Jolita Ostrauskaite

Synthesis of low melting hole conductor systems based on triarylamines and application in dye sensitized solar cells

Dye sensitized hybrid solar cells with a nanoporous TiO 2 layer and different organic hole transport materials (HTM) have been investigated. As HTMs, some novel triphenyldiamine (TDP) based polymers carrying long alkoxyl group as side chains or in the backbone were designed and synthesized using Ullmann reaction. The thermal and electrochemical properties are studied. The soft spacers lower the Tg of the polymers suitably. The penetration behavior of the HTM into the namnoporous layer in solar cells has been studied by scanning electron microscopy (SEM). The thermal treatment for getting higher penetration of HTM into nanoporous layer did not work, perhaps due to the decomposition of the dye.

Fast and stable photorefractive systems with compatible photoconductors and bifunctional NLO-dyes

Abstract Polymeric photoconductors carrying triphenyldiamine units and soft oligomethylene spacers were synthesized. Due to the flexible spacers, the polymers are highly soluble and could be obtained as film-forming materials with appreciably high molecular weights and low glass transition temperature ( T g ). They possess HOMO value of about −5.1 eV as determined from cyclic voltammetry. These photoconductors can be mixed with nonlinear optical dyes in any proportion to obtain stable photorefractive samples. The T g can be tuned about room temperature by just changing the composition of the guest–host photorefractive system and requires no additional amount of plasticizer. Fast photorefractive response time of about 1 ms was obtained for these new guest–host systems from degenerate four-wave mixing and two-beam coupling measurements. Further, a series of well-defined low molecular weight bifunctional compounds, TPA–Azos were synthesized in which the photoconductor moiety, triphenylamine (TPA) is covalently linked to an azo dye. The TPA–Azos exhibit T g values of 13, 36 and 42 °C and they can also be mixed with polymeric photoconductors to obtain stable photorefractive systems with low T g values. With a new setup to control and vary the sample temperature precisely, the temperature dependence of erasing dynamics of holographic gratings in these systems was studied. It reveals change of response time by three orders of magnitude over a temperature range of 13 K. The difference in behaviour of the photorefractive system above and below the T g of the sample allows us to differentiate between the time contributions due to the fast electro-optic effect and the slow orientational diffusion processes.

Temperature dependencies in organic photorefractive materials

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.

Temperature dependent measurements on a low-molecular-weight photorefractive glass

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.

Influence of the sensitizer on the dynamic behavior of an organic photorefractive material

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.