Chemical Distribution of Multiple Cation (Rb+, Cs+, MA+, and FA+)

Article pubs.acs.org/cm

Chemical Distribution of Multiple Cation (Rb+, Cs+, MA+, and FA+) Perovskite Materials by Photoelectron Spectroscopy Bertrand Philippe,*,†,○ Michael Saliba,*,‡,○ Juan-Pablo Correa-Baena,§,∥ Ute B. Cappel,† Silver-Hamill Turren-Cruz,§,⊥ Michael Graẗ zel,‡ Anders Hagfeldt,§,# and Håkan Rensmo† †

Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1015-Lausanne, Switzerland § Laboratory for Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1015-Lausanne, Switzerland ∥ Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ⊥ Benemérita Universidad Autónoma de Puebla, CIDS, Av. San Claudio y 18 Sur, Col. San Manuel, Ciudad Universitaria, CP 72570, P.O. Box 1067, 7200, Puebla, México # Department Chemistry - Ångström, Uppsala University, Box 523, SE-751 20 Uppsala, Sweden ‡

S Supporting Information *

ABSTRACT: Lead-based mixed perovskite materials have emerged in the last couple of years as promising photovoltaic materials. Recently, it was shown that improved material stability can be achieved by incorporating small amounts of inorganic cations (Cs+ and Rb+), partially replacing the more common organic cations (e.g., methylammonium, MA, and formamidinium, FA). Especially, a mixed cation composition containing Rb+, Cs+, MA+, and FA+ was recently shown to have beneficial optoelectronic properties and was stable at elevated temperature. This work focuses on the composition of this material using synchrotron-based photoelectron spectroscopy. Different probing depths were considered by changing the photon energy of the X-ray source providing insights on the chemical composition and the chemical distribution near the surface of the samples. Perovskite materials containing two, three, or four monovalent cations were analyzed and compared. The presence of Cs and Rb was observed both at the sample surface and toward the bulk, and we found that in the presence of three or four cations, less unreacted PbI2 remains in the sample. Interestingly, Rb and Cs appear to act jointly resulting in a different cation depth profile compared to that of the triple counterparts. Our findings provide significant understanding of the intricate depth-dependent chemical composition in perovskite materials using the common practice of cation mixing.

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INTRODUCTION Perovskite solar cells (PSCs) have shown exciting development in the past few years bringing the technology from a sensitized solar cell with a liquid electrolyte and a power conversion efficiency of 3.8% in 20091 to a solid state solar cell with a certified efficiency of 22.1% in 2016.2 The highest efficiencies reported so far are with lead based materials with the general formula AMX3 where A is a mixture of cations composed of methylammonium (CH3NH3+, MA) and formamidinium (CH3(NH2)2+, FA); M is the Pb2+ ion, and X is a mixture of halides (I- and Br-). The main improvements over the last several years were achieved through both compositional and process engineering including substrate preparation, precursors and solvents used, deposition techniques, annealing procedure, and chemistry of the hole conductors with or without doping.3−6 Focusing on the perovskite materials themselves, the partial substitution of MA by FA significantly improved solar cell performances and probably more importantly enhanced the © 2017 American Chemical Society

stability of the perovskite materials. Nevertheless, even for these double cation compounds, impurities remained the cause of long-term instability. Such impurities could be significantly suppressed by the incorporation of cesium as a third cation into the perovskite material resulting in triple cation perovskite materials stable under long-term aging conditions such as continuous light soaking for hundreds of hours.7 Recently, we extended this work by incorporating of a fourth cation into the perovskite structure: Rb+. This resulted in a new family of perovskite materials of the form RbFA, RbMAFA, and RbCsMAFA.8 One composition, in which about 5% of Cs and 5% of Rb were included, exhibited a high open circuit voltage (Voc) of 1.24 V (at a band gap of 1.63 eV) for the best solar cells, high stabilized efficiencies of up to 21.6%, and unprecedented stability properties at 85 °C under continuous Received: January 11, 2017 Revised: March 30, 2017 Published: March 30, 2017 3589

DOI: 10.1021/acs.chemmater.7b00126 Chem. Mater. 2017, 29, 3589−3596

Article

Chemistry of Materials

Figure 1. Top-view SEM images of MAFA, RbMAFA, CsMAFA, and the quadruple cation perovskite RbCsMAFA (horizontal scale bars = 500 nm).

made by addition of either 5% of cesium (Cs) or 5% of rubidium (Rb) to the MAFA reference. These compositions are called “CsMAFA” and “RbMAFA”, respectively. Finally, the quadruple cation perovskite material containing both Cs and Rb is referred to as RbCsMAFA. The exact precursor solution compositions are given in the Experimental Section. The four materials investigated covered the substrates uniformly as shown by the top-view scanning electron microscopy (SEM) images of the perovskite materials (Figure 1), where no domains of uncovered TiO2 are observed. Specifically, we used HAXPES to investigate the RbCsMAFA material first by itself and then in comparison to the perovskite materials containing two (MAFA) and three cations (CsMAFA and RbMAFA) to determine the role of the added Cs and Rb cations. The samples were studied with two photon energies to reach two analysis depths, 2100 eV highlighting the surface of the sample and 4000 eV allowing us to probe deeper, and is thus referred to as bulk analysis throughout the article as the data obtained at such energies were in line with XRD data. Finally, the results are compared and used to explain solar cell results obtained with the different materials.

illumination and load for 500 h (exceeding even industrial aging norms). Comparable results have since been reported in the literature where significant improvement of similar mixed perovskites containing the double cation MAFA were obtained after inclusion of 5% of Rb leading also to a better crystallinity, the suppression of the δ-yellow phase, and better thermal and photostability.9,10 While these compounds have been characterized in solar cells, there is still little understanding of the chemical composition and distribution of this multielement material. Therefore, an in-depth study is warranted. Photoelectron spectroscopy (PES) is one of the best techniques to investigate sample surfaces and can provide information about the chemical composition of a sample and about the chemical environment of the detected species. Apart from studies of surfaces, PES can be used to study amorphous as well as crystalline components and is therefore complementary to XRD. In this article, we use an advanced photoelectron spectroscopy approach with a synchrotron radiation source in the hard X-ray region to efficiently investigate RbCsMAFA. This method is referred to as HAXPES (Hard X-ray Photoelectron Spectroscopy). In comparison to classical AlKα based PES, higher photon energies are used, which allows for the investigation of the chemical composition and electronic structure of our material deeper within the sample due to the increased inelastic mean free path (IMFP) when electrons are emitted with a higher kinetic energy. The results therefore can provide information more characteristic of the bulk of the sample and not just of the sample surface. We previously applied this approach to the investigation of the mixed perovskite containing MA and FA and could give insights into the surface versus bulk chemistry of the material.11 Four different cation combinations were used here to make mixed perovskite materials. The reference sample referred to as “MAFA” throughout the article is composed of the two cations MA and formamidinium FA. Two triple cation perovskites were

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EXPERIMENTAL SECTION

Perovskite Sample Preparation. The perovskite samples used in this article were prepared in line with our previous reports7,8 and were made of a stacking of FTO/compact TiO2-layer/Li-doped mesoporous TiO2/mixed perovskite. The perovskite was deposited by spin coating using a one-step method. Briefly, the perovskite precursor solution was deposited by spin-coating at 1000 rpm for 10 s and then at 4000 rpm for 30 s. After spin-coating at 4000 rpm for 10 s, 200 μL of chlorobenzene was added to the spinning substrate (antisolvent method developed by Jeon et al.12). The films were then annealed under nitrogen atmosphere at 100 °C for 1 h for all different perovskite precursor solutions used as detailed below. Perovskite Precursor Solution. The precursor solutions were made from organic cation iodide salts purchased from Dyesol, the lead compounds from TCI, and the CsI and RbI salts from abcr GmbH. MA/FA Perovskite Solution. The “mixed” perovskite was deposited from a precursor solution containing FAI (1 M), PbI2 3590


Article


Figure 2. HAXPES measurement on the RbCsMAFA perovskite performed with a photon energy of 4000 eV. (a) Overview spectra and (b−g) highresolution spectra curve fitted to the main core level, i.e., the (b) Pb 4f, (c) I 3d, (d) Br 3d, (e) Cs 3d, (f) Rb 3d, and (g) N 1s core levels. (1.1 M), MABr (0.2 M), and PbBr2 (0.22 M) in anhydrous DMF/ DMSO 4:1 (v/v). Following previous works,13,14 a nonstoichiometric solution was used with an excess in the present case of 10 mol % of the lead precursors, i.e., 1.1 M of PbI2 and 0.22 of PbBr2 instead of 1 and 0.2, respectively. The nominal composition of this solution can be written as (MA0.17FA0.83)Pb(I0.83Br0.17)3, 0.1[0.83PbI2, 0.17PbBr2] and referred as to MAFA. Cs/MA/FA Perovskite Solution. CsI salt was predissolved in DMSO (1.5 M) and then added to the mixed perovskite (MA/FA) precursor to achieve the desired triple cation composition of 5% of Cs. The resulting solution labeled CsMAFA has a precursor solution with a nominal composition of 0.05 CsI and 0.95 MAFA. Rb/MA/FA Perovskite Solution. A similar procedure as that for CsMAFA was used with RbI instead of CsI. RbI was predissolved as a 1.5 M stock solution in DMF/DMSO 4:1 (v/v) and added to the MAFA solution. The corresponding precursor solution labeled CsMAFA has a nominal composition of 0.05 RbI and 0.95 MAFA. Rb/Cs/MA/FA Perovskite. The quadruple cation was prepared by the addition of 5% of the 1.5 M RbI solution to the CsMAFA perovskite solution. The corresponding precursor solution labeled RbCsMAFA has a nominal composition of 0.05 RbI, 0.95 CsMAFA, i.e., 0.05 RbI, 0.0475 CsI, and 0.9025 MAFA (rounded to 0.05 RbI, 0.05 CsI, and 0.9 MAFA). Hard X-ray Photoelectron Spectroscopy (HAXPES). The perovskite materials were investigated by HAXPES using a synchrotron radiation source. These experiments were conducted on bare perovskite films without the hole transporter layer and the gold electrode. Each sample was stored in a sealed box containing silica gel

desiccants to avoid moisture contamination. The box was only opened prior HAXPES analysis, and the sample was then directly introduced in the UHV chamber. HAXPES was carried out at BESSY II (Helmholtz Zentrum Berlin, Germany) at the KMC-1 beamline15 using the HIKE end-station16 providing a usable photon energy range from 2 to 12 keV. In this work, photon energies of 2100 and 4000 eV were used by selecting the first-order light from Si(111) and Si(311) crystals, respectively, of a double-crystal monochromator (Oxford-Danfysik). The pressure in the analysis chamber was ∼10−8 mbar. The depth sensitivity in the PES measurements depends on the inelastic mean free path (IMFP) of the photoelectrons, which is related to their kinetic energy. Therefore, changing the photon energy will modify the depth sensitivity.17 The depth sensitivity values reported in the present work were defined as three times the IMFP of the photoelectron since 96% of the PES signal in a homogeneous material comes from a layer with this thickness. Overview spectra were measured with a pass energy (Ep) of 500 eV, while 200 eV was used for core level peaks and valence band spectra. The spectra recorded at 2100 eV presented in this work were energy calibrated versus the Fermi level at zero binding energy, which was determined by measuring a gold plate in electric contact with the sample and setting the Au 4f7/2 core level peak to 84.0 eV after curve fitting. Measurements performed at 4000 eV were aligned with the ones performed at 2100 eV. The spectra used for quantification were always recorded on a fresh spot. X-ray induced damages were carefully checked by controlling the evolution of the peak position, shape, and intensity during the measurements, and the results reported below do 3591


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Chemistry of Materials not suffer from such effects. Generally, the peak positions were very stable upon long measurements with only minor fluctuations of the Fermi level position ( CsMAFA > RbMAFA > MAFA. In a previous HAXPES study on MAFA, we showed that samples containing more organic species at their grain boundaries lead to solar cells with higher 3594


Chemistry of Materials Table 1. Intensity Ratios between Different Core Levels Calculated from Experimental Results: (a) I/Pb and (b) Cs/ Pb and Rb/Pb of the Mixed Cation Perovskites as a Function of the Excitation Energy (2100 and 4000 eV)a (a) I/Pb intensity ratio (theo) 2100 eV 4000 eV (b) Rb/Pb and Cs/Pb intensity ratio (theo) Cs/ Pb

Rb/ Pb

2100 eV 4000 eV 2100 eV 4000 eV

MAFA (2.41)

RbMAFA (2.46)

CsMAFA (2.46)

RbCsMAFA (2.51)

2.95 2.19

3.12 2.34

3.28 2.32

3.40 2.45

RbMAFA (0.048)

CsMAFA (0.048)

RbCsMAFA (0.048 for Cs/Pb 0.05 for Rb/Pb)

0.061

0.084

0.061

0.077

0.008

0.030

0.018

0.029

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CONCLUSION

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ASSOCIATED CONTENT

Using HAXPES core levels analysis and quantification considerations, we have shown that the double cation perovskite material (MAFA) contains an excess of PbI2 toward its bulk, while a small amount of unreacted FAI is still present at the surface. When one or two more cations are added (Cs and Rb), the iodine distribution, i.e., the I/Pb and I/Br ratio, is significantly altered suggesting that less PbI2 is detected at higher probing depth, although an excess of iodine is observed at the surface, most likely linked to FAI. The added CsI and RbI appear to properly and preferentially react with PbI2 resulting in an excess of FAI at the surface of the sample. Regarding the sample containing rubidium as a third cation, less than 2% could be detected at 4000 eV and less than 1% at 2100 eV despite the 5% added as a precursor. These results suggested that Rb is mainly located toward the bulk of the materials. The triple cation perovskite material with Cs behaves differently, and cesium can be found homogeneously within the material. Finally, when both extra cations (Cs and Rb) are introduced, their distribution over the 18 nm surface layer seems to be rather homogeneous even with a Rb amount twice smaller than the amount of Cs, i.e., about ∼3% of Rb for ∼8% of Cs at the surface. In this last case, the better distribution of Rb is significantly impacted by the presence of Cs that is more naturally well distributed. Finally, we can observe that the outermost surface still contains the remaining organic species in an increasing amount with the increasing number of cations used. This difference seems to be the main effect in the improvement of the Voc reported. Despite these differences at the surface and at the grain boundaries, the electronic structure close to the valence band edge and the optical band gap of the materials is not significantly impacted by the inclusion of new cations, i.e., cesium (5%) and rubidium (5%).

a

The ratios are estimated from the HAXPES spectra presented in Figure 4 and the Rb 3d spectra in Figure S3. The theoretical values based on the precursor solution composition are indicated in these tables and based on the following chemical formula: MAFA = (MA0.17FA0.83)Pb(I0.83Br0.17)3, 0.1[0.83PbI2, 0.17PbBr2] RbMAFA = 0.05RbI[(MA0.17FA0.83)Pb(I0.83Br0.17)3, 0.1[0.83PbI2, 0.17PbBr2]]0.95 CsMAFA = 0.05CsI[(MA0.17FA0.83)Pb(I0.83Br0.17)3, and 0.1[0.83PbI2, 0.17PbBr2]]0.95 RbCsMAFA = 0.05RbI[0.05CsI[(MA0.17FA0.83)Pb(I0.83Br0.17)3, 0.1[0.83PbI2, 0.17PbBr2]]0.95]0.95.

Finally, Table 1b represents the distribution of Cs and Rb within the material through the Cs/Pb and Rb/Pb ratio. For RbMAFA, we can see that only 2 and 1% are detected at 4000 and 2100 eV, respectively, suggesting that Rb is mainly located toward the bulk of the materials and gets easily released at the surface due to its small ionic radius. For CsMAFA, about 6% of cesium is uniformly found at both depths. Surprisingly, the perovskite materials containing both Cs and Rb are not a mere superposition of RbMAFA amd CsMAFA. Both Cs and Rb distributions over the 18 nm surface layer investigated are rather homogeneous with larger amounts of both cations present than in the individual materials (about ∼3% of Rb for ∼8% of Cs with both photon energies). This indicates that the alkali metals act jointly and that the presence of Cs aids the integration of Rb into the perovskite compound. The performance improvement observed for the RbCsMAFA can thus also be due to a better crystal quality and especially homogeneity resulting from the good Cs and Rb distribution within the sample.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00126. High resolution C 1s core level spectra of the RbCsMAFA perovskite; characteristic binding energies and fwhm of the main core level peaks composing the RbCsMAFA perovskite; UV−vis and photoluminescence data of the triple cation CsMAFA and RbMAFA perovskite materials; Rb 3d HAXPES spectra of the RbMAFA and RbCsMAFA; Br/Pb, I/Br intensity ratios calculated from experimental results; and details on the solar cells device preparation and device data for the

Figure 5. Schematic illustration summarizing the main difference observed by HAXPES between the perovskite materials containing two (MA/FA), three (Rb/MA/FA and Cs/MA/FA), and four (Rb/Cs/MA/FA) cations. These schemes are based on the quantification tables and followed a similar methodology previously employed in ref 11. 3595


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MAFA, RbMAFA, CsMAFA and RbCsMAFA compounds (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(B.P.) E-mail: [email protected]. *(M.S.) E-mail: michael.saliba@epfl.ch. ORCID

Bertrand Philippe: 0000-0003-2412-8503 Juan-Pablo Correa-Baena: 0000-0002-3860-1149 Silver-Hamill Turren-Cruz: 0000-0003-3191-6188 Michael Grätzel: 0000-0002-0068-0195 Author Contributions ○

B.P. and M.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Swedish Research Council, the Swedish Energy Agency, the Swedish Foundation for Strategic Research, and StandUP for Energy. HZB is acknowledged for the allocation of synchrotron radiation beamtime. M.S. acknowledges support from the cofunded Marie Skłodowska Curie fellowship, H2020 Grant agreement no. 665667.

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REFERENCES

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Chemical Distribution of Multiple Cation (Rb+, Cs+, MA+, and FA+)

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