## 1 Introduction  
The past decade has witnessed increasing interest in novel materials such as graphene and its hybrids [1, 2], transition metal dichalcogenides (TMDCs) [3], black phosphorus [4], and organic-inorganic metal halide perovskites as promising optoelectronic materials for solid-state photovoltaics [5], light-emitting diodes (LEDs) [6], lasers [7], photodetectors [8, 9, 10], and photocatalysts [11]. Perovskite materials exhibit many remarkable optoelectronic properties such as broad bandgap tunability [12, 13], high absorption coefficients [5], ambipolar charge transport properties [14], long charge carrier diffusion lengths (>1 \(\upmu\)m) [15], high charge carrier mobilities [16], low non-radiative recombination rates [17, 18], and high emission color purity [19]. Aside from that, ionic conductivity in perovskites arises from ion/ defect migration, which has revealed a fascinating feature toward switchable resistance depending on the history of the last applied bias. This ability to reliably regulate ion migration and hysteresis has aided in the development of memristors devices (memory + resistor) based on the unique structural properties provided by perovskites [20]. These remarkable properties make perovskites ideal candidates for their use as light-harvesting materials in solar cells and various other devices. Perovskite structures have the stoichiometry of ABX\({}_{3}\), where \(A\) = a large organic/inorganic cation (e.g., CH\({}_{3}\)NH\({}_{3}\)\({}^{+}\), Cs\({}^{+}\)), \(B\) = an inorganic metal cation (e.g., Pb\({}^{2+}\), Sn\({}^{2+}\)), and \(X\) = a halide anion (e.g., Cl\({}^{-}\), Br\({}^{-}\), I\({}^{-}\)). The formability of the perovskite structure is determined by the geometrical tolerance factor \(t\), defined as \(t=\frac{(r_{A}+r_{X})}{\sqrt{2}\left(r_{B}+r_{X}\right)}\), where \(r_{A}\), \(r_{B}\), and \(r_{X}\) are the effective ionic radii of the \(A\), \(B\), and \(X\) ions [21]. The ideal perovskite structure is formed when \(t\) = 1, but a feasible range of values for the formability of alkali metal halide perovskites is 0.813 \(<t<\) 1.107 [22]. While the organic halide (e.g., methylammonium iodide (MAI)) component of the perovskite dissolves readily in virtually all polar solvents, finding a solvent to dissolve the metal halide (e.g., lead iodide (PbI\({}_{2}\))) component is more difficult. Some of the widely used solvents include \(N\),\(N\)-dimethylformamide (DMF) and dimethylsulfoxide (DMSO), and the solubility limit of PbI\({}_{2}\) has been experimentally determined to be \(\sim\)450 mg/mL and \(\sim\)600 mg/mL in DMF and DMSO, respectively, which is sufficient to form an adequately thick perovskite film [23; 24]. Over the years, several distinct variants of solution-processing by spin coating of perovskite materials have been studied. Such approaches have the common goal to produce perovskite films with dense and homogeneous morphology and large grain size and to achieve optimal crystallinity to yield an effective photoabsorber layer within the photovoltaic solar cell stack. The so-called one-step, two-step, and solvent engineering procedures are the most popular (Figure 1). Using the one-step method, 3D perovskite (e.g., MAPbI\({}_{3}\)) can be readily formed by spin coating the mixture of perovskite precursors (e.g., MAI and PbI\({}_{2}\)) in some polar aprotic solvents, such as \(N\),\(N\)-dimethylacetamide (DMAAc) and DMF. After spin coating, films can be thermally treated at mild temperatures to form the desired perovskite phase [25]. While this method is straightforward and remains widely used for some perovskite compositions [26; 27], the perovskite precursors may undergo chemical reactions in the solution that affect the final film structure and composition, with a concomitant impact on the solar cell performance [28]. The two-step method, in which one of the precursors is deposited first, followed by the deposition of a second precursor and thermal annealing, was popular in the early days of perovskite research [29].