Dimensional Confinement-Deconfinement Along the Crystallographic C Axis: A Desorption Mediated Band Engineering of Methylammonium Lead Iodide

Strong optical absorption, long diffusion length, benign intrinsic defects established organic-inorganic hybrid perovskites as one of the most important materials in the area of photovoltaics. Unfortunately, the formation of photogenerated halide rich trap centers during conventional halide exchange methods limits the use of hybrids perovskite in multi-junction solar cells.
In this report, we demonstrate band gap engineering of MAPbI₃ film due to systematic dimensional deconfinement along the crystallographic c axis during gas induced growth. There are some distinct interrelated events occurring during gas induced reaction, which involves (i) in situ formations of hexylamine from 2D (HA)₂PbI₄ upon exposure to methylamine, (ii) selective adsorption of hexylamine on the Pb sites of growing MAPbI₃ leading to the anisotropic growth, (iii) systematic desorption of hexylamine during exposure of methylamine leading to continuous band gap tuning (2.18 eV to 1.69 eV). Most importantly, this dimensional confinement-deconfinement leads to extremely smooth, homogeneous film without having the halide rich trap centres, thereby establishing the importance of this novel route in the context of hybrid perovskite photovoltaics.