Due to light-induced effects in CHNH-based perovskites, such as ion migration, defects formation, and halide segregation, the degradation of CHNH-based perovskite solar cells under maximum power point is generally implicated. Here we demonstrated that the effect of light-enhanced ion migration in CHNHPbI can be eliminated by inorganic Cs substitution, leading to an ultrastable perovskite solar cell. Quantitatively, the ion migration barrier for CHNHPbI is 0.62 eV under dark conditions, larger than that of CsPbIBr (0.45 eV); however, it reduces to 0.07 eV for CHNHPbI under illumination, smaller than that for CsPbIBr (0.43 eV). Meanwhile, photoinduced halide segregation is also suppressed in Cs-based perovskites. Cs-based perovskite solar cells retained >99% of the initial efficiency (10.3%) after 1500 h of maximum power point tracking under AM1.5G illumination, while CHNHPbI solar cells degraded severely after 50 h of operation. Our work reveals an uncovered mechanism for stability improvement by inorganic cation substitution in perovskite-based optoelectronic devices.

The fabrication of high-quality perovskite film highly relies on chemical composition and the synthesis method of perovskite. So far, sequentially deposited MA FA Pb(I Br ) polycrystalline film is adopted to produce high-performance perovskite solar cells with record power conversion efficiency (PCE). Fewer grain boundaries and incorporation of inorganic cation (e.g., cesium) would further increase device performance via sequential deposition. Here, cesium chloride (CsCl) is introduced into lead iodide (PbI ) precursor solution that beneficially modulates the property of PbI film, leading to larger grains with cesium incorporation in the resulting perovskite film. The enlarged crystal grains originate from a slower nucleation process for CsCl-containing PbI film when reacting with formamidine iodide, confirmed by in situ confocal photoluminescence imaging. Photovoltaic devices based on CsCl-containing PbI film demonstrate a higher averaging efficiency of 21.3% than 20.3% of the devices without CsCl additives for reverse scan. More importantly, the device stability is improved by CsCl additives that retain over 90% of their initial PCE value after 4000 min tracking at maximum power point under 1-sun illumination. This work paves a way to further improve the photovoltaic performance of mixed-cation-halide perovskite solar cells via a sequential deposition method.

Ion migration has been regarded as the major cause of photocurrent hysteresis. Here we use photoluminescence (PL) and optical images, combined with Galvanostatic measurement, to detect the ionic motion. We observe an irreversible PL and optical transmittance change after electric poling. By comparing a neat perovskite film with the sample coated by poly(methyl methacrylate) (PMMA), polyethylene glycol (PEG), and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), we found that PCBM effectively inhibits ionic motion near the surface of the perovskite.We further evidenced the donor−acceptor complex formed between PCBM and perovskite, implying the mechanism of inhibited ion migration by PCBM. We close by demonstrating that PCBM can also be introduced on the top of perovskite fim in an n−i−p TiO 2 planar structure, to achieve an average 14% steady-state output over 2.3 × 10 5 s (∼64 h). This work highlights the importance of inhibiting ionic motion in perovskite solar cells.

Organometal halide perovskites are mixed electronic−ionic semiconductors. It is imperative to develop a deeper understanding of how ion-migration behavior in perovskites impacts the long-term operational stability of solar cells. In this work, we found that ion penetration from the perovskite layer into the adjacent organic hole-selective layer is a crucial cause of performance degradation in perovskite solar cells. The monovalent cation, namely, methylammonium (MA + ), is the main ion species that penetrates into the organic hole-selective layer of Spiro-MeOTAD because of the built-in electric field during operation. The incorporation of MA + induces deep-level defects in the Spiro-MeOTAD layer and thereby deteriorates the hole-transporting ability of Spiro-MeOTAD, degrading solar cell performance. Our work points to two ways to improve the stability of perovskite solar cells: one is to insert a compact ion-blocking layer between Spiro-MeOTAD and perovskite, and the other is to find a hole-selective layer that is insensitive to extraneous ions (MA + ).