Controlled Crystal Grain Growth in Mixed CationâHalide Perovskite by

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Controlled Crystal Grain Growth in Mixed Cation-Halide Perovskite by Evaporated Solvent Vapor Recycling Method for High Efficiency Solar Cells Youhei Numata, Atsushi Kogo, Yosuke Udagawa, Hideyuki Kunugita, Kazuhiro Ema, Yoshitaka Sanehira, and Tsutomu Miyasaka ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

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ACS Applied Materials & Interfaces

Controlled Crystal Grain Growth in Mixed CationHalide Perovskite by Evaporated Solvent Vapor Recycling Method for High Efficiency Solar Cells Youhei Numata,*,† Atsushi Kogo, † Yosuke Udagawa,‡ Hideyuki Kunugita,‡ Kazuhiro Ema,*,‡ Yoshitaka Sanehira,† and Tsutomu Miyasaka*,† †

Graduate School of Engineering, Toin University of Yokohama, 1614 Kurogane-cho, Aoba,

Yokohama, Kanagawa, 225-8503 Japan. ‡

Faculty of Science and Engineering, Sophia University, Chiyoda-ku, Tokyo, 102-8554 Japan.

Keywords, perovskite solar cell, formamidinium, metal oxide, composite material, carrier transport.

ABSTRACT

We developed new and simple solvent vapor-assisted thermal annealing (VA) procedure, which can reduce grain boundaries in a perovskite film, for fabricating highly efficient perovskite solar cells (PSCs). By recycle of solvent molecules evaporated from an as-prepared perovskite film as

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a VA vapor source, named as pot-roast VA (PR-VA) method, finely controlled and reproducible device fabrication was achieved for formamidinium (FA) and methylammonium (MA) mixed cation halide perovskite, (FAPbI3)0.85(MAPbBr3)0.15. The mixed perovskite was crystallized on a low-temperature prepared brookite TiO2 mesoporous scaffold. Exposed to very dilute vapor of solvent, small grains in a perovskite film gradually unified into large grains resulting in grain boundaries which were highly reduced towards improvement of photovoltaic performance in PSC. PR-VA treated large grain perovskite absorbers exhibited stable photocurrent-voltage performance with high fill factor and suppressed hysteresis, achieving the best conversion efficiency of 18.5% for a 5 × 5 mm2 device and 15.2% for a 1.0 × 1.0 cm2 device, respectively.

1. Introduction

Perovskite solar cells (PSCs) based on organic–inorganic hybrid lead trihalide have attracted wide interests due to the high conversion efficiency and potentially low production cost.1,2 Based on significant efforts to improve qualities of materials and cell structures, the conversion efficiency of PSC has reached 22.1%,3 the same level of the best efficiencies of CdTe and CIGS solar cells. Since the high conversion efficiency was reported, widespread applications and commercialization of PSCs have been expected; however, some problems and challenges are still left such as stability, toxicity, and further improvement of efficiency.4 A key to this progress of efficiency has been engineering of solution process to achieve void-less uniform qualities of perovskite layer and contact interfaces. Well-controlled morphology of perovskite layer is significant element for realizing compatibility of high efficiency and high stability. In order to improve quality of perovskite layer, various techniques have been developed such as additives,5-7 fast deposition-crystallization (anti-solvent dripping),8.9 sequential deposition method,10,11 hot


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casting technique,12 nonstoichiometric perovskite precursor,13-16 and vapor-assisted annealing.1722

Vapor-assisted annealing (VA) treatment accelerates crystal grain growth in perovskite films via Ostwald ripening like process.19-22 Typically, VA treatment is carried out as follows; a perovskite film is put into an anneal pot (e.g., petri dish) containing various solvents. The perovskite film is kept exposed to the solvent vapor being thermally evaporated, which promotes crystal growth. This annealing process helps formation of large perovskite crystal grains in micrometer sizes, which leads to reduce grain boundaries involved in carrier recombination and interfacial resistivity and consequently improves a conversion efficiency of the cell. Additionally, this process can be combined with other method such as fast deposition-crystallization and sequential deposition. Although VA treatment seems easy and simple method, it is difficult to control the VA condition with good reproducibility. For example, for the case that solvent vapor is evaporated after film insertion into VA pot, non-uniform distribution of solvent vapor can impair reproducible perovskite formation (Figure S1a). On the other hand, for the case that a VA pot has already filled with solvent vapor before VA treatment, solvent vapor is escaped when the pot is opened to insert the perovskite film (Figure S1b). Additionally, high vapor concentration condition and longer time annealing cause excess crystal growth. This allows large pinholes to generate and/or grains to be dispersed sparsely on the substrate, leading to serious deterioration of photovoltaic performance of PSCs. Therefore, development of reliable and reproducible VA method is sought after to achieve higher conversion efficiencies. The key to the VA method is to realize uniform vapor distribution and retaining of the solvent vapor inside the VA treatment pot. We considered that if the solvent contained in an as-prepared perovskite film can be recycled as a


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VA solvent, reproducible VA treatment with accurate control of vapor concentration could be carried out (Figure 1). We called this new and simple method as pot-roast (PR)-VA method. Perovskite composition is also a key factor to achieve high conversion efficiency and durable device. By changing a composition ratio of A-site cation (MA, FA, Cs), halogen ions,23-28 and central metal ions (Pb, Sn, and others),30-32 energy levels and band gap can be finely tuned, and chemical stability as well.23-29 In this study, we chose MA, FA, I, and Br mixed perovskite, (FAPbI3)0.85(MAPbBr3)0.15 for this research because the mixed perovskite has presented significantly high conversion efficiencies beyond 20% and high stability against heat and light soaking.23 We report here a new VA method and influence of the PR-VA method on photovoltaic properties of mix cation–halide (FAPbI3)0.85(MAPbBr3)0.15 perovskite-based solar cells. 2. Experimental Section 2.1. General methods All reagents and solvents were purchased from chemical companies (Tokyo Chemical Industry Co., Ltd, Wako Pure Chemical Industries, Ltd, and Sigma-Aldrich Japan). An aqueous Brookite TiO2 suspension (PECC-B01) and FTO glass substrates were bought from Peccel Technologies. 2.2. Experimental details 2.2.1. Preparation of precursor solutions A 1.5 M solutions of formamidinium iodide (FAI) and PbI2, and methylammonium bromide (MABr) and PbBr2 in N,N-dimethylformamide (DMF) and dimethylsulfoxide (DMSO) (4:1 v/v) were mixed in a 85 : 15 molar ratio, and stirred at 70°C for 1 h. 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) (70 mM) was dissolved in chlorobenzene and stirred at 70 °C for 1 h. The solution was cooled to r.t., and 4-tert-butylpyridine (0.231 M), a stock solution of LiTFSI (0.035 M) in acetonitrile, and a


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stock solution of FK209 PF6 salt (0.002 M) in acetonitrile were added as an additive and oxidizing agents (molar ratio; 1 : 3.3 : 0.5 : 0.03).

2.2.3. Device fabrication FTO glass substrate (2.5 × 2.5 cm2) was washed with EtOH, 5%, HellmanexTM (2%), and deionized water by ultrasonic treatment for 15 min each. TiO2 compact layer (CL) was prepared by spin-coating method. The cleaned FTO substrate was treated with UV-ozone cleaner. Then, 0.15 M [Ti(acac)2(iPrO)2] (acac = acetylacetonato and iPrO = iso-propoxy) in IPA was spincoated at 3000 rpm for 30 sec, dried at 100 °C for 5 min. The substrate was cooled down, and similar operation was carried out twice by means of 0.3 M [Ti(acac)2(iPrO)2] solution. The substrate was then sintered by muffle furnace at 450 °C for 15 min. A Brookite TiO2 colloidal suspension was casted onto the TiO2 CL and spin-coated at 3000 rpm for 40 sec. The substrate was dried at 70 °C for 2 min and then, annealed at 150 °C for 1 h. Before preparation of perovskite film, a precursor solution and the mesoporous substrates were warmed at 70 °C. The precursor solution of (FAPbI3)0.85(MAPbBr3)0.15 perovskite was poured onto the TiO2 mesoporous substrate and spin-coated (kept for 60 sec, 3000 rpm for 40 sec, and 5000 rpm for 10 sec with toluene (300 µl) dripping). The as prepared film was annealed at 110 °C for various times with or w/o VA treatment. VA treatment was carried out using with a glass petri dish (φ = 45 mm, h = 18 mm), and to prevent escape of solvent vapor via surface asperity of a hot plate, we put a glass plate on the hot plate. On the perovskite films, the HTM solution was spin-coated at 6000 rpm for 30 sec and the prepared cell was kept under dry and dark condition overnight to promote oxidation. Finally, gold was vacuum-evaporated onto the HTL as a counter electrode.


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2.3. Measurements 2.3.1. SEM and XRD measurements SEM measurements were performed by SU8000 (Hitachi High-Technologies Co.) XRD patterns were measured by D8 DISCOVER (Bruker-AXS K. K.) with Cu Kα radiation under operation condition of 40kV, 40mA. 2.3.2. Photovoltaic measurements Cell active area (5 × 5 mm2) was defined by a black metal mask. Photocurrent density-voltage (J-V) curves were recorded using with PEC-L01 solar simulator (Peccell Technologies) under AM1.5G condition (100 mW cm–2). Measurement condition; Step voltage: 0.01 V, search delay: 0.05 sec, and hold time: 0.1 sec. Incident photon-to-current conversion efficiency (IPCE) spectra were observed by PEC-S20 spectrometer (Peccell Technologies). 2.3.3. Transient photoluminescent spectra measurements Photoluminescent (PL) decay measurement was carried out using the second harmonic of a pulse from an amplified mode-locked Ti:Al2O3 laser at a repetition rate of 10 kHz (Coherent RegA) and a streak camera (Hamamatsu C5680). Excitation light wavelength was 400 nm and power was 100 µW. The perovskite only film was prepared on a cleaned glass substrate, and encapsulated between glass substrates glued by a HIMILAN film (DuPont). A structure of mesoscopic TiO2/perovskite film was similar to the solar cell device without HTL and Au counter electrode.

3. Results and Discussion 3.1. Film preparation and VA treatment


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Perovskite films were prepared by spin coating plus antisolvent dripping method with toluene.9 Film preparation and cell fabrication were carried out in a dry chamber (RH < 4%). As a mesoporous scaffold, we employed a low-temperature prepared brookite TiO2 nano-particlebased mesoporous film, which was dried at 150 °C. Use of low-temperature TiO2 scaffold leads to fabrication of plastic substrate-based flexible devices in future application.33 We examined the various VA treatment conditions as followings. Thermal annealing temperatures were 110 °C and total annealing time was fixed as 20 min for a comparison. A DMSO ratio in the perovskite precursor solution was 20%, which is a key factor to control PR-VA treatment. For the perovskite film without any VA treatment (w/o-VA), only thermal annealing was carried out for 20 min. For conventional-VA treatment (c-VA) and PR-VA treatment, VA treatment was carried out for 15 min in a glass petri dish. Then, the petri dish was removed and the perovskite film was thermally annealed for additional 5 min. For c-VA treatment, a drop of DMSO was put onto the hot plate and completely evaporated in the petri dish, and then an as-prepared perovskite film was put in the saturated solvent vapor that is filled inside a petri dish. For the PR-VA treatment, an asprepared perovskite film was confined in a petri dish without adding any solvent externally. After insertion of the as-prepared perovskite film into the petri dish on the hot plate, the clear orange film quickly turned into black and solvent was evaporated from the perovskite film. Then, dew concentration was observed inside the petri dish and the perovskite film was exposed to the solvent vapor. It is expected that the VA solvent molecules in the as-prepared perovskite film are confined below crystallized surface layer by poor solvent dripping and coordinated to lead ion as a ligand, forming an intermediate phase (Figure S2).34-36

3.2. Film characterization of VA treated perovskite films


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Appearances of the prepared perovskite films with or without VA treatment were significantly different. The w/o-VA perovskite film gave a dark and mirror like very smooth surface. The c-VA treatment gave different results depending on amount of DMSO solvent. With less amount of DMSO (3 µL) for VA treatment, the perovskite surface became lackluster black. In contrast, with over 4 µL of DMSO, the film turned black, but partly melted into yellow solution. Then, recrystallization started and the film was finally turned into black perovskite (Figure S3). Sometimes the latter phenomenon can be observed on the c-VA performed using less volume DMSO, which however tends to deteriorates cell yield and performance. These experiences indicate importance of controlling the VA by adjusting the vapor concentration. The pot-roast method can control the VA by way of self-adjustment of vapor concentration. For a PR-VA film, the perovskite film turned into mirror-like black immediately after insertion of the film into a petri dish. Then, by exposure to the evaporated solvent vapor from the perovskite film, appearance of the perovskite film surface became lackluster black similar to the c-VA film with less volume solvent. These appearance changes of perovskite surface before and after VA treatment are attributed to the surface texture of the films (vide infra). Figure 2a-d and S5-9 shows cross-sectional scanning electron microscopy (SEM) images of w/o-VA, c-VA, and PR-VA PSCs. Thicknesses of each layer are 50 nm (CL), 150 nm (brookite TiO2 mesoporous layer), 600 ~ 650 nm (perovskite), 150 nm (HTL), and 70 nm (Au), respectively. Thickness of the brookite TiO2 mesoporous film is less than 150 nm, which is further thinner compared to reported brookite-based PSCs.37-39 The TiO2 nano-particles fill the valleys between FTO textures. Because of small TiO2 particle size (18.5 ± 5.0 nm),37 the TiO2 mesoporous layer is dense and forms very flat surface on the FTO substrate. A w/o-VA perovskite film consists of many small crystal grains with sizes up to 300 nm, and they are


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vertically stacked (Figure 2a). By c-VA treatment, the grains were gradually integrated and became enlarged depending on VA time. Finally, each grain size exceeds 1.5 µm as shown in Figure 2b. Each grain became large dome-like shape, which is because the solvent vapor is produced from topside of the perovskite film, and the crystal grain became spherical due to the surface tensile. Further VA treatment, the crystal grain grown further and finally, the perovskite film became discretely distributed crystals (Figure 2c). The deep valleys formed between the discrete crystals behave as pinholes and charge recombination is expected. In contrast, by PRVA treatment, solvent vapor is gradually evaporated from inside of the perovskite film and filled in the petri dish. For the PR-VA treated perovskite film, each crystal grain was unified by maintaining film character unlike in the case of c-VA treated film (Figure 2d and S9). Surface SEM images of w/o-VA, c-, and PR-VA treated perovskite films are shown in Figure 3a-d. For the w/o-VA perovskite film, many crystal grains appeared which sizes were ranging from 100 to 300 nm. In contrast, surface grain sizes of the PR-VA treated perovskite film are increased up to 800 nm. Crystal grains in a c-VA film treated with 3 µL of DMSO were grown over 1 µm because of Ostwald ripening under higher DMSO vapor concentration condition. For a c-VA film treated with 5 µL of DMSO, crystal grains were further enlarged up to 3 µm; however, comparable sized pinholes also generated (Figure 3d). Therefore, the change in surface texture before and after VA treatment is attributed to the difference in surface roughness and size of grains. Figure 4 and S4 shows X-ray diffraction (XRD) patterns of the perovskite films prepared by different VA methods. For all samples showed characteristic (111) diffraction peak of the trigonal (FAPbI3)0.85(MAPbBr3)0.15 perovskite at 2θ = 14.0°.23 The VA treatment did not affect to crystal structure of mixed perovskite. Negligible diffraction peaks corresponding to by-products; PbI2


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(2θ = 12.6°) and δ-phase of FA-based perovskite (2θ = 11.5°) are observed, regarding single αphase perovskite can be obtained. For yellow part of c-VA cells treated with high concentration DMSO vapor and PR-VA cell with high DMSO ratio showed diffraction peaks corresponding to

δ-phase (Figure S3). It indicates that under too much high DMSO vapor condition, δ-phase is partly formed via melting.

3.3. Photoluminescence lifetime measurements In order to understand influences of VA treatment on inherent property of perovskite films, we measured photoluminescence (PL) decay for the VA treated perovskite films (Figure 5 and Table S1). The perovskite films were prepared on a cleaned glass substrate and sandwiched between glass substrates by a HIMILAN® film (Du Pont) to protect the perovskite films from oxygen and humidity during the measurement. Figure 5 shows PL lifetime for different VA treated perovskite films and their integrated spectrum (excitation laser; 400 nm, 100 µW). Luminescent decay curves were clearly different depending on VA-treatment method. PL lifetime of w/o-VA film is shortest among the three perovskite films. The PL lifetime was lengthened in the order of w/oVA, PR-VR, and c-VR, which indicated that a perovskite film consisting of larger grain size presented longer PL lifetime. This result implies the photoexcited carriers are quenched at the grain boundary and/or surface defect of the crystal grains.42-44 PL lifetime of the PR-VA film was slightly and the c-VA film was significantly improved compared to that of w/o-VA film. This is due to the crystal grains in the PR-VA film are not yet perfectly unified (Figure 2b), and the remaining boundaries behave as carrier recombination sites. In contrast, the crystal grains in the c-VA film are highly unified; therefore, there is no grain boundary in a vertical direction to the substrate and less grain boundaries in a parallel direction to the substrate as shown in Figure 2c,


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leading the longest PL lifetime. PL spectra of the pristine perovskite films and perovskite/TiO2 film are shown in Figure 5b. With increasing perovskite grain size, PL intensities of the pristine perovskite films increased and the peak top was red-shifted by 3.3 and 2.6 nm, respectively. It is expected that the red shift of PL peak is attributed to improvement of crystallinity and uniform distribution of cations and halogen ions at crystal inside/grain boundaries via Ostwald ripening. By contact with an electron collecting TiO2 substrate, PL intensities of all three perovskite films were strongly suppressed, indicating that photoexcited electrons are efficiently collected by the TiO2 mesoporous layer.

3.4. Influences of VA treatment on photovoltaic properties At first, we prepared PSCs without any VA treatment (w/o-VA) and varied annealing time for 10, 15, and 20 min. A result is shown in Table 1 and Figure S11. Their averaged photovoltaic parameters do not show significant difference depending on annealing time, and large hysteresis is observed on all w/o-VA devices. The efficiencies in reverse and forward scans were approximately 15% and 9%, respectively. For c-VA devices, we examined an influence of adding DMSO amount on the photovoltaic properties as shown in Figure S12 and Table 2. We varied DMSO amount from 1 to 3 µL. Their conversion efficiencies were improved by decreasing with amount of DMSO solvent. The main reason of the improved efficiency of the c-VA device is attributed to fill factor (FF) values, and hysteresis widths of the J–V curve were suppressed as well. The FF of c-VA device was improved by low concentration DMSO vapor treatment. As shown the cross-sectional SEM images of the c-VA devices VA-treated with different DMSO volume in Table 2, the crystal grains in perovskite film VA-treated under higher concentration DMSO vapor were grown larger


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and some pinholes generated, resulting the low FF value was observed. This result indicative that (a certain level of) lower DMSO concentration give better efficiency; however, as mentioned above, it is difficult to control vapor concentration for VA treatment, in particular low concentration. We optimized PR-VA conditions; VA time and DMSO ratio to improve conversion efficiency (Table 3, S2, S3, and Figure S13 and S14). An influence of VA treatment time was examined for 5, 10, 15, and 20 min, and additional normal thermal annealing for 5 min. Short-circuit photocurrent density (JSC) was slightly improved with increase of PR-VA time. Open-circuit voltage (VOC) and FF values were also improved as well as JSC from 5 to 15 min; however, both VOC and FF values deteriorated after 20 min VA treatment. Hysteresis behavior in J-V curve also presented same trend; therefore, 15 min PR-VA treatment gave the best conversion efficiency. An influence of DMSO ratio in the perovskite solution also examined from 10 ~ 80 %. The obtained efficiencies did not show significant difference depending on DMSO content. However, under conditions of DMSO ratio above 40%, reproducibility of perovskite films deteriorated and some perovskite films were partly melted similar with the c-VA with high DMSO concentration condition (Figure S3) due to a high DMSO vapor concentration. On the other hand, for the DMSO 10% condition, photovoltaic parameters slightly decreased and hysteresis was increased. It is expected that crystal grain reconstruction via Ostwald ripening is not sufficiently promoted due to low DMSO vapor concentration. Taking the optimization results into consideration, we concluded that the highest conversion efficiency with least hysteresis could be obtained by the PR-VA condition: 20% DMSO ratio and 15 min VA treatment. We compared influences of VA treatments on photovoltaic properties for three conditions. To optimize VA, we examined various cell fabrications conditions on each method (Figure 6). Total


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annealing time and a DMSO ratio in the perovskite precursor solution were fixed as 20 min and 20%, respectively, for a comparison. All photovoltaic data of optimization process is summarized in supporting information (Table S2-S4). Photocurrent density-voltage (J-V) curves and external quantum efficiency (EQE) spectra of the examined PSCs are shown in Figure 7 and their photovoltaic parameters are summarized in Table 3. w/o-VA devices could work with efficiency up to 15%. However, they showed significantly large hysteresis in J–V curves and their averaged photovoltaic performances are 9% in forward (JSC → VOC) scan and 15% in reverse scan (VOC → JSC), respectively. Hysteresis is mainly due to large difference in FF and VOC. Low FFs are assumed to be caused by high series resistivity of the device. For c-VA cells, JSC value is comparable to w/o-VA cell. VOC and FF was slightly improved, particularly, in reverse scans, resulting a similar hysteresis gap compared with the w/o-VA cells. For PR-VA cells, VOC and FF in forward scans were further improved compared with those of c-VA cells, and in reverse scans as well. Consequently, hysteresis is fairly suppressed in the PR-VA cells. As a result, the best efficiency of 16.6% in reverse scan was achieved. In Figure 2b, external quantum efficiency (EQE) spectra of these three cells are shown. EQE onsets are perfectly same, 810 nm, and their calculated photocurrent values are ranging from 19.93 to 20.93 mA cm–2. The deviation of the photocurrent values from J–V measurement is less than approximately 5%. In the UV region, EQE of c-VA device slightly decreased compared with other two devices. This is expected that perovskite crystals came out from the mesoporous layer by c-VA treatment, and the incident light was scattered and/or reflected at an interface between FTO and TiO2 CL/mesoporous layer (vide infra). Although VA-treatment was applied, hysteretic behavior is still remained on both c- and PR-VA treated devices. The hysteresis gap in J-V curves of c-VA was larger than that of PR-VA treated devices. From the PL decay measurement, higher vapor concentration condition gave


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longer carrier lifetime for the perovskite only samples. Therefore, we expected that the remnant hysteresis in J-V curves of the VA treated PSC is mainly attributed not to quality of the perovskite film but the interfaces between perovskite and HTL or electron collecting layer (TiO2). In comparison to the reported hysteresis-less mesoscopic MAPbI3 cell based on Brookite TiO2 nano-particles,37,39,40 pore filling of the PR-VA treated (FAPbI3)0.85(MAPbBr3)0.15 cell is still poor (Figure S5), leading less interface area between perovskite and the electron collecting TiO2 meso-structure. It is considered that a loss in electron collection efficiency at the perovskite and TiO2 interface, and imperfection of inter-particle necking between brookite TiO2 particles (and/or CL) due to low temperature sintering are enhancing hysteretic behaviors.41 The c-VA device showed larger hysteresis and lower FF compared to the PR-VA device although crystal sizes were larger than those of PR-VA film as shown in Figure 6. The hysteresis gap in c-VA device was increased with increased DMSO amount. A reason why it happens is that perovskite is sucked out from mesopore, leaving some voids generated in the TiO2 mesoporous film during crystal growth via Ostwald ripening, as a result of re-dissolution by higher concentration DMSO vapor (Figure 8). This also causes less interfacial contact between perovskite and TiO2 nanoparticles. To confirm reliability of the efficiency of the hysteretic cell, we performed steady-state photocurrent measurement at the maximum power point for the PR-VA treated cell (cell area: 3 × 3 mm2). Photocurrent value was 19.4 mA cm–2 with applied bias of 0.86 V, and a resulting stabilized efficiency was 16.7%, which is comparably high with the efficiency of 17% estimated from J–V measurement in reverse scan (Figure 7c). We further optimized thickness of the Brookite TiO2 mesoporous layer on a PR-VA device. By using thinner mesoporous layer based on a further diluted Brookite suspension in EtOH (1:18, v/v), the best efficiency of 18.5% with


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JSC = 22.27 mA cm–2, VOC = 1.13 V, FF = 0.73 was achieved in reverse scan (Table S1 and Figure S16). Based on the optimized PR-VA procedure, we fabricated a larger device with an aparture area of 1.0 × 1.0 cm2. With this device, high conversion efficiency up to 15.2% was also obtained (Figure S17).

4. Conclusion In conclusion, we have successfully developed and optimized new pot-roast vapor-assisted thermal annealing method for a mixed perovskite film, (FAPbI3)0.85(MAPbBr3)0.15, by using DMSO solvent, and achieved over 18% of conversion efficiency on a 5 × 5 mm2 area cell. Without VA treatment, perovskite film was occupied by small grains of ca. 300 nm. In conventional VA treatment, although perovskite crystal was grown up to larger size over 1 µm, pinhole formed between grain boundaries of perovskite causes imperfect coverage and occupancy of mesoporous layer. Pot-roast VA treatment was found to be effective VA method to produce large and unified perovskite grains capable of high coverage of mesoporous scaffold without pinhole generation. Based on PR-VA method, VOC and FF values were improved with reduced hysteretic performance and the best efficiency reached 16.6%. By a further optimization of a mesoporous TiO2 layer, we achieved 18.5% and 15.2% of the best efficiencies on 5 × 5 mm2 and 1.0 × 1.0 cm2 area devices, respectively. The PR-VA method is highly influenced based on solvent properties such as solubility, boiling point, and coordination ability (Lewis acidity). To obtain higher conversion efficiency, we are attempting further optimization of PR-VA procedure by means of different solvents and their combinations. Additionally, Our fabrications of PSCs are based on a low-temperature prepared brookite TiO2 mesoporous layer. We have previously reported a brookite-based low-temperature fabricated PSC and flexible PSCs based on a plastic


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substrate.39,40 Our study to apply the PR-VA treated PSC to low-temperature fabrication of flexible PSCs is in progress.


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Figure 1. Schematic illustration of pot-roast vapor-assisted annealing (PR-VA) procedures.


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Figure 2. Cross-sectional SEM images of PSCs; (a) w/o-VA, with (b) PR-VA, and c-VA using (c) 1 µL, and (d) 5 µL of DMSO. Inset: Expanded images. Bars: 1 µm.

Figure 3. Surface SEM images of of PSCs; (a) w/o-VA, with (b) PR-VA, and c-VA using (c) 1 µL, and (d) 5 µL of DMSO. Inset: Expanded images. White bars: 1 µm.


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Figure 4. XRD patterns of no-, c-, and PR-VA treated Perovskite films on mesoscopic Brookite TiO2 substrates.

Figure 5. (a) Photoluminescent decay and (b) integrated PL intensities of perovskite films with or without VA treatment. Solid lines are fitting lines based on (a) double exponential and (b) gauss fit, respectively.


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Figure 6. Photovoltaic parameters of different VA-treated PSCs in forward (blue) and reverse scans (red).


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Figure 7. (a) Photocurrent–voltage curves and (b) EQE spectra of w/o-, c-, and PR-VA treated PSCs. (c) Steady-state photocurrent density with 0.86 V bias (blue) and stabilized efficiency (red). Inset; J-V curve of the PR-VA device. Black marker corresponds to the steady-state efficiency data.

Figure 8. (a) Schematic illustrations of crystal grain growth in the perovskite/mesoporous layer during VA treatment via Ostwald ripening. Cross-sectional SEM images at the interface between perovskite and mesoporous layers for (b) c- and (c) PR-VA treated PSCs.


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Table 1. Photovoltaic parameters of w/o-VA PSCs depending on annealing time. Anneal Time

scan JSC a) direction (mA cm–2)

VOC (V)

FF

η (%)

21.42±0.19

0.94±0.06

0.43±0.06

8.6±1.4

(21.47)

(0.99)

(0.51)

(10.9)

21.29±0.16

1.07±0.05

0.66±0.05

15.0±1.5

(21.36)

(1.08)

(0.67)

(15.3)

20.67±1.00

0.96±0.06

0.48±0.05

9.6±1.9

(21.87)

(1.01)

(0.52)

(11.4)

20.49±1.05

1.06±0.05

0.65±0.08

14.3±2.8

(21.72)

(1.09)

(0.69)

(16.4)

20.65±0.32

0.95±0.03

0.47±0.04

9.2±0.9

(20.60)

(0.97)

(0.52)

(10.3)

20.51±0.34

1.06±0.02

0.70±0.02

15.3±1.0

(20.43)

(1.06)

(0.70)

(15.1)

No. of cell

forward 10 min

8 reverse

forward 15 min

6 reverse

forward 20 min

8 reverse

a)

scan direction: forward JSC → VOC and reverse: VOC → JSC. In parentheses, the parameters of the best cell. Cell active area is 5 × 5 mm2 defined by black mask.


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Table 2. Photovoltaic parameters of c-VA PSCs depending on adding DMSO volume. DMSO vol. scan JSC directiona) (mA cm–2)

VOC (V)

FF

η (%)

21.17±0.51

0.98±0.05

0.51±0.07

10.2±1.9

(20.02)

(1.03)

(0.61)

(12.7)

19.98±0.49

1.07±0.03

0.67±0.04

14.4±1.1

(19.93)

(1.11)

(0.70)

(15.5)

20.26±1.40

0.97±0.05

0.47±0.06

9.2±1.7

(21.27)

(1.04)

(0.56)

(12.5)

20.17±1.04

1.06±0.03

0.62±0.07

13.2±1.9

(21.21)

(1.08)

(0.65)

(15.0)

21.58±0.38

0.95±0.06

0.41±0.10

8.5±2.4

(21.42)

(1.04)

(0.64)

(14.2)

No. of cell

forward 15

1 µL reverse

forward 16

2 µL reverse

forward 13

3 µL 21.43±0.46

1.04±0.04

0.57±0.08

12.9±2.2

(21.60)

(1.11)

(0.71)

(17.0)

reverse a)

scan direction: forward JSC → VOC and reverse: VOC → JSC. Annealing condition: VA for 15 min, and then, thermal annealing for 5min. In parentheses, the parameters of the best cell; Cell active area is 5 × 5 mm2 defined by black mask.


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Table 3. Photovoltaic parameters of w/o-VA, c-VA, and PR-VA PSCs. VA conditiona)

scan directionb)

JSC

w/o-VA

forward

reverse

c-VA

forward

reverse

PR-VA

forward

reverse

VOC (V)

FF

η (%)

No. of cell

20.77±0.38 0.95±0.05

0.46±0.05

9.0±1.3

11

(20.60)

(0.52)

(10.3)

20.66±0.40 1.06±0.03

0.70±0.02

15.3±1.0

(20.43)

(0.70)

(15.1)

20.17±0.51 0.98±0.05

0.51±0.07

10.2±1.9

(20.02)

(0.61)

(12.7)

19.98±0.49 1.07±0.03

0.67±0.04

14.4±1.1

(19.93)

(0.70)

(15.5)

20.72±1.01 1.02±0.06

0.54±0.06

11.4±1.8

(21.85)

(0.59)

(14.1)

20.60±1.03 1.08±0.03

0.68±0.03

15.0±1.0

(21.39)

(0.70)

(16.6)

(mA cm–2)

(0.97)

(1.06)

(1.03)

(1.11)

(1.09)

(1.12)

15

29

a)

Detailed annealing condition is described in experimental section; b)scan direction: forward JSC → VOC and reverse: VOC → JSC; Cell active area is 5 × 5 mm2 defined by black mask.

ASSOCIATED CONTENT Supporting Information. Photographs of VA process, SEM images of w/o-VA, c-VA and PRVA treated PSCs, steady-state measurement data, and all PV data in optimization of VA conditions. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Authors *Dr. Youhei Numata (E-mail: [email protected]). *Prof. Kazuhiro Ema (E-mail: [email protected]). *Prof. Tsutomu Miyasaka (E-mail: [email protected]). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by Advanced Low Carbon Technology Research and Development Program (ALCA) by Japan Science and Technology Agent (JST). We would like to appreciate supports for SEM and XRD measurement by Professor Hiroshi Segawa (RCAST, The University of Tokyo). REFERENCES (1)

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