Photovoltaic parameters of CH3NH3PbI3 and CH3NH3I3-xClx PSCs under simulated AM 1.5G illumination (100 mW/cm2) (reprinted with the permission from , 2016, Elsevier).
Solar cells employing organolead halide perovskite films have caught tremendous attention, and their power conversion efficiencies were stunning from 3.9% to over 22% in only 6 years. Various research reports have shown that effective controls on perovskite crystallinity, homogeneity, and surface morphology are crucial to improving the power conversion efficiencies (PCE) of perovskite solar cells. 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, investigate the growth mode and control means of perovskite films by physical characterizations, and discuss their effects on the photovoltaic performance improvements for perovskite solar cells.
- perovskite solar cell
- one-step deposition
- solvent annealing
- two-step deposition
- solvent additive
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, 2, 3, 4, 5, 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, 8, 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 , hysteresis characteristics , device structures and stability, and so on . 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 open-circuit voltage (
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  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 CH3NH3PbI3 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 CH3NH3PbI3 crystals can be further increased, thus further enhanced short-circuit current density (
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 spin-coating 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.
2. One-step method: prepared perovskite film
2.1. Film formation
The CH3NH3PbI3 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 CH3NH3I3-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 .
2.2. Results and discussion
The CH3NH3PbI3 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 CH3NH3PbI3 film is 8.28 nm; this result is consistent with the report  by using the solvent-engineering method. Introducing the IPA vapor in the annealing process, the minimum RMS value of the CH3NH3PbI3 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, CH3NH3PbI3 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 CH3NH3PbI3 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 CH3NH3PbI3 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 . In addition, there are also spots of pinholes on the film surface. The charge transport and the photovoltaic performance  are strongly influenced by these defects. The average grain size of the CH3NH3PbI3has been increased with the obvious reduction of pinholes (Figure 2b) after treating the anhydrous IPA vapor in the annealing process. Then, the CH3NH3PbI3 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 CH3NH3PbI3 film is obviously larger than that of the IPA/DMSO CH3NH3PbI3 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 CH3NH3PbI3 films. The formation of CH3NH3PbI3 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 CH3NH3PbI3. Significantly, the solvent annealing reduces the small peak at 12.8° belonging to PbI2, which is in line with the previous SEM results. The CH3NH3PbI3 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 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 . 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
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 CH3NH3PbI3 can be transported to PEDOT:PSS and collected by the anode, while electrons from CH3NH3PbI3 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 CH3NH3PbI3 and minimize the
|FF (%)||PCE (%)|
|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)|
|FF (%)||PCE (%)|
|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)|
To further improve the photovoltaic performance of inverted PSCs, the CH3NH3I3-xClx precursor and BCP interface layer have been employed, and the resulted PSCs show a structure of ITO/PEDOT:PSS/CH3NH3I3-xClx/PCBM/BCP/Ag. The measured photovoltaic parameters are summarized in Table 1. Without solvent annealing treatment, the pristine CH3NH3I3-xClx device exhibits a relatively poor performance with
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 , 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,
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 .
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, 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  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 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 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 electron-blocking 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.
Based on the high some batches of devices were fabricated, and Figure 12(a) displays typical
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
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
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 solvent-vapor annealing (100:1, v/v). The PCE of the CH3NH3PbIxCl3-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
We thank the Natural Science Foundation of Shaanxi Province (2017JQ6014), Fundamental Research Funds for the Central Universities, and Class General Financial Grant from the China Postdoctoral Science Foundation (Grant No. 2016M602771).