Hole transport

Hole transport in polymers occurs by charge transfer between adjacent donor functionalities. The functionalities can be associated with a dopant molecule, pendant groups of a polymer, or the polymer main chain. Most literature references are of doped polymers. The more common donor molecules include various arylalkane, arylamine, enamine, hydrazone, oxadiazole, oxazole, and pyrazoline derivatives. Commonly used polymers are polycarbonates, polyesters, and poly(styrene)s. Transport processes in these materials are unipolar. The mobilities are very low, strongly field and temperature dependent, as well as dependent on the dopant molecule, dopant concentration, and the polymer host This chapter reviews hole transport in polymers and doped polymers of potential relevance to xerography. The organization is by chemical classification. The discussion mainly includes molecularly doped, pendant, and 

In studies of organic materials, the materials are usually described by acronyms derived from their chemical names. In the following discussion, we will generally use the nomenclature of the original reference. There are many examples, however, where different acronyms are used for the same compound. There are also examples where the same acronym is used for different compounds. The molecular structures, chemical names, and acronyms of many of the materials described in this chapter are included in the Appendices. 

Here a denotes the average jump distance and co is the jumping frequency. In a semiclassical model (Emin, 1973) the jumping frequency can be expressed as 

Eg is the activation energy and P denotes the tunneling probability for the case that adjacent sites have the same energy level. cOq is the phonon frequency. Since the density of a liquid decreases with increasing temperature it seems probable that b increases with temperature. In the further discussion, we assume that the trapped state of the hole can be described as a small polaron (Holstein, 1959). Although this picture was developed initially for electrons in perfect crystals, it was later extended to electrons and holes in disordered materials (Emin, 1973). The mobility of the small polaron is given by. 

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Pig. 8. Stmctures of (a) hole transporter molecule W,Ar-biphenyl-W,Ar-bis(3-methylphen5l)l-lTiphen5l-4,4 diamine (TPD) (b) an oxidiazole derivative ... 

Photoconductive polymers are widely used in the imaging industry as either photosensitive receptors or carrier (electron or hole) transporting materials in copy machines and laser printers. This is still the only area in which the photoelectronic properties of polymers are exploited on a large-scale industrial basis. It is also one electronic appHcation where polymers are superior to inorganic semiconductors. 

The ordered columnar arrangement of the hexapentyloxytriphenylene molecules provides good overlap of the -electrons of the triphenylene moieties along the director axis. This results in efficient hole transport in the mesophase. The hole photocurrent shows nondispersive transport with a high mobihtyup to 1 X 10 cm /Vs (24). 

In moleculady doped polymers, charge transport is carried out by the hole-transporting molecular dopants, usually aromatic amines. The polymer merely acts as a binder. The hole mobiUty is sensitive to the dopant concentrations. For example, the hole mobiUty of... 

At very high dopant concentrations, transport occurs direcdy between the dopant molecules. The polymer acts only as a binder in most cases. Taking TPD-doped PVK as an example, at low TPD concentrations the hole mobihty first decreases from 3 x 10 cm /Vs to 10 cm /Vs with increasing TPD concentration, because TPD molecules act as hole traps (48,49). At higher TPD concentrations, new direct transport channels between the TPD molecules open up and the hole mobihty increases to lO " cm /Vs for ca 60% TPD doping (Table 1, entries 9—11) (48,49). In this case, there is no evidence for unusual interaction between TPD and PVK that affects the hole transport process. 

The main advantages that compound semiconductor electronic devices hold over their siUcon counterparts He in the properties of electron transport, excellent heterojunction capabiUties, and semi-insulating substrates, which can help minimise parasitic capacitances that can negatively impact device performance. The abiUty to integrate materials with different band gaps and electronic properties by epitaxy has made it possible to develop advanced devices in compound semiconductors. The hole transport in compound semiconductors is poorer and more similar to siUcon. Eor this reason the majority of products and research has been in n-ty e or electron-based devices. 

Most of the known charge-transport layers are -type or hole transporting. Thus this type of layered photoconductor must be charged negatively. 

Further developments in this area have included the neparation of several additional N,N -diaryl indolo[3,2-h]carbazoles with substituents such as m-tolyl, ffi-anisoyl, or triarylamine-containing species. Like 221, these compounds, possessing excellent hole-transport properties, also occurred in stable amorphous states and displayed high glass-transition temperatures. LED devices involving these systems were also constructed and showed promising characteristics [OOSMO11-112)42]]. 

Figure 9-3. Conventional multilayer light emission device (LED) indium tin oxide (ITO) electrode on a substrate, active layers A (hole transport), B (emitter), C (electron transport), and a niclat electrode. A possible encapsulation layer has been omitted, which would prevent the conjugated molecules from photo-oxidation.

There are many organic compounds with useful electronic and/or optical properties and with sufficiently high volatility to be evaporable at a temperature well below that at which decomposition occurs. Since thermal evaporation lends itself to facile multilayering, organic compounds may be selected for use in one or more function electron injection, electron transport, hole injection, hole transport, andI or emission. A complete list of materials that have been used in OLEDs is too vast to be included here. Rather, we list those that have been most extensively studied. 

Doubly doped polymers can be used to enhance two functions. For example, Wu et al. [58] added Alq3 as an election transport agent and nile red as an emitter to hole transporting PVK. 

Another approach to molecular assembly involves siloxane chemistry [61]. In this method, the electrically or optically active oligomers are terminated with tii-chlorosilane. Layers are built up by successive cycles of dip, rinse, and cure to form hole transport, emissive, and electron transport layers of the desired thicknesses. Similar methods have also been used to deposit just a molecular monolayer on the electrode surface, in order to modify its injection properties. 

There is one added layer which deserves special mention, namely a thin copper phthalocyaninc layer, which has been placed [103] between an 1TO anode and the hole transport layer. It is not an injection layer in the sense just discussed, because its HOMO is not well aligned with the 1TO Fermi energy and it slightly raises the operating voltage of the structure. It does, however, dramatically improve the stability of the device and appears to act as an adhesion layer for the organic materials above it. The inechanism(s) for these improvements is not yet well understood. 

Using a stable dopant as the emissive dye has been shown to greatly enhance the lifetime of small molecule LEDs. Rubrene doped into the Alq, electron transport layer ] 184] or into the TPD hole transport layer 1185] can extend the lifetime by an order of magnitude. Similarly, dimclhylquinacridone in Alq has a beneficial effect ]45 ]. The likely mechanism responsible for this phenomenon is that the dopant acts as a trap for the excilon and/or the charge. Thus, molecules of the host maLrix are in their excited (cationic, anionic or cxcitonic) states for a smaller fraction of the time, and therefore have lower probability to undergo chemistry. 

In contrast with conjugated polymers, such as PPV, devices employing CN-PPV 47 as the emissive layer can achieve respectable internal efficiencies (ca. 0.2%) with both calcium and aluminum electrodes. EL efficiency may be further improved by employing a hole-transporting layer such as PPV in conjunction with... 

According to that model, the net current flow in the device therefore can be increased in bilayer structures using a hole-transport layer, which possess higher hole mobility than the active polymer layer and which changes the height of the potential barrier at the interface transport layer/hole injection contact [81],... 

Figure 9-28. Trap-limited current (low ills (solid lines) lo the experimental (symbols) l/V characteristics of two typical devices with a 200 nin and 600 nm thick hole-transport layer and Alq3. Inset shows l/V curves for various different Alq3-lhicknesses. Reproduced front Ref. 82. ...

One can, nevertheless, conclude that (i) there is only a very small barrier for hole injection from ITO to PTV, if any barrier at all, (ii) a finite energy should exist for hole transport across the PTVIDASMB interface, and (iii) PBD should act as an efficient internal blockade for hole transport towards the cathode. 

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