Crystallization of Methyl Ammonium Lead Halide Perovskites: Implications for Photovoltaic Applications

Abstract

Hybrid organic/lead halide perovskites are promising materials for solar cell fabrication, resulting in efficiencies up to 18%. The most commonly studied perovskites are CH3NH3PbI3 and CH3NH3PbI3-xClx where x is small. Importantly, in the latter system, the presence of chloride ion source in the starting solutions used for the perovskite deposition results in a strong increase in the overall charge diffusion length. In this work we investigate the crystallization parameters relevant to fabrication of perovskite materials based on CH3NH3PbI3 and CH3NH3PbBr3. We find that the addition of PbCl2 to the solutions used in the perovskite synthesis has a remarkable effect on the end product, because PbCl2 nanocrystals are present during the fabrication process, acting as heterogeneous nucleation sites for the formation of perovskite crystals in solution. We base this conclusion on SEM studies, synthesis of perovskite single crystals, and on cryo-TEM imaging of the frozen mother liquid. Our studies also included the effect of different substrates and substrate temperatures on the perovskite nucleation efficiency. In view of our findings, we optimized the procedures for solar cells based on lead bromide perovskite, resulting in 5.4% efficiency and Voc of 1.24 V, improving the performance in this class of devices. Insights gained from understanding the hybrid perovskite crystallization process can aid in rational design of the polycrystalline absorber films, leading to their enhanced performance.

Efficient hole-blocking layer-free planar halide perovskite thin-film solar cells

“…Furthermore, as discussed earlier, UVO treatment to FTO substrates may also help reduce interface recombination. Cl in precursors has also been found to improve perovskite film quality such as coverage 40,41 . However, we did not observe significant difference on film coverage for CH 3 NH 3 PbI 3 and CH 3 NH 3 PbI 3-x Cl x films in our study.…”

Spatially Heterogeneous Chlorine Incorporation in Organic–Inorganic Perovskite Solar Cells

“…[8][9][10][11][12] The performance of this organic-inorganic halide cell is related to the optoelectronic properties of the organic lead halide absorber layer, most commonly CH 3 NH 3 PbX 3 (X = I and Br), and can be influenced by chemical composition, film morphology and device architecture. 7,[13][14][15][16][17] Recent reports have shown that using chloride containing salts or precursors (methylammonium chloride, MACl, and/or PbCl 2 ) can assist perovskite crystal growth because chloride ions or the formation of PbCl 2 nanocrystals (in a one-step deposition) can act as a nucleation center 11,18 due to size of Cl and PbCl 2 solubility. Additionally, the presence of MACl can also enhance the adsorption of lead iodide perovskite absorber on the TiO 2 substrate and produce large oriented crystallite domains by allowing slow crystal growth.…”

Effect of the Ultrasonic Substrate Vibration on Nucleation and Crystallization of PbI2 Crystals and Thin Films

“…By increasing the power of the vibration from 5 to 10 W, the morphology of thin film changes from irregular and small grains to grid-style large grain structures; thus, the SVPT is most effective at 5 W applied for 1 min. It is deduced that, as a result of the excitation by the ultrasonic vibration, the growth of large planar or 2D grains is suppressed due to an increase in the nucleation rate and the density of primary stable embryos [45][46][47]. In other words, in a dense population of stable seeds, the grain fronts will be confined by the neighboring cells.…”

“…This results in uniform mixing and distribution of the embryos generated on the substrate surface and within the film. This uniform and accelerated nucleation favors the size distribution, compactness, dimensionality, and the morphology of the crystallites [1,46,47]. grain fronts will be confined by the neighboring cells.…”

“…Table 1 shows that the Avarmi exponent of the SVPT sample is larger than that of the pristine sample. According to the literature, q 1 is expected to be in the range of 1 to 3, where~1 usually represents the formation of the rode-like 1D structures,~2 is associated with the formation of planar or disc-like 2D structures or lateral growth, and~3 is associated with the spatially-expanded or 3D columnar crystals [43][44][45][46][47]. The value of q 2 which is typically in the range of 0 to 1 indicates the contribution of nucleation in the crystallization process.…”