Chemical Synergic Stabilization of High Br-Content Mixed-Halide Wide-Bandgap Perovskites for Durable Multi-Terminal Tandem Solar Cells with Minimized Pb Leakage.

High Br-content lead mixed-halide perovskites with wide-bandgap (WBG) of 1.6-2.0 eV have showcased vast potential to be used in tandem solar cells. However, WBG perovskites often suffer from severe halide segregation, phase separation and ion migration under the stress of light, heat, moisture and electric bias, which would accelerate the decomposition of perovskite films and thus deteriorate the photovoltaic performance and even aggravate the lead leakage from damaged devices. Here, we report a novel chemical synergic interaction strategy to mitigate the abovementioned issues in WBG perovskites. To achieve that, a small amount of cationic β-cyclodextrin, composed of multiple ammonium cations, chlorine ions and abundant hydroxyl functional groups, was introduced into WBG perovskites, which effectively stabilized the halide ions and homogenized the phase distribution, comprehensively passivated the crystallographic defects, as well as efficiently immobilized the Pb ions. Encouragingly, the cationic β-cyclodextrin was universal and useful for different WBG perovskite compositions (i.e. 1.68 eV, 1.79 eV and 1.99 eV), which favorably boosted the efficiencies by 10 %-36 % and extended the operational stability of resultant devices to 2680 h. The four-terminal all-perovskite tandem and six-terminal all-perovskite tandem solar cells integrated with different WBG perovskite sub-cells exhibited efficiencies up to 24.39 % and 22.42 %, respectively. More importantly, we demonstrated the cationic β-cyclodextrin-assisted internal chemical encapsulation effectively prevented the Pb leakage when the devices were severely damaged and immersed in water. Surprisingly, there was only 5.63 ppb Pb leaching out for the single-junction devices, far below than the U.S. standard for safe drinking water (<15 ppb). The target tandem solar cells with cationic β-cyclodextrin modification also realized a Pb sequestration efficiency of 93.4 % under the most adverse environment.