Crystal growth of vacancy ordered triple-perovskites in silica gel Prajna Bhatt and Anna Regoutz Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, UK Lead-free halide perovskites (ABX 3 ) and perovskite-related structures, such as the vacancy ordered perovskites (A 2 BX 6 ,A 3 B 2 X 9 ) are well established for their application potential in solar cells due to their enhanced optical and electrical properties. [1] Beyond solar cells, applications are also include LEDs, X-ray detectors in healthcare and gas sensing. [2] Single crystals of perovskite materials prove to be advantageous as they are able to circumvent issues of unwanted defect inclusion in materials, provide a large active area, longer carrier diffusion lengths and higher attenuation. [3] Current single crystal growth mechanisms either require specialised apparatus (Bridgman growth) or are solution based . Liquid medium growth modes show difficulties, mainly the formation of defects as a result of convection currents - causing point defects and lattice strains. [4] We present a method of crystal growth for halide perovskites in gel based systems. A gel is described as a system with two phases (solid and liquid) wherein, the solid phases forms a loosely linked polymer that the liquid phase permeates. [5] Commonly used gels for crystal growth include agar, bentonite, gelatin, polyacrylamide and silica. Of these, this work presents crystal growth with silica gels, which has been the most extensively studied. Crystal growth in gels is described as nucleation by diffusion. As such, Fick’s law is able to formalise the gel growth mechanism. [9] Figure 1 (a), (b) illustrates two growth modes, single and dual diffusion. FIG.Schematic of (a) single diffusion and (b) dual diffusion growth systems (c) (CH 3 NH 3 ) 3 Bi 2 I 9 grown by single diffusion Using the silica gel medium, vacancy ordered triple-perovskites A 3 B 2 X 9 (A = CH 3 NH 3 , NH 2 HCNH 2 , Cs, Rb and X = Cl, Br, I) (Figure 1 (c)) are synthesised and characterised using X-ray Diffraction, Xray Photoelectron Spectroscopy and Raman Spectroscopy. Additionally, diffusion coefficients for the precursors are found and calculated using UV-visible spectroscopy and Fick’s Law. Diffusivity of the precursor solutions within the silica gel has been explored for different gel preparation methods to correlate with crystal growth. With the successful formation of single crystals, future work will include doping at the B- and X-site using the silica medium and calculating X-ray detection limits, electron transport and ion mobility analysis to assess feasibility for high performance X-ray detection and light absorption in solar cells. References 1. L. Chouhan, S. Ghimire, C. Subrahmanyam, T. Miyasaka, and V. Biju. Chem. Soc. Rev., 49(10):2869–2885, 2020. 2. H. Kim, J. Han, J. Choi, S. Kim, and H. Jang. Small Methods, 2(3):1700310, 2018. 3. L. Chen, Y. Tan, Z. Chen, T.Wang, S. Hu, Z. Nan, L. Xie, Y. Hui, J. Huang, Chao Zhan, et al. ACS, 141(4):1665–1671, 2019. 4. S K. Arora and T. Abraham. Journal of Crys. Growth, 52:851–857, 1981. 5. O. VelÅLasquez-GonzÅLalez, C. Campos-Escamilla, A. Flores-Ibarra, N. Esturau-Escofet, R. Arreguin-Espinosa, V. Stojanoff, M. CuÅLellar-Cruz, and A. Moreno. Crystals, 9(9):443, 2019.
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