Brian Collins, Obaid Alqahtani, Maxime Babics, Julien Gorenflot, Victoria Savikhin, Thomas Ferron, Ahmed Balawi, Andreas Paulke, Zhipeng Kan, Michael Pope, Andrew Clulow, Jannic Wolf, Paul Burn, Ian Gentle, Dieter Neher, Michael Toney, Frédéric Laquai, Pierre Beaujuge
It is established that the nanomorphology plays an important role in performance of bulk-heterojunction (BHJ) organic solar cells. From intense research in polymer-fullerene systems, some trends are becoming apparent. For example, small ~10 nm domains, high crystallinity, and low miscibility are typically measured in high-performance systems. However, the generality of these concepts for small-molecule (SM) BHJs is unclear. We present a comprehensive study of performance, charge generation and extraction dynamics, and nanomorphology in SM-fullerene BHJ devices to probe these critical structure-property relationships in this class of materials. In the systems investigated, small domains remain important for performance. However, devices composed of highly mixed domains with modest crystallinity outperform those consisting of pure/highly crystalline domains. Such a result points to an alternative ideal morphology for SM-based devices that involves a predominant mixed phase. This stems from SM aggregation in highly mixed domains that both maximize interface for charge generation and establish continuous pathways for efficient charge extraction. Such a morphological paradigm should be considered in future SM systems in pursuit of high-efficiency large-scale solar power production.
Organic semiconducting materials are typically subject to energetic and positional disorder and localization of electronic states. The electronic behavior of these materials are therefore strongly influenced by thermalization
of charge carriers in the localized density of states (DOS). Consequently, non-equilibrium processes play an important role in the operation of devices made of such materials.
We show as an example that measurements of recombination dynamics, conducted under
transient or steady-state conditions, can easily be misinterpreted when a detailed understanding of the interplay
of thermalization and recombination is missing. To enable adequate measurement analysis, we solve
the multiple-trapping problem for recombining charge carriers and analyze it in the transient and steady
excitation paradigm for different DOS distributions.
We show that recombination rates measured after pulsed excitation are inherently time-dependent, since
recombination gradually slows down as carriers relax in the DOS. When measuring the recombination order
after pulsed excitation, this leads to an apparent high-order recombination at short times. As times goes
on, the recombination order approaches an asymptotic value. For the Gaussian and the exponential DOS
distributions, this asymptotic value equals the recombination order under continuous
excitation. For a more general DOS distribution, the recombination order can also depend on the carrier
density, under both transient and steady-state conditions.
However, we show that there are cases where thermal equilibrium is never attained in the device.
We conclude that transient experiments can provide rich information about recombination in and out of
equilibrium and the underlying DOS occupation provided that consistent modeling of the system is performed.
The increasing interest in non-volatile memory devices has extended the exploration towards new materials, such as organic-inorganic hybrids. Devices based on organic semiconductors and embedded metal nanoparticles (MNPs) were found to display resistive bistability, suitable for programmable electronic applications. Different models were developed to explain the resistive switching mechanism occurring in the devices. Charging/ de-charging of MNPs and concomitant resistivity changes was mainly proposed as mechanism, despite the lack of solid experimental evidence.[ ] In this contribution we report on the role of the space-charge field due to charged MNPs in two-terminal devices, via electrical characterization. Devices comprise 4,4-bis[N-(1-naphthyl)-N-phenyl-amino]diphenyl (α-NPD) with embedded gold nanoparticles (AuNPs). The electrical characterization (current vs bias) of the devices was conducted with and without illumination during operation. Due to the energy level alignment of the chosen materials, the AuNPs behave as deep charge carrier traps. The induced space-charge spontaneously sets the device to the low conductivity state. The de-charging of the AuNPs can then be dynamically induced through illumination, setting the device to a high conductivity state.
Despite the ability to optically control the charging state of the AuNPs, the devices do not display any bistability. This finding provides evidence that the commonly proposed MNPs charging/de-charging mechanism can be excluded as the cause for electrical bistability in two-terminal devices, and that other mechanisms, such as filament formation, should be evoked.
The optically induced switching of material properties is important for investigations of opto-electronic effects and optomechanical
properties. Investigated organic materials contain chromophore dipole consisting of acceptor and donor
groups bridged by a delocalized &pgr;-electron system. Both calculations and experimental data show a reversible highly
dipolar photoinduced intra molecular charge transfer in indandione type molecules (DMABI) accompanied by change of
the sign and the value of the dipole moment. Investigations of optical properties of thin host-guest polymer films show
that the photoinduced process of DMABI is related to the photoinduced switching between two equally stable states of
the molecule. In this work first results of formation of the surface relief in polymer films incorporated with DMABI
derivatives will be presented. The refractive index gratings of DMABI host-guest films show that red light is less
diffracted than blue one. The reversible surface potential changes on irradiation in photoinduced intramolecular electron
transfer band in polymer host-guest films is observed. The DMABI molecules in solid state have nonlinear optical
properties, which can be used and investigated in host-guest polymer matrix. The influence of concentration of DMABI
molecules on photoinduced processes is discussed.
We report on a nearly isothermal, reversible transition of a polymer film from an isotropic solid to an anisotropic liquid state in which the degree of mechanical anisotropy can be controlled by light. The phase transition phenomenon and the related unidirectional mass-transport effect are caused by optically driven molecular motion of azobenzene-functionalised molecular units, which can be effectively activated only when their transition dipole moments are oriented close to the direction of the light polarization. We also show that the selective excitation of chromophores by linear polarized light induces an anisotropic expanding force, which can be used for polarization-selective opto-mechanical actuators and sensors. Moreover, since the molecular motions can be induced also by spatially confined non-propagating optical fields, we are able to manipulate the state and position of nanoscopic elements of matter using optical near-field approaches.
An increase in stochastic molecular vibrations of a solid due to heating above the melting point leads to a decrease in its
long-range order and a loss of structural symmetry. Therefore conventional liquids are isotropic media. We report on a
light-induced isothermal phase transition of polymer materials from an isotropic solid to an anisotropic liquid state in
which the degree of mechanical anisotropy can be controlled by light. Whereas during irradiation by circular polarized
light the materials behave as an isotropic viscoelastic fluid, they display considerable fluidity only in the direction
parallel to the light field vector under linear polarized light. In this case the polymers behave visually as a liquid along
one direction and as a solid in others (one-direction liquid-state), demonstrating thus extraordinarily anisotropic
mechanical properties. The fluidisation phenomenon is related to photoinduced motion of azobenzene-functionalised
molecular units, which can be effectively activated only when their transition dipole moments are oriented close to the
direction of the light polarization. We also show that the selective excitation of chromophores orientated parallel to the
light polarization not only has great influence on the anisotropy of the mechanical properties of the materials, but also
induces an anisotropic expanding force, which can be used for polarization-selective opto-mechanical actuators and
sensors.
We report on a light-induced isothermal transition of a polymer film from an isotropic solid to an anisotropic liquid state in which the degree of mechanical anisotropy can be controlled by light. Whereas during irradiation by circular polarized light the film behaves as an isotropic viscoelastic fluid, it displays considerable fluidity only in the direction parallel to the light field vector under linear polarized light, demonstrating thus extraordinarily anisotropic mechanical properties. The fluidisation phenomenon is related to photoinduced motion of azobenzene-functionalised molecular units, which can be effectively activated only when their transition dipole moments are oriented close to the direction of the light polarization. Along with the phenomenological finding, our work allows us to make a substantial step in understanding the mass transport effect in azobenzene containing systems under conditions of far- and near-field illumination.
The spectral characteristics of polyfluorene (PF) based light-emitting diodes (LEDs) are discussed. First, conditions that facilitate photo-oxidation of PF are investigated. We show that dense chain packing and addition of hole-trapping moieties lead to increased defect formation. Second, devices containing either a defined low concentration of keto-defects or of the polymer poly(9,9-octylfluorene-co-benzothiadiazole) (F8BT) are presented. Both types of blend layers were tested in different device configurations with respect to the relative and absolute intensities of green and blue emission components. It is shown that either blending of hole-transporting molecules into the emission layer at low concentration or the incorporation of a suitable hole-transporting layer reduces the green emission in the PF:F8BT blend, similar to what is observed for the keto-containing PF layer. We conclude that photo-oxidation leads to the formation of keto-defects that mainly constitute weakly-emissive electron traps.
Organic photovoltaic devices often show improved performances, if the active layer is made of a polymer blend. Due to the low miscibility of polymers the layer will phase separate and the lengthscale of the phase separation has a major influence on the device efficiency. We present a novel method to control the lengthscale of the phase separation, based on semiconducting polymer nanospheres (SPNs) forming the active layer. SPNs of M3EH-PPV (diameter 54nm) and CN-Ether-PPV (diameter 36nm) dispersed in water were produced by the miniemulsion process. Mono- and multilayers of these particles were fabricated by spincoating and photovoltaic devices utilizing these nanoparticles are shown to exhibit large external quantum efficiencies of up to 14%.
We have studied photogeneration, transport, trapping and recombination as the governing mechanisms for the saturation field strength and the time response of the photorefractive (PR) effect in PVK-based PR materials, utilizing xerographic discharge and photoconductivity experiments. Both the charge carrier photogeneration efficiency and the photocurrent efficiency were found to be independent of chromophore content, suggesting that the chromophore does not participate in carrier generation and trapping. The photoconductivity gain factor G defined as the number of charge carriers measured in photoconductivity in relation to the number of carriers initially photogenerated as determined by the xerographic experiments is found to be much smaller than unity, which indicates that the mean free path of the photogenerated charge carriers is less than the grating period. Photoconductivity data can be explained over 3 orders of magnitude in field, assuming a field-independent trap density. Based on the photoelectric data, PR response times have been predicted by Yeh's model for the build-up of space or by calculating the time, which is necessary to fill all traps by photogenerated holes. Only the latter model can reasonably well explain the observed field dependence of the PR growth time, suggesting that trap-filling essentially controls the PR onset behavior.
Light-emitting diodes based on chiral substituted, liquid crystalline (pi) -conjugated poly(fluorene)s have been built up which emit bluish light with a significant circularly- polarized component, despite the sub-micrometer thickness of the active layer. After proper annealing absolute values of the dissymmetry factor exceed 0.15 in absorption, and up to 0.25 in emission are measured. Compared to the first demonstration of CPEL in a conjugated polymer, a 200-fold increase in the degree of circular polarization in emission is achieved. Since polyfluorenes have been proven to be efficient and stable emitters in polymer light-emitting diodes, this concept has the potential for the construction of large-area circularly-polarized light sources for use in LCD displays and all-plastic optical storage devices.
Polymer light-emitting diodes (LED) with aligned conjugated molecules and polymers as the active layer can emit polarized light as shown recently in several reports. We have synthesized several liquid-crystalline polyfluorenes with different side chain patterns. The thin polymer films were aligned at elevated temperatures on a rubbed polyimide layer. For the most suitable substitution, a dichroic ratio of twelve was observed in absorption. This polymer was used to construct light-emitting devices with ITO and Ca electrodes. Suitable hole transport molecules were added to the polyimide in order to obtain layers with good alignment properties and large hole mobility. Polyimide and poly-fluorene layer thicknesses were varied to optimize the device performance. For those optimized devices, blue light with a dichroic ratio in emission of fifteen was measured and the luminance was approx. 100 Cd/m2 at 18 V. Even though these values are still below the requirements in application, the high dichroic ratio combined with a reasonable brightness represents an important step towards the use of organic LEDs as illumination sources in LCD displays.
The electromechanical properties of ultrathin films of polyglutamates with either a preferential polar upright orientation of the helical chains or an orientation parallel to the substrate plane are compared. The electric field- induced change in film thickness as determined by an electromechanical interferometer is used to measure of the degree of polar order and the mechanical compliance. Following the approach by Whitesell and Chang monomolecular films of 15 nanometer thickness of the helical polypeptide poly((gamma) -benzyl-L-glutamate) (PBLG) were grown directly from a flat aluminum surface. The measured PBLG film polarization was found to be comparable to that of conventional ferroelectric materials. This result demonstrates that the polymerization starting from the surface forces the helical chains into a polar arrangement. A large mechanical plate modulus of the film of approximately 35 GPa as determined by electrostriction agreed with the theoretical prediction for a single PBLG molecule along the chain axis.In contrast to this, Langmuir- Blodgett layers of poly((gamma) -octadecyl-co-methyl-L- glutamate)s, bearing long flexible side chains, exhibited only negligible piezoelectric activities, as the helices are oriented parallel to the substrate surface.In this case the mechanical properties were almost entirely determined by the structure and packing of the flexible side chains.
Changes as small as 1 ppm in the real and imaginary components of the complex permittivity resulting from flash photoexcitation of dilute solutions of (pi) -bond conjugated oligomers and polymers can be measured with nanosecond time resolution using the time-resolved microwave conductivity technique. The results provide information on the extent of delocalization of singlet and triplet state photoexcitations. Data are presented for oligomeric and polymeric derivatives of phenylene vinylene, paraphenylene and thiophene. For the polymers excess polarizability volumes, (Delta) Vp, of the singlet exciton of 2000, 180 and 570 angstroms3 are found, respectively. (Delta) Vp for the triplet exciton of the polythiophene derivative is found to be only 25 angstroms3.
Electrooptical and electromechanical effects of a PNA substituted rigid rod-like polymer have been studied experimentally. Modulated attenuated reflection spectra show a strong piezoelectric contribution to the overall field induced response. The piezoelectric coefficient dP as determined independently by a Nomarski interferometer was found to be comparable in size to the electronic electrooptical coefficient (chi) (2)zzz. This behavior is explained by the particular electronic configuration of the PNA chromophores.
We demonstrate that the Maker fringe technique can be applied to third order frequency mixing experiments designed to measure the electronic (chi) (3)(-(omega) 3;(omega) 1,(omega) 1,-(omega) 2) dispersion with (omega) 1 for a poly[3-tetradecylthiophene] thin film. Both the magnitude and the phase of the (chi) (3) were obtained. In the magnitude spectrum there was a strong three-photon resonance where the output frequency ((omega) 3equals2(omega) 1-(omega) 2) corresponds to the energy level of the first one photon state. We also found a peak which could be either a 2(omega) 1 or a (Delta) (omega) (equals(omega) 1-(omega) 2) resonance. Since this method is considered to be more sensitive to two-photon resonances that third harmonic generation, it can be used as a powerful tool to probe two photon states in the case of thin film polymer samples.
We demonstrate a new method for investigating the orientational distribution of the rod-like poly[bis(m-butoxyphenyl) silane] molecules in Langmuir-Blodgett (LB) films. The first two non-trivial in-plane orientational order parameters C2 and C4 could be easily deduced by measuring the independent tensor components of (chi) (3)(-(omega) 3;(omega) 1,(omega) 1,(omega) 2). Using this method, we show that the annealing process improves the molecular alignment along the dipping direction. We were also able to get information about the domain morphology of the film from the off-diagonal components of (chi) (3).
We demonstrate resonant third harmonic generation as a powerful tool for analyzing the quasi 2-D molecular orientational order in a Langmuir-Blodgett film of poly(bis-m- butoxyphenylsilane). By recording Maker-fringes at different angles between the polarization of the fundamental beam and the dipping direction, we were able to determine the order parameters *2> and *4> simultaneously. We found *2> equals 0.466 +/- 0.030 and *4> equal to -0.12 +/- 0.10.
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