Direct Evidence of Chlorine-Induced Preferential Crystalline

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Direct Evidence of Chlorine Induced Preferential Crystalline Orientation in Methylammonium Lead Iodide Perovskites Grown on TiO

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Mathilde Bouchard, Jan Hilhorst, Stéphanie Pouget, Firoz Alam, Maria Méndez, David Djurado, Dmitry Aldakov, Tobias U. Schulli, and Peter Reiss J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11529 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 18, 2017

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The Journal of Physical Chemistry

Direct Evidence of Chlorine Induced Preferential Crystalline Orientation in Methylammonium Lead Iodide Perovskites Grown on TiO2 Mathilde Bouchard,1 Jan Hilhorst,2‡ Stéphanie Pouget,3 Firoz Alam,1 Maria Mendez,1 David Djurado,1 Dmitry Aldakov,1 Tobias Schülli,2 Peter Reiss1* 1

CEA Grenoble, INAC, UMR5819 SyMMES (CEA-CNRS-Université Grenoble Alpes), STEP, 17 rue des Martyrs, 38054 Grenoble cedex 9 2 The European Synchrotron ESRF, BP 220, 38043 Grenoble Cedex 9, France 3 CEA Grenoble, INAC/MEM/SGX ‡ Current address: Bruker AXS, Östliche Rheinbrückenstrasse 49, 76187, Karlsruhe

* Corresponding author email address: [email protected] Abstract Mixed halide hybrid perovskites CH3NH3PbI3-xClx (MAPICl) show higher charge carrier diffusion lengths, improved solar cell characteristics and enhanced stability as compared to their single halide counterparts CH3NH3PbI3 (MAPI). This is surprising in view of the fact that the actually observed Cl content is very low (x < 0.1) as the incorporation of large amounts of Cl in the MAPI lattice is crystallographically not possible. Understanding the role of chlorine in these systems is therefore of utmost importance and has been the subject of several experimental and theoretical studies. The main conclusions are that Cl addition leads to larger grain sizes and preferential crystalline orientation, however, so far a more quantitative description of these aspects is lacking. Synchrotron X-ray diffraction microscopy is a very promising technique in this respect, as it allows for the direct imaging of crystalline domains oriented in a selected direction with a resolution of some hundreds of nanometers. When imaging perovskite thin films deposited on an FTO covered glass substrate containing a dense polycrystalline TiO2 layer at the perovskite (220) reflection, diffraction micrographs show an increase in surface coverage of oriented grains from 2.5% (MAPI) to 5.5% (MAPICl; x=0.07). This is a clear evidence of the increased preferential orientation in the mixed perovskite. At the same time the integrated intensity increases by a factor of 4, which can be related to a larger perovskite grain size. When a single crystalline TiO2 (001) substrate is used for the perovskite film deposition, both effects are dramatically enhanced in the case of MAPICl, leading to a surface coverage of 80%, a 55-fold increase in integrated diffracted intensity with respect to MAPI and a mean crystallite size of 2.5 µm. To the contrast, MAPI does not show any differences when grown on TiO2 (001), which demonstrates that chlorine locates preferentially at the TiO2-

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perovskite interface and induces the growth of larger crystallites showing preferential orientation along the (110) direction. Introduction The addition of chlorine, in form of PbCl2 or MACl (MA=methylammonium), to MAPbI3 (MAPI) has been extensively used to make high-quality perovskite films for solar cells. It is known that hybrid perovskites obtained using Cl-containing precursors result in improved photovoltaic performances in terms of efficiency and stability.1-5 For instance, Stranks and coworkers demonstrated that the electron-hole diffusion lengths in the mixed halide perovskite are around 1 micrometer, whereas the triiodide perovskite yields ten times smaller values.6 Different electrical and photophysical properties of the mixed halide perovskite CH3NH3PbI3-xClx (MAPICl) have been reported, but the localization and role of chlorine are still subject of debate and the results from groups employing different characterization techniques are hard to reconcile. One central point is whether chlorine gets incorporated into the perovskite crystalline lattice and stays there after thermal annealing. The difficulty of experimentally detecting and localizing Cl in the mixed halide perovskite is mainly due to the very low amount actually incorporated: in essentially all examples only small quantities ranging from less than 300 ppm to some per cent were observed confirming that the final Cl content in films of MAPICl is very low.7-15 This weak incorporation is consistent with theoretical studies, which predict that the MAPI lattice can tolerate only up to 3-4% of Cl because of the strong difference in ionic radii of iodine and chlorine, which does not allow for forming a stable mixed crystal.16 The much weaker content of Cl in the final material compared to the initial precursor ratio has been explained by the loss of gaseous MACl during thermal treatment of the mixed halide perovskite layer.2, 7, 10, 17 When using a mild annealing process (60°C, 60 min), Chae et al. have shown by means of photothermal induced resonance mapping that Cl ions are heterogeneously distributed in the MAPICl film with a relatively high Cl concentration (10%, x=0.3).10 This heterogeneous distribution led to local band gap heterogeneities in the film. Upon further annealing to 110°C, these heterogeneities disappeared and the band gap approached the typical value of MAPI, while the Cl content was progressively reduced (x < 0.06) by expelling MACl. A blue shift of the external quantum efficiency (EQE) spectrum compared to that of stoichiometric MAPI suggests that a small amount of Cl must have been retained in the perovskite lattice even after annealing the device at 110°C for more than 2 hours. Similarly, a recent study of the chlorine content evolution as a function of annealing time has reported a very high x value of 1.52 after the first 20 minutes of annealing, which however drops rapidly to 0.05 in the final material after 2 hours of heating.18 The


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fact that chlorine is essentially not incorporated into the crystalline lattice but nevertheless detected in the film points at its presence either on the grain boundaries of the perovskite crystallites7, 9 or at the interfaces of the perovskite film with the adjacent contact layers. DFT calculations predict the preferential location of Cl at the TiO2 interface due to an increased chloride-TiO2 surface affinity as compared to the bulk perovskite.16 By using hard X-ray photoelectron spectroscopy (HAXPES) at different photon energies Starr et al. have demonstrated that the Cl contents increases from x = 0.07 up to 0.4 while going from the mixed halide perovskite film surface down to the interface with TiO2.11 Angle-resolved XPS allowing to change the sampling depth of the perovskite films by varying take-off angles also showed that the chlorine signal is more intense deeper in the film closer to the TiO2 interface.16, 19 All these recent studies unambiguously agree on the fact that most of the chlorine present in mixed halide perovskite films is concentrated at the interface with the TiO2 contact. As mentioned before, the use of chlorine both as PbCl2 in the case of the one-step deposition or as MACl in the case of sequential deposition for the fabrication of mixed halide perovskite layers is highly beneficial for the applications in solar cells. Generally, changes in the electronic and morphological properties are put forward to explain the superior performance of chloride containing perovskites. In the first case, it is considered that chloride changes the electronic properties of the film by “doping” the MAPI or reducing the defect density. Theoretical studies predicted that perovskites grown under I-rich conditions show a high density of trap states due to the low formation energy of PbI antisite deep traps.20 The use of PbCl2 instead of PbI2 could reduce the excess iodine content in the films and thus the trap state density as confirmed by longer diffusion lengths measured. Other simulations indicated that Cl introduction reduces the lattice constant and significantly increases the formation energy of interstitial defects.21 De Quilettes et al. demonstrated that Cl-rich domains have longer fluorescence lifetimes compared to I-rich ones, which should lead to lower recombination rates.22 A lowering of the recombination rate by 1-2 orders of magnitude was earlier demonstrated by Zhao et al. in the case of MAPICl as compared to MAPI.23 Cl incorporation has also been shown to yield a 30 times increased photocurrent due to defect reduction as studied by conductive atomic force microscopy13 and to locally modify the band gap (vide supra).10 Theoretical studies by Even et al. indicate that Cl doping restricts the collective orientational motion of the organic CH3NH3+ cations yielding better transport properties and increasing the electron−hole diffusion lengths in MAPICl.24 In agreement with the hypothesis of preferential location at the interface with TiO2, Chen et al. showed that chloride may improve the carrier transport across the heterojunction interfaces.3 Mosconi et al. have calculated that the


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stronger coupling between the perovskite and TiO2 in the case of a Cl-containing interface results in improved electron injection.25 The second, “morphological” explanation of the beneficial influence of Cl incorporation is based on the observed higher quality of the obtained mixed perovskite films. One of the origins of this phenomenon is the formation of larger crystalline domains leading to fewer grain boundaries and better perovskite surface coverage.2, 17, 23, 26-28 For example, in the case of the one-step perovskite formation, PbCl2 nanocrystals present in the colloidal precursor solution can act as nucleation centers guiding the crystallization of MAPI or MAPICl.29-30 The influence of chlorine containing precursors on the perovskite nucleation dynamics has been demonstrated not only on TiO216 but also on PEDOT-PSS layers.17 Another aspect of morphological improvement upon Cl incorporation is a preferential orientation of the crystallites in the mixed halide peroskite films. It was demonstrated by GIWAXS and NEXAFS that pure MAPI deposited on TiO2 with thicknesses above 60 nm does not have any preferential orientation.31 In the case of the mixed halide, however, the perovskite preferentially orients along the (110) direction resulting in improved charge transport.19, 25, 28-29, 32 A detailed theoretical study using first principle simulations shows that chlorine atoms situated at the interface with the TiO2 anode can increase the binding energy of the perovskite (110) surface to TiO2 in MAPICl as compared to MAPI.25 In this work we use for the first time synchrotron X-ray diffraction microscopy to investigate in details the crystallites microstructure in the perovskite phase. This novel technique33 allows for the direct comparison of the crystallite size and orientation in MAPI and MAPICl perovskite films. Measurements were performed after deposition of the two types of perovskites on a dense layer of TiO2 with all processing conditions being identical to those used for the preparation of working solar cells. Furthermore, MAPI and MAPICl layers were also grown on oriented, single-crystalline TiO2 (001) substrates, which led to a much higher degree of orientation in the case of MAPICl.


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Results and Discussion MAPI and MAPICl layers have been deposited by spin-coating on glass substrates coated with fluorine doped tin oxide (FTO) on which we deposited a 60-80 nm thick compact layer of TiO2. We emphasize that this planar configuration is also relevant for understanding the behavior of perovskite solar cells based on a mesoporous titania scaffold. As has been shown recently, the socalled overlayer or capping layer of MAPI or MAPICl above the mesoporous layer containing infiltrated perovskite governs the optoelectronic properties there: only with a significant thickness of the capping layer on top of the mesoporous oxide sufficiently large crystalline grains are formed leading to high diffusion lengths and improved device performances.34 The mean chlorine content determined by EDX spectroscopy on the final MAPICl layers was x=0.06-0.07, while x=0.66 had been used during the preparation. In a first step, the different samples studied by synchrotron X-ray diffraction microscopy were characterized by scanning electron microscopy (SEM) and laboratory X-Ray thin film diffraction. Cross section SEM images (Fig. 1) show the uniform thickness of around 400 nm of the perovskite layers, while top view images reveal the significantly increased grain size when using MAPICl instead of MAPI.

Figure 1: SEM images of MAPI (a,b) and MAPICl (c,d) deposited on a dense layer of TiO2, which was spin-cast on an FTO-coated glass substrate.

Powder X-ray diffraction was carried out on a Panalytical X’Pert powder diffractometer equipped with a copper anode (λKα1= 1.5406 Å, λK α2= 1.5444 Å) and an X’Celerator 1D detector. It was


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configured in Bragg-Brentano geometry, with a variable divergence slit on the primary beam path and a set of anti-scattering slits positioned before and after the sample. Axial divergence was limited by 0.02 rad Soller slits. The experiments on thin film samples were performed with a Panalytical Empyrean diffractometer with a Cobalt anode beam tube (Co λKα1=1,789 Å), a Göbel mirror and a two-dimensional pixel detector used in both zero-dimensional and one-dimensional modes. Figure 2 displays the θ/2θ diffractograms recorded on MAPI powder and on two samples constituted of MAPI and MAPICl layers deposited on FTO/TiO2 substrates. The Le Bail profile fit of the data obtained with the powder confirmed the tetragonal I4/mcm structure, reported to be stable in the temperature range 162–327 K.35 A Rietveld refinement was also performed with a satisfactory agreement; nevertheless, some discrepancies in the relative intensities were observed, likely due to the presence of disorder in the structure. The values of the refined lattice parameters are a=8.871(5) Å and c=12.656(5) Å. The data recorded with the MAPI thin film on FTO/TiO2 were analysed similarly leading to values of the lattice parameters of a=8.885(5) Å and c=12.67(1) Å. Unlike what was observed in the case of the powder, the Rietveld refinement revealed the presence of [110] preferential orientation of the perovskite crystallites. This feature is strongly enhanced in the case of the MAPICl layer, which evidences the same tetragonal structure with a=8.868(5) Å and c=12.62(3) Å. The insets in Figure 2 display the (004) and (220) Bragg peaks of the tetragonal perovskite structure for the three samples, clearly illustrating the presence of preferential orientation for the perovskite thin films.


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Figure 2: XRD patterns of MAPI powder and MAPI and MAPICl layers deposited on FTO/TiO2. (a) Le Bail profile fit of the powder diffraction pattern confirming a single phase sample with tetragonal symmetry at room temperature. (b) and (c) The diffraction peaks associated to the substrate are denoted by #. In insets, zoom of the (004) and (220) perovskite Bragg peaks evidencing the presence of [110] preferential orientation in the samples.

To push forward the investigation of the orientation of the perovskite crystallites a relatively new and unique technique combining X-ray diffraction with microscopy was used at beamline ID-01 of the European Synchrotron (ESRF). In short, instead of capturing a diffracted beam at a detector to measure the intensity of a Bragg reflection, this technique utilizes imaging lenses in the diffracted beam to identify the origin of the diffracted intensity.33 For example, when the lenses are aligned into the MAPI (110) peak, an image of the sample is obtained where only the crystallites diffracting in the [110] direction are visible. In single crystal samples, this allows for local characterization of crystal quality. In this work, it is used for the first time to study polycrystalline thin film samples to investigate orientational preferences and crystallite grain size with a resolution down to ~500 nm. Figure 3a displays a typical image of a MAPI sample at the (220) peak measured at 11.34° 2θ at an X-Ray energy of 20 keV. To enhance visibility, the image background has been subtracted and the noise has been reduced using Gaussian blurring.


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Figure 3: Logarithmically scaled, false color diffraction micrographs of MAPI (a) and MAPICl (b) on FTO/TiO2 at the (220) reflection. In the image, a few bright dots can be seen surrounded by many tiny specks with intensities just above background level. This entire signal is a spatial representation of the perovskite grains contributing to the diffraction signal. The few high intensity spots are well aligned, strongly diffracting grains. The low-intensity signal surrounding them is attributed to small, sub-resolution grains that are present throughout the whole perovskite layer, barely giving enough intensity to be recognized. In Figure 3b, a micrograph is shown of a MAPICl sample, with the same magnification and exposure time, at the same (220) peak. This image is distinctly different from Figure 3a in several ways. First of all, the intensity scaling is a factor of ~18 higher, going up to 1000 counts per second for the brightest grain, against 56 counts per second for the MAPI sample. Second, many more distinct dots can be distinguished, indicating a general growth of crystallites, resulting in more intensity per grain. Another effect contributing to the number of visible high intensity grains is the preferential orientation of the crystallites with their (110) planes parallel to the substrate, resulting in an increase of the diffracted intensity into the [220] direction under θ-2θ conditions. The third difference between MAPI and MAPICl is that the low intensity background has been reduced, which is another indication that tiny, sub-resolution grains have disappeared and grown into larger ones. The fact that fewer sub-resolution grains can be identified and that the intensity of each of the visible grains is significantly higher than even the brightest grains found in the MAPI sample, is strong evidence that the addition of chlorine indeed aids in grain growth.


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What is harder to ascertain than the chlorine induced grain growth is the increased orientation of the crystallites, as the increase in the number of highly diffracting crystallites can in principle be fully explained by increased grain size. To identify any orientation in the samples, the surface fraction of diffracting grains was determined in an area in the center of several images of each sample. The results are presented in Table 1. The MAPICl sample has over twice higher surface coverage than MAPI and an integrated intensity roughly 4 times higher. Therefore, although the increased intensity can be partly attributed to grain growth, the higher surface coverage of (220) diffracting grains is fully attributable to increased alignment, in agreement with the lab XRD observations. On the other hand, even though MAPICl has a much higher degree of alignment than MAPI, it is clear that this still plays a relatively small role: a major part of the micrograph remains dark indicating the areas where no grains are in the Bragg condition and therefore are not aligned with the substrate.

Table 1: Sample characteristics extracted from X-ray diffraction micrographs. Maximum intensity per

Integrated

image (counts per second

(counts per second

in the brightest pixel)

per image)

MAPI on FTO/TiO2

56

1.8·105

2.4%

Direct Evidence of Chlorine-Induced Preferential Crystalline

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