Elastic Moduli of Organic Electronic Materials by the Buckling Method

Abstract

Mechanical moduli of common organic electronic materials are measured by the buckling method. The organic layers were prepared on the elastomer polydimethylsiloxane (PDMS) substrate by transfer, direct spin-coating, or thermal evaporation. When a small (∼2%) compressive strain is applied to organic/PDMS film samples, the layer becomes buckled with a characteristic wavelength. Fitting the experimentally measured data of buckling wavelength as a function of layer thickness with a model equation yields the mechanical modulus of the organic layer. The measured values compare well with those from theoretical predictions for materials such as poly(3-hexylthiophene) (P3HT) and its blend with [6,6]-phenyl C61-butyric acid methyl ether (PCBM). The modulus of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is similar to that of pure PSS, which is contrary to the common expectation that the ionic interaction between PEDOT and PSS chains may lead to a modulus value 2−3 times larger than that of the constituent polymers. The similarity is likely due to a very small amount of PEDOT added and the oligomeric nature of PEDOT. Thermally evaporated pentacene film has a modulus value of ∼15 GPa, which is an order of magnitude larger than those of other polymeric materials investigated here, and reveals delamination bukling behavior when the magnitude of compression is relatively large. The residual solvent in polyaniline (PANI) plays the role of plasticizer and leads to a very small modulus. The measured mechanical moduli of common organic electronic materials would be valuable for designing and implementing flexible and/or stretchable organic electronics.