Patterning of organic electronic devices

Electronics based on organic semiconductors, such as organic light-emitting devices, thin film transistors, photodetectors, and photovoltaic cells can be deposited on a wide range of substrates, including glass, light-weight flexible plastics, and metal foils, and offers great potentials in low-cost, large-area applications where conventional inorganic electronics becomes too expensive or impossible to practically implement. However, the weak van der Waals interaction between neighboring molecules in organic solids imposes a challenge with respect to patterning, since the bonding in these low cohesive energy solids can be easily disturbed by exposure to various physical stress induced by heating, solvation or high energy plasmas. This thesis focuses on the development of patterning techniques that are compatible with fragile organic materials, and at the same time, preserve the low-cost potential that organic electronics can offer.; The initial focus was on the patterning of a metal electrode layer on top of an organic layer. In cold-welding techniques (subtractive and additive), patterning is achieved by transfer of a metal film via pressure-induced metallic bond formation between a metal on a substrate and that on a pre-patterned rigid stamp. We demonstrate the versatility of the cold-welding techniques by fabricating various organic electronic devices and a polymer grating structure with a line width smaller than 80 nm. Significant reduction of pressure required for patterning is achieved by employing elastomeric stamps. Our finite element numerical simulations show that the reduction of pressure is predominantly due to the difference in extent of loss of contact around dust particles interposed between a stamp and a substrate.; Next, the cold-welding techniques are generalized to demonstrate the localized patterning of organic thin films based on direct material transfer from a stamp to a substrate. This process, based on van der Waals bonding between contacting organic films, extends the range of application of patterning via stamping to devices where the active organic materials must be locally deposited on the substrate. Mechanical analysis combined with finite element calculations successfully describes the details of the material transfer process over substrate irregularities and step edges, thereby providing a guide for determining processing conditions as well as device structures.