Dynamics of optical excitations in a ladder-type π-conjugated polymer containing aggregate states

Mahrt, Rainer F.; Pauck, T.; Lemmer, Uli; Siegner, U.; Hopmeier, M.; Hennig, Richard G.; Bäßler, H.; Göbel, E. O.; Bolívar, P. Haring; Wegmann, G.; Kurz, H.; Scherf, Ullrich; Müllen, Kläus

doi:10.1103/physrevb.54.1759

Cited by 104 publications

(76 citation statements)

References 13 publications

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“…Os fenômenos relacionados com a participação dos polímeros orgânicos em dispositivos eletroluminescentes são diferentes em vários aspectos quando comparados aos que envolvem a participação dos semicondutores inorgânicos 94 . Entre essas diferenças poderiam ser destacadas: a) dificuldade de formação dos pares polarônicos [95][96][97][98] porque a excitação eletrônica não leva a uma fácil separação de cargas. Essa separação requer uma energia de, aproximadamente 100 meV, muito maior do que comparada os poucos meV necessá-rios para produzir a separação de cargas em um semicondutor inorgânico; b) o transporte de carga ocorre por "saltos" entre estados localizados produzidos pelos elétrons π, e não através do transporte dentro da banda de condução, como no caso dos semicondutores inorgânicos.…”

Section: Propriedades Dos Dispositivos Eletroluminescentesunclassified

Dispositivos poliméricos eletroluminescentes

Oliveira¹,

Cossiello²,

Atvars³

et al. 2006

Quím. Nova

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Recebido em 29/9/04; aceito em 21/6/05; publicado na web em 1/12/05 POLYMERIC LIGHT EMITTING DEVICES. Here we present an overview of electroluminescent devices that use conjugated polymers as the active media. The principal components of the devices are described and we show some examples of conjugated polymers and copolymers usually employed in polymeric light emitting devices (PLED). Some aspects of the photo and electroluminescence properties as well as of the energy transfer processes are discussed. As an example, we present some of the photophysical properties of poly(fluorene)s, a class of conjugated polymers with blue emission.Keywords: polymer; photoluminescence; electroluminescence. INTRODUÇÃOOs primeiros trabalhos sobre condução eletrônica em polímeros conjugados datam de 1970 1,2 sendo que a eletroluminescência (EL) em polímeros foi relatada pela primeira vez em 1989 3 . O uso de polímeros conjugados com um sistema de elétrons π delocalizados em dispositivos optoeletrônicos, conhecidos como OPLEDs ("Organic Polymeric Light Emission Devices"), tem crescido desde então. Na atualidade, monitores coloridos na forma de matriz ativa, fabricados com polímeros orgânicos emissores de luz (OPLEDs) estão na fronteira de serem comercializados. As razões para o sucesso dos OPLEDs devem-se às múltiplas possibilidades de estruturas químicas dos compostos possíveis, às várias possibilidades de formas de montagem dos dispositivos, aliadas à alta performance e ao menor custo de produção 4 . A tecnologia de exibição não é, entretanto, o único campo em que materiais orgânicos em geral, e poliméricos conjugados em particular, aparecem com um futuro promissor. Entre esses novos campos aparecem as novas tecnologias de células solares 5-8 , transistores 9-11 , emissores tipo laser, sistemas de armazenamento e circuitos integrados poliméricos [12][13][14][15][16][17] . O uso de materiais orgânicos, poliméricos ou não, nos vários tipos de dispositivos tem, entre suas principais vantagens, a imensa possibilidade de preparação de estruturas químicas diferentes, além de poderem ser misturados, formando sistemas com outras propriedades. Têm, entretanto, alguns problemas tecnológicos importantes ainda não resolvidos, relacionados com a menor estabilidade química quando comparados com materiais inorgânicos ou organo-metálicos. Entretanto, a possibilidade de mistura física entre componentes permitindo a obtenção de sistemas com diferentes propriedades é, sem dúvida, uma forma atraente e de custo reduzido de preparação de novos materiais. Essa flexibilidade nas formas de preparação de sistemas é particularmente interessante no caso de polímeros eletroluminescentes para os quais pequenas alterações nas rotas de síntese podem levar a materiais com diferentes estruturas e, o fato de apresentarem diferentes estruturas químicas, leva a que os materiais passem a emitir em diferentes regiões do espectro [18][19][20][21] . As diferenças nos processos sintéticos podem, também, envolver reações de copolimerização, permitindo a geração de materi...

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Section: Propriedades Dos Dispositivos Eletroluminescentesunclassified

Dispositivos poliméricos eletroluminescentes

Oliveira¹,

Cossiello²,

Atvars³

et al. 2006

Quím. Nova

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“…[6][7][8] The presence of interchain electronic species has important implications for the performance of devices based on these materials: interchain species may be responsible for quenching a conjugated polymer's luminescence but also may be beneficial for promoting charge transport. [9][10][11] There is also debate concerning the physical nature of interchain electronic species, which are often referred to in the literature as "excimers", 8,[12][13][14] "aggregrates", 9,[15][16][17][18][19][20] or "polaron pairs". [6][7][8][21][22][23] The way that we distinguish among these labels is to use "excimer" to denote a neutral excitation shared equally between two or more chromophores in the electronic excited state and "aggregate" to imply delocalization of the wave function over multiple chain segments in both the ground and excited states.…”

Section: Introductionmentioning

confidence: 99%

The Nature of Interchain Excitations in Conjugated Polymers: Spatially-Varying Interfacial Solvatochromism of Annealed MEH-PPV Films Studied by Near-Field Scanning Optical Microscopy (NSOM)

Schaller¹,

Lee²,

Johnson³

et al. 2002

J. Phys. Chem. B

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The nature of interchain electronic species in conjugated polymers has been the subject of much debate. In this paper, we exploit a novel near-field scanning optical microscopy (NSOM)-based solvatochromism method to spatially image the difference in dipole moment, and hence the difference in degree of charge separation, between the ground and electronic excited states of the emissive interchain species in films of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV). The method uses NSOM to collect emission from near the surface of solid samples that are placed into contact with liquids of varying polarity. The solvatochromic spectral shifts of the interfacial luminescence are measured as a function of solvent polarity; the results are analyzed with an interfacial dielectric continuum model to determine the dipole moment of emissive excited states. Experiments performed on films of the laser dye trans-4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) in poly(methyl methacrylate) (PMMA) demonstrate that our interfacial NSOM solvatochromic method and analysis can successfully reproduce the known dipole change of DCM upon photoexcitation. With the method calibrated, we then apply it to the interchain luminescence from the surface of thermally annealed MEH-PPV films. The interfacial solvatochromic analysis reveals that the dominant interchain species in annealed MEH-PPV films is "excimer-like", exhibiting an ∼4-7 D decrease in dipole moment upon optical excitation. In a few highly localized regions of the film (ca. 1-2 µm in diameter), however, the interchain excited state exhibits a large (∼9-13 D) increase in dipole moment upon excitation, indicative of minority interchain species with a large degree of charge separation, such as exciplexes or polaron pairs. The large variation in excited-state dipole moments observed throughout the film is suggestive of an entire family of interchain species, each characterized by a different degree of charge separation. The fact that the large-dipole interchain species are found in spatially segregated domains implies that interchain charge separation in conjugated polymer films is associated with the presence of defects. When the molecular weight of the polymer is lowered, the large excited-state dipole regions increase in spatial extent, suggesting that the defects that promote charge separation are intrinsic and may be associated with the chain ends.

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“…26,34 In this material, spatial variations in E g,sp can be expected, either from the formation of aggregates 35,36 or from differences in orientation of the conjugated backbone with respect to the substrate. 37 In addition, keto defect sites can occur in the polymer chain during synthesis of the material, 38 which can also give rise to variations in E g,sp .…”

Section: Resultsmentioning

confidence: 96%

Scanning tunneling spectroscopy on organic semiconductors: Experiment and model

et al. 2004

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Scanning-tunneling spectroscopy experiments performed on conjugated polymer films are compared with three-dimensional numerical model calculations for charge injection and transport. It is found that if a sufficiently sharp tip is used, the field enhancement near the tip apex leads to a significant increase in the injected current, which can amount to more than an order of magnitude and can even change the polarity of the predominant charge carrier. We show that when charge injection from the tip into the organic material predominates, it is possible to probe the electronic properties of the interface between the organic material and a metallic electrode directly by means of tip height versus bias voltage measurements. Thus, one can determine the alignment of the molecular orbital energy levels at the buried interface, as well as the single-particle band gap of the organic material. By comparing the single-particle energy gap and the optical absorption threshold, it is possible to obtain an estimate of the exciton binding energy. In addition, our calculations show that by using a one-dimensional model, reasonable parameters can only be extracted from z-V and I-V curves if the tip apex radius is much larger than the tip height. In all other cases, the full three-dimensional problem needs to be considered.

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Dynamics of optical excitations in a ladder-type π-conjugated polymer containing aggregate states

Cited by 104 publications

References 13 publications

Dispositivos poliméricos eletroluminescentes

Dispositivos poliméricos eletroluminescentes

The Nature of Interchain Excitations in Conjugated Polymers: Spatially-Varying Interfacial Solvatochromism of Annealed MEH-PPV Films Studied by Near-Field Scanning Optical Microscopy (NSOM)

Scanning tunneling spectroscopy on organic semiconductors: Experiment and model

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