Polymer hole-transporting materials

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. 

A better PLED performance was observed by Jenekhe and coworkers [173] for ITO/PEDOT/polymer/Al devices with quinoxaline-phenylene vinylene copolymers 586 and 587 as emitting layers. The el and maximum brightness were measured as 0.012 and 0.01%, and 120 and 35 cd/m2, respectively. The turn-on voltages of these devices were reasonably low, 6.0 and 4.0 V, respectively. The performance of PLEDs with polymer 586 was further improved by blending with 5wt% of a hole transport material, 1, l-Mstdi-d-tolylami-ii ophenyI )cycIohexane (TAPC) that enhanced the d lto 0.06% and the maximum brightness to 450 cd/m2. 

S. Liu, X. Jiang, H. Ma, M.S. Liu, and A.K.-Y. Jen, Triarylamine-containing poly(perfluorocy-clobutane) as hole-transporting materials for polymer light-emitting diodes, Macromolecules, 33 3514-3517, 2000. 

Y. Zhang, R. Hreha, G. Jabbour, B. Kippelen, N. Peyghambarian, and S.R. Marder, Photo-crosslinkable polymers as hole-transport materials for organic light-emitting diodes, J. Mater. Chem., 12 1703-1708 (2002). 

A. Kimoto, J. Cho, M. Higuchi, and K. Yamamoto, Novel carbazole dendrimers having a metal coordination site as a unique hole-transport material, Macromolecular Symposia 209 (Organo-metallic and Coordination Clusters and Polymers) pp. 51-65 (2004). 

The research race aiming at solidification of DSC by replacing the liquid electrolytes with solid state materials such as conductive polymers and novel hole transport materials is still on. Tennakone disclosed the use of CuBr as an exotic hole transport material for solidification of DSC.,02) Such success would open up the possibility of a low-cost printing process to fabricate solid-state DSC. 

A solid-state solar cell was assembled with an ionic liquid—l-ethyl-3-methylimidazolium bis(trifluoromethanesulfone)amide (EMITFSA) containing 0.2 M lithium bis(trifluoromethanesulfone)amide and 0.2 M 4-tert-butylpyridine—as the electrolyte and Au or Pt sputtered film as the cathode.51,52 The in situ PEP of polypyrrole and PEDOT allows efficient hole transport between the ruthenium dye and the hole conducting polymer, which was facilitated by the improved electronic interaction of the HOMO of the ruthenium dye and the conduction band of the hole transport material. The best photovoltaic result ( 7p=0.62 %, 7SC=104 pA/cm2, FOC=0.716 V, and FF=0.78) was obtained from the ruthenium dye 5 with polypyrrole as the hole transport layer and the carbon-based counterelectrode under 10 mW/cm2 illumination. The use of carbon-based materials has improved the electric connectivity between the hole transport layer and the electrode.51... 

Triphenylamine derivatives are known to be efficient hole transport materials and are widely used in organic light-emitting devices. Thelakkat et al. reported the synthesis of a 2,2-bipyridine ligand capped with polyfvinyl-triphenylamine) at both ends.97 The polymer chain was synthesized by the atom transfer radical polymerization of 4-bromostyrene using 4,4-bis (chloromethyl)bipyridine as the initiator (Scheme 18). The bromide groups were then replaced by diphenylamine in the presence of palladium catalyst. Polymer 33 was then obtained by the metalation reaction. 

Many of the hyperbranched polymers contain aromatic chromophores in their 7r-conjugated structures and should show interesting optical properties [31]. This is indeed the case. For example, hb-P20 carries the TPA chro-mophore, which is often used as hole-transport materials in the fabrication of light-emitting diodes [120]. The Aab of triyne 20 appeared at 342 nm (Fig. 5),... 

Figure 4.46. Molecular structures of commonly used OLED/PLED materials. Shown are (a) Alq3 (tris(quinoxalinato)Al (III)) used as an electron-transport material (b) DIQA (diisoamylquinacridone) used as an emissive dopant (c) BCP (2,9-dimethyl-4,7-diphenyl-l,10-phenanthroline) used as an exciton/ hole blocking agent (d) NPB (l,4-bis(l-napthylphenyl amino)biphenyl) (e) PFO (9,9-dioctylfluorene) used as an emissive polymer in PLEDs (f) PEDOT-PSS (poly-3,4-ethylenedioxythiophene-polystyrene sulfonate) used as a hole transport material in PLEDs.

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