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Fabrication of perovskite solar cells based on vacuum-assisted linear meniscus printing of MAPbI3
Solar Energy Materials and Solar Cells 191 (2019) 148–156
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Fabrication of perovskite solar cells based on vacuum-assisted linear meniscus printing of MAPbI3
T
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Ershad Parvaziana, Amir Abdollah-zadeha, , Hamid Reza Akbarib, Nima Taghaviniab a b
Department of Materials Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran Department of Physics, Sharif University of Technology, P.O. Box 11155-9161, Tehran, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Meniscus printing Vacuum process Perovskite solar cell Ambient condition
Scale-up deposition methods in perovskite solar cell research, are mostly used under humidity environment outside the glove-box. Also, the as-printed absorbing layer before the post-annealing process is always wet. Thus, controlling the morphology and crystallization of perovskite thin-films in up-scaled deposition systems is difficult and strongly investigated by the researchers. In this work, we introduce an anti-solvent-free meniscus printing method in which, the absorbing perovskite film with optimal performance is achieved. To this end, we check the printing parameters to get to the optimized film characteristics. Also, a vacuum chamber (< 100 Pa) is used for 30 s to remove the solvent with appropriate pace from the as-printed wet perovskite films. Based on using vacuum process, instead of low surface coverage needle-like morphology, a pinhole-free dense film with appropriate grain-size (400–500 nm) and thickness (~480 nm) was obtained. The perovskite devices with optimized meniscus printed films with and without vacuum process show a PCE of 10.1% and 2.3% respectively (with active area of 0.1 cm2). Also, the conversion efficiency of 8.0% was achieved with an active area of 1 cm2. The result demonstrated the merit of using vacuum process before post-annealing step.
1. Introduction Organic-inorganic perovskite solar cells have recently attracted much attention due to their high power conversion efficiency, low product cost, and ease of fabrication. Due to their great intrinsic optoelectronic properties, such as high absorption coefficient, low defect density, long carrier life-time and diffusion length, perovskite solar cells are known as a promising candidate for the next generation of industrial photovoltaic module devices [1–3]. The current world-record efficiency for perovskite solar cells is 22.1%, which is even higher than that of multi-crystalline silicon solar cells [4]. However, today, most perovskite solar cells are fabricated by laboratory solution-based processing methods (such as spin coating) which are not designed to minimize material use and industrial cost-effective processes [5]. Although huge improvements have been made in the fabrication techniques for spin-coating, there has been a very little demonstration supporting such approaches in scalable processes. Laboratory methods are incompatible with large-area substrates and cannot be scaled to meet industrial production requirements [6]. So, in order to obtain the appropriate morphology in the lab-scale processing, the perovskite solution is required to be prepared and deposited inside an inert environment of a glovebox [7]. In lab scale systems, it is necessary to use
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an anti-solvent material for extracting the solvent content from the perovskite film to get smooth, compact and large grain perovskite thin films [8–10]. The anti-solvents need to be miscible with the solvents used to dissolve the perovskite precursors, but the perovskite itself should not be soluble in the anti-solvent. By extracting the solvent from the precursor solution, the fast precipitation of the perovskite materials can be ensured [11,12]. Because of the substrate size limitations in spin coating and due to the high-cost issue in glovebox-based deposition methods, it is important to use some scaling-up deposition methods to achieve solar PV markets. Slot-die printing [13,14], ink-jet printing [15–19], dip-coating [20–22], ultrasonic spraying [23,24], and doctor blade coating [25–27] are the most considered deposition methods by which perovskite material can be easily deposited in large-scale and cost-effective systems. Scale-up deposition methods are mostly used under the humid environment outside the glove-box [7,28]. Thus, it is important to have some tricks during these deposition techniques to get the absorbing perovskite layer with the required morphology in the ambient air. Some recent works have focused on these goals by alternative methods. In this context, to avoid the effect of humidity on the morphology and to get a smooth, uniform, dense and shinier layer, Kim et al. claimed that the post-annealing process had to be changed to the high temperature of
Corresponding author. E-mail address: [email protected] (A. Abdollah-zadeh).
https://doi.org/10.1016/j.solmat.2018.11.012 Received 10 July 2018; Received in revised form 6 September 2018; Accepted 6 November 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic illustration of the perovskite (a) solution meniscus providing and (b) wet thin-film preparation processes.
2. Experimental section
400 °C in a very short time (4s) [29]; the larger the size of perovskite grains, the shinier the layer and the better the performance of the solar cell [23]. In one other research based on the ink-jet printing of the perovskite layer, Li et al. introduced a deposition method in the lab environment however, the deposited layer was post-annealed in some N2-glovebox device not an ideal low-cost industrial process [17]. Also, in order to tackle the effect of humidity on crystallization, Yang et al. used their deposition device inside an N2-filled glovebox; to deplete the solvent content before the heat-treatment process, they dispended the as-deposited substrate into some diethyl ether bath as an anti-solvent material [30]. Unlike the spin coating, which involves a self-drying effect during the centrifugal force from spinning, films in scalable techniques are very wet due to their high solvent content [31,32]. To remove the solvent from the precursor, anti-solvent materials can be sprayed [33] or dripped [34] on the substrate during the spin coating. Unfortunately, anti-solvent dripping and spraying have been used only with spin coating, and they are difficult to employ in scalable deposition. However, there are alternative techniques such as inert (or dry) gas blowing [35,36] or anti-solvent bathing [30] to extract the extra solvent from the solution during the deposition. Cottella et al. held that to control the amount of the solvent in the perovskite ink during the slot-die printing, it was possible to apply both pre-heating the substrate surface from the bottom and cold air-knife blowing on the top. In that case, a temperature gradient could be created to prevent vertical growth, as well as the increase in wettability and perovskite nucleation [37]. Also, to get rid of wet perovskite layer in a short period of time, Ciro et al., show the dependence of substrate temperature on the morphology during the one-step slot-die printing [38]. The drying process for the wet perovskite film is very slow, resulting in overgrown crystals with very low coverage. Therefore, Vak et al. introduced a gasquenching step to speed up the film drying process [35]. Moreover, they employed both gas quenching and substrate heating processes during the slot-die printing to achieve a defect-free dense layer [36]. In the above-mentioned methods, researchers have focused on how to decrease the humidity of the deposition environment or how to extract the solvent from the precursor by using an anti-solvent material or gas blowing on the substrate for large-scale perovskite solar cells. This paper presents a new brand scale-up deposition method named linear meniscus printing to achieve the required perovskite morphology regardless of the level of humidity in the air, without using any antisolvent material. To get rid of the extra solvent in the wet layer, and to achieve a shiny, smooth and fully-coverage perovskite layer, even in 70% humidity, we have recommended a low-pressure process (less than 100 Pa) just before the post-annealing step.
2.1. Substrate preparation Fluorine-doped tin oxide glass (FTO, Solaronix, 15 Ω/square, Switzerland) substrates were patterned by etching with Zinc powder and diluting with Hydrochloric (HCl) acid. Then, the substrates were cleaned sequentially using some 0.1 M HCl (in ethanol) solution, distilled water, acetone, and 2-propanol in an ultrasonic bath for 6–10 min, before being sintered at 500 °C for 30 min. Then, the sintered substrates were exposed under UV-Ozone irradiation at room temperature for 15 min to clean the surface residual contaminants. It should be mentioned that the UV-Ozone irradiation is an essential treatment method before each deposition step in perovskite solar cells [39]. So, as-UV-Ozone treated substrates had to be deposited by the blocking material, before contamination, again. 2.2. Linear meniscus printing method In this work, a 3D-printer device (Quantum 2020, Iran) was modified by changing its printing head to deposit the absorbing layer as a linear meniscus printer. The schematic in Fig. 1(a) shows the perovskite meniscus preparation step before the printing process. This method is very similar to the slot-die printing. The main difference between these two methods lies in the way the perovskite precursor droplet is spread in the gap between the substrate and the shim (or blade). During the slot-die-coating process, a microliter pump injects a certain amount of the perovskite ink onto the substrate from a reservoir. However, in our linear meniscus printing, perovskite droplet was dripped by a microsampler in one side of the printer's tape-covered blade. Because of the chemical resistant tape's slippery surface, the droplet slipped during the gap between the blade and the substrate and a uniform perovskite meniscus will be formed. Finally, the shim (or blade) could move along the substrate, leading to the formation of a wet layer from the perovskite meniscus which its schematic can be seen in Fig. 1(b). For the perovskite precursor ink, methylamine iodide (MAI) and lead iodide (PbI2) powders were purchased from Dyesol (> 99%, Australia) and TCI (99.99%, India) respectively. The precursor perovskite solution contained MAI (1 M) and PbI2 (1.5 M) in anhydrous Dimethylformamide (DMF) and Dimethyl sulfoxide (DMSO) with various volume ratio to give the final concentration of 1.15 M. The perovskite precursor meniscus was printed by the optimized deposition parameters which are mentioned in Table 1. In order to achieve the optimum parameters shown in Table 1, the deposition was done at various times under different conditions. Also, the results of 149
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Table 1 Optimum parameters for maximum performance of a meniscus printed MAPbI3 solar cell.
Solution parameters Printing parameters
Environmental parameters
Perovskite fabrication parameters
Optimized value
Perovskite formula Blade and substrate distance Print speed Substrate temp. Perovskite droplet amount Post-annealing process Lab humidity Lab temperature
MAPbI3 1.15 M 200 µm 1.65 mm/s Room temp. 2 µl per cell Vacuum (30 s, 100 Pa) + heating (100 °C, 2 min) ~70% 20–30 °C
Fig. 2. (a) Perovskite solar cell device structure used in this work. Schematic illustration of meniscus printing method (b) with vacuum and (c) without vacuum process.
Fig. 3. Optical microscope and camera pictures of the layer with (a) MAI-PbI2-DMF-DMSO intermediate phase (after vacuum step) and (b) crystalline MAPbI3 phase (after post-annealing step).
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layer on FTO substrates, a mildly acidic (HCl, 37.5%, Merck Germany) solution of titanium isopropoxide (TTIP, 97% Merck, Germany) in ethanol (the concentration of TTIP/ 2 M. HCl/ ethanol = 369 µl/ 35 µl/ 2.54 ml) was spin coated at 2000 rpm for 30 s; this was followed by sintering at 500 °C for 30 min. Immediately after the sintering step, the hole-blocking layer was exposed under UV-Ozone irradiation for surface-treatments. Afterwards, the mesoporous TiO2 solution (TiO2 paste, IRASOL PST-20T, diluted in ethanol with the ratio of 1: 5.5) was spincoated on the hole-blocking layer at 5000 rpm for 30 s, dried at 110 °C for 6 min, and sintered at 500 °C for 30 min. The perovskite precursor was provided and printed on the substrate according to the parameters noted in Section 2.2. A thin layer of the hole-transporting material was then spin cast from a solution prepared by dissolving 72.3 mg SpiroOMeTAD (99.5%, Borun Co, China), 28.8 µl 4-tert-butylpyridin, and 17.5 µl of the LiTFSI solution (1.8 M in acetonitrile) in 1 ml chlorobenzene at the ambient atmosphere. Gold contacts (~100 nm thickness) were thermally evaporated to complete the device stack for the active area of 0.1, 1 and 1.2 cm2.
Fig. 4. J-V curves of perovskite meniscus printed solar cells with and without vacuum assisted process.
2.4. Film and device characterization experimental details for each optimal parameter can be seen in the Supporting information section.
The thickness and morphology of perovskite films were characterized using optical microscope and field emission scanning electron microscope (FESEM, TeScan- Mira 3 XMU). The perovskite films' optical properties were characterized with a diffuse reflection/transmittance spectrometer (DRS/DTS, Avaspec2048-TEC), UV–Visible spectrometer (400–1000 nm wavelength range, Perkin-Elmer Lambda25) and photoluminescence spectrometer devices. Photoluminescence (PL) was measured using a pulsed triple harmonic Nd: YAG laser for excitation
2.3. Device fabrication It is important to note that the all solution processes were conducted in the ambient air. FTO glass substrates were etched and cleaned, as mentioned in Section 2.1. To deposit the hole-blocking compact TiO2
Fig. 5. (a) Absorbance, (b) photoluminescence spectra and (c) Tauc plots of perovskite films with and without vacuum process. 151
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Fig. 6. (a) Top SEM image of the morphology and (b) cross-section SEM image of the thickness for meniscus printed MAPbI3 layer by 1.65 mm/s speed and 2 µl perovskite solution droplet at room substrate and solution temperature.
(355 nm, 1000 Hz, 2 mJ/pulse), and a fiber spectrometer (Avaspec 2048-TEC). Also, the J-V measurements were carried out under ambient condition using a solar simulator (IRASOL, SIM-1000) under 100 mWcm−2 AM 1.5 G illumination.
could gradually turn to light brown. The change in color could represent the MAI-PbI2-Solvent intermediate phase formation and the progress in the solvent-evaporation, which were all we needed from the vacuum process. Table S2 shows the facial differences of the deposited layers under different pressure conditions. As can be seen, 30 s after placing the printed layer into the vacuum device (at a pressure of around100 Pa), the yellow layer gradually turned to light brown. Also, Fig. 3 shows the optical microscope pictures of the intermediate (after vacuum process) and final perovskite (after post-annealing process) phases formation. Although the higher film thicknesses were obtained at higher substrate temperatures, this parameter had a negative effect on the crystallinity of the final perovskite layers, as mentioned in Table S3 and could be clearly observed in Fig. S2. The highest crystallinity was achieved at the substrate room temperature. Therefore, despite its low obtained thickness, we preferred to print our perovskite ink at the substrate room temperature. On the other hand, as mentioned earlier, it is fully controllable to use printing parameters via the meniscus printing method. So, in order to tackle the negative effect of the substrate room temperature on the film thickness, we changed other printing parameters such as printing speed and perovskite ink amount during the deposition. Fig. S3 shows the characteristic differences between perovskite films deposited by various printing speeds. From Table S4, it could be seen that the device champion with the maximum achieved PCE of 4.14% and the desired perovskite film thickness of ~291 nm was obtained at the printing speed of 1.65 mm/s. After checking all the controllable printing parameters and optimizing them (Table 1), we could discuss the effect of the vacuum process on the perovskite layer morphology. Focusing on the role of the vacuum process during the perovskite film formation led to some amazing results. The devices fabricated by two different perovskite film preparation conditions (with and without the vacuum process) showed completely distinct performances. The device without the vacuum process achieved the PCE of 1.8%. Compared to this device, the cell with the vacuum process had a remarkable increase in performance, with the maximum PCE of 7.7%, as shown in Fig. 4. The results showed that despite the high increase in the short circuit current density, the open circuit voltage in both cases was not significantly different. The lower current-density for without vacuum solar cell, might be due to many pinholes in the perovskite layer and lead to serious leakage and recombination. To investigate the meniscus printed perovskite film characteristics, films with and without the vacuum process were examined. It should be noted that all the printing parameters except the vacuum process were similar for both compared devices. Table 1 shows the optimized parameters used to get the perovskite absorbing layers. Fig. 5(a) and (c) display the optical absorption spectra and the corresponding Tauc plots
3. Results and discussion One of the amazing advantages of using linear meniscus printing, in comparison with spin-coating, is its decrease in the amount of the required perovskite precursor for each cell. To get the optimized amount of perovskite ink needed, the solution was printed on the substrate with various droplet amounts. It is possible to measure all our perovskite layers' thicknesses, by having the thickness of one layer via its scanning electron microscopy (SEM) cross-section image. To measure the other layers' thicknesses, it is needed to use both the diffuse transmittance graphs of each layer and the equation presented in Fig. S1(c). Also, Tauc plots in Fig. S1(b), were used to get each layers' bandgap energy. The results for thickness and bandgap energy of the layers deposited by various amounts of perovskite droplets are summarized in Table S1. From Table S1, to deposit the precursor over the substrate area of 1.4 * 2.8 cm2, 2 µl perovskite solution were needed for the linear meniscus printing. Thus, compared to the spin coating (25 µl solution need for similar substrate), in this method, we can save material precursor up to 12 times for each cell. Therefore, our method could provide a scalable, cost-effective uniform film, while minimizing the material waste. The schematic drawing of the device structure and the deposition processes used in this work can be seen in Fig. 2. In the current study, we focused on the mesoporous structure of perovskite solar cells due to its most widely-adopted geometry in research labs, ease of fabrication, and outstanding record efficiencies. This structure is shown in the schematic of Fig. 2(a) and the SEM image Fig. S1(c). In order to control the perovskite layer morphology, it is necessary to remove the solvent with the appropriate pace. In spin coating, due to the centrifugal force during the substrate spinning, the self-drying process can occur. On the contrary, meniscus printing method does not have any solvent-removal mechanism, so the wet film dries slowly in the ambient air without any control on it. On the other hand, if we only heat the as-printed layer to remove all the solvent quickly, a layer with lots of pin-holes will be formed. To overcome the problem, we decided to use a vacuum system to control the solvent removal pace. Fig. 2(b) and (c) shows the schematic of the meniscus printing method with and without the vacuum process. In the pressure below that of the solvent vapor, solvent evaporation started. Thus, to get the desired film morphology, we only needed to adjust the vacuum device pressure to a degree less than that of the solvent vapor. We examined various pressure ranges to get the appropriate pressure value in which the yellow non-crystalline layer 152
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Fig. 7. Optical microscopy and SEM images of MAPbI3 film (a, c, e) with and (b, d, f) without vacuum process at room temperature in different magnifications.
of these two compared thin films, respectively. As can be seen, relatively higher absorption (in the range of visible light) was achieved in the case of the vacuum device hired. This significant improvement in photon absorption led to producing much more excitons and consequently, better film performance. Moreover, the peak intensities in the PL spectra of perovskite films showed a great increase in film crystallinity when the vacuum process was assisted (Fig. 5(b)). Also, both PL spectra in Fig. 5(a) and the Tauc plots in Fig. 5(c) show the smaller bandgap for the vacuum-assisted perovskite layer, thereby implying the
higher absorption performance. The SEM images of the perovskite layer morphology (for the solution ink with DMF) and the device structure from the champion cell made under the printing speed of 1.65 mm/s, the substrate and solution temperature of 30 ℃ (room temperature) and the droplet amount of 2 µl per cell are shown in Fig. 6. The perovskite average grain-size of 240 nm was obtained by measuring the size of 100 grains through the digimizer software. As shown, the perovskite thickness of 150 nm was obtained by printing the layer with the above-mentioned parameters. 153
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Fig. 8. SEM images of perovskite film morphology at various solution temperatures of 30–95 °C for DMF: DMSO solvent ratio of (a-d) 9:1 and (e-h) 4:1.
Fig. 9. J-V curves for MAPbI3 solar cells with sample features mentioned in Table 2.
Fig. 10. J-V curves for MAPbI3 solar cells with sample features mentioned in Table 3.
Both high pin-hole distribution and low perovskite layer crystallinity prohibited the good device performance through poor interfacial contact with the hole transporting layer. Optical microscopy and SEM images of the layers shown in Fig. 7 demonstrated the perovskite surface coverage difference between these two layers. From Fig. 7, it could
be observed that this coverage was increased when the vacuum process was used. The vacuum process could strongly influence the morphology and the surface coverage percentage of the perovskite layers. With vacuum, a perovskite pin-hole-free film with high uniformity, crystallinity, and a round-shape morphology was achieved. On the contrary,
Table 2 Photovoltaic performance of MAPbI3 solar cells with vacuum and without vacuum process for different ratio amount of solvent. # Sample Sample Sample Sample Sample Sample
1 2 3 4 5 6
Post annealing method
Solvent
PCE (%)
Jsc (mA/cm2)
Voc (V)
FF (%)
Vacuum + (100 °C/10 min) Vacuum + (100 °C/10 min) Vacuum + (100 °C/ 10 min) Only (100 °C/1 h) Only (100 °C/1 h) Only (100 °C/1 h)
DMF: DMF: DMF: DMF: DMF: DMF:
7.7 8.2 10.1 1.8 1.9 2.3
13.3 14.6 17.8 3.1 3.4 4.1
0.92 0.86 0.83 0.85 0.78 0.81
0.63 0.63 0.68 0.68 0.71 0.69
DMSO(1:0) DMSO (9:1) DMSO (4:1) DMSO (1:0) DMSO (9:1) DMSO (4:1)
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Table 3 Photovoltaic performance of meniscus printed MAPbI3 solar cells in various active areas. Sample
Active area (cm2)
PCE (%)
Jsc (mA/cm2)
Voc (V)
FF (%)
meniscus printed MAPbI3 meniscus printed MAPbI3 meniscus printed MAPbI3
0.1 1 1.2
10.1 8.0 6.8
17.8 14.23 14.01
0.83 0.79 0.77
0.68 0.71 0.63
the solvent content ratio in the final desired perovskite morphology. Also, Fig. S6. shows the information on hysteresis of the cell champion (sample #3). It is found that there is no large performance difference in forward and reverse J-V measurements. To make clear the effects of our meniscus printing method on solar cells performance, we decided to compare the results of J-V curves of solar cell made by optimized meniscus printing method based on vacuum process and solar cell made by conventional one-step spin coating method based on Chlorobenzene antisolvent dripping. Fig. S5 and Table S6 show the J-V curve and final performance results of these two perovskite devices. As it can be seen, both deposition methods have comparable results. It should be noted that all perovskite precursor preparation and device fabrication parameters were equal for both perovskite solar cells except the perovskite deposition methods. Also, Fig. 10 and Table 3, show the photovoltaic performance of the champion cell in higher active areas of 1 and 1.2 cm2. We used a 3 * 3 mm2 mask on overlap of FTO and Au to define the active area of our devices. For larger areas (1 and 1.2 cm2) we just determined the device area by measuring the overlap area of FTO and Au. The maximum conversion efficiency achieved by the meniscus printed perovskite solar cells with vacuum process in active area of 1 and 1.2 cm2 was 8.0% and 6.8% respectively. Although the device performance with the PCE of 10.1% was not yet comparable with that of spin-coated perovskite solar cells, our study resolved the main problem with perovskite solar cells, which was their high dependence on the Ni-glovebox environment at various deposition steps. We fabricated our devices with a large-scale deposition method in the fully ambient air with the high humidity of 70%. Also, we believe that by working on the perovskite formula, it is possible to get even higher device efficiencies with the vacuum-assisted meniscus printing method.
for the layer in which the vacuum process was not involved, needle-like grains with a high pin-hole distribution in its morphology was found. This difference in the morphology could clearly explain the layer optical characteristic and facial color differences between these two layers. Also, the grain size had a profound effect on the performance parameters of the perovskite solar cells. It was such that the resistivity of the material was increased with decreasing the grain size. This was because the grain boundaries acted as some potential barriers against the motion of the carries across the grain boundaries. The other effect was that the carrier lifetime was decreased with the reduction of the grain size. Both effects led the reduction in the conversion efficiency of perovskite solar cells. Thus, we decided to increase the grain size by solvent engineering and temperature optimization during the printing process. The solvent engineering method was proposed in our perovskite solution by adding DMSO with various molar ratios to the DMF solution containing MAI and PbI2. By having a mixture of DMSO and DMF in our solvent, due to their difference in the boiling temperature, it was possible to control the solvent-removing process more easily during the vacuum step. The residual DMSO could improve the mass transport which could enhance the film quality if we slow down the evaporation rate of the solvent by vacuum process [40]. The DMSO presence led to the formation of an intermediate (PbI2-MAI-DMSO-DMF) phase (Fig. 3) preventing the crystallization process speed. SEM images of Fig. 8 show the effect of solvent engineering and temperature optimization on the perovskite grain size parameter. As can be seen in Fig. 8 and Table S5, the maximum perovskite grain size (476 nm) was achieved from the sample with the solution temperature and the solvent ratio of 55 ℃ and DMF: DMSO of 4:1, respectively. The grain size distribution graph for various solution temperatures and solvent ratios is also presented in Fig. S4. In the samples with the solution at room temperature, we did not find any difference in the perovskite grain size. However, by increasing the temperature to 55 ℃, the grain growth was observed. Also, at temperatures above 55 ℃, it was not clear why the decrease in the grain size and the increase in grain boundaries were found. Moreover, by adding some DMSO material to our ink, the increase in the perovskite grain size was obtained. However, this increase was much higher than that in the solvent with DMF: DMSO of 4:1, as compared with the ratio of 9:1. The claim could be confirmed by comparing the SEM images and the results of Fig. 8 and Table S5, respectively. To show the positive effect of the high grain growth parameter on device efficiency, we also examined the device photovoltaic performance by fabricating solar cells with various perovskite ink ratios at the temperature of 55 ℃ during the printing process. Also, we compared the performance of these devices both with and without the vacuum process. The corresponding device J-V curves with different solvent ratios and post-annealing times are shown in Fig. 9, and the device parameters are summarized in Table 2. As can be seen, adding a little DMSO in solvent increased the photovoltaic performance of the devices. Also, when the vacuum is used to remove the solvent, the efficiency is greatly increased compared to the case where the vacuum is not used. The vacuum-assisted, solvent DMF: DMSO ratio (4: 1) sample demonstrated a significant improvement in the PCE of 10.1%, whereas in the notvacuum-assisted, the PCE of 1.8% was observed for the only-DMF solvent sample. The huge difference between these two devices in terms of efficiency indicated the important role of the vacuum process as well as
4. Conclusions In Summary, we demonstrated a one-step meniscus printing method to provide pin-hole-free MAPbI3 perovskite film under air, low temperature and high relative humidity (60–70%) condition. The present study introduced a vacuum-assisted process before post-annealing treatment to mimic the self-drying behavior inherent in spin-coating. This vacuum device was introduced during the meniscus printing process to accelerate solvent evaporation which significantly improved surface coverage of the prepared perovskite films. Also, we have carefully controlled the printing parameters to get to the optimized film characteristics and final device performance. At the end, the optimized pin-hole-free perovskite film was obtained by optimal thickness and grain-size of 291 and 476 nm, respectively. To achieve to this goal, the optimized solution meniscus temperature was 55 °C during the printing speed of 1.65 mm/s on room temperature substrate. Compared to vacuum-assisted process, the not-vacuum-assisted layer had a needle-like morphology with lots of pin-holes all over the film. The champion vacuum-assisted and not-vacuum-assisted meniscus printed perovskite devices with optimal printing parameters showed a PCE of 10.1% and 2.3%, respectively. The obtained results are remarkable considering that using as-printed layers for short period of time in vacuum process can both get the required perovskite morphology and prevent the terrible effect of moisture during the post-annealing process. 155
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Funding
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Fabrication of perovskite solar cells based on vacuum-assisted linear meniscus printing of MAPbI3
T
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Ershad Parvaziana, Amir Abdollah-zadeha, , Hamid Reza Akbarib, Nima Taghaviniab a b
Department of Materials Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran Department of Physics, Sharif University of Technology, P.O. Box 11155-9161, Tehran, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Meniscus printing Vacuum process Perovskite solar cell Ambient condition
Scale-up deposition methods in perovskite solar cell research, are mostly used under humidity environment outside the glove-box. Also, the as-printed absorbing layer before the post-annealing process is always wet. Thus, controlling the morphology and crystallization of perovskite thin-films in up-scaled deposition systems is difficult and strongly investigated by the researchers. In this work, we introduce an anti-solvent-free meniscus printing method in which, the absorbing perovskite film with optimal performance is achieved. To this end, we check the printing parameters to get to the optimized film characteristics. Also, a vacuum chamber (< 100 Pa) is used for 30 s to remove the solvent with appropriate pace from the as-printed wet perovskite films. Based on using vacuum process, instead of low surface coverage needle-like morphology, a pinhole-free dense film with appropriate grain-size (400–500 nm) and thickness (~480 nm) was obtained. The perovskite devices with optimized meniscus printed films with and without vacuum process show a PCE of 10.1% and 2.3% respectively (with active area of 0.1 cm2). Also, the conversion efficiency of 8.0% was achieved with an active area of 1 cm2. The result demonstrated the merit of using vacuum process before post-annealing step.
1. Introduction Organic-inorganic perovskite solar cells have recently attracted much attention due to their high power conversion efficiency, low product cost, and ease of fabrication. Due to their great intrinsic optoelectronic properties, such as high absorption coefficient, low defect density, long carrier life-time and diffusion length, perovskite solar cells are known as a promising candidate for the next generation of industrial photovoltaic module devices [1–3]. The current world-record efficiency for perovskite solar cells is 22.1%, which is even higher than that of multi-crystalline silicon solar cells [4]. However, today, most perovskite solar cells are fabricated by laboratory solution-based processing methods (such as spin coating) which are not designed to minimize material use and industrial cost-effective processes [5]. Although huge improvements have been made in the fabrication techniques for spin-coating, there has been a very little demonstration supporting such approaches in scalable processes. Laboratory methods are incompatible with large-area substrates and cannot be scaled to meet industrial production requirements [6]. So, in order to obtain the appropriate morphology in the lab-scale processing, the perovskite solution is required to be prepared and deposited inside an inert environment of a glovebox [7]. In lab scale systems, it is necessary to use
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an anti-solvent material for extracting the solvent content from the perovskite film to get smooth, compact and large grain perovskite thin films [8–10]. The anti-solvents need to be miscible with the solvents used to dissolve the perovskite precursors, but the perovskite itself should not be soluble in the anti-solvent. By extracting the solvent from the precursor solution, the fast precipitation of the perovskite materials can be ensured [11,12]. Because of the substrate size limitations in spin coating and due to the high-cost issue in glovebox-based deposition methods, it is important to use some scaling-up deposition methods to achieve solar PV markets. Slot-die printing [13,14], ink-jet printing [15–19], dip-coating [20–22], ultrasonic spraying [23,24], and doctor blade coating [25–27] are the most considered deposition methods by which perovskite material can be easily deposited in large-scale and cost-effective systems. Scale-up deposition methods are mostly used under the humid environment outside the glove-box [7,28]. Thus, it is important to have some tricks during these deposition techniques to get the absorbing perovskite layer with the required morphology in the ambient air. Some recent works have focused on these goals by alternative methods. In this context, to avoid the effect of humidity on the morphology and to get a smooth, uniform, dense and shinier layer, Kim et al. claimed that the post-annealing process had to be changed to the high temperature of
Corresponding author. E-mail address: [email protected] (A. Abdollah-zadeh).
https://doi.org/10.1016/j.solmat.2018.11.012 Received 10 July 2018; Received in revised form 6 September 2018; Accepted 6 November 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic illustration of the perovskite (a) solution meniscus providing and (b) wet thin-film preparation processes.
2. Experimental section
400 °C in a very short time (4s) [29]; the larger the size of perovskite grains, the shinier the layer and the better the performance of the solar cell [23]. In one other research based on the ink-jet printing of the perovskite layer, Li et al. introduced a deposition method in the lab environment however, the deposited layer was post-annealed in some N2-glovebox device not an ideal low-cost industrial process [17]. Also, in order to tackle the effect of humidity on crystallization, Yang et al. used their deposition device inside an N2-filled glovebox; to deplete the solvent content before the heat-treatment process, they dispended the as-deposited substrate into some diethyl ether bath as an anti-solvent material [30]. Unlike the spin coating, which involves a self-drying effect during the centrifugal force from spinning, films in scalable techniques are very wet due to their high solvent content [31,32]. To remove the solvent from the precursor, anti-solvent materials can be sprayed [33] or dripped [34] on the substrate during the spin coating. Unfortunately, anti-solvent dripping and spraying have been used only with spin coating, and they are difficult to employ in scalable deposition. However, there are alternative techniques such as inert (or dry) gas blowing [35,36] or anti-solvent bathing [30] to extract the extra solvent from the solution during the deposition. Cottella et al. held that to control the amount of the solvent in the perovskite ink during the slot-die printing, it was possible to apply both pre-heating the substrate surface from the bottom and cold air-knife blowing on the top. In that case, a temperature gradient could be created to prevent vertical growth, as well as the increase in wettability and perovskite nucleation [37]. Also, to get rid of wet perovskite layer in a short period of time, Ciro et al., show the dependence of substrate temperature on the morphology during the one-step slot-die printing [38]. The drying process for the wet perovskite film is very slow, resulting in overgrown crystals with very low coverage. Therefore, Vak et al. introduced a gasquenching step to speed up the film drying process [35]. Moreover, they employed both gas quenching and substrate heating processes during the slot-die printing to achieve a defect-free dense layer [36]. In the above-mentioned methods, researchers have focused on how to decrease the humidity of the deposition environment or how to extract the solvent from the precursor by using an anti-solvent material or gas blowing on the substrate for large-scale perovskite solar cells. This paper presents a new brand scale-up deposition method named linear meniscus printing to achieve the required perovskite morphology regardless of the level of humidity in the air, without using any antisolvent material. To get rid of the extra solvent in the wet layer, and to achieve a shiny, smooth and fully-coverage perovskite layer, even in 70% humidity, we have recommended a low-pressure process (less than 100 Pa) just before the post-annealing step.
2.1. Substrate preparation Fluorine-doped tin oxide glass (FTO, Solaronix, 15 Ω/square, Switzerland) substrates were patterned by etching with Zinc powder and diluting with Hydrochloric (HCl) acid. Then, the substrates were cleaned sequentially using some 0.1 M HCl (in ethanol) solution, distilled water, acetone, and 2-propanol in an ultrasonic bath for 6–10 min, before being sintered at 500 °C for 30 min. Then, the sintered substrates were exposed under UV-Ozone irradiation at room temperature for 15 min to clean the surface residual contaminants. It should be mentioned that the UV-Ozone irradiation is an essential treatment method before each deposition step in perovskite solar cells [39]. So, as-UV-Ozone treated substrates had to be deposited by the blocking material, before contamination, again. 2.2. Linear meniscus printing method In this work, a 3D-printer device (Quantum 2020, Iran) was modified by changing its printing head to deposit the absorbing layer as a linear meniscus printer. The schematic in Fig. 1(a) shows the perovskite meniscus preparation step before the printing process. This method is very similar to the slot-die printing. The main difference between these two methods lies in the way the perovskite precursor droplet is spread in the gap between the substrate and the shim (or blade). During the slot-die-coating process, a microliter pump injects a certain amount of the perovskite ink onto the substrate from a reservoir. However, in our linear meniscus printing, perovskite droplet was dripped by a microsampler in one side of the printer's tape-covered blade. Because of the chemical resistant tape's slippery surface, the droplet slipped during the gap between the blade and the substrate and a uniform perovskite meniscus will be formed. Finally, the shim (or blade) could move along the substrate, leading to the formation of a wet layer from the perovskite meniscus which its schematic can be seen in Fig. 1(b). For the perovskite precursor ink, methylamine iodide (MAI) and lead iodide (PbI2) powders were purchased from Dyesol (> 99%, Australia) and TCI (99.99%, India) respectively. The precursor perovskite solution contained MAI (1 M) and PbI2 (1.5 M) in anhydrous Dimethylformamide (DMF) and Dimethyl sulfoxide (DMSO) with various volume ratio to give the final concentration of 1.15 M. The perovskite precursor meniscus was printed by the optimized deposition parameters which are mentioned in Table 1. In order to achieve the optimum parameters shown in Table 1, the deposition was done at various times under different conditions. Also, the results of 149
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Table 1 Optimum parameters for maximum performance of a meniscus printed MAPbI3 solar cell.
Solution parameters Printing parameters
Environmental parameters
Perovskite fabrication parameters
Optimized value
Perovskite formula Blade and substrate distance Print speed Substrate temp. Perovskite droplet amount Post-annealing process Lab humidity Lab temperature
MAPbI3 1.15 M 200 µm 1.65 mm/s Room temp. 2 µl per cell Vacuum (30 s, 100 Pa) + heating (100 °C, 2 min) ~70% 20–30 °C
Fig. 2. (a) Perovskite solar cell device structure used in this work. Schematic illustration of meniscus printing method (b) with vacuum and (c) without vacuum process.
Fig. 3. Optical microscope and camera pictures of the layer with (a) MAI-PbI2-DMF-DMSO intermediate phase (after vacuum step) and (b) crystalline MAPbI3 phase (after post-annealing step).
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layer on FTO substrates, a mildly acidic (HCl, 37.5%, Merck Germany) solution of titanium isopropoxide (TTIP, 97% Merck, Germany) in ethanol (the concentration of TTIP/ 2 M. HCl/ ethanol = 369 µl/ 35 µl/ 2.54 ml) was spin coated at 2000 rpm for 30 s; this was followed by sintering at 500 °C for 30 min. Immediately after the sintering step, the hole-blocking layer was exposed under UV-Ozone irradiation for surface-treatments. Afterwards, the mesoporous TiO2 solution (TiO2 paste, IRASOL PST-20T, diluted in ethanol with the ratio of 1: 5.5) was spincoated on the hole-blocking layer at 5000 rpm for 30 s, dried at 110 °C for 6 min, and sintered at 500 °C for 30 min. The perovskite precursor was provided and printed on the substrate according to the parameters noted in Section 2.2. A thin layer of the hole-transporting material was then spin cast from a solution prepared by dissolving 72.3 mg SpiroOMeTAD (99.5%, Borun Co, China), 28.8 µl 4-tert-butylpyridin, and 17.5 µl of the LiTFSI solution (1.8 M in acetonitrile) in 1 ml chlorobenzene at the ambient atmosphere. Gold contacts (~100 nm thickness) were thermally evaporated to complete the device stack for the active area of 0.1, 1 and 1.2 cm2.
Fig. 4. J-V curves of perovskite meniscus printed solar cells with and without vacuum assisted process.
2.4. Film and device characterization experimental details for each optimal parameter can be seen in the Supporting information section.
The thickness and morphology of perovskite films were characterized using optical microscope and field emission scanning electron microscope (FESEM, TeScan- Mira 3 XMU). The perovskite films' optical properties were characterized with a diffuse reflection/transmittance spectrometer (DRS/DTS, Avaspec2048-TEC), UV–Visible spectrometer (400–1000 nm wavelength range, Perkin-Elmer Lambda25) and photoluminescence spectrometer devices. Photoluminescence (PL) was measured using a pulsed triple harmonic Nd: YAG laser for excitation
2.3. Device fabrication It is important to note that the all solution processes were conducted in the ambient air. FTO glass substrates were etched and cleaned, as mentioned in Section 2.1. To deposit the hole-blocking compact TiO2
Fig. 5. (a) Absorbance, (b) photoluminescence spectra and (c) Tauc plots of perovskite films with and without vacuum process. 151
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Fig. 6. (a) Top SEM image of the morphology and (b) cross-section SEM image of the thickness for meniscus printed MAPbI3 layer by 1.65 mm/s speed and 2 µl perovskite solution droplet at room substrate and solution temperature.
(355 nm, 1000 Hz, 2 mJ/pulse), and a fiber spectrometer (Avaspec 2048-TEC). Also, the J-V measurements were carried out under ambient condition using a solar simulator (IRASOL, SIM-1000) under 100 mWcm−2 AM 1.5 G illumination.
could gradually turn to light brown. The change in color could represent the MAI-PbI2-Solvent intermediate phase formation and the progress in the solvent-evaporation, which were all we needed from the vacuum process. Table S2 shows the facial differences of the deposited layers under different pressure conditions. As can be seen, 30 s after placing the printed layer into the vacuum device (at a pressure of around100 Pa), the yellow layer gradually turned to light brown. Also, Fig. 3 shows the optical microscope pictures of the intermediate (after vacuum process) and final perovskite (after post-annealing process) phases formation. Although the higher film thicknesses were obtained at higher substrate temperatures, this parameter had a negative effect on the crystallinity of the final perovskite layers, as mentioned in Table S3 and could be clearly observed in Fig. S2. The highest crystallinity was achieved at the substrate room temperature. Therefore, despite its low obtained thickness, we preferred to print our perovskite ink at the substrate room temperature. On the other hand, as mentioned earlier, it is fully controllable to use printing parameters via the meniscus printing method. So, in order to tackle the negative effect of the substrate room temperature on the film thickness, we changed other printing parameters such as printing speed and perovskite ink amount during the deposition. Fig. S3 shows the characteristic differences between perovskite films deposited by various printing speeds. From Table S4, it could be seen that the device champion with the maximum achieved PCE of 4.14% and the desired perovskite film thickness of ~291 nm was obtained at the printing speed of 1.65 mm/s. After checking all the controllable printing parameters and optimizing them (Table 1), we could discuss the effect of the vacuum process on the perovskite layer morphology. Focusing on the role of the vacuum process during the perovskite film formation led to some amazing results. The devices fabricated by two different perovskite film preparation conditions (with and without the vacuum process) showed completely distinct performances. The device without the vacuum process achieved the PCE of 1.8%. Compared to this device, the cell with the vacuum process had a remarkable increase in performance, with the maximum PCE of 7.7%, as shown in Fig. 4. The results showed that despite the high increase in the short circuit current density, the open circuit voltage in both cases was not significantly different. The lower current-density for without vacuum solar cell, might be due to many pinholes in the perovskite layer and lead to serious leakage and recombination. To investigate the meniscus printed perovskite film characteristics, films with and without the vacuum process were examined. It should be noted that all the printing parameters except the vacuum process were similar for both compared devices. Table 1 shows the optimized parameters used to get the perovskite absorbing layers. Fig. 5(a) and (c) display the optical absorption spectra and the corresponding Tauc plots
3. Results and discussion One of the amazing advantages of using linear meniscus printing, in comparison with spin-coating, is its decrease in the amount of the required perovskite precursor for each cell. To get the optimized amount of perovskite ink needed, the solution was printed on the substrate with various droplet amounts. It is possible to measure all our perovskite layers' thicknesses, by having the thickness of one layer via its scanning electron microscopy (SEM) cross-section image. To measure the other layers' thicknesses, it is needed to use both the diffuse transmittance graphs of each layer and the equation presented in Fig. S1(c). Also, Tauc plots in Fig. S1(b), were used to get each layers' bandgap energy. The results for thickness and bandgap energy of the layers deposited by various amounts of perovskite droplets are summarized in Table S1. From Table S1, to deposit the precursor over the substrate area of 1.4 * 2.8 cm2, 2 µl perovskite solution were needed for the linear meniscus printing. Thus, compared to the spin coating (25 µl solution need for similar substrate), in this method, we can save material precursor up to 12 times for each cell. Therefore, our method could provide a scalable, cost-effective uniform film, while minimizing the material waste. The schematic drawing of the device structure and the deposition processes used in this work can be seen in Fig. 2. In the current study, we focused on the mesoporous structure of perovskite solar cells due to its most widely-adopted geometry in research labs, ease of fabrication, and outstanding record efficiencies. This structure is shown in the schematic of Fig. 2(a) and the SEM image Fig. S1(c). In order to control the perovskite layer morphology, it is necessary to remove the solvent with the appropriate pace. In spin coating, due to the centrifugal force during the substrate spinning, the self-drying process can occur. On the contrary, meniscus printing method does not have any solvent-removal mechanism, so the wet film dries slowly in the ambient air without any control on it. On the other hand, if we only heat the as-printed layer to remove all the solvent quickly, a layer with lots of pin-holes will be formed. To overcome the problem, we decided to use a vacuum system to control the solvent removal pace. Fig. 2(b) and (c) shows the schematic of the meniscus printing method with and without the vacuum process. In the pressure below that of the solvent vapor, solvent evaporation started. Thus, to get the desired film morphology, we only needed to adjust the vacuum device pressure to a degree less than that of the solvent vapor. We examined various pressure ranges to get the appropriate pressure value in which the yellow non-crystalline layer 152
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Fig. 7. Optical microscopy and SEM images of MAPbI3 film (a, c, e) with and (b, d, f) without vacuum process at room temperature in different magnifications.
of these two compared thin films, respectively. As can be seen, relatively higher absorption (in the range of visible light) was achieved in the case of the vacuum device hired. This significant improvement in photon absorption led to producing much more excitons and consequently, better film performance. Moreover, the peak intensities in the PL spectra of perovskite films showed a great increase in film crystallinity when the vacuum process was assisted (Fig. 5(b)). Also, both PL spectra in Fig. 5(a) and the Tauc plots in Fig. 5(c) show the smaller bandgap for the vacuum-assisted perovskite layer, thereby implying the
higher absorption performance. The SEM images of the perovskite layer morphology (for the solution ink with DMF) and the device structure from the champion cell made under the printing speed of 1.65 mm/s, the substrate and solution temperature of 30 ℃ (room temperature) and the droplet amount of 2 µl per cell are shown in Fig. 6. The perovskite average grain-size of 240 nm was obtained by measuring the size of 100 grains through the digimizer software. As shown, the perovskite thickness of 150 nm was obtained by printing the layer with the above-mentioned parameters. 153
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Fig. 8. SEM images of perovskite film morphology at various solution temperatures of 30–95 °C for DMF: DMSO solvent ratio of (a-d) 9:1 and (e-h) 4:1.
Fig. 9. J-V curves for MAPbI3 solar cells with sample features mentioned in Table 2.
Fig. 10. J-V curves for MAPbI3 solar cells with sample features mentioned in Table 3.
Both high pin-hole distribution and low perovskite layer crystallinity prohibited the good device performance through poor interfacial contact with the hole transporting layer. Optical microscopy and SEM images of the layers shown in Fig. 7 demonstrated the perovskite surface coverage difference between these two layers. From Fig. 7, it could
be observed that this coverage was increased when the vacuum process was used. The vacuum process could strongly influence the morphology and the surface coverage percentage of the perovskite layers. With vacuum, a perovskite pin-hole-free film with high uniformity, crystallinity, and a round-shape morphology was achieved. On the contrary,
Table 2 Photovoltaic performance of MAPbI3 solar cells with vacuum and without vacuum process for different ratio amount of solvent. # Sample Sample Sample Sample Sample Sample
1 2 3 4 5 6
Post annealing method
Solvent
PCE (%)
Jsc (mA/cm2)
Voc (V)
FF (%)
Vacuum + (100 °C/10 min) Vacuum + (100 °C/10 min) Vacuum + (100 °C/ 10 min) Only (100 °C/1 h) Only (100 °C/1 h) Only (100 °C/1 h)
DMF: DMF: DMF: DMF: DMF: DMF:
7.7 8.2 10.1 1.8 1.9 2.3
13.3 14.6 17.8 3.1 3.4 4.1
0.92 0.86 0.83 0.85 0.78 0.81
0.63 0.63 0.68 0.68 0.71 0.69
DMSO(1:0) DMSO (9:1) DMSO (4:1) DMSO (1:0) DMSO (9:1) DMSO (4:1)
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Table 3 Photovoltaic performance of meniscus printed MAPbI3 solar cells in various active areas. Sample
Active area (cm2)
PCE (%)
Jsc (mA/cm2)
Voc (V)
FF (%)
meniscus printed MAPbI3 meniscus printed MAPbI3 meniscus printed MAPbI3
0.1 1 1.2
10.1 8.0 6.8
17.8 14.23 14.01
0.83 0.79 0.77
0.68 0.71 0.63
the solvent content ratio in the final desired perovskite morphology. Also, Fig. S6. shows the information on hysteresis of the cell champion (sample #3). It is found that there is no large performance difference in forward and reverse J-V measurements. To make clear the effects of our meniscus printing method on solar cells performance, we decided to compare the results of J-V curves of solar cell made by optimized meniscus printing method based on vacuum process and solar cell made by conventional one-step spin coating method based on Chlorobenzene antisolvent dripping. Fig. S5 and Table S6 show the J-V curve and final performance results of these two perovskite devices. As it can be seen, both deposition methods have comparable results. It should be noted that all perovskite precursor preparation and device fabrication parameters were equal for both perovskite solar cells except the perovskite deposition methods. Also, Fig. 10 and Table 3, show the photovoltaic performance of the champion cell in higher active areas of 1 and 1.2 cm2. We used a 3 * 3 mm2 mask on overlap of FTO and Au to define the active area of our devices. For larger areas (1 and 1.2 cm2) we just determined the device area by measuring the overlap area of FTO and Au. The maximum conversion efficiency achieved by the meniscus printed perovskite solar cells with vacuum process in active area of 1 and 1.2 cm2 was 8.0% and 6.8% respectively. Although the device performance with the PCE of 10.1% was not yet comparable with that of spin-coated perovskite solar cells, our study resolved the main problem with perovskite solar cells, which was their high dependence on the Ni-glovebox environment at various deposition steps. We fabricated our devices with a large-scale deposition method in the fully ambient air with the high humidity of 70%. Also, we believe that by working on the perovskite formula, it is possible to get even higher device efficiencies with the vacuum-assisted meniscus printing method.
for the layer in which the vacuum process was not involved, needle-like grains with a high pin-hole distribution in its morphology was found. This difference in the morphology could clearly explain the layer optical characteristic and facial color differences between these two layers. Also, the grain size had a profound effect on the performance parameters of the perovskite solar cells. It was such that the resistivity of the material was increased with decreasing the grain size. This was because the grain boundaries acted as some potential barriers against the motion of the carries across the grain boundaries. The other effect was that the carrier lifetime was decreased with the reduction of the grain size. Both effects led the reduction in the conversion efficiency of perovskite solar cells. Thus, we decided to increase the grain size by solvent engineering and temperature optimization during the printing process. The solvent engineering method was proposed in our perovskite solution by adding DMSO with various molar ratios to the DMF solution containing MAI and PbI2. By having a mixture of DMSO and DMF in our solvent, due to their difference in the boiling temperature, it was possible to control the solvent-removing process more easily during the vacuum step. The residual DMSO could improve the mass transport which could enhance the film quality if we slow down the evaporation rate of the solvent by vacuum process [40]. The DMSO presence led to the formation of an intermediate (PbI2-MAI-DMSO-DMF) phase (Fig. 3) preventing the crystallization process speed. SEM images of Fig. 8 show the effect of solvent engineering and temperature optimization on the perovskite grain size parameter. As can be seen in Fig. 8 and Table S5, the maximum perovskite grain size (476 nm) was achieved from the sample with the solution temperature and the solvent ratio of 55 ℃ and DMF: DMSO of 4:1, respectively. The grain size distribution graph for various solution temperatures and solvent ratios is also presented in Fig. S4. In the samples with the solution at room temperature, we did not find any difference in the perovskite grain size. However, by increasing the temperature to 55 ℃, the grain growth was observed. Also, at temperatures above 55 ℃, it was not clear why the decrease in the grain size and the increase in grain boundaries were found. Moreover, by adding some DMSO material to our ink, the increase in the perovskite grain size was obtained. However, this increase was much higher than that in the solvent with DMF: DMSO of 4:1, as compared with the ratio of 9:1. The claim could be confirmed by comparing the SEM images and the results of Fig. 8 and Table S5, respectively. To show the positive effect of the high grain growth parameter on device efficiency, we also examined the device photovoltaic performance by fabricating solar cells with various perovskite ink ratios at the temperature of 55 ℃ during the printing process. Also, we compared the performance of these devices both with and without the vacuum process. The corresponding device J-V curves with different solvent ratios and post-annealing times are shown in Fig. 9, and the device parameters are summarized in Table 2. As can be seen, adding a little DMSO in solvent increased the photovoltaic performance of the devices. Also, when the vacuum is used to remove the solvent, the efficiency is greatly increased compared to the case where the vacuum is not used. The vacuum-assisted, solvent DMF: DMSO ratio (4: 1) sample demonstrated a significant improvement in the PCE of 10.1%, whereas in the notvacuum-assisted, the PCE of 1.8% was observed for the only-DMF solvent sample. The huge difference between these two devices in terms of efficiency indicated the important role of the vacuum process as well as
4. Conclusions In Summary, we demonstrated a one-step meniscus printing method to provide pin-hole-free MAPbI3 perovskite film under air, low temperature and high relative humidity (60–70%) condition. The present study introduced a vacuum-assisted process before post-annealing treatment to mimic the self-drying behavior inherent in spin-coating. This vacuum device was introduced during the meniscus printing process to accelerate solvent evaporation which significantly improved surface coverage of the prepared perovskite films. Also, we have carefully controlled the printing parameters to get to the optimized film characteristics and final device performance. At the end, the optimized pin-hole-free perovskite film was obtained by optimal thickness and grain-size of 291 and 476 nm, respectively. To achieve to this goal, the optimized solution meniscus temperature was 55 °C during the printing speed of 1.65 mm/s on room temperature substrate. Compared to vacuum-assisted process, the not-vacuum-assisted layer had a needle-like morphology with lots of pin-holes all over the film. The champion vacuum-assisted and not-vacuum-assisted meniscus printed perovskite devices with optimal printing parameters showed a PCE of 10.1% and 2.3%, respectively. The obtained results are remarkable considering that using as-printed layers for short period of time in vacuum process can both get the required perovskite morphology and prevent the terrible effect of moisture during the post-annealing process. 155
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Funding
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