Unraveling the Impact of Halide Mixing on Crystallization and Phase Evolution in CsPbX3 Perovskite Solar Cells

Abstract

•GIWAXS is utilized to probe real-time phase evolution of CsPbX3 perovskite film•The role of halide elements in phase evolution is revealed•High-quality perovskite film is obtained by ternary halide engineering•A PCE as high as 17.14% is achieved All-inorganic perovskites have attracted increasing attention because of their potentially better thermal stability. However, the film-formation process and phase-transition mechanism of all-inorganic perovskite are still unclear. There is no consensus regarding the role of halide elements in the phase-transition and crystal growth kinetics of all-inorganic perovskite. In this work, state-of-the-art in situ grazing-incidence wide-angle X-ray scattering is utilized to probe real-time crystalline and phase evolution of all-inorganic perovskite film. The roles of I, Br, and Cl in phase-transition and crystallization kinetics are revealed individually and correlated with device performance of all-inorganic perovskite solar cells, which leads to the development of a facile and brand-new means to fabricate highly efficient and phase-stable ternary halide (I, Br, Cl) all-inorganic perovskite solar cells. All-inorganic perovskite solar cells (PSCs) have attracted wide attention for their excellent thermal stability. However, the detailed crystallization process and complicated phase-transition mechanism of the CsPbX3 film with different halide compositions (I, Br, Cl) remain mysterious. In this study, systematic investigations are performed via state-of-art in situ grazing-incidence wide-angle X-ray scattering to understand the role of the halide elements in all-inorganic perovskite crystallization kinetics, phase-transition, and stabilization mechanism as well as how film morphology and grain size are affected. Based on these results, we were able to fabricate high-performance ternary halide (I, Br, Cl) all-inorganic PSCs through a precise compositional engineering. Our results provide guidance for an in-depth understanding of all-inorganic perovskite materials and pave the way to obtain high-performance all-inorganic PSCs. All-inorganic perovskite solar cells (PSCs) have attracted wide attention for their excellent thermal stability. However, the detailed crystallization process and complicated phase-transition mechanism of the CsPbX3 film with different halide compositions (I, Br, Cl) remain mysterious. In this study, systematic investigations are performed via state-of-art in situ grazing-incidence wide-angle X-ray scattering to understand the role of the halide elements in all-inorganic perovskite crystallization kinetics, phase-transition, and stabilization mechanism as well as how film morphology and grain size are affected. Based on these results, we were able to fabricate high-performance ternary halide (I, Br, Cl) all-inorganic PSCs through a precise compositional engineering. Our results provide guidance for an in-depth understanding of all-inorganic perovskite materials and pave the way to obtain high-performance all-inorganic PSCs. Recently, organic-inorganic hybrid lead halide perovskite solar cells (PSCs) have achieved rapid progress with power-conversion efficiencies (PCEs) of over 25% due to their outstanding optoelectronic properties.1Jiang Q. Zhao Y. Zhang X. Yang X. Chen Y. Chu Z. Ye Q. Li X. Yin Z. You J. Surface passivation of perovskite film for efficient solar cells.Nat. 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Zhang et al. investigated the influence of solvent composition (volume ratio of dimethyl sulfoxide [DMSO]/N,N-dimethylformamide [DMF]) on the morphology and quality of CsPbI2Br film using scanning electron microscopy (SEM).10Zhang S. Wu S. Chen W. Zhu H. Xiong Z. Yang Z. Chen C. Chen R. Han L. Chen W. Solvent engineering for efficient inverted perovskite solar cells based on inorganic CsPbI2Br light absorber.Mater. Today Energy. 2018; 8: 125-133Crossref Scopus (78) Google Scholar They pointed out that the volatilization rate and viscosity of solvent are important factors that affect the film-formation process. Chen et al. revealed the synergistic effect of gradient thermal annealing and antisolvent on CsPbI2Br film formation.11Chen W. Chen H. Xu G. Xue R. Wang S. Li Y. Li Y. 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The residual DMSO enhanced the mass transport and diffusion and slowed down the evaporation rate of the solvent, which is beneficial for film quality.7Wang P. Zhang X. Zhou Y. Jiang Q. Ye Q. Chu Z. et al.Solvent-controlled growth of inorganic perovskite films in dry environment for efficient and stable solar cells.Nat. Commun. 2018; 9: 2225https://doi.org/10.1038/s41467-018-04636-4Crossref PubMed Scopus (378) Google Scholar To date, studies on solution chemistry and crystallization mechanism of all-inorganic perovskites are still very limited. In situ grazing-incidence wide-angle X-ray scattering (GIWAXS) characterization is an effective means by which to understand the perovskite film-formation mechanism. Liu et al. proposed the in situ observation of vapor-assisted 2D-3D perovskite heterostructure formation process. They found that the 3D-to-2D transformation of MAPbI3 is initiated by butylamine vapor.13Liu Z. Meng K. Wang X. Qiao Z. Xu Q. Li S. Cheng L. Li Z. Chen G. In situ observation of vapor-assisted 2D-3D heterostructure formation for stable and efficient perovskite solar cells.Nano Lett. 2020; 20: 1296-1304Crossref PubMed Scopus (23) Google Scholar Szostak et al. explored the formation of formamidinium-based perovskite by the antisolvent method during spin-coating through in situ GIWAXS. They identified various crystalline phases and provided guidelines to manage the composition during film processing.14Szostak R. Marchezi P.E. Marques A.d.S. da Silva J.C. de Holanda M.S. Soares M.M. Tolentino H.C.N. Nogueira A.F. Exploring the formation of formamidinium-based hybrid perovskites by antisolvent methods: in situ GIWAXS measurements during spin coating.Sustain. Energy Fuels. 2019; 3: 2287-2297Crossref Google Scholar Fan et al. performed in situ GIWAXS measurements to study the structural evolution of all-inorganic perovskite films blade-coated at different processing temperatures.15Fan Y. Fang J. Chang X. Tang M.-C. Barrit D. Xu Z. Jiang Z. 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Revealing the crystal structure of metastable black-phase CsPbI3 by theory and experiment.ACS Energy Lett. 2018; 3: 1787-1794Crossref Scopus (246) Google Scholar Marronnier et al. studied the competition between all the low-temperature phases of CsPbI3 (γ, δ, β) using total energy and vibrational entropy calculations and highlighted that avoiding the order-disorder entropy term arising from double-well instabilities is a key to preventing the formation of the yellow δ phase.9Marronnier A. Roma G. Boyer-Richard S. Pedesseau L. Jancu J.-M. Bonnassieux Y. et al.Anharmonicity and Disorder in the Black Phases of Cesium Lead Iodide Used for Stable Inorganic Perovskite Solar Cells.ACS Nano. 2018; 12: 3477-3486Crossref PubMed Scopus (301) Google Scholar To hinder the unwanted phase transition, Swarnkar et al. introduced organic phenylethylammonium iodide (PEAI) additives into perovskite.22Wang K. Jin Z. Liang L. Bian H. Bai D. Wang H. et al.All-inorganic cesium lead iodide perovskite solar cells with stabilized efficiency beyond 15%.Nat. Commun. 2018; 9: 4544https://doi.org/10.1038/s41467-018-06915-6Crossref PubMed Scopus (266) Google Scholar Substituting Pb2+ ions with smaller metal ions such as Ge2+, Eu3+, and Mn2+ have been developed to enhance the phase stability.23Swarnkar A. Mir W.J. Nag A. Can B-site doping or alloying improve thermal- and phase-stability of all-inorganic CsPbX3 (X = Cl, Br, I) perovskites?.ACS Energy Lett. 2018; 3: 286-289Crossref Scopus (247) Google Scholar Regulating the halide component by introducing Br and Cl has been proved to be a viable route to improve the PSCs' stability.24Zhou Y. Zhao Y. Chemical stability and instability of inorganic halide perovskites.Energy Environ. Sci. 2019; 12: 1495-1511Crossref Google Scholar,25Wang K. Jin Z. Liang L. Bian H. Wang H. Feng J. Liu S. (Frank). Chlorine doping for black γ-CsPbI3 solar cells with stabilized efficiency beyond 16%.Nano Energy. 2019; 58: 1175-1182Crossref Scopus (95) Google Scholar Researchers devoted extensive efforts to reveal the effect of halide on the organic-inorganic hybrid perovskite system. Hieulle et al. unraveled the impact of halide mixing on organic-inorganic hybrid perovskite stability. By combining scanning tunneling microscopy and density functional theory, they determined the optimal Cl incorporation ratio for stability increase without detrimental band-gap modification.26Hieulle J. Wang X. Stecker C. Son D.Y. Qiu L. Ohmann R. Ono L.K. Mugarza A. Yan Y. Qi Y. Unraveling the impact of halide mixing on perovskite stability.J. Am. Chem. Soc. 2019; 141: 3515-3523Crossref PubMed Scopus (57) Google Scholar Fan et al. elucidated the role of chlorine in MAPbI3 perovskite. They demonstrated that the formation of a porous PbCl scaffold in the precursor film induced high-quality perovskite realization.27Fan L. Ding Y. Luo J. Shi B. Yao X. Wei C. Zhang D. Wang G. Sheng Y. Chen Y. et al.Elucidating the role of chlorine in perovskite solar cells.J. Mater. Chem. A. 2017; 5: 7423-7432Crossref Google Scholar Cheng et al. reported triple-anion CH3NH3PbI2−xBrClx perovskite film. They pointed out that the involvement of both bromide and chloride could suppress the trap states and non-radiative recombination loss.28Cheng R. Chung C.C. Zhang H. Liu F. Wang W.T. Zhou Z. et al.Tailoring triple-anion perovskite material for indoor light harvesting with restrained halide segregation and record high efficiency beyond 36%.Adv. Energy Mater. 2019; 9: 1901980https://doi.org/10.1002/aenm.201901980Crossref Scopus (61) Google Scholar Compared with the organic-inorganic hybrid perovskite system, all-inorganic perovskites process different compositions and thereby distinct film-formation processes, which require further exploration. However, to date there is no consensus regarding the role of halide elements including I, Br, and Cl in phase transition and crystal growth kinetics accompanied by device photovoltaic performance in all-inorganic perovskites. Last but not least, the fabrication of high-performance all-inorganic PSCs still mainly relies on the assistance of organic additives. Lau et al. reported a cation exchange growth method for fabricating all-inorganic perovskite CsPbI3 during the black perovskite phase by adding methylammonium iodide additives to the precursor.29Lau C.F.J. Wang Z. Sakai N. Zheng J. Liao C.H. Green M. et al.Fabrication of efficient and stable CsPbI3 perovskite solar cells through cation exchange process.Adv.Energy Mater. 2019; 9: 101685https://doi.org/10.1002/aenm.201901685Crossref Scopus (55) Google Scholar In addition, organic salts such as dimethylammonium iodide (DMAI) are commonly used to assist the crystal growth of all-inorganic perovskite in order to fabricate a stable device with PCEs of over 18%.30Ye Q. Zhao Y. Mu S. Ma F. Gao F. Chu Z. et al.Cesium lead inorganic solar cell with efficiency beyond 18% via reduced charge recombination.Adv. Mater. 2019; 31: e1905143https://doi.org/10.1002/adma.201905143Crossref PubMed Scopus (106) Google Scholar However, the employment of DMAI requires an additional presynthesis procedure and often leads to conditions of iodine excess, challenging the precise control of halide composition.31Wang Y. Zhang T. Kan M. Zhao Y. Bifunctional stabilization of all-inorganic alpha-CsPbI3 perovskite for 17% efficiency photovoltaics.J. Am. Chem. Soc. 2018; 140: 12345-12348Crossref PubMed Scopus (381) Google Scholar Therefore, developing new strategies for high-performance all-inorganic PSCs without the assistance of organic salts is still highly desirable.32Meng H. Shao Z. Wang L. Li Z. Liu R. Fan Y. Cui G. Pang S. Chemical composition and phase evolution in DMAI-derived inorganic perovskite solar cells.ACS Energy Lett. 2019; 5: 263-270Crossref Scopus (32) Google Scholar In this work, we carried out systematic synchrotron-based in situ GIWAXS measurements to investigate the influence of ternary halide mixing on the film crystallization mechanism and phase evolution of all-inorganic perovskite CsPbX3 (X = I, Br, Cl) under spin-coating (Figure 1B).16Qin M. Tse K. Lau T.K. Li Y. Su C.J. Yang G. et al.Manipulating the mixed-perovskite crystallization pathway unveiled by in situ GIWAXS.Adv. Mater. 2019; 31: e1901284https://doi.org/10.1002/adma.201901284Crossref PubMed Scopus (63) Google Scholar Benefiting from the guidance of the in situ GIWAXS, we were able to deeply understand the all-inorganic perovskite film-formation and phase-transition mechanisms and construct high-quality all-inorganic perovskite films with enhanced phase stability based on ternary halide engineering via precise regulation of crystallization kinetics. We found that pure CsPbI3 exhibits the broadest absorption spectrum but the worst film coverage due to the highest phase-formation barrier. Besides, the black phase of CsPbI3 is unstable owing to the large deviation of the tolerance factor from the cubic phase range.24Zhou Y. Zhao Y. Chemical stability and instability of inorganic halide perovskites.Energy Environ. Sci. 2019; 12: 1495-1511Crossref Google Scholar Introducing Br ions not only compensates the lattice distortion caused by the small Cs+ but also significantly expedites the nucleation and crystal growth rate, simultaneously improving the film coverage and phase stability. However, it also causes reduction of the average grain size as well as the photocurrent due to band-gap enlargement. Remarkably, the doping of Cl can improve the film crystallinity and crystalline orientation. However, it further deviates the band gap from the optimized value. Therefore, by balancing the band gap, nucleation, and crystal growth rates through fine-tuning the I/Br/Cl ratio (Figure 1C), devices with a conventional I/Br ratio of 2:1 have achieved a PCE as high as 15.04% under reverse voltage scanning. Remarkably, a new halide composition of I/Br = 4:1 has obtained a record high PCE of 17.14% with only 0.6 eV energy loss (Eloss) under reverse voltage scanning, which is to our knowledge the highest PCE reported for all-inorganic PSCs based on pure all-inorganic perovskite precursors without the assistance of organic additives to date.33Chen H. Xiang S. Li W. Liu H. Zhu L. Yang S. Inorganic perovskite solar cells: a rapidly growing field.Solar RRL. 2018; 2: 1700188https://doi.org/10.1002/solr.201700188Crossref Scopus (142) Google Scholar To uncover the impact of mixing I and Br in the phase evolution, we carried out in situ GIWAXS characterization during the film formation of CsPb(I1−xBrx)3 (x = 0%, 20%, 33%, 50%, 66%) during the spin-coating process. Different compositions of halide mixture were prepared by mixing pure inorganic CsI, PbI2, and PbBr2 with specific molar ratios in DMF/DMSO. False-color GIWAXS intensity maps versus qz and frame numbers (3 s per frame) of CsPb(I1−xBrx)3 (x = 0%, 20%, 33%, 50%, 66%) films are shown in Figures 2A–2E . The corresponding intensity profiles along the qz direction are presented in Figures S1A–S1E. The X-ray diffraction (XRD) standard patterns and characteristic diffraction peaks to distinguish α, β, γ, and δ phases are displayed in Figure S2. For the 0% Br film (Figure 2A), crystallization began at about the 52nd frame with the yellow δ phase scattering peaks appearing first at 0.71 Å−1, after which the black γ phase at 1.02 Å−1 and 1.15 Å−1 appeared at the 66th frame. For the 20% Br film (Figure 2B), the dominant black phase became β phase as indicated by the scattering peaks at 1.03 Å−1 and 2.35 Å−1, consistent with the fact that Br− could correct the tolerance factor to some extent and thereby experience less distortion in the corner-sharing of PbX6 octahedra. Moreover, the β phase appeared at the 58th frame, slightly earlier than the δ phase which appeared at the 70th frame. For the 33% Br film (Figure 2C), the scattering peak for black α phase can be clearly identified at 1.04 Å−1 starting from the 46th frame, while the δ phase appeared at the 92nd frame. When the concentration of Br further increased, to 50% and 66%, the appearance of black α phase was further advanced without the detection of δ phase during the whole spin process (Figures 2D and 2E). The emergence time (frame number) of the black perovskite phase and yellow non-perovskite phase versus different Br components are summarized in Figure 2F. An obvious trend of black phase advancing and yellow phase deferring can be observed, suggesting that Br ions play an important role in facilitating the formation of the black perovskite phase and effectively suppress the unwanted phase transition to the yellow δ phase. It was reported that the rattling of Cs, low number of Cs-I contacts, and high degree of octahedral distortion cause instability of all-inorganic perovskite.34Straus D.B. Guo S. Abeykoon A.M. Cava R.J. Understanding the instability of the halide perovskite CsPbI3 through temperature-dependent structural analysis.Adv. Mater. 2020; 32: e2001069https://doi.org/10.1002/adma.202001069Crossref PubMed Scopus (35) Google Scholar This suggests that the incorporation of Br ions, with relatively stronger electronegativity and smaller ionic size than those of I ions, could form stronger coordination of Cs ions and reduce the octahedral distortion, which would effectively contribute to the stabilization of the black phase. The time evolution of the scattering peak area at qz ≈ 1 Å−1 for CsPb(I1−xBrx)3 (x = 0%, 20%, 33%, 50%, 66%) perovskites are plotted in Figures 3A–3E to aid in further understanding the nucleation and crystal growth kinetics. Three stages of crystallization can be identified: (I) solution stage; (II) nucleation and fast growth stage; (III) slow growth stage. The mechanism of nucleation and crystal growth in the film-formation process can be explained by the LaMer theory.35Liang L. Li Z. Zhou F. Wang Q. Zhang H. Xu Z. Ding L. Liu S. Jin Z. The humidity-insensitive fabrication of efficient CsPbI3 solar cells in ambient air.J. Mater. Chem. A. 2019; 7: 26776-26784Crossref Google Scholar,36Gao L. Yang G. Organic-inorganic halide perovskites: from crystallization of polycrystalline films to solar cell applications.Solar RRL. 2019; 4: 1900200https://doi.org/10.1002/solr.201900200Crossref Scopus (29) Google Scholar As shown in Figure 3G, the crystallization starting points are gradually advanced with the increase of Br concentration. It is suggested that the incorporation of more Br could lower the nucleation barrier of the perovskite phase so as to enter stage II of nucleation and fast growth earlier. The nucleation and fast growth process quickly consume the solute concentration. The crystallization process moves into the third crystallinity saturation stage (stage III) when the nucleation stops and the crystal growth slows down. For the 0% Br film that enters stage II at the latest, the nucleation sites are too scarce to form a fully covered film. A smaller Br content gives rise to a milder supersaturation condition, leaving sparser nucleation sites for further crystal growth in stage II. Comparing the 20% and 33% Br films, their time spent in stage II is similar while the 20% film experiences much milder supersaturation conditions with fewer nucleation sites when entering stage III. Figure 3F is a cartoon that qualitatively summarizes the influence of halide content on the nucleation and crystal growth (Figures 3A–3E), based on the classical LaMer theory for monodispersed particle formation. When the Br concentration is increased from 33% to 66%, the launch time of stage II is further advanced, which is compensated by the shortening of the duration of stage II, giving rise to a similar amount of nucleation sites. The corresponding intensity profiles along the qz direction (out-of-plane) were extracted in detail (Figure 3H). The diffraction peak gradually shifts to higher q values with more Br in the film, confirming that Br indeed entered the lattice in substitution of I, inducing the lattice shrinkage to stabilize the perovskite crystal structure.37Nam J.K. Chun D.H. Rhee R.J.K. Lee J.H. Park J.H. Methodologies toward efficient and stable cesium lead halide perovskite-based solar cells.Adv. Sci. 2018; 5: 1800509https://doi.org/10.1002/advs.201800509Crossref Scopus (34) Google Scholar The film morphology of CsPb(I1−xBrx)3 (x = 0%, 20%, 33%, 50%, 66%) was further studied by SEM, shown in Figures 4A–4E and S3. All the films were annealed under the same temperature conditions as detailed in Experimental Procedures. The corresponding statistics of the grain-size distribution are shown in Figures 4F–4J. The 0% Br film exhibits dendrite morphology with observable pinholes, which originate from the very small amount of nucleation sites and a high crystallization barrier, which hinders the crystal growth. The introduction of Br ions can significantly alter the morphology of the resultant film. A continuous coverage of the film can be obtained, owing to the formation of sufficient nucleation sites promoted by the incorporation of Br. The variation in morphology versus the concentration of Br is in accordance with the crystallization kinetics observed by the in situ data. For the 20% Br film, mild nucleation leads a compact morphology with very large grains. In contrast, the 33% and higher Br films present compact morphology with much smaller grain sizes due to the relatively faster nucleation process. Notably, when the Br concentration is further increased to 66%, the pinholes appear again. We speculate that too rapid nucleation would restrain the crystal continuous growth and result in pinholes during the film-formation process. Hence, it is suggested that precisely regulating the crystallization kinetics is critical to the resultant film morphology. Figure 4K presents the ultraviolet-visible (UV-vis) absorption spectra of films with different Br concentrations. The absorption edges blue-shift noticeably with the increase of Br, demonstrating its powerful band-gap-tuning ability.38Ono L.K. Juarez-Perez E.J. Qi Y. Progress on perovskite materials and solar cells with mixed cations and halide anions.ACS Appl. Mater. Interfaces. 2017; 9: 30197-30246Crossref PubMed Scopus (304) Google Scholar This is consistent with the appearance of film color (Figure 4L), which gradually turns lighter and becomes semitransparent with the increase of Br. Further investigations on the influence of chlorine doping were performed with in situ GIWAXS. A small amount (2%) of the CsPbICl2 precursor was added to CsPb(I0.67Br0.33)3 precursor, which was denoted as CsPb(I0.67Br0.33)3:Cl. Note that here, CsPbICl2 was used to introduce Cl instead of CsPbCl3, because it is very difficult to dissolve CsCl in the DMF/DMSO solution due to the low solubility. We prepared CsPbICl2 precursor by dissolving CsI and PbCl2 in mixed solvent DMSO/DMF solution and retained the demanded stoichiometric ratio of Cs and Pb. Due to the small amount added, changes in the I/Br ratio are considered negligible, which was confirmed by UV-vis absorption spectra (Figure S6A). Compared with the film without Cl, the perovskite film with Cl presents an earlier launch of nucleation (Figures 5A and 5B ) and a deferred growth rate with higher crystallinity, as indicated by the integral area of the (100) scattering peak (Figure 5C). Two-dimensional (2D) GIWAXS patterns of the film with and