**5. Conclusions**

sequentially on metal-halide perovskite layer. So, surface roughness of metal-halide perovskite film has a significant impact on the cell's interface morphology. A roughened interface that resulted from rough metal-halide perovskite layer would strengthen internal light scattering [57]. And, a large interface area also benefits charge transport [14]. However, high surface roughness of metal-halide perovskite layer will increase short-circuiting possibility of device existing between silver electrode and metal-halide perovskite layer, in the case that electron-transporting layer or hole-transporting layer cannot fully cover the metalhalide perovskite film. In other words, a metal-halide perovskite layer with high roughness requires a thick electron-transporting layer or hole-transporting layer to eliminate shortcircuiting. On the contrary, a thin one is preferred to ensure a reasonable FF. So, there is compromise in surface roughness of metal-halide perovskite film as far as cell performance is concerned, and some exploratory works have been undertaken. For example, one the one hand, Meng et al. [58] introduced a hot-pressing method that can transform MAPbI3 film with rough surface to be a smooth one, and pinholes in original film can be cured

the preheating temperatures of room temperature (RT), 40, and 60°C were labeled as FDA-RT, FDA-40, and FDA-60,

respectively. Reproduced with permission from Ref. [56]. Copyright 2017, American Chemical Society.

**Figure 8.** (a) XRD patterns of samples CA, FDA-60, FDA-40, and FDA-RT, respectively. (b) Corresponding histograms of XRD peak intensity ratios of (110) to (310) planes and (220) to (310) planes as well as calculated Lotgering factors. (c–f) cross-sectional SEM images of samples CA, FDA-60, FDA-40, and FDA-RT. Left inset describes the stereoshape

transport and eliminate charge carrier recombination in perovskite solar cells. Moreover, much improved performances with high PCEs of 10.84 and 16.07% are thus realized in hole-transporting-layer-free and spiro-OMeTAD-based cells, respectively. Yu et al. [59] found that spray-assisted process instead of commonly used dipping process in a two-step

morphology is conductive to improve charge carrier

/FTO substrate. The facets are marked in brown for clarity. The samples with

effectively. This modified MAPbI3

model proposed for MAPbI3

194 Emerging Solar Energy Materials

grains on TiO2

In summary, metal-halide perovskite films have many excellent optoelectronic properties such as high absorption coefficient, tunable bandgap, long and balanced carrier diffusions, ambipolar transport of charge carriers, tolerance of defects, along with capacity for film deposition via either solution or vacuum-based methods including one-step spin-coating method, sequential deposition method, two-step spin-coating method vacuum coevaporation deposition method, sequential vacuum deposition method, and vapor-assisted solution deposition method. Those desired features make them promising for high-performance and low-cost perovskite solar cells. The microstructural features that mainly refer to surface coverage, grain size, texture, surface roughness, and so on are vital in determining the performance of perovskite solar cells. Specifically, the ones with full surface coverage, large grain size, textured feature, and smooth surface, are highly desirable for efficient devices. Some important progresses in microstructure engineering of metal-halide perovskite films are described in this chapter, which will promote systematically understanding the role of microstructure engineering in the progress of perovskite solar cells.
