Author Affiliations
1School of Physics and Electronic Science, Engineering Research Center of Nanophotonics & Advanced Instrument (MoE), East China Normal University, Shanghai 200062, China2Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology, Beijing 100190, China3East Chapel Hill High School, Chapel Hill, NC 27514, USAshow less
![(Color online) (a) Chemical polishing of perovskite surface to eliminate excess PbI2. Reproduced with permission[14], Copyright 2022, American Chemical Society. (b) Ligand-modulated PbI2 nanosheet in PSCs. Reproduced with permission[15], Copyright 2020, Wiley. (c) Supramolecular engineering to modulate excess PbI2 in PSCs. Reproduced with permission[16], Copyright 2022, Wiley.](/Images/icon/loading.gif)
Figure 1.(Color online) (a) Chemical polishing of perovskite surface to eliminate excess PbI2. Reproduced with permission[14], Copyright 2022, American Chemical Society. (b) Ligand-modulated PbI2 nanosheet in PSCs. Reproduced with permission[15], Copyright 2020, Wiley. (c) Supramolecular engineering to modulate excess PbI2 in PSCs. Reproduced with permission[16], Copyright 2022, Wiley.
Perovskite solar cells (PSCs) have attracted much attention due to their low cost, high efficiency and easy processing. Recently, You et al. achieved a record efficiency of 26.1% (certified 25.6%) in PSCs with excellent stability[1]. Excess PbI2 in perovskite films was converted into inactive (PbI2)2RbCl to avoid its detrimental effect to device stability, while maintaining its positive effect to device efficiency.
However, scientists found that excess PbI2 in perovskite films could cause severe stability issues of PSCs especially under illumination, despite increased efficiency[6, 7]. PbI2 tended to decompose into metallic Pb0 and I2 under illumination, yielding Pb0 defects and thus decreasing device efficiency[8]. I2 product could further promote perovskite decomposition, even destroy charge-transporting layer[9, 10] and corrode metal electrode[11, 12], accelerating device degradation. In addition, excess PbI2 can absorb adjacent MA+ (or FA+) and I− during perovskite aging, yielding MA+ and I− vacancies[13]. Theses vacancies can act as channels for ion migration in perovskite films, also leading to device degradation.
Most highly efficient PSCs were achieved with excess PbI2 in either perovskite bulk or film surface. For example, Seo et al. fabricated PSCs with >25% efficiency by using perovskite precursor solution with excess PbI2[2]. Besides, You et al. achieved planar PSCs with efficiency close to 20% in 2016 through annealing perovskite films at high temperature to induce its partial decomposition and form excess PbI2 spontaneously[3]. The increased efficiency in PSCs with excess PbI2 resulted from PbI2 passivation. Owing to the larger bandgap of PbI2, a favorable type Ⅰ band alignment was established between PbI2 and perovskite, forming energy barriers to block both electron and hole transport to grain boundaries (GBs) and repel them into perovskite grains[4, 5]. As a result, potential carrier recombination induced by defects at GBs was effectively inhibited, thus improving device efficiency. In addition, the excess PbI2 at perovskite/charge-transport layer interface can block possible interfacial recombination, further enhancing device efficiency.
To reduce the negative effect of excess PbI2, Zhu et al. reported a chemical polishing method to optimize perovskite films. They introduced excess PbI2 during perovskite growth to obtain high-quality perovskite films. Then they washed away excess PbI2 on perovskite surface through its reaction with ammonium salts in the polishing agents (Fig. 1(a))[14], avoiding PbI2-induced perovskite degradation. The PSCs exhibited over 24% efficiency with good light and air stability. In addition, Luo et al. reported a ligand-modulated method (Fig. 1(b)) to regulate the shape and distribution of excess PbI2 in perovskite films via introducing cetyltrimethylammonium bromide (CTAB)[15]. CTAB caused PbI2 into 2D nanosheet, vertically sticking among perovskite grains. As a result, over 22% efficiency was obtained. Besides, Li et al. modulated excess PbI2 with ionic liquid [BMIM]X (Fig. 1(c))[16]. [BMIM]X could interact with PbI2 and form supramolecular structures, which could relax lattice distortion and release residual tensile strain in perovskite films, thus improving device stability.
Recently, You et al. managed excess PbI2 through RbCl doping in perovskite films[1]. RbCl reacted with PbI2 to form (PbI2)2RbCl (Fig. 2(a)). (PbI2)2RbCl promoted the formation and stabilization of black FAPbI3 phase through Pb-Cl bonding. On the other hand, (PbI2)2RbCl was inactive and could not react with FA+ or I− of perovskite grains, thus inhibiting the formation of FA+ or I− vacancies and suppressing ion migration (Fig. 2(b)). Besides, (PbI2)2RbCl is an intercalated compound and is more favorable to block ion migration in perovskite. As a result, a record efficiency of 26.1% (certified 25.6%) was realized in PSCs with good stability (Fig. 2(c)). The devices retained 96% of initial efficiency after aging in N2 for 1000 h and 80% of initial value after aging at 85°C for 500 h.
In short, future efforts on PSCs will focus on improving the module efficiency and stability for real commercialization.
Figure 2.(Color online) (a) (PbI2)2RbCl and its crystal structure in PSCs. (b) Ion-migration activation energy in control perovskite or perovskite with (PbI2)2RbCl. (c) J–V curves for PSCs with (PbI2)2RbCl. Reproduced with permission[1], Copyright 2022, the American Association for the Advancement of Science.
References
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