Though the introduction of MA and Br is beneficial for producing high-quality perovskite films, it induces a blue shift of absorption, limiting the further enhancement ofJ_sc and PCE. And Br can cause phase segregation under long-term illumination^[30]. With the volatile nature of MA, introducing Br and MA into FA-based perovskite decreases the stability of PSCs^[31]. Therefore, scientists continued focusing on FAPbI₃ and devoted great efforts to obtain MA- and Br-free pure α-phase FAPbI₃^[32]. Adding methylammonium chloride (MACl) into FAPbI₃ precursor solution could overcome phase transformation of α-FAPbI₃^[33]. MACl induces the growth of (001) plane of α-FAPbI₃ and increases the crystallinity. Seok et al. added methylenediammonium dichloride (MDACl₂) into FAPbI₃ to stabilize α-FAPbI₃ and achieved a J_sc of 26.7 mA/cm^2[31]. More than 90% of the initial PCE was maintained after 600-h operation. Besides the phase transformation, anion-vacancy defects at grain boundaries and at FAPbI₃ film surface inhibit PCE improvement. Jeong et al. used pseudo-halide anion formate (HCOO−) to suppress anion-vacancy defects and to increase film crystallinity^[34]. The resulting solar cells gave a PCE of 25.6% (certified 25.2%) (Table 1).

In short, improving device stability while maintaining high PCE stays a hot topic in PSC field. Adjusting cations to enhance the performance of 2D or quasi-2D perovskite solar cells will be an interesting approach.

Since metal halide perovskites were utilized as visible-light-harvesting materials for solar cells in 2009, power conversion efficiencies (PCEs) for metal halide perovskite solar cells (PSCs) have already reached to a certified value of 25.7%, making PSCs to be a promising next-generation photovoltaic technology^[1-5]. Compositional engineering of perovskite materials is an effective approach for achieving highly efficient and stable PSCs^[6-8]. Typical perovskite materials have a general formula ABX₃, where A is a monovalent cation, B a divalent metal cation and X a halogen anion. The radii of each component in perovskite material via Goldschmidt tolerance factor (t) determine the crystallographic stability and the formation of the 3D crystal structure^[9]. Therefore, cation and anion with different size like Cs, methylammonium (MA), formamidinium (FA), I, Br, and Cl can be adopted to construct perovskite crystals, resulting in bandgap variation. In 2009, MA-based perovskites were first used as sensitizers in liquid-state solar cells, producing a PCE of 3.81% with extremely poor stability (Table 1)^[1]. Kim et al. used MAPbI₃ in solid-state mesoporous solar cells, achieving dramatically improved performance (Table 1)^{[10, 11]}. Since then, composition engineering based on MAPbI₃ has sprung up. In 2013, Seok et al. produced bandgap tunable MAPbX₃ solar cells via substituting I with Br^[6]. Combined with solvent engineering, a substitution of 10–15 mol% I with Br in MAPbI₃ greatly improved the device stability in ambient atmosphere and a certified PCE of 16.2% was achieved for MAPb(I_1–xBr_x)₃ (x = 0.1–0.15) PSCs^[12]. MAPbI_3–xCl_x perovskites exhibit much longer carrier diffusion length and the related PSCs gave PCEs >12% and >14% for mesoporous and planar structure, respectively ^{[13, 14]}. Cl can aid film crystallization to improve device performance and stability^[15-17].

In order to further improve PCE and stability, FA and Cs were successively applied in composition engineering. The bandgap of MAPbI₃ is about 1.5 eV, which is far from Shockley-Queisser (S-Q) optimum^{[18, 19]}. Substituting MA with a slightly larger monovalent cation FA could reduce the bandgap of perovskite to S-Q optimum. What’s more, FA exhibits better thermal stability than the volatile MA cation^[20]. However, the degradation of black-phase FAPbI₃ to yellow non-perovskite phase under ambient conditions restricts the development of FAPbI₃ PSCs. It was found that the incorporation of MA and Br ions into FAPbI₃ can effectively stabilize black-phase perovskite and enhance the crystallinity^[21]. As a result, FA_1–xMA_xPb(I_1–yBr_y)₃ composition drew attention and dominated for a long time^[22-24]. PCEs exceeding 22% was achieved in FA_1–xMA_xPb(I_1–yBr_y)₃ PSCs (Table 1), together with a long-term stability, especially the thermal stability^[25]. In 2016, Saliba et al. introduced Cs into FA_1–xMA_xPb(I_1–yBr_y)₃ to further improve crystallinity of the perovskite film and the thermal stability of PSCs^[26]. They found that Cs-containing PSCs could steadily work over hundreds of hours under continuous illumination. Since then, FA_0.95–xMA_xCs_0.05Pb(I_1–yBr_y)₃ composition has become one of the dominant recipes (Table 1)^{[27, 28]}. Besides Cs, other alkali metals are used in composition engineering^[29].

Though PCE has been greatly improved, the long-term stability of organic/inorganic hybrid perovskites cannot satisfy commercial requirements. To tackle this issue, all-inorganic CsPbX₃ and low-dimensional (LD) materials are tried. Without volatile organic components, all-inorganic CsPbX₃ cells exhibit excellent thermal stability and desired bandgaps for tandem solar cells^[35]. Because of more easily formed defects and poor surface morphology, the PCE for all-inorganic PSCs is still inferior to inorganic–organic hybrid counterparts^[36]. Similar to FAPbI₃, how to stabilize the black phase and passivate the defects of CsPbX₃ is very important for achieving high PCE^[37]. By using a sequential dripping method and octylammonium iodide post-treatment, Seok et al. made uniform and pinhole-free CsPbI₃ film, and the cells gave a PCE of 20.37%^[38]. Most recently, Meng et al. reported a facile and effective defect passivation method for high-quality CsPbI₃ films and efficient devices^[39]. They found that the in-situ grown phenyltrimethylammonium iodide (PTAI)-based LD perovskites (1D PTAPbI₃ and 2D PTA₂PbI₄) located at CsPbI₃ grain boundaries and the film surface, which can not only suppress non-radiative recombination but also stabilize black-phase CsPbI₃ to prevent moisture intrusion. As a result, the CsPbI₃ cells exhibited a record efficiency of 21.0% with high stability (Table 1). Owing to excellent stability in ambient environment and under operating conditions, 2D Ruddlesden–Popper (RP) perovskites with a formula of A₂B_n−1Pb_nI_3n+1 are recognized as another promising candidate for PSCs^{[40, 41]}. Their performances are still lower than 3D counterparts. The lower PCE is mainly ascribed to quantum confinement effect, the enlarged bandgap and in-plane orientation of 2D RP perovskite with respect to the substrate^[42]. Various approaches, including solvent, additives, and cations engineering have been proposed to make vertically directed 2D RP perovskite to improve device performance. Zhang et al. reported pure FA-based 2D PSCs with the assistance of MACl and PbCl₂ additives, which gave a record PCE of 21.07% (Table 1)^[43].

This work was supported by the National Natural Science Foundation of China (22179053, 21905119) and Jiangsu Six Talent Peaks Program (XNY066). L. Ding thanks the open research fund of Songshan Lake Materials Laboratory (2021SLABFK02), the National Key Research and Development Program of China (2017YFA0206600) and the National Natural Science Foundation of China (51922032, 21961160720).