Methyl-substituted Ladder-type Poly Research Articles

Effect of confinement on photoluminescence of MeLPPP/SBA-15 nanocomposite

Photoluminescence spectra and kinetics of methyl-substituted ladder-type poly(para-phenylene) (MeLPPP)/SBA-15 mesoporous silica (SBA-15) nanocomposite have been studied and compared with photoluminescence properties of the MELPPP film and solution. Relaxation processes of excited singlet states in the closed geometry of the nanocomposite and in the diluted solution compared to the film based on the temperature dependence of photoluminescence (5–220 K) were studied. It was shown that the nanocomposite has twice the photoluminescence lifetime and three times weaker temperature dependence of photoluminescence intensity in comparison with the film. These peculiarities are due to a decrease in the number of polymer chains in a low-dimensional structure and a reduced role of exciton diffusion.

Influence of the Conjugation Length on the Optical Spectra of Single Ladder-Type (p-Phenylene) Dimers and Polymers

We employ low-temperature single-molecule photoluminescence spectroscopy on a π-conjugated ladder-type (p-phenylene) dimer and the corresponding polymer methyl-substituted ladder-type poly(p-phenylene), MeLPPP, to study the impact of the conjugation length (π-electron delocalization) on their optical properties on a molecular scale. Our data show that the linear electron-phonon coupling to intramolecular vibrational modes is very sensitive to the conjugation length, a well-known behavior of organic (macro-) molecules. In particular, the photoluminescence spectra of single dimers feature a rather strong low-energy (150 cm(-1)) skeletal mode of the backbone, which does not appear in the spectra of individual chromophores on single MeLPPP chains. We attribute this finding to a strongly reduced electron-phonon coupling strength and/or vibrational energy of this mode for MeLPPP with its more delocalized π-electron system as compared to the dimer. In contrast, the line widths of the purely electronic zero-phonon lines (ZPL) in single-molecule spectra do not show differences between the dimer and MeLPPP; for both systems the ZPLs are apparently broadened by fast unresolved spectral diffusion. Finally, we demonstrate that the low-temperature ensemble photoluminescence spectrum of the dimer cannot be reproduced by the distribution of spectral positions of the ZPLs. The dimer's bulk spectrum is rather apparently broadened by electron-phonon coupling to the low-energy skeletal mode, whereas for MeLPPP the inhomogeneous bulk line shape resembles the distribution of spectral positions of the ZPLs of single chromophores.

Femtosecond hot-exciton emission in a ladder-typeπ-conjugated rigid-polymer nanowire

A hot-exciton is usually the initial elementary excitation product of the solid phase, particularly in low dimensional photonic materials, which is a bottle-neck to all subsequent processes. Measurement of hot-exciton emission (HExEm) is a great challenge due to fast EK relaxation and thus very weak transient emission. Here we report the first unambiguous observation of femtosecond HExEm from thin films of a model quasi-one-dimensional {\pi}-conjugated organic rigid-rod quantum nanowire, MeLPPP (methyl-substituted ladder-type poly(para-phenylenes), by using femtosecond time-resolved fluorescence spectroscopy. The results show the clear HExEm from the cooling hot-excitons has a lifetime of ~500 to ~800 fs, and concomitant very weak density-dependent singlet-singlet annihilation (SSA) due to this ultrashort dwelling time.

Single‐Molecule Spectroscopy on a Ladder‐Type Conjugated Polymer: Electron–Phonon Coupling and Spectral Diffusion

We employ low-temperature single-molecule spectroscopy combined with pattern recognition techniques for data analysis on a methyl-substituted ladder-type poly(para-phenylene) (MeLPPP) to investigate the electron-phonon coupling to low-energy vibrational modes as well as the origin of the strong spectral diffusion processes observed for this conjugated polymer. The results indicate weak electron-phonon coupling to low-frequency vibrations of the surrounding matrix of the chromophores, and that low-energy intrachain vibrations of the conjugated backbone do not couple to the electronic transitions of MeLPPP at low temperatures. Furthermore, these findings suggest that the main line-broadening mechanism of the zero-phonon lines of MeLPPP is fast, unresolved spectral diffusion, which arises from conformational fluctuations of the side groups attached to the MeLPPP backbone as well as of the surrounding host material.

Fluorescence vibronic analysis in a ladder-type conjugated polymer

The underlying vibronic structure of methyl-substituted ladder-type poly(para-phenylene) absorption and emission was investigated using temperature dependent fluorescence and site-selective fluorescence spectroscopy. Principal contributions in the vibronic manifold were identified from modelling of the data, and were found to be in good agreement with theoretical predictions after a re-evaluation of the position of the zero-phonon transition. A crossover to Raman dominated spectra at low excitation energy and temperature was observed. The influence of Raman effects should form an important point of consideration in the interpretation of fluorescence data.

Exciton and Polaron Contributions to Photocurrent in MeLPPP on a Picosecond Time Scale

Electric field-induced second harmonic generation was applied to probe the electric field dynamics in methyl-substituted ladder-type poly(para-phenylene) (MeLPPP). To delineate the exciton and polaron contributions to the ultrafast field dynamics, we have investigated devices with traps and without them. The traps were induced upon deliberate oxidation. The relative exciton contribution to the field dynamics is larger in oxidized films due to a lower yield of mobile charge carriers.

Fabrication and characterization of Ta2O5 photonic feedback structures

The integration of a thin layer of the high refractive index material Ta2O5 in organic laser structures is studied. A complete fabrication process of Ta2O5 based photonic crystals is described including deposition, electron beam lithography and reactive ion etching. Functional devices using methyl-substituted ladder-type poly(para-phenylene) (MeLPPP) as gain material are characterized with regard to the effectivity of the fabricated feedback structures.

Continuous-wave two-photon spectroscopy on a ladder-type conjugated polymer

Employing continuous-wave two-photon fluorescence-excitation spectroscopy on a methyl-substituted ladder-type poly( para-phenylene) we studied the symmetry properties of the vibronic wave functions of the lowest electronically excited states and found that the symmetry selection rules for optical transitions are rather strictly fulfilled in this rigid, rod-like conjugated polymer. Additionally, these data allowed us to show that the energies of the predominant vibrational modes, the inter-ring stretching vibration and the aromatic ring CC stretching mode, are significantly lower in the second excited singlet state with respect to the corresponding energies in the first excited singlet state and in the electronic ground state.

Revealing the Electron–Phonon Coupling in a Conjugated Polymer by Single‐Molecule Spectroscopy

Electron–phonon coupling in a π-conjugated polymer is revealed by single-molecule spectroscopy in combination with statistical pattern recognition techniques. The technique allows to reveal the phonon-side band in the spectra of methyl-substituted ladder-type poly(para-phenylene) (see figure). For this polymer a weak electron–phonon coupling strength is found at low temperatures. The distribution of the phonon frequencies provides strong evidence that the low-energy vibrational modes, which couple to the electronic transitions, stem from vibrations of the host matrix.

Enhanced feedback and experimental band mapping of organic photonic-crystal lasers

Photonic-crystal lasers whose feedback is enhanced by a thin layer of titanium dioxide (TiO2) have been experimentally investigated. The use of TiO2 increases the index contrast in the photonic crystal as well as the confinement in the waveguide, resulting in larger feedback of the lasing modes. This allows lower thresholds and much smaller devices. Vertically emitting laser devices, using the methyl-substituted ladder-type poly(p-phenylene) (MeLPPP) as gain material, have been fabricated according to optimized parameters, and the measured dispersion relation of the structures matches excellently with simulations. The devices feature a three to five times lower threshold than devices whose feedback structure is etched directly into the fused silica substrate.

Ultrafast dynamics of charge carrier photogeneration and geminate recombination in conjugated polymer:fullerene solar cells

We investigate the nature of ultrafast exciton dissociation and carrier generation in acceptor-doped conjugated polymers. Using a combination of two-pulse femtosecond spectroscopy with photocurrent detection, we compare the exciton dissociation and geminate charge recombination dynamics in blends of two conjugated polymers, MeLPPP [methyl-substituted ladder-type poly($p$-phenylene)] and MDMO-PPV [poly(2-methoxy,5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene], with the electron accepting fullerene derivative PCBM [1-(3-methoxycarbonyl)-propyl-1-phenyl-$(6,6){\mathrm{C}}_{61}$]. This technique allows us to distinguish between free charge carriers and Coulombically bound polaron pairs. Our results highlight the importance of geminate pair recombination in photovoltaic devices, which limits the device performance. The comparison of different materials allows us to address the dependence of geminate recombination on the film morphology directly at the polymer:fullerene interface. We find that in the MeLPPP:PCBM blend exciton dissociation generates Coulombically bound geminate polaron pairs with a high probability for recombination, which explains the low photocurrent yield found in these samples. In contrast, in the highly efficient MDMO-PPV:PCBM blend the electron transfer leads to the formation of free carriers. The anisotropy dynamics of electronic transitions from neutral and charged states indicate that polarons in MDMO-PPV relax to delocalized states in ordered domains within $500\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$. The results suggest that this relaxation enlarges the distance of carrier separation within the geminate pair, lowering its binding energy and favoring full dissociation. The difference in geminate pair recombination concurs with distinct dissociation dynamics. The electron transfer is preceded by exciton migration towards the PCBM sites. In MeLPPP:PCBM the exciton migration time decays smoothly with increasing PCBM concentration, indicating a trap-free exciton hopping. In MDMO-PPV:PCBM, however, the exciton migration time is found to show a threshold-like dependence on the PCBM concentration. This observation is explained by efficient interchain energy transfer in ordered MDMO-PPV domains in conjunction with exciton trapping.

The impact of high bias voltages on the luminance characteristics of light-emitting electrochemical cells

We report on the opto-electronic characteristics (in particular with respect to the efficiency behaviour) of organic light-emitting diodes (oLEDS) and organic light-emitting electrochemical cells (oLECS), both prepared from a blue-green emitting conjugated polymer, (methyl substituted ladder type poly ( p-phenylene), mLPPP), as electronically conductive component. Different from the oLEDs, the oLECs are characterized by a distinct decrease of efficiency upon bias voltage increase. From scan rate dependent studies we conclude that this efficiency drop is not only due to device decomposition upon operation above the window of electrochemical stability but also reflects (and thereby sheds some more light on) the intrinsic mode of oLEC operation. Due to the declining width of the intrinsic region upon ongoing pin-junction formation field induced and electrode induced luminance quenching processes also play a decisive role.

The Efficiency of Light-Emitting Electrochemical Cells

We report on the efficiency behavior of light-emitting electrochemical cells (LECs) fabricated from a methyl-substituted ladder-type poly(p-phenylene) (mLPPP) that was blended with a crown ether based solid state electrolyte. Unlike organic light-emitting diodes (oLEDs) utilizing mLPPP as an active layer, the LECs suffer from a loss of efficiency at elevated current densities. From scan rate dependent studies we deduce that this efficiency drop is not only due to device decomposition upon high voltage operation and we also reveal the intrinsic mode of LEC operation. The decreasing width of the intrinsic region between the p- and n-type doped zones upon ongoing pin-junction formation causes distinct (either field or electrode induced) luminance quenching effects.

Near‐Infrared Electroluminescence from HgTe Nanocrystals

Colloidal semiconductor nanocrystals: We present a hybrid inorganic–organic near-infrared light-emitting device (see picture), which comprises a blend of a blue–green emitting, hole-conducting conjugated polymer, methyl-substituted ladder-type poly(para-phenylene), and HgTe nanocrystals. The demonstrated device emits at the telecommunications wavelength of 1.3 μm, while the exceptionally broad spectral range covered by HgTe nanocrystals allows further spectral tunability in the near-infrared region.

The Influence of the Phase Morphology on the Optoelectronic Properties of Light‐Emitting Electrochemical Cells

AbstractWe report on the morphological aspects of thin films prepared from a blue–green light‐emitting conjugated polymer, (methyl‐substituted ladder‐type poly(p‐phenylene, mLPPP)), blended with a solid‐state electrolyte composed either by a crown ether, dicyclohexano‐18‐crown‐6 (DCH18C6), or a high‐molecular‐weight poly(ethylene oxide) (HMWPEO), and a Li salt, lithium trifluoromethanesulfonate (LiCF3SO3, Li triflate (LiTf)), as they have been successfully applied in light‐emitting electrochemical cells (LECs). The surface morphologies of the blend layers were investigated using atomic force microscopy (AFM) in tapping mode, and the ion distribution was probed using X‐ray analysis by means of energy‐dispersive X‐ray spectrometry (EDXS) in the scanning electron microscope (SEM). We show that the two different phase‐separation processes, the complexation tendencies of the ionic species as well as the ionic transport numbers, have tremendous influence on the performances of the corresponding LECs, revealing either rectifying or symmetric optoelectronic characteristics in forward and reverse bias directions. This opens up new possibilities for tuning the optoelectronic properties of ion‐supported organic electronic devices.

The photophysics of organic semiconducting nanospheres: a comprehensive study

The steady-state and non equilibrium photoexcitation dynamics of semiconducting polymer nanospheres is studied in detail and compared to a regular bulk film of the same polymer (methyl-substituted ladder-type poly(para)phenylene). It is found that the major optical properties of the polymer are left unaltered by this innovative material processing approach. However there is a clear influence of the film nanostructure on the generation, migration and recombination behavior of the involved photoexcited species.

Organic mixed ionic–electronic conductors as active layers in light-emitting electrochemical cells: vibrational spectroscopic, microscopic and electronic characterization

We report on investigations of the morphological aspects of blend layers prepared from a blue-green light-emitting conjugated polymer (methyl-substituted ladder-type poly( p-phenylene), mLPPP), blended with either a crown ether, Dicyclohexano18crown6 (DCH18C6) or a high-molecular-weight poly(ethylene oxide) (HMWPEO), and a Li salt, lithium trifluoromethanesulfonate (LiCF 3SO 3, Litriflate, LiTf for short) as a solid state electrolyte as they were successfully applied as active layers in light-emitting electrochemical cells (LECs). The surface morphologies of the blends were investigated by atomic force microscopy (AFM) in Tapping mode© and the ion distribution with X-ray analysis by energy-dispersive X-ray spectrometry (EDXS) in the scanning electron microscope (SEM). The specific phase morphologies of the blends, the complexation of the ions and the ionic transport numbers were found to have tremendous influence on the performances of the corresponding LECs revealing either rectifying or symmetric optoelectronic characteristics in forward and reverse bias directions.

New Conjugated Ladder Polymer Containing Carbazole Moieties

AbstractA new ladder polymer incorporating a polar carbazole group within the main chain, ladder‐type poly(para‐phenylene carbazole), LPPPC, was synthesized and characterized by luminescence techniques. Its properties are compared to that of the well‐known methyl‐substituted ladder‐type poly(para‐phenylene), MeLPPP. The results obtained evidence a very low energetic disorder in this new polymer, presumably due to its near‐perfect intrachain structure and low content of defects. It was found that although the density of states (DOS) distribution for neutral excitations is narrower than in MeLPPP, the manifold of charge‐transporting localized states is substantially more energetically disordered due to important dipolar disorder contributions.

A detailed study of the photophysics of organic semiconducting nanospheres

Conjugated polymers are an intriguing class of organic semiconductors which can be utilized as active medium in a wide range of electronic applications. Semiconducting polymer nanospheres fabricated from a conjugated polymer dispersed in aqueous phase have been realized successfully utilizing the miniemulsion process. This class of material combines the properties of conjugated polymers and nanostructured matter and can overcome certain limitations in the film formation of defined multi-layer structures and self assemblies and can, therefore, enable new device design concepts. We present first spectroscopic investigations to characterize nanospheres of methyl-substituted ladder-type poly( para-phenylene) in film and compare them to conventional bulk films of the same polymer. The dynamics of the various photo-excited states is probed via both steady state and transient differential transmission spectroscopy.

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

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 3SO 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 3SO 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.