Electromechanical Properties of Transparent Conducting Substrates for Flexible Electronic Displays

As opto-electronic devices become flexible, the designing of reliable devices becomes more challenging. In this paper, we focus on the degradation of flexible transparent anodes by mechanical and thermal stresses. Indium tin oxide (ITO)-coated polyethyleneterephthalate (PET) is susceptible to cracking at low strains (<2%), but has excellent electrical and optical characteristics. Conducting polymers, such as polyethylene dioxythiophene doped with polystyrene sulfonate (PEDOT:PSS), have good mechanical properties, but are severely degraded by temperature, and have inferior optical and electronic properties. We have developed a mandrel-bending automated test system to evaluate the degradation of flexible anodes in service, and find that even when the strain is below the virgin cracking threshold, there are measurable changes in ITO resistance. Cyclic loading of ITO-coated PET shows three regimes of resistance increase: 1) an increase in resistance, due to changes in sample dimension until equilibrium width is obtained (50-100 cycles); 2) a gradual linear increase in resistance, possibly due to cracking of ITO; and 3) a catastrophic failure after 50 000 cycles due to severe cracking. For PEDOT:PSS-coated PET, the resistance does not increase significantly with increasing tensile strain, and it is also less susceptible to damage from repeated bending.