Alexander Pogantsch

Green emission from poly(fluorene)s: The role of oxidation

Poly(fluorene)-type materials are widely used in polymer-based light emitting devices. In their pristine state, they emit in the deep blue spectral region. During operation there appears, however, an additional emission peak at around 2.3 eV. This observation has usually been attributed to aggregate or excimer formation. Recently, it has been shown that photo- and/or electro-oxidation of poly(fluorene) chains resulting in ketonic defects (i.e., formation of fluorenone groups) can also be held responsible for emission in that spectral region. In this contribution, we apply quantum-chemical techniques to gain a detailed understanding of the optical properties of poly(fluorene)s containing ketonic defects. In particular, we compare model systems for poly(fluorene) with their ketone-containing counterparts, focusing on the influence of excited-state localization effects. The results of the theoretical calculations are confirmed by experimental investigations on statistical copolymers of fluorene and 9-fluorenone.

Optimisation of polyfluorenes for light emitting applications

The properties of polyfluorenes for use in luminescent devices have been improved by a variety of approaches. Their conjugation length may be extended by using more planar indenofluorene units. Formation of long wavelength emitting aggregates is suppressed by attachment of bulky dendron groups to give stable blue emission. The emission colour can be tuned across the visible spectrum by incorporation of perylene dye units on the main chain or as substituents. Rod-coil copolymers have been made in order to achieve control of the supramolecular order. Polyfluorenone prepared by a precursor route shows improved electron injection and transport properties.

Low-onset organic blue light emitting devices obtained by better interface control

We demonstrate that highly efficient single-layer light emitting devices (LED) can be realized by better control in the device production process, especially regarding the interface between the active material and the metal cathode. The device cross-section was investigated using Auger depth profiling and spectroscopy. Following this approach, LEDs with para-sexiphenyl (optical gap: 3.1 eV) as active medium and aluminum as cathode were realized, which emit blue light at a bias of only 3.5 V, when the aluminum deposition rate is drastically reduced, namely from 10 to below 1 A/s. We find that the lower deposition rate of Al results in a decreased width of the interfacial region, where carbon, aluminum, and oxygen are intermixed. At the same time the relative oxygen concentration at the cathode interface is increased. However, the presence of oxygen does not lead to the formation of oxidized aluminum species, as verified by the Al local mixing model (LMM) Auger signal.

Optically written solid-state lasers with broadly tunable mode emission based on improved poly (2,5-dialkoxy-phenylene-vinylene)

We report an optical written distributed feedback (DFB) structure with broadly and continuously tunable mode emission (Δλ=30 nm) based on polycondensation-type poly[2methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH–PPV). In a Lloyd-mirror configuration we realize first-order feedback by dynamic photoinduced in-plane gratings, which can also be imprinted permanently. This technique is a versatile fabrication tool for polymeric DFB laser structures while also providing an efficient method to probe the lasing performance of organic solid-state laser materials. Furthermore, features of the photoinduced absorption spectra of improved MEH–PPV indicate favorable material properties towards further homopolymer solid-state laser applications.

Charged defects in highly emissive organic wide-band-gap semiconductors

A combined photoluminescence (PL) -detected magnetic-resonance (PLDMR) and thermally stimulated current (TSC) study of defects in wide-band-gap para-phenylene-type semiconductors is described. As TSC probes the density of mobile charge carriers after detrapping and PLDMR reveals the influence of trapped charges on the PL, their combination yields the concentration of traps, their energetic position, and their contribution to PL quenching. The reported trap densities, which are 2×1016 for the polymer and 1×1014 cm−3, for the oligomer, are the lowest reported for para-phenylene-type materials.

Long lived photoexcitation dynamics in a dendronically substituted poly(fluorene)

Polyfluorenes are possibly the most attractive class of conjugated polymers for blue polymer light emitting devices. Derivatives with dendron side chains have been found to significantly improve the color stability of the devices. Using a quasi-cw photoinduced absorption technique we present a qualitative and quantitative analysis of lifetimes and deactivation mechanisms for the triplet excited states in a dendronically substituted derivative. The triplet lifetimes increase by one order of magnitude in the dendronic derivative compared to those in a standard poly(dialkylfluorene), which has been characterized for comparison. The temperature dependence of triplet lifetimes is interpreted in terms of migration assisted deactivation of triplet excitations. We conclude that the dendrons inhibit the Dexter-transfers of triplet excitations between the polymer chains and so strongly hinder triplet migration.

Molecular fluorescent pH-probes based on 8-hydroxyquinoline

Three 5,7-pi-extended 8-benzyloxyquinolines, namely 5,7-diphenyl-, 5,7-bis(biphenyl-4-yl)- and 5,7-bis(4-dibenzothiophenyl)-8-benzyloxyquinoline were prepared and investigated as fluorescent pH-probes in nonaqueous solution. Absorption and photoluminescence spectra of the introduced compounds also including the starting material 8-benzyloxy-5,7-dibromoquinoline as well as their N-protonated counterparts were recorded and the results were rationalized by quantum-chemical calculations. A pronounced red shift of the emission occurred upon protonation of the non halogenated derivatives, while the dibromo-derivative is hardly emissive and is virtually not protonated under the conditions used. The diphenyl- and the bis(biphenyl)-derivative especially show promising photoluminescence quantum yields both in the parent and the protonated state making them candidates for the active component in pH sensing applications.

Photophysics of poly(fluorenes) with dendronic side chains

Abstract Display technology requires full red, green, and blue electroluminescence colors in polymer light emitting diodes. For the realization of blue emission poly(fluorene)s (PFs) are possibly the most attractive class of materials. The color stability of poly(fluorene)s is, however, relatively poor. Introducing bulky side chains, such as oligo-phenyl dendrons is a promising method for improving the color stability of these materials. The present study deals with the photophysics of a dendronically substituted poly(fluorene) derivative. The excited state properties are investigated by means of photoinduced absorption (PA) techniques at either quasi steady-state conditions or with femtosecond time resolution.

Polymer-based red, green, and blue emitting devices fabricated by reductive photopatterning

Using a color tuning approach reliant on reductive photopatterning, we present red-green-blue electroluminescence from a single layer polymer light-emitting device. To be able to cover the full color range, we employ a single emissive layer consisting of a blue emitter (the host polymer), as well as green and red emitting guest polymers. The energy transfer between the host and the various guest compounds is tuned via a reductive photoinitiated process in the presence of gaseous hydrazine. This process is compatible with regular film casting techniques such as spin coating, and therefore can be regarded as a promising alternative to the more complex, traditional patterning approaches.

A Comparative Study of the Photophysics in Polyfluorenes and Polyfluorenes with Polyphenylene Dendron Sidechains

The cw absorption, steady state photoluminescence (PL), photoinduced absorption (PA), PL-detected magnetic resonance (PLDMR), and the time resolved PL of a novel polyfluorene (PF) prepared with bulky polyphenylene dendrimer substituents are compared with those of (PF) with ethyl-hexyl substituents. We show that the dendronic sidechains suppress the contribution from unwanted low energetic emission, yielding a polymer with pure blue emission. The sidechains also strongly alter the dynamics of the excited entities. In particular, the time-resolved PL and temperature-dependence of the cw PL from 20-320 K reveal distinct singlet exciton (SE) dynamics in the polymer films, while the behavior in solution is essentially the same. However, the PA results show that the dynamics of polarons and triplet excitons (TEs) are similar, and the PLDMR shows that the interaction between the SEs and polarons are also similar.