Instability of rear electrodes undermines the long-term operational durability of efficient perovskite solar cells (PSCs). Here, a composite electrode of copper-nickel (Cu-Ni) alloy stabilized by in situ grown bifacial graphene is designed. The alloying makes the work function of Cu suitable for regular PSCs and Cu-Ni is the ideal substrate for preparing high-quality graphene via chemical vapor deposition, which simultaneously protects the device from oxygen, water and internal components reaction. To rivet the composite electrode with semi-device, a thermoplastic copolymer is employed as an adhesive layer during hot pressing. The resultant device achieved power conversion efficiency of 24.34% with significantly improved stability; the devices without encapsulation retained 97% of their initial efficiency after the damp heat test at 85 o C with relative humidity of 85% for 1440 hours and the encapsulated devices maintained 95% of their initial efficiencies after maximum power point tracking under continuous 1 sun illumination for 5000 hours.Metal halide perovskite solar cells (PSCs) have attracted great attention in both academia and industry owing to their excellent optoelectronic performance and low manufacturing costs 1-6 . However, for PSCs to realize commercialization, they must survive the long-term natural erosion imposed by oxygen, moisture, light and heat 7,8 . Thanks to the optimization of the perovskite materials, charge transport materials and the interface layers 9-12 , the

Perovskite solar cells (PSCs) have emerged as one of the third‐generation photovoltaic technologies. However, the toxicity issue of the lead element in perovskite absorbers hinders their large‐scale production. Thus, exploiting lead‐free perovskite materials becomes an important solution to overcome this challenge. Among all the candidates, tin perovskites have advanced rapidly in recent years due to their low toxicity, favorable bandgap, and high carrier mobility. After a few years of development, the highest power conversion efficiency (PCE) of tin PSCs has exceeded 13%, which is mainly attributed to the breakthroughs arising from additive engineering of the Sn perovskite layer. Herein, the role of additive engineering in the research community of tin PSCs is emphasized. First, the crystal structure, electronic characteristics, and the chemical instability of Sn perovskites are introduced. Next, additives used for stabilizing the Sn2+ components, purifying SnI2 sources, and improving the crystal quality of perovskite films are discussed in detail. Finally, challenges and perspectives are laid out to advance the properties of tin halide perovskites for further improving the device efficiency and stability.

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traditional power generation methods such as coal and natural gas due to the low-cost raw materials and relatively simple structures of PSCs. [14,15] Both the high efficiency and low cost make PSCs a promising candidate to realize commercialization. Currently, to further improve the performance of PSCs and accelerate the commercialization progress, there are still several challenges to be overcome. For example, the ion migration caused by weak Coulomb force in perovskite lattice is considered to be detrimental to the operational stability. [16-18] In addition, the energy loss originated from the nonradiative recombination limits the open-circuit voltage (V OC) of PSCs. [19,20] Graphene is a 2D and chemically stable material with a hexagonal honeycomb structure composed of carbon atoms, which make itself and their derivatives have multiple applications at interfaces or in functional layers of PSCs (Figure 1). [21,22] For example, the lattice parameter of graphene is 0.246 nm, which is smaller than the radius of I − (0.412 nm). [23] Therefore, it is an ideal material to block the ion migration in PSCs and improve the stability under working conditions. Furthermore, graphene derivatives have numerous derivatives with rich functional groups and adjustable band gaps, which can passivate the specific defect states by rational molecule design and therefore optimize the charge transport in PSCs and reduce the charge recombination. In addition, graphene materials have also been applied in flexible and translucent PSCs because of their semiconductor properties and foldable and transparent inherent natures. [24-26] In this paper, we introduce the history and preparation methods of graphene materials, such as the chemical vapor deposition (CVD) and oxidation-reduction methods. Then, the applications of graphene and its derivatives in PSCs are reviewed, including defect passivation, ion migration blocking, and charge transport optimization. We also summarize the progress of graphene as electrode materials in semitransparent and flexible devices. Finally, the challenges and prospects of the application of graphene materials in PSCs are proposed. 2. Graphene and Its Derivatives Carbon elements are rich in allotropes, [22] such as the 3D graphite and diamond, the 0D fullerenes (C 60) and 1D carbon Perovskite solar cells (PSCs) have attracted much attention due to their high efficiency and low manufacturing cost. However, the defects in perovskite films and the instability issue caused by ion migration limit the further improvement of its efficiency and stability. Graphene and its derivatives are potential materials to solve these problems due to their rich functional groups, environmental stability and compactness, which triggered extensive researches in recent years. In this paper, the development and preparation methods of graphene materials are summarized and then their applications in PSCs are reviewed, with particular emphasis on their function of blocking ion migration.