RII Track-4: Applying Transient Reflectance Spectroscopy to Decipher the Impact of Energetics and Electronic Coupling on Interfacial Recombination in Hybrid Halide Perovskites

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Hybrid organic-inorganic halide perovskites (HPs) are promising materials for use in next-generation solar cells and light-emitting diodes (LEDs). Further improvements in the performance and stability of these materials and devices will help enable mechanically flexible, lightweight, and inexpensive solar cells and LEDs that may be used in disaster relief areas, in military applications, and broadly deployed to help create a more secure and sustainable energy future. In both solar cells and LEDs, the HP is sandwiched between charge-transporting layers that serve to remove or inject charge carriers (either holes or electrons) into the HP. Interfaces between the HP and these transporting layers are the dominant areas where harmful non-radiative recombination processes occur, which can lower light generation efficiency and increase heat loss. Thus, it is essential to better understand these interfaces and identify means to reduce interfacial recombination rates. This research will directly probe recombination rates at HP interfaces with various transport layers using transient reflectance spectroscopy through a collaboration with Dr. Matthew Beard at the National Renewable Energy Laboratory (NREL). The Graham group has worked extensively on understanding and manipulating the surface chemistry of HPs. Through this grant, the Graham group will be able to directly probe how changes in surface chemistry impact interfacial charge recombination rates. The findings of this research will help accelerate the development of HPs for solar cells and LEDs, while the collaboration with NREL will help transfer knowledge from the nation's premier renewable energy research facility to researchers in Kentucky. Charge-transfer processes at interfaces are one of the most relevant processes determining the performance of electronic and optoelectronic devices, yet these interfacial processes remain much less understood than those occurring within the bulk of the materials. The goal of this proposed research is to use HPs as a platform to characterize how energetics and electronic coupling influence interfacial recombination processes through the application of transient reflectance spectroscopy. Transient reflectance relies on ultrafast laser pulses to monitor charge carrier recombination dynamics specifically at interfaces, thereby providing a powerful tool to differentiate interfacial recombination from recombination occurring within the bulk of the HP. Ultimately, the research will test the hypothesis that interfacial recombination rates can be significantly reduced through manipulating the interfacial energy landscape and electronic coupling at interfaces between dissimilar materials using surface modifiers. Previous research has investigated surface and bulk recombination in pure HPs in both single crystal and polycrystalline film formats, but these investigations have not been extended to include the presence of charge transport layers. Recombination rates have also been investigated within completed PV devices, but in these complete devices it is not possible to identify where recombination is occurring; thus, targeted approaches to reduce recombination are hindered. In this proposed research the principal investigator and a graduate student will work with Dr. Beard at NREL on applying transient reflectance spectroscopy to directly probe recombination rates at these important interfaces. Combining these transient reflectance measurements with ultraviolet and inverse photoelectron spectroscopy will uncover how interfacial energetics and electronic coupling impact interfacial recombination, thereby helping to guide the development of improved solar cells and LEDs based on HPs.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.