CAREER: Nanostructural strain to control stability and function in halide perovskites

Drexel University, Philadelphia PA

Investigators

Abstract

Non-technical abstract This NSF CAREER award, supported by the Solid State and Materials Chemistry program in the Division of Materials Research, fundamentally expands the palette of materials available for solar cells and other applications by rendering metastable structures stable. Metastable structures exist only temporarily before reverting to their "preferred" stable structure. In spite of this drawback there exist a vast number of metastable materials with otherwise ideal properties for a variety of technological applications. For example, among the diverse class of materials known as metal halide perovskites metastable variants are known that combine ideal properties for efficient solar cells (over 23% efficient solar-to-electricity energy conversion) with extremely low-cost and scalable synthesis. This project provides critical new avenues to exploiting such ideal behaviors in structures that have not been prepared in stable forms before. The novel techniques employed in Aaron Fafarman's research group to achieve the stabilization of these structures relies on the ability to make very small - one ten-thousandths of the thickness of a human hair - metastable materials and thereby with control induce strain in the structures. This makes it possible to selectively enhance their stability. The tremendous social dividends of the low-cost solar cells that could result from utilizing the fundamental principles established in this work include reduced emission of green-house gases, increased domestic energy security, a sustainable energy economy and growth of jobs in the development and manufacture of an important high-tech commodity. Additionally, through this project, undergraduate and graduate students receive interdisciplinary training in chemistry, solid-state physics and electrical engineering tools and concepts, emerging with skills for a twenty first century manufacturing economy. Also, as part of this work, local, public junior high students get hands-on experience synthesizing solar energy conversion materials in a format designed to bolster their interest in the STEM field. Technical abstract: This NSF CAREER award project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research, develops near-ambient-temperature chemical and physical approaches to synthesize and stabilize non-equilibrium semiconducting ionic crystals by understanding and controlling lattice strain with nanoscale precision. Through defined nanostrain, this research provides entirely novel avenues for the synthesis of halide perovskites into complex, far-from-equilibrium, functional materials - avenues that may prove generalizable to a much wider class of ionic solids. The correlations between strain and compositional stability, perovskite phase-stability and functional properties are tested in three ways: by imparting reduced dimensionality (nanostructuring), applying negative pressure and inducing spatially defined-strain perturbations. Negative pressure is achieved in a versatile scheme due to frustrated thermal contraction of a material embedded in a second material with a smaller thermal expansion coefficient. Both by nanostructuring alone and by engineered thermal stresses, low density and high symmetry crystal polymorphs are stabilized that are inaccessible in the bulk. In a related concept, spatially defined-strain perturbations are utilized as a means to modify the as-made distribution of impurity ions in doped ionic crystals. The resulting spatially engineered composition gradients can be harnessed for a variety of functional purposes; they also comprise a tool for probing the strain-structure-function relationship. The tremendous social dividends of the low-cost solar cells that could result from utilizing the fundamental principles established in this work include reduced anthropogenic emission of green-house gases, increased domestic energy security, and a sustainable energy economy. Additionally, through this project, undergraduate and graduate students receive interdisciplinary training in chemistry, solid-state physics and electrical engineering tools and concepts, emerging with skills for a twenty first century manufacturing economy. Also, as part of this work, local, public junior high students get hands-on experience synthesizing solar energy conversion materials in a format designed to bolster their interest in the STEM field. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.