AUTHOR=Fru Juvet Nche , Nombona Nolwazi , Diale Mmantsae 

TITLE=Characterization of Thin MAPb(I1–xBrx)3 Alloy Halide Perovskite Films Prepared by Sequential Physical Vapor Deposition

JOURNAL=Frontiers in Energy Research

VOLUME=9

YEAR=2021

URL=https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2021.667323

DOI=10.3389/fenrg.2021.667323

ISSN=2296-598X

ABSTRACT=<p>Lead iodide (PbI<sub>2</sub>)-rich methylammonium lead bromide-iodide (MAPb(I<sub>1–<italic>x</italic></sub>Br<sub><italic>x</italic></sub>)<sub>3</sub>) thin-films were prepared by sequential physical vapor deposition of methylammonium lead tri-bromide (MAPbBr<sub>3</sub>) on methylammonium lead tri-iodide (MAPbI<sub>3</sub>) bottom layer. The structural, optical, morphological, and electrical properties of the thin-films were studied as the thickness of methylammonium bromide (MABr) was increased from 300 to 500 nm. X-ray diffractograms confirmed transformation of tetragonal MAPbI<sub>3</sub>(<italic>x</italic> is 0.0) to the cubic-like structure of MAPbBr<sub>3</sub> (<italic>x</italic> is 1.0) as MAPb(I<sub>1–<italic>x</italic></sub>Br<sub><italic>x</italic></sub>)<sub>3</sub> (<italic>x</italic> = 0.89–0.95) and PbI<sub>2</sub> were formed. The bromine mole ratio <italic>x</italic> decreased as MABr thickness increased. UV-Vis absorption spectra showed that the bandgap of the thin alloy film decreased from 2.21 to 2.14 eV as <italic>x</italic> decreased. Scanning electron micrographs depicted densely packed grains that entirely covered the substrate and contained very few pinholes. The average grain size increased from 150 to 320 nm as <italic>x</italic> decreased. Electrical properties showed high charge carrier mobility that increased linearly with MABr thickness. FTO/MAPb(I<sub>1–<italic>x</italic></sub>Br<sub><italic>x</italic></sub>)<sub>3</sub>/Au devices using fluorine-doped tin oxide (FTO) as substrate and gold (Au) as contacts were fabricated and current-voltage characteristics were determined. Space-charge-limited current theory was applied to charge carrier mobility and trap density of MAPb(I<sub>1–<italic>x</italic></sub>Br<sub><italic>x</italic></sub>)<sub>3</sub> thin-films. The charge carrier mobility increased as <italic>x</italic> decreased. The power conversion efficiency (PCE) of FTO/MAPbBr<sub>3</sub>/Au, FTO/MAPb(I<sub>0.11</sub>Br<sub>0.89</sub>)<sub>3</sub>/Au and FTO/MAPbI<sub>3</sub>/Au solar cells were 0.56, 0.62, and 1.15%. Devices including titanium dioxide compact layer (c-TiO2) and titanium dioxide mesoporous (m-TiO<sub>2</sub>) layer as electron transport layers were also fabricated for the application of Mott-Shottky (M-S) theory. Analyses of dark current-voltage and capacitance-voltage curves of FTO/c-TiO<sub>2</sub>/m-TiO<sub>2</sub>/MAPb(I<sub>0.11</sub>Br<sub>0.89</sub>)<sub>3</sub> solar cells revealed a sizeable built-in voltage (<italic>V</italic><sub><italic>bi</italic></sub>) of 1.6 V and an accumulation of charge at interfaces for voltages greater than 0.2 V, respectively. Similar analyses for FTO/TiO<sub>2</sub>/MAPbI<sub>3</sub>/Au showed a small <italic>V</italic><sub><italic>bi</italic></sub> of 0.7 V and no charge carrier at interfaces. The work paves a way for reproducible growth of MAPb(I<sub>1–<italic>x</italic></sub>Br<sub><italic>x</italic></sub>)<sub>3</sub> for solar cells and sheds more light on the degree of ion migration in mixed halide and pure halide perovskites.</p>