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 Characterization
To determine lattice spacing, the crystals were analyzed on the University of Utah’s Crus Center’s Rigaku Miniflex 600 X-ray Diffractometer (XRD). The thin films were analyzed on an Anton Paar SAX/WAXS/GISAXS/RheoSAXS laboratory beamline at the University of Utah’s Nanofab using the GISAXS 2.0 stage with the GIWAXS Heating Module 2.0 attachment, to analyze their phase change and its effects on lattice spacing. A variety of microscopes were used to analyze the thin films. An Axioscope Zeiss inspection microscope, a Hitachi S-3000N scanning electron microscope, and a Molecular Vista PiFM, were all used to inspect the surfaces of the thin films. Results and Discussion We found that the larger the lattice spacing, the higher the transition temperature of the perovskite was. The XRD showed that (DDA)2CuBr4 had the largest lattice spacing at 27.68 Å, followed by (DA)2CuBr4 with 24.07 Å and (NA)2CuBr4 with 22.41 Å. The XRD also showed that blending two cations in a crystal effect the lattice spacing, with a 50/50 blend of DA and NA resulting in a crystal with a lattice spacing of 23.65 Å. When we preformed temperature dependent GIWAXS on thin films of these samples, we found that (DDA)2CuBr4 had the highest phase transition around 60 ℃, while (DA)2CuBr4 had a phase transition around 45 ℃, and (NA)2CuBr4 having a phase transition somewhere below 30 ℃ and below the temperature limits of the heating stage. Further research into these phase transitions and how lattice spacing effects these transitions will include using differential scanning calorimetry (DSC) of all crystals and will also include the creation and characterization of crystals with different blends of cations.
Bibliography:
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(2) Dahod, Nabeel S.; Paritmongkol, Warcharaphol; Stollmann, Alexia; Settens, Charles; Cheng, Shao-Liang; Tisdale, William A. Melting Transitions of the Organic Subphase in Layered Two-Dimensional Halide Perovskites. J. Phys. Chem. Lett. 2019, 10, 2924-2930
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(5) Li, X.; Li, B.; Chang, J.; Ding, B.; Zheng, S.; Wu, y.; Yang, J.; Yang, G.; Zhong, X.; Wang, J. (C6H5Ch2Nh3)2Cubr4: A Lead-Free, Highly Stable Two-Dimensional Perovskite for Solar Cell Applications.” ACS Appl. Energy Mater. 2018, 1, 2709-2716.