Progress and Opportunities for Cs Incorporated Perovskite Photovoltaics

To overcome phase-instability issues associated with high-performing FAPBI3 perovskites, cesium (Cs) has been incorporated into FAPBI3 precursors, exhibiting an added benefit of yielding higher power conversion efficiencies. Because Cs is nonvolatile and remains in the final perovskite thin films, clarifying the role of Cs in stabilizing and improving the performance of the lower-bandgap and higher-efficiency-potential FAPbI3 PSCs is critical. A review of the appropriate Cs concentration in the precursor for the optimal crystallization pathway, the minimal phase segregation, and the minimal density of trap states to achieve the highest device efficiency is also key. Future research opportunities include the need to characterize Cs at the nanoscale and to develop scalable deposition methods for large-area Cs-containing perovskite solar cells. Efficiencies of perovskite solar cells (PSCs) have risen unprecedentedly from 3.8% to 25.2% in just over a decade as the light absorber material has evolved from the original methylammonium (MA)- to formamidinium (FA)-dominated perovskite. While FA lead iodide (FAPbI3) has a lower bandgap and, therefore, a higher theoretical efficiency limit, it is less phase stable, although this can overcome by incorporating cesium (Cs), MA, bromide (Br), or a combination of these. Cs being a nonvolatile component remains in the perovskite film and therefore it is important to understand the effect of the amount of Cs on film properties and associated PSC performance and stability. Future research opportunities for Cs-containing PSCs including large-area demonstrations are also discussed in this review. Efficiencies of perovskite solar cells (PSCs) have risen unprecedentedly from 3.8% to 25.2% in just over a decade as the light absorber material has evolved from the original methylammonium (MA)- to formamidinium (FA)-dominated perovskite. While FA lead iodide (FAPbI3) has a lower bandgap and, therefore, a higher theoretical efficiency limit, it is less phase stable, although this can overcome by incorporating cesium (Cs), MA, bromide (Br), or a combination of these. Cs being a nonvolatile component remains in the perovskite film and therefore it is important to understand the effect of the amount of Cs on film properties and associated PSC performance and stability. Future research opportunities for Cs-containing PSCs including large-area demonstrations are also discussed in this review. a concept from band theory in solid-state physics; refers to the energy difference (typically in electron volts) between the bottom of the conduction band and the top of the valence band. Due to the quantum-mechanical effects, electrons are confined in discrete energy levels when interacting with an isolated atom. In a solid, electrons interact with huge number of surrounding particles (~1022) leading to closely spaced (at an order of 10−22 eV) energy levels, and these energy levels can be considered as a continuum band. The valence band is filled by electrons at thermal equilibrium while the conduction band is empty. The bandgap is the minimum energy required to enable the free movement in a solid. refers to the content of Cs in the perovskite precursor or the film, not necessarily the amount of Cs occupying the A site in the final perovskite crystal lattice. refers to the Cs salt added in the HOIP preparation. It should be noted that the amount of Cs salt added in the HOIP preparation does not necessarily represent the amount of Cs in the perovskite crystal; Cs may accumulate at the grain boundaries or at the interface, such as the surface of the HOIP thin film. a solution process deposition method where a blade is used to spread and remove excess solution at a height over the substrate. The thickness of the doctor-bladed film is dependent on the height of blade with respect to the substrate, the coating speed, and the concentration of the solution being used. perovskite is a crystal structure named after Russian mineralogist Lev Perovski and was discovered by Gustav Rose in 1839 in the Ural Mountains of Russia. The general chemical formula of perovskite compound is ABX3 and in an ideal cubic perovskite structure the A cation occupies the eight vertexes of cube, the B cation sits in the cube center, and the X anion locates in eight facet centers. The A cation can be MA+, FA+, Cs+, or a combination of these cations, the B cation can be lead (Pb2+), tin (Sn2+), or a combination of these cations, and the X anion can be chloride (Cl−), bromide (Br−), iodide (I−), or a combination of these anions. chemically identical substances but in different crystal forms. For hybrid organic–inorganic halide perovskite, the chemical composition is ABX3, while there may be a few stacking sequences leading to various polymorphs. also known as the detailed balance limit; the theoretical estimation of solar cell efficiency developed by William Shockley and Hans-Joachim Queisser in 1961. The limit considers the radioactive recombination and assumes that a solar cell absorbs only those photons with energy larger than the bandgap. refers to the states in a semiconductor where the carrier is trapped until it is recombined. The trap state is caused by the imperfections in crystals since the interactions of atoms at the imperfections differ, which limits the movement of carriers.