Fully Ambient-Processed Perovskite Film for Perovskite Solar Cells

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Fully ambient processed perovskite film for perovskite solar cells: the effect of solvent polarity on lead iodide Wei-Ting Wang, Sandeep K. Das, and Yian Tai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01038 • Publication Date (Web): 10 Mar 2017 Downloaded from http://pubs.acs.org on March 12, 2017

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ACS Applied Materials & Interfaces

Fully ambient processed perovskite film for perovskite solar cells: the effect of solvent polarity on lead iodide

Wei-Ting Wang, Sandeep K Das, and Yian Tai*,

Department of Chemical Engineering, National Taiwan University of Science and Technology, 10607, Taipei, Taiwan

ABSTRACT Fully ambient-processed and highly efficient methylamine lead iodide (MAPbI3) perovskite films are very desirable for industrial manufacturing of perovskite solar cells (PSCs). To date, most reported highly efficient MAPbI3 PSCs rely on the fabrication of lead iodide (PbI2) films inside the glovebox. Here we report a simple fabrication method using extra dry isopropanol (IPA100) for obtaining uniform and loosely packed PbI2 film, which leads to a uniform and highly crystalline MAPbI3 film under ambient conditions. Compared with recently reported results (10%-15%) using IPA treatment in the glovebox, we achieved over 16% efficiency of PSCs while fabricating perovskite films in fully ambient conditions. We have found the removal of even trace amounts of water from IPA to be a key factor for the successful ambient fabrication of PbI2 films, as the high polarity of water negatively influences the crystallinity and morphology of the PbI2 film.

KEYWORDS: Perovskite solar cell; Solvent engineering; Isopropyl alcohol; Polarity; Process stability

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1. INTRODUCTION During the last half-decade, Organic-inorganic hybrid methylammonium lead trihalide (MAPbX3) based perovskite solar cells (PSCs) have shown a boost in their power conversion efficiency (PCE) from 3.81%1 to over 20%2 because of thelong diffusion length and panchromatic sunlight absorption of the perovskite active layer.3 One of the most important aspect to achieve these impressive efficiencies is obtaining a uniform and highly crystalline MAPbI3 film. Several methods have been developed and reported for the fabrication of the MAPbI3 film.4-9 Despite these advances, the fabrication methods are restricted due to strict requirement of controlled environmental conditions during processing,10 i.e. a glovebox is required to avoid the moisture that degrades the quality of the MAPbI3 film. These restrictions limit the scaling of fabrication methods and hinder the industrialization of the PSCs. The fully ambient-processed planar MAPbI3 PSC was initially demonstrated by Kelly et al.11 using the sequential deposition method (SDM). In this method, first a layer of lead iodide (PbI2) is fabricated and interacts with methylamine iodide (MAI), resulting in a MAPbI3 perovskite film. However, the efficiencies of the solar cells fabricated with SDM are lower because of the relatively poor quality of perovskite film due to incomplete conversion of PbI2 to MAPbI3 and its rough surface morphology. Therefore, various solvent-engineering techniques for SDM12-15 have been developed recently to improve the quality of perovskite film. For instance, El-Henawey et al. reported that higher crystallinity and larger grain size of PbI2 (and consequently, rougher surface morphology) resulting from solvent vapor treatment, induces better contact of MAI and PbI2.12 Zheng et al. demonstrated that generation of nanopores on PbI2 film promotes the penetration of MAI, leading to the better crystallinity of the perovskite film.15 These studies increased the degree of conversion of PbI2 to perovskite. However, they ignored that rough and non-uniform PbI2 layers may cause perovskite films with pinholes and rough surface morphology.16 Thus the efficiencies (10%-15%) of these PSCs are still limited. Also, the process for the fabrication of these PbI2 films has to be carried out inside the glovebox. 2


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In this context, an advanced process that could lead to smooth PbI2 film with low crystallinity and performed in ambient atmosphere is highly desired.16 Through our present study, we have developed an ambient fabrication technique for MAPbI3 perovskite films using the solvent engineering method PbI2 shows very low solubility in the solvents such as isopropyl alcohol (IPA) or toluene and the influence of our solvent engineering comes from the interaction of uncrystallized PbI2 with these solvents. We screened the solvents from the lowest polarity to the highest polarity to treat the PbI2 film and found the extra dry IPA (called IPA100, also exhibiting the highest dispersibility of PbI2) to be the best candidate for obtaining a smooth surface and a low crystalline (small crystal sized) PbI2 film. Further, these PbI2 film resulted in the high quality of the MAPbI3 film (fully fabricated in ambiance). It is of note that highly polar solvents such as water negatively influence the PbI2 film by inducing high crystallinity and rough surface morphology, and even trace amounts of water (5%) in IPA may significantly reduce the performance of the resulting device. For the fabrication of highly efficient PSCs, we used IPA100 to realize the high-quality MAPbI3 film under ambient conditions. Using a simple and easy to use sol-gel ZnO-based PSC architecture, we achieved an impressive efficiency of 16.3%. The rationale of using sol-gel processed ZnO as the electron transport layer (ETL) was that it is a low temperature and simple process. Also, a recent study suggested that ZnO-based PSCs exhibited better efficiency deviation and lesser hysteresis than TiO2-based devices due to longer charge carrier lifetime and better electron conductivity of ZnO ETL.17 For further demonstrating the versatility and compatibility of our solvent engineering, we fabricated PSCs with a planar architecture, including conventional ni-p on a flexible substrate and inverted p-i-n devices under ambient conditions (RH: 50-60%, temperature: 25-30 °C) without using a glovebox. The low-temperature and fully ambient preparation methods make the MAPbI3-IPA100 device a promising candidate for industrialization. 2. EXPERIMENTAL SECTION 2.1. Synthesis of CH3NH3I 3



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CH3NH3I was synthesized by the reaction of methylamine (27.8 ml, 40% in methanol, TCI) and hydroiodic acid (30 ml, 57 wt% in water, Sigma-Aldrich) at 0 ˚C and stirred in ice bath for 2 h.18,19 The precipitated CH3NH3I was collected after evaporation at 50 ˚C for 1 h, washed with diethyl ether three times. Finally, the product was dried at 60 ˚C in a vacuum oven for 24 h. 2.2. Preparation of sol-gel ZnO The sol-gel ZnO was obtained by the mixed reaction of 0.5 M zinc acetate dehydrate (SigmaAldrich) in 0.5 M monoethanolamine (Arcos) and 2-methoxyethanol (Arcos).20 2.3. Fabrication of conventional PSCs and conventional flexible PSCs The patterned indium tin oxide (ITO) glass (sheet resistance = 15 Ω sq-1) was sequentially cleaned with deionized water, acetone and isopropanol in an ultrasonic bath. Afterward, they were exposed to UV-ozone for 20 min, the sol-gel ZnO was spin-coated on the ITO and PEN/ITO substrates at 6000 rpm for 40 s, followed by annealing at 150 ˚C for 5 min. A 462 mg ml-1 solution of lead iodide (PbI2, 99.998 wt%, Sigma-Aldrich) in N, N-dimethylformamide (DMF, Arcos) was stirred 80 ˚C for 1 h. The 60 µl of PbI2 solution was spin-coated on the top of the ZnO layer at 7000 rpm for 20 s without post annealing and thereafter the PbI2 films were treated with various organic solvents (IPA100 (isopropanol, 99.5%, Extra Dry, Over Molecular Sieves, AcroSeal®, Arcos), IPA95 (95%, technical grade, Sigma-Aldrich), Toluene (99.8%, for analysis, Acros Organic), H2O (deionized water, Milli-Q, Millipore Co.)) via individually spin-coating them at 6000 rpm for 20 s in sequence. After that, PbI2 films were spin-coated with 38 mg ml-1 CH3NH3I in the isopropanol solution and subsequently annealed at 80 ˚C for 5 min. For hole transport layer, the 80 mg ml-1 solution of Spiro-OMeTAD (99%, ShiFeng Technology Co., Ltd.) (28.5 µl 4-tertbutylpyridine (96%, Sigma-Aldrich) and 17.5 µl lithium-bis(trifluoromethane sulfonyl)imide (Li-TFSI, 98%, Alfa Aesar) solution (520 mg Li-TFSI in 1 ml acetonitrile) all dissolved in 1 ml chlorobenzene)11 was spin-coated on the perovskite films at 4000 rpm for 20 s in sequence. Finally, 100 nm Ag were deposited on the top of the device at a base pressure of 2 x 4


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10-5 torr. All the steps of device fabrication, except deposition of Ag electrode, were carried out under ambient conditions. (RH: 50-60%, Temperature: 25-30 ˚C) All the materials were stored in the same ambient conditions. 2.4. Inverted PSCs fabrication The inverted perovskite devices, with configurations of ITO/NiOx/CH3NH3PbI3/PC61BM/Ag, were fabricated on pre-cleaned ITO substrates. The NiOx precursor solution21 was spin-coated onto the substrate at 3000 rpm for 40 sec. The PbI2 and MAI deposition method were the same as described in the conventional device fabrication. Subsequently, the PC61BM solution21 (20 mg dissolved in 1 ml chlorobenzene) was spin-coated on top of the perovskite film at 2000 r.p.m. for 20 s. Finally, the devices were completed with deposition of 100 nm Ag (top electrode) by thermal evaporation at a pressure of 2 x 10-5 torr. The fabrication process was conducted under ambient atmosphere. (RH: 50-60%, Temperature: 25-30 ˚C) and the commercial materials were stored in ambient below 30 ˚C. 2.5. Thin films characterization X-ray diffraction (XRD) patterns were obtained for different PbI2 and respective perovskite film deposited on substrates using a D2 PHASER X-ray diffractometer (BRUKER) equipped with a focusing-graded X-ray mirror with a monochromatic CuKα (λ = 1.5405 Å) radiation source. Scans were taken with a 1 mm wide source with X-ray generator settings of 30 kV and 10 mA. Ultraviolet-visible spectroscopy (UV-vis) of the polymer films were recorded on a JASCO-V-550 spectrophotometer at room temperature in air. Photoluminescence (PL) spectra were measured with the HORIBA Jobin Yvon FluoroMax-3 spectrofluorometer. A field-emission scanning electron microscope (FESEM) of JEOL JSM-6500F was used for obtaining surface morphology and cross-sectional view of various thin films. The instrument used an electron beam accelerated to 30 kV, enabling operation at a variety of currents. The FEI Quanta 3D FEG Dual Beam

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machine combines traditional thermal SEM with focused ion beam (FIB) was used for obtaining cross-sectional view of flexible films 2.6. PSCs characterization The device area was 0.04 cm2. J-V characteristics of the photovoltaic cells were measured using a Keithley 2400 source unit under a simulated AM 1.5G spectrum. With an Oriel 9600 solar simulator, the light intensity was calibrated by a KG-5 Si diode, the measurements are carried out at ambient conditions. The PSC devices were measured with masks of 0.04 cm2 and 0.16 cm2 (for the large area devices) in the forward scan (-0.1 V to 1 V, step 0.01 V, delay time 200 ms) and reverse scan (1.2 V to -0.1 V, step 0.01 V, delay time 200 ms). The differences between reverse and forward scans of PSC devices are shown in Figure S2. The external quantum efficiency (EQE) measurements of the devices were taken by using an Enli tech (Taiwan) EQE measurement system. All the devices described in this study were measured under ambient conditions. (RH: 5060%, temperature: 25-30 ˚C)

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3. RESULTS AND DISCUSSION 3.1. Solvent effect based on polarity derivation To improve the MAPbI3 film quality in SDM, a uniform PbI2 film with a relatively low crystalline state is highly desired.16 Usually at ambient conditions, when the supersaturated PbI2 solution (in DMF) is coated onto substrates, a highly crystallized film with uneven morphology is obtained after several minutes, possibly due to the effect of moisture.22 Therefore, this procedure is typically performed inside a glovebox to prevent the PbI2 film from being contaminated with moisture. One effective approach to obtain the desired film in ambient conditions is by treating the surface of the immediate grown PbI2 film with a solvent. As such, the solvent could interact with the PbI2 film, resulting in a more uniform PbI2 with lower crystallinity before moisture could undesirably affect the film. The solvent should be relatively insoluble in the film material so that it does not destruct the PbI2 layer. Moreover, the suspensibility of the PbI2 in this solvent is highly desired; PbI2 could form small crystals in this solvent, leading to a more uniform surface morphology when such a suspension solution is spin-coated on the substrate. Moreover, this solvent should be miscible with DMF, the solvent of the PbI2 solution, to prevent the damage of the PbI2 surface due to strong phase separation. Considering these parameters, we screened various solvents for evaluating the solubility of PbI2 in them. PbI2 being of a semi-ionic, semicovalent nature has a tendency to be soluble in lower mid-polarity solvents such as DMF (relative polarity: 0.386) and DMSO (relative polarity: 0.444).23 We chose solvents having a wide polarity range from highest (water (H2O), relative polarity: 1) to the middle (IPA100, relative polarity: 0.546) and lowest (Toluene, relative polarity: 0.099).23 We individually spin-coated these solvents on just formed PbI2 films (denoted as PbI2-H2O, PbI2-IPA100, and PbI2-Toluene, respectively). Afterward, the surface morphology of untreated and solvent-treated PbI2 films was analyzed through FESEM.

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As shown in Figure 1b, the PbI2-IPA100 possesses the most uniform and homogenous morphology which is visible through both cross-sectional and top view images, whereas PbI2 treated with H2O and toluene (Figure 1a and 1d, respectively) showed much rougher morphologies even in comparison with untreated PbI2 films (Figure 1c). All these films were then further treated with MAI solution for the second step of SDM to form a perovskite films. The PbI2-IPA100 film facilitated the homogeneous penetration of the solution because of its low crystallinity and smoother morphology, leading to uniformly distributed pathways that resulted in perovskite film of uniform and large grain sizes with a smooth grain boundary (Figure 1f). Rough morphologies of untreated, H2O, and toluene treated PbI2 films produced perovskite films of nonuniform morphology with rough grain boundary and pinholes (Figure 1g, 1e, and 1h, respectively). Apart from affecting surface morphology of PbI2 films, solvent treatment also affected the crystallinity of just formed PbI2 films. The direct evidence came from the XRD peak position (2θ) and peak broadening (full width at half maximum, FWHM) of treated and untreated PbI2 and corresponding perovskite films. It was found PbI2-IPA100 film had the lowest crystallinity (Figure 2a) of 59.9% among all treated and untreated films. In addition, PbI2-IPA100 film showed the highest transparency as compared with other films in Figure 2c. This result of PbI2-IPA100 was in accordance with previous report16 which is sufficient for its effective conversion to a perovskite crystal of higher crystallinity (54.5%, Figure 2b). In comparison, untreated, H2O and toluene treated PbI2 films showed higher crystallinity (70.5%, 65.7%, and 73.8%, respectively) when compared with PbI2-IPA100. The higher crystallinity of these films hindered the efficient conversion of them to perovskite and resulted in perovskite films of lower crystallinity (40.4%, 44.7%, and 40.5%, respectively; Figure 2b). The FWHM, crystallite size, and crystallinity for major XRD peaks were summarized in Table S1 and Table S2 in the supporting information (SI). The crystalline effects of PbI2 films and correspondingly prepared perovskite film were also supported by the Ultraviolet-visible spectroscopy (UV-Vis) absorption 8


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data (Figure 2c and 2d). The lowest absorption intensity of PbI2-IPA100 indicates that PbI2IPA100 possesses a more amorphous phase than others,24 while the highest absorption intensity of MAPbI3-IPA100 indicates that MAPbI3-IPA100 possesses a higher crystallinity than others,25,26 which illustrates that the lower crystalline state of the PbI2 film is a necessary pre-condition for producing good quality perovskite film. Also noteworthy is that the perovskite film derived from PbI2-IPA100 showed an increased photoluminescence (PL) intensity as compared with those of untreated and H2O and toluene treated- PbI2 films, which are consistent with the absorption data. 3.2. The significance of utilizing extra dry IPA As the above experiments revealed, the quality of perovskite films could be positively improved with IPA treatment and negatively impacted with H2O treatment. Therefore, to explore the impact of even a trace amount of water in IPA, newly cast PbI2 film was treated with IPA having 5% water (denoted as IPA95). Figures 3a and 3b show the difference in morphology of a PbI2 film treated with IPA95 and IPA100. The IPA95, due to the presence of H2O (high polarity), resulted in a PbI2 film of rough morphology, which impacts the quality of the final perovskite film (MAPbI3-IPA95; Figure 3c) when MAI was coated on it. The optical data (shown in Figure S1 in SI) including UV-Vis, PL, and XRD spectra demonstrate the lower crystallinity of PbI2-IPA100 than PbI2-IPA95, and higher crystallinity of MAPbI3-IPA100 than MAPbI3-IPA95, which is consistent with the optical results using solvents with different polarities. In addition, we fabricated conventional devices using IPA95 and IPA100 using the solvent treatment of PbI2 films in ambient atmosphere, respectively. The forward and reverse scans of both devices are shown in Figure 3e and 3f, the MAPbI3-IPA100 based device exhibited higher device performance and no hysteresis behavior, indicating a higher quality perovskite film with fewer charge traps.27 Therefore it needs to be taken into consideration that the presence of even small amounts of H2O in IPA can adversely affect the ambient-processed PbI2.

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3.3. Device performance and process stability analysis Conventional n-i-p planar devices of ITO/ZnO/perovskite/spiro-OMeTAD/Ag configuration using toluene, IPA100, and water treated MAPbI3 films were fabricated under ambient condition and their average performance is shown in Figure S2, 4a, and summarized in Table 1. As expected, MAPbI3-IPA100 demonstrated the performance enhancement of over 23.0%, 123.6% and 48.3% on MAPbI3, MAPbI3-H2O, and MAPbI3-Toluene devices, respectively. Moreover, MAPbI3-IPA100 devices also showed almost hysteresis free forward (15.76%) and backward (15.65%) scan performances compared to other devices illustrated in Figure S2. The outstanding performance of MAPbI3-IPA100 device was found to be mainly due to the significant improvement in photon harvesting ability (reflected by high Jsc and external quantum efficiency (EQE) data, Figure S3), fill factor (FF, which was the result of improved morphology) and relatively uniform interface morphology of MAPbI3-IPA100 (cross-sectional view of FESEM, Figure 4b). To ensure the reproducibility of PSCs performances, 60 separate devices were fabricated and tested using IPA100 treated and untreated PbI2 films. The histograms of the PCEs (Figure 4c) show 17% enhancement in average PCE of MAPbI3-IPA100 PSCs over MAPbI3 PSCs, demonstrating the

superiority of IPA100 solvent engineering. Keeping future

industrialization in mind, we carried out the process stability test of the PSCs. As for the mass production of perovskite film through SDM, there would be a time gap between the first and second steps. Within this time difference, it would be mandatory to keep the PbI2 film stable under ambient conditions. As such, we compiled statistics (60 devices for each condition, Figure 4d) of untreated and IPA100 treated PSCs by delaying the second step (MAI application) of SDM for 15 min, 30 min, 1 hour and 3 hours under ambient conditions. It was found more than 60% PSCs produced with the IPA100 treated PbI2 films achieved a PCE of over 10% even when the MAI coating step was deferred by 3 hours; in comparison, only 25% of PSCs produced from untreated PbI2 films showed the same result. The test clearly demonstrates that IPA100 treatment 10


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improves not only the performances of PSCs but also the process stability, which is crucial for commercialization. 3.4. Versatility of IPA100 treatment for inverted and flexible device structures We extended the IPA100 treatment for PSCs fabricated with inverted planar p-i-n architecture (ITO/NiOx/MAPbI3/PC61BM/Ag) and to the device fabricated on flexible polyethylene naphthalate (PEN) substrate. The results of the best performing inverted and flexible PSCs with and without IPA100 treatment are shown in Figure 5, while the performance of a conventional PSC is also shown for comparison. It is noteworthy that an IPA100 treatedconventional device demonstrated the best performance of 16.27% (Figure 5a) having Voc: 1.09 V, Jsc: 20.30 mA cm-2, FF: 73.19% device parameters, which is the best performance reported for ambient processed planar PSCs using ZnO as the ETL.11,28-30 The EQE spectrum shown in Figure S3 was consistent with the current density versus voltage (J-V) curve of this best performing conventional device, while the integrated photocurrent from 300 nm to 800 nm was 19.35 mA cm2

and comprised the J-V measurement.

The MAPbI3 inverted PSC had a best PCE of 10.46% whereas the MAPbI3-IPA100 inverted perovskite device exhibited a best PCE of 13.87%, which is comparable with reported results utilizing similar device architectures,21,31-33 that demonstrated 38.5% enhancement (Figure 5b). The EQE of the highest-performing device is shown in Figure S4; the efficiency of 70-80% obtained across the visible light region (400-800 nm) highlights the exceeding performance of the PSC in comparison with devices without IPA100 treatment. Integrating the product of the photon flux within the EQE spectrum yields a Jsc of 19.16 mA cm-2, which is in agreement with the measured value of 20.46 mA cm-2. The average J-V performance data is summarized in Table S3. The high performance of the MAPbI3-IPA100 device in inverted configuration was due to the same reason as that of conventional devices, which was confirmed by XRD results present in

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Figure S5. The untreated PbI2 film shows higher crystallinity as compared with PbI2-IPA100 film while the results are inverted when comparing the XRD results of perovskite films. For the flexible devices, we had chosen PEN/ITO as substrate because of its relative high glass transition temperature (Tg: 120 °C), which eliminates the softening effect that might arise due to an annealing temperature of 80 °C required for MAPbI3 perovskite. The device performance

of

the

flexible

PSCs

fabricated

with

conventional

planar

n-i-p

(PEN/ITO/ZnO/MAPbI3/spiro-OMeTAD/Ag) configuration is shown in Figure 5c. The MAPbI3IPA100 device showed a remarkable improvement of more than 33% PCE than that of the MAPbI3 cell. The J-V curve of highest-performing device yielded Voc of 1.04 V, Jsc of 16.41 mA cm-2, FF of 65.64%, and PCE of 11.12%. A narrower EQE of the flexible solar cell is observed in Figure S6 compared with the device using glass/ITO. The absence of EQE in the range from 300400 nm is mainly due to the absorption by an aromatic moiety of PEN (the π-π* transition of naphthalene). The integrated current density of flexible device using MAPbI3-IPA100 is 16.28 mA cm-2, which is in agreement with the J-V measurement. FESEM images (Figure S7) of MAPbI3-IPA100 film on the PEN/ITO exhibited a higher degree of surface coverage with a more uniform morphology than the MAPbI3 film did. The standard fabrication process of MAPbI3 produces much more pinholes and a non-uniform perovskite crystal (Figure S7c and S7e) as compared with that of MAPbI3-IPA100 (Figure S7d and S7f), respectively, inducing current leakage and resulting in device instability. Although, the obtained PCE of 11.12% for a flexible PSC is lower than the device fabricated on glass/ITO substrates, yet it is among the highest reported so far for flexible PSCs having ZnO as the ETL and fabricated in ambient condition without using a glove box.11,34,35 Finally, we compared the performances and stability of MAPbI3 and MAPbI3-IPA100 conventional solar cells with a 0.16 cm2 active area fabricated on a glass substrate. The performance results in Figure S8 and Table S4 clearly demonstrate that the MAPbI3-IPA100 based device achieved higher performance (15.26%) than that of the standard 12


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device (11.99%). The stability analysis in Figure S9 also shows that MAPbI3-IPA100 possesses a much higher stability as compared with the MAPbI3 based device.

4. DISCUSSION When PbI2 solution (in DMF) is coated on a substrate, the PbI2 film does not dry instantly and goes through a slow crystallization process due to the high boiling point of DMF (~150 °C). As mentioned above, unlike in DMF, PbI2 has very low to no solubility in solvents like H2O, IPA100, or toluene due to their different polarities. Therefore, when these solvents are individually spin-coated on immediately formed PbI2 film, the PbI2 molecules in film crystallize instantly. Moreover, apart from the fast rate of crystallization, the polarity of these solvents was also found to be affecting the degree of crystallization and morphology of PbI2 film. The IPA100, due to its relative mid-polarity, has a partial affinity for PbI2 molecules and causes PbI2 to arrange in small grains of lower crystallinity in the film. The fact has been supported by the suspension behavior of PbI2 particles in IPA100 solvent even after keeping the solution for a long time (Figure S10). In contrast, the PbI2 molecules form large grains of higher crystallinity in the film when treated with high polarity (H2O) or low polarity (Toluene) solvents. The phenomena has been confirmed by the relatively high precipetation of PbI2 in these solvents (Figure S10). Due to the larger polarity difference between toluene and DMF/DMSO(in comparison with that of IPA100 and DMF/DMSO, the PbI2 particles demonstrated a low amount of suspension. In addition, due to polarity difference of H2O with DMF/DMSO being the largest, PbI2 showed no affinity for water molecules and thus arranged itself in enormous chunks and precipitated to the bottom of the solution. Figure 6 demonstrates the role of solvent polarities on the dispersibility of PbI2 in them and the grain sizes of PbI2 affected by the dispersibility. It shows how PbI2 arranges itself from mid to 13



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large sizes when treated with low to high polarity solvents. Figure 7 demonstrates the schematic representations of the formation mechanism of MAPbI3 and MAPbI3-IPA100 films from untreated (PbI2) and treated (PbI2-IPA100) films. As shown (Figure 7a), in the first step of SDM, IPA100 treatment results in the crystallization of the PbI2 to smaller chunks, demonstrating a smooth morphology which is absent in untreated PbI2 films. On further coating with MAI (Figure 7b), the small ordered PbI2 chunks led to large sized crystal perovskite (MAPbI3-IPA100). In short, all morphological, XRD, and optical studies indicated the requirement of low crystallinity and uniform morphology of the PbI2 layer for producing a high-quality perovskite film. This was achieved by the post-treatment of a just formed PbI2 film using IPA100 solvent. In this context, such a highly improved perovskite film would be expected to deliver higher performance PSCs.

5. CONCLUSION In summary suitable polarity of the green solvent, IPA 100, and its affinity to PbI2 facilitated the fabrication of low crystalline and smooth PbI2 films, which further helped the full ambient fabrication of a uniform and highly crystalline perovskite film. The resulting MAPbI3-IPA100 was used in planar conventional/inverted configuration, and flexible PSCs to achieve impressive PCEs which are comparable with the best-reported results using identical device architectures. Also, the results suggest that the use of the PbI2-IPA100 film is promising for preparing not only high-performance planar MAPbI3 devices but also a stable process for SDM under atmospheric conditions. Our findings indicate that IPA100 or possibly another similar solvent engineering can play a significant role in the realization of stable, ambient processed, fully solution based printing/coating techniques, which is a prerequisite for the lowcost and large-scale production of PSCs.

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Figure 1. FESEM cross-sectional and top-view images (inset) of the (a) PbI2-H2O, (b) PbI2-

IPA100, (c) PbI2 and (d) PbI2-Toluene. FESEM top-view images of (e) MAPbI3-H2O, (f) MAPbI3-IPA100, (g) MAPbI3 and (h) MAPbI3-Toluene.

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Figure 2. (a) XRD spectra of PbI2 films with and without various solvent treatments. (b) XRD spectra of MAPbI3-H2O, MAPbI3-IPA100, MAPbI3, and MAPbI3-Toluene. (c) The photograph and UV-Vis absorption spectra of PbI2, PbI2-IPA100, PbI2-Toluene, and PbI2-H2O. (d) UV-Vis absorption and PL spectra of MAPbI3, MAPbI3-IPA100, MAPbI3-Toluene, and MAPbI3-H2O.

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Figure 3. FESEM images of (a) PbI2-IPA95, (b) PbI2-IPA100, (c) MAPbI3-IPA95, and (d) MAPbI3-IPA100 films. (e) Device performance of a conventional device compiled with the configuration of ITO/ZnO/MAPbI3-IPA95/spiro-OMeTAD/Ag and (f) ITO/ZnO/MAPbI3IPA100/spiro-OMeTAD/Ag

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Figure 4. (a) Average PCEs (%) of the PSCs without or with various solvent treatment. (b) FESEM cross-sectional images of the devices based on MAPbI3 and MAPbI3-IPA100. (c) Histograms of device performance using MAPbI3 and MAPbI3-IPA100 films. (d) Process stability of SDM using untreated PbI2 and PbI2-IPA100 films for sixty devices.

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Figure 5. Efficiency measurements of best-performing PSCs: J-V curves of the best-performing devices using MAPbI3 and MAPbI3-IPA100 in (a) conventional, (b) inverted architectures, and (c) flexible device.

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Figure 6. The polarity of various solvents and crystal formation of PbI2 when treated with solvents according to the increasing polarity.

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Figure 7. Schematic description of the formation of PbI2 and MAPbI3 films with and without solvent treatments: (a) Crystallization and precipitation of the PbI2 films. (b) Crystal transformation of PbI2 to MAPbI3 with and without solvent treatment.

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Table 1. Summary of device parameters of the conventional Sol-gel ZnO-based perovskite solar cells Sample a)

Jsc (mA cm-2)

Voc (V)

FF (%)

Average PCE (%)

MAPbI3

18.78±1.28

1.03±0.05

63.97±3.28

12.46±0.57

MAPbI3-IPA100

19.77±0.74

1.06±0.02

72.66±1.25

15.32±0.47

MAPbI3-IPA95

18.14±1.33

1.03±0.02

65.28±3.31

12.45±0.82

MAPbI3-Toluene

14.77±0.49

1.00±0.03

69.65±2.98

10.33±0.71

MAPbI3-H2O

13.69±2.38

0.81±0.13

61.47±5.46

6.85±1.92

a)

Each parameter was calculated as an average value of ten devices.

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ASSOCIATED CONTENT Supporting Information. The supporting information includes the top view and cross-sectional SEM images of the PbI2 and perovskite films, XRD spectra of the PbI2 and perovskite films, J-V curves and IPCEs of the conventional, inverted perovskite solar cells. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are grateful to the Ministry of Science and Technology for the financial support.

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Graphical abstract

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Fully Ambient-Processed Perovskite Film for Perovskite Solar Cells

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