Heterovalent B-Site Co-Alloying Approach for Halide Perovskite

Sep 26, 2017 - Compositional engineering studies often focus on A+ and X– site substitutions, within the ABX3 perovskite structure, due to the relat...
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Heterovalent B-Site Co-Alloying Approach for Halide Perovskite Bandgap Engineering Ke-Zhao Du, Xiaoming Wang, Qiwei Han, Yanfa Yan, and David B. Mitzi ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00824 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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ACS Energy Letters

Heterovalent B-Site Co-Alloying Approach for Halide Perovskite Bandgap Engineering Ke-zhao Du, [a]‡ Xiaoming Wang, [c]‡ Qiwei Han,[b] Yanfa Yan,*[c] David B. Mitzi*[a][b] a

Department of Mechanical Engineering and Materials Science, Duke University, Durham 27708, North Carolina, United States b Department of Chemistry, Duke University, Durham 27708, North Carolina, United States c Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, University of Toledo, Toledo 43606, Ohio, United States

Supporting Information Placeholder ABSTRACT: Compositional engineering, which can enrich the database of prospective materials and offer new or enhanced properties, represents one of the key focal points within halide perovskite research. Compositional engineering studies often focus on A+ and X- site substitutions, within the ABX3 perovskite structure, due to the relative ease of varying these sites. However, alloying on the B site can play a more important role in generating novel properties and decreasing Pb toxicity for Pb-based systems. To date, B site substitution has primarily been confined to single element alloying. Herein, a heterovalent co-alloying strategy for the B site of halide perovskites is proposed. AgIBiIII and AgISbIII are co-alloyed into a host crystal of APbBr3 (A = Cs and methylammonium) leading to a larger range of prospective alloying elements on the perovskite B site. Density-functional theory based first-principles calculations provide a possible rational for the red shift of the bandgap and blue shift of the photoluminescence (PL) in the alloying experiments.

Halide perovskites represent a fascinating and versatile structural family, within which over 50% of the known elements have been incorporated.1 Their representative formula is ABX3, where for example A may be a large monovalent cation (e.g., Cs and methylammonium (MA)), with B as a smaller bivalent metal cation (e.g., Pb and Sn) and X as a halogen anion (e.g., I, Br and Cl). Intensive research has been focused on ABX3 in the field of solar

cells,2 light-emitting diodes3 and other applications.4, 5 The modification of material properties through compositional engineering represents one of the main topics in halide perovskite research, showing two prospective advantages—i.e., firstly, that compositional engineering can produce new structures and properties. For example, Mn alloying can convey opto-magnetic characteristics into the host structure.6 The second interesting direction is to enhance the impressive existing characteristics of the perovskite materials and to compensate for shortcomings. For example, replacing the MA with less volatile Cs can improve the material stability, and using MI and MIII instead of PbII can solve or decrease the toxicity from Pb.7, 8 Nevertheless, achieving a non-toxic and stable halide perovskite for high-performance photovoltaic (PV) devices still represents a key challenge for the PV community. Among the lead-free halide perovskites, double perovskites offer more stability than the SnII- or GeII-based perovskites and, at the same time, have higher electronic and/or structural dimensionality than those of the single-metal BiIII- or SbIII-based systems.9 However, current double perovskites generally offer large, indirect or direct forbidden bandgaps,10-12 which are not suitable for high-performance PV devices. Although some double perovskites appear promising under computational examination,13-15 these systems have not been experimentally realized.16-18 Therefore, additional compositional engineering might be needed to obtain the bandgap characteristics required for efficient PVs. In our previous work, we have reduced the bandgap of the double perovskite, Cs2AgBiBr6, by as much as 0.3 eV through single element (i.e., Sb and In) alloying.19 However, the retained indirect bandgap remains a barrier for effective solar cell application. We therefore question whether an intermediate solution may exist between the lead-free composition of the double perovskites and the direct bandgap nature of the parent halide perovskite, e.g., CsPbX3 and MAPbX3 where X = Cl, Br, I. CsPbBr3 offers strong prospects for tandem solar cells, due to improved phase and thermal stability compared with CsPbI3 and MAPbI3, respectively.20-22 Nevertheless, the large bandgap (e.g., >2.2 eV) renders CsPbBr3 less attractive in terms of light harvesting efficiency and device performance. Most of the compositional engineering approaches to tailor the CsPbBr3 bandgap were carried out by substitution on the Cs and/or Br sites, since Cs and Br sites have smaller activation energies for ionic replacement than for the Pb site.23-26 Only several experiments for single element replacement on the Pb site have been reported, including incorporation of Mn and Sn.27, 28 Alloying on the Pb site can play

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a critical role in property modification of CsPbBr3 and simultaneously decrease the toxicity from the Pb.29 In this work, we explore a heterovalent (i.e., Ag and Bi; Ag and Sb) co-alloying strategy for Pb-site compositional engineering within APbBr3 (A = MA and Cs). Using a coordinated substitution of heterovalent elements onto the divalent B site, this co-alloying strategy offers a new opportunity for simultaneous insertion of monovalent/trivalent metals (maintaining proper charge balance) into ABX3 perovskites, with useful semiconducting character. The synthetic details for the target bulk samples appear in the Experimental Section of the supporting information (SI) and in Table S1. CsPbBr3 crystallizes in the orthorhombic space group Pnma at room temperature. Cs(AgBi)x/2Pb1-xBr3 (x < 18.75%) retains the same structure with obvious powder X-ray diffraction (PXRD) peak shifting relative to the parent x=0 structure (Figure 1a and 1b). When x is ≥ 18.75%, a secondary phase, the double perovskite Cs2AgBiBr6, appears without any PXRD peak shift compared with the simulated Cs2AgBiBr6 PXRD pattern (Figure S1a). In other words, Ag and Bi appear to alloy into the host crystal lattice of CsPbBr3, while Pb does not appear to substitute significantly into the host lattice of Cs2AgBiBr6. Using Pawley fitting (Table S2 and S3), the unit cell variation of Cs(AgBi)x/2Pb1xBr3 is shown in Figure 1c. The b axis for the CsPbBr3 structure falls along the direction of Pb-Br-Pb connectivity. Therefore, when Pb is substituted by Ag and Bi together, the b axis decreases almost linearly with x (for x < 18.75%), in accordance with the smaller radii of BiIII (117 pm) and AgI (129 pm) relative to PbII (133 pm).30 The narrowing difference between a and c axis dimensions suggests a trend toward higher symmetry for AgBi coalloying, which perhaps can be rationalized given the higher symmetry of cubic Cs2AgBiBr6 relative to orthorhombic CsPbBr3.

Figure 1 a) The PXRD of Cs(AgBi)x/2Pb1-xBr3 with different chemical substitution; b) The Cs(AgBi)x/2Pb1-xBr3 structure shown in polyhedral representation; c) The variation of refined crystal lattice parameters as a function of substitution level x in Cs(AgBi)x/2Pb1-xBr3 prepared by solid state synthesis. To explore the importance of co-alloying versus single-element substitution, we also pursued analogous Ag and Bi single-alloying experiments. The maximum homogeneity range in CsAgxPb1-xBr3xVx and Cs(Bi2x/3Vx/3)Pb1-xBr3 (V = vacancy) falls below 6.25% substitution. The secondary phases, CsAgBr2 in CsAg0.0625Pb0.9375Br2.9375V0.0625 and Cs3Bi2Br9 in Cs(Bi0.0417V0.0208)Pb0.9375Br3, can be observed in Figure S1b. As shown in Figure 2a, the split PXRD peaks of unsubstituted CsPbBr3 merge and shift toward higher angle in the co-alloyed com-

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pounds, while those of the single-alloyed compounds stay the same as for the unsubstituted case. This suggests that Ag and Bi can facilitate alloying into the crystal lattice of CsPbBr3 (perhaps because the combined substitution preserves the charge balance of the structure). The absence of PXRD peak shift in Cs(Bi2x/3Vx/3)Pb1-xBr3 is consistent with the report for Bi singlealloyed CsPbBr3 nanocrystals.31

Figure 2 a) Comparison of selected-region PXRD among the compounds, CsAgxPb1-xBr3-xVx, Cs(Bi2x/3Vx/3)Pb1-xBr3, Cs(AgSb)x/2Pb1-xBr3, Cs(AgBi)x/2Pb1-xBr3 and unsubstituted CsPbBr3. The peak at ~38° is a combination of (240), (042), (321) and (123) for CsPbBr3. b) The comparison of PXRD peak (211) between the compounds MA(AgBi)x/2Pb1-xBr3 and MAPbBr3; c) The PL of Cs(AgBi)x/2Pb1-xBr3, measured using a 442 nm He-Cd laser excitation. The filters employed for the PL measurement of the unsubstituted sample and the alloyed products are 0.1% and 25%, respectively. As shown in Figure 3a, c, the bandgap of AgBi co-alloyed CsPbBr3 decreases from 2.28 to 2.09 eV, in agreement with the powder color. For analogous Bi-alloyed CsPbBr3, the bandgap decreases to 2.16 eV (Figure 3b and 3d). In the Ag-alloyed case, the bandgap changes little (Figure S2). Using single crystal X-ray diffraction (SCXRD), we have tried to determine the Bi position in the structure of the red uniform Bi-alloyed (targeted) CsPbBr3 crystal (Figure 3b and S3). The red crystal structure does not change noticeably compared with the yellow non-alloyed CsPbBr3 crystal (Table S4). In spite of the good SCXRD data quality, as shown in Table S4, the Bi position cannot be resolved, similar to the case previously reported for a Bi:CsPbBr3 nanocrystal and Bi:MAPbBr3 bulk crystal.31, 32 This difficulty may arise from the small amount of substituted Bi. In CsPbBr3 nanocrystals, Bi doping has been reported to induce an increased bandgap,31 which is different from the current results for CsPbBr3 bulk materials. The difference may arise from the nanocrystal size (~ 10 nm) and dimensionality effect on the properties. The PL intensities for all of the alloyed products (i.e., Ag-alloyed, Bi-alloyed and AgBi coalloyed) are largely quenched, especially for the AgBi co-alloyed systems (Fig. 2c and S4b), suggesting higher recombination, which would be detrimental for PV application. Note that, for Cs(AgBi)x/2Pb1-xBr3, there is a slight blue-shift of the PL peak with alloying (Fig. 2c), in contrast to the red-shift of the bandgap

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determined from diffuse reflectance (due to band structure effects, as described below, but also possibly impacted by laser heating during measurement). Broad emission (700-800 nm) also becomes evident for the AgBi- and Bi-alloyed systems (Figure 2c and S4b), likely reflecting intrinsic defects within the alloyed semiconductors,33 or minor laser beam damage during measurement.

Figure 3 Direct Tauc plots (from diffuse reflectance) and photos of Cs(AgBi)x/2Pb1-xBr3 (a, c) and Cs(Bi2x/3Vx/3)Pb1-xBr3 (b, d). To confirm the broad applicability of the heterovalent coalloying strategy for halide perovskites, AgSb co-alloyed CsPbBr3 and AgBi co-alloyed MAPbBr3 have been examined. The PXRD shift for AgSb co-alloyed products (e.g., b = 11.728 Å at x = 6.25%) is slightly larger than the analogous shift for the AgBi coalloyed system (b = 11.732 Å at x = 6.25%), as shown in Figure 2a, Figure S5 and Table S2, correlating with the smaller radius for SbIII (90 pm) compared to BiIII (117 pm).30 The bandgap of the AgSb co-alloyed CsPbBr3 decreases slightly (~ 0.03 eV) with substitution, while the PL intensity quenches substantially (Figure S6). Different from the orthorhombic CsPbBr3 versus cubic Cs2AgBiBr6 for AgBi co-alloyed CsPbBr3, MAPbBr3 and MA2AgBiBr6 both have cubic symmetry.34, 35 The PXRD peaks for MA(AgBi)x/2Pb1-xBr3 shift toward higher angle with increasing x (Figure 2b). The secondary phase, MA3Bi2Br9, appears for x ≥ 12.5% (Figure S7). Considering that the PXRD peaks of Bi-doped MAPbBr3 do not exhibit any shift,32 the synergistic effect of Ag and Bi in the co-alloyed MA(AgBi)x/2Pb1-xBr3 (as noted for the other co-alloyed systems) may also exist for this system. The bandgap of MAPbBr3 decreases from 2.24 eV to 2.14 eV after AgBi co-alloying, accompanied by PL quenching (Figure S4a and S8). In all of the co-alloyed samples in the current study, the bandgap measured by solid-state diffuse reflectance red shifts with alloying, while the PL blue shifts (or remains at the same energy) and quenches (Figure S4a, S6b and 2c). To understand this anomalous behavior, we performed first-principles densityfunctional theory (DFT) calculations. Both the Ag-Sb and Ag-Bi co-alloyed perovskites exhibit direct bandgaps, as shown in Figure 4. The incorporation of the Ag atoms slightly modifies the valence band maximum (VBM) of the co-alloyed structures by reducing the contribution of Pb 6s-Br 4p hybridization and adding contribution of Ag 4d-Br 4p hybridization. On the other hand, the Sb 5p and Bi 6p orbitals, with their antibonding character, lower the conduction band minimum (CBM). As a result, the bandgap of the co-alloyed CsPbBr3 decreases. Since the 6p antibonding states are lower in energy than those of 5p, the bandgap reduction for the Ag-Bi co-alloyed perovskite is more obvious compared to AgSb co-alloying—i.e., 0.28 eV versus 0.03 eV for Ag-Bi and AgSb co-alloyed perovskites, respectively, consistent with our experimental results (Figure 3a and S6a). Due to the localized Bi 6p

states (Figure 4e), the Bi 6p-derived CBM (Figure 4c) is flat, though there is a small contribution from Br 4p states. A slight increase of Bi concentration will not lead to obvious Bi-Bi interaction (based on Figure 4e) and, therefore, no obvious energy shift of the Bi-6p derived states is expected. Likewise, the occupied Ag 4d states are also localized. A slight increase of Ag concentration would also not introduce obvious changes to the VBM. As a result, the absorption bandgap of Ag-Bi co-alloyed samples will be relatively insensitive to slight increase of Ag and Bi concentration (for low alloying levels), in agreement with our experimental result (Figure 3a). The slight blue shift of the PL may be rationalized by the localized nature of the Sb and Bi p states in the co-alloyed perovskites (low alloying levels). Both the Sb 5p and Bi 6p bands are relatively flat compared with the Pb 6p bands, as shown in Figure 4, indicating spatial localization. The band-decomposed charge density for the CBM of the Ag-Sb and Ag-Bi co-alloyed perovskites are calculated. As shown in Figure 4d and 4e, charges are confined on the Sb or Bi sites, mixing with the surrounding Br/Pb sites. According to our experimental results, the partially localized Sb and Bi p states in the band structures might trap the excited electrons and dissipate the energy into phonons. As a result, these localized states would not lead to substantial photoluminescence. The PL therefore may still (as for the non-alloyed sample) arise from the delocalized Pb 6p states, which are pushed up due to reduced band width of Pb 6p states. Since the shift of the Pb 6p states is much larger than the corresponding shift of the VBM (Figure 4b and 4c), the PL peak exhibits a blue shift (Figure 2c and S6b). In addition, effects of dual direct-indirect transition, as reported for the bismuth halide perovskite,36 resonant defect states above the band edge,37 and laser heating during the measurement may also play a role in the observed PL blue shift. The PL quenching likely arises from charge trapping by the localized Sb and Bi p states. In conclusion, we have demonstrated a new heterovalent coalloying strategy for compositional engineering on the B site of halide perovskites—e.g., AgBi co-alloyed CsPbBr3, AgSb coalloyed CsPbBr3 and AgBi co-alloyed MAPbBr3. Compared with the invariant PXRD peaks in the single-alloyed compounds, the obvious PXRD peak shift for the co-alloyed products suggests a synergistic effect between the heterovalent Ag and Bi (or Sb), enabling them to enter into the crystal lattice of APbBr3 together. The alloyed products retain a direct bandgap structure with reduced bandgap value. Through first-principles DFT calculations, we find that the bandgap reduction is attributed to a combination of VBM increase induced by the Ag 4d orbital and CBM decrease induced by the Sb 5p or Bi 6p orbitals. The blue shift of the quenched PL may result, at least in part, from the more-localized Sb 5p and Bi 6p states, which dissipate the energy into phonons and push up the Pb 6p states responsible for the observed weak PL (other effects such as dual direct-indirect transition,36 resonant trap states above the band edge37 and laser heating may also play a role). This work significantly extends the known compositional engineering flexibility within the halide perovskite family.

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The information, data, or work presented herein was funded in part by the Office of Energy Efficiency and Renewable Energy (EERE), U.S. Department of Energy, under Award Number DEEE0006712. This work was performed in part at the Duke University Shared Materials Instrumentation Facility (SMIF), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI). This research used the resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC0205CH11231. All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and views of DOE or NSF. See the Supporting Information section of this article for more information.

REFERENCES

Figure 4 The band structure of (a) CsPbBr3, (b) Cs(AgSb)0.0625Pb0.875Br3 and (c) Cs(AgBi)0.0625Pb0.875Br3, calculated by MBJ functional with spin-orbit interactions. Different colors denote the different orbital contributions (as noted in the figure). The band structures are aligned with respect to the Cs 5s states for easy comparison. Band decomposed charge density for the conduction band minimum of (d) Cs(AgSb)0.0625Pb0.875Br3 and (e) Cs(AgBi)0.0625Pb0.875Br3. The green isosurface shows the charge density. The black, brown, yellow, blue and magenta balls denote the Pb, Br, Ag, Sb and Bi atoms, respectively. The Cs atoms are omitted for clarity.

ASSOCIATED CONTENT Supporting Information The details of the experiments, instruments, summary of the reactions, the PXRD refinement parameters, the SCXRD data and structure refinement of the red Bi-alloyed CsPbBr3 crystal, direct Tauc plots, PXRD and PL of the other compounds are supplied as Supporting Information.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Author Contributions ‡These authors contributed equally. Kezhao Du conducted all the experiments. Xiaoming Wang performed the computational work. Qiwei Han provided help on the PL measurement. All the authors discussed and commented on the manuscript. Prof. David B. Mitzi and Prof. Yanfa Yan supervised all the experiments, computation, discussion and manuscript revision.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

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Perovskite Nanocrystals through Cation Exchange. J. Am. Chem. Soc. 2017, 139, 4087-4097. (29) Eperon, G. E.; Ginger, D. S. B-Site Metal Cation Exchange in Halide Perovskites. ACS Energy Lett. 2017, 2, 1190-1196. (30) Shannon, R. t. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr. Sect. A 1976, 32, 751-767. (31) Begum, R.; Parida, M. R.; Abdelhady, A. L.; Murali, B.; Alyami, N. M.; Ahmed, G. H.; Hedhili, M. N.; Bakr, O. M.; Mohammed, O. F. Engineering Interfacial Charge Transfer in CsPbBr3 Perovskite Nanocrystals by Heterovalent Doping. J. Am. Chem. Soc. 2017, 139, 731737. (32) Abdelhady, A. L.; Saidaminov, M. I.; Murali, B.; Adinolfi, V.; Voznyy, O.; Katsiev, K.; Alarousu, E.; Comin, R.; Dursun, I.; Sinatra, L.; Sargent, E. H.; Mohammed, O. F.; Bakr, O. M. Heterovalent Dopant Incorporation for Bandgap and Type Engineering of Perovskite Crystals. J. Phys. Chem. Lett. 2016, 7, 295-301. (33) Nitsch, K.; Hamplová, V.; Nikl, M.; Polák, K.; Rodová, M. Lead Bromide and Ternary Alkali Lead Bromide Single Crystals--Growth and Emission Properties. Chem. Phys. Lett. 1996, 258, 518-522. (34) Wei, F.; Deng, Z.; Sun, S.; Zhang, F.; Evans, D. M.; Kieslich, G.; Tominaka, S.; Carpenter, M. A.; Zhang, J.; Bristowe, P. D.; Cheetham, A. K. Synthesis and Properties of a Lead-Free Hybrid Double Perovskite: (CH3NH3)2AgBiBr6. Chem. Mater. 2017, 29, 1089-1094. (35) Weber, D. CH3NH3PbX3, ein Pb (II)-System mit Kubischer Perowskitstruktur/CH3NH3PbX3, a Pb (II)-System with Cubic Perovskite Structure. Z. Naturforsch. B 1978, 33, 1443-1445. (36) Zhang, Y.; Yin, J.; Parida, M. R.; Ahmed, G. H.; Pan, J.; Bakr, O. M.; Bredas, J. L.; Mohammed, O. F. Direct-Indirect Nature of the Bandgap in Lead-Free Perovskite Nanocrystals. J. Phys. Chem. Lett. 2017, 8, 31733177. (37) Sebastian, M.; Peters, J. A.; Stoumpos, C. C.; Im, J.; Kostina, S. S.; Liu, Z.; Kanatzidis, M. G.; Freeman, A. J.; Wessels, B. W. Phys. Rev. B 2015, 92, 235210.

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