**4. Microstructure engineering of metal-halide perovskite films**

The microstructural features of metal-halide perovskite films such as surface coverage, grain size, texture, surface roughness, and so on are previously revealed to play extremely vital roles for determining ultima device performances and even stability. In the past year, there have been rapid progresses in the research field of microstructure engineering of metal-halide perovskite films. In this section, we will focus our attention on the recently developed strategies on microstructure engineering of metal-halide perovskite films aiming to realize highperformance and stable perovskite solar cells.

#### **4.1. Surface coverage engineering of metal-halide perovskite films**

The reasons for ensuring surface coverage of metal-halide perovskite film in perovskite solar cells come from the following two reasons. On the one hand, if there are some regions without metal-halide perovskites padding, light will travel directly without absorption, which leads to decreased photocurrent in turns. On the other hand, any existed pinholes inevitably result in direct contact of electron transport layer with hole transport layer, thus resulting in the formation of shutting paths. They will form additional parallel resistors, causing declines in performance parameters of the cell.

The traditional one-step spin-coating method faces the difficulty to yield a uniform and fullcoverage metal-halide perovskite film in large areas. Aiming to overcome this issue, some effective modifications have been reported. For example, as given in **Figure 6(a)**, Yu et al. [45] introduced a recrystallization process via DMF vapor fumigation to induce the self-repair of one-step deposited MAPbI3 films with poor coverage and low crystallinity. By adjusting the cycle of recrystallization process, they found that MAPbI3 films with dendritic structures spontaneously transformed to the uniform ones with full coverage and high crystallinity (**Figure 6(b–e)**). Solar cells with these modified MAPbI3 films yielded reproducible average PCE of 10.25 ± 0.90% and the optimal one of 11.15%, which is both much higher than that of non-modified MAPbI3 films (**Figure 6(f–d)**). In addition, the J-V hysteresis in the measurement of cell performance can also be effectively alleviated. The authors attributed this desired feature to improve the quality of MAPbI3 films in the optimized devices.

**4.2. Grain size engineering of metal-halide perovskite films**

by modifying deposition technologies of metal halide perovskite films.

thin-film PV devices such as CdTe and Cu(InGa)Se2

**Figure 6.** (a) Schematic processing procedure for MAPbI3

from Ref. [45]. Copyright 2015, Royal Society of Chemistry.

growth, and prevent composition loss of MAPbI3

MAPbI3

Increasing theoretical and experimental evidences indicate that, similar to well-developed

fumigation for 2 (C2), 4 (C4), and 6 (C6) cycles. (b–e) Surficial SEM images of samples C0, C2, C4, and C6. (f) J-V curves and (g) IPCE spectra of the devices based on samples of C0, C2, C4, and C6, respectively. Reproduced with permission

cells is also ascribed to non-radiative recombination of carriers at undesirable trap states. In general, for polycrystalline perovskite films trap states mainly come from crystal imperfections especially such as grain boundaries and intragranular defects [49, 50]. While number of grain boundaries is inversely proportional to average grain size for polycrystalline film, so numerous works have been focused on increasing grain size of metal-halide perovskite films

For example, as shown in **Figure 7**, Yu et al. [49] demonstrated that a homogeneous cap-mediated crystallization with face-to-face configuration can control the crystallization kinetics of

cially the ones with low surface roughness, can effectively retard the nucleation rate, promote

be formed, which have many desirable features, such as greatly enlarged grains, significantly

films in one-step spin-coating method. They found that homogeneous caps, espe-

, primary energy loss in perovskite solar

film deposition (C0) and recrystallization *via* DMF vapor

Microstructure Engineering of Metal-Halide Perovskite Films for Efficient Solar Cells

http://dx.doi.org/10.5772/intechopen.74225

191

grains. Thus, pinhole-free MAPbI3

films can

Cui et al. [46] discovered that methylamine (CH3 NH<sup>2</sup> ) gas can trigger defect-healing of MAPbI3 films via room-temperature ultrafast, reversible chemical reaction of MAPbI3 with CH3 NH<sup>2</sup> gas. They revealed that healing of MAPbI3 films can be ascribed to the formation and reconstruction of an intermediate MAPbI3 ·xCH3 NH<sup>2</sup> liquid phase during perovskite-gas interaction. MAPbI3 film processed by one-step spin-coating method using DMF as solvent is composed of dendrite-like MAPbI3 crystals. And voids with size up to several micrometers between them can be clearly found. After CH3 NH<sup>2</sup> induced defect-healing treatment, dendrite-like crystals and voids almost disappeared and a dense, smooth MAPbI3 film has formed. And, AFM measurement further revealed that the healed film has a very dense and smooth surface, with a RMS roughness of ~6 nm. Benefiting from improved surface coverage of MAPbI3 films by methylamine-induced defect-healing, obvious increase of PCE from 5.7 to 15.1% were realized for the cells. Afterwards, this interesting chemical reaction of CH3 NH<sup>2</sup> gas with metal-halide perovskite was investigated in detailed and extended for further uses such as reduction of intrinsic defect concentration of MAPbI3 films [47], realization of solventand vacuum-free deposition of MAPbI3 films [48], and so on.

Microstructure Engineering of Metal-Halide Perovskite Films for Efficient Solar Cells http://dx.doi.org/10.5772/intechopen.74225 191

**Figure 6.** (a) Schematic processing procedure for MAPbI3 film deposition (C0) and recrystallization *via* DMF vapor fumigation for 2 (C2), 4 (C4), and 6 (C6) cycles. (b–e) Surficial SEM images of samples C0, C2, C4, and C6. (f) J-V curves and (g) IPCE spectra of the devices based on samples of C0, C2, C4, and C6, respectively. Reproduced with permission from Ref. [45]. Copyright 2015, Royal Society of Chemistry.

#### **4.2. Grain size engineering of metal-halide perovskite films**

roles for determining ultima device performances and even stability. In the past year, there have been rapid progresses in the research field of microstructure engineering of metal-halide perovskite films. In this section, we will focus our attention on the recently developed strategies on microstructure engineering of metal-halide perovskite films aiming to realize high-

The reasons for ensuring surface coverage of metal-halide perovskite film in perovskite solar cells come from the following two reasons. On the one hand, if there are some regions without metal-halide perovskites padding, light will travel directly without absorption, which leads to decreased photocurrent in turns. On the other hand, any existed pinholes inevitably result in direct contact of electron transport layer with hole transport layer, thus resulting in the formation of shutting paths. They will form additional parallel resistors, causing declines in

The traditional one-step spin-coating method faces the difficulty to yield a uniform and fullcoverage metal-halide perovskite film in large areas. Aiming to overcome this issue, some effective modifications have been reported. For example, as given in **Figure 6(a)**, Yu et al. [45] introduced a recrystallization process via DMF vapor fumigation to induce the self-repair

spontaneously transformed to the uniform ones with full coverage and high crystallinity

PCE of 10.25 ± 0.90% and the optimal one of 11.15%, which is both much higher than that of

ment of cell performance can also be effectively alleviated. The authors attributed this desired

films via room-temperature ultrafast, reversible chemical reaction of MAPbI3

formed. And, AFM measurement further revealed that the healed film has a very dense and smooth surface, with a RMS roughness of ~6 nm. Benefiting from improved surface coverage

gas with metal-halide perovskite was investigated in detailed and extended for further uses

to 15.1% were realized for the cells. Afterwards, this interesting chemical reaction of CH3

dendrite-like crystals and voids almost disappeared and a dense, smooth MAPbI3

·xCH3

films with poor coverage and low crystallinity. By adjusting

films (**Figure 6(f–d)**). In addition, the J-V hysteresis in the measure-

films in the optimized devices.

NH<sup>2</sup>

NH<sup>2</sup>

NH<sup>2</sup>

films by methylamine-induced defect-healing, obvious increase of PCE from 5.7

films [48], and so on.

film processed by one-step spin-coating method using DMF as solvent

crystals. And voids with size up to several microm-

films with dendritic structures

films yielded reproducible average

) gas can trigger defect-healing of

liquid phase during perovskite-gas

induced defect-healing treatment,

films [47], realization of solvent-

films can be ascribed to the formation

with

film has

NH<sup>2</sup>

performance and stable perovskite solar cells.

190 Emerging Solar Energy Materials

performance parameters of the cell.

of one-step deposited MAPbI3

feature to improve the quality of MAPbI3

non-modified MAPbI3

interaction. MAPbI3

MAPbI3

of MAPbI3

CH3 NH<sup>2</sup>

**4.1. Surface coverage engineering of metal-halide perovskite films**

the cycle of recrystallization process, they found that MAPbI3

(**Figure 6(b–e)**). Solar cells with these modified MAPbI3

Cui et al. [46] discovered that methylamine (CH3

eters between them can be clearly found. After CH3

and reconstruction of an intermediate MAPbI3

is composed of dendrite-like MAPbI3

and vacuum-free deposition of MAPbI3

gas. They revealed that healing of MAPbI3

such as reduction of intrinsic defect concentration of MAPbI3

Increasing theoretical and experimental evidences indicate that, similar to well-developed thin-film PV devices such as CdTe and Cu(InGa)Se2 , primary energy loss in perovskite solar cells is also ascribed to non-radiative recombination of carriers at undesirable trap states. In general, for polycrystalline perovskite films trap states mainly come from crystal imperfections especially such as grain boundaries and intragranular defects [49, 50]. While number of grain boundaries is inversely proportional to average grain size for polycrystalline film, so numerous works have been focused on increasing grain size of metal-halide perovskite films by modifying deposition technologies of metal halide perovskite films.

For example, as shown in **Figure 7**, Yu et al. [49] demonstrated that a homogeneous cap-mediated crystallization with face-to-face configuration can control the crystallization kinetics of MAPbI3 films in one-step spin-coating method. They found that homogeneous caps, especially the ones with low surface roughness, can effectively retard the nucleation rate, promote growth, and prevent composition loss of MAPbI3 grains. Thus, pinhole-free MAPbI3 films can be formed, which have many desirable features, such as greatly enlarged grains, significantly

with relatively excellent reproducibility and the optimal efficiency of 19.24% were realized by

But, they found that similar process is ineffective when replacing MABr with MAI. This phenomenon mainly comes from the fact that low-concentration MAI solution was used and low post-treatment temperature was adopted in their experiments. So, further investigations are needed to clarify those factors. More recently, Jen et al. [53] reported a simple pseudohalide-

that the retreatment process yields a controllable decomposition-to-recrystallization evolu-

As to polycrystalline films, orientation of crystal axis in each grain is another important microstructural feature that dominates their physical properties. Films with aligned crystal axes are so-called textured ones. They possess a single-crystal-like nature along crystal axis, so an enhancement in physical properties is expected for them. In general, ordinarily prepared polycrystalline films are composed of grains with random orientation. Methodology that is explored to develop texture to improve functional properties of polycrystalline films is known as texture engineering. Specifically, one-step deposited metal-halide perovskite films are similarly characterized with randomly oriented grains. Hence, texture engineering is of particular importance to modify their electrical and optical properties, and hence further

large Pb–N binding energy of ~80.04 kJ mol−1 results in a liquefied state after MA adhesion

and delivered an impressive average efficiency of 16.63 ± 0.49% and champion efficiency of 17.22%. Yu et al. [56] demonstrated that face-down annealing of one-step deposited precursor

well-ordered, micrometer-sized grains that span vertically the entire film thickness, as shown in **Figure 8**. Such microstructural features induced dramatically decreased nonradiative recombination sites as well as greatly improved transport property of charge carries in the films compared with that of the non-textured ones obtained by conventional annealing route. As a consequence, planar heterojunction perovskite solar cells with these textured MAPbI3

Metal-halide perovskite film was usually sandwiched between electron-transporting layer and hole-transporting layer in perovskite solar cell. And, one of them has to be deposited

released. Cao et al. [55] found that MACl-containing precursor can yield MAPbI3

exhibit much improved PCE along with small hysteresis and excellent stability.

**4.4. Surface roughness engineering of metal-halide perovskite films**

thin films following an Ostwald ripening process as reported by Zhao et al. [52].

Microstructure Engineering of Metal-Halide Perovskite Films for Efficient Solar Cells

film. Corresponding, it remarkably enlarges grain size of the film in all direc-

film with high crystallinity. The film exhibits much higher both thermal and

thin films to high-quality

http://dx.doi.org/10.5772/intechopen.74225

with low partial pressure MA gas can form a

film is formed when excess MA expeditious are

films were used for typical planar solar cells

. Further investigation revealed that

films consisting of high-crystallinity,

film with

films

film. They found

193

Yu. et al. [50]. Afterward, a similar MABr treatment converts MAPbI3

induced film retreatment technology as passivation for preformed MAPbI3

tions, as well as improving crystallinity and hindering trap density.

**4.3. Texture engineering of metal-halide perovskite films**

improve the performance of ultimate cells.

Yan et al. [54] reported that reaction of HPbI3

strong (110) preferred orientation. The MAPbI3

moisture stability than the one prepared from MAI + PbI2

. And, a highly textured MAPbI3

films can enable the formation of (110) textured MAPbI3

MAPbI3−*<sup>x</sup>*

tion of MAPbI3

textured MAPbI3

on MAPbI3

Br*<sup>x</sup>*

**Figure 7.** (a) Illustration of homogeneous cap-mediated crystallization configuration, where a crystallized MAPbI3 on TiO2 /FTO substrate is placed face to face on precursor MAPbI3 film. (b) Illustration of conventional crystallization configuration. Top-view and cross-sectional SEM images of MAPbI3 films prepared by (c, e) homogeneous capmediated crystallization and (d, f) conventional crystallization, respectively. Reproduced with permission from Ref. [49]. Copyright 2016, Royal Society of Chemistry.

improved crystallinity, preferred (110) orientation, vertically aligned grain boundaries, and proper stoichiometry. As a result, planar-resultant heterojunction solar cells yielded a much enhanced average PCE of 17.87%. It should be noted that large fill factors (FFs) were observed in these efficient cells. In subsequent work, they revealed that PbI2 heterogeneous cap can also realize MAPbI3 films with large-sized grains [51]. Improved PCE was thus realized because of more efficient transport of charge carriers and decreased non-radiative recombination in corresponding devices. Overall, those works suggest a promising strategy to engineer grain size of metal-halide perovskite films.

In addition to crystallization process control, post-treatment strategies were also developed to engineer grain size of metal-halide perovskite films. For example, obvious grain coarsening via Ostwald ripening in one-step deposited MAPbI3 film can be realized by post-synthesis high-temperature heating treatment assisted with additionally deposited CH3 NH<sup>3</sup> I layer [50]. The grain coarsening via Ostwald ripening was revealed to be related to the heating treatment parameters (temperature and time). By optimizing them, the film with average grain size of ~2 μm, much increased crystallinity, and proper stoichiometry can be achieved. Due to those characteristics, defect states along with recombination centers were greatly reduced, and carrier transport and injection properties were much improved. So, efficiency of corresponding planar heterojunction solar cells can be boosted from 14.54 to 16.88%. Then, the same post-treatment recipe was used for thick MAPbI3 films, and a same grain coarsening was observed in them. So post-treatment recipe gives the fact that thickening the absorb layer of cells to realize more sufficient absorption avoids serious aggravation of charge recombination. By further optimizing the thickness of coarsened MAPbI3 films, highly efficient cells with relatively excellent reproducibility and the optimal efficiency of 19.24% were realized by Yu. et al. [50]. Afterward, a similar MABr treatment converts MAPbI3 thin films to high-quality MAPbI3−*<sup>x</sup>* Br*<sup>x</sup>* thin films following an Ostwald ripening process as reported by Zhao et al. [52]. But, they found that similar process is ineffective when replacing MABr with MAI. This phenomenon mainly comes from the fact that low-concentration MAI solution was used and low post-treatment temperature was adopted in their experiments. So, further investigations are needed to clarify those factors. More recently, Jen et al. [53] reported a simple pseudohalideinduced film retreatment technology as passivation for preformed MAPbI3 film. They found that the retreatment process yields a controllable decomposition-to-recrystallization evolution of MAPbI3 film. Corresponding, it remarkably enlarges grain size of the film in all directions, as well as improving crystallinity and hindering trap density.

#### **4.3. Texture engineering of metal-halide perovskite films**

improved crystallinity, preferred (110) orientation, vertically aligned grain boundaries, and proper stoichiometry. As a result, planar-resultant heterojunction solar cells yielded a much enhanced average PCE of 17.87%. It should be noted that large fill factors (FFs) were observed

**Figure 7.** (a) Illustration of homogeneous cap-mediated crystallization configuration, where a crystallized MAPbI3

mediated crystallization and (d, f) conventional crystallization, respectively. Reproduced with permission from Ref.

of more efficient transport of charge carriers and decreased non-radiative recombination in corresponding devices. Overall, those works suggest a promising strategy to engineer grain

In addition to crystallization process control, post-treatment strategies were also developed to engineer grain size of metal-halide perovskite films. For example, obvious grain coarsen-

The grain coarsening via Ostwald ripening was revealed to be related to the heating treatment parameters (temperature and time). By optimizing them, the film with average grain size of ~2 μm, much increased crystallinity, and proper stoichiometry can be achieved. Due to those characteristics, defect states along with recombination centers were greatly reduced, and carrier transport and injection properties were much improved. So, efficiency of corresponding planar heterojunction solar cells can be boosted from 14.54 to 16.88%. Then, the

was observed in them. So post-treatment recipe gives the fact that thickening the absorb layer of cells to realize more sufficient absorption avoids serious aggravation of charge recombi-

high-temperature heating treatment assisted with additionally deposited CH3

films with large-sized grains [51]. Improved PCE was thus realized because

heterogeneous cap can also

NH<sup>3</sup>

films, highly efficient cells

I layer [50].

film can be realized by post-synthesis

film. (b) Illustration of conventional crystallization

films prepared by (c, e) homogeneous cap-

films, and a same grain coarsening

in these efficient cells. In subsequent work, they revealed that PbI2

/FTO substrate is placed face to face on precursor MAPbI3

configuration. Top-view and cross-sectional SEM images of MAPbI3

ing via Ostwald ripening in one-step deposited MAPbI3

same post-treatment recipe was used for thick MAPbI3

nation. By further optimizing the thickness of coarsened MAPbI3

realize MAPbI3

on TiO2

192 Emerging Solar Energy Materials

size of metal-halide perovskite films.

[49]. Copyright 2016, Royal Society of Chemistry.

As to polycrystalline films, orientation of crystal axis in each grain is another important microstructural feature that dominates their physical properties. Films with aligned crystal axes are so-called textured ones. They possess a single-crystal-like nature along crystal axis, so an enhancement in physical properties is expected for them. In general, ordinarily prepared polycrystalline films are composed of grains with random orientation. Methodology that is explored to develop texture to improve functional properties of polycrystalline films is known as texture engineering. Specifically, one-step deposited metal-halide perovskite films are similarly characterized with randomly oriented grains. Hence, texture engineering is of particular importance to modify their electrical and optical properties, and hence further improve the performance of ultimate cells.

Yan et al. [54] reported that reaction of HPbI3 with low partial pressure MA gas can form a textured MAPbI3 film with high crystallinity. The film exhibits much higher both thermal and moisture stability than the one prepared from MAI + PbI2 . Further investigation revealed that large Pb–N binding energy of ~80.04 kJ mol−1 results in a liquefied state after MA adhesion on MAPbI3 . And, a highly textured MAPbI3 film is formed when excess MA expeditious are released. Cao et al. [55] found that MACl-containing precursor can yield MAPbI3 film with strong (110) preferred orientation. The MAPbI3 films were used for typical planar solar cells and delivered an impressive average efficiency of 16.63 ± 0.49% and champion efficiency of 17.22%. Yu et al. [56] demonstrated that face-down annealing of one-step deposited precursor films can enable the formation of (110) textured MAPbI3 films consisting of high-crystallinity, well-ordered, micrometer-sized grains that span vertically the entire film thickness, as shown in **Figure 8**. Such microstructural features induced dramatically decreased nonradiative recombination sites as well as greatly improved transport property of charge carries in the films compared with that of the non-textured ones obtained by conventional annealing route. As a consequence, planar heterojunction perovskite solar cells with these textured MAPbI3 films exhibit much improved PCE along with small hysteresis and excellent stability.

#### **4.4. Surface roughness engineering of metal-halide perovskite films**

Metal-halide perovskite film was usually sandwiched between electron-transporting layer and hole-transporting layer in perovskite solar cell. And, one of them has to be deposited

spin-coating method can effectively reduce roughness of MAPbI3

mately. One the other hand, Chen et al. [60] reported a novel MAPbI3

layer and mesoporous TiO2

sity exceeded 22 mA cm−2.

**Acknowledgements**

**Conflict of interest**

engineering in the progress of perovskite solar cells.

Science Foundation (Grant No. 2016M602771).

The authors declare no competing financial interest.

**5. Conclusions**

demonstrated that the cells' average *V*oc can be enhanced from 0.823±0.105 V to 0.940±0.008 V by spray-assisted process. It benefits from low leakage possibility between hole-transport

formed. Finally, average PCEs of mesoporous cells could be promoted by 25% approxi-

under-layer and a porous upper-layer that was formed by using a thin mesoporous TiO2 seeding layer and a gas-assisted crystallization method. This novel multitiered nanostructure allows for greatly improved light harvesting for wavelengths exceeding 500 nm, as well as a more effective interfacial charge separation for perovskite solar cells. The combination of these factors culminated in average PCEs over 15% and average short-circuit current den-

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

This work was supported primarily by National Natural Science Foundation of China under Grant 61334002 and 61106063, and Class General Financial Grant from the China Postdoctoral

layer when a smooth and pinhole-free MAPbI3

Microstructure Engineering of Metal-Halide Perovskite Films for Efficient Solar Cells

films. Experimental results

http://dx.doi.org/10.5772/intechopen.74225

has successfully

195

film with a dense

**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 model proposed for MAPbI3 grains on TiO2 /FTO substrate. The facets are marked in brown for clarity. The samples with 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.

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 effectively. This modified MAPbI3 morphology is conductive to improve charge carrier 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 spin-coating method can effectively reduce roughness of MAPbI3 films. Experimental results demonstrated that the cells' average *V*oc can be enhanced from 0.823±0.105 V to 0.940±0.008 V by spray-assisted process. It benefits from low leakage possibility between hole-transport layer and mesoporous TiO2 layer when a smooth and pinhole-free MAPbI3 has successfully formed. Finally, average PCEs of mesoporous cells could be promoted by 25% approximately. One the other hand, Chen et al. [60] reported a novel MAPbI3 film with a dense under-layer and a porous upper-layer that was formed by using a thin mesoporous TiO2 seeding layer and a gas-assisted crystallization method. This novel multitiered nanostructure allows for greatly improved light harvesting for wavelengths exceeding 500 nm, as well as a more effective interfacial charge separation for perovskite solar cells. The combination of these factors culminated in average PCEs over 15% and average short-circuit current density exceeded 22 mA cm−2.
