# Developments on Perovskite Solar Cells (PSCs): A Critical Review
## 5 Fabrication Methods
### One-Step Method  
The one-step deposition method involves spin coating a perovskite precursor solution onto a pre-treated transparent conductive oxide (TCO) substrate followed by annealing to form the perovskite active layer, as shown in Figure 10a. This is the widely used deposition method due to its simplicity and low production cost for small, lab-based PSCs, and is not suitable for large-surface-area device fabrications. To form a perovskite layer of crystal structure ABX\({}_{3}\), organic (e.g., MAI/FAI) and inorganic (e.g., PbX\({}_{2}\)/SnX\({}_{2}\)) halides are dissolved into a common dipolar aprotic solvent, such as dimethyl sulfoxide (DMSO), dimethylformamide (DMF), or \(\gamma\)-butylrolactone (GBL), then spin coated onto an ETM-bearing conductive substrate. The early PSC fabrication that triggered research interest was  
Figure 9: (**a**) Schematic diagram showing the basic device operation in a PSC. (**b**) Simplified energy alignment of possible ETM and HTM materials in PSC applications [19; 60]. achieved through one-step spin coating of organic salts (MABr and MAI) together with inorganic salts (PbBr\({}_{2}\) and PbCl\({}_{2}\)), reaching efficiencies of 3.31% and 3.81% in the resulting MAPbBr\({}_{3}\) and MAPbI\({}_{3}\) PSCs, respectively [5]. In a later study, [PERSON] (2012) reported efficiencies of up to 10.9% by spin coating a solution of mixed-halide perovskite precursor over a mesoporous Al\({}_{2}\)O\({}_{3}\) scaffold [7]. In the one-step deposition method, the solution's ionic strength and post-annealing improve the perovskite crystal formation [61]. However, drawbacks such as lack of control over film uniformity and morphology are observed. [PERSON] (2015) _fabricated_ an inverted (p-i-n) perovskite device with a 210 nm-thick perovskite by one-step spin coating and acquired PCE = 11.99%, V\({}_{\text{oc}}\) = 0.81 V and J\({}_{\text{sc}}\) = 21.9 mAcm\({}^{-2}\). Figure 11a shows XRD peaks located at 28.39\({}^{\circ}\) and 31.86\({}^{\circ}\) diffraction angles, indicating perovskite crystal formation and suggesting that the deposition solvents did not destroy the perovskite crystal structure [62]. Despite showing good device efficiencies from 3.8 to 11.99%, the SEM surface image of the perovskite film shown in Figure 11b demonstrates the presence of pinholes caused by incomplete crystallization and distribution of non-uniform grains along the film morphology in the one-step method. [PERSON] (2019) worked on improving the perovskite surface morphology by using binary solvent engineering systems during the perovskite one-step deposition. The technique offers control over the perovskite crystal formation, allowing complete and well-oriented homogeneous perovskite films [39]. This unique control over the perovskite morphology is achieved by a mixture of solvents such as the above mentioned DMF, DMSO, and GBL [39; 63]. Figure 11c-e show AFM images at different DMF/DMSO solvent ratios of 1:0, 3:1, and 1:3, denoted as FO10, FO31, and FO13, respectively. With the presence of only DMF in Figure 11c, the AFM image reveals similar results reported in Figure 11b. The addition of a second solvent at a solvent ratio of 3:1 and 1:3 assists the film to eliminate the observed morphology cracks, pinholes, and incomplete crystallization defects. The volatile DMF solvent activates a homogeneous perovskite crystal nucleation and the presence of DMSO helps with crystal growth in an ordered orientation. Hence, the observed large, smooth, and closely packed perovskite grain morphologies in Figure 11e. In a similar study using a solvent mixture of GBL/DMSO at a solvent ratio of 7:3, [PERSON] (2014) acquired 16.2% efficiencies with no hysteresis in forward and reverse bias I-V measurements [63]. Figure 10: Schematic representation of perovskite solar cell fabrication methods. (**a**) One-step method; (**b**) two-step method; (**c**) vapor-assisted solution method; (**d**) thermal vapor deposition method.