We present a number of polyfluorene based conjugated polymers with crosslinkable acrylate and oxetane units. These polymers can be crosslinked by free radical polymerization in the case of acrylates and by cationic ring opening polymerization for oxetanes. Upon polymerization densely crosslinked networks are formed which are completely insoluble. We show that the diffusion coefficient of C60 in polyfluorene is reduced by a factor of 1000 by crosslinking. MIS-CELIV measurements are used to monitor changes in the charge carrier mobility upon crosslinking. It shows that using appropriate conditions, e.g. low initiator concentrations or thermal crosslinking, the charge carrier mobility is not reduced by crosslinking. Solution processed three layer organic solar cells were realized with a crosslinkable fluorene based copolymer containing acrylate groups. The efficiency is increased from 1.4% for the reference to 1.8% in the three layer cell with a crosslinked exciton blocking layer. A critical issue of BHJ cells is the instability of the morphology of the polymer:fullerene blend over long operation times at elevated temperature. We present a crosslinkable derivative of the low bandgap polymer PFDTBT which contains oxetane units. BHJ cells with the crosslinked PFDTBT derivative and PCBM were tested in accelerated aging experiments at 100 °C for times up to 100 h. Stabilization was clearly observed in crosslinked BHJ cells compared to the non-crosslinked reference. We show for the first time that oxetane containing polymers can be thermally crosslinked without any added initiator. Initiator free crosslinking is particularly attractive as it avoids the formation of decomposition products, and thus potential electron traps and quenching sites from the initiator.
In the presented work solvent-free film preparation from molecular glass resists, the evaluation of the patterning performance using thermal scanning probe lithography (tSPL) and an efficient etch transfer process are demonstrated. As the presented materials have a high tendency to crystallize and thus form crystalline films of bad quality when processed by solution casting, two component mixtures prepared by coevaporation were investigated. Stable amorphous films were obtained by selecting compatible material pairs for the coevaporation. One optimized material pair is based on trissubstituted, twisted resist materials with a distinct difference in molecular design. Here a high resolution tSPL prepared pattern of 18 nm half pitch in a 10 nm thick film is demonstrated.
A further optimization is reported for “small” cubic silsequioxane molecules. Again single component films show independent to applied film preparation techniques bad film forming properties due to the high crystallinity of the symmetric cubic silsequioxane molecules. But coevaporation of the phenyl substituted octaphenylsilsequioxane combined with the fully aromatic 2,2',7,7'-tetraphenyl-9,9'-spirobi[fluorene] results in stable amorphous thin films. tSPL investigations demonstrate the patternability by writing high resolution line features of 20 nm half pitch. An important advantage of such a silicon rich resist material is that it can be directly converted to SiO2, yielding to a patterned hardmask of SiO2. This proof of principle is demonstrated and an efficient pattern transfer of 60 nm half pitch line into the underlying HM8006 is reported.
In the presented work solvent-free film preparation from tailored molecular glass resists, their thermal analysis, the characterization of etch resistance for plasma etching transfer processes, and the evaluation of the patterning performance using scanning probe lithography (SPL) tools, in particular electric field and thermal based SPL, are demonstrated. Therefore a series of fully aromatic spiro-based and tris-substituted twisted resist materials were systematically investigated. The materials feature very high glass transition temperatures of up to 173 °C, which allows solvent-free thin film preparation by physical vapor deposition (PVD) due to their high thermal stability. The PVD prepared films offer distinct advantages compared to spin coated films such as no pinholes, defects, or residual solvent domains, which can locally affect the film properties. In addition, PVD prepared films do not need a post apply bake (PAB) and can be precisely prepared in the nanometer range layer thickness. An observed sufficient plasma etching resistance is promising for an efficient pattern transfer even by utilizing only 10 nm thin resist films. Their lithographic resolution potential is demonstrated by a positive and a negative tone patterning using electric field, current controlled scanning probe lithography (EF-CC-SPL) at the Technical University of Ilmenau or thermal scanning probe lithography (tSPL) investigations at the IBM Research - Zurich. High resolution tSPL prepared patterns of 11 nm half pitch and at 4 nm patterning depth are demonstrated.
The presented work deals with molecular glass resist materials based on (i) calixresorcinarene resist systems, (ii) twisted fully aromatic biscarbazole-biphenyl materials, and (iii) fully aromatic spiro resist materials as new promising materials for Scanning Probe Lithography (SPL). Because of the non-chemically amplified resist nature and the absence of corresponding material diffusion, the novel SPL resists have the potential to increase the patterning resolution capabilities at a simultaneous reduction of the edge roughness (LER). In addition, these low molecular weight molecular glasses offer the advantage of solvent-free film preparation by physical vapor deposition (PVD). The PVD prepared films offer a number of advantages compared to spin coated ones such as no more pinholes, defects, or residual solvent domains, which can locally affect the film properties. These high-quality PVD films are ideal candidates for the direct patterning by SPL tools. Presented highlights are the thermal scanning probe lithography (tSPL) investigations at IBM Research - Zurich and the patterning by using electric field, current controlled scanning probe lithography (EF-CC-SPL) at the Technical University of Ilmenau. Further investigations on film forming behavior, etch resistance, and etch transfer are presented. Owing to the high-resolution probe based patterning capability in combination with their improved etch selectivity compared to reference polymeric resists the presented molecular glass resists are highly promising candidates for lithography at the single nanometer digit level.
Photocrosslinking is known as a suitable method for patterning organic semiconductors in organic light emitting diodes. We extend this concept to the field of organic solar cells using conjugated polymers bearing sidechains with photocrosslinkable oxetane units. By UV irradiation in the presence of a photo acid generator the oxetane groups polymerize, leading to the formation of a densely crosslinked, and thus insoluble, network of a low-bandgap polymer. In this paper we present the synthesis of two novel photocrosslinkable low-bandgap polymers PFDTBTOx and PCDTBTOx and discuss several strategies for the fabrication of organic solar cells taking advantage of the novel crosslinkable materials.
We present two classes of host materials for blue phosphors. The first are carbazole substituted biphenyls 1-9. In these CBP-type materials the triplets are confined to one half of the molecules by using either twisted biphenyls or by a metalinkage of the carbazoles to the biphenyl. We obtained high triplet energies of 2.95-2.98 eV and high glass transition temperatures in the range of 100-120 °C. OLEDs were fabricated using the host material 6 and the carbene emitter Ir(dbfmi) with pure blue emission at 450 nm. The devices achieved an external quantum efficiency of 8.7% at 100 cd/m2 and 6.1% at 1000 cd/m2. MBPTRZ with an electron transporting biscarbazolyltriazine that is separated from the hole transporting carbazole by a non-conjugated, meta-linked biphenyl unit is an example for a bipolar matrix material. The excellent glass forming properties and the high Tg of 132 °C ensure morphological stability in OLEDs. The meta-linkage and the additional twist at the biphenyl unit, which is achieved by two methyl groups in the 2- and 2’-position of the biphenyl in MBPTRZ leads to a decoupling of the electron accepting and electron donating part and therefore to a high triplet energy of 2.81 eV. DFT calculations show a clear separation of the electron and hole transporting moieties. A phosphorescent OLED with MBPTRZ as host and FIrpic as emitter reached a maximum external quantum efficiency of 7.0%, a current efficiency of 16.3 cd/A and a power efficiency of 6.3 lm/W.
Within this work we present the synthesis and applications of a novel material designed for n-type self-assembled monolayer field-effect transistors (SAMFETs). Our novel perylene bisimide based molecule was obtained in six steps and is functionalized with a phosphonic acid linker which enables a covalent fixation on aluminum oxide dielectrics. The organic field-effect transistors (OFETs) were fabricated by submerging predefined transistor substrates in a dilute solution of the molecule under ambient conditions. Investigations showed a thickness of about 3 nm for the organic layer which is coincides to the molecular length. The transistors showed bulk-like electron mobilities up to 10-3 cm2/Vs. Due to the absence of bulk current high on/off-ratios were achieved. An increase of the electron mobility with the channel length and XPS investigations point to a complete coverage of the dielectric with a dense monolayer. In addition, a p-type SAMFET based on a thiophene derivative and our new n-type SAMFET were combined to the first CMOS bias inverter based solely on SAMFETs.
PCTrz, a new bipolar host material containing a phenoxy-carbazole separated from a biscarbazolyl-triazine by a non-conjugated
ether bond is presented. Computational calculations demonstrated the separation of PCTrz into an oxidation
and a reduction site. A phosphorescent OLED with PCTrz as host and FIrpic as blue emitter yielded high current
efficiencies of up to 16.2 cd/A. Additionally two electron transporting host materials DBFTaz and DBFTazC, both
containing 1,2,4-triazole moieies, were synthesized and characterized. The triazole moiety in DBFTaz was formed by a
classical ring closure reaction between a N-benzoylbenzimidate and a hydrazine. For DBFTazC we used another
synthetical pathway which involves subsequent coupling of a carbazole and a triazole moiety to a dibenzofuran core.
Both triazoles posses high triplet energies of 2.95 eV for DBFTaz and 2.97 eV for DBFTazC, which make the
compounds interesting as matrix materials for blue phosphorescent OLEDs.
Quinoxaline model compounds are interesting materials for Organic Light Emitting Devices (OLEDs) because of their thermal stability and their higher ionization potential in comparison to other electron transporting/hole blocking materials. In this work we studied a starburst trisphenylquinoxaline by means of Ultraviolet Photoelectron Spectroscopy (UPS) and Thermally Stimulated Luminescence (TSL). UPS provided not only the characterization of the valence band structure but also parameters like ionization potential, 6 eV, and, combined with absorption spectroscopy, electron affinity, that are of crucial importance in designing optimized OLED configurations. On the other hand, a wide distribution of localized states occurs in organic layers due to several factors and TSL can investigate these states. The combination of the used techniques together with semi-empirical quantum chemical calculation, gave a detailed description not only of the valence and conduction band of the studied materials, i.e. the energy position of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), but also of the trap distributions localized in the band gap: a shallow one at 0.06 eV and a deeper state centered at 0.24 eV. The full spectroscopic and electrical characterization of the material formed the background for understanding its behavior in heterolayer devices.
Eight novel low molecular weight liquid crystalline (LC) vitrifying materials with photopolymerizable acrylate groups have been synthesized. The molecules have a star-shaped topology with three and four arms. The mesogenic units were varied by the addition of lateral groups in different positions. In the case of the three-armed star-molecules crystallization from the melt was strongly suppressed. One of the three-armed stars (Triple-BTB) exhibits a nematic phase at room temperature and does not recrystallize for at least nine months. The photopolymerization behavior was studied over the whole liquid crystalline temperature range, and the novel nematic star-molecules were used as components for colored cholesteric polymer networks. Additionally, we describe the formation of holographic gratings in photopolymerizable cholesteric mixtures by using polarization holography, where we used two writing beams with perpendicular polarization instead of the usual setup with two writing beams with parallel polarization.
In this paper the monodomain-alignment of new photocrosslinkable fluorene bisacrylate model compounds and oligomers is reported. The orientation of the nematic LC- phase is preserved by photopolymerization in an oriented LC- network. Monodomain orientations of crosslinked fluorene bisacrylates are investigated by UV-VIS and photoluminescence spectroscopy comparing different alignment layers. OLEDs were fabricated using oriented fluorene bisacrylates and polyfluorenes leading to state-of-the-art polarization ratios for electroluminescence of up to 25 and brightnesses of about 250 Cd/m2.
Most photorefractive (PR) materials require plasticizers in order to decrease the glass transition temperature, allowing for orientational enhancement by the chromophores. Introduction of the plasticizer, however, alters not only the viscosity but also the photoconductive properties of the material. This can be shown by comparing two different plasticizers which were introduced into a bifunctional low-molecular-weight PR glass and into a polyfluorene guest-host polymer. The latter reaches response times down to 600 microsecond(s) at a writing intensity of 1 W/cm2. We have recently improved the concept of low-molecular-weight PR glasses. A suitable, photoconducting unit allows the synthesis of a bifunctional system with a glass transition of 22.6 degree(s)C. Therefore, no plasticizer is needed. The material is based on a triphenyldiamine (TPD) moiety to which a nonlinear-optical chromophore is directly attached. The system is the first representative of a whole class of TPD molecules and polymers for photorefractive applications.
We report on holographic investigations of several organic photorefractive materials based on monolithic low molar mass glasses. The addition of different plasticizing agents and the variation of three NLO-units prove to change the photorefractive performance and the thermal properties of the resulting compounds dramatically. Very fast holographic response times down to 2.5 ms could be reached at refractive index modulations of 6 10- and two-beam coupling gains of up to 140 cm-1.
Liquid crystalline conjugated polymers have potential applications as inexpensive, easy to process polarized back lights for liquid crystal displays. Two 'Hairy rod' type conjugated polymers have been investigated as polarized light emitting materials. In this paper, we present polarized absorption and photoluminescence for films aligned on rubbed precursor PPV alignment layers. We also report electroluminescence measurements for oriented multi-layer devices with rubbed PPV hole transporting alignment layers that produced polarization ratios of up to 25:1 and a luminance of 250 cd/m2. Sample preparation, device fabrication and characterization are described. Preliminary investigations have been made into the performance of PPV as an alignment layer with respect to thermal degradation.
A series of novel bifunctional cyclosiloxanes with pendant photoconducting and nonlinear-optical moieties was synthesized in a two step procedure. The variation of the spacer length between the siloxane ring and the functional carbazole and azo moieties leads to glass transition temperatures between 33 degrees C and 61 degrees C. These well defined low-molar mass compounds are characterized with regard to their chemical, thermal and electrooptical properties. Compared to commonly used guest-host systems crystallization of the chromophore dopants is diminished due to the covalent bonding of the azo-dye to the cyclosiloxane backbone.
We present an organic photorefractive material based on a low molar mass glass with both photoconductive and nonlinear optical properties. By implementing a novel plasticizer and doping with the well known sensitizer C60 we obtained a composite material, which shows extremely fast initial response times of 2.5 ms at writing beam intensities of Iwrite equals 1 W/cm2 and 450 microsecond(s) at Iwrite equals 10.8 W/cm2. Combined with high refractive index modulations of up to (Delta) n equals 6 X 10-3 and sample lifetimes of over 6 months this material exhibits an excellent overall performance.
Seven novel photorefractive glasses have been prepared in an effective four-step synthesis. The low molar mass glasses consist of a triphenylamine-core substituted with two carbazole moieties and a NLO-chromophore (NLO-DCTAs). In a second series of compounds (NLO-DATAs) the carbazole groups are replaced by diphenylamine. The compounds from stable glasses with Tgs of about 120 degrees C and 85 degrees C. The photorefractive properties such as the diffraction efficiency (eta) , the modulation amplitude of the refractive index (Delta) n, and the gain coefficient (Gamma) were determined by means of degenerate four-wave mixing and two- beam coupling experiments. We observed diffraction efficiencies of (eta) equals 90 cm-1. The samples have excellent properties with respect to optical quality and shelf-lifetime.
We investigated polymeric materials based on polysiloxane (PSX), polymethylmethacrylate (PMMA), polyurethane (PU), as well as a triphenylamin-based glass (DRDCTA) with respect to their photorefractive properties. Electric-field dependencies of the two-beam coupling gain, diffraction efficiencies, refractive index amplitudes and holographic rise-times could be obtained by means of two-wave mixing and degenerate four-wave mixing measurements. The examined PSX polymer systems were composed of a photoconducting polysiloxane host doped with trinitrofluorenone (TNF) as a sensitizing moiety and various chromophores, namely, an azo derivative, a stilben derivative and a tolan derivative. Due to their comparatively low glass-transition temperatures Tg, an orientational enhancement of the photorefractive properties was observed. Furthermore, the influence of photoisomerisation (based on trans-cis-trans cycles) on the holographical properties could be determined for the different chromophores. In addition, a class of fully functionalized polymers with azo chromophores and carbazole-units covalently attached to PMMA- and PU-backbones was synthesized. These systems show comparatively high glass transition temperatures of more than 80 degrees C. The third type of materials investigated is a glass of triphenylamin with attached carbazole and NLO-chromophore moieties. It has a glass transition temperature of 120 degrees C. For the high-Tg materials, poling procedures--essential for the photorefractive properties--could be monitored in-situ by second-harmonic generation. Absolute values for the nonlinear Pockets coefficients χ(2) (-ω; ω, 0) have been obtained by electro-optical measurements.
In this paper we report on the synthesis of a number of photorefractive materials with high glass transition temperatures. We have developed a series of fully-functionalized polymethacrylates with both photoconducting and NLO-active moieties. Secondly, low molecular weight glasses based on triphenylamine and carbazole units together with different NLO-chromophores have been designed which show no tendency towards crystallization. We now report on the synthetic approach to these novel materials and their properties with regard to photorefractive applications.
Chromophore relaxation is still one of the major problems of NLO polymers. Two strategies have been developed to overcome this problem. One of them is crosslinking after poling. We synthesized new polymers with methylfuryl acryloyl side groups. Upon irradiation with UV- light, they undergo a (2 + 2) cycloaddition which leads to a highly crosslinked material. Compared to cinnamoyl gorups frequently used in such polymers, the absorption of the novel crosslinker is shifted to longer wavelengths. This allows to use a cutoff filter during irradiation and therewith minimizes chromophore decomposition. The second way to suppress chromophore relaxation is the use of polymers with high glass transition temperatuers. We have prepared a number of polymers by copolymerization of NLO-active methacrylates with the bulky adamantyl methacrylate. This leads to polymers with glass transition temperatures up to 190 degrees C. The synthesis and properties of these polymers are reported in detail. The polymers shown a good long term stability of the electro-optic coefficient (r33) with its relaxation approximately following a KWW-function.
Photobleaching experiments on thin films of azo chromophore functionalized methacrylates were carried out to investigate bleaching kinetics and its underlying mechanisms. The polymer film's spatial index variation was measured and compared with a theoretical model of bleaching dynamics. From IR-spectroscopy and from GPC measurements it was found, that bleaching causes a destruction of the chromophore and a broadening of the polymer molecular weight distribution. The generated low molecular weight fractions from both polymer and chromophore photodecay evaporate during annealing above Tg, resulting in a reduction of the film's thickness. Surface roughness increases during photobleaching, causing an increase in waveguide loss. Film surface can be smoothed completely by annealing, thus lowering the optical loss induced by bleaching.
Four high Tg side chain polymers have been investigated based on polyimide and acrylate backbones functionalized with DR1 and heterocyclic thiophene chromophores. UV-VIS studies revealed chromophore stability up to 210 degree(s)C. Maximum poling efficiency was found approximately 15 K above Tg. From dielectric relaxation studies and from poling dynamics it is obvious that chromophore reorientation follows an Arrhenius law at temperatures well above Tg leading into a WLF-behavior in the vicinity of Tg. Relaxation of the EO-coefficient could be interpreted in terms of a KWW function. The average relaxation times strongly deviate from the WLF-function at temperatures well below Tg and can be described with an Arrhenius law. Activation energies here are significantly smaller than those in the high temperature limit above Tg, indicating that the chromophore dipoles are incompletely coupled to the polymer (alpha) process. Very good stability was observed for the polyimide P3 with average relaxation times of 104 years at 50 degree(s)C and 4 months at 120 degree(s)C. EO coefficients of up to 12 pm/V at 1541 nm were realized using a poling field strength of 1 MV/cm. EO-coefficient at constant poling field was found to be approximately linearly dependent on chromophore content. Channeled waveguides were fabricated by selective reactive ion etching with small losses of 1 dB/cm at 1318 nm. However, loss varies strongly between 1 dB/cm and 3 dB/cm depending on film quality.
Because of their excellent optical properties, a variety of polymethacrylates with pendant NLO-chromophores has been prepared and investigated by different research groups. The method normally used for the synthesis of these polymers is the free radical polymerization of the corresponding methacrylates with NLO-active side groups. However, the NLO- chromophores, usually large conjugated molecules with an electron donor and an electron acceptor substituent, often contain a number of functional groups, e.g., nitro- or azo groups. These may act as retarders or inhibitors in a free radical polymerization. So in many cases the yields are not quantitative and the molecular weights are quite low. We present an alternative method for the preparation of polymethacrylates with pendant NLO-chromophores, the polymeranalogous esterification of poly(methacryloyl chloride). In a first step, reactive prepolymers are prepared by the free radical polymerization of methacryloyl chloride (MAC1) or by copolymerization of MAC1 with methyl methacrylate (MMA). These prepolymers are esterified using NLO-active side groups with a hydroxy-terminated spacer. Well defined, high molecular weight polymethacrylates with high dye contents can be prepared by this method. A copolymer with 19 mole% of azochromophores exhibits an electro-optical coefficient of 9 pm/V at 1300 mm after poling, whereas 19 pm/V (1500 nm) were measured for a polymer with 90 mole% of NLO active azobenzene side groups. In addition, the novel method provides easy access to some novel copolymers with both NLO-active azobenzene units and photocrosslinkable cinnamoyl groups.