Ionic Liquid for Perovskite Solar Cells: An Emerging Solvent Engineering Technology

Abstract

Conspectus Perovskite solar cells (PSCs) have, in recent years, become one of the most in-depth photovoltaic materials in many different disciplines due to their long-range charge carrier diffusion lengths, strong light absorption, easy tuning of the band gaps, low defect density, and solution processability. The power conversion efficiency (PCE) of single-junction PSCs has reached a certified value of 25.5%, which has caught up with or even surpassed traditional photovoltaic technologies, such as silicon (Si) solar cells, thin film solar cell, etc. In addition to the performance of PSCs comparable to traditional photovoltaic technology, its biggest feature is that it can be prepared by a solution method. The preparation process of the solution method simplify the film preparation process and greatly reduce the preparation cost. In addition, the solution method provides a favorable option for the energy supply of flexible wearable devices in the future. At present, the preparation of PSCs mainly uses organic molecular solvents, including N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), and their mixed solvents. However, most of these commonly used organic solvents are unsafe and are likely to cause water and soil pollution, harm to the human body, and safety accidents. In addition, most solvents have a high coordination number, which limits the crystallization of perovskite. Furthermore, high-quality perovskite films require the introduction of a large amount of high boiling point, polar and aprotic antisolvents to assist crystallization, such as chlorobenzene, ether, and ethyl acetate. In addition to the toxicological problems of the antisolvent, the technology has a relatively narrow operating time window, relatively high operating environment requirements, and relatively low repeatability, which greatly limits the large-scale production of PSCs. Therefore, It is very necessary to develop a new solvent engineering technology for PSCs, which is green and pollution-free, is a simple process, and has high efficiency, long-term stability, and low cost. Recently, we have developed a series of ionic liquids, including MAAc, MAFa, BAAc, etc., which can be used as solvents to prepare efficient and stable PSCs. It allows the production of smooth and continuous polycrystalline perovskite thin films in ambient air through a simple spin coating method In this Account, we started with the discovery of ionic liquids as solvents to PSCs. We have proposed new strategies for improving and enhancing ionic liquid-based PSCs, including interface engineering, solvent engineering, and additive engineering. Furthermore, the stable and efficient mechanism of ionic liquid-based perovskite solar cells has been revealed. In view of the designable characteristics of ionic liquids, some functional ionic liquids have been designed and applied to different systems of halide perovskites, which has important guiding significance for the development of more functional ionic liquids for PSCs. Finally, the challe...

Efficient and Moisture‐Stable Inverted Perovskite Solar Cells via n‐Type Small‐Molecule‐Assisted Surface Treatment

“…The reduction of defect states in the perovskite films by Y6 or JY16 treatment was further confirmed by capacitance‐frequency measurements (Figure 4c ). [ 5b ] The distribution of the trap density of state (tDOS) was obtained from the derivative of the capacitance with respect to the frequency according to Equation ( 2 ): [ 18 ] where C is the capacitance, ω is the angular frequency, q is the elementary charge, k B is the Boltzmann's constant, T is the temperature, and W is the depletion width. The built‐in potential ( V bi ) was obtained using Mott–Schottky analysis (Figure S12 , Supporting Information).…”

“…In addition, the crystal growth kinetics of perovskite can be properly modulated by additives such as polymers, ammonium salts, low‐dimensional perovskites, Lewis acids, Lewis bases, and ionic liquids. [ 3 , 5 ] The growth of perovskite can be regulated by incorporating Lewis base semiconducting materials with electron‐donating functional groups such as sulfur (S), nitrogen (N), and oxygen (O) into the perovskite films to take advantage of Lewis acid–base interactions between the additive material and perovskite layer, resulting in large grain‐size perovskite films with high crystallinity. [ 5 ] Moreover, the insertion of Lewis base semiconducting additives with electron‐donating functional groups into perovskite can passivate the defect states of perovskite films, thereby further enhancing the PCE and stability of PeSCs.…”

Inkjet printing for scalable and patterned fabrication of halide perovskite-based optoelectronic devices

“…Besides, some ionic liquid based solvents, such as methylammonium acetate (MAAc), methylammonium formate (MAFa), and butylammonium acetate (BAAc), have been shown to be beneficial for the production of smooth and continuous perovskite thin films with improved phase stability, lower toxicity, and ambient air printing compatibility. 39 Generally, solvents with high dielectric constants or Gutmann's donor number (DN) 40 will have good solubility for the perovskite precursors.…”

Solvent Engineering of Ionic Liquids for Stable and Efficient Perovskite Solar Cells

“…The application of ILs is not only limited to 3D PSCs, but also be extended to the field of 2D perovskite. [ 91 ] Currently, the obtained 2D‐layered perovskites through solution methods easily produce a mixture of multiple quantum wells (MQWs) with a random well width distribution, which impedes the development of 2D‐based PSCs. [ 92 ] Liang and colleagues introduced IL BAAc as spacer to replace the traditional halide spacer n‐butylamine iodide (BAI) to acquire phase‐pure quantum wells in 2D Ruddlesden–Popper layered PSCs.…”