A study of structural and dielectric properties of Ba2+ doped CH3NH3PbI3 crystals

Abstract

An understanding of the effects of doping in lead perovskites may allow for improved solar cell performance or reduction in toxicity of the materials used. Ba 2+ doped CH 3 NH 3 PbI 3 (CH 3 NH 3 Pb 1−x Ba x I 3 with x = 1%, 5% and 10%) are successfully synthesized by cooling down a concentrated aqueous solution containing HI, CH 3 NH 2 and metal acetates and their properties are investigated using x-ray diffraction, differential scanning calorimetry, and impedance spectroscopy (IS). No new structures are formed upon doping with Ba 2+ ions, however the lattice expands. The IS measurements show a strong increase in the conductivity and dielectric constants of the doped crystals. The conductivity arises mainly from vacancy mediated iodide ion (I −) migration and the increase in ionic conductivity with dopants is ascribed to increased defect formation due to lattice distortion. In addition, the bulk dielectric constant increases with increasing Ba 2+ dopant. The increase in the dielectric constant is consistent with ordering of CH 3 NH 3 dipoles resulting from distortions of the Ba sites in the lattice. The temperature dependence of the dielectric constants is attributed to thermal effects on the orientation of CH 3 NH 3 molecules.

Doping Strategies for Promising Organic–Inorganic Halide Perovskites

“…Figure a shows the classical 3D crystal structure of OIHPs. The theory often mentioned is the tolerance factor ( t ) and the octahedral factor ( µ ) which can be described as: [ 6,48–50 ] $$\[t = \frac{{{R_{\rm{A}}} + {R_{\rm{B}}}}}{{\sqrt 2 \left( {{R_{\rm{B}}} + {R_{\rm{X}}}} \right)}}\]$$ $$\[ \begin{array}{*{20}{c}}{m = {R_{\rm{B}}}{\rm{/}}{R_{\rm{x}}}}\end{array} \]$$where R A , R B , and R X are the ionic radii of A, B, and X ions, respectively. For a perfect stable cubic structure, the tolerance factor must be in the range of 0.89 ≤ t ≤ 1.0.…”

“…[ 57 ] Doping Si 2+ into MAPbI 3 forms a pseudo‐cubic structure. [ 91 ] Doping cations such as Sr 2+ , [ 57,95 ] Cd 2+ , [ 48,57 ] Ba 2+ , [ 6 ] and Mn 2+ , [ 90 ] can preserve the tetragonal structure which is also the structure of pure MAPbI 3 . Incorporation of La 2+ into MAPbI 3 can lead to the orthorhombic structure.…”

“…Notably, the I − vacancies are produced by I − clearance (Frenkel pair) due to lattice strain and deformation, which promotes migration of I − . [ 6 ]…”

“…The MA orientation affected by thermal conditions contributes to the temperature dependence of dielectric constants. [ 6 ] The theoretical research predicts a ferromagnetic transition caused by doping Ti 2+ into MAPbX 3 , where X is halogen (Cl, Br, and I). [ 41 ]…”

“…Notably, the modification of properties through ion doping is an effective way to improve the performance and broaden the application field of OIHPs in recent years, [1,[35][36][37][38][39][40] involving solar cells, light-emitting diodes, transistors, and so on. [1,4,6,41] In order to better understand doping, it is important to study the structural characteristics of perovskites. There are two main kinds of perovskites 3D and 2D perovskites, which were extensively explored in recent years.…”