Dessislava Sainova

Control of color and efficiency of light-emitting diodes based on polyfluorenes blended with hole-transporting molecules

Adding low-molecular-weight hole-transporting molecules (HTM) with different oxidation potentials to the polyfluorene emission layer of single-layer light-emitting diodes causes significant changes in the device properties. The pronounced increase in luminance efficiency combined with a decrease in current is attributed to significant hole trapping, as further suggested by thermoluminescence experiments. Using a oligo-triphenylamine HTM with an ionization potential of ∼4.9u200aeV, light-emitting diodes with stable blue emission, a brightness of 800u200acd/m2 and an efficiency of 0.87 cd/A were realized. Further, the red-emitting contribution to the spectra as observed in the pure polymer devices could be fully suppressed.

Photoaddressable Alignment Layers for Fluorescent Polymers in Polarized Electroluminescence Devices

Liquid-crystalline (LC) polyfluorenes have been successfully aligned on photoaddressable polymers (PAPs). This is the first example of the alignment of a LC main chain polymer on a photoaligned layer. The degree of molecular alignment in the fluorescent polyfluorene layer on top of an ultra-thin PAP layer is shown to depend strongly on the chemical nature of the PAP. Good alignment with dichroic ratios of more than 10 was only achieved with PAPs containing liquid-crystalline side chains. Patterning with laterally structured alignment was realized in several ways, utilizing reorientation with orthogonally polarized light. Thin PAP layers have further been utilized as hole-conducting alignment layers in polymer light-emitting diodes (LEDs) with polarized emission. In order to facilitate hole transport through the alignment layer, different concentrations of a hole-transporting molecule (HTM) have been mixed into the PAP layer. These hole-conducting alignment layers retained their aligning abilities even at HTM concentrations of 20 wt.-%. LEDs with photometric polarization ratios in emission of up to 14 at a brightness of up to 200 cd/m2 and an efficiency of 0.3 cd/A could be realized.

Liquid crystalline polyfluorenes for blue polarized electroluminescence

A series of 9,9-dialkyl-poly(fluorene-2,7-diyl)s containing linear and branched alkyl substituents with a Mn of up to 200000 g/mol has been synthesized. Moreover, some of the polymers were end capped with a suitable hole transport functionality, such as a triphenylamine derivative, to improve their charge transport properties and to control the molecular weight. The thermal alignment of these novel polymers on a rubbed polyimide layer led to highly anisotropic film formation with dichroic ratios (absorption parallel and perpendicular to the rubbing direction) of up to 26 in absorption and 21 in emission.

Some new electron-affine polymers for organic photovoltaics

The aim of this work is to develop some new polymer materials with typical n-type semiconducting properties and low reduction peak potentials. Therefore new synthetic routes are presented leading to organo-soluble polyquinolines and polyquinoxalines. The results of the synthesis and characterization of the obtained new organo-soluble polymers are shown. The electrochemical reduction and oxidation behavior of these polymers is studied by cyclovoltammetric measurements. All studied polymers can be reversibly oxidized and reduced. In addition the absorption and luminescence properties are investigated. Polymer/polymer solar cells are prepared from blends of the new polyquinoxalines and poly-[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenvinylen] as active single layer. First results are discussed.

The effect of hole traps on the performance of single layer polymer light emitting diodes

The performance of single layer LED devices based on a ladder-type poly para-phenylene (LPPP) with a small percentage of a hole transport compound (TPTE) added is examined. TPTE is expected to act as a hole trap with little influence on electron injection and transport. The addition of 1 wt% TPTE to LPPP reduces the current through the device at a given voltage and simultaneously increases the device efficiency. We believe that the trapping of holes creates a space charge field which influences the injection of charge carriers into the polymer layer.

New Concepts for the Development of Active Functional Polymers for p and n-Type OFET- Applications

Here we present a concept to improve the field effect transistor performance of P3HT in terms of threshold voltage stability as well as the stability in ambient atmosphere by introducing a strong acceptor dopant in the main polymer chain. In our concept the direct introduction of the acceptor dopant in the polymer main chain ensures the strucural stability against diffusion processes. P3HTs with different contents of acceptor molecules which are fixed linked in the main chain of the polymer , have been synthesized using the McCullough Grignard metathesis method. As acceptor unit has been integrated tetrafluorbenzene (TFB). The introduced dopant amount has been varied in order to obtain an optimum between the processability of the polymers and the resultant transistor performance. Compared to the p-type semionducting polymers the n-type organic materials are markedly less developed. Recently an interesting solution to this task has been proposed in the form of a conjugated ladder-type poly (benzo-bisimidazobenzo-phenanthroline) (BBL) showing either ambipolar or n-type field effect properties dependent upon the sample preparation and processing. However this rigid-chain ladder polymer is not soluble in the common organic solvents resulting in a rather complicated technological transfer. We report the significant improvement of the BBL-processing utilizing aqueous colloidal dispersions and their OFET-application. The resultant devices demonstrate ambipolar electronic transport with charge carrier mobilities in the range of 10−5 cm 2 /Vs without specific optimization procedures..

Increase the field-effect mobility of regioregular poly(3-hexylthiophene) by introducing fixed acceptor molecules

It is well known of regioregular poly(3-hexylthiophene) (P3HT) to self-assemble on hydrophobic surfaces, because the regular arrangement of its side chains allows an efficient π-stacking of the conjugated backbones. This should be the reason for the very high reported field effect mobilities (0.2 cm2/Vs)1. We present an alternative approach to increase the field effect mobility and the transistor stability by introducing a strong acceptor dopant in the main chain of P3HT while preserving the regioregularity of the 3-hexylthiophene segments in the polymer chain. P3HTs with different contents of acceptor molecules which are fixed linked in the main chain of the polymer, were synthesized using the McCullough Grignard metathesis method. As acceptor unit has been integrated 9-dicyanomethane-fluorene. The introduced dopant amount has been varied in order to obtain an optimum between the processability of the polymers and the resultant transistor performance. The utilized organic field effect transistor (OFET) substrates with SiO2 gate dielectric were always pre-treated with a silylating agent (HMDS) to facilitate the self-organization properties of the polymers by hydrophobization of the SiO2 surface. The optimized doped structures showed higher field effect mobility by one order of magnitude compared to the conventional P3HT, (μFE~9 x 10-3 cm2/V.s for the doped compound vs. μFE~5 x 10-4 cm2/V.s for P3HT), combined with a marked current modulation with ON/OFF ratio of ~ 7 x 103 and a better operational stability of the resultant OFET-devices.