Abstract
CH3NH3PbI3 (MAPbI3) perovskite layers can be obtained by thermal annealing thin films of the intermediate dimethyl sulfoxide (DMSO) intercalated complex, MA2Pb3I8·2DMSO. In the present work, the formation, structure, and thermal transformation of the intermediate complex in both bulk and thin film form is examined in detail. The grain size and orientation of the intermediate crystallites in the solvent-intercalated thin film material is shown to directly influence the flatness of the annealed perovskite layer. Flat-lying orientation of the small needle-like intermediate crystallites is found to yield dense and flat perovskite layers. Optimized spin coating and annealing processes are developed for the formation and thermal conversion, respectively, of the intermediate film. Based on these methods, MAPbI3 perovskite solar cells with high power conversion efficiency (maximum ∼20.3%) were obtained with high reproducibility.
1. Introduction
ABX3-type organic-inorganic perovskites (A-site: methyl ammonium (MA+), formamidinium (FA+), Cs+; B-site: Pb2+, Sn2+; X-site: I−, Br−, Cl−, etc.) are promising materials for cost effective, printable photovoltaics. Perovskite-based solar cells with 3.8% power conversion efficiency (PCE) were first reported in 2009 by Miyasaka et al.1 Following intense research and development, device efficiencies now exceed 24%.2,3 Much of this rapid improvement may be attributed to refinements in perovskite layer fabrication protocols. Several techniques have been developed, such as the one-step4–6 and two-step solution methods,7 solvent engineering method,8–10 as well as vacuum11 and vapor-assisted deposition methods.12 Among these, solution methods are considered to be the most viable when taking into consideration the future high-volume manufacture of perovskite solar cells and modules.13
Regardless of the method, flat and dense perovskite layers are desired for high efficiency solar conversion. High film planarity allows for thinner, more efficient charge transport layers to be deposited on the top surface, while high film densities reduce the likelihood of conducting pinholes short-circuiting the perovskite layer. Recently, triple-cation or mixed composition perovskites have demonstrated high device efficiencies and good stability.14–16 Smooth films of these materials are also relatively easy to fabricate, directly transforming to perovskite at the spin coating stage.16 In this work, we focused on MAPbI3 as a representative metal halide perovskite, which is often used for fundamental investigations. Morphological control of MAPbI3 films is more difficult than mixed composition materials. Here, based on our long experience working with this material, we disclose how to fabricate dense and flat layers of MAPbI3 perovskite using the solvent engineering method, via the formation of a transparent, solvent-intercalated intermediate film of MA2Pb3I8·2DMSO. In typical solvent engineering methods, precipitation of the perovskite is induced by dripping antisolvent (e.g. toluene, chlorobenzene, or ether) on to a wet film of precursor solution.9,11 We noted, however, that when precursor solutions of MAI and PbI2 are dissolved in mixed dimethylformamide (DMF) and DMSO solvents a transparent intermediate forms, which is shown to be made up of needle-shaped crystallites of MA2Pb3I8·2DMSO complex having the crystal structure as previously determined by Yao et al.17 We also find that DMF can be intercalated in a similar manner as MA2Pb3I8·2DMF. The affinity for complexation of the perovskite starting materials with DMSO and DMF is in marked contrast to other common organic solvents, such a gamma-butyrolactone (GBL), from which perovskite is precipitated directly. Furthermore, we noted that with careful optimization of the spin coating parameters perovskite films of exceptional flatness could be obtained from the transparent MA2Pb3I8·2DMSO layers, yielding significant improvements in device efficiencies compared to the two step method used previously.18,19
Even in mixed solvents, DMSO is found to be preferentially intercalated in DMF-rich DMF:DMSO up to a 3:1 molar ratio. At higher DMF fractions, the DMF-intercalated intermediate complex, MA2Pb3I8·2DMF, is formed instead. Antisolvent dripping significantly influences the size and orientation of the crystallites, compared to fabrication without antisolvent addition.
Heating the transparent intermediate film removes the intercalated solvent molecules and drives a structural rearrangement to the black perovskite phase. The best film quality is obtained by slowly ramping the temperature to 100 °C. Examining the films at different stages of the temperature ramp, we show that solvent loss, crystal phase change, and grain ripening take place sequentially as the temperature increases. The quality of the perovskite layer depends on the intermediate layer grain size. While small crystallites can be successfully converted to MAPbI3 perovskite, complete conversion is no longer possible once the crystal dimensions exceed a certain size. Improved processes for the spin coating and annealing stages were developed based on these detailed observations in order to reliably fabricate dense and flat MAPbI3 perovskite layers. Applying these optimized fabrication methods, we achieved MAPbI3-based solar cells with maximum PCE values over 20% and high reproducibility.
2. Experimental
Materials
Methylammonium iodide (MAI, 99.0%), lead (II) iodide (PbI2, 99.99%),16 and titanium(IV) diisopropoxide bis(acetylacetonate) (75 wt% in isopropanol) were purchased from Tokyo Chemical Industry Co., Ltd. (TCI). Titanium(IV) tetrachloride (TiCl4, 99.0%) and lithium(I) bis(trifluoromethanesulfonyl)imide (LiTFSI) were purchased from Wako Pure Chemical Industries, Ltd. Co(III) (4-tert-butylpyridyl-2-1H-pyrazole)3·3bis(trifluoromethanesulfonyl)imide (FK 209, Co(III)TFSI, 98%) was purchased from Sigma-Aldrich Co., Ltd. 2,2′,7,7′-Tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) was purchased from Merck Co., Ltd. Mesoporous titanium(IV) oxide (PST-18NR, average particle size: ca. 20 nm) was purchased from JGC Catalysts and Chemicals Ltd. These materials were used as received.
Acetone (99%), ethanol (EtOH, super dehydrated), dimethylsulfoxide (DMSO, super dehydrated), dimethylformamide (DMF, super dehydrated) were purchased from Wako Pure Chemical Industries, Ltd. Toluene (super dehydrated) was purchased from Kanto Chemical. Co., Inc. 4-tert-butylpyridine (TBP, 96%) was purchased from Sigma-Aldrich Co., Ltd., which was distilled over CaH2 under Ar before use. All of the other solvents were degassed by Ar gas bubbling for 1 h and further dried over molecular sieves (3A) in a N2-filled glove box (O2 < 1 ppm) before use.
Equipment
Thermogravimetric analysis (TGA) was performed on a Shimadzu TGA-50 apparatus. Single crystal X-ray structure analysis was performed on a Bruker Single Crystal CCD X-ray Diffractometer SMART APEX II with Mo Kα radiation (λ = 0.71073 Å) and graphite monochromater (Bruker Co.). Thin film X-ray diffraction (XRD) measurements were performed on a Rigaku RINT 2500 (Rigaku Co.). Scanning electron microscope (SEM) images were recorded on SM 6500F (JEOL Ltd.), SU8010 (Hitachi High-Technologies Co.), or S4800 (Hitachi High-Technologies Co.) instruments. Optical microscopy was recorded on a VHX-6000 (Keyence Co.). Inductively coupled plasma mass spectrometry (ICP-MS) was conducted at NANO SCIENCE Co. High resolution transmission electron microscopy (HRTEM) images were recorded at Toray Research Center Inc.
The J–V curves and IPCE spectra of perovskite solar cells were measured in air with OTENTO-SUNIII and OTENTO-SUN-P1G instruments (Bunkoukeiki, Co. Ltd.), respectively. The scans were performed in both forward and reverse directions, with 50 mV voltage step and 1 s dwell time. IPCE spectra were recorded on a Bunkoukeiki SMO-250III system. The light intensity of the illumination source was calibrated against standard BS520 and SiPD S1337-1010BQ silicon photodiodes for the J–V and IPCE measurements, respectively. During the J–V and IPCE measurement, a shadow mask was used giving an active area of 0.1 cm2.
Preparation of Solvent-Intercalated Bulk Crystals
1.0 mmol solutions of PbI2 and MAI in either DMF or DMSO were prepared. After filtering with a 0.45 µm PTFE membrane filter, 100 µL of the solution was placed on a cover glass heated to either 55 °C (for DMF solution) or 73 °C (for DMSO solution). Colorless, needle-shaped crystals formed which afterwards partially transformed to a black solid. After cooling, the crystals were washed with acetonitrile to retrieve the colorless material. The structure of the colorless needle crystal was determined by X-ray crystallography to be MA2Pb3I8·2DMF and MA2Pb3I8·2DMSO for the DMF and DMSO solutions, respectively.
Growth of Compact-TiO2 and Mesoporous-TiO2 Layers
Glass-FTO (10 Ω/sq.) and quartz glass substrates were purchased from Asahi Glass Co., Ltd. The FTO layer was partially etched with zinc powder and HCl aqueous solution (6 M in de-ionized water). The patterned glass-FTO substrates (25 mm × 25 mm) were treated with ultrasonic cleaning for 10 min with a 1 wt% neutral aqueous detergent solution, acetone, 2-propanol, and finally distilled water, before being treated with UV-ozone for 15 min.
To deposit the compact-TiO2 layer, the FTO substrate was covered with stainless masks and heated on a hot plate to 450 °C. A 0.05 M solution of titanium di-isopropoxide bis(acetylacetonate) (75 wt% in 2-propanol) in ethanol was sprayed over the substrates with a sweep speed of ca. 20 cm/s, from a distance of 30 cm. After six spray cycles, spaced 3 minutes apart, the substrates were cooled to room temperature. The substrates were then immersed in a 0.77 wt% aqueous solution of TiCl4 at 70 °C for 30 min, followed by rinsing twice with distilled water. They were then sintered at 500 °C for 20 min.
100 nm mesoporous-TiO2 layers (average particle size: ca. 20 nm) were deposited on the substrate by spin-coating (5000 rpm, 30 s) a 1:8 wt. ratio suspension of TiO2 paste (PST–18NR) in ethanol, followed by sintering at 500 °C for 30 min. After cooling to room temperature, the substrates were treated with UV-ozone cleaning for 15 min immediately before the perovskite layer fabrication.
Fabrication of Perovskite Thin Films
All the procedures described in this section were conducted in an inert gas-filled glove box with the exception of the fabrication of the oxide layers, which was done in air.
Fabrication of Perovskite Layer (2 Step Method):18
PbI2 (461 mg, 1.0 mmol) was dissolved in DMF (1.0 mL) at 70 °C. The resulting solution was deposited on the mesoporous-TiO2 substrate by spin-coating (slope 5 s, 6500 rpm 5 s, slope 5 s). The resulting yellow film was annealed on a hot plate at 70 °C for 1 h. The film was dipped in a 0.060 M 2-propanol solution of MAI for 40 s. The formed perovskite film was then annealed on a hot plate at 70 °C for 1 h.
Exposure of the Perovskite Layer (2 Step Method) to DMF, DMSO, GBL, TBP, and IPA Vapor
The perovskite film was allowed to stand in a lidded petri dish at room temperature for 10 minutes together with four droplets of solvent. The sample was removed from the petri dish, heated at 100 °C for 10 minutes, and then cooled to room temperature.
One-Step Solution Method with Toluene Dripping and with Annealing
MAI (596 mg, 3.75 mmol) and PbI2 (1729 mg, 3.75 mmol) were dissolved in DMSO (2.5 mL) as a 1.5 M precursor solution. After stirring at 50 °C for 20 min, the resulting solution was filtered with a 0.45 µm PTFE filter. 190 µL of the solution was placed on the mesoporous-TiO2 substrates and spread by spin-coating (slope 1 s, 1000 rpm 40 s, slope 1 s, 0 rpm 30 s, slope 5 s, 5000 rpm 20 s, slope 1 s) to make a thin film. Optionally, during the last 2 s of spin-coating, 0.5 mL of toluene was dripped over the rotating substrate. The antisolvent was introduced from a pipet tip cut to widen the exit pupil so as to reduce the force of the solvent stream impacting on the precursor film. The tip of the pipet was held at a distance of ca. 2 cm from the substrate. The resulting transparent films with and without the antisolvent addition were annealed on a hot plate at 100 °C for 30 min.
Fabrication of Solar Cell Devices
All the procedures described in this section were conducted in an inert gas-filled glove box.
MAI (717 mg, 4.5 mmol) and PbI2 (2076 mg, 4.5 mmol) were dissolved in DMF (2.175 mL) and DMSO (0.725 mL) to form a 1.55 M precursor solution. After stirring at 40 °C for 1 h, the resulting solution was filtered with a 0.45 µm PTFE filter. 200 µL of the solution was placed on the mesoporous-TiO2 substrate and spread by spin-coating (slope 5 s, 5000 rpm 5 s, slope 1 s) to make a thin film. At 2 s before the end of spin-coating, 0.5 mL of toluene was slowly dripped over the rotating substrate (Figure S1, video is also available as part of the Supporting Information.) The resulting transparent film was annealed on a hot plate at 40 °C for 5 min, 55 °C for 5 min, 75 °C for 5 min and 100 °C for 30 min to form the perovskite layer.
Spiro-OMeTAD (73.5 mg, 0.060 mmol), FK 209 Co(III) TFSI (13.5 mg, 0.0090 mmol), TBP (27.0 µL, 0.20 mmol), and Li TSFI (8.6 mg, 0.030 mmol) were dissolved in chlorobenzene (1.0 mL). After stirring at 70 °C for 30 min, the resulting suspension was filtered with a 0.45 µm PTFE filter. 100 µL of the solution was placed on the perovskite layer and spread by spin-coating (slope 5 s, 4000 rpm, 30 s, slope 5 s), followed by annealing at 70 °C for 30 min. The devices were then transferred under inert atmosphere to a vacuum chamber where 80 nm gold electrodes were thermally evaporated on the spiro-OMeTAD layer through a shadow mask.
3. Results and Discussion
Solvent Vapor Exposure Test
The unusual interaction of DMF and DMSO solvent with perovskite first came to our attention when we observed the reversible transformation of MAPbI3 films under DMF or DMSO vapor. Exposure to the vapor of either of these solvents causes the black perovskite films to become colorless and translucent. Similar exposure to other organic solvents had little or no effect. Annealing the colorless films causes the black perovskite phase to re-appear, but with a significantly altered film morphology (Figure S2). X-ray diffraction (XRD) measurements confirm that both the initial and final black films are MAPbI3 (Figure S3). In solution phase, multiple transformations between intermediate solvent-intercalated complexes of MAPbI3 in DMF and DMSO have been carefully elucidated by Nakamura et al.20 In our work, we are concerned with elaborating the solvent-perovskite interplay in the solid state, and controlling the crystallinity and morphology of these transparent intermediate films to yield flat and dense perovskite layers.
Characterization of the Intermediate Complexes
First, in order to characterize the crystal structure and thermal characteristics of the transparent compounds, bulk samples of the solvent-intercalated materials were prepared under rapid timescales approximating the thin film growth conditions.
PbI2 (461 mg, 1.0 mmol) and MAI (159 mg, 1.0 mmol) were dissolved in 1.0 mL of DMF, DMSO, or GBL. A few drops of each solution were placed at the center of a slide glass placed on a hot plate at 100, 120, and 120 °C for DMF, DMSO, and GBL solution, respectively. (Videos are available as part of the Supporting Information.). As shown in Figure 1 for the case of GBL, cubic, black perovskite solid crystallizes out of solution directly — the expected result. For DMSO and DMF, however, long, colorless, needle-like crystals formed initially. Only after additional heating did the crystals transform into black MAPbI3. The intermediate compounds were isolated, and washed with acetonitrile to remove residual solvent prior to characterization.
Photographs of crystal growth process upon heating of several precursor solutions. MAI + PbI2 (1:1) in GBL, DMF, and DMSO. The circular images are about 3 cm wide. Videos are available as part of the Supporting Information.
Single-crystal X-ray structure analysis revealed that precursor compounds dissolved in DMF and DMSO form MA2Pb3I8·2DMF and MA2Pb3I8·2DMSO intermediates, respectively (Figure 2). In both cases the solvent molecule is intercalated between a planar [Pb3I8]2− layer and the MA+ cation. Note that neither complex maintains a perovskite stoichiometry: Both are deficient by one MAI formula unit for every three PbI2 units, unlike the MAPbI3·DMF complex we reported previously.21,22 The different structure most likely arises from the faster rate of crystallization in the present work.
X-ray crystal structures and TGA curves of (top) MA2Pb3I8·2DMF and (bottom) MA2Pb3I8·2DMSO.
The loss of solvent molecules from the bulk intermediates was monitored by thermogravimetric analysis (TGA) as shown in Figure 2. Solvent loss begins at 77 °C and 73 °C, for MA2Pb3I8·2DMF and MA2Pb3I8·2DMSO, respectively. The total weight loss of 8.1 wt% and 8.5 wt% is in good agreement with the theoretical values for complete solvent extraction, 7.9 wt% and 8.4 wt%. We note in passing that MA2Pb3I8·2DMF shows different behavior from MAPbI3·DMF. In the latter case solvent loss began at a much lower temperature, around 58 °C.21,22
Since solvent engineering protocols frequently make use of mixed DMF/DMSO solvents, it is important to determine the composition and structure of the solvent-intercalated intermediate complexes for different solvent ratios.23–25 To examine this, precursor solutions of 1.0 M MAI and 1.0 M PbI2 were made up with DMF:DMSO ratios from 1:0 to 0:1. The results, confirmed by X-ray structural analysis, are compiled in Table 1. MA2Pb3I8·2DMSO formed in DMF-rich solvents up to 3:1 DMF:DMSO, demonstrating the clear preference for the intercalation of DMSO over DMF in the intermediate complexes. Only DMSO and DMF single crystal complexes were isolated, no mixed solvent complexes were observed.
Intermediate Complexes Formed from MAI and PbI2 in Mixed Solvents
| Ratio of DMF:DMSO[a] | Structure of solvent–intercalated intermediate complex |
|---|---|
| 1:0 | MA2Pb3I8·2DMF |
| 7.7:1 | MA2Pb3I8·2DMF |
| 3:1 | MA2Pb3I8·2DMSO |
| 1:1 | MA2Pb3I8·2DMSO |
| 1:3 | MA2Pb3I8·2DMSO |
| 1:7.7 | MA2Pb3I8·2DMSO |
| 0:1 | MA2Pb3I8·2DMSO |
| Ratio of DMF:DMSO[a] | Structure of solvent–intercalated intermediate complex |
|---|---|
| 1:0 | MA2Pb3I8·2DMF |
| 7.7:1 | MA2Pb3I8·2DMF |
| 3:1 | MA2Pb3I8·2DMSO |
| 1:1 | MA2Pb3I8·2DMSO |
| 1:3 | MA2Pb3I8·2DMSO |
| 1:7.7 | MA2Pb3I8·2DMSO |
| 0:1 | MA2Pb3I8·2DMSO |
[a] Molar ratio.
Intermediate Complexes Formed from MAI and PbI2 in Mixed Solvents
| Ratio of DMF:DMSO[a] | Structure of solvent–intercalated intermediate complex |
|---|---|
| 1:0 | MA2Pb3I8·2DMF |
| 7.7:1 | MA2Pb3I8·2DMF |
| 3:1 | MA2Pb3I8·2DMSO |
| 1:1 | MA2Pb3I8·2DMSO |
| 1:3 | MA2Pb3I8·2DMSO |
| 1:7.7 | MA2Pb3I8·2DMSO |
| 0:1 | MA2Pb3I8·2DMSO |
| Ratio of DMF:DMSO[a] | Structure of solvent–intercalated intermediate complex |
|---|---|
| 1:0 | MA2Pb3I8·2DMF |
| 7.7:1 | MA2Pb3I8·2DMF |
| 3:1 | MA2Pb3I8·2DMSO |
| 1:1 | MA2Pb3I8·2DMSO |
| 1:3 | MA2Pb3I8·2DMSO |
| 1:7.7 | MA2Pb3I8·2DMSO |
| 0:1 | MA2Pb3I8·2DMSO |
[a] Molar ratio.
Thin Film Fabrication of Intermediate Complexes
We now turn to examining the properties of the intermediate MA2Pb3I8·2DMSO in the solid thin film state, including the influence of the film fabrication method, the morphology and orientation of the crystallites, and the details of the thermal conversion to MAPbI3 perovskite. Attention is drawn to the difference in stoichiometry between MA2Pb3I8·2DMSO and the annealed perovskite, MAPbI3. This complex is deficient in one MAI formula per three PbI2 units, so it is important to consider whether complete conversion of the film to MAPbI3 is achieved, and what side products may be formed.
The thin films studied in the following sections were prepared from precursor solutions of 1.55 M MAI and PbI2 (1:1) dissolved in 3:1 DMF:DMSO. The 3:1 DMF:DMSO solvent ratio was found to give an optimal balance between good solubility of the precursor materials and long drying times of the intermediate films.21 The precursor solution was spin-coated on a mesoporous TiO2 substrate, and toluene antisolvent was dripped over the spinning substrate for 1 s starting precisely 8 s after the 5000 rpm spin program started, and just before the spinning stopped. Full details of the fabrication process are given in the experimental section and illustrated schematically in Figure S1.
By inducing rapid precipitation of the intermediate crystallites the toluene antisolvent plays a critical role in determining the flatness of the intermediate film, and therefore the morphology of the final perovskite layer as well. Scanning electron microscope (SEM) images of the films before annealing reveal that toluene antisolvent addition facilitates the formation of needle-shaped microcrystals approximately 50 nm wide and 500 nm long (Figure 3a). When the toluene dripping was omitted and the films were allowed to dry naturally, the films were opaque or translucent (Figure 3b), rather than transparent. The intermediate crystals were found to be much larger, over 500 nm in width and to over a micrometer in length. With antisolvent, a black perovskite film with a flat and uniform surface was observed after annealing. Without antisolvent, the color of the film changed to dark-brown while retaining the rough morphology of the intermediate film. Such a rough and granular film is not suitable for efficient solar cells. Thus, antisolvent is necessary to control the crystal size of the intermediate complex, MA2Pb3I8·2DMSO, and ensure full conversion of the intermediate film to perovskite. In controlling the crystal size, and, as we will demonstrate later, the orientation of the needles with respect to the substrate, the antisolvent addition is directly responsible for forming dense and flat perovskite layers.
Top-view SEM images and XRD patterns of the films fabricated by (a) with or (b) without toluene dripping before and after annealing. Inserted photographs show a top view of each film. Asterisks indicate peaks of FTO substrate. Black arrows in (b) imply unconverted MA2Pb3I8·2DMSO.
The XRD measurements shown in Figure 3 confirm that the MA2Pb3I8·2DMSO intermediate film precipitated with the aid of toluene antisolvent is completely converted to perovskite on annealing. While the desirable result, it is surprising, considering that as a result of the 2:3:8 stoichiometry of MA:Pb:I, there is insufficient MAI present within the intermediate crystals for full conversion. Given that our precursor solution contained MAI and PbI2 in exactly 1:1 ratio, it seems reasonable to speculate that residual, amorphous MAI exists in the dried film and is taken up into the perovskite during thermal conversion. However, the possibility of balancing quantities of amorphous PbI2 cannot be ruled out as an alternate explanation. Regardless of which scenario is correct, the key point is that, for the intermediate film prepared without antisolvent, the MA2Pb3I8·2DMSO diffraction peaks remain after annealing, indicating that the films have only partially converted to perovskite. This is reasonable considering that in the bulk experiment described earlier, only the surface of the intermediate complex transformed to perovskite after annealing. Antisolvent addition therefore ensures full conversion of the intermediate material to perovskite, again through control of the crystallite size.
The perovskite film morphology is also likely influenced by the orientation of the intermediate crystallites. The difference in the relative peak intensities between the simulation and the observed XRD pattern of MA2Pb3I8·2DMSO intermediate indicates the presence of a preferred crystal orientation in the as-spun film. Single-crystal X-ray structural analysis confirmed that the long axis of the needle-shaped MA2Pb3I8·2DMSO crystal corresponds to the [2 –1 1] direction, which coincides closely with the direction of the Pb-I network, as shown in Figure 4a. Cross-sectional (Figure 4b) SEM images of the as-spun precursor film show that the individual needle-shaped crystals tend to stack with their long axes nearly parallel to the substrate. The effect of the preferred orientation was evaluated by Rietveld refinement of the XRD pattern using March-Dollase function26 in the RIETAN-FP software.27 The pattern could be reproduced by setting the preferred-orientation vector to the [2 –1 1] direction, and the March distribution parameter r converged to 5.52(7). Figure 4c shows the dependence of the relative peak intensities on the value r. Generally, r = 1 represents the absence of any preferred-orientation effect, and r > 1 (<1) represents the preferred orientation vector being parallel (perpendicular) with respect to the substrate. Thus, it can be concluded that the MA2Pb3I8·2DMSO crystal has a preferred orientation with its long-axis direction (Pb–I network direction) in the substrate surface plane. The crystal growth of the perovskite most likely preferentially proceeds following the same horizontal directions, filling voids in the film rather than creating roughness-inducing stacks or whiskers perpendicular to the substrate. Thus, the horizontal orientation of the intermediate crystallites may be an important additional factor to ensure the growth of a smooth perovskite layer. (Figure 4d)
(a) Photograph of the mounted MA2Pb3I8·2DMSO crystal showing the orientation of the crystal planes. (b) Cross-sectional SEM image of the as-spun film with toluene dripping. (c) Rietveld refinement of the intermediate phase. XRD patterns of as-spun film (blue line), several calculated XRD profiles (black line), and their difference (green line). The large r value indicates that the crystals align parallel to the substrate. (d) Illustration of the formation of aligned MA2Pb3I8·2DMSO crystallites on the substrate during spin coating.
Thermal Conversion of the Intermediate Complex to Perovskite
Examining the TGA results of the bulk intermediate crystals as shown in Figure 2, it might be expected that solvent loss might begin around 70 °C. Instead, for the intermediate film samples prepared with toluene antisolvent dripping, significant color change was observed after heating up to 55 °C. In the film, both solvent loss and the transformation to perovskite crystal structure occurs below that of the bulk solid.
The various stages of film growth were explored by heating the intermediate film on a hot plate in a nitrogen-filled glove box and stopping the annealing at different points on a temperature ramp to a maximum of 100 °C. Figure 5 shows top-view SEM images at each of the holding temperatures. Prior to annealing, the intermediate layer is made up of MA2Pb3I8·2DMSO microcrystals about 50 nm wide (Figure 5a). During the first heating stage at 40 °C, the films began to change from colorless and transparent to brown, indicating the onset of conversion to perovskite (Figure 5b). The conversion onset is significantly lower than for the bulk MA2Pb3I8·2DMSO crystals (see Figure 2). The difference in temperature is almost certainly due to the much smaller crystal sizes present in the intermediate film. We found that 55 °C is the critical temperature where the intermediate complex fully changes to the deep black typical of perovskite (Figure 5c). Despite the apparent complete conversion, we also found the overall crystal size increased and the number of grain boundaries decreased during additional “ripening” at around 75 °C (Figure 5d). Finally, a finishing step at 100 °C creates large grains which extend across the full height of the film (Figure 5f). The SEM results clearly show that multiple processes occur when the film is annealed in a stepwise manner over a range of temperature from 55 °C–100 °C.
Top view SEM images of (a) the as-spun perovskite layer with toluene antisolvent dripping, and of similar films after annealing at (b) 40 °C, (c) 55 °C, and (d) 75 °C. For 100 °C, the final annealing temperature, (e) top view and (f) cross-sectional SEM images are given. Photographs are inset in each top view image to show the corresponding change in color/transparency.
When the as-spun film was heated immediately to 100 °C for 30 min, the resulting perovskite layer had poor uniformity and contained many voids (Figure S4). This generally resulted in moderate device performance with lower average VOC and FF (Table S1). As seen in Figure 5, the stepwise-annealed films showed significantly higher quality. Based on the above results, a stepwise temperature ramp with fixed holding times: 40 °C for 5 min, 55 °C for 5–15 min, 75 °C for 5 min, and 100 °C for 30 min, was adopted as our standard, optimized annealing process for converting the MA2Pb3I8·2DMSO intermediate to perovskite layers.
Evaluation of Solar Cells Devices
To show the advantage of our method of fabricating dense and flat perovskite layers, glass-FTO/compact-TiO2 (20 nm)/mesoporous-TiO2 (∼100 nm)/MAPbI3 (480 nm)/2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) (200 nm)/Au (80 nm) solar cell devices were fabricated (Figure 6a) and characterized. The cross-sectional SEM image is shown in Figure 6b. Large perovskite grains can be readily observed which span the distance between charge transporting layers. These grains are not single crystals, however, showing instead multiple crystal domains according to high resolution transmission electron microscopy (HRTEM) (Figure S5) as noted by Kim et al,28 with the measurement area constrained to 0.1 cm2 using a shadow mask. The forward scan J-V curve of the best performing device is shown in Figure 6c, with JSC = 23.4 mA cm−2, VOC = 1.16 V, and FF = 0.75. The PCE is 20.3%. For a MAPbI3 perovskite device, with a typical structure using commonly available charge transport layers, this recorded open circuit voltage and efficiency are exceptionally good results. In addition, our fabrication method yielded highly reproducible devices (average PCE = 19.1 ± 0.6%). The best and average photovoltaic parameters are summarized in Table 2, and the incident photon to current conversion efficiency (IPCE) is shown in Figure 6d. These devices were confirmed to have similar stability to those reported in our related paper.22
(a) Device structure and (b) cross-sectional SEM image of the perovskite solar cells. (c) Best performing J–V curves (forward scan: red, reverse scan: blue) and (d) IPCE spectrum (black line) together with integrated JSC (red line). The integrated JSC from the IPCE measurement was 23.2 mA cm−2.
Photovoltaic Parameters of the Perovskite Solar Cells
| JSC (mA cm−2) | VOC (V) | FF | PCE (%) | |
|---|---|---|---|---|
| Best | 23.4 | 1.16 | 0.75 | 20.3 |
| Average[a] | 23.1 ± 0.6 | 1.13 ± 0.04 | 0.73 ± 0.02 | 19.1 ± 0.6 |
| JSC (mA cm−2) | VOC (V) | FF | PCE (%) | |
|---|---|---|---|---|
| Best | 23.4 | 1.16 | 0.75 | 20.3 |
| Average[a] | 23.1 ± 0.6 | 1.13 ± 0.04 | 0.73 ± 0.02 | 19.1 ± 0.6 |
[a] Average result for 85 devices.
Photovoltaic Parameters of the Perovskite Solar Cells
| JSC (mA cm−2) | VOC (V) | FF | PCE (%) | |
|---|---|---|---|---|
| Best | 23.4 | 1.16 | 0.75 | 20.3 |
| Average[a] | 23.1 ± 0.6 | 1.13 ± 0.04 | 0.73 ± 0.02 | 19.1 ± 0.6 |
| JSC (mA cm−2) | VOC (V) | FF | PCE (%) | |
|---|---|---|---|---|
| Best | 23.4 | 1.16 | 0.75 | 20.3 |
| Average[a] | 23.1 ± 0.6 | 1.13 ± 0.04 | 0.73 ± 0.02 | 19.1 ± 0.6 |
[a] Average result for 85 devices.
4. Conclusion
The mechanism of MAPbI3 perovskite layer formation by solvent engineering from mixed DMF:DMSO precursor solution and toluene antisolvent was examined, paying particular attention to the role of the solvent-intercalated complex in determining the morphology of the annealed perovskite layer. Control of the grain size and orientation of the MA2Pb3I8·2DMSO intermediate are found to be key requirements to form flat and dense perovskite layers. Control of the annealing process also significantly influences the perovskite film morphology. Based on these observations, fabrication methods were developed to reliably obtain high quality perovskite layers, resulting in excellent reproducibility in the fabricated solar cell devices and high performance. These findings are also informative for the fabrication of other devices using perovskite layers, such as perovskite light-emitting diodes.29
Acknowledgment
This study was partially supported by NEDO, JST–COI, JST–ERATO (Grant Number JPMJER 1302) and JST–ALCA (Grant Number JPMJAL 1603) programs. Additionally, the AUN-KU Student Mobility Program cooperated with Prof. Hideaki Ohgaki (Kyoto University) and Dr. Lim Chee Ming (Universiti Brunei Darussalam). We would like to thank Prof. Hiroshi Kageyama and Dr. Tomoko Aharen (Kyoto University) for measurement of the X-ray diffraction, and Prof. Kazunari Matsuda, Prof. Toshiharu Teranishi, Dr. Ryota Sato, and Dr. Tokuhisa Kawawaki (Kyoto University) for advice on the measurement of SEM images, and Dr. Masanori Tomozawa (Toray Research Center Inc.) for HRTEM observation.
Supporting Information
Details of the X-ray diffraction experiments and additional information (movies) about the perovskite film growth and structure are available at https://doi.org/10.1246/bcsj.20190241.
References
Best Research Cell Efficiencies Chart. National Renewable Energy Laboratory (NREL), 2019. https://www.nrel.gov/pv/cell-efficiency.html.
Atsushi Wakamiya
Atsushi Wakamiya received his Ph.D. degree from Kyoto University in 2003. He started his academic carrier at Nagoya University as an Assistant Professor in 2003. In 2010, he moved to Kyoto University as an Associate Professor, and was promoted to Full Professor in 2018. He has received several awards: The Chemical Society of Japan Award for Young Chemist in 2009, The Young Scientists’ Prize (MEXT, Japan) in 2012, The Nozoe Memorial Award for Young Organic Chemists in 2015, etc. He is a group leader of the ALCA (Advanced Low Carbon Technology Research and Development) project and COI (Center of Innovation) project of the Japan Science and Technology Agency (JST).
