Using Spacer Molecular Structure to Control Energetics, Stability, Charge-Carrier Transport, and Photovoltaic Performance in 2D Organic Metal Halide Perovskites: Participant Support Costs Scope

Hybrid organic-inorganic materials provide the potential for exciting new combinations of properties that neither pure organic nor pure inorganic materials display. One of the most exciting hybrid material classes for optoelectronic applications is hybrid organic-metal halide perovskites (HPs). The unparalleled increase in photovoltaic performance of 3D Pb-based HPs over the previous decade, combined with their low-cost, ability to be printed from solution, and attractive optical and electronic properties, has spurred tremendous research interest in HPs. However, 3D HPs suffer from instability, inclusion of the toxic element Pb, and more constrained options for future materials development owing to the requirements on ion sizes to maintain the 3D perovskite structure. Reduced dimensionality HPs, specifically those where the 3D HP structure is broken into 2D sheets by large organic spacer cations, are promising materials for improving stability, permitting the replacement of Pb with less toxic elements such as tin (Sn), and discovering materials with novel combinations of properties. In this proposal we will investigate targeted families of organic spacer molecules to decipher how the structure of the spacer molecule influences ionization energies, electron affinities, exciton binding energies, charge transport, photovoltaic performance, and stability in Pb- and Sn-based reduced dimensionality HPs. The research will take advantage of our low-energy ultraviolet and inverse photoelectron spectroscopy instrumentation to determine critical relationships between spacer structure, crystal structure, ionization energy, and electron affinity. The fundamental structure-property relationships established will provide guidance for designing spacers with increased functionality to yield reduced dimensionality HPs with improved optical and electronic properties. Intellectual Merit Understanding the influence of the spacer molecule on the optical and electronic properties of reduced dimensionality HPs is critical to enable the design of high-performance materials with targeted properties. Currently, it is hard to predict how molecular parameters of the spacer molecule, such as electrostatics, will influence the optical and electronic properties of reduced dimensionality HPs. Furthermore, there has been no work to establish fundamental relationships between the molecular parameters of the spacers and the ionization energies, electron affinities, and exciton binding energies of the reduced dimensionality HPs. These parameters are critical to designing materials and device structures for photovoltaics and light emitting diodes. Our proposed research will determine important structure-property relations with regards to the influence of the spacer molecule’s structure on ionization energies, electron affinities, exciton binding energies, and material stability. Thereby, these investigations will provide crucial information for accelerating the development of new materials. The guidelines established will be used to develop Sn-based 2D materials with improved charge transport properties and higher photovoltaic performance. Broader Impacts The educational goals of this proposal are to promote the development and dissemination of high-quality experiments to improve education and interest in STEM at the middle and high school, while emphasizing participation of traditionally underrepresented groups. These educational goals will be accomplished through hosting an annual workshop for middle and high-school teachers from around Kentucky, where they will conduct experiments in our organic electronics fabrication laboratory and leave with the materials required to make dye sensitized solar cells with their classes. The graduate students involved in this research and I will participate in STEM Camp and Chem Camp, where we will perform experiments with middle and high school students from around Kentucky that are participating in the camps. The experiments are designed to teach students about the role of chemistry in energy and technology, particularly in the technologies that these students interact with every day. The research results are also likely to have broader impacts by facilitating the development of low-cost photovoltaic cells and light emitting diodes.