NSF Award
2428901
Nontechnical<br/><br/>Organic semiconductors and metal halide perovskites are intensively studied materials for a range of clean energy and consumer applications such as solar cells, flexible electronics, and sensors. Although organic semiconductors have great promise, they exhibit significantly different electrical and optical properties depending on the crystal structure created when assembled into solid films. This impacts their potential for use in optoelectronic devices. Although there is a well-recognized need to precisely control crystal structure to manipulate or optimize material behaviors, there is a very limited experimental toolkit for doing so. In this project, the researchers aim to use perovskites as a templating layer that can control the crystal properties of organic films deposited on top. X-ray scattering and ultrafast spectroscopy are used to evaluate the structural properties of organic thin films, and their effect on the optical and electronic properties. This approach allows investigators to finely tune the underlayer periodicity and chemistry simultaneously and thereby optimize the efficiency of energy and charge transport. This paradigm can be used to design better solar cells, light-emitting diodes, and other optoelectronic devices. Students are engaged in the research through training and guidance by the principal investigators and participate in regular meetings between the groups. In addition, the project team is committed to promoting diversity by encouraging recruitment and retention of underrepresented groups to the project, and leading outreach efforts in the community targeted towards middle and high school students.<br/><br/>Technical<br/>The goal of this project is to develop two-dimensional metal halide perovskites (2D MHPs) as a crystallization templating tool for organic semiconductor (OSC) thin films, to reduce the disorder in crystalline packing, and to control packing geometries for tuning optoelectronic behaviors, including singlet fission and exciton transport. These controlled heterostructures can then be utilized for photovoltaic devices. Numerous research thrusts have focused on how changing the OSC chemistry can impact exciton transport and energy transfer, which generally result in large crystal structure changes, but there is relatively less information on how to finely control the solid-state structure of the OSC to control optoelectronic behavior, and by extension, device performance. This understanding is necessary for controlling processes such as singlet fission and exciton transport, as sub-Angstrom changes in molecular packing can cause significant changes in these behaviors. This project aims to address three objectives: 1) Understanding how 2D MHP thin films can control various OSC thin-film crystalline properties (order, polymorphism, orientation) through lattice registry, 2) Controlling exciton transfer (including singlet fission) and transport in 2D MHP templated OSCs by utilizing these sub-Angstrom changes, and 3) Controlling the heterostructure transport between the 2D MHP layer and the OSC. The research focuses on the prototypical OSC molecule perylenediimide, but the knowledge gained is relevant for other OSCs as well. The success of this project can result in a novel method of controlling the order, packing, and orientation of OSCs and understanding and manipulating structure-property relationships between templated OSCs and optoelectronic properties. There is a scarcity of fundamental knowledge relating to sub-Angstrom changes in OSC solid-state packing and their resulting optoelectronic behavior. The work addresses this gap and advances the knowledge of crystal structure and optoelectronics. The use of a 2D MHP template to reduce defects as well as change the crystal packing while still creating stable structures would be a significant departure from the current understanding of creating ordered OSC thin films, and the optoelectronic control afforded by these structures can open new doors for device applications.<br/><br/>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.
