Over the next three year period Smilgies will supervise projects for CBE masters of engineering students as well as work closely with faculty and students, to counsel them in the use of x-ray scattering methods for their research. In addition he will work with CBE faculty on methodology, on how to tackle frontier chemical engineering problems with x-ray scattering methods.
Spin coating is a standard technique, commonly used to prepare organic thin films from a solution. In order to achieve high-performance optoelectronic devices, such as solar cells based on spin-coated organic semiconductors, there is a need to control the structure, the morphology and the degree of phase separation of the organic semiconductor.
The crucial property of a BHJ is a self-organized interspersed network of donor and acceptor materials that transport the charge produced by the absorption of sunlight to the respective electrodes. In semiconducting molecules and polymers the charge splitting works via a bound electron-hole pair, a so-called exciton. The exciton has a finite lifetime corresponding to a diffusion length of only 10 nm, before it recombines, and thus the exciton has to reach a donor-acceptor interface within this range; otherwise it will not contribute to the photo current.
Global research team unveils new mechanism of controlled crystallization yielding high-performance organic transistors
A classic example of such a molecule is TIPS-pentacene with a pentacene backbone and two bulky tri-isopropyl silyl ethynyl (TIPS) side groups that render the molecule soluble in toluene. A feature of this class of materials is low lattice symmetry and polymorphism, i.e., depending on the crystallization conditions, different crystal structures and types of molecular packing may form. The distance between the p-orbitals is all-important for device performance - the closer the better.
RIM experiments at CHESS have reached a degree of complexity that a permanent lab was set up, in which users can test and develop their experimental set-ups as well as can be trained in the use of CHESS sample cells and ancillary equipment. A mock-up of the D1 station sample goniomenter is set up on an optical table (Figure 1) and can be equipped with a sample microscope, an optical spectrometer, or an optical film thickness monitor, so that complex experiments can be prepared and tested, before the equipment is transferred to the beamline.
Materials were as varied as conjugated small molecules [5,10,18], pharmaceutical molecules , metal-organic frameworks , cellulose nanocrystals , and hybrid organic-inorganic perovskites [2,3,8,9,13] (all in GIWAXS mode) as well as monodisperse nanoparticles [1,14,17], and block copolymers, polymer membranes, and polymer brushes [4,6,11,12,15] in GISAXS mode. In GIWAXS mode length scales probed are 1-3 nm, in the GISAXS modes 2-20 nm for nanoparticles and 20-80 nm for block copolymers.
In the SNIPS method a block copolymer film is coated onto a substrate, left to evaporate for a magic time period, and then plunged into water. Graduate students Yibei Gu and Rachel Dorin of the Wiesner group at the Cornell Department of Materials Science and Engineering set out to investigate what happens during the evaporation period in the top surface separation layer. They used an in-situ doctor blade coater developed by CHESS staff scientist Detlef Smilgies to study the membrane formation in real time at CHESS D1 station. Their results were recently published in Macromolecules .
However, typical thermal annealing is slow, and polymers may degrade at high annealing temperature. Recently a group at Cornell University devised a scheme for ultrafast heating of polymer films using laser spike annealing . Alan Jacobs from Mike Thompson’s group and Clemens Liedel from Chris Ober’s group, both at the Department of Materials Science and Engineering, brought samples to CHESS D1 station for a detailed analysis of laser annealed traces.
They are joined by their theory collaborator Igor I. Potemkin (Lomonosov Moscow State University) and former Papadakis postdoc Jianqi Zhang who is now at the National Center of Nanoscience and Technology in Beijing, as well as current postdoc Anatoly Berezkin (TUM).
Spreading the Butter on the Toast Just So – High-Performance Organic Transistors by Novel Deposition Technique
A joint team of scientists from the Cornell High Energy Synchrotron Source (CHESS), King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, and Stanford University just reported an experimental break-through for studying the structural evolution of organic transistor layers during the coating process in-situ and in real-time with x-ray scattering.