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

非技术综述：大规模制造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受到限制，具体取决于特定的工艺和要沉积的材料。概念上有吸引力的替代方法包括预制前体溶液(或半导体油墨)，然后通过喷涂层、辊对辊或丝网印刷或浸渍涂层将油墨简单且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，使得通过简单的溶解形成油墨非常困难。这凸显了无机半导体需要新的“通用溶剂”，或称碱类化合物。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来容易地溶解广泛的块状无机材料，以制造可溶液加工的半导体油墨。通过溶液沉积产生的半导体油墨，研究小组正在证明，可以在温和的条件下获得高质量的半导体薄膜。在这项研究计划中纳入了一个专门针对当地社区大学生的外展计划。社区大学生是传统化学推广项目中最不受关注的群体之一；然而，大洛杉矶地区是美国社区大学生人数最多的地区。首席调查员与Cerritos社区学院合作，提供材料研究方面的实习机会。Cerritos社区学院拥有大量未被充分代表的学生。这项为期8周的年度外展计划的目标是为这些学生提供社区大学水平无法提供的STEM研究机会，从而提高他们到四年制院校的转化率。技术摘要：尽管固态和材料化学领域已有50多年的发展，但沉积无机半导体薄膜的方法仍然有限，其中大多数方法要求苛刻和能源密集型条件。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范围的块状无机硫化物半导体，以制造可溶液处理的半导体油墨。这一点值得注意，因为这些材料通常不溶于普通溶剂。这种溶剂体系的好处有四个：(I)它在环境条件下具有高的溶剂力，(Ii)它相对无害，(Iii)溶解速度快，(Iv)它有足够的挥发性，使其易于溶液沉积。通过溶液沉积半导体墨水，可以在温和的条件下获得目标无机相的高质量结晶薄膜。在这个项目中，首席研究员正在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)探索可以溶解的块状材料的范围，以及使用硫醇-胺溶剂混合物进行溶液处理的范围。重点是通过检查分子溶质来研究溶解机制；(2)通过光电化学表征技术评估溶液处理过的薄膜，以衡量半导体薄膜对太阳能转换的效用。如果满足某些要求(例如，强烈的感生光电流，禁带宽度在~1.0-1.5 eV之间，由富含地球的元素组成)，那么第一代固态太阳能电池正在制造和测试；以及(Iii)将半导体墨水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“按设计获得材料”的方法成为可能，在这种方法中，可以从块状材料合理地合成新的功能薄膜和纳米晶体，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.