**3. Two-step method: Prepared perovskite film**

#### **3.1. Film formation**

0.85 M PbI2 and 0.15 M PbCl2 were dissolved in the solvent of DMF and stirred for 2 h at 75°C. Forty milligrams of MAI were dissolved in the solvent of IPA with/without additionally 0.9 vol% DMF or GBL, respectively. Around 60ul PbX2 precursor solution preheated to 75°C was transferred by pipettes to the ITO substrates. Briefly, the spin-coating process was programmed to run at 3000 rpm for 45 s, and a yellow transparent dense PbI2 film was deposited. Then, MAI was spin-coated on top of the dried PbI2 layer at room temperature at 3000 rpm for 45 s. All of the films were thermally annealed on the hot plate at 100°C for 10 min. And, the perovskite film is formed by interdiffusion process. **Figure 7** shows the photographs of perovskite films deposited by MAI/IPA and MAI/(IPA-0.9%DMF) solutions. It can be seen that the perovskite film shows a heterogeneous and whitish surface morphology if the pure MAI/IPA solution was used. And, the concentration variation of the MAI/IPA solution cannot reverse the situation. However, by introducing proper DMF (0.9%) solvent additive into the MAI/IPA solution, the dark brown perovskite film is obtained, and the optimized MAI concentration is 40 mg/L. To further know their difference, the morphology and crystalline quality of perovskite films were measured by scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests, as well as their optical property by UV–visible spectrophotometer and photoluminescence spectra. More experimental details can be found in our previous work [36].

#### **3.2. Results and discussion**

**Figure 8(a)** and **(b)** displays the scanning electron microscopy (SEM) images of perovskite films. The perovskite film without DMF additive shows the small grain size and many pinholes between the grain boundaries (marked with the red circles). These defects increase recombination probability and severely hamper the charge transport and the device performance. However, when a small amount of DMF is added to MAI/IPA precursor, those pinholes among the grain boundaries are effectively eliminated in the resulted perovskite films, as shown in **Figure 8(b)**; also, the average grain size of perovskite is obviously increased.

**Figure 8(c)** and **(d)** displays the cross-sectional images of perovskite films deposited on glass substrate. While using MAI/IPA solution, the perovskite film shows the low-quality, incom-

**Figure 8.** SEM images of perovskite films. Higher-magnification SEM image of the perovskite film without (a) and with (b) DMF additive, cross-sectional SEM image of the perovskite film without DMF additive (c) and with (d) DMF additive

(reprinted with the permission from [31], 2017, the Royal Society of Chemistry).

sor solution, a perovskite film with large grain size could be observed from **Figure 8(d)**. **Figure 9** shows the forming process of perovskite film by two-step deposition method. It is obvious that the controlled perovskite film seemed a bit low quality with the little crystal and more defects when the bare MAI/IPA solution were used. However, with proper DMF solvent additive doped into the MAI/IPA solution, the crystal quality of perovskite film could

According to the high solubility of MAI and PbI2 in DMF, we present a possible mechanism that [31] the small amount of DMF solvent provides a "wet" environment so that PbI2

MAI could react with each other and later a high-quality perovskite could be obtained after annealing. As we know, the DMF has a higher boiling point (152.8°C) that of IPA; thus, the presence time of DMF is relatively long during the 100°C annealing process. During the crystal growth process, the DMF additive could drive the MAI penetrating into the thick PbI2

form larger crystal grains by slowing down the perovskite crystallization rate, and a thick film with a pure phase since perovskite can be totally but very slowly dissolved in DMF, and the dissolving process depends on the amount of DMF. Moreover, proper DMF solvent vapor annealing could increase thin-film crystallinity, and crystalline domain size since the

and small grain size. However, by adding proper DMF into MAI precur-

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and

to

plete-reaction PbI2

significantly be improved.

**Figure 7.** Photographs of perovskite films by spin-coating MAI/IPA solution (top) and MAI/(IPA-0.9%DMF) (bottom), respectively, with 10, 20, 30, 40, 50, and 60 mg/ml MAI concentrations (from left to right) (reprinted with the permission from [31], 2017, The Royal Society of Chemistry).

High-Quality Perovskite Film Preparations for Efficient Perovskite Solar Cells http://dx.doi.org/10.5772/intechopen.75103 227

**3. Two-step method: Prepared perovskite film**

0.9 vol% DMF or GBL, respectively. Around 60ul PbX2

Then, MAI was spin-coated on top of the dried PbI2

grammed to run at 3000 rpm for 45 s, and a yellow transparent dense PbI2

were dissolved in the solvent of DMF and stirred for 2 h at

precursor solution preheated to 75°C

layer at room temperature at 3000 rpm

film was deposited.

75°C. Forty milligrams of MAI were dissolved in the solvent of IPA with/without additionally

was transferred by pipettes to the ITO substrates. Briefly, the spin-coating process was pro-

for 45 s. All of the films were thermally annealed on the hot plate at 100°C for 10 min. And, the perovskite film is formed by interdiffusion process. **Figure 7** shows the photographs of perovskite films deposited by MAI/IPA and MAI/(IPA-0.9%DMF) solutions. It can be seen that the perovskite film shows a heterogeneous and whitish surface morphology if the pure MAI/IPA solution was used. And, the concentration variation of the MAI/IPA solution cannot reverse the situation. However, by introducing proper DMF (0.9%) solvent additive into the MAI/IPA solution, the dark brown perovskite film is obtained, and the optimized MAI concentration is 40 mg/L. To further know their difference, the morphology and crystalline quality of perovskite films were measured by scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests, as well as their optical property by UV–visible spectrophotometer and photolumi-

nescence spectra. More experimental details can be found in our previous work [36].

**Figure 8(a)** and **(b)** displays the scanning electron microscopy (SEM) images of perovskite films. The perovskite film without DMF additive shows the small grain size and many pinholes between the grain boundaries (marked with the red circles). These defects increase recombination probability and severely hamper the charge transport and the device performance. However, when a small amount of DMF is added to MAI/IPA precursor, those pinholes among the grain boundaries are effectively eliminated in the resulted perovskite films, as shown in **Figure 8(b)**; also, the average grain size of perovskite is obviously increased.

**Figure 7.** Photographs of perovskite films by spin-coating MAI/IPA solution (top) and MAI/(IPA-0.9%DMF) (bottom), respectively, with 10, 20, 30, 40, 50, and 60 mg/ml MAI concentrations (from left to right) (reprinted with the permission

and 0.15 M PbCl2

**3.1. Film formation**

226 Emerging Solar Energy Materials

**3.2. Results and discussion**

from [31], 2017, The Royal Society of Chemistry).

0.85 M PbI2

**Figure 8.** SEM images of perovskite films. Higher-magnification SEM image of the perovskite film without (a) and with (b) DMF additive, cross-sectional SEM image of the perovskite film without DMF additive (c) and with (d) DMF additive (reprinted with the permission from [31], 2017, the Royal Society of Chemistry).

**Figure 8(c)** and **(d)** displays the cross-sectional images of perovskite films deposited on glass substrate. While using MAI/IPA solution, the perovskite film shows the low-quality, incomplete-reaction PbI2 and small grain size. However, by adding proper DMF into MAI precursor solution, a perovskite film with large grain size could be observed from **Figure 8(d)**. **Figure 9** shows the forming process of perovskite film by two-step deposition method. It is obvious that the controlled perovskite film seemed a bit low quality with the little crystal and more defects when the bare MAI/IPA solution were used. However, with proper DMF solvent additive doped into the MAI/IPA solution, the crystal quality of perovskite film could significantly be improved.

According to the high solubility of MAI and PbI2 in DMF, we present a possible mechanism that [31] the small amount of DMF solvent provides a "wet" environment so that PbI2 and MAI could react with each other and later a high-quality perovskite could be obtained after annealing. As we know, the DMF has a higher boiling point (152.8°C) that of IPA; thus, the presence time of DMF is relatively long during the 100°C annealing process. During the crystal growth process, the DMF additive could drive the MAI penetrating into the thick PbI2 to form larger crystal grains by slowing down the perovskite crystallization rate, and a thick film with a pure phase since perovskite can be totally but very slowly dissolved in DMF, and the dissolving process depends on the amount of DMF. Moreover, proper DMF solvent vapor annealing could increase thin-film crystallinity, and crystalline domain size since the

**Figure 9.** The schematic of interdiffusion procedure for preparing the uniform and dense perovskite film (reprinted with the permission from [31], 2017, the Royal Society of Chemistry).

for the perovskite without DMF additive, the low quenching efficiency can be attributed to

**Figure 10.** (a) XRD spectra of perovskite with MAI/IPA and MAI/IPA-0.9% DMF, respectively. (b) Normalized intensity

temperature. Perovskite films grown on different substrates including glass or PEDOT:PSS. (d) Absorption spectra of the

perovskite films, which were fabricated by spin-coating MAI/IPA and MAI/IPA-0.9%DMF on dried PbI2

plays the absorption spectra of the perovskite films. It is clear that the light absorption in the perovskite film with 0.9% DMF additive is more efficient than the perovskite film without DMF additive at all absorption wavelength range. **Figure 11** exhibits the device structure of the perovskite solar cell and the corresponding energy diagram. In the device the highest occupied molecular orbital (HOMO) level and the lowest unoccupied molecular orbital (LUMO) level of PEDOT:PSS are 3.0 and 5.2 eV, respectively. So, the PEDOT:PSS layer plays the role of electronblocking layer and hole transport layer. Correspondingly, the PCBM acts as hole-blocking layer and electron transport layer with the HOMO level of 4.0 eV and LUMO level of 6.2 eV. The BCP is used as the interface modification layer with the HOMO level of 7.0 eV. The Ag film and the

Based on the high some batches of devices were fabricated, and **Figure 12(a)** displays typical *J*– *V* characteristics of the fabricated PSCs. Without DMF additive, the PSC exhibits a short- circuit

(*FF*) of 71.2% and a corresponding PCE of 11.4%. It is obvious that the low *J*SC and *FF* are the main factors limiting the PCE. Compared with the PSC without DMF additive, the PSC performance has been greatly improved as shown in **Figure 12(a)**, when the DMF is first introduced in

to 77.1% with a slightly increased *V*OC (1.00 V), which enhances the PCE to 15.5%. It can be inferred that the DMF solvent in the MAI solution can help to enhance the reaction between

the MAI solution. Consequently, *J*SC is greatly improved to 20.06 mA/cm2

at perovskite/PEDOT:PSS interface. **Figure 10(c)** dis-

and MAI, respectively. (c) Photoluminescence (PL) spectra at room

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, an open-circuit voltage (*V*OC) of 0.99 V, and a fill factor

crystallinity and grain size, and improve the device

, and *FF* is improved

films (Reprinted

the charge block effect of residual PbI2

with the permission from [31], 2017, the Royal Society of Chemistry).

of X-ray diffraction spectra of perovskite, PbI2

ITO are chosen as the top and bottom electrodes.

current density (*J*SC) of 16.16 mA/cm2

and MAI, improve the MAPbI3

PbI2

DMF solvent could induce a second perovskite dissolution and recrystallization process. As a result, the large-size crystal grains and high-quality perovskite films are achieved, which can be partly supported by the SEM images. To verify this mechanism, the crystal quality, light absorption ability, and charge transport property are discussed as follows.

**Figure 10(a)** shows the XRD results of PbI2 , MAI, and perovskite films. As expected, the PbI2 displays a characteristic diffraction peak at 2θ of 12.8°. And, the diffraction peaks of MAI at 2θ of 9.8, 19.65, and 29.65° are consistent with reported results. For the perovskite film without DMF additive, the diffraction peak at 12.8° means the PbI2 residues in this film. However, when a 0.9 vol% DMF is added to MAI/IPA precursor, the PbI2 diffraction peak disappears, and the peak intensity of perovskite is enhanced; both of them demonstrate the higher crystal quality of perovskite film. It is suggested that the presence of a small amount of DMF solvent could improve the complete conversion of PbI2 to perovskite by promoting the reaction between PbI2 and MAI. **Figure 10(b)** displays the steady-state PL spectra of the perovskite films on the glass or glass/ITO/PEDOT:PSS substrates. For the perovskite films on glass, the same peak position at 759 nm is observed; the PL peak intensity is enhanced after adding the DMF additive in MAI precursor, which demonstrates the improved perovskite film quality. Furthermore, for the perovskite/PEDOT:PSS/ITO/glass sample, the more obvious PL quenching in perovskite with DMF additive means the more efficient charge transfer from the perovskite to the PEDOT:PSS layer, which agrees the XRD discussion of the complete conversion of PbI2 to perovskite. While

**Figure 10.** (a) XRD spectra of perovskite with MAI/IPA and MAI/IPA-0.9% DMF, respectively. (b) Normalized intensity of X-ray diffraction spectra of perovskite, PbI2 and MAI, respectively. (c) Photoluminescence (PL) spectra at room temperature. Perovskite films grown on different substrates including glass or PEDOT:PSS. (d) Absorption spectra of the perovskite films, which were fabricated by spin-coating MAI/IPA and MAI/IPA-0.9%DMF on dried PbI2 films (Reprinted with the permission from [31], 2017, the Royal Society of Chemistry).

for the perovskite without DMF additive, the low quenching efficiency can be attributed to the charge block effect of residual PbI2 at perovskite/PEDOT:PSS interface. **Figure 10(c)** displays the absorption spectra of the perovskite films. It is clear that the light absorption in the perovskite film with 0.9% DMF additive is more efficient than the perovskite film without DMF additive at all absorption wavelength range. **Figure 11** exhibits the device structure of the perovskite solar cell and the corresponding energy diagram. In the device the highest occupied molecular orbital (HOMO) level and the lowest unoccupied molecular orbital (LUMO) level of PEDOT:PSS are 3.0 and 5.2 eV, respectively. So, the PEDOT:PSS layer plays the role of electronblocking layer and hole transport layer. Correspondingly, the PCBM acts as hole-blocking layer and electron transport layer with the HOMO level of 4.0 eV and LUMO level of 6.2 eV. The BCP is used as the interface modification layer with the HOMO level of 7.0 eV. The Ag film and the ITO are chosen as the top and bottom electrodes.

DMF solvent could induce a second perovskite dissolution and recrystallization process. As a result, the large-size crystal grains and high-quality perovskite films are achieved, which can be partly supported by the SEM images. To verify this mechanism, the crystal quality, light

**Figure 9.** The schematic of interdiffusion procedure for preparing the uniform and dense perovskite film (reprinted with

displays a characteristic diffraction peak at 2θ of 12.8°. And, the diffraction peaks of MAI at 2θ of 9.8, 19.65, and 29.65° are consistent with reported results. For the perovskite film without

peak intensity of perovskite is enhanced; both of them demonstrate the higher crystal quality of perovskite film. It is suggested that the presence of a small amount of DMF solvent could

and MAI. **Figure 10(b)** displays the steady-state PL spectra of the perovskite films on the glass or glass/ITO/PEDOT:PSS substrates. For the perovskite films on glass, the same peak position at 759 nm is observed; the PL peak intensity is enhanced after adding the DMF additive in MAI precursor, which demonstrates the improved perovskite film quality. Furthermore, for the perovskite/PEDOT:PSS/ITO/glass sample, the more obvious PL quenching in perovskite with DMF additive means the more efficient charge transfer from the perovskite to the PEDOT:PSS

, MAI, and perovskite films. As expected, the PbI2

to perovskite by promoting the reaction between PbI2

residues in this film. However, when

diffraction peak disappears, and the

to perovskite. While

absorption ability, and charge transport property are discussed as follows.

layer, which agrees the XRD discussion of the complete conversion of PbI2

**Figure 10(a)** shows the XRD results of PbI2

the permission from [31], 2017, the Royal Society of Chemistry).

228 Emerging Solar Energy Materials

improve the complete conversion of PbI2

DMF additive, the diffraction peak at 12.8° means the PbI2

a 0.9 vol% DMF is added to MAI/IPA precursor, the PbI2

Based on the high some batches of devices were fabricated, and **Figure 12(a)** displays typical *J*– *V* characteristics of the fabricated PSCs. Without DMF additive, the PSC exhibits a short- circuit current density (*J*SC) of 16.16 mA/cm2 , an open-circuit voltage (*V*OC) of 0.99 V, and a fill factor (*FF*) of 71.2% and a corresponding PCE of 11.4%. It is obvious that the low *J*SC and *FF* are the main factors limiting the PCE. Compared with the PSC without DMF additive, the PSC performance has been greatly improved as shown in **Figure 12(a)**, when the DMF is first introduced in the MAI solution. Consequently, *J*SC is greatly improved to 20.06 mA/cm2 , and *FF* is improved to 77.1% with a slightly increased *V*OC (1.00 V), which enhances the PCE to 15.5%. It can be inferred that the DMF solvent in the MAI solution can help to enhance the reaction between PbI2 and MAI, improve the MAPbI3 crystallinity and grain size, and improve the device

bias 1.1 V)). **Figure 12(a)** displays the *J–V* characteristics of PSCs under different scan directions. Regardless of scan directions, highly consistent *J–V* curves and negligible photocurrent hysteresis are observed. This indicates the validity of our measured device performance. To further verify that the measured efficiency is reliable, we also measured steady-state outputs of current density and PCE. The measurements are set at the maximum power point for the steady-state PCE and *J* outputs. As shown in **Figure 12(c)**, the device without DMF additive also shows a stable current density, yielding a stabilized PCE of 11.4%. Meanwhile, the device with DMF additive also shows a stable current density, yielding a stabilized PCE of over 15% as shown in **Figure 12(d)**. This result further verifies the validity of our measured

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**Figure 13** displays the statistic results of the fabricated PSCs. Those statistic parameters clearly reveal that the DMF additive in the MAI solution could significantly enhance the photovoltaic performance of PSCs. It should be noted that the statistic results were based on 20 perovskite solar cell devices in several batches, which indicates that our experiments are reproducible. It confirms the validity of above discussion. Surprisingly, a champion device with a PCE of 19.2% is obtained during the optimization. The corresponding device exhibited the

, *V*OC = 1.03 V, and *FF* = 79.6%, while a *J*SC = 16.3 mA/cm2

and *FF* = 74.7% belong to the champion cell without DMF, as displayed in **Figure 14**. At the same time, it should be noted that although the above discussion is on the device with DMF additive, devices with the GBL additive has the same trend, which shows that the method is a

**Figure 13.** Comparison of histograms of photovoltaic parameters for the perovskite solar cells based on MAI/IPA (red) and MAI/(IPA-0.9%DMF) (black) condition. Data from 20 cells were used for the histogram (reprinted with the

, *V*OC = 1.01 V,

device performance.

*J*SC = 23.4 mA/cm2

general method to enhance the PSC performance.

permission from [31], 2017, the Royal Society of Chemistry).

**Figure 11.** Device architecture of the perovskite solar cell (glass/ITO/PEDOT:PSS/perovskite/PC61BM/BCP/Ag) and the corresponding energy level diagram of corresponding materials used in the device (reprinted with the permission from [31], 2017, the Royal Society of Chemistry).

**Figure 12.** (a) *J–V* curves with different scanning directions at the condition of IPA and IPA- 0.9%DMF. Reverse (1.1 V → -0.2 V) and forward scan (−0.2 V → 1.1 V) measurement: the voltage step is 0.01 V. (b) IPCEs for the PSCs from MAI/IPA (red curves) and MAI/(IPA- 0.9%DMF)(black curves). Steady-state current density and PCE of the devices without (c) and with (d) DMF additive in MAI solution (Reprinted with the permission from [31], 2017, The Royal Society of Chemistry).

performance. The incident photo-to-electron conversion efficiency (IPCE) curves of the PSCs with/without DMF additive are shown in **Figure 12(b)**. The device with DMF additive shows a higher IPCE value than the device without the DMF additive, which is same as the improved efficiency of the device with DMF additive obtained from the *J*–*V* curve measurement.

As the photocurrent hysteresis behavior is a common issue in accurate characterization of device efficiency, the photocurrent hysteresis behaviors of PSCs with/without DMF additive were measured by changing the scanning directions (reverse scan (from a positive bias 1.1 V to a negative bias −0.2 V) and forward scan (from a negative bias −0.2 V to a positive bias 1.1 V)). **Figure 12(a)** displays the *J–V* characteristics of PSCs under different scan directions. Regardless of scan directions, highly consistent *J–V* curves and negligible photocurrent hysteresis are observed. This indicates the validity of our measured device performance. To further verify that the measured efficiency is reliable, we also measured steady-state outputs of current density and PCE. The measurements are set at the maximum power point for the steady-state PCE and *J* outputs. As shown in **Figure 12(c)**, the device without DMF additive also shows a stable current density, yielding a stabilized PCE of 11.4%. Meanwhile, the device with DMF additive also shows a stable current density, yielding a stabilized PCE of over 15% as shown in **Figure 12(d)**. This result further verifies the validity of our measured device performance.

**Figure 13** displays the statistic results of the fabricated PSCs. Those statistic parameters clearly reveal that the DMF additive in the MAI solution could significantly enhance the photovoltaic performance of PSCs. It should be noted that the statistic results were based on 20 perovskite solar cell devices in several batches, which indicates that our experiments are reproducible. It confirms the validity of above discussion. Surprisingly, a champion device with a PCE of 19.2% is obtained during the optimization. The corresponding device exhibited the *J*SC = 23.4 mA/cm2 , *V*OC = 1.03 V, and *FF* = 79.6%, while a *J*SC = 16.3 mA/cm2 , *V*OC = 1.01 V, and *FF* = 74.7% belong to the champion cell without DMF, as displayed in **Figure 14**. At the same time, it should be noted that although the above discussion is on the device with DMF additive, devices with the GBL additive has the same trend, which shows that the method is a general method to enhance the PSC performance.

performance. The incident photo-to-electron conversion efficiency (IPCE) curves of the PSCs with/without DMF additive are shown in **Figure 12(b)**. The device with DMF additive shows a higher IPCE value than the device without the DMF additive, which is same as the improved

**Figure 12.** (a) *J–V* curves with different scanning directions at the condition of IPA and IPA- 0.9%DMF. Reverse (1.1 V → -0.2 V) and forward scan (−0.2 V → 1.1 V) measurement: the voltage step is 0.01 V. (b) IPCEs for the PSCs from MAI/IPA (red curves) and MAI/(IPA- 0.9%DMF)(black curves). Steady-state current density and PCE of the devices without (c) and with (d) DMF additive in MAI solution (Reprinted with the permission from [31], 2017, The Royal

**Figure 11.** Device architecture of the perovskite solar cell (glass/ITO/PEDOT:PSS/perovskite/PC61BM/BCP/Ag) and the corresponding energy level diagram of corresponding materials used in the device (reprinted with the permission from

[31], 2017, the Royal Society of Chemistry).

230 Emerging Solar Energy Materials

Society of Chemistry).

As the photocurrent hysteresis behavior is a common issue in accurate characterization of device efficiency, the photocurrent hysteresis behaviors of PSCs with/without DMF additive were measured by changing the scanning directions (reverse scan (from a positive bias 1.1 V to a negative bias −0.2 V) and forward scan (from a negative bias −0.2 V to a positive

efficiency of the device with DMF additive obtained from the *J*–*V* curve measurement.

**Figure 13.** Comparison of histograms of photovoltaic parameters for the perovskite solar cells based on MAI/IPA (red) and MAI/(IPA-0.9%DMF) (black) condition. Data from 20 cells were used for the histogram (reprinted with the permission from [31], 2017, the Royal Society of Chemistry).

**Acknowledgements**

**Author details**

**References**

Shangzheng Pang and Dazheng Chen\*

\*Address all correspondence to: dzchen@xidian.edu.cn

Microelectronics, Xidian University, Xi'an, China

6050-6051. DOI: 10.1021/ja809598r

970. DOI: 10.1126/science.aaa5760

10.1038/nature14133

Petrozza A, Snaith HJ. Science. 2013;**342**:341-344

We thank the Natural Science Foundation of Shaanxi Province (2017JQ6014), Fundamental Research Funds for the Central Universities, and Class General Financial Grant from the

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233

State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, School of

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China Postdoctoral Science Foundation (Grant No. 2016M602771).

**Figure 14.** *J–V* curves for the best device (with or without 0.9%DMF) under standard AM1.5 simulated illumination (reprinted with the permission from [31], 2017, the Royal Society of Chemistry).

#### **4. Conclusions**

Organolead halide perovskites are emerging photovoltaic materials for next-generation solar cells. To obtain high-performance PSCs with good stability, the perovskite film with improved crystallinity, homogeneity, and surface morphology is of great importance. This chapter introduces the solvent treatment mechanisms of mixed-solvent-vapor annealing and polar solvent additive in the typical one-step and two-step perovskite deposition methods. These treatments effectively improve the perovskite film quality as well as the photovoltaic performance of planar PSCs with inverted structure. In details, compared to the alone IPA solvent annealing in one-step method, the introduction of a little polar aprotic solvent such as DMF is effective to improve the device performance. The XRD and SEM analysis demonstrates that the average grain size and crystallinity of perovskite film have been increased via IPA/DMF mixed solventvapor annealing (100:1, v/v). The PCE of the CH3 NH3 PbIxCl3 -x planar heterojunction solar cell increases from 14.0% of the pristine PSC to 17.3% of the IPA PSC and further to 18.0% of the IPA/ DMF PSC and shows negligible *J* − *V* hysteresis. In addition, the PSC stability is significantly improved treated by IPA/DMF mixed-solvent vapor. Our results show that the mixed-solventvapor annealing is a simple and promising method for PSCs and other photoelectric devices. For the interdiffusion of two- step sequential deposition, a small ratio of DMF solvent addicted into the MAI/IPA solution can help the complete conversion of PbI2 into perovskite, which leads to the reduced pinholes, improved film morphology, increased grain sizes, enhanced film light absorption, and charge transport ability. The improved perovskite film quality is responsible for the enhancement of JSC and PCE for PSCs with inverted structure. Using this method, an optimized PCE as high as 19.2% was acquired for CH3 NH3 PbIxCl3 -x PSCs. In short, these two solvent treatment strategies could provide guidelines to further improve the perovskite quality and fabricate more efficient perovskite solar cells with good stability, which is essential to realize their commercialization in the future.
