Electronic transport properties

In order to discuss electron transport properties we need to know about the electronic distribution. This means that, for the case of metals and semimetals, we must have a model for the Fermi surface and for the phonon spectrum. The electronic structure is discussed in Chap. 5. We also need to estimate or determine some characteristic lengths. 

It would be preferable to incorporate both fluorescent and electron transport properties in the same material so as to dispense entirely with the need for electron-transport layers in LEDs. Raising the affinity of the polymer facilitates the use of metal electrodes other than calcium, thus avoiding the need to encapsulate the cathode. It has been shown computationally [76] that the presence of a cyano substituent on the aromatic ring or on the vinylene portion of PPV lowers both the HOMO and LUMO of the material. The barrier for electron injection in the material is lowered considerably as a result. However, the Wessling route is incompatible with strongly electron-withdrawing substituents, and an alternative synthetic route to this class of materials must be employed. The Knoevenagel condensation... 

C. Electron Transport Properties of Nanocrystals Either Isolated or Self-Assembled in 2D and 3D Superlattices... 

The electron transport properties described earlier markedly differ when the particles are organized on the substrate. When particles are isolated on the substrate, the well-known Coulomb blockade behavior is observed. When particles are arranged in a close-packed hexagonal network, the electron tunneling transport between two adjacent particles competes with that of particle-substrate. This is enhanced when the number of layers made of particles increases and they form a FCC structure. Then ohmic behavior dominates, with the number of neighbor particles increasing. In the FCC structure, a direct electron tunneling process from the tip to the substrate occurs via an electrical percolation process. Hence a micro-crystal made of nanoparticles acts as a metal. 

Li Z, Zou B, Wang C (2006) Electronic transport properties of molecular bipyridine junctions effects of isomer and contact structures. Phys Rev B 73 075326-075327... 

In the early 1970s, Spear and coworkers (Spear, 1974 Le Comber et al., 1974), although unaware of the presence of hydrogen, demonstrated a substantial reduction in the density of gap states (with a corresponding improvement in the electronic transport properties) in amorphous silicon films that were deposited from the decomposition of silane (SiH4) in an rf glow discharge. 

Although MEH-PPV 13 (at the time of discovery) was one of the most efficient soluble polymers for PLEDs application, its performance is not high enough for commercialization as LEP. One of the reasons is unbalanced hole-electron mobility in MEH-PPV (the mobility of holes is 100 times faster than the mobility of electrons) [133]. Copolymerization with other conjugated monomers, to some extent, can improve the electron-transporting properties and increase the EL performance. 

As we already mentioned, electron-transporting properties of PPV polymers can be adjusted by introduction of an oxadiazole moiety in the polymer structure. A variety of PPV copolymers... 

The tuning of electron injection and transport in PF has been undertaken by Shu s group [354], who introduced electron-deficient oxadiazole units as pendant groups in fluorene copolymer 257. The introduction of oxadiazole units into the PF can potentially improve the electron transport properties of the polymer, while their bulkiness can help to suppress aggregation effects (Chart 2.68). 

Efficient blue emission and good electron affinity and electron-transporting properties were demonstrated for two fluorene copolymers with dicyanobenzene moiety in the main chain, 271a,b (Scheme 2.41) [363], Due to improved electron transport properties, the device... 

Excellent electron-transporting properties of quinoxaline (also demonstrated for noncon-jugated quinoxaline-containing polymer 588 [684] and quinoxaline-based polyether 589 [685]) resulted in a substantially decreased turn-on voltage of PPV/590 PLED (3.6 V), which is much lower than that of pure PPV in the same conditions (7 V). These diodes showed a maximum luminance of 710 cd/m2 (ca. 40 times brighter than the PPV diode at the same current density and voltage) [686]. 

Inspired by good electron transport properties and high PL of PBD and, particularly, a claim by Heeger and co workers [68] of exceptional performance of PBD MEH-PPV mixtures (EL of up to 50% of the PL yield), Bryce and coworkers [697] reported the first poly(PBD) homopolymer (604) and its aza-derivative (605). The device ITO/PEDOT/MEH-PPV 604/A1 showed cT>i" of 0.26%, compared to 0.01% obtained with MEH-PPV alone in an identically prepared device. 

Yamaguchi and coworkers [709] have prepared luminescent silole polymers 615a-e. The blue emission of the homopolymers 615a,b can be shifted into the red region by copolymerization with other conjugated units (but for the price of lowered PLQY). Although no device studies have been reported yet, excellent electron-transport properties are expected from such materials [710]. 

Introducing heteroaromatic moieties (mainly with N and, to a lesser extent, with O and S) in the backbone of the polymer or as a pendant group, can substantially modify the LUMO level of the materials, improving their electron-transport properties and facilitating electron injection in PLEDs, but the efficiencies still lag behind the other systems. 

Methods to determine or justify the utility of the electron transport properties of ETM are TOF electron mobility and electron-only diode device measurement as well as the overall OLED performance. 

A perfluorinated, para-conjugated oligophenylene with high EA exhibited improved electron transport properties and was investigated as an ETM (100-103) (Scheme 3.32) [155]. The electron mobility of NPF-6 (100), determined by the TOF technique, is much higher than that... 

Compared to Alq3, the EAs of these compounds are slightly higher (3.3 eV), which in part explains their superior electron transport properties. 

The broad PL emission spectra of some metal chelates match the requirements for white emission. Hamada et al. investigated a series of Zn complexes and found bis(2-(2-hydroxy-phenyl)benzothiazolate)zinc (Zb(BTZ)2, 246) is the best white emission candidate. An OLED with a structure of ITO/TPD/Zn(BTZ)2/OXD-7/Mg In showed greenish-white emission with CIE (0.246, 0.363) with a broad emission spectrum (FWHM 157 nm) consisting of two emission peaks centered at 486 and 524 nm (Figure 3.14) [277], A maximum luminance of 10,190 cd/m2 at 8 V was achieved. The electronic and molecular structure of Zn(BTZ)2 have been elucidated by Liu et al. [278]. There is evidence that the dimeric structure [Zn(BTZ)2]2 in the solid state is more stable than its monomer Zn(BTZ)2. They also found that the electron transport property of Zn(BTZ)2 is better than that of Alq3. 

Without using Alq3 as the ETM, BCP has been used as an ETL and was demonstrated in DPVBI-based blue OLEDs [347], BCP has superior electron transport properties and its electron mobility is around 5.2 x 10-4 cm2/(V s) (5.5 x 105 V/cm) as measured by the TOF method [348], The concept of using BCP has been extended into doped OLEDs. 

Y. Sakamoto, T. Suzuki, A. Miura, H. Fujikawa, S. Tokito, and Y. Taga, Synthesis, characterization, and electron-transport property of perfluorinated phenylene dendrimers, J. Am. Chem. Soc., 122 1832-1833 (2000). 

K. Okita, Y. Harima, K. Yamashita, and M. Ishikawa, Synthesis and optical, electrochemical, and electron-transporting properties of silicon-bridged bithiophenes, Organometallics, 18 1453-1459 (1999). 

We shall briefly discuss the electrical properties of the metal oxides. Thermal conductivity, electrical conductivity, the Seebeck effect, and the Hall effect are some of the electron transport properties of solids that characterize the nature of the charge carriers. On the basis of electrical properties, the solid materials may be classified into metals, semiconductors, and insulators as shown in Figure 2.1. The range of electronic structures of oxides is very wide and hence they can be classified into two categories, nontransition metal oxides and transition metal oxides. In nontransition metal oxides, the cation valence orbitals are of s or p type, whereas the cation valence orbitals are of d type in transition metal oxides. A useful starting point in describing the structures of the metal oxides is the ionic model.5 Ionic crystals are formed between highly electropositive... 

Electronic transport properties of individual chemically reduced graphene oxide sheets, Nano Letters, 7 (2007) 3499-3503. 

C. Gomez-Navarro, R. T. Weitz, A. M. Bittner, M. Scolari, A. Mews, M. Burghard, K. Kern, Electronic transport properties of individual chemically reduced graphene oxide sheets, Nano Lett., vol. 7, pp. 3499-3503, 2007. 

Further, the electron-transport properties of the polymer 7 were enhanced by extending the separation between the redox center and backbone from a single Os—amino linkage to one that extends over 17 bonds. The goal was to provide mobility of the redox center independently of backbone motion, which is necessarily restricted by cross-linking. The mobility of the redox center can be characterized by an apparent diffusion coefficient, Z app- According to the relation proposed by Blauch and Saveant ... 

Many published results on electronic transport properties of organic materials, where metal contacts are usually made by evaporation of metals, do not describe the quality of the organic/metal interface, and some exotic observed features may perhaps be ascribed to extrinsic effects such as metal diffusion. The relatively simple contact lamination technique may become an alternative, since it provides a means for establishing electrical contacts without the potential disruption of the organic material associated with metal evaporation. The method consists in bringing the organic layer into mechanical contact with an elastomeric element coated with a thin metal film, which can also be patterned. The contacts are robust and reversible... 

Electronic Transport Properties of Single Fullerene Molecules. 132... 

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