Ionic Liquid‐Tuned Crystallization for Stable and Efficient Perovskite Solar Cells

Abstract

Organometal halide perovskites as the core of the next-generational photovoltaic technologies have attracted remarkable attention due to their advantages of low material cost, easy fabrication, and outstanding photovoltaic properties. [1] Significant efforts have been made to enhance the photovoltaic performance of perovskite solar cells (PSCs). [2] The power conversion efficiency (PCE) has been boosted to about 26%, [3] which is comparable to that of conventional photovoltaic technology based on crystalline silicon. Nevertheless, the device instability suffering from moisture exposure and heat attack is still a significant barrier on the road toward commercialization. [4] It is known that halide perovskites with low formation energy are intrinsically unstable so that they are easy to deteriorate under various external forces. [5] Therefore, how to stabilize the perovskite structure and films is the most important concern in the PSCs.Recently, several effective strategies, including interfacial engineering, additive engineering, dimensional manipulation, and so on, have been applied to improve device stability. [6] Caffeine (1,3,7-trimethylxanthine) was employed as an additive to increase the activation energy of perovskite films through a molecular lock between carboxyl groups and bivalent Pb, yielding PSCs with good thermal stability at 85 C. [7] Lin et al. proved that the introduction of a piperidinium salt into the perovskites could slow down degradation of the perovskite layer under continuous illumination. [8] Additionally, by depositing n-hexyl trimethyl ammonium bromide on the top of the perovskite film, a thin layer of wide-bandgap halide perovskites was formed through an in situ reaction, leading to a high efficiency of 22.7% and good stability both at 85% relative humidity and under one-sun illumination. [9] Despite these progresses, few studies have considered crystallization control of the halide perovskites, which could be a critical approach to concurrently improve both PCE and device stability. Given that the nucleation and crystallization of perovskites during both initial spin-coating and subsequent annealing process are correlated with the PbI 2 precursor solution, [10,11] it is necessary to tune crystallization of perovskite by coordinating the precursor solution to enhance chemical bonding interactions to deliver high-quality and stable perovskite films.

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

“…Organic-inorganic halide perovskite solar cells (PSCs) have attracted great research interest due to their superb optoelectronic properties. [1][2][3][4][5][6][7] In the past decade, the record power conversion efficiency (PCE) of all-solid-state single-junction thin-film PSCs has jumped from 9.7% to 25.7%, mainly benefiting from the enormous compositional and interface engineering. [8][9][10][11][12][13][14] For PSCs achieving record PCE and stability, the perovskite layer was turned from the original MAPbI 3 -to FAPbI 3 -based compositions.…”

Role of Ionic Liquids in Perovskite Solar Cells

“…The IL BAAC was also employed as an additive to tune the crystallization of Cs 0.05 (FA x MA y ) 0.95 Pb(I x +0.05 Br y Cl y ) 3 perovskite films by Ran and coworkers. [ 15 ] It was found that the BAAc can react with PbI 2 by forming chelate bonds (C=O–Pb) and hydrogen bonds (N–H···I), retarding perovskite crystal growth. Therefore, the average grain size increased from about 476–620 nm, and the PCE elevated from 18.4% to 20.1%.…”

“…In addition to those mentioned above, many other functional groups have been introduced into ILs to improve the efficiency and stability of PSCs (Table 1), such as benzal groups (-CH 2 C 6 H 5 ), [7,36] vinyl groups (-CH=CH 2 ), [7,33,36] amide groups (-CONH 2 ), [37] and carboxyl groups (-COOH). [8,[11][12][13]15,19,38] In conclusion, to design an effective IL, first, an electron-rich chemical structure is necessary, such as imidazole rings, pyridine rings, and pyrrolidine rings, which can provide electrons to neutralize trap states. Second, the spatial hindrance effect should be considered.…”

3D/1D Architecture Using a 1-Hexyl-3-methylimidazolium Lead Triiodide Interlayer for Robust and Highly Performing Perovskite Solar Cells

“…Their employment as layers between the different interfaces of the devices, especially between the perovskite and the hole transporting material (HTM), promotes the extraction of the charge carriers by creating an intermediate energy level between the charge transport layer and the perovskite. − Their incorporation as solvents and inside the perovskite solution slows down the crystallization of the film as a result of their nonvolatile character, which helps the formation of larger, highly oriented, and almost defect-free crystals . While their hydrophobic character (hydrophobic alkyl chain) protects from the penetration of the humidity, ILs assemble periodic structures with halides, rendering the perovskite more stable by a strong chemical bonding and, in parallel, mitigating the trap-state density at the interfaces and grain boundaries. , Many groups have reported studies on how ILs passivate the perovskite layers and the interfacial reactions among them. − …”