2, Argonne National Laboratory, Lemont, Illinois, United States
3, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
4, Ulsan National Institute of Science and Technology, Ulsan, , Korea (the Democratic People's Republic of)
5, Sandia National Laboratories, Albuquerque, New Mexico, United States
Methylammonium lead trihalides (MAPbX3) have great potential as light harvesters for perovskite solar cells due to their unique optical and electronic properties. In general, conventional growth techniques apply spin-coated precursors on a substrate followed by annealing for the processing of the lead halide perovskites; however, use of toxic solvents and high temperature hinder device stability and performance. To avoid annealing processes, the solution-based methods have been developed, which involve the formation of perovskite colloidal particles in solution. I will introduce a new one-step solution technique to facilitate in situ crystal formation of methylammonium lead bromide and methylammonium lead chloride perovskites at the micron (~1-10 µm) to nano scale (< 500 nm).1 As a substrate-free approach, the crystal pre-growth allows crystallization in alcohols (methanol, ethanol, 2-propanol, 1-butanol, and 2-butanol) at room temperature followed by a direct precipitation of the perovskite material for a large-area deposition. This room-temperature processable technique, however, differs from the in situ growth method of methylammonium lead iodide in alcohols that eliminates treatment at the boiling point of the alcohols.2
The techniques used to characterize the perovskite crystals involve high-energy synchrotron XRD, wide angle x-ray scattering (WAXS), Fourier transform infrared (FTIR) in reflection (ATR, % R), UV-Vis-NIR (% R), micro Raman spectroscopy, and the solid state 1H, 13C, and 207Pb –MAS NMR. Based on the analysis results, the perovskites show improvement in air/moisture for their chemical stability (<1.5 months, in a fume hood under varying humidity level). Thermogravimetric analysis and in situ techniques of powders also determine their thermal stability (~150°C -MAPbCl3, ~250°C -MAPbl3, ~350°C -MAPbBr3). The poor yield of methylammonium lead iodide in toluene confirms that the alcohols catalyze the growth process through a substitutional reaction mechanism but the mechanism in toluene follows a different path. Indeed, the theoretical calculations reveal that the growth reaction in alcohols is exothermic. Moreover, I will discuss the role of solvent polarity (polar, apolar, non-polar), the type of solvent (protic vs. aprotic), the reactivity order of the alcohols, effect of their different binding affinity, and other reaction parameters on the growth mechanisms. 1M. Acik, et al. in prep., 2M. Acik, et al. Adv. Energy Mater., 1701726 (2017).