Mary E. Galvin

Modeling the influence of charge traps on single-layer organic light-emitting diode efficiency

We investigate theoretically the role of carrier trapping on the efficiency of single-layer organic light-emitting diodes (OLEDs) by incorporating traps into the OLED device model of Davids et al. [J. Appl. Phys. 82, 6319 (1997)]. Carrier trapping directly affects the density and mobility balance between electrons and holes through its effects on injection and mobility. In addition, trap-mediated changes in density alter recombination rates and spatial profiles of recombination that become important when excited state quenching at metallic contacts is considered. We illustrate these various influences of traps on device efficiency through computations on a series of model devices. Good agreement is obtained with previous experiments by Menon et al. [Chem. Mater. 14, 3668 (2002)], where energetic disorder from transport traps was shown to reduce device efficiency. Our model, however, predicts circumstances where traps will improve device efficiency as well and can assist with selection of contacts to reali...

Regulation of electronic behavior via confinement of PPV-based oligomers on peptide scaffolds

Intermolecular interactions dramatically affect the structure/property relationships of electroactive molecules in device environments and have been the subject of significant research effort. Purposeful manipulation of the distance between chromophores is a key parameter in such investigations, although such manipulation has proven difficult. Systems in which distances between chromophores can be controlled serve both as model systems for understanding structure/property relationships in more depth, as well as potential active components in devices. In this work, oxadiazole-containing poly(phenylenevinylene) (Oxa-PPV) oligomers have been chemically attached in specific positions to an α-helical peptide scaffold, which permits the presentation of the Oxa-PPV side-chains at distances of nominally 6 A and 11 A apart on the same side and also 7 A apart on opposite sides of the peptide. The Oxa-PPV side-chains were attached to the peptide scaffolds via straightforward Heck coupling strategies; the resulting modified peptides were characterized by a variety of spectroscopic methods. Circular dichroic spectroscopy (CD) confirmed the helical conformation of the scaffolds modified with the bulky conjugated side-chains, and suggested that side-chain interactions may improve the thermal stability of the molecules. Exciton-coupled CD measurements confirmed the interactions of the conjugated side-chains and detected subtle apparent differences in side-chain orientation that were also indicated in computational modeling of the molecules. Photoluminescence spectroscopy confirmed the electronic interaction of the side-chains; the results clearly captured differences in luminescence as a function of chromophore presentation, and further indicated differences in excited state species with differences in chromophore presentation. The observed effects indicate not only that bulky chromophores can be presented on helical peptide templates without loss of template conformation, but also that such presentation may accurately capture details of electronic transport between molecules commonly employed in organic electronic applications.

Synthesis and electrochemical investigation of oligo-(arylenevinylenes) intended for the preparation of two-dimensional polymer networks

Four phenylenevinylene oligomers (1/4 )h ave been synthesized and characterized by cyclic voltammetry, absorption spectroscopy and photoluminescence (PL). Two of the oligomers contain a tri-substituted central benzene ring, while one contains a tetrasubstituted central ring. These oligomers were prepared in an effort to increase hole and electron mobility in organic materials via extension of conjugation to two dimensions. Optical and electrochemical evidence is given for significant interactions of the alkoxysubstituted styryl units of 3 that are attached to the central benzene ring in meta positions. The tetra-substituted oligomer (4) also exhibits strong interactions between the pair of para substituted arms. The photoluminescence (PL) quantum yield for the tetrasubstituted oligomer is high, indicating that it is an excellent candidate for the active layer in organic light-emitting diodes. # 2002 Elsevier Science B.V. All rights reserved.

The role of sidechains in the luminescence of an electroactive polymer

We report here results of luminescence studies on a series of poly(2,5-dialkoxy-1,4-phenylene vinylene) derivatives incorporating an oxadiazole moiety in the conjugated backbone. While there is no difference in the photoluminescence (PL) quantum yield in solution, the photoluminescence intensity of thin films of the polymers shows a strong dependence on the length of the linear dialkoxy sidechains as it is varied from hexyloxy to dodecyloxy. We attribute this to differences in the packing of the chains and the interchain spacings in the polymers. The side group determines the spacing between the conjugated backbones and thereby, its luminescence efficiency. While there is a direct correlation between chain spacing and PL efficiency, the correlation with electroluminescence (EL) efficiency is subtler. A wider separation between the backbone chains, while suppressing formation of bound polaron pairs and thus aiding luminescence, may hinder interchain charge transport, carrier injection or carrier balance. Hence the actual dependence of EL efficiency on the sidechain is an interplay between these two opposing factors.

The effects of energetic disorder and polydispersity in conjugation length on the efficiency of polymer-based light-emitting diodes

Energetic disorder plays a critical role in governing the performance of organic light-emitting diodes (OLEDs). This is particularly true for polymer-based OLEDs in which disorder can be controlled but not eliminated. Contrary to the common assumption that energetic disorder has only a negative effect on performance, we provide experimental evidence of improved efficiency when the energetic disorder is increased upon deliberate addition of traps via an increase in polymer chain length distribution. We use numerical calculations to extract the mechanisms responsible for the observed improvements and discuss the general conditions under which traps can be introduced to increase device efficiency.