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 ofJsc 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 Jsc of 26.7 mA/cm2[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(I1–xBrx)3 (x = 0.1–0.15) PSCs[12]. MAPbI3–xClx 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, FA1–xMAxPb(I1–yBry)3 composition drew attention and dominated for a long time[22-24]. PCEs exceeding 22% was achieved in FA1–xMAxPb(I1–yBry)3 PSCs (Table 1), together with a long-term stability, especially the thermal stability[25]. In 2016, Saliba et al. introduced Cs into FA1–xMAxPb(I1–yBry)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, FA0.95–xMAxCs0.05Pb(I1–yBry)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 A2Bn−1PbnI3n+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).