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

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Nontechnical DescriptionOrganic metal halide perovskites (HPs) are attractive materials for high-performing inexpensive optoelectronic devices, such as photovoltaic cells and light emitting diodes, that can be fabricated using high-throughput approaches such as solution-based printing methods. Research scale photovoltaic cells based on HPs display record power conversion efficiencies that are on par with the most widely used photovoltaic material, silicon, but can be produced at a fraction of the cost of silicon photovoltaics. Although HPs have similar efficiencies to silicon, there are two major barriers to commercialization and wide scale deployment – inadequate stability and the presence of lead (Pb), a well-known toxic element. Reduced dimensionality HPs, which are a class of HPs where the three-dimensional (3D) perovskite structure is broken into 2D sheets by bulky organic cations, are promising materials for improving stability and enabling the replacement of Pb with tin (Sn), which is much less toxic. At present, photovoltaic devices based on reduced dimensionality HPs show significantly lower performance than their 3D counterparts, while Sn-based HPs also underperform their Pb-based counterparts. This project develops new reduced dimensionality HPs, with a stronger focus on the less toxic Sn-based materials, and determines key relationships between spacer structure and material properties, thus providing the understanding necessary to accelerate material development for more stable and less toxic photovoltaics and light emitting diodes. The research team promotes STEM education and interest at the middle and high school levels while emphasizing participation of traditionally underrepresented groups. This goal relies largely on an annual workshop for middle and high-school teachers from around Kentucky, where teachers conduct experiments in the organic electronics fabrication laboratory at the University of Kentucky and leave with the materials required to make dye sensitized solar cells with their middle and high school classes.Technical DescriptionThe performance of reduced dimensionality HPs is limited by their high exciton binding energies and poor electronic transport relative to their 3D counterparts, while the development of Sn-based HPs in general is limited by defect states resulting from Sn oxidation. Currently, it is hard to predict how molecular parameters of the spacer molecule, such as electrostatics, polarizability, or size, influence the optical and electronic properties of reduced dimensionality HPs. In part, these predictions are difficult because fundamental relationships between the molecular parameters of the spacer molecule and the ionization energy, electron affinity, and exciton binding energy of the reduced dimensionality HPs are largely unexplored. However, these parameters are critical to designing materials and device structures for photovoltaics and light emitting diodes. This research project systematically probes how the spacer molecule’s structure impacts these important material properties as well as stability, charge-carrier transport, and photovoltaic performance in both Pb- and Sn-based reduced dimensionality HPs. The project uses low-energy ultraviolet and inverse photoelectron spectroscopies to uncover critical relationships between spacer structure, crystal structure, ionization energy, and electron affinity in reduced dimensionality HPs. Next, the research focuses on how the spacer structure influences exciton binding energies in reduced dimensionality HPs and uses this knowledge to design materials with lower exciton binding energies. Third, spacer molecules are designed to inhibit Sn(II) oxidation and thereby provide a potential route for developing improved Sn-based HPs. Finally, selected reduced dimensionality HPs are incorporated into photovoltaic devices to determine how material properties such as exciton binding energies impact photovoltaic performance. Charge-carrier mobility measurements are conducted to establish more complete structure-property relationships that help to advance both material and device design.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.