**1. Introduction**

Organolead halide perovskites are emerging materials with outstanding optoelectronic properties of high absorption coefficient, broad absorption, range, adjustable band gap, solution processing, and so on [1–6]. Employing this kind of material, solar cells have caught tremendous attention, and their power conversion efficiencies (PCEs) have dramatically increased from 3.8% to over 22% in only 6 years [1, 7–9]. This great progress mainly comes from the effective controls on perovskite crystallinity, homogeneity, and surface morphology, and many researchers have focused on the first-principles modeling molecular motion and dynamic crystal structure [10, 11], defect physics [12, 13], ionic conductivity [14], hysteresis

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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.

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

xClx precursor solution was prepared by mixing 1.26 M PbI2

experimental details can be found in our previous work [24].

root-mean-square (RMS) roughness value of the pristine CH3

increase to 10.51 and 9.04 nm, respectively. As we all know, CH3

It is shown in the SEM image (**Figure 2a**) that the pristine CH3

induce a recrystallization process of CH3

which is likely to be less conductive PbI2

the IPA vapor in the annealing process, the minimum RMS value of the CH3

precursor solution was prepared by mixing 1.4 M PbI2

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

microscopy (AFM) and scanning electron microscopy (SEM). As is shown in **Figure 1**, the

result is consistent with the report [23] by using the solvent-engineering method. Introducing

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

in DMSO and DMF, and a trace of DMSO or DMF introduced in the annealing process can

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

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.

size in the range of 100–300 nm. Bright portions at the grain boundaries can be observed,

also spots of pinholes on the film surface. The charge transport and the photovoltaic perfor-

mance [26] are strongly influenced by these defects. The average grain size of the CH3

NH3 PbI3

the film quality. As discussed above, the DMSO vapor will be released by the PbI2

film morphologies and surface textures are investigated by atomic force

NH3 PbI3

> NH3 PbI3

NH3 PbI3

as in the previous reports [23]. In addition, there are

leading to the change of the morphology

dissolved in the co-solvent of DMSO:GBL (3:7 v/v) and stirred for 2 h at 70°C. The CH3

and 1.35 M MAI

, and 1.35 M MAI

film is 8.28 nm; this

is easily dissolved


NH3 PbI3

film has a small grain

film is

NH3 PbI3

, 0.14 M PbCl2

High-Quality Perovskite Film Preparations for Efficient Perovskite Solar Cells

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

NH3 I3 - 219

**2.1. Film formation**

NH3 PbI3

**2.2. Results and discussion**

NH3 PbI3

The CH3

The CH3

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 the annealing treatments are crucial in one-step method to transform PbI2 -MAI-DMSO intermediate 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-solvent-vapor annealing in the one-step solution method. Generally, the CH3 NH3 PbI3 possesses 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 process, the average grain size of CH3 NH3 PbI3 crystals can be further increased, thus further enhanced short-circuit current density (*J*SC), suppressed reverse dark current, reduced recombination loss in PSCs, and improved device stability. All devices with planar heterojunction structure show the efficiency over 15%. What is more, by employing CH3 NH3 I3 -xClx perovskite precursor and interface modifying layer, the device PCE reaches around 19%.

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 sequential deposition method. As we all know, DMF could easily dissolve PbI2 film while spincoating MAI solution, and it has never been used in two-step method to fabricate perovskite film. Although DMF is a typical polar solvent for PbI2 and perovskites, it has been found that a small ratio of DMF in the MAI solution could provide a beneficial atmosphere to promote MAI molecules diffusing into the bottom PbI2 film and avoiding the PbI2 residue, which is helpful to 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 performance improvements for perovskite solar cells.
