Improving the Uniformity\nand Stretchability of Inkjet-Printed\nFilms by Adding the Surfactant Triton X

Stretchable organic light-emitting diodes (OLEDs) are\na key component\nof stretchable electronics. Inkjet printing is a potential processing\nmethod for stretchable and flexible OLEDs. However, improving the\nuniformity and stretchability of the emission layer (EML) prepared\nby inkjet printing is challenging. Here, we propose a strategy to\nsimultaneously improve the uniformity and stretchability of inkjet-printed\nfilms by tuning the Marangoni flow and increasing the free volume.\nTo verify our idea, Triton X (TX) with a lipophilic alkyl end and\na hydrophilic hydroxyl end was added to the Super Yellow (SY)/polystyrene-<i>block</i>-polybutadiene-<i>block</i>-polystyrene (SBS)\nblend film. TX played two roles. (1) To inhibit the coffee ring effect.\nThe surface tension of the solution decreased because the hydrophilic\nends of TX repelled with the nonpolar solvent toluene to decrease\nthe cohesion of toluene molecules on the surface. Thus, the surface\ntension at the edges was lower than in the middle due to the high\nevaporation rate at the edges during solvent evaporation. This resulted\nin the generation of the inward Marangoni flow to drive the solute\ntoward the middle. Therefore, the coffee ring effect was inhibited,\nand a uniform film was formed. (2) To improve the stretchability.\nWith TX, the glass transition temperature decreased because TX acted\nas a plasticizer to insert between the polymer chains due to the attraction\nbetween the lipophilic ends of TX and the alkyl side chains of SY.\nThis provided more free volume for the polymer chains to move and\norientate under strain, which is beneficial for the stretchability.\nFinally, we fabricated OLEDs with the inkjet-printed stretchable EML.\nAt 100% strain, the luminance kept 70% of the initial luminance, much\nhigher than that without the surfactant (33%).