College of Science
83 Controlling Phase Transitions in Lead-free 2D Metal Halide Perovskites
Emily Dalley
Faculty Mentor: Connor Bischak (Chemistry, University of Utah)
Introduction
Two-dimensional (2D) metal halide Ruddlesden-Popper (RP) perovskite crystals have been shown to have a number of optoelectronic properties, making them useful materials for a wide range of devices, such as light emitting diodes (LEDs), solar panels, and photodetectors (1-3). Additionally, RP perovskites are efficient barocaloric materials, as they undergo reversible phase transition in response to changes in pressure or temperature (3-4). This phase transition is possible due to the structure of RP perovskites, where two inorganic layers sandwich an organic layer. As the organic layer “melts,” the inorganic layer remains solid, allowing the crystal to undergo a solid-solid phase transition. These phase transitions can be used for room-temperature solid-state barocaloric cooling, which could allow perovskites to replace hydrofluorocarbon as more eco-friendly refrigerants (3-4). However, much of the research on RP perovskites to date has been on lead-based halide perovskites, which would be too toxic to be feasible for industrial level production (5). This is why our study specifically focuses on copper (II) bromide perovskites, which are much more environmentally friendly. We look into controlling the phase transitions in lead-free metal halide perovskites by altering the length of the organic cation. We did this by synthesizing copper (II) bromide with nonylammonium ((NA)2CuBr4), decylammonium ((DA)2CuBr4), and dodecylammonium ((DDA)2CuBr4), and then used a variety of methods including X-ray diffraction (XRD), temperature dependent grazing incident wide angle x-ray scattering (GIWAXS), bright field microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM), to characterize and analyze the resulting perovskite crystals and their thin films.
Procedure
I. Synthesis
First, we brominated the alkylamines (nonylamine, decylamine, and dodecylamine) by combining the amine with hydrobromic acid in a one-to-one ratio in ethanol to create white salts, using a procedure adapted from a previous study (5). We then combined these salts with copper (II) bromide in a solution of hydrobromic acid and recrystallizing the resulting solution overnight resulting in dark purple flake-like crystals, again following a similar procedure to the previous study (5). To create thin films, we dissolved the crystals in ethanol in ratios equivalent to 100 mg crystal to 1 mL solvent. We then syringe filtered this precursor solution and spin-coated 40 𝜇L of solution onto a clean substrate at 2000 rpm for 30 s, then annealed on a hotplate at 80 ℃ for 10 min. The substrates were cleaned in 1% alconox, DI water, acetone, and IPA sequentially for 10 min each. They were then dried using an N2 gun and then cleaned in a plasma cleaner for 15 min before being spin-coated.
II. Analysis and