Solution Processing of Bulk Semiconductors with a Thiol-Amine Solvent Mixture

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NON-TECHNICAL SUMMARY: Making low-cost semiconductor thin films on a large scale holds promise for improving a number of high-tech applications, including solar cells. High vacuum techniques can achieve excellent quality semiconductor thin films, but low deposition speeds, high cost, difficult scalability, and/or high processing temperatures can be limiting depending on the particular process and material to be deposited. A conceptually attractive alternate route involves pre-formation of a precursor solution (or semiconductor ink) followed by simple and inexpensive solution deposition of the ink onto a substrate by spray coating, roll-to-roll or screen printing, or dip coating. This provides a powerful driving force for the development of semiconductor inks; however, most bulk inorganic semiconductors are totally insoluble in common solvents, making ink formation by simple dissolution very difficult. This highlights the need for new "universal solvents," or alkahests, of inorganic semiconductors. With support from the Solid State and Materials Chemistry program in the Division of Materials Research, the principal investigator is utilizing a novel solvent mixture discovered by his research group to readily dissolve a broad scope of bulk inorganic materials to make solution processible semiconductor inks. Upon solution deposition of the resulting semiconductor inks, the research team is demonstrating that high-quality semiconductor thin films can be achieved using mild conditions. Integrated into this research plan is an outreach program specifically aimed at local community college students. The community college student demographic is among the least targeted in traditional chemistry outreach programs; however, the greater Los Angeles area is home to the largest number of community college students in the U.S. The principal investigator has partnered with Cerritos Community College, an institution with a large number of underrepresented students, to provide internships on materials research. The objective of this annual 8-week outreach program is to provide these students with STEM research opportunities that are not afforded to them at the community college level, and thereby increase their transfer rate to 4-year institutions.TECHNICAL SUMMARY: Despite over fifty years of developments in the field of solid-state and materials chemistry, there are still only a limited number of ways to deposit inorganic semiconductor thin films - the majority of which require harsh and energy intensive conditions. This project addresses this challenge with the principal investigator's discovery that a binary solvent mixture of thiol and amine can readily dissolve a wide scope of bulk inorganic chalcogenide semiconductors to make solution processible semiconductor inks. This is notable because these materials are typically insoluble in common solvents. The benefits of this solvent system are four-fold: (i) it possesses high solvent power under ambient conditions, (ii) it is relatively nonhazardous, (iii) dissolution is kinetically fast, and (iv) it has sufficient volatility such that it is amenable to solution deposition. Upon solution deposition of the semiconductor ink, high-quality crystalline thin films of the target inorganic phases can be achieved under mild conditions. In this project, the principal investigator is leveraging his group's expertise in inorganic material synthesis to meet the following objectives: (i) Explore the scope of bulk materials that can be dissolved and solution processed with the thiol-amine solvent mixture. An emphasis is being placed on studying the mechanism of dissolution through an examination of the molecular solutes; (ii) Assess the solution-processed films by photoelectrochemical characterization techniques to gauge the utility of the semiconductor films for solar energy conversion. If certain requirements are met (e.g., strong induced photocurrent, band gap between ~1.0-1.5 eV, comprised of earth abundant elements), then first-generation solid-state solar cells are being fabricated and tested; and (iii) Apply the semiconductor inks toward the synthesis and ligand exchange of inorganic nanocrystals. These objectives will ultimately enable a "materials by design" approach to be taken, in which new functional thin films and nanocrystals can be rationally synthesized from bulk materials to meet specific applications.