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

非技术总结：大规模生产低成本半导体薄膜有望改善包括太阳能电池在内的许多高科技应用。高真空技术可以获得高质量的半导体薄膜，但低沉积速度、高成本、难以扩展和/或高加工温度可能会限制特定的工艺和沉积材料。概念上有吸引力的替代途径包括预先形成前驱体溶液（或半导体油墨），然后通过喷涂，卷对卷或丝网印刷或浸渍涂层将油墨简单而廉价的溶液沉积到基材上。这为半导体油墨的发展提供了强大的动力；然而，大多数块状无机半导体完全不溶于普通溶剂，使得通过简单溶解形成油墨非常困难。这突出了对无机半导体的新型“通用溶剂”或碱的需求。在材料研究部固态和材料化学项目的支持下，首席研究员正在利用他的研究小组发现的一种新型溶剂混合物，可以很容易地溶解大量无机材料，以制造溶液可加工的半导体油墨。在半导体墨水的溶液沉积过程中，研究小组证明了在温和的条件下可以获得高质量的半导体薄膜。纳入这项研究计划的是一个专门针对当地社区大学生的外展计划。在传统的化学拓展项目中，社区大学的学生是最没有针对性的；然而，大洛杉矶地区是美国社区大学生人数最多的地区。首席研究员与Cerritos社区学院（一个拥有大量代表性不足的学生的机构）合作，提供材料研究方面的实习机会。这项为期8周的年度拓展计划的目标是为这些学生提供在社区大学无法获得的STEM研究机会，从而提高他们转学到4年制大学的几率。技术总结：尽管在固态和材料化学领域有了50多年的发展，但沉积无机半导体薄膜的方法仍然有限，其中大多数需要苛刻和能源密集型的条件。该项目解决了这一挑战，主要研究人员发现，硫醇和胺的二元溶剂混合物可以很容易地溶解大量无机硫族半导体，以制造溶液可加工的半导体油墨。这是值得注意的，因为这些材料通常不溶于普通溶剂。这种溶剂系统的优点有四方面：(i)在环境条件下具有高溶剂功率，（ii）相对无害，（iii）溶解速度快，（iv）具有足够的挥发性，因此适合溶液沉积。在半导体墨水的溶液沉积后，在温和的条件下可以获得高质量的目标无机相晶体薄膜。在这个项目中，首席研究员利用他的团队在无机材料合成方面的专业知识来实现以下目标：(i)探索可以用硫醇胺溶剂混合物溶解和溶液加工的散装材料的范围。重点是通过对分子溶质的考察来研究溶解机理；（ii）利用光电化学表征技术评估溶液处理薄膜，以衡量半导体薄膜在太阳能转换方面的效用。如果满足一定的要求（例如，强感应光电流，带隙在~1.0-1.5 eV之间，由地球丰富的元素组成），那么第一代固态太阳能电池将被制造和测试；（三）将半导体油墨应用于无机纳米晶体的合成和配体交换。这些目标最终将使“设计材料”的方法成为可能，在这种方法中，新的功能薄膜和纳米晶体可以从块状材料中合理地合成，以满足特定的应用。

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.