丁香园AV

How does a material work? What makes one material better than another? How does one design the best material? Simply trying them all is not possible, not even via computer simulation. The number of possible synthetic targets for carbon-based molecules with 30 or less atoms is greater than the number of atoms in the solar system. In the face of such a vast unknown, one needs insight into the mechanisms responsible for the properties of a material. We use the tools of physical chemistry to answer this question. Our signature methods are in-situ spectroscopy and self-assembly in supercritical fluids. In-situ spectroscopy uses light to gain molecular-level information during device operation by coupling electrical measurements with spectroscopic techniques like FTIR or UV-Vis. We are also working to control nanoscale material structure by using the unique properties of supercritical fluids to grow thin films in a process we call physical supercritical fluid deposition (p-SFD). Our research spans the breadth of physical chemistry from solution thermodynamics, to polymer physics, to quantum mechanics and spectroscopy. We do thin film characterization and develop software tools for data analysis and modeling. We are always looking for students with good chemical intuition that are mechanically inclined and not afraid of a challenge. Check the group website for the most up-to-date information on what we are doing.

Kaake, L.G. Towards the Organic Double Heterojunction Solar Cell. Chem. Rec. 2019, 19, 6, 1131-1141. DOI: 10.1002/tcr.201800180

Chemistry in higher dimensions! The diversity of chemical compounds can only be captured by describing chemical parameter space in a very high number of dimension. A small picture of high-dimensional space that illustrates the challenge of finding a compound with good properties (illustrated with green to red) from a given starting point (circled with blue). 

Ion Transport in Organic Electrochemical Devices

Organic electronic materials are pi-conjugated polymers and oligomers that have the ability to transport electronic charge carriers. These materials are useful in many applications like solar cells, light emitting diodes and printed semiconductor logic devices. They are also capable of transporting both ions and electronic charge carriers. This property has applications in printed batteries, electrochemical transistors, electrochromic devices, light emitting electrochemical cells, sensors, and neuromorphic computing elements. Our research is focused on understanding the physical processes responsible for device function and relating chemical structure to device performance. For example, we investigated the rate limiting step that allows an organic electrochemical transistor to be switched from the insulating (off) state to the conducting (on) state. In many cases, the diffusion of ions in the organic semiconducting layer is slower than the movement of electrons in the semiconductor and the movement of ions in the electrolyte. In collaboration with the University of Stuttgart, we demonstrated that this process also determines the kinetics of standard electrochemical experiments, like cyclic voltammetry. We are developing a structure-property relationship for this process by combining concepts from polymer mixing thermodynamics and ion transport theories from aliphatic polymers.

Shiri, P.;  Neusser, D.;  Malacrida, C.;  Ludwigs, S.; Kaake, L. G., Mixed Ion-Carrier Diffusion in Poly(3-hexyl thiophene)/Perchlorate Electrochemical Systems. J. Phys. Chem. C 2021, 125, 536-545. DOI: 10.1021/acs.jpcc.0c09527

Shiri, P.;  Dacanay, E. J. S.;  Hagen, B.; Kaake, L. G., Vogel鈥揟ammann鈥揊ulcher model for charging dynamics in an organic electrochemical transistor. J. Mater. Chem. C 2019, 7, 12935-12941. DOI: 10.1039/C9TC02563D

Shiri, P.;  Dacanay, E. J. S.;  Hagen, B.; Kaake, L. G., Dynamic Charging Mechanism of Organic Electrochemical Devices Revealed with In Situ Infrared Spectroscopy. J. Phys. Chem. C 2019, 123, 19395-19401. DOI: 10.1021/acs.jpcc.9b05739

Schematic of in-situ FTIR technique (top) spectra collected as a function of time (middle) and integrated spectra as a function of time for several different organic semiconductor films (bottom).

Self-Assembly in Supercritical fluids

Supercritical fluids are compounds held at temperatures and pressures that allow them to exhibit properties intermediate to liquids and gasses. We are developing the field of self-assembly in supercritical fluids to grow materials. Despite the rich history of liquid-phase self-assembly and the science of supercritical fluids, this topic is largely unexplored, giving our group a unique opportunity to make discoveries about the process. The key to the process is that at constant pressure, polymers first increase their solubility with increasing temperature, but their solubility decreases at higher temperatures. This property can be used to form thin polymer films when the solution is held at the solubility maximum and the substrate is held at a higher temperature. The ability to direct material deposition by heating the substrate has led to a patented additive manufacturing process that couples solution phase self-assembly and photolithography. This provides a key to realizing many of the early promises of nanotechnology. We are working to produce materials via this process and study their properties, providing a relationship between sample morphology, supercritical growth conditions, and material properties.