Perovskite solar cells (PSCs) have achieved impressive power conversion efficiencies (PCE) of up to 26.1%,^[1] however their commercialization is hindered by significant stability challenges, particularly in the presence of oxygen/moisture, and under operating conditions such as voltage bias, light exposure, and temperature fluctuations. These issues are often associated with migration of ions, which can lead to a degradation of optoelectronic properties, and, in turn can adversely affect both the performance and long-term stability of PSCs. Inherent defects and grain boundaries are the main ion migration pathways within the perovskite layer. Notably, unreacted lead iodide (PbI₂) at these sites, including surface of perovskite grains, and the interface between the perovskite layer and the charge transport layer, is a major cause of intrinsic instability under illumination. PbI₂ can break down into iodine and metallic lead, acting as recombination centers, further promoting ion migration, and accelerating degradation.^[2] In stark contrast for the operation of PSCs, it has been shown that enhancing device performance often involves adding a small excess of PbI₂ to the precursor solution.

The conflicting observations that PbI₂ can both enhance performance but also degrade stability suggests a need for research efforts directed toward simultaneously mitigating crystallization of residual PbI₂ while improving device stability, ensuring that efficiency gains do not compromise the long-term reliability of perovskite solar cells.

This presentation addresses these challenges through supramolecular complex engineering by introducing beta-cyclodextrin (β-CD) into a triple cation perovskite layers to effectively prevent the crystallization of residual PbI₂. This approach results in uniform crystal growth and the passivation of undercoordinated lead cation defects, as confirmed by XRD and SEM analyses. The use of β-CD leads to a PSC with an improved PCE of 21.36%, surpassing the control (19.4%), and enhanced stability against aggressive thermal stress and high humidity (85% RH), likely attributed to defect passivation as evidenced by PL, SCLC and the dependence of V_OC on the incident light intensity studies. Notably, in comparison to the β-CD-free control, the β-CD-treated sample exhibited minimal optical bandgap shifts of 3 meV after 1170 hours of moisture exposure. In addition, the devices treated with a 0.5% of β-CD showcased improved stability, maintaining over 73% of their initial PCE even after undergoing 320 hours of testing at 50-60℃. XPS and NMR observations reveal that β-CD effectively anchors uncoordinated Pb²⁺ ions, preventing the formation of metallic Pb and enhancing film stability under environmental stress. Furthermore, this method not only passivates unreacted PbI₂ but also provides valuable insights into the role of β-CD in PSCs. Additional tests with Maltose as a non-cyclic control were conducted and confirm the superior ability of β-CD to enhance perovskite film stability under harsh conditions.^[3] The formation of a supramolecular system between β-CD and perovskite holds promise as a strategy to control perovskite precursor chemistry, material structure, and subsequent device performance and stability.