## 1 Introduction
Most commonly, the first precursor (e.g., PbI\({}_{2}\)) is deposited by spin coating, while the second one (e.g., MAI) can be deposited by dipping the sample into a solution [30] or by spin coating on top of the first layer [31]. For a time, devices fabricated using the two-step method performed better than those made using the one-step approach [32], but the situation drastically changed with the advent of the solvent engineering approach [33], which remains the most commonly used method to date. The solvent engineering method is a variation of the one-step approach, in which all perovskite precursors are deposited in a single step, followed by the application of an antisolvent, which causes the rapid crystallization of the perovskite layer [34]. This method provides the ease of the one-step method and, at the same time, results in better film morphology, similar to that observed for the two-step method. The use of a suitable amount of DMSO (Lewis base) in combination with DMF can form the Lewis acid-base adduct with PbI\({}_{2}\) (Lewis acid), which results in the formation of a dense perovskite layer via the MAI-PbI\({}_{2}\)-DMSO intermediate phase. The antisolvent treatment enables the elimination of the host solvent(s) and initiates crystallization of the perovskite film during this intermediate phase, which helps regulate crystal development and morphology of the film. However, the use of an excessive amount of DMSO may lead to pinholes in the perovskite film, and thus, a small volume of DMSO is usually preferred with DMF; here, we used DMF:DMSO (4:1) [33; 35]. The rate at which the antisolvent is applied is crucial and one of the decisive steps to determine film quality; thus, an appropriate rate of antisolvent dropping should be optimized for different antisolvents [36]. The convenience and simplicity of solution-processed perovskite layers by spin coating, which can be conducted in practically any laboratory without the need of expensive and sophisticated vacuum apparatus, is one of the primary advantages of this method. Another important benefit is the ability to introduce additives to the perovskite precursor solution, which has proven to have a beneficial impact on the final perovskite structure [37]. Therefore, an organized study to optimize varying equimolar concentrations during the spin coating of perovskite precursors is highly beneficial in making MAPbJ\({}_{3}\) PSCs since these are often still used as a benchmark to compare with the growing family of other perovskite-based photoabsorbers for PSCs. In the present report, we have conducted a systematic study in tuning the equimolar precursor ratio of the organic halide (MAI = 1 M, 1.2 M, 1.5 M, 1.6 M) and metal halide (lead iodide: PbI\({}_{2}\) = 1 M, 1.2 M, 1.5 M, 1.6 M) in a fixed solvent mixture of DMF:DMSO. Hereafter, the fabricated films and perovskite solar cells (PSCs) with these four different concentrations will be denoted as (1.0:1.0), (1.2:1.2), (1.5:1.5) and (1.6:1.6) in this study. These films were studied and discussed on the basis of their surface morphology, optical properties and crystallinity. Additionally, the n-i-p PSCs fabricated from all four formulations were analyzed for recombination losses and operational stability study, which we also discuss here.