Imidazole-Based Ionic Liquid Engineering for Perovskite Solar Cells with High Efficiency and Excellent Stability.

Despite the remarkable progress of perovskite solar cells (PSCs), the substantial inherent defects within perovskites restrict the achievement of higher efficiency and better long-term stability. Herein, we introduced a novel multifunctional imidazole analogue, namely, 1-benzyl-3-methylimidazolium bromide (BzMIMBr), into perovskite precursors to reduce bulk defects and inhibit ion migration in inverted PSCs. The electron-rich environment of -N- in the BzMIMBr structure, which is attributed to the electron-rich adjacent benzene ring-conjugated structure, effectively passivates the uncoordinated Pb2+ cations. Moreover, the interaction between the BzMIMBr additive and perovskite can effectively hinder the deprotonation of formamidinium iodide/methylammonium iodide (FAI/MAI), extending the crystallization time and improving the quality of the perovskite precursors and films. This interaction also effectively inhibits ion migration to subsequent deposited films, leading to a noteworthy decrease in trap states. Various characterization studies show that the BzMIMBr-doped films exhibit superior film morphology and surface uniformity and reduced nonradiative carrier recombination, consequently enhancing crystallinity by reducing bulk/surface defects. The PSCs fabricated on the BzMIMBr-doped perovskite thin film exhibit a power conversion efficiency of 23.37%, surpassing that of the pristine perovskite device (20.71%). Additionally, the added BzMIMBr substantially increased the hydrophobicity of perovskite, as unencapsulated devices still retained 93% of the initial efficiency after 1800 h of exposure to air (45% relative humidity).

Stable and efficient perovskite solar cells via hydrogen bonding and coordination.

The instability of perovskite solar cells (PSCs) is primarily caused by the unavoidable ion migration in the perovskite layer. Ion migration and accumulation influence the properties of perovskite and functional layers, resulting in severely degraded device performance. Herein, we introduced an n-type, low optical gap-conjugated organic molecule (i.e., COTIC-4F or COTIC-4Cl) to serve as the perovskite photoactive layer in a perovskite precursor solution for broadening the near-infrared spectral response and enhancing the efficiency of PSCs. Various characterization studies have determined that COTIC-4F forms hydrogen bonds with perovskites, thereby remarkably enhancing the anchoring ability of MA+, suppressing ion migration, and reducing photocurrent hysteresis. Meanwhile, the carbonyl (CO) group of COTIC-4F and COTIC-4Cl can donate a lone electron pair to passivate the Pb trap, avoiding possible carrier recombination. The COTIC-4F- and COTIC-4Cl-treated perovskite films exhibit an optical response in the near-infrared region and an excellent morphology. Through ultraviolet photoelectron spectroscopy, it has been determined that COTIC-4F can facilitate more charge transfer than COTIC-4Cl, which results in a larger photocurrent from the PSCs. The PSCs of the COTIC-4F-treated perovskite films demonstrate a maximum power conversion efficiency of 21.72%. They exhibit a high fill factor of 82.02% and possess long-term stability under an air atmosphere.