Multi-cation Synergy Suppresses Phase Segregation in Mixed-Halide Perovskites

•Reveal phase segregation in mixed-halide perovskite films via in situ observation•Cs+ and Rb+ additions dictate the crystallization pathways and solar cell performance•Direct formation of perovskite phase is beneficial in minimizing halide segregation Having recognized the potential of hybrid organic-inorganic perovskites solar cells, in recent years the photovoltaic community has shifted its focus away from efficiency improvements to simplifying the processing and improving the stability of devices. In this work, we utilize in situ and time-resolved X-ray scattering to track various phase evolutions during the perovskite film solidification to link the microstructure to the composition. In particular, we unravel the crucial roles of Cs+ and Rb+ in promoting the in situ formation of the perovskite phase prior to thermal annealing, thus preventing segregation of halides and cations. Our study points to a significant new guideline for designing mixed-halide mixed-cation perovskites: the sol-gel formulation must possess the ability to convert directly into the targeted perovskite phase without transitioning through compositionally distinct intermediate phases in order to minimize halide segregation and yield-homogenized films. Mixed lead halide perovskite solar cells have been demonstrated to benefit tremendously from the addition of Cs+ and Rb+, but its root cause is yet to be understood. This hinders further improvement, and processing approaches remain largely empirical. We address the challenge by tracking the solidification of precursors in situ and linking the evolutions of different crystalline phases to the presence of Cs+ and Rb+. In their absence, the perovskite film is inherently unstable, segregating into MA-I- and FA-Br-rich phases. Adding either Cs+ or Rb+ is shown to alter the solidification process of the perovskite films. The optimal addition of both Cs+ and Rb+ drastically suppress phase segregation and promotes the spontaneous formation of the desired α phase. We propose that the synergistic effect is due to the collective benefits of Cs+ and Rb+ on the formation kinetics of the α phase and on the halide distribution throughout the film. Mixed lead halide perovskite solar cells have been demonstrated to benefit tremendously from the addition of Cs+ and Rb+, but its root cause is yet to be understood. This hinders further improvement, and processing approaches remain largely empirical. We address the challenge by tracking the solidification of precursors in situ and linking the evolutions of different crystalline phases to the presence of Cs+ and Rb+. In their absence, the perovskite film is inherently unstable, segregating into MA-I- and FA-Br-rich phases. Adding either Cs+ or Rb+ is shown to alter the solidification process of the perovskite films. The optimal addition of both Cs+ and Rb+ drastically suppress phase segregation and promotes the spontaneous formation of the desired α phase. We propose that the synergistic effect is due to the collective benefits of Cs+ and Rb+ on the formation kinetics of the α phase and on the halide distribution throughout the film. On the quest for high-efficiency solar cells at affordable cost, hybrid organic-inorganic metal-halide perovskites have emerged as promising thin-film photovoltaic materials, owing to their outstanding optoelectronic properties for photon-to-electron conversion and tunable bandgap.1Colella S. Mazzeo M. Rizzo A. Gigli G. Listorti A. The bright side of perovskites.J. Phys. Chem. Lett. 2016; 7: 4322-4334Crossref PubMed Scopus (104) Google Scholar, 2Correa-Baena J.-P. Abate A. Saliba M. Tress W. Jesper Jacobsson T.J. Grätzel M. Hagfeldt A. The rapid evolution of highly efficient perovskite solar cells.Energy Environ. 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Record efficiencies require the perovskite to crystallize in the α-phase, which has a trigonal symmetry (space group P3m1, often called black phase,9Zhou Y. Zhou Z. Chen M. Zong Y. Huang J. Pang S. Padture N.P. Doping and alloying for improved perovskite solar cells.J. Mater. Chem. A. 2016; 4: 17623-17635Crossref Scopus (131) Google Scholar, 12Stoumpos C.C. Malliakas C.D. Kanatzidis M.G. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties.Inorg. Chem. 2013; 52: 9019-9038Crossref PubMed Scopus (3935) Google Scholar, 13Weller M.T. Weber O.J. Frost J.M. Walsh A. Cubic perovskite structure of black formamidinium lead iodide, α-[HC(NH2)2]PbI3, at 298 K.J. Phys. Chem. Lett. 2015; 6: 3209-3212Crossref Scopus (363) Google Scholar for the color of the perovskite film). Unwanted non-perovskite phases such as the hexagonal symmetry (P63mc) δ-phase (or yellow phase9Zhou Y. Zhou Z. Chen M. Zong Y. Huang J. Pang S. Padture N.P. Doping and alloying for improved perovskite solar cells.J. Mater. Chem. A. 2016; 4: 17623-17635Crossref Scopus (131) Google Scholar, 12Stoumpos C.C. Malliakas C.D. Kanatzidis M.G. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties.Inorg. Chem. 2013; 52: 9019-9038Crossref PubMed Scopus (3935) Google Scholar) often lead to poor photovoltaic performance. Currently, optimal perovskite compositions are based on FA+ as the majority cation. The cubic perovskite α-FAPbI3 phase has a band gap of 1.48 eV, which is closer to the ideal single-junction device Shockley-Queisser bandgap (1.34 eV) than MAPbI3 (1.57 eV).13Weller M.T. Weber O.J. Frost J.M. Walsh A. Cubic perovskite structure of black formamidinium lead iodide, α-[HC(NH2)2]PbI3, at 298 K.J. Phys. Chem. 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Ed. 2014; 53: 3151-3157Crossref PubMed Scopus (1037) Google Scholar In addition, mixing well-controlled Br− concentrations in the MAPbI3 lattice results in mixed-halide (I− and Br−) perovskites with a more stable crystal structure and better long-term durability. This can be attributed to the smaller ionic radius of Br− compared to I−, which transforms the tetragonal phase I4/mcm symmetry of MAPbI3 into the Pm3m cubic phase.16Zarick H.F. Soetan N. Erwin W.R. Bardhan R. Mixed halide hybrid perovskites: a paradigm shift in photovoltaics.J. Mater. Chem. A. 2018; 6: 5507-5537Crossref Google Scholar Therefore, despite the fact that the widened bandgap somewhat shifts away from the Shockley-Queisser optimum, the mixed-halide (I− and Br−) perovskites offer better performance when compared to triiodide perovskites as well as tunable bandgaps, which are critical in tandem application of wider bandgap perovskites. The latter depends on the halide ratio in the precursors. It is noted, however, that mixed-halide compositions are not a necessity for achieving high-performance devices, as recent reports on FAPbI3-based devices demonstrate.17Zhao Y. Tan H. Yuan H. Yang Z. Fan J.Z. Kim J. Voznyy O. Gong X. Quan L.N. Tan C.S. et al.Perovskite seeding growth of formamidinium-lead-iodide-based perovskites for efficient and stable solar cells.Nat. Commun. 2018; 9: 1607Crossref PubMed Scopus (250) Google Scholar Instead, mixed halides appear to significantly facilitate the processing and manufacture of high-quality perovskite thin films. Solar cells prepared from mixed-cation (MA+ and FA+) and mixed-halide (I− and Br−) precursors are more reproducible and thermally stable when adding ∼ 5% (molar) Cs+ to the perovskite precursor formulation.18Saliba M. Matsui T. Seo J.Y. Domanski K. Correa-Baena J.P. Nazeeruddin M.K. Zakeeruddin S.M. Tress W. Abate A. Hagfeldt A. et al.Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency.Energy Environ. Sci. 2016; 9: 1989-1997Crossref PubMed Google Scholar Addition of Cs+ was found to enhance phase stabilization by tuning the tolerance factor through solid-state alloying.8Li Z. Yang M. Park J.-S. Wei S.-H. Berry J.J. Zhu K. Stabilizing perovskite structures by tuning tolerance factor: formation of formamidinium and cesium lead iodide solid-state alloys.Chem. Mater. 2016; 28: 284-292Crossref Scopus (1279) Google Scholar In parallel, Cs+ suppresses the photo-inactive δ-phase of pure FAPbI3 and facilitates the formation of the desired photoactive α-phase, resulting in more homogenous, low-defect perovskite films.18Saliba M. Matsui T. Seo J.Y. Domanski K. Correa-Baena J.P. Nazeeruddin M.K. Zakeeruddin S.M. Tress W. Abate A. Hagfeldt A. et al.Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency.Energy Environ. Sci. 2016; 9: 1989-1997Crossref PubMed Google Scholar Gratia et al. showed that the crystallization sequence of mixed-halide perovskite films is 2H-4H-6H-3C and unveiled that the addition of 3% Cs+ provides a shortcut to the 3C phase by inhibiting the hexagonal intermediate phases.19Gratia P. Zimmermann I. Schouwink P. Yum J.H. Audinot J.N. Sivula K. Wirtz T. Nazeeruddin M.K. The many faces of mixed ion perovskites: unraveling and understanding the crystallization process.ACS Energy Lett. 2017; 2: 2686-2693Crossref Scopus (106) Google Scholar We adopt here the Ramsdell notation, widely used for oxide perovskites and also used by Gratia et al.,19Gratia P. Zimmermann I. Schouwink P. Yum J.H. Audinot J.N. Sivula K. Wirtz T. Nazeeruddin M.K. The many faces of mixed ion perovskites: unraveling and understanding the crystallization process.ACS Energy Lett. 2017; 2: 2686-2693Crossref Scopus (106) Google Scholar to accurately describe the intermediate phases during perovskite film formation. The classical AMX3 perovskite structure shows various polytypes depending on the stacking sequence of the close-packed AX3 layers. For example, the well-known yellow δ phase observed in lead halide perovskites is composed of pure hexagonal closed-packed AX3 layers, resulting in a 1D array of BX6 face-sharing octahedra, namely the 2H phase. Moreover, the photoactive α phase is composed of cubic closed-packed AX3 layers, resulting in a 3D framework of BX6 corner-sharing octahedral, namely the 3C phase. The perovskite can also crystallize in a 3R phase, resembling the 3C phase but with rhombohedral symmetry instead of cubic symmetry. The other two common intermediate phases, 4H and 6H, are composed of a combination of a hexagonal and cubic closed-packed AX3 stacking sequence, resulting in a 3D framework of both face-sharing and corner-sharing BX6 octahedra.19Gratia P. Zimmermann I. Schouwink P. Yum J.H. Audinot J.N. Sivula K. Wirtz T. Nazeeruddin M.K. The many faces of mixed ion perovskites: unraveling and understanding the crystallization process.ACS Energy Lett. 2017; 2: 2686-2693Crossref Scopus (106) Google Scholar Recently, Rb+ addition has been reported to further improve the performance of mixed-halide mixed-cation lead perovskite solar cells, exhibiting a PCE of 21.6% with remarkable stability under continuous illumination at elevated temperature.20Saliba M. Matsui T. Domanski K. Seo J.Y. Ummadisingu A. Zakeeruddin S.M. Correa-Baena J.P. Tress W.R. Abate A. Hagfeldt A. et al.Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance.Science. 2016; 354: 206-209Crossref PubMed Scopus (2765) Google Scholar Subsequently, several studies investigated the effect of Rb+ and Cs+ on the resulting perovskite films.21Kubicki D.J. Prochowicz D. Hofstetter A. Zakeeruddin S.M. Grätzel M. Emsley L. Phase segregation in Cs-, Rb- and K-doped mixed-cation (MA)x(FA)1–xPbI3 Hybrid Perovskites from Solid-State NMR.J. Am. Chem. Soc. 2017; 139: 14173-14180Crossref PubMed Scopus (262) Google Scholar, 22Jacobsson T.J. Svanström S. Andrei V. Rivett J.P.H. Kornienko N. Philippe B. Cappel U.B. Rensmo H. Deschler F. Boschloo G. Extending the compositional space of mixed lead halide perovskites by Cs, Rb, K, and Na doping.J. Phys. Chem. C. 2018; 122: 13548-13557Crossref Scopus (63) Google Scholar, 23Philippe B. Saliba M. Correa-Baena J.-P. Cappel U.B. Turren-Cruz S.-H. Grätzel M. Hagfeldt A. Rensmo H. 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Rensmo H. Chemical distribution of multiple cation (Rb+, Cs+, MA+, and FA+) perovskite materials by photoelectron spectroscopy.Chem. Mater. 2017; 29: 3589-3596Crossref Scopus (156) Google Scholar Using a combination of characterization techniques, Hu et al.24Yinghong H. M. Philipp H.E. Irene R. Jonas G. H. F. G. Matthias H.A. Thomas H. et al.Understanding the role of cesium and rubidium additives in perovskite solar cells: trap states, charge transport, and recombination.Adv. Energy. Mater. 2018; 8Google Scholar found that Rb+ addition enhances the film’s carrier mobility whereas Cs+ addition leads to a significant reduction of trap density in the perovskite crystals. In a recent study,26Yadav P. Dar M.I. Arora N. Alharbi E.A. Giordano F. Zakeeruddin S.M. Grätzel M. The role of rubidium in multiple-cation-based high-efficiency perovskite solar cells.Adv. Mater. 2017; 29Crossref Scopus (103) Google Scholar improvements in the VOC and fill factor (FF) were found for solar cells with Rb+ addition, which was argued to originate from lower recombination and improved extraction of holes at the interface of perovskite and spiro-OMeTAD. However, collectively, these studies have only probed the residual effect of added Cs+ and Rb+ on the properties of perovskite films and devices via ex situ characterization techniques, i.e., by characterizing the final state of the resulting films. In this work, we investigate in detail the impact of Cs+ and Rb+ addition upon perovskite film formation by tracking the evolution of the crystal phases in situ with the help of time-resolved grazing incidence wide-angle x-ray scattering (GIWAXS) measurements performed during spin coating.27Yang B. Keum J.K. Geohegan D.B. Xiao K. Kumar C.S.S.R. In-situ characterization techniques for nanomaterials. Springer, 2018: 33-60Crossref Scopus (1) Google Scholar, 28Munir R. 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Amassian A. Morphology development in solution-processed functional organic blend films: an in situ viewpoint.Chem. Rev. 2017; 117: 6332-6366Crossref PubMed Scopus (133) Google Scholar Supported also by time-of-flight secondary ion mass spectrometry (ToF-SIMS) halide mapping, we reveal that the phase-segregation-induced chemical segregation—which inherently occurs in pristine formulations—can be suppressed when Cs+ and Rb+ are added together in the right amount. In doing so, Cs+ and Rb+ also promote the direct transformation of the precursor into the photoactive 3C (often known as α) phase without requiring thermal annealing to initiate the conversion process. This is reflected in the significantly improved photovoltaic performance of devices made from the optimal combinations of Cs+ and Rb+, reaching 20.1% PCE as opposed to 15.3% from the pristine precursor. We go on to show a direct link between the ease of direct transformation of the perovskite phase and the achievable PCE within mixed-halide mixed-cation solar cells. Such detailed knowledge of the crystallization processes highlights the key role of controlling phase segregation and benefits the development of new design rules for compositions containing atomic additives intended to mitigate halide segregation, facilitate phase transformation and overall ease of processing, and ultimately yield hybrid perovskite films with excellent optoelectronic properties and reproducibility. We synthesized a set of perovskite films containing different amounts of CsI and/or RbI added into the pristine precursor. The pristine perovskite precursor serving as the control sample has a stoichiometry of (FA0.83MA0.17)Pb(I0.83Br0.17)3. Thin films resulting from the addition of Cs+ and/or Rb+ are coded for convenience as (% Cs and % Rb), where % Cs and % Rb are, respectively, molar percentages of added CsI and RbI solution relative to the mixed cations. All test samples have a total addition of 10% of Cs+ and/or Rb+ [e.g., (1,9), (3,7), (5,5), (7,3), and (9,1)], with the exception of three cases: (5,0), (0,5), and the control (0,0). All perovskite films are prepared using a one-step spin-coating process with so-called antisolvent drip and used chlorobenzene as quenching solvent,10Xiao 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. Int. Ed. 2014; 53: 9898-9903Crossref PubMed Scopus (1296) Google Scholar followed by annealing at 100°C for 20 min. We refer to the Experimental Procedures for more details on solution preparation and device fabrication. To unveil the mechanism underlying the improved efficiency, as well as the roles of added Cs+ and Rb+, we took advantage of time-resolved GIWAXS to diagnose the solution-to-solid transformation of the various perovskite precursors, both with and without the addition of Cs+ and/or Rb+. To mimic the perovskite crystallization in as-cast films, we performed these measurements during spin coating using an earlier developed setup,28Munir R. Sheikh A.D. Abdelsamie M. Hu H. Yu L. Zhao K. Kim T. Tall O.E. Li R. Smilgies D.-M. et al.Hybrid perovskite thin-film photovoltaics: in situ diagnostics and importance of the precursor solvate phases.Adv. Mater. 2017; 29PubMed Google Scholar, 32Richter L.J. DeLongchamp D.M. Amassian A. Morphology development in solution-processed functional organic blend films: an in situ viewpoint.Chem. Rev. 2017; 117: 6332-6366Crossref PubMed Scopus (133) Google Scholar, 33Chou K.W. Yan B. Li R. Li E.Q. Zhao K. Anjum D.H. Alvarez S. Gassaway R. Biocca A. 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Stable high-performance perovskite solar cells via grain boundary passivation.Adv. Mater. 2018; 30Crossref Scopus (551) Google Scholar, 36Li J. Munir R. Fan Y. Niu T. Liu Y. Zhong Y. Yang Z. Tian Y. Liu B. Sun J. et al.Phase transition control for high-performance blade-coated perovskite solar cells.Joule. 2018; 2: 1313-1330Abstract Full Text Full Text PDF Scopus (138) Google Scholar, 37Zhang X. Munir R. Xu Z. Liu Y. Tsai H. Nie W. Li J. Niu T. Smilgies D.-M. Kanatzidis M.G. et al.Phase transition control for high performance Ruddlesden–Popper perovskite solar cells.Adv. Mater. 2018; 30Google Scholar, 38Quintero-Bermudez R. Gold-Parker A. Proppe A.H. Munir R. Yang Z. Kelley S.O. Amassian A. Toney M.F. Sargent E.H. Compositional and orientational control in metal halide perovskites of reduced dimensionality.Nat. Mater. 2018; 17: 900-907Crossref PubMed Scopus (271) Google Scholar See Experimental Procedures and Figure S1 for more details on GIWAXS measurements. The time evolution of the 2D intensity map with respect to the scattering vector q and process time t for the pristine (control) precursor is shown in Figure 1A. The 2D intensity map was constructed by cake integration (Figure S2). Prior to CB dripping, the initial dominant scattering feature is characterized by a halo at low q values (∼4–7 nm−1) associated with the colloidal state of the sol-gel precursor.28Munir R. Sheikh A.D. Abdelsamie M. Hu H. Yu L. Zhao K. Kim T. Tall O.E. Li R. Smilgies D.-M. et al.Hybrid perovskite thin-film photovoltaics: in situ diagnostics and importance of the precursor solvate phases.Adv. Mater. 2017; 29PubMed Google Scholar Upon CB dripping (indicated in Figure 1A with a vertical arrow), the sol-gel halo disappears and diffraction peaks appear instead. Scattering peaks at lower q, namely ∼5 and ∼7 nm−1 are associated with the presence of a solvated crystalline phase (S solvate phase).28Munir R. Sheikh A.D. Abdelsamie M. Hu H. Yu L. Zhao K. Kim T. Tall O.E. Li R. Smilgies D.-M. et al.Hybrid perovskite thin-film photovoltaics: in situ diagnostics and importance of the precursor solvate phases.Adv. Mater. 2017; 29PubMed Google Scholar, 39Barrit D. Sheikh A.D. Munir R. Barbé J.M. Li R. Smilgies D.-M. Amassian A. Hybrid perovskite solar cells: in situ investigation of solution-processed PbI 2 reveals metastable precursors and a pathway to producing porous thin films.J. Mater. Res. 2017; 32: 1899-1907Crossref Scopus (22) Google Scholar We detect a comparatively faint diffraction signal at ∼10 nm−1, where the 6H and 3C phases are known to be.28Munir R. Sheikh A.D. Abdelsamie M. Hu H. Yu L. Zhao K. Kim T. Tall O.E. Li R. Smilgies D.-M. et al.Hybrid perovskite thin-film photovoltaics: in situ diagnostics and importance of the precursor solvate phases.Adv. Mater. 2017; 29PubMed Google Scholar In the region between 8–9 nm−1, we see a broad scattering feature composed of two contributions at q = 8.4 and 8.8 nm−1. These two phases have distinct peak positions and compositions and will later be discussed to be the 2H and 4H hexagonal phases. Our GIWAXS results are supported by X-ray diffraction (XRD) measurements of the as-prepared thin films (see Figure S3) where solvated, 4H, 2H, and 6H crystalline phases are, respectively, located at 2θ values of ∼(6.8° and 9.2°), (11.6° and 13.0°), 11.8°, and (12.2° and 14.0°). We note that peaks of the 6H (102) and 3C (100) phases, both observed at q ∼ 10 nm−1 in the GIWAXS, are positioned at very close 2θ values of 14.0° and 14.1° respectively, in the XRD of thin films (Figure S3). We therefore distinguish the two phases by the presence of an additional 6H (101) peak at 12.2°. Briefly, we found that the 3C phase is observed in films prepared from Cs+-added precursors whereas the 6H phase is observed in other films (pristine, as well as 5% Rb+- and 10% Rb+-added films). Figure 1B shows 2D GIWAXS snapshots at select moments (10, 40, and 350 s) after initiating the spin coating of the precursor, revealing more details about the in-plane texture of the different phases in the as-cast film. The identity of the solvate (S) and intermediates, including so-called δ phases, are not always well-defined in the literature,7Jeon N.J. Noh J.H. Yang W.S. Kim Y.C. Ryu S. Seo J. Seok S.I. Compositional engineering of perovskite materials for high-performance solar cells.Nature. 2015; 517: 476-480Crossref PubMed Scopus (4901) Google Scholar, 40Kim J. Saidaminov M.I. Tan H. Zhao Y. Kim Y. Choi J. Jo J.W. Fan J. Quintero-Bermudez R. Yang Z. et al.Amide-catalyzed phase-selective crystallization reduces defect density in wide-bandgap perovskites.Adv. Mater. 2018; 30Google Scholar, 41Deng Y. Dong Q. Bi C. Yuan Y. Huang J. Air-stable, efficient mixed-cation perovskite solar cells with Cu electrode by scalable fabrication of active layer.Adv. Energy. Mater. 2016; 6Crossref Scopus (247) Google Scholar mainly because of the singular composition of the studied perovskite precursors and the presence of only one diffr