Solar cells based on metal halide perovskites continue to approach their theoretical efficiency limits thanks to worldwide research efforts. The next challenge is to develop perovskite devices that can retain these efficiencies but exhibit acceptable degradation and decent stability for real-life practical applications. The degradation can be triggered and significantly affected by external environmental factors, such as moisture, oxygen, light, and heat. Although the encapsulation allows effective suppression of moisture- and oxygen-induced degradation, the reduction of light degradation and heat degradation is primarily dependent on the improvement of materials and interfaces of cells. Herein, the degradation mechanisms caused by light and heat are elucidated for each of the major layers in the device. The methodologies for the corresponding degradation reduction and stability enhancement are interpreted from compositional and interfacial engineering strategies with quantitative analysis including the site-based substitution in perovskite lattice, doping in charge transporting layers, passivation by using various materials (small molecules, polymers, ligands, perovskite quantum dots, and low-dimensional perovskites), and a protective layer for vulnerable layers. This Review will provide important insight into degradation suppression and stability enhancement of perovskite solar cells and give a clue to optimal design toward high-efficiency and stable devices.

Realization of high efficiency along with long-term stability of perovskites solar cells has been a goal of researchers in this field. Breakthroughs in efficiency have been achieved in the past decade, exceeding those of most thin film solar cells. However, the challenge of degradation and instability of perovskite solar cells is currently the bottleneck for their real-life application. Extensive research has been conducted to achieve simultaneously high efficiency and high stability, leading to considerable progress in device performance. A significant source causing degradation is the ingression of external materials, such as moisture and oxygen. In this review, the mechanisms of moisture and oxygen degradation occurring in perovskite absorber and charge transporting layers are interpreted. Primary approaches to simultaneously achieve high efficiency and high stability focus on encapsulation, interfacial layer engineering, process engineering, ion engineering, and dopant and alternative engineering. In brief, we review the materials development and engineering strategies to improve performance and identify the obstacles that hinder the realization of high stability along with high efficiency.

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Perovskite quantum dots (QDs) preserve the attractive properties of perovskite bulk materials and present additional advantages, owing to their quantum confinement effect, leading to their suitability as an absorber in perovskite solar cells. In this Review, the issues and advantages of perovskite QDs are analyzed in the context of purification, device fabrication with perovskite QDs, light absorption, charge transport, and stability. In addition, promising strategies to enhance perovskite QDs and QD‐based solar cells are elucidated based on exchange chemistry (ion and ligand exchange), passivation engineering (ion and ligand passivation), and structure engineering (conventional/inverted, planar/mesoscopic and dimensionally graded structures). These discussions will give a clue to the further development of perovskite QDs and thus the advancement of QD‐based solar cells.

The efficiency of perovskite solar cells (PSCs) has undergone rapid advancement due to great progress in materials development over the past decade and is under extensive study. Despite the significant challenges (e.g., recombination and hysteresis), both the single‐junction and tandem cells have gradually approached the theoretical efficiency limit. Herein, an overview is given of how passivation and crystallization reduce recombination and thus improve the device performance; how the materials of dominant layers (hole transporting layer (HTL), electron transporting layer (ETL), and absorber layer) affect the quality and optoelectronic properties of single‐junction PSCs; and how the materials development contributes to rapid efficiency enhancement of perovskite/Si tandem devices with monolithic and mechanically stacked configurations. The interface optimization, novel materials development, mixture strategy, and bandgap tuning are reviewed and analyzed. This is a review of the major factors determining efficiency, and how further improvements can be made on the performance of PSCs.