**3. Film formation technologies**

**Figure 3.** (a) UV-Vis absorbance spectra of FAPbIy

the photocurrent of the cell with FAPbI3

1.73 eV [31]. For X site, after replacing I−

decreases to 1.2 eV by replacing the Pb<sup>+</sup>

*2.3.2. Long-range ambipolar charge transport*

extremely similar to hole effective mass (m<sup>h</sup>

to 1.30 eV for CsSnI3

\* and m<sup>h</sup> \*

indicates that this fact that electron effective mass (m<sup>e</sup>

MAPbBr3

variation of FAPbIy

186 Emerging Solar Energy Materials

1.67 eV for CsPbI3

to 550 nm for FAPbBr3

ferent effective masses (m<sup>e</sup>

light absorption range. While a smaller Cs<sup>+</sup>

or 3.12 eV for MAPbCl3

Br3−y metal-halide perovskite films with different y values. (b) Steady-

state photoluminescence spectra for the corresponding films. (c) Photographs of the films with y increasing from 0 to 1

an absorption edge extended from 800 to 840 nm, as firstly reported by Eperon et al. [30]. And,

or Cl−

Br3−y (0 < y < 1) films. Its absorption onset changes from 840 nm for FAPbI3

, corresponding the bandgap increases from 1.48 to 2.13 eV. For B site,

with Br−

a smaller cation size usually results in less bandgap. For example, the bandgap of MAPbI3

For most of semiconductors, the transport of electrons and holes is unbalanced, due to dif-

studies have revealed that well-balanced electron and hole transport is established in metalhalide perovskites, namely ambipolar transport property. First principle calculation also

has an effect on bulk polarization during charge transport and collection; in turns affects PV parameters of perovskite solar cells. Moreover, metal-halide perovskites can convey both n-type and p-type properties when they are used in thin-film devices with different interfacial layers [22]. In addition, long carrier diffusion length of ~100 nm and ~1 μm were revealed

and 1.08 eV for CsGeI3

\* = 0.29 m<sup>0</sup>

with Sn<sup>+</sup>

film can reach up to 23 mA cm−2, because of extended

, respectively. **Figure 3(a–c)** shows the continuous bandgap

[24].

). Surprisingly, numerous independent experimental

\* = 0.23 m<sup>0</sup>

at A site gives rise to an increased bandgap of

[24]. Similarly, the bandgap decreases from

, the bandgap increases to 2.30 eV for

) of this type materials is

) [32]. Balanced ambipolar transport

(left to right). Reproduced with permission from Ref. [30]. Copyright 2014, Royal Society of Chemistry.

Up to now, various processing techniques have been developed to prepare metal-halide perovskite films, as summarized in **Figure 4**. They mainly include one-step spin-coating method, sequential solution deposition method, two-step spin-coating method, vacuum coevaporation deposition method, sequential vacuum deposition method, and vapor-assisted solution deposition method [36].

The one-step spin-coating method involves spin-coating of a precursor solution containing PbX2 with a certain amount of MAX firstly (X = I, Br, and Cl). Then, the metal-halide perovskites formed and grew during solvent evaporation. A post-annealing recipe with a temperature of ~100°C was usually required to remove residual solvents and complete crystallization. For this method, the film morphology and quality strongly depend on the processing conditions such as annealing temperature, solution concentration, precursor composition, solvent choice, etc. [36]. Although it is extremely simple, the one-step spin-coating method faces the difficulty to the deposition of pinhole-free metal-halide perovskite films. Solvent engineering is proved to be one of effective routes to overcome this obstacle. Spiccia et al. [37] proposed a fast deposition crystallization method to induce the crystallization of MAPbI3 during spin-coating process. This method includes the spin-coating of MAPbI3 precursor with N,N-dimethylformamide (DMF) as the solvent, followed by dropping anti-solvent such as toluene and chlorobenzene to complete the crystallization of MAPbI3 . The anti-solvent decreased the MAPbI3 solubility in DMF solvent, and thereby promoting fast nucleation and crystallization [6]. Later, Jeon et al. [38] designed a mixed γ-butyrolactone and dimethyl sulfoxide (DMSO) as the processing solvent followed by toluene drop-casting. The difference is

**Figure 4.** Illustrations of developed processing techniques for deposition of metal-halide perovskite films. Reproduced with permission from Ref. [36]. Copyright 2015, Royal Society of Chemistry.

that a dark brown MAPbI3 film formed immediately after dropping of toluene solvent, while a transparent MAI-PbI2 -DMSO intermediate phase formed firstly after dropping of toluene using the mixed solvent. Both X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) measurements verified the formation of intermediate phase. After being thermal annealed at 100°C for 10 min, intermediate phase film can transform to an extremely uniform and compact MAPbI3 layer [6]. Similarly, Park et al. [39] further developed the intermediate phase route based on Lewis acid adduct of PbI2 , as shown in **Figure 5**. This method eventually resulted in an average PCE of 18.3% from 41 cells and the best value of 19.7%.

The sequential deposition procedure was firstly introduced by Grätzel et al. [40]. And in surprise, it yielded a high PCE of 15%. This method includes the spin-coating of a PbX2 solution on substrate, followed by drying, and then dipping the substrate into an isopropanol (IPA) solution of MAX to accomplish the reaction in few minutes. Typically, the solutions are 1 M PbI2 in DMF and MAI in IPA with relative low concentration from 0.004 to 0.006 M. After the dipping step, the residual MAI was rinsed out by IPA. With this method, the films' homogeneity can be dramatically improved and their morphology becomes more controllable than the one-step spin-coating method.

contrast with that of one-step solution process. Due to its low control accuracy in precursor ratio, the sequential vacuum evaporation technology was further developed via depositing

that in panels (a) and (b), but diethyl ether was dripped during the spinning process. Reproduced with permission from

However, the requirement of high vacuum degree of them inevitably increases the device

pre-deposited via spin-coating route reacted with MAI vapor under atmospheric pressure.

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

Vapor-assisted solution deposition is firstly used by Yang et al. [44]. For it, PbI2

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

a pure-phase, homogeneous, and pinhole-free CH3

**Figure 5.** Schematic illustration of processing procedures for MAPbI3

or (b) MAI, PbI2

scanning electron microscopy (SEM) images. The MAPbI3

Ref. [39] . Copyright 2015, American Chemical Society.

solution containing (a) MAI and PbI2

polycrystallites, delivering to a PCE of 12%.

cost, and thus partially restricts their practical applications.

and MAI layer by layer [43]. By heating the substrate during precursor evaporation,

NH<sup>3</sup>

film was revealed to be extremely smooth, and composed of microscale

PbI3 <sup>−</sup> <sup>x</sup> Cl<sup>x</sup>

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

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

189

film can be synthesized.

films deposition as well as the corresponding

films were deposited by one-step spin-coating of the DMF

, and DMSO. Panels (c) and (d) were prepared by same solution as

film that was

PbCl2

The resultant MAPbI3

The two-step spin-coating method that is also known as interdiffusion method was proposed by Huang et al. [41]. It can be seen as the one derived from sequential deposition procedure. But, the difference is that metal-halide perovskite is formed by spin-coating an MAI layer on PbX2 precursor film followed by thermal annealing at 100°C for a relative long time up to 2 h. This method is materials saving, and can synthesize more uniform films than sequential deposition.

Snaith et al. [42] firstly reported the vacuum co-evaporation deposited planar MAPbI3−xCl<sup>x</sup> perovskite solar cells by co-evaporating PbCl2 and MAI in a vacuum thermal evaporation system. The PCEs with a narrow distribution and optimal one of 15.4% were observed in Microstructure Engineering of Metal-Halide Perovskite Films for Efficient Solar Cells http://dx.doi.org/10.5772/intechopen.74225 189

**Figure 5.** Schematic illustration of processing procedures for MAPbI3 films deposition as well as the corresponding scanning electron microscopy (SEM) images. The MAPbI3 films were deposited by one-step spin-coating of the DMF solution containing (a) MAI and PbI2 or (b) MAI, PbI2 , and DMSO. Panels (c) and (d) were prepared by same solution as that in panels (a) and (b), but diethyl ether was dripped during the spinning process. Reproduced with permission from Ref. [39] . Copyright 2015, American Chemical Society.

that a dark brown MAPbI3

uniform and compact MAPbI3

the one-step spin-coating method.

perovskite solar cells by co-evaporating PbCl2

mediate phase route based on Lewis acid adduct of PbI2

with permission from Ref. [36]. Copyright 2015, Royal Society of Chemistry.

a transparent MAI-PbI2

188 Emerging Solar Energy Materials

PbI2

film formed immediately after dropping of toluene solvent, while

layer [6]. Similarly, Park et al. [39] further developed the inter-

, as shown in **Figure 5**. This method

and MAI in a vacuum thermal evaporation

solution


using the mixed solvent. Both X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) measurements verified the formation of intermediate phase. After being thermal annealed at 100°C for 10 min, intermediate phase film can transform to an extremely

**Figure 4.** Illustrations of developed processing techniques for deposition of metal-halide perovskite films. Reproduced

eventually resulted in an average PCE of 18.3% from 41 cells and the best value of 19.7%.

prise, it yielded a high PCE of 15%. This method includes the spin-coating of a PbX2

The sequential deposition procedure was firstly introduced by Grätzel et al. [40]. And in sur-

on substrate, followed by drying, and then dipping the substrate into an isopropanol (IPA) solution of MAX to accomplish the reaction in few minutes. Typically, the solutions are 1 M

The two-step spin-coating method that is also known as interdiffusion method was proposed by Huang et al. [41]. It can be seen as the one derived from sequential deposition procedure. But, the difference is that metal-halide perovskite is formed by spin-coating an MAI layer on PbX2 precursor film followed by thermal annealing at 100°C for a relative long time up to 2 h. This method is materials saving, and can synthesize more uniform films than sequential deposition.

Snaith et al. [42] firstly reported the vacuum co-evaporation deposited planar MAPbI3−xCl<sup>x</sup>

system. The PCEs with a narrow distribution and optimal one of 15.4% were observed in

 in DMF and MAI in IPA with relative low concentration from 0.004 to 0.006 M. After the dipping step, the residual MAI was rinsed out by IPA. With this method, the films' homogeneity can be dramatically improved and their morphology becomes more controllable than contrast with that of one-step solution process. Due to its low control accuracy in precursor ratio, the sequential vacuum evaporation technology was further developed via depositing PbCl2 and MAI layer by layer [43]. By heating the substrate during precursor evaporation, a pure-phase, homogeneous, and pinhole-free CH3 NH<sup>3</sup> PbI3 <sup>−</sup> <sup>x</sup> Cl<sup>x</sup> film can be synthesized. However, the requirement of high vacuum degree of them inevitably increases the device cost, and thus partially restricts their practical applications.

Vapor-assisted solution deposition is firstly used by Yang et al. [44]. For it, PbI2 film that was pre-deposited via spin-coating route reacted with MAI vapor under atmospheric pressure. The resultant MAPbI3 film was revealed to be extremely smooth, and composed of microscale polycrystallites, delivering to a PCE of 12%.
