**2. One-step method: prepared perovskite film**

### **2.1. Film formation**

characteristics [15], device structures and stability, and so on [16]. A high-quality perovskite film with low point defects and grain boundaries is necessary to obtain higher device PCEs. This could greatly avoid the non-radiative recombination which could cause the loss of opencircuit voltage (*V*OC) and decreased carrier lifetime [17–19]. On the other hand, a high-quality perovskite should also have good charge transport properties and slow ionic transport so that the free carriers could be effectively collected by the electrode and the current hysteresis behavior in current–voltage sweep measurements could be effectively avoided. In order to achieve high-quality perovskite films, a lot of deposition categories have been developed, such as onestep solution method, two-step solution method, and vapor deposition method [20–22]. And, this chapter will describe two effective solvent treatment mechanisms in typical one-step and two-step solution methods to obtain perovskite film with high-quality and relatively high PCEs. Firstly, the early presented one-step method has still been widely used due to the advantages of low cost, simple, and more compatible with the roll-to-roll process. It is well known that

mediate phase [23] and deposit perovskite films, and the stand-alone solvent annealing or anti-solvent annealing has been proven to be efficient for improving the perovskite quality. Here, we would like to introduce a novel solvent-engineering method, namely, the mixed-sol-

a poor solubility in anhydrous isopropanol, and the annealing in this vapor environment can result in a dense uniform and pinhole-free perovskite film. When a little polar aprotic DMF or DMSO vapor is mixed with the isopropanol vapor, after the mixed-solvent-vapor annealing

ther enhanced short-circuit current density (*J*SC), suppressed reverse dark current, reduced recombination loss in PSCs, and improved device stability. All devices with planar hetero-

Secondly, by incorporating a certain ratio of polar solvent such as N,N′-Dimethylformamide (DMF) into MAI/IPA precursor solution, we introduce a modified interdiffusion two-step

coating MAI solution, and it has never been used in two-step method to fabricate perovskite

small ratio of DMF in the MAI solution could provide a beneficial atmosphere to promote MAI

form perovskite with high quality. Simultaneously, it can also improve the surface morphology efficiently and enlarge the size of the perovskite crystal. Further, a PCE of 19.2% is achieved by the related planar heterojunction perovskite solar cells. And, this mechanism of polar solvent addition provides a facile way toward the high-quality perovskite film and high-performance devices. As we all know, the performance of perovskite solar cells (PSCs) is strongly depending on the quality of perovskite layer. Here, based on the typical one-step and two-step deposition methods, we would like to introduce the solvent treatment mechanisms of mixed-solvent-vapor annealing and polar solvent additive to investigate the growth mode and control the means of perovskite films by physical characterizations and discuss their effects on the photovoltaic

film and avoiding the PbI2

NH3 PbI3

junction structure show the efficiency over 15%. What is more, by employing CH3

sequential deposition method. As we all know, DMF could easily dissolve PbI2

perovskite precursor and interface modifying layer, the device PCE reaches around 19%.


NH3 I3 -xClx

film while spin-

residue, which is helpful to

possesses

NH3 PbI3

crystals can be further increased, thus fur-

and perovskites, it has been found that a

the annealing treatments are crucial in one-step method to transform PbI2

vent-vapor annealing in the one-step solution method. Generally, the CH3

process, the average grain size of CH3

218 Emerging Solar Energy Materials

molecules diffusing into the bottom PbI2

film. Although DMF is a typical polar solvent for PbI2

performance improvements for perovskite solar cells.

The CH3 NH3 PbI3 precursor solution was prepared by mixing 1.4 M PbI2 and 1.35 M MAI dissolved in the co-solvent of DMSO:GBL (3:7 v/v) and stirred for 2 h at 70°C. The CH3 NH3 I3 xClx precursor solution was prepared by mixing 1.26 M PbI2 , 0.14 M PbCl2 , and 1.35 M MAI was dissolved in the co-solvent of DMSO:GBL (3:7 v/v), and was stirred for 2 h at 70°C. The solution was then spin-coated onto the PEDOT:PSS layer with solvent-engineering method. Briefly, the spin-coating process was programmed to run at 1000 rpm for 15 s and then 5000 rpm for 25 s. When the spinning was at 37 s, 350 μl anhydrous toluene was injected onto the substrates. The perovskite films were solvent or thermally annealed on the hot plate at 100°C for 20 min. For the film treated with solvent annealing, the perovskite films were put on top of a hot plate and covered by a glass Petri dish. Around 40 μl of IPA, IPA:DMF (100:1 v/v) or IPA:DMSO (100:1 v/v) solvent was added around the substrates during the thermal annealing process, so that the solvent vapor could make contact with the perovskite films. More experimental details can be found in our previous work [24].

#### **2.2. Results and discussion**

The CH3 NH3 PbI3 film morphologies and surface textures are investigated by atomic force microscopy (AFM) and scanning electron microscopy (SEM). As is shown in **Figure 1**, the root-mean-square (RMS) roughness value of the pristine CH3 NH3 PbI3 film is 8.28 nm; this result is consistent with the report [23] by using the solvent-engineering method. Introducing the IPA vapor in the annealing process, the minimum RMS value of the CH3 NH3 PbI3 film is achieved. The introduced liquid anhydrous isopropanol on the hot plate turns to gas rapidly in a confined space which produces a certain anti-solvent vapor pressure and retards the crystal formation of perovskite to improve the crystalline quality [25, 26]. When the polar aprotic solvents of DMSO and DMF are introduced in the IPA vapor annealing process, the RMS values increase to 10.51 and 9.04 nm, respectively. As we all know, CH3 NH3 PbI3 is easily dissolved in DMSO and DMF, and a trace of DMSO or DMF introduced in the annealing process can induce a recrystallization process of CH3 NH3 PbI3 leading to the change of the morphology and surface. The film quality can improve by precise control of the recrystallization process. However, an excessive polar aprotic solvent vapor will produce a negative effect and reduce the film quality. As discussed above, the DMSO vapor will be released by the PbI2 -MAI-DMSO intermediate phases. With extra DMSO introduced in the annealing process, the DMSO vapor will be excessive. This causes the largest RMS value in the perovskite film, which may be one of the reasons for the lower device performance than the IPA PSCs. Therefore, the introduced DMF is more suitable than DMSO, and the corresponding devices show a better performance.

It is shown in the SEM image (**Figure 2a**) that the pristine CH3 NH3 PbI3 film has a small grain size in the range of 100–300 nm. Bright portions at the grain boundaries can be observed, which is likely to be less conductive PbI2 as in the previous reports [23]. In addition, there are also spots of pinholes on the film surface. The charge transport and the photovoltaic performance [26] are strongly influenced by these defects. The average grain size of the CH3 NH3 PbI3

has been increased with the obvious reduction of pinholes (**Figure 2b**) after treating the anhy-

and dense, and the pinholes disappear as shown in **Figure 2c** and **d** when the polar aprotic solvent of DMSO or DMF is further introduced in the annealing process. However, there is an obvious difference between the IPA/DMSO and IPA/DMF that resulted in perovskite films.

the high crystalline, large grain size, and a small grain boundary area. This will benefit the charge transport and charge collection, which could be another reason for the better perfor-

which are assigned to the (110), (220), and (310) lattice planes of the tetragonal perovskite structure, respectively. And, the improved crystallinity of the perovskite films annealed in IPA and IPA/DMF vapor has been confirmed by the stronger and sharper XRD diffraction peaks

film treated by the mixed IPA/DMF vapor shows stronger and sharper peaks, which reveals the higher crystallization. This again explains why the IPA-/DMF-treated devices acquire the

To fabricate perovskite solar cells, there are two typical device structures of mesoporous and conventional planar structure. Mesoporous device structures employing an n-type TiO2

**Figure 3.** XRD patterns of CH3NH3PbI3 films with pristine (blue), IPA/DMSO vapor (green), IPA vapor (red), and IPA/

DMF vapor (black) annealing (reprinted with the permission from [24], 2016, Elsevier).

NH3 PbI3

**Figure 3** shows the XRD patterns of pristine, IPA, and IPA/DMF CH3

NH3 PbI3

High-Quality Perovskite Film Preparations for Efficient Perovskite Solar Cells

film. The boundary defects and related recombination are reduced for

is proven by the diffraction peaks around 14.21, 28.51, and 31.88°,

, which is in line with the previous SEM results. The CH3

. Significantly, the solvent annealing reduces the small peak

films become more compact

http://dx.doi.org/10.5772/intechopen.75103

films. The for-

221

NH3 PbI3

film is obviously larger than that of the IPA/

NH3 PbI3

drous IPA vapor in the annealing process. Then, the CH3

NH3 PbI3

The grain size of the IPA/DMF CH3

NH3 PbI3

mance of IPA/DMF devices.

than that of pristine CH3

at 12.8° belonging to PbI2

best performance.

NH3 PbI3

DMSO CH3

mation of CH3

**Figure 1.** AFM images of perovskite films via (a) pristine, (b) IPA, (c) IPA/DMSO, and (d) IPA/DMF vapor annealing. The measured RMS values are (a) 8.28 nm, (b) 7.87 nm, (c) 10.51 nm, and (d) 9.04 nm (reprinted with the permission from [24], 2016, Elsevier).

**Figure 2.** SEM images of perovskite films via (a) pristine, (b) IPA, (c) IPA/DMSO, and (d) IPA/DMF vapor annealing (reprinted with permission from [24], 2016, Elsevier).

has been increased with the obvious reduction of pinholes (**Figure 2b**) after treating the anhydrous IPA vapor in the annealing process. Then, the CH3 NH3 PbI3 films become more compact and dense, and the pinholes disappear as shown in **Figure 2c** and **d** when the polar aprotic solvent of DMSO or DMF is further introduced in the annealing process. However, there is an obvious difference between the IPA/DMSO and IPA/DMF that resulted in perovskite films. The grain size of the IPA/DMF CH3 NH3 PbI3 film is obviously larger than that of the IPA/ DMSO CH3 NH3 PbI3 film. The boundary defects and related recombination are reduced for the high crystalline, large grain size, and a small grain boundary area. This will benefit the charge transport and charge collection, which could be another reason for the better performance of IPA/DMF devices.

**Figure 3** shows the XRD patterns of pristine, IPA, and IPA/DMF CH3 NH3 PbI3 films. The formation of CH3 NH3 PbI3 is proven by the diffraction peaks around 14.21, 28.51, and 31.88°, which are assigned to the (110), (220), and (310) lattice planes of the tetragonal perovskite structure, respectively. And, the improved crystallinity of the perovskite films annealed in IPA and IPA/DMF vapor has been confirmed by the stronger and sharper XRD diffraction peaks than that of pristine CH3 NH3 PbI3 . Significantly, the solvent annealing reduces the small peak at 12.8° belonging to PbI2 , which is in line with the previous SEM results. The CH3 NH3 PbI3 film treated by the mixed IPA/DMF vapor shows stronger and sharper peaks, which reveals the higher crystallization. This again explains why the IPA-/DMF-treated devices acquire the best performance.

To fabricate perovskite solar cells, there are two typical device structures of mesoporous and conventional planar structure. Mesoporous device structures employing an n-type TiO2

**Figure 3.** XRD patterns of CH3NH3PbI3 films with pristine (blue), IPA/DMSO vapor (green), IPA vapor (red), and IPA/ DMF vapor (black) annealing (reprinted with the permission from [24], 2016, Elsevier).

**Figure 2.** SEM images of perovskite films via (a) pristine, (b) IPA, (c) IPA/DMSO, and (d) IPA/DMF vapor annealing

**Figure 1.** AFM images of perovskite films via (a) pristine, (b) IPA, (c) IPA/DMSO, and (d) IPA/DMF vapor annealing. The measured RMS values are (a) 8.28 nm, (b) 7.87 nm, (c) 10.51 nm, and (d) 9.04 nm (reprinted with the permission from [24],

(reprinted with permission from [24], 2016, Elsevier).

2016, Elsevier).

220 Emerging Solar Energy Materials

layer as the bottom electron transport layer. A high-temperature (>450°C) sintering process for the TiO2 scaffold, which is a great limitation on the substrate and increases the cost, is required. On the other hand, the conventional planar structures based on TiO2 usually suffer from a large degree of J−V hysteresis. In 2013, Guo developed the first planar heterojunction perovskite solar cell with inverted structure design. The p-type layer was deposited before the perovskite film, while the n-type layer was deposited after the perovskite film [27]. This architecture was defined as p-i-n structure or inverted structure. Recent studies have shown that the inverted planar PSCs adopted in this study show negligible *J*−*V* hysteresis and promising device performance [28, 29]. Thus, the PSCs in this work adopt the inverted structure of ITO/PEDOT:PSS/CH3 NH3 PbI3 /PCBM/Ag (shown in **Figure 4(a)**), where the PCBM and PEDOT:PSS act as electron and hole transport layers, respectively.

The corresponding energy band diagram is illustrated in **Figure 4(b)**. PEDOT:PSS has the conduction band energy of around −3.0 eV and the valance band of around −5.2 eV, which suggests that holes from CH3 NH3 PbI3 can be transported to PEDOT:PSS and collected by the anode, while electrons from CH3 NH3 PbI3 can be blocked. In other words, this PEDOT:PSS acts as an electron-blocking layer and a hole extraction layer. At the same time, the PCBM layer plays the role of electron extraction layer, and it can effectively aid the electron transport to the cathode. Furthermore, it has been reported that PCBM can effectively passivate CH3 NH3 PbI3 and minimize the *J*−*V* hysteresis [30]. This structure is expected to obtain better photovoltaic performance for perovskite solar cells.

The *J*–*V* characteristics of the fabricated CH3 NH3 PbI3 PSCs are shown in **Figure 5(a)**, and their photovoltaic parameters are summarized in **Table 1**. The "pristine" represents the PSCs without the vapor treatment in the perovskite annealing process. The "IPA," "IPA/DMF (100:1 v/v)," and "IPA/DMSO (100:1v/v)" represent the PSCs treated by corresponding vapors. It can be seen that the pristine PSCs exhibit an average PCE of 11.5%, with *J*SC = 17.1 ± 0.7 mA/cm2 , *V*OC = 0.96 ± 0.02 V, and FF = 70.1 ± 1.6%, and the best one shows a PCE of 12.2% with *J*SC = 18.1 mA/cm2 , *V*OC = 0.96 V, and FF = 70.1%. It is obvious that the PCE is mainly limited by the relatively low *J*SC, which is in line with the relatively low PCE for inverted planar PSCs [23]. However, by introducing the solvent vapor in the annealing process, the resulted PSCs show a significant improvement in

performance (see **Figure 5b**). For the IPA PSCs, the greatly improved *J*SC of 19.8 ± 0.5 mA/cm2

PCE increased to 13.2% and the highest PCE of 14.2%(*J*SC = 20.9 mA/cm2

NH3 PbI3

density of IPA PSCs (blue) and IPA/DMF PSCs (red). (c) *J*–*V* characteristics of CH3

(reprinted with the permission from [24], 2016, Elsevier).

solvent annealing under the simulated AM 1.5G illumination of 100 mW/cm2

Since the perovskite formation is reversible, the transform of PbI2

DMSO vapor would further affect the recrystallization of CH3

IPA vapor treatment can help enhance the CH3

**Figure 5.** (a) *J*–*V* characteristics of CH3

to the PbI2

100 mW/cm2

cm<sup>2</sup>

obtained, as well as a slightly increased FF (70.4 ± 1.2) and *V*OC (0.98 ± 0.01 V); thus, the average

annealing and with IPA, IPA/DMSO, or IPA/DMF solvent annealing under the simulated AM 1.5G illumination of

cess and thus improve the photovoltaic performance of PSCs. When the DMSO vapor is further introduced, the IPA/DMSO PSCs show an average PCE of 12.3% with *J*SC = 19.0 ± 0.7 mA/cm2

*V*OC = 0.99 ± 0.01 V, and FF = 65.7 ± 1.8%, which is relatively inferior to the IPA PSCs. This is related

phases will release extra DMSO vapor, combining with the introduced DMSO, and the excessive

along the decomposition direction. However, when the IPA-/DMF-mixed vapor is adopted in the annealing process, the corresponding PSCs show an obviously improved *J*SC of 20.8 ± 0.6 mA/

, *V*OC of 1.02 ± 0.01 V, and FF of 67.0 ± 1.5% and the PCE average values of 14.2%, and the


NH3 PbI3

PSCs without solvent annealing and with IPA, IPA/DMSO, or IPA/DMF

High-Quality Perovskite Film Preparations for Efficient Perovskite Solar Cells

http://dx.doi.org/10.5772/intechopen.75103

223

NH3 I3

NH3 PbI3 is

,

). It is suggested that the

−xClx PSCs without solvent


by shifting the reaction

crystallinity during the annealing pro-

. (b) IPCE curves and integrated current

**Figure 4.** (a) Schematic structure of the devices in this study: ITO/PEDOT:PSS/CH3 NH3 PbI3 /PCBM/Ag. The thickness of each layer was not in scale with the real thickness for clarity. (b) Schematic illustration of energy band diagram of studied devices (reprinted with the permission from [24], 2016, Elsevier).

High-Quality Perovskite Film Preparations for Efficient Perovskite Solar Cells http://dx.doi.org/10.5772/intechopen.75103 223

layer as the bottom electron transport layer. A high-temperature (>450°C) sintering process

from a large degree of J−V hysteresis. In 2013, Guo developed the first planar heterojunction perovskite solar cell with inverted structure design. The p-type layer was deposited before the perovskite film, while the n-type layer was deposited after the perovskite film [27]. This architecture was defined as p-i-n structure or inverted structure. Recent studies have shown that the inverted planar PSCs adopted in this study show negligible *J*−*V* hysteresis and promising device performance [28, 29]. Thus, the PSCs in this work adopt the inverted structure

The corresponding energy band diagram is illustrated in **Figure 4(b)**. PEDOT:PSS has the conduction band energy of around −3.0 eV and the valance band of around −5.2 eV, which

acts as an electron-blocking layer and a hole extraction layer. At the same time, the PCBM layer plays the role of electron extraction layer, and it can effectively aid the electron transport to the cathode. Furthermore, it has been reported that PCBM can effectively passivate

> NH3 PbI3

photovoltaic parameters are summarized in **Table 1**. The "pristine" represents the PSCs without the vapor treatment in the perovskite annealing process. The "IPA," "IPA/DMF (100:1 v/v)," and "IPA/DMSO (100:1v/v)" represent the PSCs treated by corresponding vapors. It can be seen that the

and FF = 70.1%. It is obvious that the PCE is mainly limited by the relatively low *J*SC, which is in line with the relatively low PCE for inverted planar PSCs [23]. However, by introducing the solvent vapor in the annealing process, the resulted PSCs show a significant improvement in

each layer was not in scale with the real thickness for clarity. (b) Schematic illustration of energy band diagram of studied

and minimize the *J*−*V* hysteresis [30]. This structure is expected to obtain better

required. On the other hand, the conventional planar structures based on TiO2

scaffold, which is a great limitation on the substrate and increases the cost, is

/PCBM/Ag (shown in **Figure 4(a)**), where the PCBM and

can be transported to PEDOT:PSS and collected by the

NH3 PbI3

can be blocked. In other words, this PEDOT:PSS

PSCs are shown in **Figure 5(a)**, and their

, *V*OC = 0.96 ± 0.02 V,

/PCBM/Ag. The thickness of

, *V*OC = 0.96 V,

usually suffer

for the TiO2

222 Emerging Solar Energy Materials

CH3 NH3 PbI3

of ITO/PEDOT:PSS/CH3

suggests that holes from CH3

anode, while electrons from CH3

NH3 PbI3

photovoltaic performance for perovskite solar cells.

The *J*–*V* characteristics of the fabricated CH3

PEDOT:PSS act as electron and hole transport layers, respectively.

NH3 PbI3

> NH3 PbI3

pristine PSCs exhibit an average PCE of 11.5%, with *J*SC = 17.1 ± 0.7 mA/cm2

**Figure 4.** (a) Schematic structure of the devices in this study: ITO/PEDOT:PSS/CH3

devices (reprinted with the permission from [24], 2016, Elsevier).

and FF = 70.1 ± 1.6%, and the best one shows a PCE of 12.2% with *J*SC = 18.1 mA/cm2

**Figure 5.** (a) *J*–*V* characteristics of CH3 NH3 PbI3 PSCs without solvent annealing and with IPA, IPA/DMSO, or IPA/DMF solvent annealing under the simulated AM 1.5G illumination of 100 mW/cm2 . (b) IPCE curves and integrated current density of IPA PSCs (blue) and IPA/DMF PSCs (red). (c) *J*–*V* characteristics of CH3 NH3 I3 −xClx PSCs without solvent annealing and with IPA, IPA/DMSO, or IPA/DMF solvent annealing under the simulated AM 1.5G illumination of 100 mW/cm2 (reprinted with the permission from [24], 2016, Elsevier).

performance (see **Figure 5b**). For the IPA PSCs, the greatly improved *J*SC of 19.8 ± 0.5 mA/cm2 is obtained, as well as a slightly increased FF (70.4 ± 1.2) and *V*OC (0.98 ± 0.01 V); thus, the average PCE increased to 13.2% and the highest PCE of 14.2%(*J*SC = 20.9 mA/cm2 ). It is suggested that the IPA vapor treatment can help enhance the CH3 NH3 PbI3 crystallinity during the annealing process and thus improve the photovoltaic performance of PSCs. When the DMSO vapor is further introduced, the IPA/DMSO PSCs show an average PCE of 12.3% with *J*SC = 19.0 ± 0.7 mA/cm2 , *V*OC = 0.99 ± 0.01 V, and FF = 65.7 ± 1.8%, which is relatively inferior to the IPA PSCs. This is related to the PbI2 -MAI-DMSO intermediate phases and can be understood from the annealing process. Since the perovskite formation is reversible, the transform of PbI2 -MAI-DMSO intermediate phases will release extra DMSO vapor, combining with the introduced DMSO, and the excessive DMSO vapor would further affect the recrystallization of CH3 NH3 PbI3 by shifting the reaction along the decomposition direction. However, when the IPA-/DMF-mixed vapor is adopted in the annealing process, the corresponding PSCs show an obviously improved *J*SC of 20.8 ± 0.6 mA/ cm<sup>2</sup> , *V*OC of 1.02 ± 0.01 V, and FF of 67.0 ± 1.5% and the PCE average values of 14.2%, and the


Besides the efficiency of PSCs, the stability is another critical limitation for their commercial applications. The structural chemical stability of perovskite could be damaged by many factors such as interaction with moisture and oxygen especially at high temperatures. For the ITO/PEDOT:PSS/perovskite/PCBM/Ag structure, the hydrophilic and acidic nature of PEDOT:PSS is considered an unstable transport layer, also the possible oxidation of silver electrode. Here, we mainly discuss the device stability issue related to the perovskite layers. As we know, the stability of perovskite is related to its material nature [21], and also the preparation process and treatment have direct effects on the crystalline quality. For the un-encapsulated PSCs processed at different annealing conditions, we tested them in an ambient environment at 22°C with about 30% humidity. The degradation of key photovoltaic parameters of PCE, *J*SC, *V*OC, and FF are shown in **Figure 6**. After 8 days in air, the PCE of the pristine PSC kept 40% of the initial efficiency, with FF and *J*SC reduced to 59 and 70% of the original values. It should be noted that the device stability was significantly improved for the PSCs treated by IPA vapor and IPA/DMF mixed solvent vapor. And, the PCEs could keep 65 and 74% of the initial values after 8 days. This indicates the relationship of perovskite quality and device stability and provides a strategy to obtain high-efficient

High-Quality Perovskite Film Preparations for Efficient Perovskite Solar Cells

http://dx.doi.org/10.5772/intechopen.75103

225

**Figure 6.** Stability of the devices with the structure of ITO/PEDOT:PSS/perovskite/PCBM/Ag: (a) normalized PCE, (b) normalized *V*OC, (c) normalized *J*SC, and (d) normalized FF (reprinted with the permission from [24], 2016,

PSCs with good stability.

Elsevier).

**Table 1.** Photovoltaic parameters of CH3 NH3 PbI3 and CH3 NH3 I3 -xClx PSCs under simulated AM 1.5G illumination (100 mW/cm2 ) (reprinted with the permission from [24], 2016, Elsevier).

best one obtains a PCE of 15.1% and a *J*SC of 22.7 mA/cm2 . Compared with the PSCs with alone IPA vapor, it is clear that both *J*SC and *V*OC are strikingly enhanced for PSCs treated with IPA-/ DMF-mixed vapor. In this work, the optimized ratio of IPA/DMF is 100:1(v/v), while the excessive DMF will be also destructive to the perovskite formations [24]. Additionally, the incident phototo-electron conversion efficiency (IPCE) curves and the integrated current density are shown in **Figure 5b**. It is clear that the PSC treated with IPA-/DMF-mixed vapor shows a higher IPCE value at most wavelengths, as well as the largely integrated current density of 20.3 mA/cm2 , which is very close to the measured *J*SC in *J–V* characteristic.

To further improve the photovoltaic performance of inverted PSCs, the CH3 NH3 I3 -xClx precursor and BCP interface layer have been employed, and the resulted PSCs show a structure of ITO/PEDOT:PSS/CH3 NH3 I3 -xClx/PCBM/BCP/Ag. The measured photovoltaic parameters are summarized in **Table 1**. Without solvent annealing treatment, the pristine CH3 NH3 I3 -xClx device exhibits a relatively poor performance with *J*SC = 19.0 ± 0.82 mA/cm2 , *V*OC = 0.98 ± 0.01 V, and FF = 79.2 ± 0.6% and an average PCE of 14.0%. With anti-solvent vapor treatment in the annealing process, the performance of CH3 NH3 I3 -xClx device has been significantly improved compared with the pristine devices. For the IPA CH3 NH3 I3 -xClx PSCs, the *J*SC is greatly improved to 20.83 ± 0.77 mA/cm2 with nearly unchanged *VOC* and FF. Thus, the average PCE of 17.3% and the highest PCE of 18.1% are achieved. Compared to the IPA CH3 NH3 I3 -xClx device, IPA/DMF CH3 NH3 I3 -xClx device shows a higher average PCE of 18.0% (the best device shows PCE of 18.9%) with *V*OC = 1.02 ± 0.01 V, *J*SC = 22.23 ± 0.50 mA/ cm<sup>2</sup> , and FF = 80.6 ± 1.3%.

Besides the efficiency of PSCs, the stability is another critical limitation for their commercial applications. The structural chemical stability of perovskite could be damaged by many factors such as interaction with moisture and oxygen especially at high temperatures. For the ITO/PEDOT:PSS/perovskite/PCBM/Ag structure, the hydrophilic and acidic nature of PEDOT:PSS is considered an unstable transport layer, also the possible oxidation of silver electrode. Here, we mainly discuss the device stability issue related to the perovskite layers. As we know, the stability of perovskite is related to its material nature [21], and also the preparation process and treatment have direct effects on the crystalline quality. For the un-encapsulated PSCs processed at different annealing conditions, we tested them in an ambient environment at 22°C with about 30% humidity. The degradation of key photovoltaic parameters of PCE, *J*SC, *V*OC, and FF are shown in **Figure 6**. After 8 days in air, the PCE of the pristine PSC kept 40% of the initial efficiency, with FF and *J*SC reduced to 59 and 70% of the original values. It should be noted that the device stability was significantly improved for the PSCs treated by IPA vapor and IPA/DMF mixed solvent vapor. And, the PCEs could keep 65 and 74% of the initial values after 8 days. This indicates the relationship of perovskite quality and device stability and provides a strategy to obtain high-efficient PSCs with good stability.

best one obtains a PCE of 15.1% and a *J*SC of 22.7 mA/cm2

NH3 PbI3

) (reprinted with the permission from [24], 2016, Elsevier).

**CH3NH3PbI3 PSCs** *V***OC (V)** *JSC*

224 Emerging Solar Energy Materials

CH3NH3I3-xClx PSCs *V*OC (V) *JSC*

very close to the measured *J*SC in *J–V* characteristic.

the *J*SC is greatly improved to 20.83 ± 0.77 mA/cm2


structure of ITO/PEDOT:PSS/CH3

The average results were based on ten devices.

**Table 1.** Photovoltaic parameters of CH3

(100 mW/cm2

, and FF = 80.6 ± 1.3%.

CH3 NH3 I3

CH3 NH3 I3

cm<sup>2</sup>

IPA vapor, it is clear that both *J*SC and *V*OC are strikingly enhanced for PSCs treated with IPA-/ DMF-mixed vapor. In this work, the optimized ratio of IPA/DMF is 100:1(v/v), while the excessive DMF will be also destructive to the perovskite formations [24]. Additionally, the incident phototo-electron conversion efficiency (IPCE) curves and the integrated current density are shown in **Figure 5b**. It is clear that the PSC treated with IPA-/DMF-mixed vapor shows a higher IPCE value

NH3 I3

**mA/cm2**

mA/cm2

Pristine 0.96 ± 0.02 17.14 ± 0.71 70.1 ± 1.6 11.5(12.2) IPA 0.98 ± 0.01 19.85 ± 0.54 70.4 ± 1.2 13.2(14.7) IPA/DMSO 0.99 ± 0.01 19.03 ± 0.73 65.7 ± 1.8 12.3(13.1) IPA/DMF 1.02 ± 0.01 20.81 ± 0.56 67.0 ± 1.5 14.2(15.1)

Pristine 0.98 ± 0.01 19.00 ± 0.82 79.2 ± 0.6 14.0(14.3) IPA 1.00 ± 0.01 20.83 ± 0.77 81.5 ± 0.5 17.3(18.1) IPA/DMSO 1.00 ± 0.01 20.30 ± 0.58 78.0 ± 1.5 15.9(16.3) IPA/DMF 1.02 ± 0.01 22.23 ± 0.50 80.6 ± 1.3 18.0(18.9)

and CH3

precursor and BCP interface layer have been employed, and the resulted PSCs show a

parameters are summarized in **Table 1**. Without solvent annealing treatment, the pristine

*V*OC = 0.98 ± 0.01 V, and FF = 79.2 ± 0.6% and an average PCE of 14.0%. With anti-solvent

the average PCE of 17.3% and the highest PCE of 18.1% are achieved. Compared to the IPA

18.0% (the best device shows PCE of 18.9%) with *V*OC = 1.02 ± 0.01 V, *J*SC = 22.23 ± 0.50 mA/

NH3 I3


at most wavelengths, as well as the largely integrated current density of 20.3 mA/cm2

To further improve the photovoltaic performance of inverted PSCs, the CH3

NH3 I3

significantly improved compared with the pristine devices. For the IPA CH3

vapor treatment in the annealing process, the performance of CH3

. Compared with the PSCs with alone


**FF (%) PCE (%)**

FF (%) PCE (%)


NH3 I3

with nearly unchanged *VOC* and FF. Thus,


, which is

,

NH3 I3 -xClx



NH3 I3

**Figure 6.** Stability of the devices with the structure of ITO/PEDOT:PSS/perovskite/PCBM/Ag: (a) normalized PCE, (b) normalized *V*OC, (c) normalized *J*SC, and (d) normalized FF (reprinted with the permission from [24], 2016, Elsevier).
