This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Letter http://pubs.acs.org/journal/aelccp Exploiting the Lability of Metal Halide Perovskites for Doping Semiconductor Nanocomposites Mariano Calcabrini, Aziz Genç, Yu Liu, Tobias Kleinhanns, Seungho Lee, Dmitry N. Dirin, Quinten A. Akkerman, Maksym V. Kovalenko, Jordi Arbiol, and Maria Ibáñez* Cite This: ACS Energy Lett. 2021, 6, 581−587 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Cesium lead halides have intrinsically unstable crystal lattices and easily transform within perovskite and nonperovskite structures. In this work, we explore the conversion of the perovskite CsPbBr3 into Cs4PbBr6 in the presence of PbS at 450 °C to produce doped nanocrystal-based composites with embedded Cs4PbBr6 nano- precipitates. We show that PbBr2 is extracted from CsPbBr3 and diffuses into the PbS lattice with a consequent increase in the concentration of free charge carriers. This new doping strategy enables the adjustment of the density of charge carriers between 1019 and 1020 cm−3, and it may serve as a general strategy for doping other nanocrystal-based semiconductors. − R esearch on metal halide perovskites is advancing rapidly, owing to the compelling electronic, optical and structural properties of these ionic semiconduc- tors, such as long diffusion lengths and carrier lifetimes,1−3 low does not contain [PbX6]4 octahedra, CsPb2X5, can be described as layers (2D) of [Pb2X5] ̅ clusters separated by Cs+ ions.13,15−17 Because of the large difference in the involved atoms’ electronegativities, all these cesium lead halides exhibit exciton binding energies,4 low number of trap states despite mixed bonding nature. The lead halide framework is the high concentration of vacancies,1,5 composition-tunable dominated by covalent bonds and balanced by ionically bandgap,6,7 and ease of processability.8−11 Because of the bound Cs+ cations. The lack of covalency between Cs+ and the relatively labile crystal structure of metal halide perovskites, the anionic units enables crystal lability.18 As a direct consequence, understanding and control of the chemical and structural the ions move easily and allow transformation between the transformations that these compounds readily undergo are different structuresprovided that the stoichiometry is some of the most pressing questions.12 compensated. Cesium lead halides can adopt perovskite and nonperovskite Very common are the transformations between CsPbBr3 and structures with different dimensionalities. Perovskites are − Cs4PbBr6 structures especially in nanocrystals (NCs). The composed of Cs+ cations stabilizing [PbX6]4 octahedra in a transformation of Cs4PbBr6 into CsPbBr3 is achieved by cubic or, upon slight distortion, tetragonal or orthorhombic adding PbBr2 to the structure,19 extracting CsBr by chemical − phases, where all the corners of the [PbX6]4 octahedra are complexation or intercalation,20 or diffusion and dissolution of shared. 13 All these compounds adopt regular CsPbX 3 CsBr in water.21 The chemical transformation from CsPbBr3 to stoichiometry (3D). Nonperovskite structures include poly- Cs4PbBr6 can be induced by removing PbBr2 from the crystal morphs with various stoichiometries, but all of them lose the lattice by its complexation with thiols or amines.22−24 In all corner-sharing motif in the lattice. The first example of these structures is a polymorph that despite having the same CsPbX3 Received: November 24, 2020 stoichiometry crystallizes in an orthorhombic phase (δ-phase) Accepted: January 11, 2021 with chains of edge-sharing octahedra (1D).14 Another Published: January 21, 2021 example is the lead-depleted Cs4PbX6 structure where the − [PbX6]4 octahedra are isolated (0D).13,15−17 A third closely related nonperovskite structure that has lower Cs+ content and © 2021 American Chemical Society https://dx.doi.org/10.1021/acsenergylett.0c02448 581 ACS Energy Lett. 2021, 6, 581−587 ACS Energy Letters http://pubs.acs.org/journal/aelccp Letter these reactions purification of the NCs is required to separate the undesired byproducts: either CsBr or PbBr2. PbS NC-based solids find application in various fields including transistors,25−28 solar cells,29−32 photodetec- tors,31,33,34 and thermoelectrics.35−37 In most of these applications adjusting the number of free charge carriers to control charge transport is crucial. The conventional n-type dopants for bulk lead chalcogenides are halides. Incorporation of halide ions (X= Cl−, Br−, I−) in the chalcogenide (Y= S2−, Se2−, Te2−) sublattice results in the addition of one electron in the conduction band per halide to compensate for the different valency of halides and chalcogenides.38 Strategies to dope bulk semiconductors require control over the composition at the impurity level; the straightforward translation into doping bottom-up assembled NC solids is to use doped NCs. However, impurity doping of NCs is energetically39 and kinetically unfavorable, because the diffusion path of impurity atoms to the surface is short.40 Although tuning NCs composition has been attempted,26,41 the introduction of a controlled amount of impurities in small structures is problematic for the preparation of heavily doped semi- conductors.42−44 Alternative approaches to dope NC-based solids have focused on changing the NC surface chem- Figure 1. TEM images of the used NCs and scheme of the bottom- istry,30,36,38,45−48 inducing partial cation exchange,49,50 and up assembly process to produce doped PbS−Cs4PbBr6 nano- composites. The nanocomposites are prepared by blending the blending with other NCs.35,51,52 NCs in solution, annealing the dried NCs at 450 °C to remove the PbY−perovskite nanocomposites have been used in photo- organic ligands and induce the chemical transformation between detectors,53,54 LEDs,55,56 and photovoltaics.57,58 Remarkably, CsPbBr3 and Cs4PbBr6, and finally pressing the annealed powder PbS NC inclusions can provide stability to the cubic phase of at 45 MPa and 500 °C for 5 min to produce the pellets. perovskites suppressing the cubic−orthorhombic phase tran- sition,59 and perovskite passivation layers enhance charge carrier separation in PbS NCs by providing adequate energy To follow the chemical transformation of the perovskites, we alignment.60 Furthermore, PbY nanocomposites with secon- performed in situ temperature-dependent X-ray diffraction dary phases might be of great importance for thermo- (XRD) measurements of a mixture of PbS NCs and 30% wt. electrics.35,61 CsPbBr3 NCs (Figures 2 and S1). This mixture was heated to Herein we propose a strategy to produce doped PbS 450 °C and kept at that temperature for 60 min, mimicking the nanocomposites that makes use of the byproduct of the annealing step. Upon heating to 150 °C, CsPbBr3 NCs show transformation of CsPbBr3 into Cs4PbBr6 and PbBr2, upon strong sharpening of the reflections explained by accelerated heating a mixture of CsPbBr3 and PbS NCs (Scheme 1). Our ion migration between particles causing grain growth.62−64 approach simultaneously introduces dopant ions (Br )̅ and a After reaching 450 °C, CsPbBr3 reflections progressively lose secondary phase (Cs4PbBr6) in PbS nanocomposites (Figure intensity, and peaks corresponding to Cs4PbBr6 become 1). visible, shifted to lower angles because of thermal expansion We prepared nanocomposites by mixing PbS NCs with a of the lattice. This experiment demonstrates that CsPbBr3 controlled amount of CsPbBr3 NCs in toluene. This NCs converts to Cs4PbBr6, necessarily releasing PbBr2. This blend was dried under vacuum and annealed with forming gas reaction could be enhanced by the binding affinity of PbBr2 (5% H2 in N2, 1 bar) to yield a powder that was then pressed to PbS surface dangling bonds.65,66 Control experiments with into pellets with relative densities of ∼92% (Table S1) using CsPbBr3 and Cs4PbBr6 pure phases showed that both phases spark plasma sintering (Figure 1). are recovered after heating to 500 °C and cooling to room temperature and do not undergo any transformation on their Scheme 1. Chemical Transformation of CsPbBr3 into own (Figures S9 and S10). Besides, annealing induces crystal Cs4PbBr6 Triggered by the Dissolution of PbBr2 in PbS grain growth in pure PbS NCs, evidenced by the narrowing of the XRD reflections and scanning electron microscopy (SEM) images (Figures S2 and S5). The addition of CsPbBr3 enhances the growth of the PbS crystal domains leading to even narrower reflections (Figures S3 and S4 and Tables S3 and S4). This enhancement can be explained by the presence of PbBr2, a phase with high solubility in PbS67 and a large difference in melting point, which lowers the activation energy for diffusion.68−70 Because no reflections associated with PbBr2 are visible, we considered the possibility of other phases in the PbS−PbBr2 phase diagram being formed. Still, Pb7S2Br10, the only stable phase, was not observed in the diffraction patterns.67 Recently, the metastable phase Pb4S3Br2 was reported in NCs; this phase is not present in the XRD 582 https://dx.doi.org/10.1021/acsenergylett.0c02448 ACS Energy Lett. 2021, 6, 581−587 ACS Energy Letters http://pubs.acs.org/journal/aelccp Letter Figure 3. (a) Electrical conductivity and (b) Seebeck coefficient of the nanocomposites with increasing starting CsPbBr3 concen- tration: 0% (black squares), 0.25% (red circles), 0.5% (blue triangles), 1% (purple triangles), 2% (green triangles), and 3% (dark blue triangles). Figure 2. X-ray diffraction patterns of a mixture of PbS NCs and CsPbBr3 NCs at different temperatures together with the reference electrical conductivity increases with the starting concentration patterns for the different crystal phases. In the 450 °C patterns, t of CsPbBr3, proving the doping effect of the transformation indicates the time held at 450 °C before measurement. The patterns are overlapped with a 2D intensity plot to help to identify (Supporting Information, page S8).72 Besides, the negative low intensity reflections (PbS PDF 00-002-0699, CsPbBr3 PDF 00- sign of the Seebeck coefficient confirms the n-type character of 054-0753, Cs4PbBr6 PDF 01-075-0412). the obtained nanocomposites. These results are consistent with the charge carrier concentrations measured by Hall effect (Table S2), which correspond exactly to one-half of the total patterns, and we disregarded it because of the high Br concentration, as expected from the reaction in Scheme 1. temperature and long reaction time, which are not compatible The doping effect of CsPbBr3 is independent of the use of with such a kinetically stabilized phase.71 Previous studies NCs; ground perovskite crystals react with PbS in the same showed pure CsPbBr3 NCs undergo partial transformation to way. However, NCs provide a much shorter ion diffusion path the lead-rich CsPb2Br5; this phase is not present in our and better mixing with PbS, which lead to faster kinetics experiments.64 These observations suggest that bromide (Supporting Information, page S10). substitutes sulfide in PbS, doping the matrix as expressed in The observed tendencies in the electrical conductivity of the chemical reactions in Scheme 1. nanostructured materials are a trade-off between many effects. To test the doping efficiency, we prepared pellets with the On the one hand, the thermal excitation of carriers through proper nominal CsPbBr3 concentrations to achieve charge energy barriers (grain boundaries) leads to an increase of the carrier concentrations between 1019 and 1020 cm−3, which is conductivity with temperature. This effect dominates at low close to the optimal charge carrier concentration for thermo- temperatures in samples with smaller grains. On the other electrics.72,73 The molar amount of Br introduced is referred to hand, an increase in temperature enhances charge carrier the amount of S, X% = 0.25, 0.50, 1, 2, 3 (see the Supporting scattering. This is the typical behavior of heavily doped Information, page S1). semiconductors and metals. Additionally, as the temperature Panels a and b of Figure 3 show the electrical conductivity increases, thermal excitation of minority carriers (bipolar and Seebeck coefficient of the samples between room effect) occurs. Consequently, the absolute value of the Seebeck temperature and 900 K, respectively. At low temperatures, coefficient starts decreasing. The higher the doping level, the the absolute value of the Seebeck coefficient decreases and the higher the temperature at which this effect is perceptible.61 583 https://dx.doi.org/10.1021/acsenergylett.0c02448 ACS Energy Lett. 2021, 6, 581−587 ACS Energy Letters http://pubs.acs.org/journal/aelccp Letter The XRD patterns of the produced pellets show neither of the matrix and the precipitates, along with satellite spots of CsPbBr3 nor Cs4PbBr6 reflections (Figure S6), which we the Moiré fringes, which are formed because of orientation associate with the low content of perovskite required for the differences between the matrix and precipitates. Inverse doping level targeted. To demonstrate that the pelletized Fourier (frequency) filtering of the power spectrum allowed material contains Cs4PbBr6, we evaluated the microscopic mapping the matrix and the nanodomains. The matrix phase structure of the bulk nanocomposites (Figure 4) by high- (in red) is identified as FCC PbS (space group Fm3̅m) with a resolution transmission electron microscopy (HRTEM). A lattice parameter a = 0.5936 nm visualized along its [011] axis. 0.5% pellet was thinned to electron transparency by Ar+ The nanodomain (in green) corresponds to the same PbS polishing to produce a self-suspended lamella. Figure 4d phase visualized along the [001] zone axis. We performed the shows an HRTEM micrograph obtained from a large PbS grain same analysis on the precipitate marked in Figure 4e. The (defined as the matrix) containing multiple precipitates. The diffraction spots observed in the power spectrum correspond power spectrum of the highlighted area shows diffraction spots to the matrix phase (in red) visualized along its [011] axis. The spots of the precipitate correspond to the hexagonal Cs4PbBr6 phase (space group R3̅c:H) with lattice parameters a = 1.373 nm and c = 1.732 nm, which is visualized along the [11̅02] zone axis. Because of the difficulty in analyzing all the precipitates crystallographically, to evaluate their nature we performed energy dispersion X-ray line scans across many precipitates. We found that they show no difference in composition with the matrix (Figure S7), proving that the vast majority of precipitates are PbS. The scans also show a small content of Br evenly distributed through the nano- composites as expected in its role as a dopant. In short, HRTEM revealed that the doped samples are composed of a PbS matrix with a large amount of PbS nanodomains and randomly distributed Cs4PbBr6 nanoprecipitates (Figure 4), confirming the chemical transformation observed by XRD. In summary, we showed that CsPbBr3 and PbS react to give the lead depleted Cs4PbBr6 phase. The byproduct of the transformation, PbBr2, dissolves in the PbS matrix, leading to doped nanocomposites and inducing grain growth while Cs4PbBr6 forms nanoprecipitates. We made use of this transformation to simultaneously dope and introduce a secondary phase in n-type PbS nanocomposites. These results validate this new doping strategy for nanocomposites based on NC blending that involves the reaction of one of the components. This approach is not limited to the preparation of bulk samples at high pressures but could also be applied to the preparation of films or other nanocomposites because the reaction that leads to doping is also demonstrated at ambient pressure. Finally, our results reassess the intrinsic instability of metal halide perovskites and are therefore of high relevance for applications where these materials are exposed to high energy densities. ■ ASSOCIATED CONTENT * Supporting Information sı The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.0c02448. Figure 4. Structural analysis of a PbS−Cs4PbBr6 nanocomposite. Chemicals, syntheses and preparation of the pellets. The TEM sample is a self-suspended lamella prepared by Ar+ ion Details on the structural characterization and transport milling. (a) Photograph of the pellets used to measure the measurements (PDF) electrical conductivity and the Seebeck coefficient; (b) HRTEM micrograph and (c) low-magnification high-angle annular dark field scanning transmission microscopy (HAADF-STEM) micro- graph revealing the presence of small precipitates along the grains; (d) PbS nanodomains, detail of the white squared area and its ■ AUTHOR INFORMATION Corresponding Author corresponding power spectrum and structural maps of the PbS Maria Ibáñez − Institute of Science and Technology Austria, matrix (in red) and nanodomains (in green) along with the applied Klosterneuburg 3400, Austria; orcid.org/0000-0001- maps for inverse Fourier filtering; (e) detail of a Cs4PbBr6 5013-2843; Email: mibanez@ist.ac.at nanoprecipitate and its corresponding power spectrum and structural maps of the PbS matrix (in red) and a Cs4PbBr6 Authors precipitate (in green) are shown along with the applied maps for Mariano Calcabrini − Institute of Science and Technology inverse Fourier filtering. 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Metal Halide Perovskites for Light-Emitting Diodes. Nat. Mater. Science and Technology, Zurich CH-8600, Switzerland; 2021, 20, 10−21. (9) Noculak, A.; Noculak, A.; Morad, V.; Morad, V.; McCall, K. M.; orcid.org/0000-0002-6396-8938 Yakunin, S.; Shynkarenko, Y.; Yakunin, S.; Shynkarenko, Y.; Wörle, Jordi Arbiol − Catalan Institute of Nanoscience and M.; Kovalenko, M. V. Bright Blue and Green Luminescence of Sb(III) Nanotechnology (ICN2), CSIC and BIST, Campus UAB, in Double Perovskite Cs2MInCl6 (M = Na, K) Matrices. Chem. Mater. Bellaterra, Barcelona 08193, Catalonia, Spain; ICREA, 2020, 32 (12), 5118−5124. Barcelona 08010, Catalonia, Spain; orcid.org/0000- (10) Jena, A. K.; Kulkarni, A.; Miyasaka, T. Halide Perovskite 0002-0695-1726 Photovoltaics: Background, Status, and Future Prospects. Chem. Rev. Complete contact information is available at: 2019, 119 (5), 3036−3103. (11) Brenner, P.; Bar-On, O.; Jakoby, M.; Allegro, I.; Richards, B. S.; https://pubs.acs.org/10.1021/acsenergylett.0c02448 Paetzold, U. W.; Howard, I. A.; Scheuer, J.; Lemmer, U. Continuous Wave Amplified Spontaneous Emission in Phase-Stable Lead Halide Notes Perovskites. Nat. Commun. 2019, 10 (1), 1−7. The authors declare no competing financial interest. ■ (12) Akkerman, Q. A.; Rainò, G.; Kovalenko, M. V.; Manna, L. Genesis, Challenges and Opportunities for Colloidal Lead Halide ACKNOWLEDGMENTS Perovskite Nanocrystals. Nat. Mater. 2018, 17 (5), 394−405. M.C. has received funding from the European Union’s Horizon (13) Bertolotti, F.; Protesescu, L.; Kovalenko, M. V.; Yakunin, S.; Cervellino, A.; Billinge, S. J. L.; Terban, M. W.; Pedersen, J. S.; 2020 research and innovation programme under the Marie Masciocchi, N.; Guagliardi, A. Coherent Nanotwins and Dynamic Skłodowska-Curie Grant Agreement No. 665385. ICN2 Disorder in Cesium Lead Halide Perovskite Nanocrystals. ACS Nano acknowledges funding from Generalitat de Catalunya 2017 2017, 11 (4), 3819−3831. SGR 327. ICN2 is supported by the Severo Ochoa program (14) Aebli, M.; Benin, B. M.; McCall, K. M.; Morad, V.; Thöny, D.; from Spanish MINECO (Grant No. SEV-2017-0706) and is Grützmacher, H.; Kovalenko, M. V. White CsPbBr3: Characterizing funded by the CERCA Programme/Generalitat de Catalunya. the One-Dimensional Cesium Lead Bromide Polymorph. Helv. Chim. This project has received funding from the European Union’s Acta 2020, 103 (7), e2000080. Horizon 2020 research and innovation programme under grant (15) Piveteau, L.; Aebli, M.; Yazdani, N.; Millen, M.; Korosec, L.; agreement No 823717 − ESTEEM3. M.V.K. acknowledges the Krieg, F.; Benin, B. M.; Morad, V.; Piveteau, C.; Shiroka, T.; Comas- support by the European Research Council under the Horizon Vives, A.; Copéret, C.; Lindenberg, A. M.; Wood, V.; Verel, R.; 2020 Framework Program (ERC Consolidator Grant SCALE- Kovalenko, M. V. Bulk and Nanocrystalline Cesium Lead-Halide Perovskites as Seen by Halide Magnetic Resonance. ACS Cent. Sci. HALO Grant Agreement No. 819740) and by FET-OPEN 2020, 6 (7), 1138−1149. project no. 862656 (DROP-IT). ■ (16) Yin, J.; Maity, P.; De Bastiani, M.; Dursun, I.; Bakr, O. M.; Brédas, J. L.; Mohammed, O. F. Molecular Behavior of Zero- REFERENCES Dimensional Perovskites. Sci. Adv. 2017, 3 (12), e1701793. (1) Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; (17) Akkerman, Q. A.; Manna, L. What Defines a Halide Perovskite? Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; Losovyj, Y.; ACS Energy Lett. 2020, 5 (2), 604−610. Zhang, X.; Dowben, P. A.; Mohammed, O. F.; Sargent, E. H.; Bakr, O. (18) Kovalenko, M. V.; Protesescu, L.; Bodnarchuk, M. I. Properties M. Low Trap-State Density and Long Carrier Diffusion in Organolead and Potential Optoelectronic Applications of Lead Halide Perovskite Trihalide Perovskite Single Crystals. 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