Protocol for isothermal crystallization of photovoltaic perovskite films based on volatile solvents.

The efficient functioning and stability of a perovskite photoactive layer are paramount to the performance of solar cell devices. Here, we present a protocol for the synthesis of a high-performance exemplified methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) perovskite photoactive layer. We describe steps for preparing the requisite ratios of the precursor powders, synthesizing MAPbI3 single crystals, and selecting a suitable preparation technique. We then detail a flexible doping strategy for the perovskite photoactive layer. For complete details on the use and execution of this protocol, please refer to Wang and Wu (2020, 2022, 2023).1,2,3.

Balancing Lattice Strain by Embedded Ionic Liquid for the Stabilization of Formamidinium-Based Perovskite Solar Cells.

Formamidinium (FA)-based perovskites remained state-of-the-art in the field of perovskite solar cells (PSCs) owing to the exceptional absorption and carrier transport properties, while the transition from photoactive (α-) to photoinactive (δ-FAPbI3) phase is the impediment that causes performance degradation and thus limits the deployment of FA-based PSCs. The unfavorable phase transition originates from tensile strain in the FAPbI3 crystal lattice, which undergoes structural reorganization for lattice strain balancing. In this work, we found that the ionic liquid (IL) could be used as the strain coordinator to balance the lattice strain for stability improvement of FAPbI3 perovskite. We theoretically studied the electronic coupling between IL and FAPbI3 and unraveled the originality of the IL-induced compressive strain. The strain-relaxed α-FAPbI3 by IL showed robust stability against environmental factors, which can withstand ambient aging for 40 days without any phase transition or decomposition. Moreover, the strain-relaxed perovskite films showed a lower trap density and resulted in conversion efficiency improvement from 18.27 to 19.88%. Based on this novel strain engineering strategy, the unencapsulated PSCs maintained 90% of their initial efficiency under ambient-air aging for 50 days.

Avoid Pitfalls in Identifying Perovskite Grain Size.

Perovskite grain size has been used as one of the indicators to evaluate the morphological quality of perovskite films. Large grain size is customarily regarded as an indicator of high photovoltaic performance because it is thought to result in superior charge carrier transport properties due to less carrier scattering by the grain boundaries (GBs). Consequently, the characterization of perovskite grain size has become routine in perovskite solar cell research, and large grain size is in general pursued. However, grain size estimation relying on the prevailing methods, such as scanning electron microscopy (SEM), can be viewed only as "apparent grain size", which is incomplete or possibly wrong for evaluating GB distribution. To avoid the pitfalls and advocate for accurate methodologies in identifying perovskite grain size, this Viewpoint highlights the pitfalls by demonstrating specific examples and then provides an appropriate platform to accurately evaluate perovskite grain information.

ß-Alanine-Anchored SnO2 Inducing Facet Orientation for High-Efficiency Perovskite Solar Cells.

SnO2 films as a promising electron transport layer (ETL) have been widely used in planar-type perovskite solar cells to achieve an impressive improvement in the conversion efficiency. However, compared with a mesoporous ETL, the interfacial charge carrier transfer of the SnO2 ETL is severely limited due to the issues of oxygen vacancy defects and crystal lattice mismatch between SnO2 and the perovskite, which generally leads to the growth of randomly stacked and porous perovskite layers and subsequently impacts the charge transport and transfer properties. In this work, we developed a facile approach by inducing a bifunctional molecule, ß-alanine, into the SnO2 ETL, which can serve as a bridge to modulate the interfacial charge transfer and the perovskite crystallization kinetics. Benefited by the interfacial ß-alanine, we grew a highly orientational perovskite layer that exhibited superior charge transport properties. Meanwhile, the ß-alanine caused an intimate connection between the perovskite and SnO2 to enhance the interfacial charge transfer. As a result, the power conversion efficiency (PCE) of the ß-alanine-modified device achieved a much-improved value of 19.67% and showed high reproducibility. This work provides a way for developing a high-performance ETL toward the scalable fabrication of highly efficient PSCs.