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Optical amplification in polymer based materials and devices


New materials based on a copolymer of benzothiadiazole and a substituted triarylamine were developed with improved efficiency, which exhibit high charge mobility. These materials show p-type charge motilities of up to 3x10-3cm2V-1s-1 which is a factor of 102 more than light emitting polymers currently being developed for display applications.
Tyndall and Nanocomms have developed polymer-based planar and multimode waveguides doped with organo-lanthanide complexes for emission in the visible and the near-infrared. Planar waveguides incorporating Eu and Er complexes as luminescent dopants were fabricated using a novel UV-processable fluorinated polymer. Thin films doped with each of the complexes were fabricated and their spectroscopic properties investigated. The films act as low loss multi-mode planar waveguides capable of guiding visible and near infrared light emitted following optical excitation of the lanthanide dopants. Judd-Ofelt parameters were calculated for the Eu complex dopants and effects of the polymer host environment on the photophysical properties of the chelates were identified. The radiative properties of the Eu complexes were also determined viz. their potential for use in optical amplification applications. Multi-mode planar channel waveguides, doped with Eu(dbm)3(Phen), and Er(dbm)3(Phen) complexes, were fabricated by polymer hot embossing. A low-viscosity UV-processable host material, ethylene glycol dimethacrylate (EGDMA), with high optical transparency across the visible and near infrared spectral range is used as the waveguide matrix, while the thermoplastic, optically transparent polymethylmethacrylate (PMMA) material is used as an embossed substrate and superstrate material. The optical properties of the polymer materials were investigated and the filled, embossed structures were demonstrated to act as multi-mode channel waveguides capable of successfully guiding visible and near infrared light emitted following optical excitation of the organo-lanthanide dopants. In order to demonstrate the versatility of the hot embossing process with regard to integrated optics, further multilevel-embossed organo-lanthanide doped waveguides were fabricated with self-aligned integrated optical fibres, which were shown to successfully couple waveguided luminescence. We believe that this is the first time lanthanide complexes have been incorporated in hot embossed, polymer waveguide devices.
Merck developed copolymers containing emissive comonomers and charge transporting comonomers. The emissive and charge transport properties were quantified, and an ability to balance these properties through the appropriate selection of comonomer ratios was demonstrated. Based on the naphthalimide moiety which has a maximum emission at around 530nm and taking advantage of the Förster energy transfer process, TRT-France has developed three fluorescent side chain polystyrenes which emit in the green, the yellow and the red. For the green emission, the best candidates are the polystyrene functionalised with a bulky naphthalimide group (PST-NI) and this homopolymer doped with 1.5% of quinacridone. Quantum fluorescence yield as high as 65% in the solid state are thus obtained. Copolymer derived from PST-NI and grafted with 5% in mole of DCM has led to a yellow emission (lem = 578nm) with quite high PL quantum yield (58%). Finally, introduction of 1% in mole of perylene imide in PST-NI gave rise to a red emission with a PL quantum yield of 35%.
The Leuven group has been (and still is) mainly involved in synthesis and screening of novel lanthanide complexes. Up till now, a whole range of compounds has been developed, including novel quinolinate, betadiketonate, sulfonyl imide and other complexes of mainly erbium, neodymium and ytterbium ions for NIR applications. In inorganic waveguides, the erbium ion shows 1.53µm emissions with very long luminescence lifetime. This feature, along with the luminescence quantum yield close to 1, lead to very efficient optical amplification. However, the transfer of these properties in an all organic device is difficult. Organic molecules have harmonics of vibration close to 1.5µm, and the excited erbium level can couple to the vibrations and allow efficient non-radiative transfers to take place, which explain the shorter luminescence lifetime and lower luminescence quantum yield. As a result, the population inversion threshold in such materials is very high (estimated to be 930mW for an erbium doped polymer with 0.8µs luminescence lifetime). To overcome these problems, it is possible to use an energy transfer from the ligand to the lanthanide ion, using the very intense absorption transition of the organic part of the complex. Since ligands are often rather simple organic molecules, their main absorption band lies in the UV to blue part of the electromagnetic spectrum, a region where commercially available pumping diodes are expensive. For luminescence in the near-infrared, it is interesting to push further the absorption of the ligand into the red, where pumping diodes are commercially available. We chose tetra tert-butyl phthalocyanine, for its high absorption cross section in the red. The structure of the erbium phthalocyanine was characterized using mass spectroscopy (MALDI-TOF), and the characteristic lifetimes of the singlet, triplet and erbium states were measured, along with the branching ratios and therefore the sensitisation efficiency.
Technion has developed a small pixel LED structure that enables a match with the equivalent electrical circuit of the polymer LED to that of a commercial voltage driver). Using the state of the art materials provided by Merck, Technion has constructed efficient LEDs with low turn on voltage and which are bright at 3V. These LEDs were driven by a 50MHz pulse train and produced -12ns pulse width with - 5ns rise and fall time. These results indicate that our 100MHz milestone was achieved. The next generation of polymers that are now being studied show a much higher mobility (orders of magnitude) compared to the original “state of the art” materials and hence should achieve a modulation frequency of at least 500MHz. Technion have also started to characterize polymer doped LEDs. High PL efficiency and long emission lifetime have already been verified. To facilitate LED design, Technion have developed a new theoretical approach for the analysis of contact phenomena in low mobility disordered materials (i.e. organic) within a device model framework.
Merck has further developed air-stable p-type organic semiconductors, which are amorphous in nature and therefore charge motilities are isotropic. Motilities achieved are in the 10-3 - 10-2 cm2/v.s region. A synthetic route to the attachment of cross-linkable groups to the materials has also been developed, although the cross-linking conditions remain to be optimised. The technology should facilitate OSC layer integrity in multi-layer emissive devices.
We present an analytic description for the loss of photocurrent efficiency at moderate light intensities and demonstrate a simple technique for extracting the mobility of electrons in semiconducting polymer layers. The underlying theoretical analysis, which is based on a simple drift-recombination scheme, shows good agreement with the measured intensity dependent photocurrent quantum efficiency over 5 orders of magnitude in intensity. The electron mobility extraction is demonstrated for pristine MEH-PPV. We use the combination of theoretical and experimental study to discuss the role of recombination and space charge effects in reducing photocurrent efficiency. We apply the analytical results to device design criteria and deduce that the minimum, low field, mobility value of the slow carrier required to achieve close to ideal fill- factor is 10-2cm2v-1s-1
We chose tetra tert-butyl phthalocyanine, for its high absorption cross section in the red. The structure of the erbium phthalocyanine was characterized using mass spectroscopy (MALDI-TOF), and the characteristic lifetimes of the singlet, triplet and erbium states were measured, along with the branching ratios and therefore the sensitisation efficiency. Our results show that the complex offers an absorption cross section of 4.42 x 10-22 m² at 670nm, a sensitisation efficiency of 70%, and an erbium lifetime of nearly 4µs. Moreover the complex seems very stable under light flux, and can be added to a fluorinated polymer in a high concentration (1-1 complex-monomer ratio). Calculation of the population inversion threshold showed that 0.8mW pump power is needed, which is half the power needed for previously published structure, at a wavelength convenient for an industrial device.
Tyndall has developed (doped) polymer and nanocrystal based LED structures for emission in the visible and near-infrared. Electroluminescence spectra of Eu-doped LED devices exhibited a well-resolved emission peak centred at 612nm, assigned to the 5D0 -> 7F2 transition of the Eu3+ ion. Best results obtained for the Eu-doped devices gave an EQE of 0.03% and a maximum brightness of 130 cd/m2 at 380 mA/cm2 and 25 V. A series of Nd-doped LED devices were also fabricated and tested. EL spectra exhibited characteristic emission centred at ca. 1065 nm, assigned to the 4F3/2 ®4I9/2 transition of the Nd3+ trivalent ion, with a FWHM of ca. 35nm. A maximum EQE of 0.007% and a maximum luminance of 8.5 nW at - 480 mA/cm2 and 42 V was measured for these devices. Tyndall has also developed and tested a series of LED devices based on HgTe quantum dots. Near-infrared EL was successfully detected from ITO/PEDOT/HgTe/Al devices, with an intensity maximum at - 1660 nm and a FWHM > 250nm. A maximum EQE of 0.02 % and a maximum light output of 150nW/mm2 at 50mA and 2.5V was measured for these devices.
Routes to high purity monomers and an improved polymerisation process to produce high purity polymers for use in optical amplification devices. The new route gives high quality material with improved molecular weight. This work was subject to a patent application, although it was decided not to pursue this.