Annealing-induced chemical and structural changes in tri-iodide and mixed-halide organometal perovskite layers

Abstract

Annealing process is crucial in obtaining high-quality perovskite layers used for highly efficient planar perovskite solar cells. In this study, we have investigated annealing-induced chemical and structural changes of tri-iodide (TI) and mixed-halide (MH) organometal perovskite layers using infrared absorption spectroscopy, scanning electron microscopy and x-ray diffraction measurements. For TI layers, the solvent molecule, dimethylformamide (DMF), remained in the form of PbI2/ DMF compound after drying at room temperature. During annealing, the DMF evaporated to form PbI2 crystals. When the MH perovskite film was annealed, both CH3NH3PbCl3 and CH3NH3PbI3 crystals were initially formed from an amorphous phase. With further annealing, the CH3NH3PbI3 crystals gradually grew through the incorporation of source materials supplied from the CH3NH3PbCl3 crystals and the amorphous phase and the slow evaporation of methylammonium (MA) and chloride ions. The resultant MH perovskite layer after annealing was mainly composed of large CH3NH3PbI3 grains with a trace of chloride ions. We suggest that the difference in composition and structure leads to different charge transporting properties of the TI and MH perovskite layers.

Structure and Growth Control of Organic–Inorganic Halide Perovskites for Optoelectronics: From Polycrystalline Films to Single Crystals

“…[ 88 ] Niwano and colleagues later found that upon thermal annealing, both CH 3 NH 3 PbCl 3 and CH 3 NH 3 PbI 3 crystals in mixed-halide perovskite layer were initially formed from an amorphous phase. [ 89 ] Such amorphous phase and MAPbCl 3 crystals served as ions supply for the growth of MAPbI 3 crystals when the MA and chloride ions were evaporated from the mixed-halide perovskite layer, resulting in a thin layer composed of large MAPbI 3 grains.…”

Effects of Solution-Based Fabrication Conditions on Morphology of Lead Halide Perovskite Thin Film Solar Cells

“…On the other hand, short-term annealing of 15 minutes of trihalide perovskites does not improve the crystallinity of the perovskite film in Figure 4(a). A recent study by Ma and colleagues concluded that the crystallinity is influenced by the volatility of DMF used in the formation of PbI 2 solution [22]. Further, they suggest that using a mixed halide solution will add excess chloride ions that help improve crystallinity and morphology.…”

“…Excess chlorine ions may form the chloride-containing amorphous phase that melts at around 100 ∘ C in mixed halide perovskite layers. During prolonged annealing, the "melted amorphous phase with highly moveable ions may supply ions needed for the growth of MAPbI 3 crystals" [22]. The evaporation of excess ions, methylammonium and chloride, promotes the transformation of MAPbCl 3 from MAPbI 3 .…”

“…Inset shows SEM of perovskite film annealed for 15 mins at 100 ∘ C. (b) XRD data for mixed halide perovskite annealed at 100 ∘ C with varying annealing times. SEM of perovskite film annealed for 150 mins at 100 ∘ C. Reproduced from[22] with permission from the Royal Society of Chemistry. Plot of highest percentage of grain size present in each film with increasing thermal annealing time.…”

Visualization and Investigation of Charge Transport in Mixed‐Halide Perovskite via Lateral‐Structured Photovoltaic Devices

“…Organometal halide perovskites (OHPs) have the chemical structure ABX 3 (A: organic cation, B: metal cation, and X: halide anion) and have been used as efficient active layers in perovskite solar cells (PeSCs) . The control of the X‐site halide composition in OHPs affects their electrical characteristics including the bandgap energy, perovskite structure, and film morphology . The X‐site halide can be composed of two different halide ions, i.e., I − and Cl − ions, yielding a mixed‐halide perovskite, CH 3 NH 3 PbI 3− x Cl x .…”