Unraveling Passivation Mechanism of Imidazolium-Based Ionic Liquids on Inorganic Perovskite to Achieve Near-Record-Efficiency CsPbI2Br Solar Cells

Abstract

The application of ionic liquids in perovskite has attracted wide-spread attention for its astounding performance improvement of perovskite solar cells (PSCs). However, the detailed mechanisms behind the improvement remain mysterious. Herein, a series of imidazolium-based ionic liquids (IILs) with different cations and anions is systematically investigated to elucidate the passivation mechanism of IILs on inorganic perovskites. It is found that IILs display the following advantages: (1) They form ionic bonds with Cs+ and Pb2+ cations on the surface and at the grain boundaries of perovskite films, which could effectively heal/reduce the Cs+/I− vacancies and Pb-related defects; (2) They serve as a bridge between the perovskite and the hole-transport-layer for effective charge extraction and transfer; and (3) They increase the hydrophobicity of the perovskite surface to further improve the stability of the CsPbI2Br PSCs. The combination of the above effects results in suppressed non-radiative recombination loss in CsPbI2Br PSCs and an impressive power conversion efficiency of 17.02%. Additionally, the CsPbI2Br PSCs with IILs surface modification exhibited improved ambient and light illumination stability. Our results provide guidance for an in-depth understanding of the passivation mechanism of IILs in inorganic perovskites."Image missing"

Construction of Efficient and Stable FAPbI3 Perovskite Solar Cells through Bifunctional Ionic Liquid‐Assisted Crystallization and Defect Passivation

“…It can be modified with various functional groups to passivate the surface and boundary defects. [39][40][41] For example, by adding 1-hexyl-3-methylimidazolium iodide into the FAPbI 3 DOI: 10.1002/solr.202200364 FAPbI 3 is considered one of the most ideal perovskite materials to construct highly efficient perovskite solar cell (PSC), but the poor phase stability and filmforming properties hinder further performance improvements. Herein, an ionic liquid 3-(2-amino-2-oxoethyl)-1-methyl-1H-imidazol-3-ium chloride (AOMCl) with multifunctional cations and Cl anion to assist crystallization, reduce defects, and stabilize α-FAPbI 3 is reported.…”

Superhalogen Passivation for Efficient and Stable Perovskite Solar Cells

“…Liu et al (2021) reported the use of imidazolium‐based IL containing BF 4 (1‐butyl‐2,3‐dimethylimidazolium tetrafluoroborate) that was applied as a surface passivation agent on top of CsPbI 2 Br perovskites. [ 27 ] The authors suggested that BF 4 − of the IL played an important role in the passivation of under‐coordinated Cs + /Pb 2+ by forming strong ionic interactions (Pb−F, Cs−F). The calculation result corroborated the effectiveness of superhalogen in passivation by showing its higher relative binding affinity than halide to I − vacancy, and also higher binding energy to Pb‐I antisite as shown in Figure 3c,d, respectively.…”

“…Although the authors did not deal with the detail of the underlying reasons for the suppression of ion migration, we conjecture that the presence of BF 4 − at the grain boundary might contribute to the suppression of the ion migration because of the high binding affinity of BF 4 − to iodide vacancy (V I + ). [ 27 ] The same research group (2020) reported the use of a different ionic compound, 1‐butyl‐1‐methylpiperidinium tetrafluoroborate ([BMP] + [BF 4 ] − ) to stabilize perovskites (Cs 0.17 FA 0.83 Pb(I 1− x Br x ) 3 ). [ 28 ] The authors addressed the detrimentality of the formation of iodine (I 2 ) under illumination on the device stability, [ 29 ] and explained that it forms due to the formation of Frenkel defects consisting of iodide vacancies and/or their interstitial pairs.…”

“…In contrast, Liu et al suggested defect passivation by superhalogen only at the surface, but without being incorporated into the perovskite lattice. [ 27 ] In contrast, Stranks et al strongly corroborated no atomic‐scale interaction of superhalogen with the under‐coordinated Pb 2+ in the perovskite lattice by solid‐state NMR, which led to their conclusion that superhalogen rather scavenges unreacted MAI. [ 35 ]…”

“…À to iodide vacancy (V I þ ). [27] The same research group (2020) reported the use of a different ionic compound, 1-butyl-1-methylpiperidinium tetrafluoroborate ([BMP] þ [BF 4 ] À ) to stabilize perovskites (Cs 0.17 FA 0.83 Pb(I 1Àx Br x ) 3 ). [28] The authors addressed the detrimentality of the formation of iodine (I 2 ) under illumination on the device stability, [29] and explained that it forms due to the formation of Frenkel defects consisting of iodide vacancies and/or their interstitial pairs.…”