Effect of Water Vapor, Temperature, and Rapid Annealing on

May 10, 2016 - National Renewable Energy Laboratory, Golden, Colorado 80401, United States. ‡ Advanced Materials Research Institute, University of N...
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Effect of Water Vapor, Temperature, and Rapid Annealing on Formamidinium Lead Triiodide Perovskite Crystallization Jeffery A Aguiar, Sarah Wozny, Nooraldeen R. Alkurd, Mengjin Yang, Libor Kovarik, Terry G. Holesinger, Mowafak Al-Jassim, Kai Zhu, Weilie Zhou, and Joseph J. Berry ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00042 • Publication Date (Web): 10 May 2016 Downloaded from http://pubs.acs.org on May 13, 2016

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Effect of Water Vapor, Temperature, and Rapid Annealing on Formamidinium Lead Triiodide Perovskite Crystallization Jeffery A. Aguiar1,§, Sarah Wozny2,§, Nooraldeen R. Alkurd2, Mengjin Yang1, Libor Kovarik3, Terry G. Holesinger4, Mowafak Al-Jassim1, Kai Zhu1, Weilie Zhou2, and Joseph J. Berry1,* 1

National Renewable Energy Laboratory, Golden, CO, 80401, USA

2

Advanced Materials Research Institute, University of New Orleans, New Orleans, LA, 70148,

USA 3

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O.

Box 999, Richland, Washington 99352, USA 4

Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, New Mexico 80465, USA

* Corresponding authors: J.A.A. E-mail: [email protected], tel: +1 (303) 384-6485. J.J.B. E-mail: [email protected], tel: +1 (303) 384-7611 § Joint authorship

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Abstract. Perovskite based solar cells are one of the emerging candidates for radically lower cost photovoltaics. Herein, we report on the synthesis and crystallization of organic inorganic formamidinium lead-triiodide perovskite films under controlled atmospheric and environmental conditions. Using in situ (scanning) transmission electron microscopy, we make observations of the crystallization process of these materials in nitrogen and oxygen gas with and without the presence of water vapor. Complementary planar samples were also fabricated in the presence of water vapor and characterized by in situ X-ray diffraction. Direct observations of the material structure and final morphology indicate the exposure to water vapor results in a porous film that is metastable, regardless of the presence argon, nitrogen, or oxygen. However, the optimal crystallization temperature of 175°C is unperturbed across conditions. Rapid modulation about the annealing temperature of 175°C in ±25°C steps (150°C to 200°C) promotes crystallization and significantly improves the film morphology by overcoming the presence of impregnated water trapped in the material. Following this processing protocol, we demonstrate substantial growth to micron-size grains via observation inside an environmentally controlled transmission electron microscope. Adapting this insight from our in situ microscopy, we are able to provide an informed materials protocol to control the structure and morphology of these organic inorganic semiconductors, which is readily applicable to benchtop device growth strategies. TOC Image

Halide perovskite-based solar cells (HPSCs) with an ABX3 hybrid structure hold the promise for a new era in the photovoltaic industry due to (i) their excellent light harvesting properties,1-3 (ii) easy fabrication by simple solution-based techniques,4-8 (iii) low annealing temperature (15%).11-14 Given their nascent state in the present literature the fundamental understanding of the material properties and processing conditions are lacking.15-19 Fundamental studies are therefore necessary to understand the material’s intrinsic properties as a function of their processing environment. Currently, the majority of studies on perovskite materials rely on cesium (Cs),20-21 methyl ammonium (MA, CH3NH3+)8, 14 and formamidinium (FA, CH(NH2)2+)22 cations and their alloys, 2 ACS Paragon Plus Environment

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paired typically with PbI3. By interchanging cations, the structural, optical, and electrical properties of the perovskite absorber material can be tuned. Among the myriad of cations, FA stands out due to a higher absorption coefficient, an optimum direct band gap (1.47eV), high device efficiency (>15%), and elevated crystallization temperature than its MA counterpart (̴ 170°C). FA-based perovskite alloys have also demonstrated improved device efficiency and their material stability is upgraded over their MA counterparts.23-29 Despite high efficiencies and better stabilities reported in a short period of time, HPSCs remain largely unstable under certain processing conditions. It has been widely reported that their performance varies with time, temperature, humidity, and processing.30-32 Reports on the MA-based perovskite material have shown that humidity leads to eminent degradation of the absorber material into secondary phases, lead, and lead iodide (PbI2).18, 33-35 Similar reports on FA-based HPSCs, measure no hydration of the absorber material upon humidity exposure during the fabrication process. However, conflicting literature also reports that relative humidity leads to changes in the morphology, optical and electrical properties of the absorber material, resulting in decreasing power conversion efficiency of all HPSCs as a function of increasing humidity.32 From these studies, it is clear that mitigation of environmental factors is critical to enabling HPSCs technologies, but a detailed taxonomy of different mechanisms at the device level still requires further research. Observing the solidification of perovskite–based solar cells under controlled environmental conditions is vital to furthering our ability to develop robust processes for fabricating higher efficiency solar cells. Recently, Wozny et al. used ex situ SEM imaging to report the development of pinholes and increased grain growth following exposure to relatively high water vapor pressures.32 Furthermore, researchers have also determined the rough chemical composition and texture of MAI and FAI following hot casting.36-37 The inherent challenge to each of these studies is reporting on the evolving processes as these organic inorganic perovskite materials transform. The techniques required are in situ and ex situ environment studies that take advantage of the latest staging and experimental setups for help identifying and tracking effects of different processing steps on the nanoscale, including the atomic structure and chemistry. Revealing the reaction mechanisms between organic-inorganic perovskite material precursors and different liquid and gas environments remains to be addressed. Specifically, there is a need for research on environmental influences impacting crystallization processes. In situ gas 3 ACS Paragon Plus Environment

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scanning transmission electron microscopy (STEM) is a method that enables the atomic mass contrast imaging and microanalysis of dynamic processes occurring within a gaseous environment at high spatial and energy resolution. Using either “closed-form” in situ gas cells or an open cell environmental transmission electron microscopy, new insights have been gained regarding the mechanism(s) of nanoparticle nucleation and growth, structural imaging of catalysts, and the dynamics of redox reactions.38-40 For studying the formation and crystallization of perovskite solar cells, heating experiments inside an environmental scanning transmission electron microscope (ETEM) with in situ gas affords the structural and chemical imaging sensitivity to track the material over various environmental regimes. ETEM is a high-resolution materials characterization platform that allows for both real-time observation and detailed studies on the morphological, chemical, and structural evolution of materials under a variety of experimental conditions. For this study, we performed a series of controlled in situ gas studies using a differentially pumped ETEM with a heating stage for the direct observation of the formation of FA-based perovskite on compact-titania treated silicon nitride window in the presence of water vapor, nitrogen and oxygen gases. Moisture in correlation with temperature is known to directly impact the solution-processed morphology of organic-inorganic perovskite-based materials. Based on the results of this study, we have shown that the porous microstructure is indeed dependent on the amount of water vapor. Repeated in situ experiments in the presence of N2 and O2 show that the perovskite material is also fairly materials tolerant and can be solidified into a large micron-size anhydrate material following sequential rapid annealing. These series of results highlight the reaction kinetics between the FAPbI3 and provide insight to enable modification of the films via thermal processing. The final outcome of these experiments is an anhydrous film of largely crystalline material that is the preferred morphology for incorporation of these absorbers into solar cell device stacks. Leveraging the capabilities of in situ ETEM, we conclude the study by providing a device-level demonstration of measureable improvements on the initial porous microstructure and morphology of FAI-based perovskite material, to form largely micron-sized anhydrous crystalline material following sequential rapid annealing about 175°C.

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In situ Environmental Transmission Electron Microscopy. To explore the transformation of FAPbI3 perovskite material to a crystalline structure upon heating, we performed in situ gas ETEM heating experiments in humid atmosphere. Figure 1 shows a schematic of an in situ experiment. For our experiments, a compact titania-treated, electron transparent SiNx grid served as our template for studying in situ crystallization and growth. FA-based perovskite solution was drop deposited on the UV treated compact-titania treated silicon nitride grid inside a dry cell. Details of the layered structure and synthetic approaches can be found in the Methods section. The grid was then loaded on a Gatan holder with heating capability ranging from 25°C to 1000°C and inserted into the environmental sample chamber of the ETEM. A mixture of water vapor and argon gas was then introduced inside the chamber and monitored externally by differential vacuum pressure gauges. The gas concentration was then checked with an attached mass spectrometer. Samples were therefore impregnated with water that condenses on the surface of the sample and presumably finds pathways into the material, as well. Pending the experiment, the partial pressure of water was controlled to less than 10 mbar. For studying the structure and chemistry of the material, the transmission electron microscope was operated at 300 kV under low dose conditions in TEM, diffraction, and STEM mode. STEM mode was operated with less than 10 e- nm-2*s-1. Low and high magnification annular bright and dark field images were collected.

Samples were held at room temperature for at least 10 minutes prior to observation. Water vapor was introduced into the chamber over these initial 10 minutes. Before the sample was heated, minimal images and selected area electron diffraction (SAED) patterns were collected. Upon commencing the experiment, we acquired STEM image snapshots where the beam was blanked between each acquire to otherwise minimize the total exposed sample beam dose. STEM images and SAED patterns from various regions were also recorded throughout the experiment. Following the conclusion of each experimental run, STEM images were taken and the samples were stored and further analyzed to assure that the electron beam did not affect our observations.

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Figure 1. Schematic of the heating sample holder inserted in the environmental scanning transmission electron microscope chamber used for the controlled environmental study of FAbased PSCs with heating capability from 50°C to 275°C with flowing humid argon gas.

Tracking environmental effects on the structure and morphology associated with the FA-based PSCs, we performed ETEM using an array of imaging conditions, including annular bright field (BF) and dark field (DF) STEM as well as selected area electron diffraction (SAED) patterns. Upon commencing the experiment, we acquired image and diffraction snapshots. The beam was blanked between acquisitions to otherwise minimize the total beam dose. Plane-view STEM images of the perovskite precursor material under deposition and introduction of water vapor are shown in Figure 2. The sample was then heated to 50°C and maintained at this temperature for 10 min under 1 mbar of water pressure measured through a differential pumping gauge. Figure 2a and 2b are the bright and dark field STEM images at the beginning of the experiments, where there is observed uniformly dispersed porosity throughout the material. After maintaining the gas flow and 50°C temperature for 10 minutes, Figure 2c and 2d are STEM plane-view images of the same area simultaneously taken under the bright and dark field conditions. No significant change is observed in the sample. This data indicates that at low temperature and water vapor pressure alone do not cause significant changes in the precursor materials. Some water vapor

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condensation is presumed to occur on the surface and deeper into the bulk possible, facilitated by the visible pores and surface blisters observed in Figure 2.

Figure 2. STEM plane-view images taken at 50°C from time (a-b) 0 to (c-d) 10 minutes, under dark and bright field conditions, respectively. The arrows indicate the pores in the sample.

In situ Electron Microscopy in the Presence of Water Vapor. From this point onward, we performed our in situ heating experiments to expose the role of temperature on the crystallization process of FAPbI3 in the presence of water vapor and inert argon. Figure 3 shows a series of STEM plane-view micrographs taken under bright field conditions as a function of temperature, and under 1 mbar of partial pressure. The experiment was carried out for a temperature ranging from 25°C to 275°C. Raising the temperature to 50°C (Figure 2) for roughly 10 minutes, we observe no changes in morphology. Increasing the temperature to 150°C (Figure 3a) the initial morphology is maintained. However, after 10 min at 150°C (Figure 3b) there is a noticeable coarsening of the structure on length scales smaller than ~60 nm.

Further increasing the

temperature to 160°C (Figure 3c) triggers an immediate and more dramatic change in the morphology and coarsening of material into smaller nanocrystals. After an additional 10 minutes at 160°C (Figure 3d), no change in the size of the nanocrystal is observed, but voids appear in

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near grain boundaries. We note the direction of voids are along grain boundaries cannot be determined from our images. The presence of voids in the material is attributed to the initial introduction of water vapor in the chamber. In the absence of water vapor (Figure S1 and S2, Electronic Supporting Information), when the perovskite is crystallized in situ under dry atmosphere, with the same heating conditions, no noticeable voids are observed. These results are consistent with the previous exsitu SEM experiments by Wozny et al.32 To further observe the effect of water vapor on the material, we increased the temperature to 175°C (Figure 3e) and kept it at this temperature for 10 minutes (Figure 3f). The number of voids in the material does not change, however the void size is slightly amplified, and they demonstrate well-defined boundaries. A rise to 200°C (Figure 3g and 3h) and 225°C(Figure 3i and 3j), with 10 min intervals, we observe an augmentation in the number of voids and their sizes, leading to more rapid degradation of the material and loss. During this same interval, however the grain size increases. Nanocrystalline domains on the order 30 nm grow to nearly 100 nm. From this, it is clear that humidity, in conjunction with temperature, results in material with pores that are mobile and can coalesce under heating. On the other hand, their coalescence coincidentally ties other regions of the sample together resulting in an increase in grain size. We suspect with more material, we would be able to remove pores and solidify the material with controlled sequential rapid thermal annealing steps. The rate is critically important as our work, along with that of others have found the kinetics for crystallization occur on a relatively fast temporal scale ( 1 µm) crystalline organic inorganic PSCs. Materials. All solvents and reagents were of analytical grade and used as received. Lead Iodide (PbI2, 99.999%) was purchased from Alfa Aesar. Formamidinium iodide (FAI) was purchased from Lumtec. Anhydrous N,N’-dimethylformamide (DMF) was purchased from Sigma Aldrich.

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Formamidinium lead triiodide precursor solution preparation. A 0.07M perovskite solution was prepared from stoichiometric amount of PbI2 and FAI in DMF. The solution was vortexed for 30 min to dissolve PbI2 and FAI powders in DMF, leading to a clear bright yellow solution. The solution was filtered twice using a PTFE 0.45 µm syringe filter to remove any undissolved starting materials. Electron microscopy sample preparation. Commercial silicon nitride grids (50 nm silicon nitride (SixNy) windows, Sigma) were spray coated with a compact titania layer. Briefly, a 0.2 M solution of Ti(IV) bis(ethyl acetoacetate)-diisopropoxide in 1-butanol was sprayed on the hot grid held at 450°C for 12 cycles. The silicon nitride grid was then subsequently annealed in air at 450°C for 1 hour. Following annealing, a 30 minute ultraviolet (UV) treatment was performed to roughen the active surface. A less than 1 um3 small drop of perovskite solution was immediately deposited on the surface of the grid using a micropipette. The following stack layering from top to bottom was achieved: perovskite/compact titania/amorphous silicon nitride (SiNx). Acknowledgements. This work was supported by the National Renewable Energy Laboratory as a part of the Non-Proprietary Partnering Program under Contract No. DE-AC36-08-GO28308 within the U.S. Department of Energy. A portion of the research is part of the Chemical Imaging Initiative at Pacific Northwest National Laboratory (PNNL). It was conducted under the Laboratory Directed Research and Development Program at PNNL, a multi-program national laboratory operated by Battelle for the U.S. Department of Energy under Contract DE-AC0576RL01830. The work was performed at EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at PNNL. The in situ X-ray diffraction experiments were performed at the National Renewable Energy Laboratory. TGH and in situ (S)TEM work was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division under grant number 2013LANL8400. Supporting Information Available. In situ selected area electron diffraction and low voltage STEM imaging of dry perovskite material is provided in the supplementary file. References.

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