Enhanced reproducibility of inkjet printed organic thin film transistors based on solution processable polymer-small molecule blends

Abstract

The positive effects on the inkjet drop ejection process, thin film morphology and subsequent thin film transistor performance caused by the blending a low molar mass semi-crystalline organic semiconductor, 6,13-triisopropylsilylethynylpentacene (TIPS-pentacene), with a low ε dielectric polymer, polystyrene (350 kDa) are reported. The presence of the high molecular weight polymer at a reduced concentration c/c* = 0.44 gives greater control of the drop generation process (when compared to low molar mass semi-crystalline organic semiconductor alone) due to stabilisation of the ligament break-up process with pinch off occuring at the nozzle plate and satellites coalescing within the main droplet. The addition of a small fraction of a high boiling point solvent results in the removal of droplet drying artefacts by generating a Marangoni flow in which double convection counteracts coffee stain formation. This results in more uniform solute coverage over the substrate, simplifying the print process. Whilst TIPS-pentacene and amorphous polystyrene do not undergo gross phase separation irrespective of deposition process the degree of crystallinity and extent of TIPS-pentacene stratification to the active interface was found to be strongly process dependant. Whilst a lower degree of crystallinity is observed when a dual solvent mixture is used there is strong evidence of greater stratification of the TIPS-pentacene at the active interface resulting in higher saturated hole mobility.