices used for the EL and PL measurements were fabricated in the thin film sandwich configuration anode/polymer/cathode. The electronic structure of alkoxy derivatives of PPV has been studied via internal field emission [42], internal photoemission [76], cyclovoltammetric spectroscopy [159]) and photoelectron spectroscopy [160]. The data indicate that the bottom of the Tr -band and the top of the ir-band are at 3 eV and 5 eV respectively, with respect to the vacuum. Thus, relatively good hole and electron injection can be achieved by using transparent indium/tin-oxide (ITO) as the anode and calcium (Ca) or barium (Ba) as the cathode [42,161]. In practice, however, hole injection is very sensitive to the quality of the ITO. A thin layer (= 300 A) of conducting polymer provides excellent, reproducible hole injecting contacts [70,71,162]. 

For devices using ITO/PEDOT anodes, because the PEDOT layer is much more conductive than the MEH-PPV film, the major energy barrier for hole injection depends on the PEDOT/MEH-PPV interface. If the organic solvent used to dissolve the MEH-PPV does not dissolve the PEDOT layer, it is expected that the resulting PEDOT/MEH-PPV interface will be similar to those obtained by spin-coating the polymer on top of a metal electrode. This is true (or nearly true) in most cases, because PEDOT has a very limited solubility in many commonly used organic solvents. In fact, it was found that the MEH-PPV film spun on top of the PEDOT layer could be easily pealed off from the PEDOT surface by a piece of Scotch tape,... 

On the other hand, the poor anode/polymer contact shown in Fig. 6.12b can be improved to some extent by using higher spin speeds (Fig. 6.17). At high spin speeds, the polymer coils are stretched open, allowing the conducting polymer backbone to settle closer to the ITO surface. This results in a better contact and thus a lower hole-injection barrier and lower Vl on-... 

The height of the barrier for hole injection is determined by the difference between the work function of the anode and the energy level of the jt (valence) band the height of the barrier for electron injection is determined by the energy difference between the work function of the cathode and the energy level of the it (conduction) band. The barrier height can be determined in this simple way because conjugated polymers do not have surface states that would pin the Fermi level. 

The simplest idea is to use orthogonal solvents for the individual layers, i. e., such that the solvent used in a deposition does not dissolve the previous layer(s). The most prominent example of such an approach is the conductive polymer PEDOT, which is commonly used as a hole-injecting anode and is deposited from an aqueous suspension. Organic layers can be deposited on top of the PEDOT layer without problem. 

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