Perovskite solar cells (PSCs) have been recognized as the best candidates for next-generation photovoltaics. However, it is still challenging to fabricate PSCs that are both efficient and stable. Ionic liquids (ILs) are a kind of molten salt at room temperature, possessing unique advantages that enable their widespread application in many fields. Notably, ILs have been shown to play versatile functions in realizing efficient and stable PSCs. Herein, we summarize advanced progress in ILs-based perovskite photovoltaics, focusing on the crucial functions of ILs in the processing of PSCs. First, the characteristics of ILs are systematically introduced to address their merits for application in PSCs. Sequentially, the key roles ILs play in the modification of functional layers in PSCs are categorically discussed, including film-forming dynamics control, chemical passivation, stability improvement, and innovative alternatives to traditional materials. Finally, we give some enlightening viewpoints for the design of ILs toward high-performing PSCs.

Environment-friendly protic amine carboxylic acid ionic liquids (ILs) as solvents is a significant breakthrough with respect to traditional highly coordinating and toxic solvents in achieving efficient and stable perovskite solar cells (PSCs) with a simple one-step air processing and without an antisolvent treatment approach. However, it remains mysterious for the improved efficiency and stability of PSCs without any passivation strategy. Here, we unambiguously demonstrate that the three functions of solvents, additive, and passivation are present for protic amine carboxylic acid ILs. We found that the ILs have the capability to dissolve a series of perovskite precursors, induce oriented crystallization, and chemically passivate the grain boundaries. This is attributed to the unique molecular structure of ILs with carbonyl and amine groups, allowing for strong interaction with perovskite precursors by forming C=O…Pb chelate bonds and N-H…I hydrogen bonds in both solution and film. This finding is generic in nature with extension to a wide range of IL-based perovskite optoelectronics.

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Because of the narrow bandgap and superior thermal stability, FAPbI3 is considered the most promising perovskite material for high-performance single-junction PSCs. Nevertheless, the metastable properties of the photoactive α-FAPbI3 becomes a primary obstacle for the development of FA-based PSCs. The main reasons for the instability of α-FAPbI3 are the rotation disorder of the FA cation and large anisotropic lattice strain, which lead to the high formation energy of α-FAPbI3. In this Perspective, we review various strategies for preparing phase-pure α-FAPbI3, such as engineering, intermediate phase engineering, and dimensionality engineering. These strategies can stabilize α-FAPbI3 by reducing the system energy, regulating the phase transition process and energy barrier, reinforcing the lattice structure, and passivating film defects. In addition, we investigate fundamental challenges of α-FAPbI3 PSCs and propose our perspective on preparing high-quality and high-purity α-FAPbI3.

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