Use of Nanoconfinement to Control Metal-Halide Perovskite Crystallization and Stability

We present a systematic study of the effect of nanoconfinement on the crystallization of methylammonium lead halide (MAPbI$_{\mathrm{3}})$ perovskite crystallization. MAPbI$_{\mathrm{3}}$ was spin coated onto anodized aluminum oxide (AAO) templates with uniaxially-aligned pores ranging from 20 -- 200 nm in diameter and examined using 2-D X-ray diffraction and scanning electron microscopy. X-ray diffraction patterns revealed the presence of a transient precursor phase that converts to the MAPbI$_{\mathrm{3}}$ crystal structure upon thermal annealing. The orientation of the precursor phase and conversion rate to the MAPbI$_{\mathrm{3}}$ crystal structure were found to depend on the pore size of the AAO template. The stability of MAPbI$_{\mathrm{3}}$ in air also depends on the extent of nanoconfinement. When deposited on flat SiO$_{\mathrm{2}}$ surfaces, MAPbI$_{\mathrm{3}}$ degraded into PbI$_{\mathrm{2}}$ and MA after 21 days. When deposited in AAO templates exhibiting 20-nm pore sizes, however, MAPbI$_{\mathrm{3}}$ crystals were stable for longer than 16 days. These findings suggest that nanoconfinement of MAPbI$_{\mathrm{3}}$ crystals may be a promising strategy for improving the stability of perovskite-based solar cells.