Morphology, electrical and electronic properties of inkjet printed PEDOT:PSS

• Main Author: Wilson, Peter
• Published: University of Surrey 2012
• Subjects: 620.192042972
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.582859

This thesis investigates the effect of inkjet printing on the physical, electrical and electronic properties of PEDOT:PSS, focussing on an interpretation of the granular morphology and its effect on the conductivity in both the in-plane and transverse direction. Initially, a comprehensive review of the current theories and work is outlined. A programme of experimental work is developed to monitor the film properties as a function of a common conduction aid, the addition of di-methyl sulfoxide (DMSO) and the inclusion of Surfynol, a common processing additive. An inkjet printing unit was designed and constructed. Despite a limited budget, the printer was able to meet its design criteria for accuracy, speed and ink flexibility. Through profilometry of a variety of samples, the optimal inkjet printing parameters were found as a function of drive wave voltage, drop and line separation, with the crowning effect substrate temperature has on the drop and line profile carefully monitored. Atomic force microscopy demonstrated how inkjet printing returned microstructural surface roughness similar to samples which had been spin coated, here a decrease in roughness was observed as DMSO was incrementally added. The addition of Surfynol was shown to have little effect on the conductivity due to having almost no effect on the microstructural morphology, yet samples which included Surfynol returned a dramatically increased macrosctructural roughness. The addition of DMSO generated a 103 increase in in-plane conductivity accompanied with a shift in the conduction mode from Mott variable range hopping in 3 dimensions to a pseudo 1 dimensional mode as DMSO concentration reached Sw! .%. The dramatic change is generated by an increase in grain size and an increase in the PEDOT portion of the polymer at the surface. The increase of PEDOT at the surface generated a lowering of the work function by 160 eV, a much lower change than similar spin cast films, where the rapid drying and high forces generate a thermodynamically unfavourable granular morphology. By observing the granular structure in the transverse direction it was noted how inkjet printing promotes a drying process which alters the alignment of the grains towards the periphery from a flat horizontal morphology to a near vertical geometry. The result was shown to generate an in- plane to transverse electrical conduction parity which compares favourably to spin coating and returns a 103 times lower conductivity in the transverse direction compared with the in- plane conduction.