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Room-Temperature Molten Salt for Facile Fabrication of Efficient and Stable Perovskite Solar Cells in Ambient Air
Article
Room-Temperature Molten Salt for Facile Fabrication of Efficient and Stable Perovskite Solar Cells in Ambient Air Lingfeng Chao, Yingdong Xia, Bixin Li, Guichuan Xing, Yonghua Chen, Wei Huang [email protected] (Y.C.) [email protected] (W.H.)
HIGHLIGHTS Room-temperature molten salt for facile fabrication of perovskite films in air No post-deposition treatment is needed (e.g., anti-solvent) The resulting perovskite solar cells are highly efficient and stable A universal solvent for all common salts used in perovskite devices
A solvent is vital to the control of crystallization and crystal growth in state-of-theart solution-processed hybrid organic-inorganic perovskites. We demonstrate an alternative environmentally friendly room-temperature molten salt, methylammonium acetate (MAAc), as a solvent characterized by high viscosity, negligible vapor pressure, and nonhazardous nature, which can be used to produce highly efficient perovskite solar cells (PSCs) in ambient air. The resulting PSCs exhibited excellent stability under light and dark conditions.
Chao et al., Chem 5, 1–12 April 11, 2019 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.chempr.2019.02.025
Please cite this article in press as: Chao et al., Room-Temperature Molten Salt for Facile Fabrication of Efficient and Stable Perovskite Solar Cells in Ambient Air, Chem (2019), https://doi.org/10.1016/j.chempr.2019.02.025
Article
Room-Temperature Molten Salt for Facile Fabrication of Efficient and Stable Perovskite Solar Cells in Ambient Air Lingfeng Chao,1,6 Yingdong Xia,1,6 Bixin Li,2 Guichuan Xing,3 Yonghua Chen,1,* and Wei Huang1,4,5,7,*
SUMMARY
The Bigger Picture
Here, we report an alternative environmentally friendly room-temperature molten salt, methylammonium acetate (MAAc), as a novel solvent for facile fabrication of perovskite solar cells (PSCs) in ambient air. MAAc possesses excellent chemical properties along with high viscosity, negligible vapor pressure, and a nonhazardous nature. Complete solubility of both methylammonium and lead salts by hydrogen bonds in MAAc was observed. Dense and pinholefree perovskite films with high reproducibility can be readily prepared by a simple one-step method without an anti-solvent even under a relative humidity of over 80%. Under optimized processing conditions, we achieved an average power conversion efficiency of 18.42% and a maximum efficiency of 20.05% in CH3NH3PbI3-based planar heterojunction structure. In addition, devices without encapsulation remained above 93% of their original efficiency for more than 1,000 h in ambient air. These findings may open up the possibility of developing a new approach for further improving PSC performance with higher reproducibility and reliability in ambient atmosphere.
Organic-inorganic hybrid perovskites have recently attracted extensive attention for their potential uses in photovoltaics. However, solventhandling issues and toxicology concerns represent a major challenge in solution-processed perovskite thin films. Here, we demonstrate that an environmentally- and industrially friendly room-temperature molten salt (RTMS), methylammonium acetate (MAAc), can produce high-quality perovskite films in ambient air, leading to the development of perovskite solar cells (PSCs) with 20% efficiency and stability for more than 1,000 h. MAAc is a universal solvent for all common perovskite-based salts and may open up the potential of using RTMS solvent for facile fabrication of PSCs. Moreover, this work represents a new direction in the development of efficient and stable PSCs and other optoelectronic devices.
INTRODUCTION Because of their high absorption coefficient,1,2 high carrier mobility,3 long carrier diffusion length,4,5 suitable band gap,6 small exciton binding energy,7 and low trap densities,8,9 organic-inorganic hybrid perovskites have recently attracted extensive attention for thin-film photovoltaics. Perovskite solar cells (PSCs) have demonstrated unprecedented progress in power conversion efficiency (PCE) from 3.8% to a certified 23.3% in just a few years.10,11 The low cost and ease of solution processing are promising indicators for solar energy at scale,12,13 which allow PSCs to outperform inorganic thin-film solar cells and become leaders in the field of solar photovoltaics with lower energy payback time than their crystalline silicon counterparts.14 Despite the success in obtaining excellent photovoltaic performance, the toxic, flammable, and highly coordinated solvents, e.g., N,N-dimethylformamide (DMF),15,16 dimethyl sulfoxide (DMSO),17 N-methyl-2-pyrrolidone (NMP),18 g-butyrolactone (GBL),19 and dimethylacetamide (DMA),20 still present an open problem. To date, highly efficient PSCs have to be fabricated via solution techniques that involve solvents with high boiling points, polar solvents, and aprotic solvents. However, DMF, DMA, NMP, and GBL are highly toxic solvents that can cause serious harm to the human body and the environment, and DMSO is a flammable solvent that can easily cause accidents.21 Moreover, high-quality perovskite or perovskite intermediate films are achieved by the timely application of an anti-solvent, such as toluene or chlorobenzene, to the precursor solution.22 On one hand, these solvents
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pose an irreversible danger. On the other hand, with the generally complex processes involved in perovskite formation,23 the process for making high-quality perovskite films is technically hard to control because of the very narrow time window for properly adding the anti-solvent, which results in serious reproducibility problems and the notorious variations among different groups. Therefore, solvent-handling issues and toxicology concerns represent a major challenge in the perovskite community. Recently, an ionic liquid methylammonium formate and a methylamine (MA)-acetonitrile mixed solvent were employed to prepare perovskite films, which paves the way to advanced solvent engineering for the perovskite community.24,25 However, high-quality perovskite films have to be processed in inert atmosphere, which hampers the real production and applications of PSCs. Here, we report an alternative environmentally friendly room-temperature molten salt (RTMS), methylammonium acetate (MAAc), as a solvent characterized by high viscosity, negligible vapor pressure, and a nonhazardous nature for facile fabrication of PSCs in ambient air. The MAAc allows the perovskite film to be produced in a simple one-step process by directly dissolving lead and organic ammonium salts. We found a quick liquid-to-solid transition of perovskite through a deformed perovskite intermediate structure, which is remarkably different from the traditional perovskiteprocessing solvents (e.g., DMF and DMSO), involving plumbate intermediates during crystallization. A dense, pinhole-free, and high-quality perovskite film is obtained in the absence of any auxiliary crystallizing solvent. Moreover, the film undergoes no obvious changes under the humidity of 80% for more than 5 months. Upon optimization, we achieved a PCE of 20.05% in the CH3NH3PbI3-based planar heterojunction structure. Most importantly, the device is stable in air for more than 1,000 h without encapsulation.
RESULTS AND DISCUSSION Synthesis and Characterization of RTMS MAAc The synthetic route of RTMS MAAc is shown in Figure S1A. The chemical structure of MAAc is similar to that of methylammonium iodide (MAI). Specifically, acetic acid (HAc) and MA were mixed in a molar ratio of 1:1 and stirred for 2 h under an ice-water bath. The experimental details are shown in the Experimental Procedures. In order to determine the final synthesized product, proton nuclear magnetic resonance (1H NMR) spectroscopy was conducted by dissolving the product in deuterated DMSO (Figure S1B). The hydrogen on the MA nitrogen atom (a), the methyl hydrogen on the acetic acid (b), and the methyl hydrogen on the MA (c) exhibited an area ratio of around 1:1:1, confirming that the substance structure is the same as that shown in Figure S1A. MAAc is a colorless, transparent, viscous liquid (Figure S1B, inset), which is actually an ionic liquid. In order to investigate the properties of MAAc, the ultraviolet-visible (UV-vis) spectrum was characterized, and a distinct absorption peak at 324 nm was observed (Figure S1C). This is mainly due to the electron transition between the nitrogen atom and the carbonyl group (C=O). The thermal stability of MAAc was also characterized by thermogravimetric analysis (TGA), as shown in Figure S1D. We found that MAAc reached the maximum rate of weight loss at 164 C, indicating that MAAc is more stable in the liquid state than the crystalline state, as demonstrated in our previous report.26 In order to demonstrate the TGA plot is indicative of the evaporation of an ionic liquid, we conducted detailed analysis of the TGA plot of MAAc in comparison to HAc (Figure S1E) and the mixture of HAc and MA (Figure S1F). The HAc starts to lose weight at the very beginning and reaches the maximum rate of weight loss at 99.92 C, which is near its boiling point (117.9 C). Moreover, two inflection points at 95.3 C and 158.2 C emerged in the
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1Key
Laboratory of Flexible Electronics (KLOFE) & Institution of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), Nanjing, Jiangsu 211816, China
2Department
of Educational Science, Laboratory of College Physics, Hunan First Normal University, Changsha, Hunan 410205, China
3Institute
of Applied Physics and Materials Engineering, University of Macau, Macau, Macao SAR 999078, China
4Shaanxi
Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), Xi’an, Shaanxi 710072, China
5Key
Laboratory for Organic Electronics & Information Displays (KLOEID), and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, Nanjing, Jiangsu 210023, China
6These 7Lead
authors contributed equally
Contact
*Correspondence: [email protected] (Y.C.), [email protected] (W.H.) https://doi.org/10.1016/j.chempr.2019.02.025
Please cite this article in press as: Chao et al., Room-Temperature Molten Salt for Facile Fabrication of Efficient and Stable Perovskite Solar Cells in Ambient Air, Chem (2019), https://doi.org/10.1016/j.chempr.2019.02.025
Figure 1. Characteristics of Perovskite Precursor Based on RTMS MAAc (A and B) DLS spectra of (A) MAAc and (B) DMF perovskite precursor solution with different concentrations from 10 to 300 mg mL 1 . (C) A picture of perovskite precursor solution (300 mg mL 1 ) of traditional solvents DMF, DMSO, and NMP as a comparison to MAAc. (D and E) FTIR spectra of perovskite precursor (D) DMF and (E) MAAc solution.
mixture of HAc and MA solvent, indicative of a large amount of decomposition or residual by the derivative thermogravimetry curve. The two inflection points at 95.3 C and 158.2 C are near the boiling points of HAc and MAAc, respectively, which indicates that a little MAAc was generated in the mixture of HAc and MA solvent. The decomposition of MA is not detected because of the extremely low boiling point (6.8 C). Therefore, it is concluded that MAAc is an ionic liquid and does not undergo degradation to HAc and MA during TGA measurement. Characteristics of Perovskite Precursor Based on RTMS MAAc The uniform and dense film is highly related to the dispersion of the crystal nuclei in the solution.27,28 When the crystal nuclei are uniformly dispersed in the precursor solution, they have the same growth ability during the film formation process, leading to uniform crystals with similar size. On the contrary, crystal grains with different sizes are finally formed if the crystal nuclei size is different, which may cause a rough surface. We configured different concentrations (10 to 300 mg mL1) of perovskite precursor solution in MAAc and DMF solvents. Under room temperature, the perovskite solutions in different solvents were analyzed by dynamic light scattering (DLS), as shown in Figures 1A and 1B. The micellar size in MAAc and DMF increased with increasing concentration. However, the distribution is not uniform in DMF solvent. Interestingly, the micellar size in MAAc is concentrated at certain sizes, demonstrating the uniform distribution of micellar in MAAc solution. Most importantly, MAAc has a higher total colloid concentration in the same concentration of perovskite precursor solution. This finding strongly proves that MAAc has weak interaction with the perovskite precursor PbI2 and MACl to form agglomerates. Furthermore, the perovskite precursor in DMF, DMSO, and NMP show a clear yellow solution,
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whereas it appears as light yellow in MAAc, as shown in Figure 1C. This is mainly due to the fact that DMF, DMSO, and NMP molecules intercalate into the layered structures of PbI2 in traditional solvents and induce a little expansion in the PbI2 lattice along the c axis,26 which improves the solubility of PbI2. However, a large number of MAAc molecules form hydrogen bonds with PbI2 and MACl by the O atom at the end of MAAc molecules because the O atom is more electronegative than the N atom,29 which in turn increases the colloid concentration of the precursor solution (Figure S2). The characteristics of the perovskite-MAAc solution were investigated by Fourier transform infrared spectroscopy (FTIR), as shown in Figure 1D. It can be seen that the perovskite-MAAc solution showed a broad peak at 2,500– 3,500 cm1 with respect to the original sole MAAc solvent. This is mainly because the H----O-C hydrogen bond in the MAAc solvent is replaced by stronger hydrogen bonds formed by the addition of PbI2 and MACl, where multiple molecules were involved with uniform distribution, consistent with the DLS measurement. However, the DMF-dissolved perovskite did not show similar peaks but only the C-H vibration peak (Figure 1E). Furthermore, the formation of MAPbI3-xAcx alloy with PbI2 cannot be ruled out since MA+ and Ac can be incorporated into the PbI2 lattice.26 In addition, the perovskite precursor MAAc solution has good stability. We compared the color changes of the two MAAc and DMF solutions after 15 days. The MAAc solution remained in its original state without color change (Figures S3A and S3B). However, the solution in DMF turned from yellow to orange after 15 days of storage (Figures S3C and S3D), which can be attributed to the oxidation of the iodide from I to I3. Morphology of Perovskite Films Fabricated from RTMS MAAc RTMS MAAc, with a better thermal stability over traditional solvent, possesses negligible vapor pressure until near its boiling point as an ionic liquid,25,30 which is important for a perovskite crystallization medium. These important properties of RTMS MAAc allow for easy control of crystalline film formation by temperature. At present, production of high-quality perovskite films based on traditional solvents is strictly limited to both the nitrogen atmosphere and the use of a large number of anti-solvents.31–33 Here, pinhole-free, smooth-surfaced, and completely covered films can be produced by a one-step spin-coating process from our MAAc-solvent-based perovskite precursor solutions (PbI2:MACl = 1:1 mole ratio, MACl: CH3NH3Cl) under ambient air conditions. We developed a simple and effective annealing-assisted solution process (AASP) method that keeps a constant substrate temperature during the solution processing and incorporates excess solvent into the annealing process under the crystallization temperature. The basic steps for preparing perovskite film are schematically described in Figure S4. To illustrate the quality of the films, the surface scanning electron microscope (SEM) images of films fabricated at different substrate temperatures were investigated, as shown in Figure 2A. Highly dense films with large crystal grain and without pinholes were obtained at temperatures of 70 C, 80 C, 90 C, and 100 C. The surface roughness of the perovskite films at temperatures of 100 C was 32.81 nm, which corresponds to the SEM images (Figure S5). However, the film showed cracked grain at 110 C, while the cracking of the film at 120 C was prominent and a nonuniform surface was observed. We proposed that the constant substrate temperature allows the solvent to evaporate immediately and simultaneously leads to supersaturated perovskite solution, which promotes fast nucleation and growth of the crystals. An immediate color change from colorless to light brown was observed in the first 3–4 s during spin coating, indicating the formation of perovskite. Moreover, the low vapor pressure of MAAc solvent allows the perovskite crystal to be enclosed by excess solvent under the crystallization temperature, enabling well-controlled crystal growth. However, when the temperature (110 C and 120 C) is higher than the perovskite crystallization temperature, the
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Figure 2. Characteristics of Perovskite Films Fabricated from RTMS MAAc (A) SEM images of perovskite films fabricated under different substrate temperatures. The scale bar represents 500 nm. (B) XRD pattern evolution of perovskite films placed in air. (C) 1 H NMR spectrum of perovskite powder. (D) FTIR spectra of perovskite powder.
crystal growth spins out of control. The perovskite growth process is obviously controlled by substrate temperature. Furthermore, the perovskite film produced by MAAc solvent was placed in the natural environment to verify the moisture and oxygen stability of this film. Surprisingly, no significant degradation of perovskite was observed for more than 3,000 h (humidity 60%–80%) as is evident from the absence of PbI2 peaks, indicating remarkable improved stability (Figure 2B). Therefore, a mechanism was proposed. In the perovskite film formation process, MAAc would prefer to self-assemble with PbI2 to form an intermediate phase quickly, as is evident from perovskite-like phase formation (Figure S6). The intermediate phase formed by MAAc is unstable, which is easily decomposed to PbI2 because of the evaporation of MAAc under continuous heat (Figure S6). The
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supersaturation of the precursor can thus be produced with MAAc evaporation, and MACl then enters the crystal lattice at the same time, accompanied by perovskite crystal formation on the substrate. Accordingly, MAAc not only plays the role of solvent but also promotes the rapid crystallization of perovskite as a media. It should be noted that the whole process is accomplished in ambient air irrespective of the humidity (humidity ranges from 30% to 80%), breaking the limitations of traditional solvents. This may be attributed to the high viscosity of MAAc, which shows little variation with humidity, protecting the perovskite from moisture. One thing that needs to be stated is that a small amount of MAAc residue in the final perovskite film was verified by 1H NMR (Figure 2C) and FTIR (Figure 2D), where the clear methyl hydrogen spectrum of the acetic acid at 2.38 ppm and the C=O stretching vibration from CH3COO were observed. Moreover, this was further confirmed by the presence of a small amount of O element on the surface of the film, which was detected by energy dispersive spectrometer (EDS) mapping (Figure S7). This is because it is difficult for MAAc to escape from the final perovskite film because of its high viscosity as previously reported.34 Crystal Structures and Optical Properties The substrate temperature plays an important role in the morphology of perovskite films. Accordingly, the X-ray diffraction (XRD) patterns of perovskite films at different substrate temperatures were investigated (Figure S8A). Sharp diffraction peaks of the (110), (220), and (330) diffractions at 14.21 , 28.55 , and 43.33 , respectively, were observed, arising from the tetragonal MAPbI3 perovskite.25 Moreover, two distinct peaks at 15.87 and 31.98 appeared as the representative of (110) and (220) diffractions of the MAPbCl3 phase, respectively, which is consistent with the previous report employing the same raw materials of PbI2 and MACl.35 These findings demonstrated that perovskite films fabricated by MAAc solvents have the same crystal structure as conventional solvents,36,37 indicating that MAAc-based perovskite film were well fabricated and highly oriented with a-axis self-assembly. Interestingly, the ratio of (110) diffractions peak intensity of MAPbCl3/MAPbI3 is gradually increased, and MAPbCl3/MAPbI3 reaches a ratio of 1:1 at 100 C, which declines to become stable when the temperature is further increased (Figure S9). It has been suggested that MAPbCl3 is the kinetically favored phase, whereas MAPbI3 is the thermodynamically favored product,38 demonstrating that the AASP is necessary in preparing perovskite films based on the raw materials of PbI2 and MACl. MAPbCl3 has a wide band gap (3.11 eV),39 and the presence of MAPbCl3 can be recognized as an ‘‘impurity’’ from the crystal and could cause heterojunction structure with the MAPbI3 main phase, thereby resulting in variations in the optical and electronic properties (Figure S8B)39,40 and lowering the device efficiency. Therefore, it is highly desirable to lower the MAPbCl3 perovskite phase as little as possible. Considering the perovskite crystallization temperature, the ratio of the two perovskites, as well as the low-temperature preparation process, we fixed the substrate temperature at 100 C to further characterize the XRD patterns and absorption of perovskite films from different loadings of MAAc solution. The change in perovskite concentrations has a little impact on the nature of the crystals (Figure S8C) and a significant increase of absorption intensity because of the increased thickness (Figure S8D); however, the same effect was observed as the substrate temperature increased, in that the MAPbCl3 phase was gradually increased with increasing the concentration of perovskite MAAc solution (Figure S10). Photovoltaic Performance Having shown that high-quality films with flat, dense, pinhole-free, and excellent optoelectronic properties can be fabricated and applied, we move on to integrate
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Figure 3. Device Performance for MAPbI3-Based PSCs (A) Device structure of planar heterojunction PSCs. (B) J-V curves of best-performing PSCs measured in ambient air under 100 mW cm 2 AM1.5G illumination. (C) J-V curves under reverse and forward scanning directions. (D) J-V curves under different voltage sweep rates. (E) Stability of device without encapsulation under dark and light conditions. (F) PCE histogram of 30 devices fitted with a Gaussian distribution.
these MAAc-based perovskite films into PSCs. We first fabricate PSCs with a planar n-i-p heterojunction configuration of ITO, 30 nm C60 pyrrolidine tris-acid (CPTA), 5 nm butylamine hydrochloride (BACl), 500 nm perovskite, 100 nm 2,20 ,7,70 -tetrakis (N,N-di-methoxyphenylamine)-9,90 -spirobifluorene (Spiro-OMeTAD), 1 nm MoO3, and 100 nm Au (Figure 3A). Note that the perovskite films were prepared by a simple AASP approach under ambient air conditions. The photovoltaic performance of PSCs was measured under the simulated air mass 1.5 global (AM1.5G) solar irradiation. It should be noted that the substrate temperature (Figure S11; Table S1), the perovskite precursor solution concentration (Figure S12; Table S2), and the annealing temperature and time (Figure S13; Table S3) play an important role in the device’s performance. The best performance was achieved from the photocurrentvoltage (J-V) curve with a maximum PCE of 20.05%, a high open-circuit voltage (Voc) of 1.11 V, a short-circuit current density (Jsc) of 23.16 mA cm2, and a fill factor
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Table 1. Photovoltaic Parameters of PSCs with n-i-p and p-i-n Configurations Device Configuration
Scanning Direction
Voc (V)
Jsc (mA cm2)
FF (%)
PCE (%)
n-i-p
forward
1.090 G 0.02
22.36 G 0.8
76.01 G 2.0
18.5 G 0.8 (20.05)
reverse
1.090 G 0.02
22.25 G 0.9
75.41 G 2.0
18.3 G 0.9 (19.88)
forward
1.042 G 0.01
21.83 G 0.4
72.10 G 0.8
16.4 G 0.6 (17.06)
reverse
1.047 G 0.01
22.10 G 0.3
72.35 G 0.5
16.6 G 0.5 (17.25)
p-i-n
(FF) of 78.01% (Figure 3B). The high incident photon-to-electron conversion efficiency (IPCE) over the whole visible spectrum confirmed the high current density (Figure S14). The corresponding device parameters are summarized in Table 1. The superior performance of PSCs is based on the combination of excellent film morphology, low-defect density, high carrier lifetime, and long carrier diffusion length. It is known that 1/3MAPbCl3 and 2/3MAPbI3 would generate with the perovskite precursor of MACl and PbI2 with a ratio of 1:1.35,36,41,42 On the one hand, the wide band gap MAPbCl3 indeed cannot contribute to photocurrent. On the other hand, the Cl inclusion can modulate perovskite crystal formation as a result of induced lattice distortion leading to increased conductivity and increased carrier lifetime.4,37,43,44 Moreover, Snaith and co-workers discovered that the charge carrier diffusion length of MAPbIXCl3X can extend up to over 1 mm by including Cl into perovskite, which is an enhancement of an order of magnitude as compared to MAPbI3 without creating an impact on the energy gap.4 The absorption edge can reach 760 nm, which means that MAPbCl3 does not affect light absorption in our work. Therefore, high device performance can be achieved. Furthermore, we found that our device exhibits photocurrent hysteresis-free J-V curves with different scanning directions and voltage sweep rates (Figures 3C and 3D), indicating that the efficiencies measured for the devices with stable J-V curves are reliable. Moreover, unpackaged devices can remain above 93% of the original device efficiency by placing them in air for more than 1,000 h, and the device maintains over 80% of its original PCE after 700 h operation under light stress in the glove box, which exhibits great long-term stability (Figure 3E). Possible contributions can be explored by surface MAAc passivation. Moreover, our results indicate that high-performance PSCs can be repeatedly fabricated employing the MAAc-based perovskites. A histogram of the PCE from 30 individual devices is given in Figure 3F, which shows good reproducibility with a typical value of over 18% for more than 85% of the devices. Electrical Properties of Devices Based on Different Solvents In order to further demonstrate the merit of MAAc solvent over the conventional DMF solvent, we compared our device with a device employing the perovskite fabricated from conventional DMF solvent in a popular anti-solvent method. As shown in Figure 4A, the dark current density of the MAAc-based device is obviously lower than that of the DMF-based device, indicating that the MAAc-based perovskite films can prevent the current leakage to a certain degree. In addition, the VOC VS light intensity characterization confirmed that the charge recombination of the MAAcbased device (n = 0.348) was substantially less than that of the DMF-based device (n = 0.522) (Figure 4B). Furthermore, the photoluminescence (PL) lifetimes of perovskite films from MAAc and conventional anti-solvent method (in DMF) were investigated, which were 70.23 and 23.31 ns, respectively (Figure S15; Table S4), demonstrating the low trap density of perovskite films from MAAc. A conventional PSC with p-i-n configuration was further fabricated with the device structure of ITO, 30 nm poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
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Figure 4. Comparison of Electric Performances of Devices Based on MAAc and DMF Solvents (A) J-V curves of PSCs with MAAc and DMF under dark conditions. (B) Variation of V oc with light intensity dependence.
(PEDOT:PSS), 500 nm perovskite, 100 nm [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), 5 nm LiF, and 100 nm Al, as shown in Figure S16A. The best-performing device exhibited a PCE of 17.25%, a Voc of 1.06 V, a Jsc of 22.40 mA cm2, and an FF of 72.85% at a substrate temperature of 100 C with no hysteresis (Figures S16B and S16C) and showed good reproducibility with a typical value of over 16% for more than 85% of the devices (Figure S16D), which indicated that the MAAc solvent has excellent wetting properties on different substrates, e.g., polymer and metal oxide, for high-quality perovskite films. Moreover, the impressive Voc based on both MAPbI3 perovskite and PEDOT:PSSjPCBM-selective contacts is one of the best values as far as we know (Table S5), which indicated the reduced charge recombination and leakage current at the charge extraction interface. To further demonstrate the versatility of MAAc solvent on the effect of different MA salts (e.g., MAI and MABr) and lead sources (e.g., PbBr2) for the construction of perovskites, we fabricated different perovskites by using the MAAc solvent, e.g., MAPbI3 (PbI2 + MAI), MAPbI2Br (PbI2 + MABr), and MAPbBr2I (PbBr2 + MAI). The perovskite films were monitored by absorption (Figure S17) and XRD (Figure S18). The absorption spectrum with typical onsets and characteristic diffraction peaks of different perovskites, together largely with the lack of diffraction features intrinsically associated with the MAI, MABr, and PbBr2 (Figure S19), suggests a complete transformation of the different lead sources into perovskites. The device performance was also investigated on the basis of three perovskites, as shown in Figure S20 and Table S6. Note that all three devices were not in their optimized conditions. High PCEs of 15.23% for MAPbI3, 13.43% for MAPbI2Br, and 6.57% for MAPbBr2I were obtained. Moreover, the MAAc solvent proved to be applicable to the mixed cation system, e.g., FAI and CsI, in terms of better stability and performance over MAPbI3. As shown in Figure S20 and Table S6, the devices based on FAPbI3 and Cs0.1FA0.9PbI3 perovskites (confirmed by XRD; Figure S18) exhibited PCEs of 18.68% and 19.31%, further demonstrating that our approach is a universal method for perovskite growth. It should be noted, however, that it is difficult to confirm if MA+ is doped into FAPbI3 to form a mixed MA-FA cationic perovskite because MA+ in the MAAc solvent may take part in the reaction. More research is needed, and device performance optimization is underway. Conclusion In summary, we demonstrate an environmentally friendly RTMS MAAc solvent as an alternative to traditional toxic and highly coordinating solvents for preparing highquality perovskite thin films. RTMS-MAAc possesses unique features of high viscosity, negligible vapor pressure, and nonhazardousness. The use of RTMS MAAc
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simplifies the perovskite thin-film preparation process with complete management in ambient air irrespective of the humidity, one-step solution processing without anti-solvent, and low precursor solution concentration (300–400 mg mL1, a half of the DMF solution). By exploiting the unique properties of MAAc, we prepared ultra-smooth, pinhole-free, and highly stable perovskite films over 3,000 h without appreciable PbI2-impurity under humidity of 80%. The best PCE of 20.05% was achieved with excellent stability in air. Together with the successful fabrication of perovskites by different MA salts (e.g., MAI and MABr) and lead sources (e.g., PbBr2), we demonstrated that MAAc is a universal solvent for the perovskite community. The device performance can be further improved by optimizing the device structure, interface engineering, and the perovskite components. This work is promising for the adaptability of photovoltaics industrial production and opening up the potential of using the RTMS solvent for processing with fast throughput in manufacturing.
EXPERIMENTAL PROCEDURES The experimental procedures are included in the Supplemental Information.
SUPPLEMENTAL INFORMATION Supplemental Information can be found with this article online at https://doi.org/10. 1016/j.chempr.2019.02.025.
ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China, Fundamental Studies of Perovskite Solar Cells (grant 2015CB932200); the Natural Science Foundation of China (grants 51602149, 61705102, and 91733302); the Natural Science Foundation of Jiangsu Province, China (grants BK20150064, BK20161011, and BK20161010); the Young 1000 Talents Global Recruitment Program of China; the Jiangsu Specially Appointed Professor Program; the ‘‘Six Talent Peaks’’ Project in Jiangsu Province, China; and a startup from Nanjing Tech University.
AUTHOR CONTRIBUTIONS Y.C. conceptualized and designed the experiments. Y.C. and W.H. supervised the work. L.C. and Y.X. carried out the device fabrication and characterizations. B.L. synthesized MAAc. Y.C., L.C., Y.X., and B.L. wrote the first draft of the manuscript. G.X. and W.H. participated in data analysis and provided major revisions. All authors discussed the results and commented on the manuscript.
DECLARATION OF INTERESTS The authors declare no competing interests. Received: July 21, 2018 Revised: November 23, 2018 Accepted: February 26, 2019 Published: March 28, 2019
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Room-Temperature Molten Salt for Facile Fabrication of Efficient and Stable Perovskite Solar Cells in Ambient Air Lingfeng Chao, Yingdong Xia, Bixin Li, Guichuan Xing, Yonghua Chen, Wei Huang [email protected] (Y.C.) [email protected] (W.H.)
HIGHLIGHTS Room-temperature molten salt for facile fabrication of perovskite films in air No post-deposition treatment is needed (e.g., anti-solvent) The resulting perovskite solar cells are highly efficient and stable A universal solvent for all common salts used in perovskite devices
A solvent is vital to the control of crystallization and crystal growth in state-of-theart solution-processed hybrid organic-inorganic perovskites. We demonstrate an alternative environmentally friendly room-temperature molten salt, methylammonium acetate (MAAc), as a solvent characterized by high viscosity, negligible vapor pressure, and nonhazardous nature, which can be used to produce highly efficient perovskite solar cells (PSCs) in ambient air. The resulting PSCs exhibited excellent stability under light and dark conditions.
Chao et al., Chem 5, 1–12 April 11, 2019 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.chempr.2019.02.025
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Article
Room-Temperature Molten Salt for Facile Fabrication of Efficient and Stable Perovskite Solar Cells in Ambient Air Lingfeng Chao,1,6 Yingdong Xia,1,6 Bixin Li,2 Guichuan Xing,3 Yonghua Chen,1,* and Wei Huang1,4,5,7,*
SUMMARY
The Bigger Picture
Here, we report an alternative environmentally friendly room-temperature molten salt, methylammonium acetate (MAAc), as a novel solvent for facile fabrication of perovskite solar cells (PSCs) in ambient air. MAAc possesses excellent chemical properties along with high viscosity, negligible vapor pressure, and a nonhazardous nature. Complete solubility of both methylammonium and lead salts by hydrogen bonds in MAAc was observed. Dense and pinholefree perovskite films with high reproducibility can be readily prepared by a simple one-step method without an anti-solvent even under a relative humidity of over 80%. Under optimized processing conditions, we achieved an average power conversion efficiency of 18.42% and a maximum efficiency of 20.05% in CH3NH3PbI3-based planar heterojunction structure. In addition, devices without encapsulation remained above 93% of their original efficiency for more than 1,000 h in ambient air. These findings may open up the possibility of developing a new approach for further improving PSC performance with higher reproducibility and reliability in ambient atmosphere.
Organic-inorganic hybrid perovskites have recently attracted extensive attention for their potential uses in photovoltaics. However, solventhandling issues and toxicology concerns represent a major challenge in solution-processed perovskite thin films. Here, we demonstrate that an environmentally- and industrially friendly room-temperature molten salt (RTMS), methylammonium acetate (MAAc), can produce high-quality perovskite films in ambient air, leading to the development of perovskite solar cells (PSCs) with 20% efficiency and stability for more than 1,000 h. MAAc is a universal solvent for all common perovskite-based salts and may open up the potential of using RTMS solvent for facile fabrication of PSCs. Moreover, this work represents a new direction in the development of efficient and stable PSCs and other optoelectronic devices.
INTRODUCTION Because of their high absorption coefficient,1,2 high carrier mobility,3 long carrier diffusion length,4,5 suitable band gap,6 small exciton binding energy,7 and low trap densities,8,9 organic-inorganic hybrid perovskites have recently attracted extensive attention for thin-film photovoltaics. Perovskite solar cells (PSCs) have demonstrated unprecedented progress in power conversion efficiency (PCE) from 3.8% to a certified 23.3% in just a few years.10,11 The low cost and ease of solution processing are promising indicators for solar energy at scale,12,13 which allow PSCs to outperform inorganic thin-film solar cells and become leaders in the field of solar photovoltaics with lower energy payback time than their crystalline silicon counterparts.14 Despite the success in obtaining excellent photovoltaic performance, the toxic, flammable, and highly coordinated solvents, e.g., N,N-dimethylformamide (DMF),15,16 dimethyl sulfoxide (DMSO),17 N-methyl-2-pyrrolidone (NMP),18 g-butyrolactone (GBL),19 and dimethylacetamide (DMA),20 still present an open problem. To date, highly efficient PSCs have to be fabricated via solution techniques that involve solvents with high boiling points, polar solvents, and aprotic solvents. However, DMF, DMA, NMP, and GBL are highly toxic solvents that can cause serious harm to the human body and the environment, and DMSO is a flammable solvent that can easily cause accidents.21 Moreover, high-quality perovskite or perovskite intermediate films are achieved by the timely application of an anti-solvent, such as toluene or chlorobenzene, to the precursor solution.22 On one hand, these solvents
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1
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pose an irreversible danger. On the other hand, with the generally complex processes involved in perovskite formation,23 the process for making high-quality perovskite films is technically hard to control because of the very narrow time window for properly adding the anti-solvent, which results in serious reproducibility problems and the notorious variations among different groups. Therefore, solvent-handling issues and toxicology concerns represent a major challenge in the perovskite community. Recently, an ionic liquid methylammonium formate and a methylamine (MA)-acetonitrile mixed solvent were employed to prepare perovskite films, which paves the way to advanced solvent engineering for the perovskite community.24,25 However, high-quality perovskite films have to be processed in inert atmosphere, which hampers the real production and applications of PSCs. Here, we report an alternative environmentally friendly room-temperature molten salt (RTMS), methylammonium acetate (MAAc), as a solvent characterized by high viscosity, negligible vapor pressure, and a nonhazardous nature for facile fabrication of PSCs in ambient air. The MAAc allows the perovskite film to be produced in a simple one-step process by directly dissolving lead and organic ammonium salts. We found a quick liquid-to-solid transition of perovskite through a deformed perovskite intermediate structure, which is remarkably different from the traditional perovskiteprocessing solvents (e.g., DMF and DMSO), involving plumbate intermediates during crystallization. A dense, pinhole-free, and high-quality perovskite film is obtained in the absence of any auxiliary crystallizing solvent. Moreover, the film undergoes no obvious changes under the humidity of 80% for more than 5 months. Upon optimization, we achieved a PCE of 20.05% in the CH3NH3PbI3-based planar heterojunction structure. Most importantly, the device is stable in air for more than 1,000 h without encapsulation.
RESULTS AND DISCUSSION Synthesis and Characterization of RTMS MAAc The synthetic route of RTMS MAAc is shown in Figure S1A. The chemical structure of MAAc is similar to that of methylammonium iodide (MAI). Specifically, acetic acid (HAc) and MA were mixed in a molar ratio of 1:1 and stirred for 2 h under an ice-water bath. The experimental details are shown in the Experimental Procedures. In order to determine the final synthesized product, proton nuclear magnetic resonance (1H NMR) spectroscopy was conducted by dissolving the product in deuterated DMSO (Figure S1B). The hydrogen on the MA nitrogen atom (a), the methyl hydrogen on the acetic acid (b), and the methyl hydrogen on the MA (c) exhibited an area ratio of around 1:1:1, confirming that the substance structure is the same as that shown in Figure S1A. MAAc is a colorless, transparent, viscous liquid (Figure S1B, inset), which is actually an ionic liquid. In order to investigate the properties of MAAc, the ultraviolet-visible (UV-vis) spectrum was characterized, and a distinct absorption peak at 324 nm was observed (Figure S1C). This is mainly due to the electron transition between the nitrogen atom and the carbonyl group (C=O). The thermal stability of MAAc was also characterized by thermogravimetric analysis (TGA), as shown in Figure S1D. We found that MAAc reached the maximum rate of weight loss at 164 C, indicating that MAAc is more stable in the liquid state than the crystalline state, as demonstrated in our previous report.26 In order to demonstrate the TGA plot is indicative of the evaporation of an ionic liquid, we conducted detailed analysis of the TGA plot of MAAc in comparison to HAc (Figure S1E) and the mixture of HAc and MA (Figure S1F). The HAc starts to lose weight at the very beginning and reaches the maximum rate of weight loss at 99.92 C, which is near its boiling point (117.9 C). Moreover, two inflection points at 95.3 C and 158.2 C emerged in the
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1Key
Laboratory of Flexible Electronics (KLOFE) & Institution of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), Nanjing, Jiangsu 211816, China
2Department
of Educational Science, Laboratory of College Physics, Hunan First Normal University, Changsha, Hunan 410205, China
3Institute
of Applied Physics and Materials Engineering, University of Macau, Macau, Macao SAR 999078, China
4Shaanxi
Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), Xi’an, Shaanxi 710072, China
5Key
Laboratory for Organic Electronics & Information Displays (KLOEID), and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, Nanjing, Jiangsu 210023, China
6These 7Lead
authors contributed equally
Contact
*Correspondence: [email protected] (Y.C.), [email protected] (W.H.) https://doi.org/10.1016/j.chempr.2019.02.025
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Figure 1. Characteristics of Perovskite Precursor Based on RTMS MAAc (A and B) DLS spectra of (A) MAAc and (B) DMF perovskite precursor solution with different concentrations from 10 to 300 mg mL 1 . (C) A picture of perovskite precursor solution (300 mg mL 1 ) of traditional solvents DMF, DMSO, and NMP as a comparison to MAAc. (D and E) FTIR spectra of perovskite precursor (D) DMF and (E) MAAc solution.
mixture of HAc and MA solvent, indicative of a large amount of decomposition or residual by the derivative thermogravimetry curve. The two inflection points at 95.3 C and 158.2 C are near the boiling points of HAc and MAAc, respectively, which indicates that a little MAAc was generated in the mixture of HAc and MA solvent. The decomposition of MA is not detected because of the extremely low boiling point (6.8 C). Therefore, it is concluded that MAAc is an ionic liquid and does not undergo degradation to HAc and MA during TGA measurement. Characteristics of Perovskite Precursor Based on RTMS MAAc The uniform and dense film is highly related to the dispersion of the crystal nuclei in the solution.27,28 When the crystal nuclei are uniformly dispersed in the precursor solution, they have the same growth ability during the film formation process, leading to uniform crystals with similar size. On the contrary, crystal grains with different sizes are finally formed if the crystal nuclei size is different, which may cause a rough surface. We configured different concentrations (10 to 300 mg mL1) of perovskite precursor solution in MAAc and DMF solvents. Under room temperature, the perovskite solutions in different solvents were analyzed by dynamic light scattering (DLS), as shown in Figures 1A and 1B. The micellar size in MAAc and DMF increased with increasing concentration. However, the distribution is not uniform in DMF solvent. Interestingly, the micellar size in MAAc is concentrated at certain sizes, demonstrating the uniform distribution of micellar in MAAc solution. Most importantly, MAAc has a higher total colloid concentration in the same concentration of perovskite precursor solution. This finding strongly proves that MAAc has weak interaction with the perovskite precursor PbI2 and MACl to form agglomerates. Furthermore, the perovskite precursor in DMF, DMSO, and NMP show a clear yellow solution,
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whereas it appears as light yellow in MAAc, as shown in Figure 1C. This is mainly due to the fact that DMF, DMSO, and NMP molecules intercalate into the layered structures of PbI2 in traditional solvents and induce a little expansion in the PbI2 lattice along the c axis,26 which improves the solubility of PbI2. However, a large number of MAAc molecules form hydrogen bonds with PbI2 and MACl by the O atom at the end of MAAc molecules because the O atom is more electronegative than the N atom,29 which in turn increases the colloid concentration of the precursor solution (Figure S2). The characteristics of the perovskite-MAAc solution were investigated by Fourier transform infrared spectroscopy (FTIR), as shown in Figure 1D. It can be seen that the perovskite-MAAc solution showed a broad peak at 2,500– 3,500 cm1 with respect to the original sole MAAc solvent. This is mainly because the H----O-C hydrogen bond in the MAAc solvent is replaced by stronger hydrogen bonds formed by the addition of PbI2 and MACl, where multiple molecules were involved with uniform distribution, consistent with the DLS measurement. However, the DMF-dissolved perovskite did not show similar peaks but only the C-H vibration peak (Figure 1E). Furthermore, the formation of MAPbI3-xAcx alloy with PbI2 cannot be ruled out since MA+ and Ac can be incorporated into the PbI2 lattice.26 In addition, the perovskite precursor MAAc solution has good stability. We compared the color changes of the two MAAc and DMF solutions after 15 days. The MAAc solution remained in its original state without color change (Figures S3A and S3B). However, the solution in DMF turned from yellow to orange after 15 days of storage (Figures S3C and S3D), which can be attributed to the oxidation of the iodide from I to I3. Morphology of Perovskite Films Fabricated from RTMS MAAc RTMS MAAc, with a better thermal stability over traditional solvent, possesses negligible vapor pressure until near its boiling point as an ionic liquid,25,30 which is important for a perovskite crystallization medium. These important properties of RTMS MAAc allow for easy control of crystalline film formation by temperature. At present, production of high-quality perovskite films based on traditional solvents is strictly limited to both the nitrogen atmosphere and the use of a large number of anti-solvents.31–33 Here, pinhole-free, smooth-surfaced, and completely covered films can be produced by a one-step spin-coating process from our MAAc-solvent-based perovskite precursor solutions (PbI2:MACl = 1:1 mole ratio, MACl: CH3NH3Cl) under ambient air conditions. We developed a simple and effective annealing-assisted solution process (AASP) method that keeps a constant substrate temperature during the solution processing and incorporates excess solvent into the annealing process under the crystallization temperature. The basic steps for preparing perovskite film are schematically described in Figure S4. To illustrate the quality of the films, the surface scanning electron microscope (SEM) images of films fabricated at different substrate temperatures were investigated, as shown in Figure 2A. Highly dense films with large crystal grain and without pinholes were obtained at temperatures of 70 C, 80 C, 90 C, and 100 C. The surface roughness of the perovskite films at temperatures of 100 C was 32.81 nm, which corresponds to the SEM images (Figure S5). However, the film showed cracked grain at 110 C, while the cracking of the film at 120 C was prominent and a nonuniform surface was observed. We proposed that the constant substrate temperature allows the solvent to evaporate immediately and simultaneously leads to supersaturated perovskite solution, which promotes fast nucleation and growth of the crystals. An immediate color change from colorless to light brown was observed in the first 3–4 s during spin coating, indicating the formation of perovskite. Moreover, the low vapor pressure of MAAc solvent allows the perovskite crystal to be enclosed by excess solvent under the crystallization temperature, enabling well-controlled crystal growth. However, when the temperature (110 C and 120 C) is higher than the perovskite crystallization temperature, the
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Figure 2. Characteristics of Perovskite Films Fabricated from RTMS MAAc (A) SEM images of perovskite films fabricated under different substrate temperatures. The scale bar represents 500 nm. (B) XRD pattern evolution of perovskite films placed in air. (C) 1 H NMR spectrum of perovskite powder. (D) FTIR spectra of perovskite powder.
crystal growth spins out of control. The perovskite growth process is obviously controlled by substrate temperature. Furthermore, the perovskite film produced by MAAc solvent was placed in the natural environment to verify the moisture and oxygen stability of this film. Surprisingly, no significant degradation of perovskite was observed for more than 3,000 h (humidity 60%–80%) as is evident from the absence of PbI2 peaks, indicating remarkable improved stability (Figure 2B). Therefore, a mechanism was proposed. In the perovskite film formation process, MAAc would prefer to self-assemble with PbI2 to form an intermediate phase quickly, as is evident from perovskite-like phase formation (Figure S6). The intermediate phase formed by MAAc is unstable, which is easily decomposed to PbI2 because of the evaporation of MAAc under continuous heat (Figure S6). The
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supersaturation of the precursor can thus be produced with MAAc evaporation, and MACl then enters the crystal lattice at the same time, accompanied by perovskite crystal formation on the substrate. Accordingly, MAAc not only plays the role of solvent but also promotes the rapid crystallization of perovskite as a media. It should be noted that the whole process is accomplished in ambient air irrespective of the humidity (humidity ranges from 30% to 80%), breaking the limitations of traditional solvents. This may be attributed to the high viscosity of MAAc, which shows little variation with humidity, protecting the perovskite from moisture. One thing that needs to be stated is that a small amount of MAAc residue in the final perovskite film was verified by 1H NMR (Figure 2C) and FTIR (Figure 2D), where the clear methyl hydrogen spectrum of the acetic acid at 2.38 ppm and the C=O stretching vibration from CH3COO were observed. Moreover, this was further confirmed by the presence of a small amount of O element on the surface of the film, which was detected by energy dispersive spectrometer (EDS) mapping (Figure S7). This is because it is difficult for MAAc to escape from the final perovskite film because of its high viscosity as previously reported.34 Crystal Structures and Optical Properties The substrate temperature plays an important role in the morphology of perovskite films. Accordingly, the X-ray diffraction (XRD) patterns of perovskite films at different substrate temperatures were investigated (Figure S8A). Sharp diffraction peaks of the (110), (220), and (330) diffractions at 14.21 , 28.55 , and 43.33 , respectively, were observed, arising from the tetragonal MAPbI3 perovskite.25 Moreover, two distinct peaks at 15.87 and 31.98 appeared as the representative of (110) and (220) diffractions of the MAPbCl3 phase, respectively, which is consistent with the previous report employing the same raw materials of PbI2 and MACl.35 These findings demonstrated that perovskite films fabricated by MAAc solvents have the same crystal structure as conventional solvents,36,37 indicating that MAAc-based perovskite film were well fabricated and highly oriented with a-axis self-assembly. Interestingly, the ratio of (110) diffractions peak intensity of MAPbCl3/MAPbI3 is gradually increased, and MAPbCl3/MAPbI3 reaches a ratio of 1:1 at 100 C, which declines to become stable when the temperature is further increased (Figure S9). It has been suggested that MAPbCl3 is the kinetically favored phase, whereas MAPbI3 is the thermodynamically favored product,38 demonstrating that the AASP is necessary in preparing perovskite films based on the raw materials of PbI2 and MACl. MAPbCl3 has a wide band gap (3.11 eV),39 and the presence of MAPbCl3 can be recognized as an ‘‘impurity’’ from the crystal and could cause heterojunction structure with the MAPbI3 main phase, thereby resulting in variations in the optical and electronic properties (Figure S8B)39,40 and lowering the device efficiency. Therefore, it is highly desirable to lower the MAPbCl3 perovskite phase as little as possible. Considering the perovskite crystallization temperature, the ratio of the two perovskites, as well as the low-temperature preparation process, we fixed the substrate temperature at 100 C to further characterize the XRD patterns and absorption of perovskite films from different loadings of MAAc solution. The change in perovskite concentrations has a little impact on the nature of the crystals (Figure S8C) and a significant increase of absorption intensity because of the increased thickness (Figure S8D); however, the same effect was observed as the substrate temperature increased, in that the MAPbCl3 phase was gradually increased with increasing the concentration of perovskite MAAc solution (Figure S10). Photovoltaic Performance Having shown that high-quality films with flat, dense, pinhole-free, and excellent optoelectronic properties can be fabricated and applied, we move on to integrate
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Figure 3. Device Performance for MAPbI3-Based PSCs (A) Device structure of planar heterojunction PSCs. (B) J-V curves of best-performing PSCs measured in ambient air under 100 mW cm 2 AM1.5G illumination. (C) J-V curves under reverse and forward scanning directions. (D) J-V curves under different voltage sweep rates. (E) Stability of device without encapsulation under dark and light conditions. (F) PCE histogram of 30 devices fitted with a Gaussian distribution.
these MAAc-based perovskite films into PSCs. We first fabricate PSCs with a planar n-i-p heterojunction configuration of ITO, 30 nm C60 pyrrolidine tris-acid (CPTA), 5 nm butylamine hydrochloride (BACl), 500 nm perovskite, 100 nm 2,20 ,7,70 -tetrakis (N,N-di-methoxyphenylamine)-9,90 -spirobifluorene (Spiro-OMeTAD), 1 nm MoO3, and 100 nm Au (Figure 3A). Note that the perovskite films were prepared by a simple AASP approach under ambient air conditions. The photovoltaic performance of PSCs was measured under the simulated air mass 1.5 global (AM1.5G) solar irradiation. It should be noted that the substrate temperature (Figure S11; Table S1), the perovskite precursor solution concentration (Figure S12; Table S2), and the annealing temperature and time (Figure S13; Table S3) play an important role in the device’s performance. The best performance was achieved from the photocurrentvoltage (J-V) curve with a maximum PCE of 20.05%, a high open-circuit voltage (Voc) of 1.11 V, a short-circuit current density (Jsc) of 23.16 mA cm2, and a fill factor
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Table 1. Photovoltaic Parameters of PSCs with n-i-p and p-i-n Configurations Device Configuration
Scanning Direction
Voc (V)
Jsc (mA cm2)
FF (%)
PCE (%)
n-i-p
forward
1.090 G 0.02
22.36 G 0.8
76.01 G 2.0
18.5 G 0.8 (20.05)
reverse
1.090 G 0.02
22.25 G 0.9
75.41 G 2.0
18.3 G 0.9 (19.88)
forward
1.042 G 0.01
21.83 G 0.4
72.10 G 0.8
16.4 G 0.6 (17.06)
reverse
1.047 G 0.01
22.10 G 0.3
72.35 G 0.5
16.6 G 0.5 (17.25)
p-i-n
(FF) of 78.01% (Figure 3B). The high incident photon-to-electron conversion efficiency (IPCE) over the whole visible spectrum confirmed the high current density (Figure S14). The corresponding device parameters are summarized in Table 1. The superior performance of PSCs is based on the combination of excellent film morphology, low-defect density, high carrier lifetime, and long carrier diffusion length. It is known that 1/3MAPbCl3 and 2/3MAPbI3 would generate with the perovskite precursor of MACl and PbI2 with a ratio of 1:1.35,36,41,42 On the one hand, the wide band gap MAPbCl3 indeed cannot contribute to photocurrent. On the other hand, the Cl inclusion can modulate perovskite crystal formation as a result of induced lattice distortion leading to increased conductivity and increased carrier lifetime.4,37,43,44 Moreover, Snaith and co-workers discovered that the charge carrier diffusion length of MAPbIXCl3X can extend up to over 1 mm by including Cl into perovskite, which is an enhancement of an order of magnitude as compared to MAPbI3 without creating an impact on the energy gap.4 The absorption edge can reach 760 nm, which means that MAPbCl3 does not affect light absorption in our work. Therefore, high device performance can be achieved. Furthermore, we found that our device exhibits photocurrent hysteresis-free J-V curves with different scanning directions and voltage sweep rates (Figures 3C and 3D), indicating that the efficiencies measured for the devices with stable J-V curves are reliable. Moreover, unpackaged devices can remain above 93% of the original device efficiency by placing them in air for more than 1,000 h, and the device maintains over 80% of its original PCE after 700 h operation under light stress in the glove box, which exhibits great long-term stability (Figure 3E). Possible contributions can be explored by surface MAAc passivation. Moreover, our results indicate that high-performance PSCs can be repeatedly fabricated employing the MAAc-based perovskites. A histogram of the PCE from 30 individual devices is given in Figure 3F, which shows good reproducibility with a typical value of over 18% for more than 85% of the devices. Electrical Properties of Devices Based on Different Solvents In order to further demonstrate the merit of MAAc solvent over the conventional DMF solvent, we compared our device with a device employing the perovskite fabricated from conventional DMF solvent in a popular anti-solvent method. As shown in Figure 4A, the dark current density of the MAAc-based device is obviously lower than that of the DMF-based device, indicating that the MAAc-based perovskite films can prevent the current leakage to a certain degree. In addition, the VOC VS light intensity characterization confirmed that the charge recombination of the MAAcbased device (n = 0.348) was substantially less than that of the DMF-based device (n = 0.522) (Figure 4B). Furthermore, the photoluminescence (PL) lifetimes of perovskite films from MAAc and conventional anti-solvent method (in DMF) were investigated, which were 70.23 and 23.31 ns, respectively (Figure S15; Table S4), demonstrating the low trap density of perovskite films from MAAc. A conventional PSC with p-i-n configuration was further fabricated with the device structure of ITO, 30 nm poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
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Figure 4. Comparison of Electric Performances of Devices Based on MAAc and DMF Solvents (A) J-V curves of PSCs with MAAc and DMF under dark conditions. (B) Variation of V oc with light intensity dependence.
(PEDOT:PSS), 500 nm perovskite, 100 nm [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), 5 nm LiF, and 100 nm Al, as shown in Figure S16A. The best-performing device exhibited a PCE of 17.25%, a Voc of 1.06 V, a Jsc of 22.40 mA cm2, and an FF of 72.85% at a substrate temperature of 100 C with no hysteresis (Figures S16B and S16C) and showed good reproducibility with a typical value of over 16% for more than 85% of the devices (Figure S16D), which indicated that the MAAc solvent has excellent wetting properties on different substrates, e.g., polymer and metal oxide, for high-quality perovskite films. Moreover, the impressive Voc based on both MAPbI3 perovskite and PEDOT:PSSjPCBM-selective contacts is one of the best values as far as we know (Table S5), which indicated the reduced charge recombination and leakage current at the charge extraction interface. To further demonstrate the versatility of MAAc solvent on the effect of different MA salts (e.g., MAI and MABr) and lead sources (e.g., PbBr2) for the construction of perovskites, we fabricated different perovskites by using the MAAc solvent, e.g., MAPbI3 (PbI2 + MAI), MAPbI2Br (PbI2 + MABr), and MAPbBr2I (PbBr2 + MAI). The perovskite films were monitored by absorption (Figure S17) and XRD (Figure S18). The absorption spectrum with typical onsets and characteristic diffraction peaks of different perovskites, together largely with the lack of diffraction features intrinsically associated with the MAI, MABr, and PbBr2 (Figure S19), suggests a complete transformation of the different lead sources into perovskites. The device performance was also investigated on the basis of three perovskites, as shown in Figure S20 and Table S6. Note that all three devices were not in their optimized conditions. High PCEs of 15.23% for MAPbI3, 13.43% for MAPbI2Br, and 6.57% for MAPbBr2I were obtained. Moreover, the MAAc solvent proved to be applicable to the mixed cation system, e.g., FAI and CsI, in terms of better stability and performance over MAPbI3. As shown in Figure S20 and Table S6, the devices based on FAPbI3 and Cs0.1FA0.9PbI3 perovskites (confirmed by XRD; Figure S18) exhibited PCEs of 18.68% and 19.31%, further demonstrating that our approach is a universal method for perovskite growth. It should be noted, however, that it is difficult to confirm if MA+ is doped into FAPbI3 to form a mixed MA-FA cationic perovskite because MA+ in the MAAc solvent may take part in the reaction. More research is needed, and device performance optimization is underway. Conclusion In summary, we demonstrate an environmentally friendly RTMS MAAc solvent as an alternative to traditional toxic and highly coordinating solvents for preparing highquality perovskite thin films. RTMS-MAAc possesses unique features of high viscosity, negligible vapor pressure, and nonhazardousness. The use of RTMS MAAc
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simplifies the perovskite thin-film preparation process with complete management in ambient air irrespective of the humidity, one-step solution processing without anti-solvent, and low precursor solution concentration (300–400 mg mL1, a half of the DMF solution). By exploiting the unique properties of MAAc, we prepared ultra-smooth, pinhole-free, and highly stable perovskite films over 3,000 h without appreciable PbI2-impurity under humidity of 80%. The best PCE of 20.05% was achieved with excellent stability in air. Together with the successful fabrication of perovskites by different MA salts (e.g., MAI and MABr) and lead sources (e.g., PbBr2), we demonstrated that MAAc is a universal solvent for the perovskite community. The device performance can be further improved by optimizing the device structure, interface engineering, and the perovskite components. This work is promising for the adaptability of photovoltaics industrial production and opening up the potential of using the RTMS solvent for processing with fast throughput in manufacturing.
EXPERIMENTAL PROCEDURES The experimental procedures are included in the Supplemental Information.
SUPPLEMENTAL INFORMATION Supplemental Information can be found with this article online at https://doi.org/10. 1016/j.chempr.2019.02.025.
ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China, Fundamental Studies of Perovskite Solar Cells (grant 2015CB932200); the Natural Science Foundation of China (grants 51602149, 61705102, and 91733302); the Natural Science Foundation of Jiangsu Province, China (grants BK20150064, BK20161011, and BK20161010); the Young 1000 Talents Global Recruitment Program of China; the Jiangsu Specially Appointed Professor Program; the ‘‘Six Talent Peaks’’ Project in Jiangsu Province, China; and a startup from Nanjing Tech University.
AUTHOR CONTRIBUTIONS Y.C. conceptualized and designed the experiments. Y.C. and W.H. supervised the work. L.C. and Y.X. carried out the device fabrication and characterizations. B.L. synthesized MAAc. Y.C., L.C., Y.X., and B.L. wrote the first draft of the manuscript. G.X. and W.H. participated in data analysis and provided major revisions. All authors discussed the results and commented on the manuscript.
DECLARATION OF INTERESTS The authors declare no competing interests. Received: July 21, 2018 Revised: November 23, 2018 Accepted: February 26, 2019 Published: March 28, 2019
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Please cite this article in press as: Chao et al., Room-Temperature Molten Salt for Facile Fabrication of Efficient and Stable Perovskite Solar Cells in Ambient Air, Chem (2019), https://doi.org/10.1016/j.chempr.2019.02.025
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