U. Scherf

Temperature dependent photoluminescence of organic semiconductors with varying backbone conformation

We present photoluminescence studies as a function of temperature from a series of conjugated polymers and a conjugated molecule with distinctly different backbone conformations. The organic materials investigated here are: planar methylated ladder type poly para-phenylene, semi-planar polyfluorene, and non-planar para hexaphenyl. In the longer-chain polymers the photoluminescence transition energies blue shift with increasing temperatures. The conjugated molecules, on the other hand, red shift their transition energies with increasing temperatures. Empirical models that explain the temperature dependence of the band gap energies in inorganic semiconductors can be extended to explain the temperature dependence of the transition energies in conjugated molecules.

Population and decay of keto states in conjugated polymers

Using time-resolved and steady-state photoluminescence techniques, fluorene/fluorenone copolymers have been studied to investigate the role of keto defects in degraded polyfluorene. Keto sites can be populated via migration from polyfluorene singlets, thereby quenching the polyfluorene fluorescence, and via direct photon absorption. In the former case, the migration process dominates all thermal and interchain variability in the efficiency of quenching. No annihilation process of fluorenone triplets and no interchain processes such as excimer formation participate in the defect emission itself.

The role of keto defect sites for the emission properties of polyfluorene-type materials

Abstract In this paper we report on the identification of a dominant degradation mechanism of blue light emitting polyfluorene (PFs) type polymers, which turns the desired blue emission color of these polymers into an undesired blue–green emission. As shown the low energy emission band at 2.2–2.3xa0eV can be identified as the emission from exciton and/or charge trapping keto defect sites (9-fluorenone sites), which in fact can be regarded as emission of a fluorescing guest effect incorporated into the π-conjugated PF backbone. We present spectroscopic evidence for the formation of keto defect sites in 9-monoalkylated polyfluorenes (PF-a) which are identified as the source of the strong low energy emission band. Furthermore, we show that the keto defects can be generated directly during polymer synthesis as shown for the PF-a or can be formed as main channel of an (photo-, thermal- or electro-)oxidative degradation process as shown for the 9-bisalkylated PF.

Comparison of thermal and electrical degradation effects in polyfluorenes

Abstract The origin of the low energy photoluminescence (PL) and electroluminescence (EL) peaks emerging upon degradation of highly blue emissive polyfluorenes (PFs) has been intensively debated in literature during the last years and has recently been identified as the emission from an exciton and/or charge trapping on-chain keto defect. In this work we compare several polyfluorenes with respect to their stability against degradation, namely a poly(9,9 dialkylfluorene) with two hexahydrofarnesyl sidebranches (PF 111/12), a slightly branched PF 2/6 with tetrabromospiro(fluorene-9,9′-fluorene) as branch (PF A193) and the same polyfluorene with triphenylamine endcappers (PF A207). The degradation of these polymers upon storage at an elevated temperature in vacuum and in air is compared using PL and infrared (IR) spectroscopy. We find that the stability of the spiro-type PFs is significantly improved compared to the regular PF. Essentially the same trend is observed in operated devices.

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 electrophosphorescent devices utilizing a ladder-type poly(para-phenylene) host

The properties of light-emitting diodes based on the electrophorescent platinum-porphyrin dye PtOEP blended into a ladder-type poly(para-phenylene) (LPPP) polymer host are presented. Due to the small difference between the highest occupied molecular orbital (HOMO) levels of the guest and the host, the operating voltage of single layer devices is almost independent of the dopant concentration in clear contrast to what has been observed in the case of wide-band gap host polymers. However, the efficiency and the color purity of these single-layer devices is quite poor, which can be attributed to the weak trapping of carriers on the phosphorescent dye. Incorporating an electron-transporting/hole-blocking layer greatly increases the efficiency of the devices but at the same time emission from the host becomes more significant. Adding a hole-transporting/electron-blocking layer further increases the efficiency of the devices. Pure red emission is, however, only obtained if the HOMO of the hole-transporting laye...

The influence of keto defects on photoexcitation dynamics in polyfluorene

Abstract The optical properties of two differently substituted types of polyfluorenes, 9-monoalkylated PF (mono-PF) and 9,9-dialkylated PF (bi-PF) where studied by means of photo-induced absorption (PIA) and ultrafast pump and probe measurements. The photo-induced absorption was complemented by measurements on a fluorene–fluorenone copolymer, which can be seen as a model substance for the polyfluorenes containing keto-defect sites. By differential transmission measurements we show that for the 9-monoalkylated PF measurements the singlet and triplet signal is strongly reduced compared to the 9,9-dialkylated PF. Instead, the polaron signal becomes the dominant feature.

Space-charge-mediated delayed electroluminescence from polyfluorene thin films

In polyfluorene thin films, an electrically induced short-lived delayed fluorescence (of the order of μs) is observed, which grows in intensity as the temperature is increased from 30 K to 290 K. From bias-offset-dependent electroluminescence intensity measurements and decay kinetics measurements, we conclude that the delayed recombination of space-charge carriers, along with the long-lasting tail of the prompt electrofluorescence, give rise to the short-lived delayed fluorescence.

The influence of spatial disorder of the ion distribution on the surface morphology in thin films of blend based organic mixed ionic-electronic conductors

Abstract We report on investigations of the morphology of spin-coated thin films of an organic mixed ionic-electronic conductor consisting of the conjugated polymer methyl substituted ladder type poly(para- phenylene) (mLPPP) blended with a solid state electrolyte (Dicyclohexano18crown6 (DCH18C6), complexed with lithium trifluoromethanesulfonate (LiCF 3 SO 3 )). This blend system was successfully applied as active layer in light-emitting electrochemical cells (LECs). While thin films blends of the conjugated polymer and the pristine crown ether show a very smooth surface, the addition of LiCF 3 SO 3 causes a pronounced surface roughening. Since such a distinct surface roughness can be the reason for a device failure mechanism that limits the device lifetime, this attitude is investigated by tapping mode atomic force microscopy (AFM). These studies are complemented with X-ray analysis by means of energy dispersive X-ray spectrometry (EDXS), in the scanning electron microscope (SEM), in order to get a better insight into the ion distribution within the blend layer and its influence on the surface roughness formed.

The Raman spectra of methyl substituted ladder type poly(p-phenylene): Theoretical and experimental investigations

We report on the Raman behavior of methyl substituted ladder type poly(p-phenylene) (mLPPP), a conjugated polymer with a high application potential for optoelectronic devices. A funded explanation of the experimentally observed Raman modes based on ab initio calculations is given, which takes also the influence of the side chains into account. Experimental investigations were performed with different laser excitation wavelengths on mLPPP in solution and films drop-casted on silicon substrates. From the studies of the polymer in solution the differential Raman scattering cross-section of mLPPP were determined on the basis of the known values of different solvents.