Doped Polymers and Polymer Blends

Fluorescent small molecules are used as dopants in either electron- or holetransporting binders. These emitters are selected for their high photoluminescent quantum efficiency and for the color of their emission. Typical examples include perylene and its derivatives [44], quinacridones [45], pentaphenylcyclopentene [46], dicyanomethylene pyrans [47, 48], and rubrene [36, 49]. The emissive dopant is chosen to have a lower excited state energy than the host, such that if an exciton forms on a host molecule it will spontaneously transfer to the dopant. Relatively small concentrations of dopant are used, typically in the order of 1%, in order to avoid concentration quenching of their luminescence. 

Small organic molecules doped into polymer binders have also been employed in DEED devices. Coating is done using inexpensive solution techniques. A commonly used polymer is polyvinylcarbazole [50-52], which is a hole transport material with moderate mobility. Polysilanes have also been used as hole transport agents [53]. Common laser dyes may be employed as the emissive molecular dopant, for example coumarins [50, 51], rhodamines [50] or pyrans [51, 53]. Similar considerations of energetics and concentration apply to doped polymers as to small molecule dopant-host systems the dopant should provide the lowest energy site for the exciton and concentration quenching should be minimized. 

A small amount of work has been undertaken to explore polymer blends, where one polymer is chosen for its charge transport characteristics and the other for its luminescence. Best efficiencies are obtained when small concentrations of the emitter are present in the transporting polymer, e.g. blending one PPV deriva- 

13 The Chemistry, Physics and Engineering of Organic Light-Emitting Diodes 

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

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Matienzo, L. J. Egitto, F. D., Transformation of Poly(dimethylsiloxane) into Thin Surface Films of SiO by UV/Ozone Treatment. Part II Segregation and Modification of Doped Polymer Blends. J. Mater. Sci. 2006, 41, 6374-6384. 

J.E. Cochran, M.J.N. Junk, A.M. GlaudeU, P.L. Miller, J.S. Cowart, M.F. Toney, CJ. Hawker, B.F. Chmelka, M.L. Chabinyc, Molecular interactions and ordering in electrically doped polymers blends of PBTTT and F4TCNQ, Macromolecides... 

Some of the polymers can be processed like ordinary polymers even in the doped state, which is one of their virtues. Like ordinary polymers, blending of dilferent polymers (e.g., a conducting polymer and an ordinary polymer) is possible. The mutual compatibility of the two polymers can be improved by choosing in the conductive polymer a tenside-type dopant ion that has a tail having affinity to the non-conductive polymer. 

Stabilization of PP, HDPE, LDPE and LDPE/L-LDPE blends is the main application field of HAS [12]. The polymer matrix influences, to some extent, the efficiency. For example, the light stabilizing effect in LDPE/L-LDPE increased with the relative content of L-LDPE [140]. Differences in the rate of the oxygen uptake were found between 28, R = H doped PP and PE oxidized at 90 °C or irradiated with light having A >290 nm [141]. 

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