The synthesis and analysis of solution processable polymers for organic solar cells is crucial for innovative solar cell
technologies such as printing processes. In the field of donor materials for photovoltaic applications, polymers based on
tetraphenylamine (TPA) are well known hole conducting materials. Here, we synthesized two conjugated TPA
containing copolymers via Suzuki polycondensation. We investigated the tuning of the energy levels of the TPA based
polymers by two different concepts. Firstly, we introduced an acceptor unit in the side chain. The main-chain of this
copolymer was built from TPA units. The resulting copolymer 2-(4-((4'-((4-(2-ethylhexyloxy)phenyl)(paratolyl)
amino)biphenyl-4-yl)(para-tolyl)amino)benzylidene) malononitrile P1 showed a broader absorption up to 550 nm.
Secondly, we used a donor-acceptor concept by synthesizing a copolymer with alternating electron donating TPA and
electron withdrawing Thieno[3,4-b]thiophene ester units. Consequently, the absorption maximum in the copolymer
octyl-6-(4-((4-(2-ethylhexyloxy)phenyl)(p-tolyl)amino)phenyl)-4-methylthieno[3,4-b]thiophene-2-carboxylate P2 was
red shifted to 580 nm. All three polymers showed high thermal stability. By UV-vis and Cyclic voltammetry
measurements the optical and electrochemical properties of the polymers were analyzed.
The concept of solid-state dye-sensitized TiO2 solar cell in which the hole transport medium is an organic semiconductor is critically studied by examining the anode-TiO2 interface and dye-hole conductor interface. The importance and the role of a compact hole-blocking TiO2 layer in between the anode and the mesoporous layer is extensively studied by preparing this layer by spray pyrolysis using an automated procedure which guarantees reproducibility in obtaining constant thickness and quality of this crucial layer as seen in the current-voltage characteristics of the solar cells. To characterize the rectifying behavior of the blocking layer, cells with the structure, fluorinated tin oxide (FTO)/blocking TiO2 layer/hole conductor/Au, were prepared and their current (I)-voltage (U) properties were investigated. Solid state solar cells were also prepared with different blocking layer thicknesses and their photovoltaic properties were investigated in order to study the influence of the blocking layer thickness on solar cell performance. In order to improve the dye-hole conductor interface, novel multifunctional molecules carrying dye units and triphenylamine moieties were synthesized and their influence as interface modifiers were studied. This interface modification results in doubling the external quantum efficiency of current conversion via improved charge transfer at the dye-hole conductor interface. Moreover, the recombination processes at this interface is drastically suppressed which leads to higher open circuit voltage and consequently higher power conversion efficiencies.
Predictions made by two models describing dynamic processes in organic photorefractive materials are verified for a
photorefractive polymer, consisting of a Poly-TPD, a dicyano dye
and the sensitizer C60. One of the models can describe our
measurements under the assumption of optically active traps as
dominant trap species. This is confirmed by altering the
sensitizer concentration, which has an influence on the charge
transport properties of the material. Also, increasing the
sensitizer concentration yielded a significant increase of the
grating erasure rate in our material.
We describe two methods by which discotic liquid crystalline (LC) triphenylene materials can be sensitized for wavelengths of the visible spectrum and the near IR. One way of doping is more conventional and uses bi-layers between the LC-system and an adjacent pigment layer. The second doping is achieved by a molecular doping with C70. Here the influence of doping on the measured mobilities is much smaller as compared to the influence of doping measured in amorphous systems. We attribute this smaller influence of the C70-doping on the mobility to a partial phase separation occuring between the LC-host material and the C70-guest molecules. In our comparative experiments on doped dimethyl-triphenyldiamines we can show taht the magnitude of the trap depths for hole-trapping has a very large influence on the measured transport parameters.
Altering the sample temperature in a photorefractive material changes the rotational mobility of the chromophores. A change of three orders of magnitude in the response times over a temperature change of 12 K has been observed. In the photorefractive experiment, however, the chromophore orientation is induced by the non-instantaneous change of the space charge field. The finite speed of the latter causes the chromophore answer to be different from their normal relaxation behaviour to an instantaneous change. This effect is most pronounced when both time constants are in the same range.
The erasing dynamics of holographic gratings in a low molecular weight photorefractive glass depending on the sample temperature were investigated. Changes in the overall speed of the material by three orders of magnitude over a temperature range of 13 K were observed. We identified two distinct processes below the glass transition temperature Tg, a fast one on time scales of seconds and a slower one with lifetimes around 103 s. We attribute the fast process to the electro-optic effect and the slower one to orientational diffusion processes of the glass molecules. Above Tg, the fast process vanishes, whereas the diffusional processes accelerate up to time constants in the range of seconds. This study shows, that an accurate temperature control is indispensable when measuring photorefractive dynamics, especially in the temperature range around Tg.
Dye-sensitized hybrid solar cells with a nanoporous TiO2 layer and different organic hole conducting polymers have been investigated. These hole transport materials (HTM) with low glass temperature (Tg) are based on triphenyldiamines (TPD). To enhance the power conversion efficiency, the HTMs were doped. New low Tg TPD-based polymers were designed and synthesized with long alkoxyl groups as sidechains or in the backbone, respectively, to investigate the influence of the soft alkoxyl chains on the penetration behavior into the nanoporous layer which has been studied by SEM. The effect of the penetration depth and wetting of the dye sensitized TiO2 layer by HTM on the solar cell efficiencies have been investigated by I-V- characteristics and steady-state measurements. To improve the penetration, the polymers were heated above Tg. The performance of the cell is decreasing probably due to degradation of the dye during thermal treatment. To enhance the conductivity of the hole transport materials, a Li salt has been used and this doped system was compared to a doped standard Gratzel cell with OMeTAD as HTM. Crystallization on the surface could be seen after storing the standard cell for some weeks.
We synthesized lithium quinolate complexes, 8- hydroxyquinolinolato lithium (Liq) and 2-methyl-8- hydroxyquinolinolato lithium (LiMeq) as emitter and electron injection/transport materials to be used in conventional two layer organic light emitting diodes in combination with N,N'-bis(p-methoxyphenyl)-N,N'-diphenyl benzidine (DMeOTPD) as hole transport material (HTL). The lithium complexes were also examined as interface materials in combination with 8- hydroxyquinolinolato Al(III) as emitter material. The device efficiency using these complexes were optimized using combinatorial methods. We also compared the electron injection, transport and emission properties of Li-complexes with the well known emitter Alq3 in the same experiment taking advantage of the combinatorial approach. The Li- quinolates are found to be efficient emitter molecules. But the efficiencies of lithium quinolate devices are lower than that of Alq3 devices. Contrary to the Alq3 emission, the Li-quinolates exhibit bathochromic shift of emission compared to the respective photoluminescence spectra. No clear evidence for exciplex formation was seen by comparing the photoluminescence spectrum of an equimolar mixture of Li-quinolate and DMeOTPD with the observed EL spectrum. However, the lithium complexes increase the efficiency of an optimized ITO/DMeOTPD/Alq3/Al device considerably when used as a thin interface layer between Alq3 and aluminum. The improvement of device characteristics using lithium quinolates is similar to that obtained using LiF salt. The mechanism of improvement of efficiency using a lithium complex interface layer seems to be of chemical doping nature.
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.
An experimental set-up was used to optimize the layer thicknesses of hole transport materials and electron transport/emitter material in multi-layer light emitting diodes by combinatorial methods. The method is based on a movable mask/shutter technique and simultaneous evaporation of organic molecules resulting in linear gradients of layer thickness. This allows the preparation of different devices in one single experiment under identical conditions. In the first experiment, we studied the influence of the Alq3 layer thickness on photometric and power efficiency in two layer devices using various TPD derivatives as hole transport material at a constant thickness. Some new low molecular weight TPDs and a polymeric TPD were utilized. Both photometric and power efficiencies depend considerably on the thickness of the Alq3 layer. In a second experiment, the efficiency dependence on both the TPD and Alq3 layer thickness was investigated simultaneously by preparing a landscape library with two orthogonal linear gradients of TPD and Alq3. It was observed that the device efficiencies depend on both TPD and Alq3 layer thicknesses and additionally on the total thickness of the organic layer.
A combinatorial approach combining vapor deposition of organic molecules and a mask technique was used to prepare on one substrate a matrix of 49 organic light emitting diodes (OLEDs) with different configuration and layer thickness. A landscape library with two orthogonal, linear gradients of an emitter and a hole blocking electron transport material on top of a hole transport layer of constant thickness was prepared. The aim of this experiment was to investigate the influence of an additional electron transport material on the efficiency. Using a semi-automated measurement set-up, the device parameters for each of the 49 OLEDs were evaluated. The existence of an optimum Alq3 layer thickness for two-layer devices ITO/TPD/Alq3/Al is confirmed and such an optimized two-layer structure could not be improved by adding an additional hole blocking layer to the optimum Alq3 layer. But an improvement of photometric efficiency can be obtained by replacing the optimum Alq3 layer thickness by certain combinations of Alq3/spiro-Quinoxaline layers.
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.
Uwe Hofmann, Stefan Schloter, Andreas Schreiber, K. Hoechstetter, G. Bauml, Stephan Zilker, Dietrich Haarer, Mukundan Thelakkat, Hans-Werner Schmidt, Kai Ewert, C.-D. Eisenbach
Photorefractive guest-host systems based on photoconducting polymers such as poly(N-vinylcabazol) or polysiloxane generally exhibit large diffraction efficiencies and photorefractive gain coefficients. Their response times however, are limited by the orientation of the nonlinear optical chromophores and by the photoconducting properties of the polymer. Rise times down to 50 ms have been observed until now.
Various fluoro-functionalized aromatic 1,3,5-triazine monomers were prepared. A series low molar mass and poly- (1,3,5-triazine)-ethers were synthesized by a condensation reaction. The polymers as well as the low molar mass compounds have excellent thermal stability and are amorphous. In order to examine the potential to apply these compounds in organic electroluminescent devices, the redox properties were studied by cyclic voltammetry. It was found that the monomers have high electron affinity and reach LUMO values in the range of -2.7 to -3.1 eV. Additionally high oxidation stability with HOMO values lower than -6.4 eV follows hole blocking capabilities. This opens the possibility to utilize 1,3,5-triazine containing materials as electron injection/hole blocking layer in LEDs. First LED results are in agreement to these high electron affinities.
We synthesized low molecular weight triphenyldiamines (TPDs), novel 1,3,5-tris(diarylamino)benzenes (TDABs), polymeric triphenyldiamines and insoluble triphenylamine networks based on tris(4-ethynylphenyl)amine as hole transport materials for electroluminescent displays. The HOMO energy values as determined from cyclic voltammetry measurements for TPDs and TDABs are between -4.97 and -5.16 eV. By using a polymeric TPD as hole transport layer and tris(8-quinolinolato) aluminium as emitter, LEDs with an onset voltage of 3V and a luminance up to 900 cs?m2 were obtained under ambient conditions, using airstable Al-electrode as cathode and ITO as anode.
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