In this project, funded by the Chemical Structure, Dynamics and Mechanisms program of the NSF Chemistry Division, Professor Martin Zanni from the University of Wisconsin-Madison will utilize two-dimensional infrared (2D IR) and heterodyned sum-frequency generation (SFG) spectroscopies to study electron transfer across molecule/semiconductor interfaces. This process is one of the most fundamental aspects of solar-to-electrical and solar-to-fuel conversion processes, but is still not fully understood largely due to the difficulties in characterizing molecular structures at interfaces. These techniques will be used to resolve multiple dye conformations, map their individual kinetics by measuring the time-dependence of their respective cross peaks, and determine their conformations by measuring the angles of molecular bonds on single crystal interfaces.
The broader impacts of this proposal include the training of undergraduates and graduate students in this multi-disciplinary field, the dissemination of our results at national ACS meetings and through refereed publications, and the development of a website to make it easy and efficient to keep up-to-date on developments in multidimensional spectroscopy. The goal is to provide a level of molecular detail not previously attainable for interfacial charge transfer, thereby leading to a better understanding of charge separation and transfer at molecule-semiconductor interfaces that is necessary for the rational design of organic dye- and quantum-dot sensitized solar and fuel cells.
Many different technologies require molecules to interact with surfaces. For example, the next generation of solar cells will probably be small inorganic molecules that are absorbed to crystalling semiconductors. Solar cells like these would be lightweight and flexible so that they might be rolled out like a rubber mat or painted onto the side of a house. Another technology is chemical sensors, such as for detecting explosives or hormones in human blood. However, it is extremely difficult to study molecules on surfaces, because existing techniques are either not sensitive enough or cannot easily be applied to materials. The aims of this research project were to study the structures of several dyes that are often used as models for the types of molecules in next generation solar cells. We discovered that these dyes tend to stick together when applied to the crystallin substrate. These little aggregates have different electronic properties than when solo, which we discovered altered the speed at which the dyes can injection electrons, which is an important finding because it provides insight into how these next generation solar cells might work. We also realized that we need to be able to study a single layer of dyes, not just a polycrystallin substate. Since so few molecules exist on a single layer, we had to use a technique that was new to my laboratory, called sum-frequency generation (SFG) spectroscopy. As we learned about this technique, we realized that we could improve it, and so we spent quite a lot of time developing a new 2D version of SFG. The results have been terrific. We now have a quantitative tool for studying molecular monolayers. Thus, our research is having impact on the scientific community by providing insights into exciting new solar cell materials and also by developing a new analytical technique that might be used by other researchers on these topics and many others.
