Real Time In Situ X-Ray Studies Of Organic Heterostructure Formation
This thesis presents several studies of the deposition and growth of thin films of organic semiconductors and the formation of organic-organic heterostructures. Organic semiconductors are of great interest due to their usefulness in electronic, optical and photovoltaic devices, compatibility with flexible substrates, and they can be processed at low temperatures compared to traditional, inorganic semiconductors. Thus, they are desirable for low cost device applications such as transistors, OLEDs, and photovoltaics. An organic photovoltaic device requires combining two different organic semiconductors into a single structure. An interface between a donor material and an acceptor material is necessary for a functioning organic photovoltaic device. In this work, the use of supersonic molecular beams to deposit organic thin films of pentacene and diindenoperylene on SiO2 is directly compared to the use of thermal evaporation, and the effects on the nucleation of organic thin films are examined. Incident kinetic energy is found to have no influence on the critical nucleus size for pentacene and diindenoperylene. Supersonic molecular beams and in situ synchrotron x-ray scattering are used to investigate the mechanism of adsorption and dynamics of thin film growth of three perylene derivatives: N,N'-dipentylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C5 ), N,N'-dioctylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C8 ), and N,N'-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13 ) on surfaces modified with organic self-assembled monolayers. Small changes in molecular structure, like changing the length of alkyl side chains on PTCDICn molecules, results in significant changes for thin film growth. Shorter side chains result in smoother, more prolonged layer by layer growth. This thesis also reports on the growth of heterostructures of two different organic semiconductors, both simple bilayer stacks and more complex multilayer structures. While growth of PTCDI-Cn on films of pentacene results in smooth layer-bylayer growth, when pentacene is deposited on PTCDI-Cn films, the resulting films are extremely rough, and exhibit Volmer-Weber growth. When growing heterostructures by depositing alternating layers of pentacene and PTCDI-Cn , this effect causes increases in roughness after the deposition of pentacene. Interestingly, the roughness can be decreased after depositing a layer of PTCDICn . This behavior suggests that the two materials are forming separate domains rather than a superlattice structure, which could be favorable for photovoltaic devices. This behavior is driven by differences in surface energy of pentacene and PTCDI-Cn . This result has implications for future work attempting to form heterostructures of two different organic semiconductors, highlighting the important of surface energy considerations.
organic semiconductors; x-ray scattering; supersonic molecular beam
Engstrom, James R
Hanrath, Tobias; Brock, Joel Donald; Woll, Arthur R.
Ph.D. of Chemical Engineering
Doctor of Philosophy
dissertation or thesis