Functional polymer blends

Chrisitian Kasparek1,  Davood Abbaszadeh2,  Alexander Kunz1,  Thomas Lenz1,  Jasper Michels1,  Gert-Jan Wetzelaer1,  Irina Craciun1,  Kamal Asadi1,  Dago de Leeuw1,  Paul Blom1
1Max Planck Institute for Polymer Research, 2University of Groningen


Abstract

Conjugated polymers are attractive candidates for electronic applications since they can be processed from solution. This enables the production of polymer light-emitting diodes (PLEDS) or solar cells using a fast roll-to-roll newspaper-printing-style process. However, their solution processibility enables another option that until now has been less exploited. Electronic properties of a conjugated polymer can be changed, or even new ones created, by blending the polymer with other functional materials. A fundamental disadvantage of semiconducting polymers is that their charge transport is unbalanced because electron transport is hindered by traps. This electron trapping seems to be universal in organic semiconductors and is dominated by a trap located at ~3.6 eV below vacuum. We have found that by blending poly(p-phenylene vinylene) (PPV) derivatives with wide band gap polyfluorene based polymers the electron traps are deactivated. PLEDs made from such a blend exhibit a balanced transport and enhanced efficiency due to the strong reduction of non-radiative trap-assisted recombination. In order to further enhance the efficiency a multilayer structure is required to prevent quenching of excitons at the electrodes and improve optical outcoupling. We show that by adding a few percent of a crosslinkable host material the spin coated MEH-PPV becomes completely insoluble, without affecting its electronic properties. Using this approach we have fabricated PLEDs that consist of 4 subsequently spin coated layers with an efficiency 3-4 times higher as compared to a single layer PLED. An example of creating new functional properties is blending polymers with incompatible functionalities like ferroelectricity and conductivity. In these blends non-volatile bistable ferroelectric Schottky diodes are realized where the polarisation field of the ferroelectric modulates the injection barrier at the semiconductor–metal contact. The properties of these diodes are strongly dependent on the phase separation and therefore processing conditions of the polymers. There is a strong need for a technology where well defined polymer structures can be realized. Standard IC processes such as photolithography might deteriorate the integrity of the polymer. Therefore, alternative patterning technologies as solution micromoulding are being developed. We demonstrate nano-patterned gratings of the ferroelectric P(VDF-TrFE) polymer.