Effect of Solvent Residue in the Thin-Film Fabrication on Perovskite Solar Cell Performance

Organic-inorganic Pb-based halide perovskite photoelectrical materials, especially perovskite solar cells (PSCs), have attracted attention due to the significant efforts in improving the power conversion efficiency (PCE) to above 25%. However, the stability issue of the PSCs restricts their further development for commercialization. Strategies are designed to keep moisture and oxygen out of the perovskite films, such as additive, surface passivation, and solvent engineering; however, usually, the corrosion of active films by the residual solvent is mostly ignored. Solvent residue is the paramount factor influencing the stability of the perovskite film prepared by the solution method, and most solvents can be easily absorbed and accelerate the perovskite film decomposition. Here, we studied the residual solvent effect on two kinds of perovskite films obtained by different annealing processes: hot air annealing and hot bench annealing. Several detection techniques were used to study the performance of two different annealing methods, including time-of-flight secondary ion mass spectrometry (ToF-SIMS), thermogravimetric analysis (TGA), and field-emission scanning electron microscopy (FESEM). The perovskite film obtained by hot air annealing shows less residual solvent and better device performance than the hot bench annealing method. This method is expected to provide insight into reducing solvent residue to improve the stability of the PSCs, especially for future commercialization.

Growth mechanism of CH3NH3I in a vacuum processed perovskite

In the field of halide perovskite research, the growth of high quality films has been a critical issue. Among the reported growth methods, vacuum processes have attracted much attention due to their accurate controllability and high reproducibility, as proven in the manufacture of vacuum deposited organic-light-emitting-diode industry. In a vacuum process, the major difficulty for growing a perovskite film is control of a precursor, methylammonium iodide (MAI), originating from its uncontrollable behavior i.e., a high working pressure and poor adsorption characteristics. Thus, it is crucial to understand the growth mechanism of MAI vapor for the successful application of vacuum processes in the growth of halide perovskite films. In this paper, we report the growth mechanism and deposition kinetics of MAI in a vacuum. Unlike that of conventional materials evaporated in a vacuum, the deposition rate of MAI was found to be much faster on the reactive surface, PbI₂, compared to other non-reactive materials. Surprisingly, a very thin (2 nm-thick) PbI₂ layer increased the initial growth rate of MAI 2.7-fold. Based on the real-time monitored data from a quartz microbalance and surface study, we suggest dipole-induced adsorption as the MAI growth mechanism on PbI₂ and the perovskite in the vacuum process. We believe that this work will provide meaningful insight into film growth in vacuum processed perovskites.

High Versatility and Stability of Mechanochemically Synthesized Halide Perovskite Powders for Optoelectronic Devices

We show that mechanochemically synthesized halide perovskite powders from a ball milling approach can be employed to fabricate a variety of lead halide perovskites with exceptional intrinsic stability. Our MAPbI₃ powder exhibits higher thermal stability than conventionally processed thin films, without degradation after more than two and a half years of storage and only negligible degradation after heat treatment at 220 °C for 14 h. We further show facile recovery strategies of nonphase-pure powders by simple remilling or mild heat treatment. Moreover, we demonstrate the mechanochemical synthesis of phase-pure mixed perovskite powders, such as (Cs_0.05FA_0.95PbI₃)_0.85(MAPbBr₃)_0.15, from either the individual metal and organic halides or from readily prepared ternary perovskites, regardless of the precursor phase purity. Adding potassium iodide (KI) to the milling process successfully passivated the powders. We also succeeded in preparing a precursor solution on the basis of the powders and obtained uniform thin films for integration into efficient perovskite solar cells from spin-coating this solution. We find the KI passivation remains in the devices, leading to improved performance and significantly reduced hysteresis. Our work thus demonstrates the potential of mechanochemically synthesized halide perovskite powders for long-time storage and upscaling, further paving the way toward commercialization of perovskite-based optoelectronic devices.

Recent Advances in Flexible Perovskite Solar Cells: Fabrication and Applications

Flexible perovskite solar cells have attracted widespread research effort because of their potential in portable electronics. The efficiency has exceeded 18 % owing to the high-quality perovskite film achieved by various low-temperature fabrication methods and matching of the interface and electrode materials. This Review focuses on recent progress in flexible perovskite solar cells concerning low-temperature fabrication methods to improve the properties of perovskite films, such as full coverage, uniform morphology, and good crystallinity; demonstrated interface layers used in flexible perovskite solar cells, considering key figures-of-merit such as high transmittance, high carrier mobility, suitable band gap, and easy fabrication via low-temperature methods; flexible transparent electrode materials developed to enhance the mechanical stability of the devices; mechanical and long-term environmental stability; an outlook of flexible perovskite solar cells in portable electronic devices; and perspectives of commercialization for flexible perovskite solar cells based on cost.