Wet imprinting of polymer light-emitting electrochemical cells

Organic light-emitting electrochemical cells (LECs) are likely to become the low-cost andeco-friendly alternatives to organic light emitting diodes (OLEDs) if it is possible to enhancetheir efficiency. In OLED devices losses due to poor light-outcoupling could be reduced bytransferring nanopatterns to the different layers of the device. Applying the experience madewith OLEDs to the similar structured LECs, Nikolai Kaihovirta and co-workers were able toimprove the light-outcoupling efficiency of LECs by adding patterned surface structures tothe devices [3]. Since patterning the surface of an LEC was successful, the next step wouldbe transferring patterns to the internal layers of an LEC and investigate their impact on thedevice efficiency. However, before it must be investigated if it is possible to imprint the activematerial of an LEC containing a conjugated polymer (Super Yellow), a salt (KCF3SO3) andan ion-transporter (TMPE-OH).This thesis shows that wet imprinting can be used as a cheap and fast technique to transfernanopatterns to the active material of LECs. In the wet imprinting setup a stamp is low-ered down onto a wet film and afterwards pressed down by a 3 kg weight. To obtain a wetfilm which then can be imprinted, the active material is spin coated onto a substrate. Thebest patterns were obtained when using a spin coating speed of 1000 rpm, an acceleration of1000 rpm/s and a spinning duration of 30 s.From the successfully imprinted samples, LEC devices were fabricated and characterised.AFM and SEM images show that a pattern with a depth of 24.5 nm could be transferred toan approximately 150 nm thick active material film. The results of the optoelectronic char-acterisation of the imprinted devices were compared to identically fabricated non-imprintedreference devices. During operation, the imprinted devices exhibit a star shaped emissiondue to their hexagonal pattern. Two imprinted devices showed a higher luminance than theirrespective reference devices. The highest luminance measured for an imprinted device was524 cd/m2 compared to 373 cd/m2 for the reference device. At this point further investigationis needed to confirm if the efficiency improvement is repeatable. However, these results showthat it is possible to imprint the active material of an LEC and imply that patterning theactive material could improve their efficiency