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

Hybrid organic-inorganic halide perovskites (HPs) are an emerging class of inexpensive semiconductors that can be processed using high-throughput and low-cost solution processing techniques to yield high-performing materials and optoelectronic devices. In optoelectronic devices, such as photovoltaics (PVs) and light-emitting diodes (LEDs), the HPs are sandwiched between transporting layers that serve to remove or inject either holes or electrons. Interfaces between these transporting layers and the HP are critical to the performance of the optoelectronic device, as these interfaces can limit charge extraction or injection and serve as recombination sites for charge-carriers. In PVs, even the highest performing HPs fall 25% or more below their theoretical limits when accounting for unavoidable losses. To enable further material and device development, it is essential to understand the fundamental recombination processes taking place in these materials and within the PV devices. The proposed research will directly probe recombination rates in the bulk of HPs and at interfaces with both organic and inorganic transport layers using transient reflectance (TR) spectroscopy through a collaboration with Dr. Matthew Beard at the National Renewable Energy Laboratory (NREL). The Graham group will apply their expertise in the surface modification of HPs to systematically manipulate interfacial energetics and electronic coupling between the transport layers and the HP, while using their ultraviolet and inverse photoelectron spectroscopy (UPS and IPES, respectively) systems to probe these energetics. Following manipulation of interfacial properties, the interfacial recombination rates will be directly measured at NREL. Furthermore, PV devices will be fabricated at NREL to allow characterization of recombination rates through transient photovoltage (TPV) spectroscopy within the complete PV cell.