Andrea Montali

Polarizing energy transfer in photoluminescent materials for display applications

Combinations of sheet polarizers and colour filters form the basis of numerous products — most notably colour liquid-crystal displays, — that require polarized chromatic light. But this combination of elements does not make efficient use of light, as a substantial fraction of the incident light is converted into thermal energy,, limiting the brightness and energy efficiency of the resulting devices. Here we show that these limitations can be overcome by using polymer-based photoluminescent polarizers. Our polarizers operate on a two-step principle: randomly orientated ‘sensitizer’ molecules harvest the incident light by isotropic absorption and then efficiently transfer the energy to a uniaxially orientated photoluminescent polymer, from which coloured light with a high degree of linear polarization is emitted. In principle, isotropic-to-polarized conversion efficiencies approaching unity could be attainable by this approach.

Poly(p-phenylene ethynylene)-based light-emitting devices

Abstract We here report polymer light-emitting diodes with substituted poly(p-phenylene ethynylenes) as the emissive layer. Yellow—green electroluminescence was observed for different poly(p-phenylene ethynylene) derivatives. Surprisingly, and importantly in view of stability issues, devices with an aluminium cathode were found to have a higher external quantum efficiency (0.035%) and a lower onset voltage for electroluminescence (10.8 V) than those with a calcium cathode. These results are explained in terms of a lower energy barrier for electron injection than for hole injection, consistent with the ionization potential of poly(p-phenylene ethynylenes) which was determined to be 6.3 eV below vacuum level with ultraviolet photoelectron spectroscopy and 5.8 eV with cyclovoltammetry.

Patterning of Oriented Photofunctional Polymer Systems Through Selective Photobleaching

Many applications of semiconducting conjugated polymers in (opto)electronic devices require the patterning of these functional materials into structures with feature sizes of typically between 1 and 100 μm, ideally by simple and reliable methods. We demonstrate that selective photobleaching, i.e., a spatially resolved change of the chemical structure of the active species by irradiation through an appropriate mask, is an extremely simple and versatile technique that satisfies this need. The process is particularly attractive for structuring oriented materials that exhibit anisotropic properties and is of potential interest for a broad range of applications.

Ultra-high performance photoluminescent polarizers based on melt-processed polymer blends

Photoluminescent polarizers that comprise uniaxially oriented photoluminescent species which absorb and emit light in highly linearly polarized fashion, can efficiently combine the polarization of light and the generation of bright colors. We here report the preparation and characterization of such polarizers by simple melt-processing and solid-state deformation of blends of a photoluminescent guest and a thermoplastic matrix polymer. The orientation behavior of a poly(2,5-dialkoxy-p-phenyleneethynylene) derivative (EHO-OPPE), 1,4-bis(phenylethynyl)benzene, and 1,4-bis(4-dodecyloxyphenylethynyl)benzene was systematically compared in different polyethylene grades. Experiments suggest that if phase-separation between the photoluminescent guest and the matrix polymer is reduced during the preparation of the pristine (i.e. unstretched) blend films, photoluminescent polarizers can be produced which exhibit unusually high dichroic properties at minimal draw ratios. In connection with this finding, an optimized, melt-processed blend based on 1,4-bis(4-dodecyloxyphenylethynyl)benzene and linear low-density polyethylene was developed that allows efficient manufacturing of photoluminescent polarizers which at draw ratios of only 10 exhibit dichroic ratios exceeding 50.

Depolarizing energy transfer in photoluminescent polymer blends

The occurrence of polarizing energy transfer in uniaxially oriented polymer blend films is investigated. A poly(2,5-dialkoxy-p-phenylene ethynylene) derivative (EHO-OPPE) and poly[2-methoxy-5-[2′-ethyl-hexyloxy]-p-phenylene vinylene] (MEH-PPV) were used as the acceptors, and various sensitizers were used as donors. Some of the properties of the chromophores required for polarizing energy transfer to occur efficiently are elucidated, such as form-isotropy and thermal characteristics. The energy transfer efficiency is quantified, and for the present, optimized systems, values as high as 85% were demonstrated.

Time-resolved fluorescence study on the mechanism of polarizing energy transfer in uniaxially oriented polymer blends

A time-resolved study of polarizing energy transfer in oriented blends of a conjugated polymer [a dialkoxy-substituted poly(p-phenyleneethynylene) derivative] and an organic laser dye (7-diethylamino-4-methylcoumarin) in ultra-high molecular weight polyethylene is presented. The transfer is described in terms of a Forster mechanism, based on long-range dipole–dipole interactions. Forster radii were determined in oriented blend films and in chloroform solutions. It was found that the transfer process is critically influenced by the phase behavior of the system under investigation. A depolarizing homotransfer between donor molecules was found to be a key step in the polarizing nature of the transfer which, ultimately, allows excitation light polarized perpendicularly to the film orientation direction to be emitted with the polarization direction parallel to its orientation.

Phase behavior and anisotropic optical properties of photoluminescent polarizers

The phase behavior and anisotropic optical properties of tensile deformed blends of a photoluminescent polymer guest in an ultra-high molecular weight polyethylene matrix were studied on the level of single molecules by means of scanning confocal optical microscopy. It is shown that upon tensile deformation of the blends, the system transforms from a phase-separated system into a quasi-molecular solid solution. The influence of this phase transition on the anisotropic optical properties of oriented blend films was also investigated with polarized steady-state photoluminescence spectroscopy. We show that well-dissolved guest molecules tend to reach higher degrees of orientation at lower draw ratios of the blend films compared to guests that phase-separate from the matrix polymer. Dichroic ratios in emission in the range of 50 were observed in optimized blend films based on photoluminescent oligomers and linear low density polyethylene.

Electroluminescence in poly(p-phenyleneethynylene)-based devices

In recent years there has been a considerable interest int he photoluminescence (PL) and electroluminescence (EL) properties of conjugated polymers, because of their potential application as the emitting layer in light- emitting devices. While poly(p-phenylenevinylene) (PPV) and its derivatives have been investigated most intensively among Al conjugated polymers, only few studies have been made on poly(p-phenyleneethynylene)s (PPEs), which feature a triple rather than a double bond in the conjugated backbone. Here we present out recent experiments on polymer light- emitting diodes (LEDs) based on poly(2,5-dialkoxy-p- phenyleneethynylene)s. The devices under investigation consist of the PPE emitting layer which is sandwiched between an indium tin oxide (ITO) and an aluminium, calcium or chromium electrode. Yellow-green electroluminescence with a brightness of up to 20 cd/m2 was observed for different PPEs. The EL intensity follows the applied bias and current as expected, and the EL emission essentially matches the PL spectrum. Interestingly, no significant difference in device performance was observed with respect to internal quantum efficiencies and onset-voltage, when comparing Al and Ca electrodes. These results suggest, that in the case of PPEs the hole injection barrier seems to be a limiting factor, while the injection of electrons from the low work function electrode is facilitated. Consequently, these devices favorably comprise an electron injecting contact with moderate work function which, as expected, was also found to lead to an improved device stability.

Novel liquid crystal displays based on highly polarized photoluminescent polymer films

Since the early 90s, much research has focused on the photoluminescence (PL) and electroluminescence (EL) properties of conjugated polymers, because of their potential application as emitting layer in EL devices. The introduction of uniaxial molecular orientation into films of luminescent polymers was naturally found to yield structures that emit polarized light. Rather surprisingly, the photoluminescence properties of oriented, conjugated polymers have attracted substantially less attention, especially from an application point of view. In this paper the authors report the fabrication of highly-polarized photoluminescent polymer films based on poly(2,5-dialkoxy-p-phenyleneethynylene)s (PPE), and their use in a new family of liquid crystal displays (LCDs). As one relevant example, a back-lit twisted-nematic configuration of an LCD was built, in which one of the absorbing polarizers was replaced by a polarized PL film, characterized by a dichroic ratio in excess of 70. Such devices can exhibit a substantial improvement in brightness, contrast and viewing angle, since the polarized photoluminescent films can combine two separate features, i.e., the functions of a polarizer and an efficient color filter.