Abstract
Lead iodide (PbI2)-rich methylammonium lead bromide-iodide (MAPb(I1–xBrx)3) thin-films were prepared by sequential physical vapor deposition of methylammonium lead tri-bromide (MAPbBr3) on methylammonium lead tri-iodide (MAPbI3) 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 MAPbI3(x is 0.0) to the cubic-like structure of MAPbBr3 (x is 1.0) as MAPb(I1–xBrx)3 (x = 0.89–0.95) and PbI2 were formed. The bromine mole ratio x 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 x 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 x decreased. Electrical properties showed high charge carrier mobility that increased linearly with MABr thickness. FTO/MAPb(I1–xBrx)3/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(I1–xBrx)3 thin-films. The charge carrier mobility increased as x decreased. The power conversion efficiency (PCE) of FTO/MAPbBr3/Au, FTO/MAPb(I0.11Br0.89)3/Au and FTO/MAPbI3/Au solar cells were 0.56, 0.62, and 1.15%. Devices including titanium dioxide compact layer (c-TiO2) and titanium dioxide mesoporous (m-TiO2) 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-TiO2/m-TiO2/MAPb(I0.11Br0.89)3 solar cells revealed a sizeable built-in voltage (Vbi) of 1.6 V and an accumulation of charge at interfaces for voltages greater than 0.2 V, respectively. Similar analyses for FTO/TiO2/MAPbI3/Au showed a small Vbi of 0.7 V and no charge carrier at interfaces. The work paves a way for reproducible growth of MAPb(I1–xBrx)3 for solar cells and sheds more light on the degree of ion migration in mixed halide and pure halide perovskites.
Highlights
Hybrid perovskite solar cells are gaining much attention in the photovoltaic community because of the excellent blend of stability, efficiency, and bandgap tunability
The thin MAPb(I1−xBrx)3 film diffractograms (Figures 2A,B) show the presence of all the prominent peaks of cubic MAPbBr3 and the (001) peak corresponding to PbI2, but do not contain the principal (110) peak characterizing tetragonal MAPbI3
This points to the fact that the tetragonal phase of MAPbI3 is transformed to the cubic phase of MAPbBr3 as MAPb(I1−xBrx)3 is formed, consistent with previous reports (Noh et al, 2013; Gilescrig et al, 2015; Pistor and Burwig, 2018)
Summary
Hybrid perovskite solar cells are gaining much attention in the photovoltaic community because of the excellent blend of stability, efficiency, and bandgap tunability. The used a sequential deposition of methylammonium iodide (MAI) and a mixture of MAI + methylammonium bromide (MABr) solutions on a solution-prepared lead (II) iodide (PbI2) thinfilm They showed that the MAPb(I1−xBrx) (x is 0.4) and MAPbI3 (x is 0.0) degraded under concentrated sunlight while MAPbBr3 (x is 1) did not degrade. Zhang et al (2016) grew MAPb(I1−xBrx) single crystals by the inverse temperature crystallization method, using separately optimized precursor solutions of MAPbBr3 and methylammonium lead tri-iodide (MAPbI3) They were able to tune absorption in the entire visible spectrum by varying x from 0.0 to 1.0. The heat generated from heating the crucibles raised the temperature of the substrates to 140◦C during the deposition of PbBr2 and 95◦C during the deposit of MABr. Figure 1C represents the schematic of FTO/MAPb(I1−xBrx)3/Au devices fabricated solely for the determination of charge carrier mobility and trap density using the space-charge-limited current theory. The light measurements were performed under a solar simulator (Oriel LCS-100TM Small Area So11A Series, Newport) with simulated solar output conditions of 100 mW/cm and AM1.5 G reference spectral filtering
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