This collaborative research project is to study the nucleation and growth of small-molecule crystalline semiconductors, to discern ways to precisely control crystal grain size, surface coverage, and donor/acceptor interfaces, with the ultimate goal to realize low-cost, highly efficient solar cells. High-purity, small-molecule organic materials promise significant advantages in photovoltaic performance. By using soluble small-molecule semiconductors, the creation of appropriate thin-film morphologies is expected to provide superior charge transport properties, potentially achieving high performance of photovoltaic devices. The project requires collaboration of mathematicians for modeling nucleation, crystal growth and phase separation of the blended semiconductors; chemists for the design, synthesis and tuning of appropriate donor and acceptor semiconductors; materials researchers for the analysis of films; and engineers for construction and evaluation of solar cells. The team will work synergistically to understand the mechanisms of grain growth, tailor donor and acceptor to maximize voltage and current, and improve surface treatments and deposition methods to allow the formation of large-area materials and devices. NON-TECHNICAL SUMMARY: An understanding of nucleation and growth of soluble small-molecule semiconductors is expected to have significant impact in an array of endeavors across the spectrum of electronics, including solar cells, solid-state lighting, flexible displays and radio-frequency identification tags. To maximize the training impact of this research project, the participating research groups at Kentucky and Princeton exchange researchers to enhance the cross-disciplinary training of participating students. This exchange introduces engineering researchers to organic synthesis, and allows mathematics graduate students to learn film growth and device fabrication techniques. The PIs plan to run an annual summer camp, to immerse all participants in a joint review of the project. The scientists will also continue outreach to local Schools and mentor high school students in research projects. This project is co-funded by the Divisions of Materials Research, Chemistry, and Mathematical Sciences.
Carbon-based materials used in electronic devices such as solar cells and transistors offer many potential advantages over current generation materials based on minerals. Carbon compounds are light weight, can be processed at low temperatures, and can be engineered to be flexible, foldable and bendable, which can significantly increase durability. The performance of carbon molecules in electronics is directly related to how crystalline the materials are, and to the size of the crystals of the molecule. The intellectual merit of this proposal was to design materials, processes, and mathematical models to describe the crystallization in systems containing two kinds of molecules (essential for solar cells), with the overall goal of developing a model to predict which molecules would form the best films for highly efficient solar cells. Along with the broader impact of developing lower-cost solar panels, this project had the important scientific impact of developing a better understanding of crystal growth parameters of an important class of carbon semiconductors, understanding how these semicondutors transfer charge between each other when under bright light, the development of new light-harvesting materials from compounds once thought to be too unstable to work with, and devising new methods of purifying carbon-based semiconductors to yield significantly improved performance. Working with mathematicians Jabbour and Man, a new model was indeed developed that explained the phenomena that were observed with the single semiconductor TES ADT. The Loo group discovered that binary blends of semiconductors could indeed be tuned to control the rate and degree of crystallization, and that the impact of the mixture of compounds was strongly related to the similarity in crystal structure for the two materials in the blend. Using these models, the Loo group developed methods to exercise amazing control over the crystallization of carbon semicondutors, even guiding the growing crystal around bends and corners. The Anthony group explored new light-harvesting materials for this effort, focusing on a class of green-colored compounds called hexacenes. The chemical literature suggested that this compound class could never be made stable or soluble enough for use in devices such as transistors or solar cells, but we were able to create a number of highly stable derivatives, and demonstrate that they could indeed be used in thin-film devices (image 1). As our research progressed, we suspected that the fact that our carbon molecule core, an anthradithiophene, was synthesized as an inseparable mixture of two isomers, called the syn and anti compounds (see figure - image 2). After more than 1.5 years of work, we finally devised a simple way to separate the isomers, first in a simplified compound, where we found the performance in devices varied by a factor of 100 depending on the isomer studied (see figure - image 3) We have recently devised a way to use this same method to obtain isomer-pure versions of current commercially available versions of these anthradithiophene materials, and working with the Jurchescu group at Wake Forest, have shown that performance is indeed enhanced for the pure anti isomer compound (see figure - image 4). We are now woring with many of the research groups that had published results with the isomeric mixture, to see if this increase in performance is seen in all cases. Along with the training of numerous undergraduate, graduate and post-doctoral researchers, this project provided the funds to support the research efforts of several high school students. One of these students, Valerie Sarge, exhibited her research in my group at the 2013 Intel International Science and Engineering Fair. Her work earned her the top honors from the American Chemical Society for a chemistry-based poster, and from Intel she won a trip to CERN in Switzerland.
