in Situ X-ray Diffraction Study of

Article pubs.acs.org/cm

Radiative Thermal Annealing/in Situ X‑ray Diffraction Study of Methylammonium Lead Triiodide: Effect of Antisolvent, Humidity, Annealing Temperature Profile, and Film Substrates Benjia Dou,†,‡ Vanessa L. Pool,§ Michael F. Toney,*,§ and Maikel F. A. M. van Hest*,† †

National Renewable Energy Laboratory, Golden, Colorado 80401, United States Department of Electrical, Computer, and Energy Engineering, University of Colorado Boulder, Boulder, Colorado 80309, United States § Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States ‡

S Supporting Information *

ABSTRACT: Organic−inorganic hybrid halide perovskites are one of the most promising emerging photovoltaic materials due to their high efficiency and potentially low processing cost. Here, we present a well-controlled, manufacturing relevant annealing method, radiative thermal annealing, for the methylammonium lead triiodide (MAPbI3) films formed by a solvent engineering process, with dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) as solvent and diethyl ether as the antisolvent. Radiative thermal annealing can produce high quality perovskite films, evidenced by high efficiency solar cell devices (∼18% power conversion efficiency), in a shorter time than the widely used hot plate annealing. Using in situ X-ray diffraction during the radiative annealing, we show that the role of the antisolvent is not to form an important intermediate compound (a PbI2-MAIDMSO complex) by washing of the main solvent (DMF), but to achieve a pinhole free, uniform film of MAPbI3 with minimal intermediate compound. Importantly, we show that having a PbI2-MAI-DMSO intermediate compound does not guarantee a high quality (pinhole free) perovskite film. We directly show that humidity induces MAPbI3 to decompose into PbI2 more rapidly and, as such, negatively impacts the reproducibility of the device performance. The study is extended to reveal the effect of annealing temperature profile and deposition substrate to demonstrate the complexity of perovskite processing parameters. This coupled experimental approach allows a better understanding of the effect of processing protocols, including antisolvent, humidity, and annealing profile, on MAPbI3 film quality and the resultant solar cell performance.

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INTRODUCTION

ing and printing. With the combination of the high PCE and low material and processing costs, perovskite solar cells (PSCs) are one of the most promising emerging photovoltaic technologies. To date, a number of solution-to-film deposition routes1 have been developed to produce full coverage, pinhole free, preferred morphology, and highly crystallized perovskite films. The most popular deposition route is a one-step deposition using solvent engineering,7,8 where source materials, such as

The discovery of organic−inorganic hybrid halide perovskites in the semiconductor research field, particularly in optoelectronics such as solar cells and light emitting diodes, has been extraordinary.1 The materials, such as methylammonium lead halide (MAPbI3), exhibit good absorption coefficient,2 long carrier lifetime,3 low recombination rate,4 and long diffusion length,3,5 which make them excellent solar cell materials, as demonstrated by the realization of over 22% certified power conversion efficiency (PCE) within only six years of research.6 These materials can be solution processed, which means that perovskites are compatible with large yield, low cost, industrial relevant manufacturing techniques such as roll-to-roll process© 2017 American Chemical Society

Received: April 10, 2017 Revised: June 19, 2017 Published: June 19, 2017 5931

DOI: 10.1021/acs.chemmater.7b01467 Chem. Mater. 2017, 29, 5931−5941

Article

Chemistry of Materials

PbI2-MAI-DMSO. Performance enhancement with use of the antisolvent is investigated, and devices are fabricated to demonstrate the improved efficiencies. The effect of processing environment (nitrogen and air) is studied, and it was found that humidity induces MAPbI3 to decompose into PbI2 more rapidly and as such hurts the reproducibility of the device performance. We further investigate the effect of annealing temperature profile and substrate choice on MAPbI3 crystallization and device performances, and demonstrate the complex processing parameters of solution processed perovskites. Using the in situ XRD characterization, these studies offer a systematic understanding of MAPbI3 crystallization and performance when annealed by RTA.

lead iodide (PbI2) and methylammonium iodide (MAI) in the case of MAPbI3, are dissolved in a mixture of solvents, which include dimethylformamide (DMF), γ-butyrolactone (GBL), or dimethyl sulfoxide (DMSO). An antisolvent, typically toluene,7 chlorobenzene,9,10 or diethyl ether,11 is applied while the film is deposited. After the film is deposited, a heat source, usually a hot plate, is used to anneal the film to remove excess solvent, crystallize the film, and in some cases enhance the grain size.12 This process, though successful, is complicated due to the fact that the solvents not only perform as a solvent dissolving the source materials, but also form compounds such as PbI2DMSO,13 PbI2-MAI-DMSO,13 MAPbI3-DMF,13 or PbI2DMF.13,14 The complexity of perovskite fabrication is further increased with variants in the annealing process, including the annealing environment,15,16 annealing temperature,17,18 annealing protocol,8,19 and the heat source (hot plate, light bulb,21,22 laser,23 microwave,24 etc.). To elucidate what effect each aspect of this complicated fabrication has on the perovskite crystallization, in situ X-ray diffraction (XRD) can be an effective tool.22 Despite numerous in situ XRD studies15,18,25−30 on MAPbI3, to the best of our knowledge, all of them are performed with effectively a hot plate as the annealing source and different time scale. Although using a hot plate is easy to use, it is energy inefficient and hard to incorporate to industrial-level processing, such as roll-to-roll processing, where controlled annealing ramp rate might be critical. In contrast, radiative thermal annealing (RTA) is scalable and more relevant in the commercially competitive semiconductor industry.31 Watson et al.20,32 previously applied radiative annealing to methylammonium based perovskite solar cells but found the devices fabricated with radiative annealing were not as good as the devices made with a hot plate. Later, Druffel et al.21 flash annealed MAPbI3 devices producing a champion cell with a 12.3% PCE, although this was lower than the over 15% PCE that most laboratories were reporting at the time. We speculate that this is due to the pulsed nature of their processing. Here we apply a well-controlled continuous radiative annealing process to MAPbI3 by using both commercially available RTA furnace (for device) and inhouse built RTA33 (for in situ XRD study). We have previously applied RTA22 with formamidinium lead iodide (FAPbI3) and have shown that RTA processed FAPbI3 solar cells have comparable optical properties and device performance to hot plate annealed devices. In addition, with an increasing number of perovskite studies applying an antisolvent (or similar methods such as vacuum assisting34 or nitrogen/argon gas flow35−37 that evaporate certain solvent(s) before annealing), it is imperative to understand how solvent engineering impacts the crystal dynamics using in situ XRD. Fully understanding radiative annealing induced perovskite crystallization is made possible by the recent development of a controlled RTA for use with a synchrotron X-ray source.33 In this work, the scalable annealing method, RTA, was applied to make high efficiency MAPbI3 solar cells. This annealing technique can fabricate uniform films with large grains in a short period of time, presumably due to its efficient heat transfer process. By applying the RTA/in situ XRD system, we study the film formation process to reveal crystallization differences with and without the use of the antisolvent diethyl ether. We show the role of the antisolvent is not to wash off the DMF solvent, but to assist forming large grain uniform films, consisting of mainly MAPbI3 with only a minimal amount of

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EXPERIMENTAL SECTION

Materials. Methylammonium iodide (MAI) was synthesized as reported elsewhere.38 Lead iodide (PbI2, 99.999%) was purchased from Alfa Aesar. Spiro-MeOTAD (99.5%) was purchased from Lemtec. Titania (TiO2) paste is from DyeSol. All other materials are from Sigma-Aldrich. MAPbI3 Film Deposition. Optimizing the procedure reported by Ahn et al.,11 a 1:1.05:1 molar ratio of MAI/PbI2/DMSO was mixed in DMF with a concentration of 1.1 M. The solution was vortex mixed for 1 h, which resulted in a clear bright yellow solution. Before usage, the solution was filtered with a 0.20 μm PVDF filter. The as-prepared solution was spin-coated (60 μL per substrates) on the substrates (square inch size), glass or FTO-coated glass with compact TiO2 or porous TiO2, with following spinning parameters: 4000 rpm for 25 s, with 400 μL of diethyl ether quickly dripped on the film 18−19 s before the end of the spinning cycle. The MAPbI3 films for devices and scanning electronic microscopy (SEM) were then annealed with a commercially available radiative thermal annealing equipment (Ulva MILA-3000 minilamp Annealer). The MAPbI3 films for in situ X-ray were annealed with RTA/in situ equipment described by Ahmad et al.33 Solar Cell Fabrication. Patterned FTO coated glass substrates (Thin Film Devices, Inc., 15 Ω/sq) were ultrasonic bathed in acetone and isopropanol, 10 min for each, and treated with ultraviolet-ozone 15 min. A 0.2 M solution of titanium diisopropoxide bis(acetylacetonate) in 1-butanol was spin-coated (700 rpm for 10 s, 1000 rpm for 10 s, and 2000 rpm for 30 s) and annealed (130 °C for 5 min and 450 °C for 1 h) to form a thin (∼30 nm) TiO2 blocking layer.22 For devices with mesoscopic structure, a ∼150 nm TiO2 nanoparticle layer (∼30 nm particles) was spin-coated (700 rpm for 10 s, 1000 rpm for 10 s, and 2000 rpm for 30 s) on the compact TiO2 layer after annealing the as deposited compact TiO2 for 5 min. The as spin-coated TiO2 nanoparticle layer was then annealed for 1 h at 450 °C in air. The TiCl4 treatment was performed on the TiO2 films as reported.11 The TiO2 film was UV-ozone cleaned for 15 min before spin-coating the MAPbI3 layer as stated above. The hole transport solution, containing 72 mg mL−1 of Spiro-MeOTAD, 28.8 μL mL−1 of 4-tert-butylpyridine, and 17.5 μL mL−1 of LiTFSI stock solution (520 mg/mL LiTFSI in acetonitrile), was spin coated (3000 rpm, 30 s) on the perovskite film. Finally, a 100 nm Au back contact was deposited by thermal evaporation. The thickness of each layer of the device was glass/FTO/compact TiO2 (30 nm)/[optional: mesoscopic TiO2 (150 nm)]/MAPbI3 film (400 nm)/Spiro-MeOTAD (200 nm)/Au (100 nm). Characterizations. The J−V curves were measured under simulated AM 1.5 sunlight at 100 mW/cm2 with a solar simulator (Newport, Oriel Sol3A), calibrated with an NREL calibrated KG2 filtered Si reference cell. The active area of the solar cell was 0.059 cm−2. The J−V reverse scans were taken from 1.3 V to −0.2 V and forward scans from −0.2 to 1.3 V, with scan rates of 100 mV/s. The EQE was measured using a Newport Oriel IQE-200. After processing film XRD were recorded with a Rigaku D-Max 2200 with Cu Kα radiation. SEM images were taken with a FEI Nova NanoSEM 630. A difference between Jsc calculated from EQE and Jsc from J−V scan was 5932


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Figure 1. SEM images of the film annealed with RTA and hot plate. (a) RTA for 3 min. (b) Hot plate for 3 min. (c) Hot plate for 10 min.

Figure 2. Device performance for hot plate anneal versus RTA. (a) J−V characteristics. (b) Statistics (24 samples in each condition) of device performance from the reversely scanned J−V curves.

the films fabricated with RTA and a hot plate, SEM images are shown in Figure 1a,b. The average grain size was determined with the method described by Hoffman et al.,41 and it was found the average grain sizes of the film made with RTA and a hot plate, both at 100 °C for 3 min, are 380.9 ± 17 nm and 156.8 ± 3.2 nm, respectively. The larger grain size for samples processed with RTA (with the same annealing profile) is presumably the result of more efficient heat transfer, since in the RTA heat comes from all directions, particularly directly annealing the absorber, while for the hot plate the heat transfer is unidirectional and only came through glass substrates. This assumption was tested by annealing the hot plate film longer, 10 min at 100 °C. SEM on the resulting films (Figure 1c) shows the average grain size is increased to 347.8 ± 13 nm, which is very close to that obtained by using RTA for 3 min at 100 °C. This grain size enhancement with increasing annealing time indicates that the film growth mechanism in MAPbI3 is different from FAPbI3, where the film grain size is set once the film is deposited,22 while in the case of MAPbI3, the small grains merge with each other to form larger grains with longer annealing time. We hypothesize such difference in grain growth mechanism is induced by the differences in activation energy for the grain boundary migration. For MAPbI3, the grain boundary migration activation energy is likely equivalent or lower than its crystallization activation energy, therefore grain enhancement during annealing can be observed over time during crystallization. While in FAPbI3, the grain boundary migration activation energy is likely higher than the decomposition activation energy of FAPbI3, so decomposition occurs before the grain growth.

observed, particularly in no antisolvent case (Figure 4). It is likely the mismatch between solar simulator spectrum and the EQE light source.39 Also, one key reason for the large inconsistency in no antisolvent case is due to the light soaking problem of the device, as in EQE measurement, no light soaking was performed. RTA/in Situ X-ray. RTA/in situ synchrotron X-ray data were taken at SSRL beamline 7−2 at SLAC National Accelerator Laboratory using the RTA/in situ X-ray described in ref 33. The diffraction was taken using a Pilatus 300k and there were 60 scans in each run and each scan took 0.5 s, with a 25 s ± 1 s interval (for a 20 min run) or a 4 s ± 0.5 s interval (for a 3 min run) (with the beam shutter closed between scans to reduce beam exposure and prevent beam damage). The data were converted from 2-D to 1-D using WxDiff, a code written by Stefan Mannsfield.40 Once converted into 1-D data, the data were loaded into Igor where select peaks were fit using the Multipeak fit with a Gaussian peak shape and a linear background subtraction. Since the integrated peak intensities were interpreted as proportional to phase fractions, it was important that the film texture did not change over the course of the annealing. This is demonstrated in Figure S3 and from the 2D XRD images (Figure S3c), which show uniform rings.

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RESULTS AND DISCUSSION Radiative Thermal Annealing of MAPbI3. To compare perovskite films annealed with RTA and a hot plate, MAPbI3 films were deposited with the antisolvent method (or Lewis acid−base adduct approach11) with diethyl ether as the antisolvent and annealed for 3 min at 100 °C in dry N2. The perovskite solution consists of MAI, PbI2 and DMSO in a molar ratio of 1:1.05:1 in DMF. The details of the film deposition are described in the Experimental Section. The temperature ramp rate of the RTA was chosen to be 10 °C/s to mimic the hot plate annealing process as reported in our earlier FAPbI3 study.22 To compare the morphology and grain size of 5933


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understand the MAPbI3 film crystallization process when deposited with the solvent engineering and annealed with RTA, we applied RTA/synchrotron in situ XRD,33 which can monitor film crystal phase dynamics with intervals as short as 100 ms. The films are deposited with two types of deposition methods, one with diethyl ether dripping and one without diethyl ether dripping. The film structural evolution is characterized with RTA/in situ XRD in dry N2, with the temperature profile shown in Figure 3a. A two-step annealing, 1 min at 65 °C and 20 min at 100 °C, is adopted from a modified Park et al.11 annealing procedure with an annealing ramp of 10 °C/s. The X-ray scan

To investigate the effect of the RTA annealing time on cell efficiency, a series of MAPbI3 based planar perovskite solar cells was fabricated. The device structure is glass/FTO/TiO2/ MAPbI3/Spiro-MeOTAD/Au, and the details of the device fabrication are presented in the Experimental Section. Previously, several groups12,42 have argued that larger grain size results in higher device performance, as it reduces the trap assisted recombination centers in the film. The current density−voltage (J−V) curves corresponding to the most efficient devices fabricated from RTA for 3 min at 100 °C, on a hot plate 3 min at 100 °C, and on a hot plate 10 min at 100 °C are shown in Figure 2a, and the performance parameters are presented in Table S1. As Figure 2a and Table S1 show, devices with the active layer annealed by RTA at 100 °C for 3 min produce cells with much better performance than those annealed at 3 min 100 °C using a hot plate, especially in terms of fill factor (FF), which likely due to larger grains resulted in fewer defects and less grain boundary related resistance. The performance of hot plate annealed devices does improve, both FF and open circuit voltage (Voc), after annealing the active layer for 10 min. This observation is further confirmed with the device performance statistics (24 devices for each condition) that are presented in Figure 2b. These results together with the SEM (Figure 1) suggest that the larger grain size does indeed lead to better device performance. The high performance obtained from RTA in a shorter time compared to hot plate annealing shows that the radiative heating technique offered by the RTA heats the films faster and more uniformly than conductive heating such as hot plate. Together with RTA’s compatibility in practical roll-to-roll processing, this demonstration affirms the promise of RTA as an alternative way of processing MAPbI3 in a future perovskite photovoltaic industry. While achieving high performance devices is important, equally important is the understanding of the film structural dynamics that directly affect the performance of the devices. To compare the structure of the films annealed with RTA and hot plate, XRD data have taken after processing on MAPbI3 film fabricated with three different annealing routes: hot plate for 3 min, RTA for 3 min, and hot plate for 10 min. As shown in Figure S1, we observed the main MAPbI3 (110) and (220) XRD peaks. Both of these peaks shifted toward lower two theta when annealed with RTA for the same amount of time (3 min) as compared to a hot plate. Annealing the sample longer (10 min) on the hot plate, the XRD peak shift is similar to the shift observed for the 3 min RTA anneal. This indicates that the final material for annealing on a hot plate for 10 min is similar to RTA annealing for 3 min. This is consistent with our observation in SEM and device data, which suggest that the differences between hot plate and RTA are limited to a variation in processing time, that is, a shorter annealing time with RTA and a longer hot plate annealing will give similar results. Hence, the conclusions made from this study are not limited to studies with RTA and are relevant for studies with hot plate annealing as well. Role of Antisolvent in Making Highly Efficient MAPbI3 Devices. Since the first solvent engineering report from Seok et al.,7 most of the high efficient PSCs reported in the literature were fabricated using solvent engineering. Despite the huge success in achieving high efficiency devices with solvent engineering, details of perovskite solution-to-crystallized film processing with solvent engineering require more careful exploration due to the complexity (multiple intermediate compounds appear13 due to complexation). To thoroughly

Figure 3. In situ XRD of the effect of antisolvent on the film structure evolution. (a) Temperature profiles (dots represent X-ray scan position). (b) RTA/in situ XRD data of no antisolvent sample. (c) RTA/in situ XRD data for with antisolvent sample. (d) First scan for with and without antisolvent films, with MAI-PbI2-DMSO and MAPbI3 X-ray patterns as references. (e) Time dependence of MAIPbI2-DMSO peak. Symbols represent XRD positions for PbI2 (∗) (Q ≈ 0.90 A−1), cubic MAPbI3 (100) (#) (Q ≈ 1.00 A−1), cubic MAPbI3 (110) (•) (Q ≈ 1.41 A−1), PbI2-MAI-DMSO (Δ), tetragonal MAPbI3 (211) (Ω) (Q ≈ 1.66 A−1), and cubic MAPbI3 (111) (+) (Q ≈ 1.72 A−1). 5934


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Figure 4. Effect of antisolvent on device performance. (a) J−V characteristics, (b) statistics (from reversely scanned J−V curves, 24 devices in each case), (c) EQE. and (d) stabilized photocurrents and efficiencies of devices with (blue) and without (red) antisolvent.

film surface and so inhibits facile evaporation of the PbI2-MAIDMSO intermediate compound trapped within the film. It should be noted that the film formation process discussed here is largely limited to the specific solvent engineering method (DMF and DMSO as solvents, and diethyl ether as antisolvent) we used for this study. For instance, in Seok’s7 solvent engineering procedure (solvents: GBL and DMSO), the role of antisolvent (toluene) is speculated to wash excess DMSO after forming the PbI2-MAI-DMSO compound; in Spiccia’s9 procedure (solvent: DMF), the role of the antisolvent (chlorobenzene) is assumed to wash off DMF to induce fast crystallization of MAI and PbI2. In our study, the role of the antisolvent is likely promotion of uniform MAPbI3 film with minimal amount of the intermediate compound. Changing the solvents (antisolvent) and the resulting changes in polarities, proticities, boiling points, and solubilities between the antisolvent (solvents) will greatly affect the film formation dynamics. As the film is annealed longer, MAPbI3 (110) can be detected. This peak cannot be seen earlier due to the film crystallite orientation or texture.26,43−45 To determine the crystallite orientation change (texture) over the annealing time, we tracked the XRD peak intensity (I) ratios of MAPbI3 XRD peaks, including I(100/I(110), I(100)/I(111), I(111)/I(110) of Figure 3c as Figure S3. A texture change is observed before the temperature reaches 100 °C and after the sample starts to cool down, as shown by I(100)/ I(110) and I(100)/I(111). However, over the course of annealing at 100 °C, the texture of the film remains unchanged. This shows that the phase fraction is proportional to the integrated peak intensities (which are plotted below).

results for the film with and without antisolvent are respectively presented in Figure 3b,c. In both cases, the X-ray data show strong peaks of MAPbI3 (100), and MAPbI3 (111), before heating is started (first scan in Figure 3b,c). This indicates that MAPbI3 was partially formed and crystallized before annealing. In addition to the easily recognizable MAPbI3 peaks, the films deposited without antisolvent have a more complicated XRD spectrum, that is, additional peaks, as compared to the films deposited with antisolvent, as shown in Figure 3d. We compared the additional peaks with all reported13 possible crystalline compounds, including PbI2-DMSO, PbI2-MAI-DMSO, MAI-PbI2-DMF, PbI2-DMF, and we found that the additional peaks in the no antisolvent case matched well with PbI2-MAI-DMSO (Figure 3d), that is, no crystallized compound with DMF is detected. This finding offers new insight into the role that the antisolvent plays. In an earlier study,11 it was speculated that the role of diethyl ether (antisolvent) was removal of DMF. Here we find that the antisolvent seems to promote the formation of uniform films (Figure S2) consisting of mainly MAPbI3, with only a minimal amount of the crystalline PbI2MAI-DMSO compound (Figure 3d) as shown by the presence of only the strongest PbI2-MAI-DMSO peaks. To gain insight into the evolution of PbI2-MAI-DMSO, we tracked its peak area (shown in Figure 3e) and found that along with other MAI-PbI2-DMSO peaks, it disappeared within 250 s of annealing in the no antisolvent case. However, with antisolvent, this peak was present during the whole annealing process (up to ∼1540 s). This further confirmed that the antisolvent helps to form uniform MAPbI3 films with better coverage, that is, less porosity and fewer cracks. We believe the antisolvent stabilizes the PbI2-MAI-DMSO compound on the 5935


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Figure 5. Effect of humidity on annealing perovskite characterized by in situ RTA/XRD. (a) Temperature profile (dots represent X-ray scan positions). (b) In situ diffraction from MAPbI3 film annealed in dry N2. (c) PbI2 peak from the MAPbI3 film annealed in lab air (blue) and wet N2 (green). XRD peak symbols are same as Figure 3.

Figure 4, having such an intermediate state alone does not guarantee a high-quality film. It is worth noting that in addition to poorer device efficiency, the devices without antisolvent suffer from serious light soaking issues, which are widely discussed7 in the PSC research field. As shown in Figure S4 and Table S3 in the Supporting Information, the efficiency of devices without antisolvent increased by 175% over 200 s of light soaking, while the devices with antisolvent remain unchanged during light soaking. Note, the solar cell performances reported in Figure 4a, b, and d are after light soaking. The improved device performance under light is observed in all device parameters but is most significant in the Jsc and FF, which are improved by 62% and 57%, respectively. The inherent reason why perovskite solar cells suffer from the light soaking is still a matter of debate. Many researchers43,48−52 believe the origin is the same as that causing the hysteresis in the J−V scans (e.g., Figure 4a). Reasons such as the reorientation of ferroelectric organic cations, ion migration, and trapping/detrapping of charge carriers at the perovskite/TiO2 interface have been suggested. The presence of pin-holes (Figure S2a, Supporting Information), and PbI2-MAI-DMSO (Figure 3) in the without antisolvent films, along with the reduced performance of these cells may support the hypothesis that the light soaking effect is due to trapping/detrapping of charge carriers at the nonperovskite centers in the film. These centers may be surface recombination sites at the pin holes. Elucidation of the RTA annealed MAPbI3 film formation process with solvent engineering and its implications in the final device performance prompts us to question how film deposition conditions affect this film formation process. Therefore, in the following sections, we will extend our investigation to study the effect of annealing environments, annealing profile, and substrates on MAPbI3 formation. Impact of MAPbI3 Annealing Environment. From the early stage of MAPbI3 research, processing environment, including both deposition and annealing environment, became a key parameter in determining the performance and

Figure 3 shows that PbI2 starts forming around 110 s, independent of solvent processing. As seen in Figure 3a, there is a difference in the temperature profiles of with and without antisolvent cases when cooling. We expect to observe the cubic to tetragonal phase transition of MAPbI3 on cooling from 100 °C. As shown in Figure 3c, a peak at Q ≈ 1.66 A−1 (labeled as “Ω”) appeared between the fifth from last scan (56.7 °C) and the fourth from last scan (52.2 °C). This temperature coincides with the cubic to tetragonal phase transition temperature of MAPbI3 that has been reported as 54 °C (ref 46) and 57 °C (refs 44 and 47). In Figure 4a and Table S2, we show the J−V curve comparison between MAPbI3 based solar cells with the active layer fabricated with and without antisolvent to illustrate the effect of antisolvent on the final device. For these samples, the annealing profile of the active layers is 65 °C for 1 min, and 100 °C for 2 min, following the report from Park et al.;11 all anneals are performed in ambient air (relative humidity, RH ≈ 40 ± 10%). The devices with antisolvent produce cells with much better performance. The Voc increased by 17%, the Jsc improved by 7%, FF improved by 24%, and the overall PCE improvement of 55% when reversely scanned. Statistical studies (Figure 4b), EQE characterization (Figure 4c), and steady-state current density and power conversion efficiency (Figure 4d) also confirm the significantly reduced device performance without antisolvent. SEM images (Figure S2 in the Supporting Information) show that without antisolvent, there are a lot of pin-holes in the film that can be local recombination centers or even form short circuits, which will reduce the final device performance by significantly reducing the fill factor. Many of the earlier studies on this materials7,11,45 found that the formation of such an intermediate compound PbI2-MAIDMSO is important for high efficiency devices as it allows decoupling of the film formation and crystallization process by retarding the rapid reaction between PbI2 and MAI.7 However, here we observe that the important factor is the formation of a pinhole free film, which seems to be assisted by the intermediate compound as discussed above. As shown in 5936


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Figure 6. Humidity affected device performance. (a) J−V characteristics of the best devices and (b) stabilized photocurrents and efficiencies of devices RTA in N2 (red) and lab air (blue).

XRD, which could be due to them being noncrystalline or below the detection threshold. With dry N2 flowing through the annealing chamber, the cooling is more rapid than in the lab air run where gas was not flowing (the gas in the chamber was the lab air already in the chamber upon closing it). Thus, the temperature decreased below the cubic to tetragonal transition point in the dry N2 run earlier. Figure 3c shows the tetragonal phase of MAPbI3. The (211) peak at Q ≈ 1.66 A−1 appearing in the last five X-ray scans (starts ∼1440 s, 56.6 °C, Figure 5a) and Figure 5b shows the tetragonal phase appearing in the last eight X-ray scan (∼1360 s, 56.4 °C, Figure 5a). Our observation of the cubic to tetragonal phase transition of MAPbI3 around 56° is in agreement with the study performed by Baikei et al.44 study, which again confirms that temperature control of the RTA used in this study is accurate. To understand the effect of humidity induced structural differences on device performance, two groups of devices were fabricated, one in air (RH ≈ 40 ± 10%) and one in N2. The active layers were deposited with Lewis acid−base adduct approach and then RTA at 65 °C for 1 min and 100 °C for 2 min, with a ramping rate of 10 °C/s. The J−V curves of the most efficient device in each group are presented in Figure 6a and Table S4. The device fabricated in N2 resulted in a slightly better current density, Jsc, and FF. Stabilized photocurrents and efficiency (Figure 6b) also confirmed an approximately 1% improvement of PCE, from 16.3% to 17.3%, with RTA in N2 compared to air. Aside from the device performance, the photovoltaic statistics (24 devices in each case) of Figure S5 show that reproducibility of devices processed in dry N2 is improved: PCE standard deviation decreased from 1.8% to 0.9% in dry N2 devices compared to the lab air annealed devices. One of the key reasons for such difference is likely resulted from the MAPbI3 degradation to PbI2 that is shown in Figure 5c. However, the fact that there are only small performance differences between the champion devices in lab air and dry N2 suggest the appearance of a small amount of PbI2, though harmful on device reproducibility, is not catastrophic to device performance. In conclusion, we found it is humidity, not oxygen, that induces MAPbI3 to degrade faster during radiative annealing. While humidity induced more rapid degradation of MAPbI3 into PbI2, the device performance is not catastrophically impacted Implications of Multiple Annealing Steps in Making MAPbI3 Film. To further enhance our understanding of the

reproducibility of the devices. Two prominent factors in the processing environment that affect the quality of perovskite deposition are humidity53,54 and oxygen.55 However, the role of humidity and oxygen in the formation and degradation the perovskite film is still debated.56−60 For instance, in terms of oxygen, a recent study by Aristidou et al.61 proposed that oxygen can quickly diffuse into MAPbI3 to form superoxide species that induce degradation of MAPbI3, and light further promotes such degradation. As our annealing was performed under light, a study to decouple the effect of humidity and oxygen with light is important for understanding of the growth and degradation of MAPbI3. As a result, RTA/in situ XRD experiments were performed. Films were deposited with antisolvent and annealed in different environments, namely air (relative humidity, RH ≈ 40 ± 10%), dry N2,, and wet N2. Figure 5a shows the temperature profile for the study. The as spun-coated film, using the Lewis acid−base adduct approach,11 was set to anneal at 65 °C, ramped from room temperature with a ramp rate of 10 °C/s, and held for 1 min then ramped to 100 °C (again at 10 °C/s) and held for 20 min. As shown in Figure 5a, the temperature in both cases starts to cool at ∼1280 s, but in the dry N2 case, it cools faster due to the flow of N2 through the RTA. Figure 5a shows the temperature profile, while Figure 3c and Figure 5b, respectively, show the diffraction data of MAPbI3 in the lab air and dry N2. In both series, a strong peak of cubic MAPbI3 (100) and MAPbI3 (111) is detected immediately, and MAPbI3 (110) appeared a little later, similar to the antisolvent study discussed earlier. PbI2 starts to appear at about 114 s ± 25 s (Figure 3c, PbI2 peak tracked in Figure 5c) in the air study but does not appear in 20 min of annealing in dry N2 (Figure 5c), indicating some components in the air is inducing MAPbI3 decomposition. To understand what is causing the MAPbI3 decomposition in air, in situ XRD was performed using dry N2 bubbled through a bubbler filled with water before entering the RTA chamber. The PbI2 peak areas in both air and wet N2 are tracked and shown in Figure 5c. Both air and wet N2 samples show the formation of PbI2, with almost identical appearance start time (∼120 s) and initial crystallization rates (Figure 5c), whereas the sample annealed in dry N2 does not have any PbI2, indicating that humidity (H2O) is inducing the decomposition. The MAPbI3 decomposition mechanism under humidity is widely debated and some possible paths including formation of MAI-H2O,62 (MA)4PbI6-2H2O,15,54 or MAPbI3·H2O,63 though we are unable to detect any of the above compounds in in situ 5937


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crystallinity of films with different annealing routes by directly comparing the intensity of peak areas in each condition, this comparison would be flawed due to the variation of peak areas we observed in each different XRD run due to small variations in sample alignment. Therefore, the discussion will mainly focus on the shape and trend of the XRD peak area. The MAPbI3 peak area (Figure 7c) reaches the highest intensity, both in the two step and one step case, at ∼100 °C, indicating at 100 °C, the film is most crystallized. The PbI2 (Figure 7d) does not appear when annealed at 65 °C, even after 3 min, but it appeared within 3 min when annealed at 100 °C. The 65 °C hold time does appear to influence PbI2 degradation though since the sample held first at 65 °C for 1 min then at 100 °C shows degradation within 2 min. Device performances from cells made with absorber layers annealed with these varying annealing profiles are shown in Figure S6 and Table S5. Four controlled annealing profiles were used to understand the effect of crystallization variation on the device performance: 65 °C for 1 min and 100 °C for 2 min (A), 100 °C for 3 min (B), 65 °C for 3 min (C), and 65 °C for 1 min and 100 °C for 3 min (D). As shown in Figure S6 and Table S5, when the films were annealed at 65 °C for 3 min (C), the device performance is much worse than in any of the other three cases (A, B, or D). This is expected since at 65 °C the film does not fully crystallize into MAPbI3, which is evidenced by Figure 7c (green line). However, in all other cases with 100 °C final anneals, with or without the 65 °C annealing step, the device performances are very similar, indicating that the modest structural differences observed do not cause significant difference in the device performance. Statistical analysis (Figure S7, Supporting Information) of three of the four controlled annealing profiles also shows similar trends (20 devices in each case). Only the constant 65 °C annealed film (Figure 7b and 7c green trace), where MAPbI3 is not fully crystallized, shows a different trend with more scatter. These results indicate that for the annealing method studied here, two step annealing is not required to fabricate high quality perovskite films. This suggests that it is not straightforward to apply optimizations from different perovskite solution-to-film deposition processes to a new film formation method. The processing parameters that effect the perovskite formation are diverse and intertwined and so new processing methods may require different optimizations. This is supported by MAPbI3 growth on different substrates (compact TiO2, meso TiO2, and glass) where we found planar structure devices perform better than mesostructured devices that are discussed in detail in the Supporting Information.

effects of the processing conditions on formation of MAPbI3 film and devices, we shift our attention to the annealing steps during the processing. The study by Park et al.11 on using diethyl ether as the antisolvent reports the two-step annealing, 65 °C for 1 min and 100 °C for 2 min, produces high quality films, which echo some earlier studies11,19 finding that using multiple annealing steps form films with improved surface coverage (i.e., pinhole free), better crystallization, and better morphology. A common reasoning for using two steps is the first helps to nucleate the film and the second step creates the desired morphology. Here, we try to understand the effect of those annealing steps in the perspective of crystalline structure and its effect on device performance. Figure 7 shows the temperature profile and area peak tracking of MAPbI3 film with various annealing routes: 65 °C

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CONCLUSION In this study, we applied a well-controlled, more energy efficient, and industrially compatible radiative thermal annealing to MAPbI3 perovskite films that were fabricated through the solvent engineering method, with diethyl ether as the antisolvent. Devices with ∼18% PCE using radiative annealing have been demonstrated. Using an in situ RTA chamber compatible with a synchrotron X-ray source, we comprehensively investigated the effect of antisolvent, humidity, annealing protocol, and film substrate, with radiative thermal annealing. The use of RTA/in situ X-ray allowed for a better understanding of MAPbI3 formation in solvent engineering with diethyl ether as the antisolvent, and DMSO and DMF as the solvent. It was found that the role of the antisolvent is likely not to wash off the DMF solvent as suggested in the literature,

Figure 7. Effect of annealing step from RTA/in situ X-ray runs. (a) Temperature profiles. Peak area tracking for (b) MAI-PbI2-DMSO, (c) MAPbI3 (100), and (d) PbI2.

for 3 min; 65 °C for 1 min, followed by 100 °C for 2 min; and 100 °C for 3 min. In each condition, MAI-PbI2-DMSO (Q ≈ 1.47 A−1), MAPbI3 (100) (Q ≈ 1.41 A−1), and PbI2 (Q ≈ 0.90 A−1) peak areas from the in situ RTA XRD data were tracked. MAI-PbI2-DMSO, as shown in Figure 7b, does not change over the 3 min of annealing at 65 °C (green lines). Park et al.11 found 65 °C for 1 min is sufficient to remove all DMSO; however, we observe the DMSO contained intermediate compound, does not evaporate at 65 °C. Once the temperature approaches 75 °C, MAI-PbI2-DMSO decomposition occurs (blue and red lines). Though it is tempting to compare the 5938


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Government of India subcontract IUSSTF/JCERDC-SERIIUS/2012 dated 22nd Nov. 2012. We thank Dr. Z. Li (NREL) for assistance on SEM and Dr. J. A. Christians (NREL) for the fruitful discussion. We thank Dr. B. Johnson (SLAC) and Dr. D. Van Campen (SLAC) for assistance with SSRL beamline 7-2. B.D. thanks Prof. F. S. Barnes and Prof. S. E. Shaheen for providing academic supports.

since no crystallized compounds with DMF are found in both with and without antisolvent. It is suggested that the antisolvent assists in forming large grain uniform films, consisting of mainly MAPbI3 with only a minimal amount of PbI2-MAI-DMSO. Importantly, our results show that although having PbI2-MAIDMSO is critical in forming high quality MAPbI3 films, as it decouple the process of film formation and crystallization, the presence of PbI2-MAI-DMSO alone does not guarantee good efficiency solar cells. Furthermore, we found the no antisolvent devices suffer greatly from light soaking, which is likely due to trapping/detrapping of charge carriers at pinholes. We further extended the investigation to the effect of annealing environment on MAPbI3 formation and directly show humidity caused MAPbI3 to decompose into PbI2 more rapidly and as such hurts the reproducibility of the device performance. Our study demonstrates that due to the complex nature of MAPbI3 film formation, it may not always be correct to assume specific methodology of previous work is applicable when formulating a new annealing method. Here, with the combination of radiative annealing, in situ XRD, SEM, and device performance, we have endeavored to understand the underlying cause of differences in MAPbI3 film and device qualities to inform future work in developing perovskites as next generation solar materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01467. XRD data; SEM images; device performances. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Benjia Dou: 0000-0001-6038-5561 Michael F. Toney: 0000-0002-7513-1166 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.

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ACKNOWLEDGMENTS This work was supported by the Bridging Research Interactions through collaborating the Development Grants in Energy (BRIDGE) program under the SunShot initiative of the Department of Energy (DE-EE0005951). Stanford Synchrotron Radiation Light source at the SLAC National Accelerator Laboratory is a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences under Contract No. DE-AC0276SF00515. This research is based upon work supported in part by the Solar Energy Research Institute for India and the U.S. (SERIIUS) funded jointly by the U.S. Department of Energy Subcontract No. DE AC36-08G028308 (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology Program, with support from the Office of International Affairs) and the 5939


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