**1. Introduction**

To solve the energy crisis coming with the rapid development of the society, solar energy is a promising renewable energy source which can convert light to electricity in the photovoltaic devices [1]. Among all the photovoltaic devices, perovskite solar cell (PSC) attracts researchers' attention most for its power conversion efficiency (PCE) has been increased from 3.8 to 25.2% in just 10 years [2–6]. With strong optical absorption, high carrier mobility and small exciton binding energy, organic-inorganic perovskite materials are semiconductors with remarkable optical and electrical properties [7–9]. The advantages of solution manufacturing and processing are widely used by researchers in solar cells with various structures [10]. However, one of the major reasons why high-performance perovskite solar cells have not been applied to practical application is the instability of the materials.

The instability of organic-inorganic hybrid perovskite is caused by many intrinsic and extrinsic factors. The external environmental factors include moisture, heat, oxygen and many other factors. Moisture is considered to be an important factor for the instability of perovskite materials, while the presence of light and oxygen accelerates the degradation process [11–13]. In addition to the influence of these external environmental factors, some intrinsic properties of the perovskite material itself also directly lead to its instability, such as composition and ion migration [14]. The perovskite materials with excellent photoelectric properties have a strong ionic property, indicating that the activation energy of ion migration inside the crystal

is low, and molecular dissociation and ion migration are prone to occur within the structure, which limits the structural stability of these materials [15].

In recent years, the two-dimensional (2D) perovskite structure formed by introducing large-size organic cations is proved to be more stable than its threedimensional (3D) counterpart and it has become a potential light-absorber in the PSCs. There are many reasons for the 2D perovskite to exhibit higher stability. The 2D perovskite has higher formation energy and it is more difficult to be oxidized than the 3D structure [16]. Compared with 3D perovskite crystals, the bonding forces between organic ions and [PbI6] octahedral units such as van der Waals forces and hydrogen bonds are stronger [17]. Due to the presence of large size organic cations, ion migration is blocked [18]. Meanwhile, the 2D perovskite layer can work as passivation layer and blocking layer of moisture and oxygen to enhance the stability of perovskite [19, 20].

Although 2D perovskite materials show great potential in terms of stability, the relatively lower PCE needs to be improved. In this chapter, based on the structural and photophysical properties of 2D perovskite, the latest progress made in 2D PSCs in recent years and strategies to improve the performance of 2D PSCs are summarized, which is of great significance for the further development of PSCs based on 2D perovskite materials. Finally, a brief conclusion and outlook is promoted.
