Subscriber access provided by Universitaetsbibliothek | Johann Christian Senckenberg
Article
The Exploration of Crystallization Kinetics in Quasi Twodimensional Perovskite and High Performance Solar Cells Ning Zhou, Yiheng Shen, Liang Li, Shunquan Tan, Na Liu, Guanhaojie Zheng, Qi Chen, and Huanping Zhou J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11157 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 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.
Journal of the American Chemical Society 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 8 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
Journal of the American Chemical Society
The Exploration of Crystallization Kinetics in Quasi Two-dimensional Perovskite and High Performance Solar Cells Ning Zhoua, Yiheng Shena, Liang Lia, Shunquan Tana, Na Liub, Guanhaojie Zhenga, Qi Chenb, Huanping Zhoua* a.
Beijing Key Laboratory for Theory and Technology of Advanced Battery Materials, Department of Materials Science and Engineering, College of Engineering, Peking University No.5 Yiheyuan Road Haidian District, Beijing 100871, P. R. China b. School of Materials Science and Engineering, Beijing Institute of Technology 5 Zhongguancun South Street, Beijing 100081, P. R. China
ABSTRACT: Halide perovskites with reduced-dimensionality (e.g. quasi-2D, Q-2D) are promising to be stable while remaining the high performance as compared to their three-dimensional counterpart. Generally, they are obtained in (A1)2(A2)n−1PbnI3n+1 thin films by adjusting A site cations, however, the underlying crystallization kinetics mechanism is less explored. In this manuscript, we employed ternary cations halides perovskite (BA)2(MA,FA)3Pb4I13 Q-2D perovskites as an archetypal model, to understand the principles that links the crystal orientation to the carrier behavior in the polycrystalline film. We reveal that appropriate FA+ incorporation can effectively control the perovskite crystallization kinetics, which reduces non-radiative recombination centers to acquire high-quality film with limited non-orientated phase. We further developed an in situ photoluminescence technique to observe that the Q-2D phase (n = 2, 3, 4) was formed first followed by the generation of n=∞ perovskite in Q-2D perovskites. These findings substantially benefit the understanding of doping behavior in Q-2D perovskites crystal growth, and ultimately leads to the highest efficiency of 12.81% in (BA)2(MA,FA)3Pb4I13 Q-2D perovskites based photovoltaic devices.
INTRODUCTION
Organic−inorganic halide perovskite materials have attracted tremendous interest in recent years due to their excellent optoelectronic properties, such as high absorption coefficient1, tunable band gap2, small exciton binding energy3, excellent ambipolar charge mobility4, and long charge-carrier diffusion lengths5. These advantages combined with the extremely low fabrication cost make this kind material suitable as light absorber for photovoltaic applications. Also, remarkable progress in power conversion efficiency (PCE) from 3.81%6 to 22.1%7 has been achieved in perovskite solar cells during the past seven years. However, long-term stability of the materials and devices remain to be the biggest challenge for the realistic implementation of perovskite solar cell. The instability of devices mainly comes from the intrinsic materials property8-13, where perovskites are susceptible to degradation by exposing to moisture14, UV light15,16, temperature17 and electrical field18, etc. Therefore, continuous development on the perovskite materials is urgently needed to realize long-term stable and high efficiency perovskite solar cells. Recently, in contrast to their three-dimensional counterparts, ruddlesden–popper phases layered quasi two-dimensional (Q-2D) perovskite based films and devices have shown the potential to exhibit improved stability while remaining the high performance.19-24 Initial work has demonstrated that a layered (PEA)2(MA)2Pb3I10 (where PEA=phenylethylammonium, MA= CH3NH3+) perovskite was used as light absorber and was relatively stable in air containing 52% relative humidity for up to 46 days. However, the corresponding solar cell exhibited a relatively low PCE of 4.73%,21 probably due to the inhibition of out-of-plane
charge transport by the insulating spacer cations. Similarly, a different kind of organic cation n-C4H9NH3+ (BA+) was introduced into MAPbI3 to form a Q-2D (BA)2(MA)n-1PbnI3n+1 perovskite, in which the perovskites with n=1–4 was detailed characterized and a PCE of 4.02% was obtained from the n=3 compound.19 The quality of Q-2D (BA)2(MA)3Pb4I13 perovskites has been substantially improved by using a more sophisticated deposition method of “hot-casting”, with crystalline planes aligned along the out-of-plane orientation, which facilitates e-cient charge transport along the perovskite planes and enables an efficiency of 12.52% with high reproducibility and excellent operating stability.20 Moreover, the Q-2D structure was employed in lead free materials system, and a highly oriented tin halide perovskites (PEA)2(FA)n−1SnnI3n+1 based device yielded a 5.94% PCE with no appreciable decay in efficiency over 100 h.25 In contrast to the exploration on new materials, the (BA)2(MA)3Pb4I13 Q-2D are found to exhibit unique multiple perovskite phases with n = 2, 3, 4 and ≈ ∞ naturally aligning in the order of n along the direction perpendicular to the substrate, which leads to spontaneous charge (electron/hole) separation property driven by band alignment. These findings suggests that the relatively poor transportation or undesirable energy loss in the Q-2D layered hybrid perovskite materials could be overcomed by tuning the crystal orientation and the internal phases. Though the Q-2D perovskite can be obtained by cation engineering on the A site in (A1)2(A2)n−1PbnI3n+1, e.g. FA+, MA+, Cs+, or Rb+ inspired by the composition engineering in 3D APbX326-36, the underlying crystallization kinetics mechanism in Q-2D is largely unexplored. It thus requires enormous endeavors to establish a corresponding fundamental understanding of the 1
ACS Paragon Plus Environment
Journal of the American Chemical Society 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
crystallization kinetics and crystal microstructure, which eventually contributes to improved carrier behaviors of Q-2D perovskite materials and remarkably efficient devices. Here, we design a ternary organic cation based Q-2D perovskites of (BA)2(MA,FA)3Pb4I13 as an archetype, to understand the principles for maintaining the crystal orientation with improved carrier behavior, which is essential to device optimization. The (BA)2(MA1-xFAx)3Pb4I13 (x=0, 0.2, 0.4, 0.6) Q-2D perovskites were explored in a systematic way by gradually replacing MA by FA, and the phase, morphology, crystallographic behavior, as well as the optical properties were carefully analyzed. An in situ PL technique was employed to investigate (BA)2(MA,FA)3Pb4I13 perovskite materials system for the first time, to probe the formation sequence of different phases. We also revealed a substantially different crystallographic orientations and crystallization kinetics in (BA)2(MA,FA)3Pb4I13 Q-2D perovskite induced by cation substitution, compared to conventional 3D structure. By introducing 20% FA+, the doped BA2(MA)3Pb4I13 film exhibit improved carrier lifetime and remain high crystal orientation, resulting the best efficiency of 12.81%. The understanding on the crystallization kinetics and the doping effect could significantly benefit the rational design of Q-2D perovskite materials and the solar cell performance.
RESULTS AND DISCUSSIONS
The Q-2D (BA)2(MA)3Pb4I13 perovskite powder were synthesized from a stoichiometric reaction between PbO, MAI, and BAI in appropriate ratios in a HI/H3PO2 solvent mixture as described previously.19 To obtain (BA)2(MA1-xFAx)3Pb4I13 (x=0, 0.2, 0.4, 0.6) family of perovskite compounds, we replaced MAI with FAI and adjusted the reaction temperature. We dissolved the perovskite crystal in DMF solvent and adopting spin-coating method to form films, and the detailed information can be found in supporting information. To simplify the chemical formula, we used FA0, FA0.2, FA0.4, FA0.6 to represent (BA)2(MA1-xFAx)3Pb4I13 x=0, 0.2, 0.4, 0.6, respectively. In order to confirm the actual x value in the final perovskite film, 1H nuclear magnetic resonance (NMR) study of the (BA)2(MA1-xFAx)3Pb4I13 samples scraped from the film with different FA ratio dissolved in the dimethyl sulfoxide-d6 (DMSO-d6) solvent was undertaken. (Figure S1) We found that the actual x value in the final perovskite film is quite close to the ratio from the starting precursor material. We investigated the crystal phase and crystallinity of the (BA)2(MA,FA)3Pb4I13 series perovskite films using X-ray diffraction (XRD) measurement, as shown in Figure 1a. Two dominant planes were observed at diffraction angles (2θ) of 14.1ο, 28.4ο, representing the (111), (222) crystallographic planes of (BA)2(MA,FA)3Pb4I13 series perovskite, respectively. The (111) peak is shifted from 14.1ο, to 13.9ο and 13.8ο for FA0, FA0.2 and FA0.4 respectively, which is due to the bigger size of FA cation
Page 2 of 8
with respect to MA cation that expand the crystal lattice.37,38 This gradual shift in diffraction peak position indicated that mixed (BA)2(MA,FA)3Pb4I13 perovskites are formed with FA cations inserted in the crystal lattice. However, when the amount of FA cation was further increased, a new set of periodic (0k0) peaks were found in FA0.6 film. The crystallinity of the FA0.6 film was substantially decreased, where the presented peaks was obtained after normalizing the peak intensity. It was assigned to the unorientated BA2(MA,FA)Pb2I7 Q-2D (n=2) perovskite in FA0.6 film19, which was also evidenced by the XRD peak shift between BA2(MA)Pb2I7 and BA2FAPb2I7 (Figure S2). This limited replacement of MA with FA during 2D (BA)2(MA,FA)3Pb4I13 perovskites growth is quite different from the conventional formation of 3D (MA1-xFAx)PbI3 perovskite film over the full compositional space. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements are then used to study the quality of (BA)2(MA,FA)3Pb4I13 series perovskite films. Different MA:FA ratio also presents significant differences in film uniformity and coverage, as shown in the SEM images (Figure 1b-e). Accordingly, FA0 and FA0.2 film had adequate surface coverage and densely packed grains without visible cracks. The introduction of 20% FA enlarges the crystal grains from several hundred nanometers to several micrometers significantly. When substituting MA with 40% FA in (BA)2(MA,FA)3Pb4I13, the film has larger crystal grains but is less dense with considerably increased amount of pinholes and cracks. Further substituting MA with 60% FA, lots of microparticals appeared on the film surface, which was attributed to BA2(MA,FA)Pb2I7 according to XRD results. Figure S3 provides the AFM images of the (BA)2(MA1-xFAx)3Pb4I13 (x=0, 0.2, 0.4, 0.6) perovskite film. The root mean square (RMS) roughness of the FA0 and FA0.2 films across a scanned area of 5×5 µm2 were determined to be 7.1 and 3.0 nm respectively, which suggests the superior film quality of FA0.2. The RMS roughness of FA0.4 and FA0.6 film were 5.8 and 18.1 nm respectively, with randomly distributed domains and co-existed pinholes. We consider this phenomenon as a result of the growth kinetic difference between Q-2D and 3D perovskite. Generally, 3D perovskite are likely to follow the uncontrolled crystal growth in 3D space that lead to rough surface, while Q-2D perovskite films are smoother as a result of the space confined effect.25 It also confirmed that the introduction of FA with a ratio less than 40% would not change the Q-2D orientation structure. However, if the FA ratio is higher than 40%, the MA in Q-2D structure can’t be stoichiometrically replaced by FA. This indicates a different but interesting effect in crystal structure evolution induced by dimensional variation due to the crystal growth kinetic difference between MA and FA based Q-2D perovskite films, and the underlying crystal growth kinetics mechanism will be discussed as follows.
2
ACS Paragon Plus Environment
Page 3 of 8 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
Journal of the American Chemical Society
Figure 1. (a) XRD patterns and (b)-(e) SEM images of the (BA)2(MA1-xFAx)3Pb4I13 x=0, 0.2, 0.4, 0.6 films, respectively.
Figure 2. GIWAXS patterns of the perovskite films of (BA)2(MA1-xFAx)3Pb4I13 x=0, 0.2, 0.4, 0.6 films, respectively. To confirm the orientation of the (BA)2(MA,FA)3Pb4I13 perovskite films, we performed grazing incidence wide angle X-ray scattering (GIWAXS) analysis using synchrotron radiation.39 The scattering patterns of different FA ratio in the (BA)2(MA,FA)3Pb4I13 films are shown in Figure 2a-d. The FA0 sample shows sharp and discrete Bragg spots, which indicates the highly oriented crystal grain with (111) planes parallel to the substrate surface.20 The orientated perovskite films that are vertically grown on the substrate would form an efficient carrier transport pathway, probably leading to improved photovoltaic behavior.25 Films with 20% FA ratios exhibit the same set of diffraction spots, but with even stronger intensity in the qz direction, as compared to FA0. The integrated intensity plots of the GIWAXS patterns at 14.1° diffraction angle
(Figure S4) further confirmed the retained orientation of the crystal domains, albeit of considerable change in relative intensity of the crystallographic plane. However, when FA ratio was higher (40% or more), diffraction rings instead of diffraction spots were observed, suggesting the highly orientated 2D layered structure transformed to less orientated structure. The less apparent diffraction rings in the FA0.6 film compared to FA0.4 film is due to its decreased crystallinity. The presented variation in film quality induced by FA incorporation is consistent with the XRD, SEM and AFM results. Further, we carried out absorption and photoluminescence (PL) spectroscopy measurements to detect the absorption and emission 3
ACS Paragon Plus Environment
Journal of the American Chemical Society 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
property of the perovskite film. All the films were spin-coated on the glass substrate. We found that multiple perovskite phases with various n values coexisted in the 2D perovskite films (although nominally prepared as “n=4”), as evident by multiple absorption and emission peaks in Figure S5 and Figure 3a (front-excitation), respectively. When the laser beam hit the perovskite from the glass substrate side (back- excitation), multiple emission peaks of small n value were more obvious (Figure S6). The differences in the PL spectra between front- (Figure 3a) and back-side excitations (Figure S6) suggested that the small-n phases may majorly locate near substrate side, and the large-n phase locates at the upper surface of the film, which is consistent with recent reported work.40 Besides, the strong emission peak gradually shifted from 736 nm for FA0, to 773, 790, and 784 nm for FA0.2, FA0.4 and FA0.6, respectively. This emission peak was defined as n=∞ which is colse to 3D perovskite. The red-shift observed in FA0, FA0.2 and FA0.4 is associated to the reduced bandgap (Eg) of the (BA)2(MA,FA)3Pb4I13 perovskite, due to the larger ionic radius of FA+. However, a 6 nm blue-shift in emission peak was unexpectedly observed by increasing FA content from 0.4 to 0.6. This indicates that the FA ratio in the highly orientated (BA)2(MA,FA)3Pb4I13 films has a limit and the structure of FA0.6 becomes different from the FA0.4 structure which is in accordance to the XRD measurement. We measured the photoluminescence lifetime of the perovskite films deposited on nonconductive glass and the time decay of the fluorescence signals were fitted to two exponentials in Figure 3b. The carrier lifetimes for the two components were summarized in Table 1 and we used the longer carrier lifetime for comparison. Strikingly, the emission of FA0.2 perovskite film decayed with a long lifetime of 30.34 ns, which is approximately 10 times as that of FA0 based film with 3.18 ns. Further increasing the FA amount would lead to even longer carrier lifetime of over 100 ns. The prolongation of the lifetime in the mixed-cation perovskite most likely contributes to a better carrier-transport behavior in the device. While to be noted, the film with longest carrier lifetime may not guarantee the best efficiency in the resulting device, by considering the significantly changed structure and poor film morphology in the (BA)2(MA,FA)3Pb4I13 with high FA amount, which will be discussed in the following paragraph. On the basis of the above phenomenon, it suggests an effective and unique doping effect in Q-2D structure that is different from 3D structure, due to the crystallization kinetic difference. In conventional ABX3 3D perovskite structure, the A, B, X ions all obey the goldschmidt tolerance factor, and the radii of A cation is preferred to be ~ 0.16 – 0.22 nm to couple with PbI6 octohedral. In this circumstance, MA and FA can replace each other in any given ratio, whereas the phase and optoelectronic properties can be tuned systematically. While in the present study, we observed that the replacement of MA with FA in the Q-2D structure only occurring in a limited composition window to remain the orientation structure (x
