Crystal Growth and Dissolution of Methylammonium Lead Iodide

Subscriber access provided by Fudan University

Article

Crystal Growth and Dissolution of Methylammonium Lead Iodide Perovskite in Sequential Deposition: Correlation between Morphology Evolution and Photovoltaic Performance Tsung Yu Hsieh, Chi Kai Huang, Tzu-Sen Su, Cheng-You Hong, and Tzu-Chien Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12303 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Crystal Growth and Dissolution of Methylammonium Lead Iodide Perovskite in Sequential Deposition: Correlation between Morphology Evolution and Photovoltaic Performance

Tsung Yu Hsieh, Chi Kai Huang, Tzu-Sen Su, Cheng-You Hong and Tzu-Chien Wei*

Department of Chemical Engineering, National Tsing-Hua University, Hsin-Chu, Taiwan.

Address: Department of Chemical Engineering, National Tsing Hua University, 101, Section 2, Kuang Fu Road, Hsin-Chu, Taiwan, 300, Republic of China E-mail: [email protected] Tel: +886-35715131 ext.33661 Fax number: +886-35715408

*To whom correspondence should be addressed

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Crystal morphology and structure are important for improving the organic-inorganic lead halide perovskite semiconductor property in optoelectronic, electronic and photovoltaic devices. In particular, crystal growth and dissolution are two major phenomena in determining the morphology of methylammonium lead iodide perovskite in the sequential deposition method for fabricating a perovskite solar cell. In this report, the effect of immersion time in the seconds step, i.e. methlyammonium iodide immersion in the morphological, structural, optical and photovoltaic evolution, is extensively investigated. Supported by experimental evidence, a five-staged, time-dependent evolution of the morphology of methylammonium lead iodide perovskite crystals is established and is well connected to the photovoltaic performance. This result is beneficial for engineering optimal time for methylammonium iodide immersion and converging the solar cell performance in the sequential deposition route. Meanwhile, our result suggests that large, well-faceted methylammonium lead iodide perovskite single crystal may be incubated by solution process. This offers a low cost route for synthesizing perovskite single crystal.

KEYWORDS: perovskite solar cells, sequential deposition, crystal development, dissolution-recrystallization process, morphology evolution 2


Page 2 of 36

Page 3 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction

Organo-lead halide perovskite semiconductors with favorable optical band gaps were first applied as the sensitizer in perovskite solar cells (PSC) by Kojima et al.1 After overcoming the problem of perovskite corrosion by liquid electrolyte, M. Lee et al developed a cornerstone PSC using solid-state hole transport material with remarkable 10.9% power conversion efficiency (PCE).2 Encouraged by this work, extensive research has investigated including the material properties of organo-lead halide perovskite3-6, optimization of the fabricating processes7-10, new device architecture11-12 and even scale-up engineering13-14, boosting the PCE to 22% within five years. Because high PCE PSC can be fabricated by a simple solution process, it is considered a feasibly scalable photovoltaic technology, which may change the landscape of future photovoltaics. Reviewing PSC fabrication methods, early studies applied the one-step solution process, which involves mixing both organic (typically CH3NH3I) and inorganic (typically PbI2) sources into a single precursor solution and then depositing onto the substrate via spin coating.1-2, 15 The growth mechanism of perovskite film was found to be related with the supersaturation of the deposited precursors and interfacial energy difference between substrate and precursors.16 Consequently, the method of solvent

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

removal tremendously impacts the nucleation and crystal growth of perovskite film. Furthermore, widely used solvent evaporation, exploiting anti-solvent17-18 and degassing of a liquid precursor19 have also been reported to extract solvent efficiently, leading to fully covered perovskite film with large grains on the substrate. Differing from the one-step process, Burschka and coworkers demonstrated a novel two-step process called sequential deposition to fabricate PSC; this procedure involves depositing PbI2 film firstly and subsequently converting it into perovskite film by contacting the PbI2 film with CH3NH3I8. This method has opened a new route for the preparation of efficient PSC with better reproducibility. Further development of the two-step process can be categorized into three types depending on the filming method in either step: (1) solution process, in which both inorganic and organic precursors are wet-processed like spin-coating20-22 or dip-coating23-24; (2) vapor process, in which both organic and inorganic precursors are sequentially deposited by vacuum deposition such as thermal evaporation25, (3) solution-vapor hybrid process, in which spin-coating may be used to deposit inorganic precursor followed by vacuum depositing of an organic precursor to form final perovskite crystals.10 Among the above three strategies, solution process is considered most attractive for its simplicity and scalability. Regarding the second step in sequential deposition, in which the reaction between PbI2 film and CH3NH3I solution occurs, it is generally accepted that nucleation of CH3NH3PbI3 takes place rapidly at the PbI2 and CH3NH3I interface, forming a dense 4


Page 4 of 36

Page 5 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


perovskite overlayer.20, 26 This dense perovskite overlayer layer retards further reaction between unreacted PbI2 and CH3NH3I, resulting in incomplete PbI2 conversion24. The amount and morphology of unreacted PbI2 is uncontrollable; this impacts the photovoltaic performance and reproducibility24, 27-28 of the PSC. To avoid PbI2 residuals, extending reaction time and applying thermal induced interdiffusion29 are widespread strategies to ensure a high conversion of perovskite. Consequently, dipping PbI2 film in CH3NH3I/isopropanol (CH3NH3I/IPA) is a crucial step in sequential deposition because it affects the quality of resultant perovskite film significantly. A number of studies have confirmed that large perovskite crystals appear in the cases of prolonged CH3NH3I/IPA dipping time or in a concentrated CH3NH3I/IPA bath.22,

30-31

Although the formation of such large perovskite crystals in sequential

deposition has been explained as process of dissolution and recrystallization32 during the perovskite growth, detailed examinations of this argument, especially in relation to PSC performance, have never been reported. With this in mind, this paper systematically studied the morphological, structural and spectroscopic development of the CH3NH3PbI3 made with different CH3NH3I dipping times. The correlation between the exterior change of the perovskite layer and the photovoltaic properties was properly linked. Elemental mapping in SEM showed that perovskite crystals in mesoporous-TiO2 (meso-TiO2) scaffold gradually diminished, while the surface cap of perovskite gradually rose during prolonged CH3NH3I dipping, evidencing dissolution-recrystallization dominates the 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 36

photovoltaic performance of the perovskite layer.

Experimental Synthesis of CH3NH3I CH3NH3I

was

synthesized

by

mixing

equimolar

hydroiodic

acid

and

methylammonium in a flask. Hydroiodic acid (57 wt% in water, Wako) was added to methylammonium solution (40% in ethanol, Wako) drop by drop with gentle stirring and nitrogen purging in an ice bath. The reaction was quenched after 2 hours and the white CH3NH3I precipitation

was

recovered

by carefully evaporating the solvent.

Recrystallization was conducted to purify the raw CH3NH3I product via ethanol (good solvent) and diethyl ether (poor solvent) at -7°C for 10 hours. The final product was washed by diethyl ether several times and then dried at 45°C in a vacuum oven overnight. Preparation of FTO/meso-TiO2/CH3NH3PbI3 Film A piece of fluoride-doped tin oxide glass substrate (FTO, 10 Ω/sq, NSG, Japan) was patterned by a laser etcher (LMF-020F, Taiwan), cleaned by commercial detergent (PK-LCG46, Parker Corporation, USA) and deionized water under an ultrasonic bath in sequence. After drying in air, the FTO substrate was further cleaned by UV-ozone. A dense TiO2 compact layer was spin-coated using a solution consisting of 0.15 M titanium dissopropoxide bis-(acetylacetonate) (75 wt% in 2-propanol, Sigma-Aldrich) in 6


Page 7 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2-propanol (99.9% Sigma-Aldrich) at 1500 rpm for 30 seconds. Then, such film was dried on a 120°C hotplate for 5 minutes and then annealed at 450°C for 30 minutes. A 200 nm-thick mesoporous TiO2 layer (meso-TiO2) composed of anatase nano-particles was spin-coated at 6000 rpm for 30 seconds using an ethanol-diluted commercial paste (30-TS, G24 Power Ltd, 30-TS/ethanol=1/5, w/w). After drying at 120°C, the TiO2 film was annealed at 500°C in air for 30 minutes and cooled to room temperature. Prior to use, a TiCl4 (≥98%, Fluka) chemical bath deposition was employed by treating the meso-TiO2 coated substrate in a 40 mM TiCl4 aqueous solution at 70°C for 30 minutes. After rinsing with deionized water and ethanol, the substrate was dried by air flow and annealed again at 450°C for 30 minutes. The preparation of the perovskite layer basically followed the previous literature.8 In short, lead iodide (PbI2, 99.9985%, Alfa Aesar) was dissolved in the mixture of N,N-dimethylmethanamide (DMF, 99.8%, Merck) and DMSO (dimethyl sulfoxide, 99.9%, J. T. Baker) (DMF/DMSO, 9/1, V/V) at a concentration of 1.2 M. The PbI2 solution was continually stirred at 70°C on a hotplate during the entire process. PbI2 was spin-coated on meso-TiO2 at 6000 rpm for 10 seconds. The purpose of 10% DMSO addition is to increase the solubility of PbI2 so that a uniform film can be obtained (as shown in Figure S1). Although the DMSO in the precursor solution may form amorphous PbI2-DMSO complex and therefore influence later discussion24, XRD pattern of PbI2 film coated by using DMF/DMSO mix solvent still shows very strong intensity of PbI2 at the 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

peak of 2θ=12.7°(shown in Figure S2). This evidence indicates that the possible existing amorphous PbI2-DMSO complex on the morphology of perovskite crystals can be reasonably neglected. The PbI2-infiltrated meso-TiO2 film was dried at 70°C for 10 minutes. After cooling to room temperature, the films were then immersed into a solution composed of 62.5 mM CH3NH3I in 2-propanol for different time periods, while the concentration of CH3NH3/IPA was fixed and the whole procedure was conducted at room temperature. Finally, the films were dried in air and heated at 85°C for 20 minutes. All processes were fabricated under steady conditions with 25±2°C and 30-40% RH.

Fabrication of PSC A

HTM

solution

composed

of

75

mM

2,2',7,7'-Tetrakis-(N,N-di-4-

methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD, >99%, , Taiwan), 25 mM lithium bis(trifluoromethylsulphonyl)imide (Li-TFSI, 99.95%, Sigma-Aldrich) and 120 mM tert-butylpyridine (tBP, >96%, Sigma-Aldrich) in chlorobenzene (99.8%, Sigma-Aldrich) was spin-coated onto the afore-prepared FTO/meso-TiO2/CH3NH3PbI3 at 4000 rpm for 30 seconds. Li-TFSI was pre-dissolved in acetonitrile (99.5%, Merck) at a concentration of 340 mg/ml. Finally, a 100 nm-thick gold electrode was thermally evaporated on the top of spiro-OMeTAD to complete the fabrication.

Characterizations 8


Page 8 of 36

Page 9 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Field-emission scanning electron microscopy (FESEM) images were performed by Hitachi SU-8010 operating at an accelerating voltage of 5 kV. The elemental analysis was done by using the EDS, EMAX-ENERGY (Horiba) system model and the accelerating voltage was fixed at 15 kV. The operating time was fixed at 200 seconds for each examination. For XRD measurement, the diffraction pattern was measured by a Rigaku Ultima IV X-ray diffractometer equipped with a ceramic tube (Cu, Kα, λ=1.5418 Å) and an Optional D/teX Ultra high-speed, position-sensitive detector system. The optical property of as-grown FTO/meso-TiO2/CH3NH3PbI3 films in visible light region was collected by an HP-8453 UV-visible spectrophotometer. Current-voltage (I-V) characterization of PSC was examined using a solar simulator (PEC-L15, Peccell, Japan) with a computer-controlled digital source meter (Keithley 2400, USA) under 100 mW/cm2 intensity and a standard AM1.5G spectrum. The light intensity was calibrated by a reference monocrystalline silicon photodiode (91150V, Newport, USA). All measurements were conducted applying a non-reflective metal mask with a 0.1 cm2 aperture area to precisely control the active area of PSCs. I-V curves were recorded both forward (short-circuit to open-circuit) and backward (open-circuit to short-circuit) scans. The scan step was controlled at 10 mV and the delay time was 50 ms. Moreover, no light or voltage biasing condition was conducted prior to the measurement.

Results and Discussion 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Evolution of CH3NH3PbI3 morphology Figure 1a shows the morphology of the as-deposited PbI2 layer on the top of meso-TiO2. Figure 1b to 1i are the FESEM topographies of CH3NH3PbI3 capped meso-TiO2 film on FTO glasses (FTO/meso-TiO2/CH3NH3PbI3) made from various CH3NH3I/IPA dipping times, from 10 seconds to 24 hours. As can be seen in Figure 1b, a large number of CH3NH3PbI3 crystals are uniformly distributed on the image, forming an intact cap on the meso-TiO2 within only 10 seconds dipping time; this result is consistent with findings of rapid nucleation of CH3NH3PbI3 reported elsewhere.20, 26 A zoom-in image of the CH3NH3PbI3 cap on the inset of Figure 1b reveals the crystal of CH3NH3PbI3 is irregular, with size distributed from 50 to 200 nm. As the dipping time extends to 1 minutes (Figure 1c), no obvious morphological difference is distinguishable from Figure 1b, implying the conversion of PbI2 to CH3NH3PbI3 takes place underneath the CH3NH3PbI3 skin during this period. After a dipping time of 10 minutes, a small portion of surface CH3NH3PbI3 crystals deforms to a cuboid shape and stretches out perpendicularly from the surface, increasing the roughness of the CH3NH3PbI3 cap (Figure 1d). The density and size of cuboidal crystals continue to grow in the sample with 20 minutes dipping time (Figure 1e). When the dipping time reaches 1 hour (Figure 1f), these cuboidal crystals keep coarsening and transform into either well-faceted rod-like or plate-like crystals with sizes over 2 µm. Despite the tremendous change in

10


Page 10 of 36

Page 11 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


crystal appearance, the capping of CH3NH3PbI3 remains visually intact and flawless until 3 hours dipping time (Figure 1g). When the dipping time reaches 8 hours (Figure 1h), lots of giant CH3NH3PbI3 cuboids are situated on the top of CH3NH3PbI3 capping layer. Inset of Figure 1h reveals the CH3NH3PbI3 cap is flawed with many breaches, rendering the bottom meso-TiO2 scaffold exposed and upper CH3NH3PbI3 crystals discontinuous. For an extreme condition of 24 hours dipping time (Figure 1i), the CH3NH3PbI3 cap becomes island-like, most capping CH3NH3PbI3 crystals are isolated, accompanied by shrunk crystal sizes. On the other hand, the coarsened crystals grow into 5 µm in size, which is ten times larger when compared with the newborn crystals in Figure 1b. From SEM observation, the evolutions in CH3NH3PbI3 morphologies clearly reveal that dissolution and recrystallization take place simultaneously in prolonged CH3NH3I/IPA dipping,

resulting

in

tremendous

morphological

FTO/meso-TiO2/CH3NH3PbI3 structure.

11


change

of

the


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Time-resolved SEM topographies of the CH3NH3PbI3 layer with (a) 0 second, (b) 10 seconds, (c) 1 minutes, (d) 10 minutes, (e) 20 minutes, (f) 1 hour, (g) 3 hours, (h) 8 hours, and (i) 24 hours immersion time in CH3NH3I/IPA; the insets are images of zoom-in images. (j) Correlation between dipping time and maximum grain size found in each SEM image. 12


Page 12 of 36

Page 13 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As noted above, the tremendous change in CH3NH3PbI3 morphology can be explained by the dissolution-recrystallization mechanism during CH3NH3PbI3 crystal ripening. As proposed by Fu32 and Yan33, the thermodynamically favorable formation of lead from CH3NH3PbI3 in iodide-rich solution (such as CH3NH3I/IPA) provides the driving force for CH3NH3PbI3, or unconverted PbI2 dissolution, which generates lead polyiodide complex, and eventually PbI42-. Meanwhile, due to relatively concentrated CH3NH3+ ions in the bulk solution, CH3NH3PbI3 is recrystallized as long as the lead polyiodide complex exists in the system. In other words, the process of dissolution and recrystallization is the root cause that dominates the crystal growth of CH3NH3PbI3. The reactions involved in this step are believed to be:

Dissolution: ( ) + → + Pb( ) + 2 → Recrystallization: + → +

In TiO2 scaffold-type PSC, initially-formed CH3NH3PbI3 film is anisotropic because of the presence of mesoporous structure. As dissolution-recrystallization begins, small grains preferably dissolve into the bulk solution because of their high chemical potential; on the other hand, recrystallization preferably happens on large grains to minimize Gibbs 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

free energy. Consequently, the favorably oriented growth results in well-faceted crystals with rod-like or plate-like shape. Based on the observation of SEM images in Figure 1, the size and density of coarsened crystals obviously raises when dipping time surpasses 20 minutes. According to the dissolution-recrystallization mechanism, the appearance of coarsened crystals must be accompanied by dissolution of small crystals somewhere in the CH3NH3PbI3 cap or more likely from the mesoporous TiO2 scaffold.26 In other words, dissolution-recrystallization continuously consumes CH3NH3PbI3 inside the mesoporous scaffold and re-generates on the coarsened CH3NH3PbI3 crystals. Although mass conservation is still obedient, the morphology of CH3NH3PbI3 changes gradually. For instance, when extending dipping time to 3 hours, defects appear in the CH3NH3PbI3 capping layer, suggesting that most CH3NH3PbI3 at the mesoporous structure has exhausted. As a result, the CH3NH3PbI3 grains on the capping layer start to be dissolved, rendering a hollow CH3NH3PbI3 cap (see the insets of Figure 1h and 1i). Figure 1j illustrates the relationship between dipping time and the largest observed crystal size in each sample. It can be found the crystal size grows rapidly from 10 minutes to 1 hour of dipping time (there are no obvious large crystals present on the top at the initial stage) and then gradually slows down. After 24 hours of dipping, the crystal size approaches 7 to 8 µm. The curve in Figure 1j tends to reach a plateau after a prolonged period, which suggest that the growth of large crystals may slow down owing to the depleted small crystals. 14


Page 14 of 36

Page 15 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Evolution of CH3NH3PbI3 structure In addition to morphological investigation, the structure of CH3NH3PbI3 was studied by XRD. As summarized in Figure 2, the diffraction peak at 2θ=12.7° represents (001) the lattice plane of PbI2 and the diffraction peaks at 2θ=14.1° and 28.5° are diffraction features of CH3NH3PbI3.

Figure 2. (a)XRD analysis of as-grown CH3NH3PbI3 film and (b)zoom-in images of the peak at 2θ=14.1° and (c) zoom-in images of the peak at 2θ=28.5°.

As can be seen in Figure 2, the peak intensities at 2θ=12.7° and 14.1° are comparable for the sample with 10 seconds of dipping time (red curve), indicating a significant amount of unconverted PbI2 residuals in this sample. The diffraction characteristic of PbI2 fades away as dipping time extends to 10 minutes, revealing that a long reaction time is

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

mandatory to ensure complete conversion from PbI2 to CH3NH3PbI3. Therefore, we speculate that the seed CH3NH3PbI3 crystals form instantaneously on the top of meso-TiO2 because the swift interfacial reaction somehow obstructs further fast conversion of the inner porous structure. The further conversion relies on the diffusion of CH3NH3+ and I- from bulk solution to unconverted PbI2, which is way slower than the initial interfacial reaction and is regarded as the rate-determining step to full conversion. Furthermore, it is apparent that two distinct peaks located closely at 2θ=14.1° and 28.5° individually for the samples with dipping time beyond 20 minutes. By the looking closely to the samples with dipping time below 20 minutes (Figure 2b, c), it can be seen that these four peaks exist in all samples. Although low crystallinity of perovskite crystal leads to a vague peak separation (Figure 2b), the asymmetric peak with a small shoulder on the left imply a combination of distinct peaks. The four peaks located at 14.00°, 14.14°, 28.18° and 28.50° are indexed as the tetragonal phase CH3NH3PbI3 of (002), (110), (004) and (220) lattice plane, respectively,34 and the peak intensity show a significant enhancement persisting along with increasing dipping time. Reminiscent of the well-faceted crystals appearing in SEM observation at the 20 minutes dipping time (Figure 1e), this matches perfectly with the peak split in XRD characterization. The result suggests that, rather than swift interfacial reaction, the newborn CH3NH3PbI3 crystals governed by dissolution-recrystallization display a high crystallinity. Moreover, the enhancement on peak intensity is only observed on (002), (110), (004) and (220) 16


Page 16 of 36

Page 17 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lattice planes, instead, almost stagnant of others, such as (202) located at 24.8°, which indicates

a

crystal

growth

on

(002),

(110)

preferential

orientation

by

dissolution-recrystallization and echoes the result proposed by L. Oesinghaus et al35.

Evolution of CH3NH3PbI3 optical properties The optical property of FTO/meso-TiO2/CH3NH3PbI3 during ripening was examined and presented in Figure 3a.

Figure 3. (a) UV-visible patterns of as-grown CH3NH3PbI3 film. (b) Normalized absorbance at 450 nm as a function of dipping time.

From Figure 3a, it can be seen that an on-set wavelength of 780 nm corresponding to 1.59 eV bandgap of CH3NH3PbI3 was clearly verified in the first 10 seconds of dipping. As dipping time extends, the absorbance increases, echoing the result in XRD (Figure 2) regarding the increasing conversion of PbI2 to CH3NH3PbI3, which promotes light harvesting. It is worth noting that a noticeable elevation of the baselines is observed for the samples beyond 1 hour dipping. The result indicates the presence of light-scattering 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

effect in these samples, which originates in the coarsened crystals that poke out from the surface. Moreover, the substantial upside shift of the baseline in long-wavelength rather than short-wavelength region conforms to the Mie scattering effect.36 From Figure 3a, although the absorptions of the samples with long dipping time are weak, their on-set wavelength position remains immobile at 780 nm, indicating the CH3NH3PbI3 does not degrade. Moreover, the loss of light harvesting in prolonged CH3NH3I dipping can therefore be attributed to the fragmental CH3NH3PbI3 cap and vacancies in meso-TiO2, which allow direct passage of incident light through FTO/meso-TiO2/CH3NH3PbI3. In order to better present the relationship between crystal coarsening and film deterioration, the absorbance (subtracting the effect of scattering on the baseline) at 450 nm (where the largest change of optical density was observed) were normalized and plotted against dipping time. As seen in Figure 3b, the result clearly indicates the absorbance increased sharply in the initial 1 minute of dipping even though a significant amount of unreacted PbI2 remains in this stage, and then decreased monotonically along with dipping time.

Evolution of photovoltaic performance

As presented above, CH3NH3PbI3 crystals with the sizes of few micrometers and high crystallinity in the tetragonal phase can be fabricated by immersing PbI2 film into CH3NH3I/IPA for a long time. A similar result reported by Fu et al. confirms this

18


Page 18 of 36

Page 19 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solution-synthesized, well-faceted CH3NH3PbI3 is a single crystal32, which is considered to possess superior optoelectronic properties such as high carrier mobility, high carrier lifetime, long diffusion length and low defect density.37-38 Therefore, the presence of these large CH3NH3PbI3 crystals performs in photovoltaic devices is a topic of interest. Figure 4 and Table 1 summarize the photovoltaic parameters of PSCs under standard AM1.5G, 1 Sun illumination. All photovoltaic parameters are averaged from 3 to 8 devices. The PSC employed with a mere 10 seconds of CH3NH3I dipping exhibited an impressive average short circuit current density (JSC) of 17.63 mA/cm2 despite of a lot of unreacted PbI2 remaining in the structure. Combined with an average high open-circuit voltage (VOC) of 1.05 V and average high fill factor (FF) of 0.73, which were beneficial to the uniform, fully-covered CH3NH3PbI3 cap as confirmed in SEM observation, average PCE of 13.52% was achieved. The highest PCE in this study was obtained in the sample made with 1 minute of CH3NH3I dipping. This device exhibited average JSC, VOC and FF among 6 samples of 18.27 mA/cm2, 1.07 V and 0.77, respectively, rendering an average PCE of 15.00%.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 36

Figure 4. Photovoltaic parameters extracted by I-V measurement as a function of CH3NH3I/IPA dipping time: (a) JSC (black), and PCE (red), (b) VOC (blue) and FF (green). The inset in (a) is the zoom-in curve of dipping time below 30 minutes.

Table 1. Photovoltaic parameters of devices obtained from backward scan of different dipping times in CH3NH3I/IPA during sequential deposition.

Dipping time

JSC (mA/cm2)

VOC (V)

10 seconds

17.63 ± 0.392

1.05 ± 0.027

0.73 ± 0.028

13.52 ± 1.091

1 minute

18.27 ± 0.178

1.07 ± 0.002

0.77 ± 0.003

15.00 ± 0.168

10 minutes

17.55 ± 0.697

1.04 ± 0.013

0.69 ± 0.012

12.59 ± 0.687

20 minutes

5.78 ± 2.680

0.89 ± 0.110

0.54 ± 0.087

2.82 ± 1.712

1 hour

6.19 ± 0.798

0.85 ± 0.045

0.51 ± 0.036

2.70 ± 0.616

3 hours

3.93 ± 0.569

0.85 ± 0.087

0.52 ± 0.036

1.77 ± 0.456

8 hours

3.65 ± 0.214

0.61 ± 0.211

0.48 ± 0.097

1.12 ± 0.511

24 hours

2.35 ± 0.511

0.52 ± 0.114

0.44 ± 0.033

0.55 ± 0.201

FF

PCE (%)

From SEM and XRD, the appearance of cuboidal CH3NH3PbI3 crystals with nearly zero PbI2 residues occurred in the sample with 10 minutes of CH3NH3I dipping; these

20


Page 21 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


two features appear to be advantageous to device performance. However, PSC made by this sample did not echo these advantages, as JSC, VOC and FF all dropped from the sample made with 1 minute of CH3NH3I dipping, resulting in an average low PCE of 12.59%. In the PSCs made with more and larger CH3NH3PbI3 cuboids (>20 minutes dipping), their photovoltaic performance became miserably poor, with less than 5% in PCE. For the extreme case with 24 hours of CH3NH3I dipping, the PCE of the PSC was only 0.55%, although it contained many perovskite single crystals.

Mechanism of CH3NH3PbI3 crystal evolution during CH3NH3I dipping Examining the poor IV parameters among the devices made with large CH3NH3PbI3 single crystals (dipping times from 20 minutes to 24 hours), it is obvious that all IV parameters, including JSC, VOC and FF, became worsened dramatically along with the ripening of CH3NH3PbI3 single crystals. This runs counter to the perception of the advantages of superior properties for single crystals. To find out the root cause of this result and most importantly, to obtain insight into the evolution of CH3NH3PbI3 crystals during CH3NH3I/IPA dipping, elemental mapping of the cross-sectional SEM images on FTO/meso-TiO2/CH3NH3PbI3 made with 1 minute, 1 hour and 24 hours of CH3NH3I dipping was conducted and presented in Figure 5. It should be noted the TiO2 scaffold in this examination was purposely made to be 2 µm-thick in order to attain better visual

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 36

presentation. In Figure 5, red, and green pixels represent Pb and I, respectively, and the white dashed line illustrates the upper boundary of the TiO2 scaffold. It can be seen that for the sample with 1 minute of CH3NH3I/IPA dipping (Figure 5a), Pb and I are mainly distributed within the meso-TiO2 scaffold, with the exception of few green and red pixels observed above the white line; those pixels above the TiO2 scaffold represent the relatively thin CH3NH3PbI3 capping layer. After 1 hour CH3NH3I/IPA dipping (Figure 5b), many rod-like and plate-like crystals are identified stretching out vertically through the meso-TiO2, which is evidenced by the outline of colored pixels. In addition, the pixel populations above and below the white line are almost equal. A careful comparison of Figure 5a and 5b on the area inside the TiO2 scaffold reveals that the density of both red and green pixels became significantly lower in Figure 5b when compared to Figure 5a, indicating a certain portion of Pb and I migrated from meso-TiO2 to the surface cap. For the extreme condition of 24 hours of dipping time (Figure 5c), it is very clear that both red and green pixels were concentrated in the area above the white dash line, and only very

few

pixels

were

left

in

dissolution-recrystallization

is

the

the sole

TiO2

scaffold,

dominant

solidly

mechanism

proving

that

determining

FTO/meso-TiO2/CH3NH3PbI3 structure in sequential deposition for fabricating PSC. Consequently, determining the dipping time in CH3NH3I/IPA is an important part examining the final morphology as well as photovoltaic performance of PSC configured with meso-TiO2 scaffold structure. Although the design parameters of the device such as 22


Page 23 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the thickness of the TiO2 scaffold, concentration of PbI2 in DMF and concentration of CH3NH3I in IPA are different from one research group to another, this study provides a universal index for the determination of optimal dipping time: that is, once the cuboidal CH3NH3PbI3 appears on the surface cap, it is most likely that the small CH3NH3PbI3 grains in the mesoporous scaffold have suffered from severe dissolution and hence the dipping time should be shortened. According to Figure 5a-c, it is also revealed that those large CH3NH3PbI3 single crystals are isolated from the TiO2 scaffold and consequently lose the connection with electron transport material, resulting in a tragic photovoltaic performance. In other words, they are crystallized in the wrong location in a photovoltaic device.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. Elemental analysis by EDS of FTO/meso-TiO2/PbI2 samples immersed into CH3NH3I/IPA for (a) 1 min, (b) 1 hour and (c) 24 hours, where Pb and I are represented as red and green pixels, respectively. The white dash line illustrates the upper boundary of the meso-TiO2 layer.

Consolidating the findings from SEM, XRD, UV-vis spectroscopy, elemental mapping and IV curves, an imaginary time trajectory of the structure for FTO/meso-TiO2/CH3NH3PbI3 dipping in CH3NH3I/IPA is cartooned in Figure 6 and elaborated as follows: Stage 1: Interfacial reaction between PbI2 and CH3NH3I: Within a very short CH3NH3I/IPA dipping time of 10 seconds, noticeable JSC (17.63 mA/cm2 in this study) can be generated because of the swift conversion from PbI2 to CH3NH3PbI3 in relatively concentrated CH3NH3I. On the other hand, JSC is also limited by 24


Page 24 of 36

Page 25 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


insufficient light-harvesting ability due to incomplete conversion of CH3NH3PbI3. As long as the quality of previously coated PbI2 film is adequately good, VOC over 1.0 V and FF over 0.7 can be achieved. Combining these three parameters, efficient (but not the best) PCE can be accomplished in merely 10 seconds CH3NH3I dipping. This result strongly suggests the fast interfacial reaction between PbI2 and CH3NH3I forms a continuous CH3NH3PbI3 skin among PbI2 grains very rapidly. Assuming the conversion of CH3NH3PbI3 is 0.5 at this stage (according to XRD examination, the ratio of peak intensity of PbI2 and CH3NH3PbI3 is approximately 1:1) and the PbI2 is spherical with a diameter of 100 nm, the thickness of CH3NH3PbI3 skin is merely 20.63 nm. Thanks to the super-high extinction coefficient of CH3NH3PbI3, significant photocurrent can be still generated in such a thin light harvesting domain. Stage 2: Increase of CH3NH3PbI3 conversion: When the CH3NH3I dipping lasts longer to 1 minute, JSC increases from 17.63 to 18.27 mA/cm2. This can be attributed to the enhancement of CH3NH3PbI3 conversion underneath the CH3NH3PbI3 skin and in the inner meso-TiO2 layer, which agrees with the observations of UV-visible spectra and XRD. Moreover, the highly pure CH3NH3PbI3 and tiny PbI2 residues render a smoother pathway for electron transport, thereby elevating FF. The VOC immobilizing over 1.0 V evidences the CH3NH3PbI3 cap over meso-TiO2 is dense, which therefore prevents the efficient shunting between p- and n-type carrier transport layers. In addition, the remnant PbI2 may also passivate the FTO/perovskite 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

interface, resulting in an additional hole-blocking effect.30 Both effects guarantee high VOC and FF. Stage 3: Dissolution of CH3NH3PbI3 in the mesoporous scaffold: In fact, as long as there exists CH3NH3PbI3 or unreacted PbI2 in CH3NH3I/IPA, the dissolution reaction occurs as explained previously (in the discussion of the evolution of CH3NH3PbI3 morphology), even during stages 1 and 2. However, owing to the fast interfacial reaction in stage 1, the dissolution reaction is macroscopically hard to observe. Once the PbI2 is totally depleted, the interfacial reaction halts and the evolution of CH3NH3PbI3 ripening enters a new period, in which the dissolution-recrystallization takes over the control of the subsequent morphology of CH3NH3PbI3 crystals. As mentioned previously, small CH3NH3PbI3 grains in meso-TiO2 tend to dissolve firstly and then re-crystallize as large CH3NH3PbI3 grains. This effect results in the formation of large, well-faceted CH3NH3PbI3 crystals. As this phenomenon proceeds, CH3NH3PbI3 in the mesoporous scaffold gradually depletes, accompanied by enlargement of a large cuboid or rod-like CH3NH3PbI3 on or above the surface cap. During this stage, significant loss on JSC and FF can be observed on the IV characteristic because of poor contact between the light absorber (the CH3NH3PbI3) and electron transfer layer (ETL, the TiO2 scaffold). On the other hand, the surface cap is still intact in this stage because the cap is composed of CH3NH3PbI3 with grain size larger than those in the mesoporous scaffold and is thus 26


Page 26 of 36

Page 27 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


not subjected to damage due to dissolution reaction. Consequently, VOC may stay high because the surface cap can still isolate HTM and ETL physically and efficiently. Stage 4: Dissolution of CH3NH3PbI3 in surface cap: When the small CH3NH3PbI3 grains in meso-TiO2 have dissolved completely but the dipping in CH3NH3I/IPA proceeds, the CH3NH3PbI3 grains in the surface cap start to dissolve because it is thermodynamically feasible. Although the grains in the cap are usually assumed to be uniform in the literature; however, the cap definitely contains different CH3NH3PbI3 grain sizes. In this stage, the smaller grains of the cap dissolve prior to the rest of the larger gains of the cap, resulting in the formation of breaches on the CH3NH3PbI3 cap. Meanwhile, the well-faceted CH3NH3PbI3 single crystal may grow up to several micrometers. The photovoltaic performance suffers from significant VOC loss because HTM can easily contact the ETL or even FTO substrate and thus shunt the device. This type of charge recombination further deteriorates JSC and FF18,

39

,

rendering an extremely low PCE of approximately 1% despite the existence of well-faceted CH3NH3PbI3 single crystals. Stage 5 Ripening of well-faceted CH3NH3PbI3 single crystals: As the dissolution-recrystallization continues, the erosion of the CH3NH3PbI3 cap intensifies and the growth of single crystal continues, leading to a collapsed cap and bulky CH3NH3PbI3 single crystals (as shown in Figure 1g-i), respectively. Unfortunately, 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

these large, well-faceted CH3NH3PbI3 single crystals fail to contribute any photovoltaic property because they are isolated from the ETL and conductive substrate. During this stage, the XRD pattern reports a CH3NH3PbI3 signal of very high intensity of but UV-visible spectroscopy shows very weak characteristics. As a result, PSC made by this FTO/meso-TiO2/CH3NH3PbI3 delivers almost no photovoltaic properties.

28


Page 28 of 36

Page 29 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Schematic representation of the proposed mechanism of CH3NH3PbI3 crystal growth along with CH3NH3I sequential deposition.

Conclusions In conclusion, we have demonstrated distinct progress in CH3NH3PbI3 morphology along with CH3NH3I dipping time in sequential deposition to fabricate PSC; the formation and ripening of CH3NH3PbI3 crystals converting from PbI2 and CH3NH3I involves an initial dominant period of interfacial reaction, followed by a dominant period of dissolution-recrystallization. The optimal condition in terms of photovoltaic property 29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

is found at the transition between the above-mentioned two periods because (1) the conversion of perovskite is completed and (2) the small perovskite grains in the meso-TiO2 structure have not suffered from dissolution. Moreover, with prolonged CH3NH3I dipping, large, well-faceted perovskite crystals dominate the appearance of the surface cap. However, these large perovskite crystals fail to contribute photovoltaic properties because of poor electron transport channels. In spite of deficient PSC performance, perovskite single crystals were displayed a good lasing material and may be suitable for other application such as, optoelectronic or sensing devices40. On the basis of this new understanding of the morphological change of perovskite crystals in sequential deposition in this work, we expect that the reproducibility and tunability of the process engineering can be significantly controlled, which is very important for the practical application of organic-inorganic lead halide perovskite semiconductors.

30


Page 30 of 36

Page 31 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

Acknowledgments This work was supported by the Ministry of Science and Technology, Taiwan (MOST103-2221-E-007-121-MY2 and 105-2628-E-007-012-MY3) and by a grant from National Tsing-Hua University, Taiwan (104N2023E1).

Supporting Information XRD patterns of PbI2 film prepared by using pure DMF and mix solvent (10% DMSO) and extra photovoltaic parameters of device performance extracted by forward and backward scan directions are provided in Supporting Information. This supporting information is available free of charge on the ACS Publications website. References

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.

Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T., Organometal Halide Perovskites

as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. 2. Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J., Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. 3. Lin, Q.; Armin, A.; Nagiri, R. C. R.; Burn, P. L.; Meredith, P., Electro-Optics of Perovskite Solar Cells. Nat. Photonics 2015, 9, 106-112. 4. Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J., Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341-344. 5. Snaith, H. J.; Abate, A.; Ball, J. M.; Eperon, G. E.; Leijtens, T.; Noel, N. K.; Stranks, S. D.; Wang, J. T.-W.; Wojciechowski, K.; Zhang, W., Anomalous Hysteresis in Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 1511-1515. 6. Kim, H.-S.; Mora-Sero, I.; Gonzalez-Pedro, V.; Fabregat-Santiago, F.; Juarez-Perez, E. J.; Park, N.-G.; Bisquert, J., Mechanism of Carrier Accumulation in Perovskite Thin-Absorber Solar Cells. Nat. Commun. 2013, 4. 7. Liu, M.; Johnston, M. B.; Snaith, H. J., Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-398. 8. Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M., Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316-319. 9. Xiao, Z.; Dong, Q.; Bi, C.; Shao, Y.; Yuan, Y.; Huang, J., Solvent Annealing of Perovskite-Induced Crystal Growth for Photovoltaic-Device Efficiency Enhancement. Adv. Mater. 2014, 26, 6503-6509. 10. Chen, Q.; Zhou, H.; Hong, Z.; Luo, S.; Duan, H.-S.; Wang, H.-H.; Liu, Y.; Li, G.; Yang, Y., Planar Heterojunction Perovskite Solar Cells Via Vapor-Assisted Solution Process. J. Am. Chem. Soc. 2013, 136, 622-625. 11. Wang, K.; Liu, C.; Du, P.; Zheng, J.; Gong, X., Bulk Heterojunction Perovskite Hybrid Solar Cells with Large Fill Factor. Energy Environ. Sci. 2015, 8, 1245-1255. 12. Shi, J.; Dong, J.; Lv, S.; Xu, Y.; Zhu, L.; Xiao, J.; Xu, X.; Wu, H.; Li, D.; Luo, Y., Hole-Conductor-Free Perovskite Organic Lead Iodide Heterojunction Thin-Film Solar Cells: High Efficiency and Junction Property. Appl. Phys. Lett. 2014, 104, 063901. 13. Qiu, W.; Merckx, T.; Jaysankar, M.; de la Huerta, C. M.; Rakocevic, L.; Zhang, W.; 32


Page 32 of 36

Page 33 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Paetzold, U.; Gehlhaar, R.; Froyen, L.; Poortmans, J., Pinhole-Free Perovskite Films for Efficient Solar Modules. Energy Environ. Sci. 2016, 9, 484-489. 14. Heo, J. H.; Han, H. J.; Kim, D.; Ahn, T. K.; Im, S. H., Hysteresis-Less Inverted CH3NH3PbI3 Planar Perovskite Hybrid Solar Cells with 18.1% Power Conversion Efficiency. Energy Environ. Sci. 2015, 8, 1602-1608. 15. Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E., Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. 16. Williams, S. T.; Zuo, F.; Chueh, C.-C.; Liao, C.-Y.; Liang, P.-W.; Jen, A. K.-Y., Role of Chloride in the Morphological Evolution of Organo-Lead Halide Perovskite Thin Films. ACS nano 2014, 8, 10640-10654. 17. Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I., Solvent Engineering for High-Performance Inorganic–Organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897-903. 18. Xiao, M.; Huang, F.; Huang, W.; Dkhissi, Y.; Zhu, Y.; Etheridge, J.; Gray-Weale, A.; Bach, U.; Cheng, Y. B.; Spiccia, L., A Fast Deposition-Crystallization Procedure for Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells. Angew. Chem. 2014, 126, 10056-10061. 19. Zhou, Z.; Wang, Z.; Zhou, Y.; Pang, S.; Wang, D.; Xu, H.; Liu, Z.; Padture, N. P.; Cui, G., Methylamine-Gas-Induced Defect-Healing Behavior of CH3NH3PbI3 Thin Films for Perovskite Solar Cells. Angew. Chem. Int. Ed. 2015, 54, 9705-9709. 20. Im, J.-H.; Jang, I.-H.; Pellet, N.; Grätzel, M.; Park, N.-G., Growth of CH3NH3PbI3 Cuboids with Controlled Size for High-Efficiency Perovskite Solar Cells. Nat. Nanotechnol. 2014, 9, 927-932. 21. Wu, C.-G.; Chiang, C.-H.; Tseng, Z.-L.; Nazeeruddin, M. K.; Hagfeldt, A.; Grätzel, M., High Efficiency Stable Inverted Perovskite Solar Cells without Current Hysteresis. Energy Environ. Sci. 2015, 8, 2725-2733. 22. Chen, W.; Wu, Y.; Liu, J.; Qin, C.; Yang, X.; Islam, A.; Cheng, Y.-B.; Han, L., Hybrid Interfacial Layer Leads to Solid Performance Improvement of Inverted Perovskite Solar Cells. Energy Environ. Sci. 2015, 8, 629-640. 23. Pellet, N.; Gao, P.; Gregori, G.; Yang, T. Y.; Nazeeruddin, M. K.; Maier, J.; Grätzel, M., Mixed-Organic-Cation Perovskite Photovoltaics for Enhanced Solar-Light Harvesting. Angew. Chem. Int. Ed. 2014, 53, 3151-3157. 24. Wu, Y.; Islam, A.; Yang, X.; Qin, C.; Liu, J.; Zhang, K.; Peng, W.; Han, L., Retarding the Crystallization of PbI2 for Highly Reproducible Planar-Structured Perovskite 33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Solar Cells Via Sequential Deposition. Energy Environ. Sci. 2014, 7, 2934-2938. 25. Chen, C. W.; Kang, H. W.; Hsiao, S. Y.; Yang, P. F.; Chiang, K. M.; Lin, H. W., Efficient and Uniform Planar-Type Perovskite Solar Cells by Simple Sequential Vacuum Deposition. Adv. Mater. 2014, 26, 6647-6652. 26. Schlipf, J.; Docampo, P.; Schaffer, C. J.; Kö rstgens, V.; Bießmann, L.; Hanusch, F.; Giesbrecht, N.; Bernstorff, S.; Bein, T.; Mü ller-Buschbaum, P., A Closer Look into Two-Step Perovskite Conversion with X-Ray Scattering. J. Phys. Chem. Lett. 2015, 6, 1265-1269. 27. Wakamiya, A.; Endo, M.; Sasamori, T.; Tokitoh, N.; Ogomi, Y.; Hayase, S.; Murata, Y., Reproducible Fabrication of Efficient Perovskite-Based Solar Cells: X-Ray Crystallographic Studies on the Formation of CH3NH3PbI3 Layers. Chem. Lett. 2014, 43, 711-713. 28. Jiang, C.; Lim, S. L.; Goh, W. P.; Wei, F. X.; Zhang, J., Improvement of CH3NH3PbI3 Formation for Efficient and Better Reproducible Mesoscopic Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 24726-24732. 29. Xiao, Z.; Bi, C.; Shao, Y.; Dong, Q.; Wang, Q.; Yuan, Y.; Wang, C.; Gao, Y.; Huang, J., Efficient, High Yield Perovskite Photovoltaic Devices Grown by Interdiffusion of Solution-Processed Precursor Stacking Layers. Energy Environ. Sci. 2014, 7, 2619-2623. 30. Cao, D. H.; Stoumpos, C. C.; Malliakas, C. D.; Katz, M. J.; Farha, O. K.; Hupp, J. T.; Kanatzidis, M. G., Remnant PbI2, an Unforeseen Necessity in High-Efficiency Hybrid Perovskite-Based Solar Cells? APL Mater. 2014, 2, 091101. 31. Zheng, E.; Wang, X.-F.; Song, J.; Yan, L.; Tian, W.; Miyasaka, T., PbI2-Based Dipping-Controlled Material Conversion for Compact Layer Free Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 18156-18162. 32. Fu, Y.; Meng, F.; Rowley, M. B.; Thompson, B. J.; Shearer, M. J.; Ma, D.; Hamers, R. J.; Wright, J. C.; Jin, S., Solution Growth of Single Crystal Methylammonium Lead Halide Perovskite Nanostructures for Optoelectronic and Photovoltaic Applications. J. Am. Chem. Soc. 2015, 137, 5810-5818. 33. Yan, K.; Long, M.; Zhang, T.; Wei, Z.; Chen, H.; Yang, S.; Xu, J., Hybrid Halide Perovskite Solar Cell Precursors: Colloidal Chemistry and Coordination Engineering Behind Device Processing for High Efficiency. J. Am. Chem. Soc. 2015, 137, 4460-4468. 34. Baikie, T.; Fang, Y.; Kadro, J. M.; Schreyer, M.; Wei, F.; Mhaisalkar, S. G.; Graetzel, M.; White, T. J., Synthesis and Crystal Chemistry of the Hybrid Perovskite (CH3 NH3) PbI3 for Solid-State Sensitised Solar Cell Applications. J. Mater. Chem. A 2013, 1, 34


Page 34 of 36

Page 35 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5628-5641. 35. Oesinghaus, L.; Schlipf, J.; Giesbrecht, N.; Song, L.; Hu, Y.; Bein, T.; Docampo, P.; Müller-Buschbaum, P., Toward Tailored Film Morphologies: The Origin of Crystal Orientation in Hybrid Perovskite Thin Films. Adv. Mater. Interfaces 2016, 3, 1600403. 36. Yang, J.; Siempelkamp, B. D.; Liu, D.; Kelly, T. L., Investigation of CH3NH3PbI3 Degradation Rates and Mechanisms in Controlled Humidity Environments Using in Situ Techniques. ACS nano 2015, 9, 1955-1963. 37. Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K., Low Trap-State Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519-522. 38. Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J., Electron-Hole Diffusion Lengths> 175 μm in Solution-Grown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967-970. 39. Li, G.; Ching, K. L.; Ho, J. Y.; Wong, M.; Kwok, H. S., Identifying the Optimum Morphology in High-Performance Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1401775. 40. Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, M. V.; Trinh, M. T.; Jin, S.; Zhu, X., Lead Halide Perovskite Nanowire Lasers with Low Lasing Thresholds and High Quality Factors. Nat. Mater. 2015, 14, 636-642.

35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents Graphic

36


Page 36 of 36

Crystal Growth and Dissolution of Methylammonium Lead Iodide

Recommend Documents