Photoluminescence polyfluorenes

Keywords Chemical defects Electroluminescence Light-emitting diodes Photoluminescence Polyfluorene-type polymers... 

L.M. Herz and R.T. Phillips, Effects of interchain interactions, polarization anisotropy, and photo-oxidation on the ultrafast photoluminescence decay from a polyfluorene, Phys. Rev. B, 61 13691-13697, 2000. 

Rhenium(I) tricarbonyl-2,2 -bipyridine moieties were used to cap both ends of a poly fluorine, yielding Re-capped Re(bpy)(CO)3(py)-X-(py)(CO)3(bpy)Re 2+ polymers, where X = polyfluorene [51, 52], The polymers with and without the Re caps were spin-coated from their solutions in CH2C12 onto an ITO surface previously modified with a layer of poly(styrene sulfonic acid), doped with poly(ethylenedioxythiophene). The LED (light-emitting device) was then topped with a layer of Ca/Al. The photoluminescence (PL) and electroluminescence seen were consistent with the presence of [Re(bpy)(CO)3(py)]+ [158],... 

Fig. 2 Normalized photoluminescence spectra of thin films of polymers before annealing (o), and after annealing for 2h at 150°C ( ), 4h at 200°C (x) and 24h at 250 °C (+) under ambient light and atmosphere, a Polyfluorene PF8. b Poly(dibenzosilole) 12...

Polyfluorene with on-chain ruthenium complex (polymer 49) has been synthesized by Suzuki polycondensation [77]. The photoluminescence of the copolymer was slightly blue-shifted as the concentration of dipyridy-lamine increased. The introduction of dipyridylamine and the ruthenium complex into the polymer significantly improved the photoluminescence efficiency. 

Fig. 1 Normalized photoluminescence spectra of pristine polyfluorene (solid line) and after degradation under intense UV irradiation (dashed line modified from [18])...

Fig. 6 Photoluminescence spectra of a pristine polyfluorene film thermally degraded in air (1 h at given temperature excitation wavelength 390 nm). The insert shows the corresponding IR spectrum (modified from [18])...

The photophysics of jt-conjugated polymers are reviewed in detail in other chapters of this book. (See, for example, Chapter 3.) Here, we focus on the electronic and photophysical phenomena that occur at the heterojunction between two different semiconductor polymers. The heteroj unctions are formed by combining four different polyfluorene copolymers in blend or bilayer thin films and are investigated using time-resolved and steady-state, temperature- and elec-tric-field-dependent photoluminescence measurements as well as electroluminescence and time-resolved spectroscopy. We review a body of work carried out in our laboratories over the last few years, and published in numerous journal articles (see refs. [13-17]). 

We show that the exciplex states that form at the interface between F8 and PFB can be thermally excited to form bulk excitons. This is observed as delayed PFB exciton emission in time-resolved photoluminescence measurements. These findings are analogous to those presented for PFB F8BT and TFB F8BT interfaces in the previous section and therefore support the generality of the phenomenon of endothermic exciplex-to-exciton energy transfer in polyfluorene blends. 

In Section 2.1.4, the electronic and photophysical properties of donor-acceptor or type-II heterojunctions between two semiconductor polymers were investigated. Exciplex formation was found to be a general phenomenon for type-II heterojunctions between polyfluorene copolymers. These are among the most widely used materials in polymer optoelectronics, but the presence of exciplexes had not yet been fully appreciated. This is because the exciplex population is often depopulated via efficient endothermic transfer towards the exciton, which leads to bulk exciton instead of exciplex emission. However, via time- and temperature-depen-dent photoluminescence (PL) spectroscopy the exciplex states can be identified. Employing a relationship known from small-molecule solution systems that relates the relative HOMO and LUMO levels of the molecules to the exciplex emis-... 

FIGURE 5.5 Absorption, photoluminescence emission, and photoinduced absorption spectrum of a typical polyfluorene-type polymer film. The inset shows the actual chemical structure with R typically being a branched or linear alkyl chain. 

The various functional layers in PLED devices are outlined in Figure 10.14. By applying a voltage across the device, electrons and holes are injected into the photoluminescent polymer, where they recombine to form excitons and emit light. Suitable, commonly used emissive polymers are conjugated materials such as poly(phenylenevinylene)s (PPVs) and polyfluorenes. 

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