Thermo-mechanical behavior of conjugated polymer films for flexible and stretchable organic semiconductors

Abstract

Conjugated polymers, which are semi-conductive organic materials have signature advantages such as light-weight, low-cost and solution-process ability compared to existing inorganic semiconducting materials. However, flexible and stretchable electronics consisted of conjugated polymer thin films have poor mechanical reliability (Cracking or delamination) due to severe mechanical deformations under bending and stretching. Recently, elongation at fracture of conjugated polymer thin films has been mainly quantified by using pseudo-freestanding tensile tests to solve the mechanical reliability issues. Nevertheless, flexible and stretchable semiconductor devices inevitably undergo temperature changes and thermal deformation during processing or operation. Thus, thermo-mechanical reliability problems such as thermal cracking and delamination occur due to coefficient of thermal expansion (CTE) mismatch between conjugated polymer thin films and adjacent layers. Therefore, quantification of CTE of conjugated polymer thin films is essential to improve thermo-mechanical reliability of flexible and stretchable semiconducting devices. However, “ellipsometry”, which is a conventional method for measuring CTE based on measuring polarization of reflected beam, has critical limitations to measure intrinsic thermo-mechanical behavior of thin films due to constraints of rigid substrates. In this study, film on water (FOW) method, which utilizes surface tension of water was used to measure intrinsic thermal deformation of conjugated polymer thin films in pseudo-freestanding state without effect of rigid substrates. By using this method, several studies were conducted focusing on quantitative CTE measurement for conjugated polymer thin films. First, thermal deformation of poly (3-hexylthiophene) (P3HT) thin film that has been widely studied as a representative conjugated polymer was measured by using water surface. Thermal shrinkage of as-cast P3HT thin films was observed, and it was confirmed that the thermal shrinkage was occurred due to a rearrangement and crystallization of unaligned polymer chains by fast-solidification in spin-coating. In addition, a method of preheating on liquid surfaces was introduced to measure the CTE of conjugated polymer thin films. Next, the measurable temperature range of thermal strain was extended up to 200 ℃ by using ionic liquid rather than water. Then the effect of crystallinity and confinement on CTE and melting point changes of conjugated polymer thin films was analyzed by controlling the regioregularity and concentration of P3HT polymers. Moreover, in order to investigate the relationship between CTE of the conjugated polymer thin films and thermo-mechanical reliability of stretchable semiconductors, CTE changes by molecular weight of n-type conjugated polymer thin films were investigated and the mechanism of thermo-mechanical problems (thermal cracking and wrinkling) in the stretchable devices were elucidated. Through this, it was confirmed that thermo-mechanical reliability of the stretchable device can be improved by using conjugated polymer with high molecular weight exceeding its critical molecular weight. Finally, design guidelines of acceptor type of polymer solar cells for stability of the solar cell were presented by comprehensively considering the photovoltaic, morphological, mechanical and thermo-mechanical properties of polymer blend thin films. The method demonstrated in this study for measuring CTE values of conjugated polymer thin films using liquid surfaces and analysis of thermo-mechanical behavior of flexible and stretchable organic semiconductors are expected to contribute to the commercialization of the flexible and stretchable devices with superior thermo-mechanical reliability and electrical performances.