Thin-Film Light-Emitting Devices Based on Sequentially Adsorbed Multilayers of Water-Soluble Poly(p-phenylene)s

Abstract

The discovery that conjugated polymers can be utilized as efficient emitters in thin-film electroluminescent devices has created a flurry of research effort over the past few years. [1] Current efforts have produced devices with high quantum efficiencies (1±4 %) and brightness, respectable lifetimes, and the full spectrum of colors, including white light. [2] While poly(phenylenevinylene) (PPV) and its derivatives are the most well-studied conjugated polymers for use in light-emitting devices; poly(p-phenylene) (PPP) based molecules have also received attention as emitting materials. [3±9] Due to its large bandgap, PPP has the desired quality of emitting in the blue region of the spectrum, which is not easily achieved with other conjugated polymers. However, because of the poor processibility of PPP in its underivatized form, previous workers have synthesized PPP ladder±type polymers with chemical bridging units between phenylene rings [10] as well as ring-functionalized PPPs [11] to promote solubility. The planar laddertype structures often have to be carefully manipulated to avoid the formation of molecular aggregates, which can lead to a red-shifted component of the photoluminescence. [12] Recently, one of us synthesized polyanionic and polycationic versions of PPP that are soluble in water, do not contain bridging units, and do not red-shift from their blue photoluminescence. [13±15] Because of their charged nature and related water solubility, these molecules also have the added advantage that they can be processed at the molecular level by the extremely versatile layer-by-layer sequential adsorption technique.The layer-by-layer sequential adsorption technique has been shown to produce uniform thin films with molecularlevel thickness control. [16] This technique is based on the alternating adsorption of a positively charged molecule and a negatively charged molecule to form a bilayer building block. By repeating this adsorption cycle, a film can be made with a thickness that is determined by the thickness of the individual bilayers (typically 10±60 for polyelectrolytes) and the total number of bilayers adsorbed. Furthermore, by adjusting simple solution parameters such as the amount of added salt [17,18] and the solution pH, [19] the thickness, composition and layer interpenetration of the bilayer building block can be systematically varied. Because of the technique's flexibility and ease of use, multilayer sequential adsorption has been successfully carried out with a variety of charged molecules, both organic and inorganic. [20] Within our research group, the ability to readily construct complex multilayer heterostructures and control electrode interfaces via the sequential adsorption technique, has been exploited with films containing PPV to both optimize and understand the operation of light-emitting thin-film devices. [21,22] More recent work has focused on an optimization of the device performance of PPV and polymeric tris(bipyridyl) ruthenium(II)±containing multilayers through manipulation...

Star-branched cationic light-emitting dot with silsesquioxane core, synthesis, and light scattering studies

“…The screening effect is usually described by a Debye-Huckel screening length k D , related to the presence of free counterions in the solution [7]. Figure 8 shows the effect of pH on the values of R g and R h for sample W at low ionic strength (0.01 M NaCl).…”

“…The aggregation of conjugated polyelectrolyte solution depends on the polymer concentration, salt concentration, solution pH, and polymer charge density. The changes in solvent environment, ionic strength, and temperature have a significant impact on the coil size, aggregation, and the structure of the polymer because of its polyelectrolyte character [4][5][6][7].…”

Regular poly(para-phenylene) films bound to gold surfaces through the electrochemical reduction of diazonium salts followed by electropolymerization in an ionic liquid

“…Among these conjugated polymers (polythiophenes, polyphenylenevinylenes, polyaniline….) [18], poly(para-phenylene) (PPP) is particularly interesting [19,20], for its high electrical conductivity when doped [21], its blue light emission [22][23][24][25][26], its thermal stability [27], its mechanical properties [28]. PPP is an excellent candidate for many optoelectronics applications in light-emitting diodes (LEDs) [14], field-effect transistors [29], or photovoltaic cells [30].…”

Covalent Layer‐by‐Layer Assembly of Conjugated Polymers and CdSe Nanoparticles: Multilayer Structure and Photovoltaic Properties

“…[12] The presence of inert layers might limit charge transport within films which in turn influences the performance of the resulting thin-film devices. [13] More recently, our group and others have reported organic-solvent based LbL approaches for the generation of conductive multilayer films based solely on p-conjugated polymers. [14] The ability to fabricate multilayer thin films in organic media broadens the range of LbL techniques and provides a new avenue to assemble functional materials with poor water solubility for complicated structures.…”