Emissive Bi-Doped Double Perovskite Cs2Ag1âxNaxInCl6

Subscriber access provided by GUILFORD COLLEGE

Letter

Emissive Bi-doped Double Perovskite Cs2Ag1-xNaxInCl6 Nanocrystals Federico Locardi, Emanuela Sartori, Joka Buha, Juliette Zito, Mirko Prato, Valerio Pinchetti, Matteo Luca Zaffalon, Maurizio Ferretti, Sergio Brovelli, Ivan Infante, Luca De Trizio, and Liberato Manna ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b01274 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Emissive Bi-doped Double Perovskite Cs2Ag1-xNaxInCl6 Nanocrystals Federico Locardia,b*, Emanuela Sartoria,b, Joka Buhab, Juliette Zitob, Mirko Pratoc, Valerio Pinchettid, Matteo L. Zaffalond, Maurizio Ferrettia, Sergio Brovellid*, Ivan Infanteb,e*, Luca De Triziob*, Liberato Mannab* aDipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova, Via Dodecaneso 31, 16146 Genova, Italy bNanochemistry Department and cMaterials Characterization Facility, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova,

Italy dDipartimento di Scienza dei Materiali, Università degli Studi di Milano-Bicocca, via R. Cozzi 55, 20125 Milano, Italy eDepartment of Theoretical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands ABSTRACT: We report the composition-dependent optical properties of Bidoped Cs2Ag1-xNaxInCl6 nanocrystals (NCs) having a double perovskite crystal structure. Their photoluminescence (PL) was characterized by large Stokesshifts and the PL quantum yield increased with the amount of Na up to ~22% for the Cs2Ag0.4Na0.6InCl6 stoichiometry. The presence of Bi3+ dopants was crucial to achieve high PLQYs, as non-doped NC systems were not emissive. Density Functional Theory calculations revealed that the substitution of Ag+ with Na+ leads to a localization of AgCl6 energy levels above the valence band maximum, whereas doping with Bi3+ creates BiCl6 states below the conduction band minimum. As such, the PL emission stems from a trapped emission between states localized in the BiCl6 and AgCl6 octahedra, respectively. Our findings indicated that both the partial replacement of Ag+ with Na+ ions and doping with Bi3+ cations are essential in order to optimize the PL emission of these systems.

In the last decade, lead halide perovskite (LHP) APbX3 (A = CH3NH3+, Cs+; X = Cl-, Br-, I-) materials have been widely studied for their excellent optoelectronic properties, which are particularly suitable for photodetectors, solar cells and light emitting diodes.1–3 Unfortunately, these materials suffer from two major drawbacks, namely the toxicity of Pb and their poor stability against degradation induced by heating, moisture and device operation conditions.4,5 Among the procedures to replace lead with less toxic elements, the heterovalent substitution of Pb2+ ions with a combination of a monovalent (B+) and a trivalent (B3+) metal cation is one of the most promising strategies.4,5 The resulting compounds, typically referred to as double perovskites (DPs) or elpasolites, are characterized by a 3D network of alternating [B+X6] and [B3+X6] corner-sharing octahedra, with an overall A2B(I)B(III)X6 stoichiometry.6 Although many elpasolite materials have been known for their ferroelectric properties since the sixties, only recently interesting optoelectronic features have been discovered in DP compounds, comprising Cs2AgBiX6,7,8 Cs2AgInX6 9–11 and Cs2AgSbCl6.10,12–14 Unfortunately, their light emission efficiency is still lower than that of LHPs, mainly because they are characterized by either indirect band gaps8,15 or, in the case of direct bandgap systems, by weakly emissive parity forbidden optical transitions.16–19 To enhance the photoluminescence (PL) efficiency of these compounds, different doping strategies, using Cu2+,20 Mn2+ 18,21–24 and lanthanide24,25 ions, have been explored. Alternative approaches, mainly aimed at engineering the bandgap of DPs, involve the synthesis of quinary

compounds by employing a combination of B+ or B3+ ions. For example, the use of both In3+ and Bi3+ cations led to the formation of Cs2AgIn1-xBixBr6 15 and Cs2NaIn1-xBixCl6 22 compounds with a direct bandgap. The inclusion of Na+ and a minimal amount of Bi3+ ions in the Cs2AgInCl6 lattice was shown to reduce the crystal symmetry of the system, which is suggested to make the transition of emission parity allowed.9,14,15,26 As reported by Luo et al., Bi-doped Cs2Ag0.60Na0.40InCl6 powders exhibit a record PL quantum yield (QY) as high as ΦPL~86%,27 representing, thus, promising candidate materials for both down-conversion and solar harvesting devices. Motivated by the work of Luo et al.27 we have extended the synthesis of Bi-doped Cs2Ag1-xNaxInCl6 to the nanoscale, with the aim of studying such materials in the form of colloidal nanocrystals (NCs). We have therefore developed a colloidal approach to prepare, under air, Bi-doped Cs2Ag1-xNaxInCl6 NCs with a mean size of 10nm, a tunable Na content (with x ranging from 0 to 1) and a fixed amount of Bi dopants (0.5 at.% in respect to In) (Scheme 1). The NCs featured a broad PL emission peaked at about 1.8 eV, a large Stokes-shift (on average 1.7eV), and the PLQY depended on the stoichiometry of the NCs: it increased with the amount of Na+ up x=0.6 (i.e. Cs2Ag0.4Na0.6InCl6), and subsequently decreased by a further inclusion of Na+ (x ≥ 0.7). Also, when no Bi was employed in the synthesis of Cs2Ag1-xNaxInCl6 NCs, no PL emission was observed. DFT calculations revealed that Bi3+ and Ag+ ions in Narich DP systems act as centers that can localize electrons and holes,

ACS Paragon Plus Environment

ACS Energy Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

respectively, at the band edges. Under these conditions, trapped excitons can recombine via the BiCl6AgCl6 transition, leading to an efficient PL emission. Scheme 1. Colloidal Synthesis of Bi-doped Cs2Ag1-xNaxInCl6 (0≤x≤ 1) NC samples.

For the synthesis of the NCs, Cs2CO3 and metal carboxylate precursors (Na, Ag, In and Bi acetates) were mixed with oleic acid and oleylamine in dioctyl ether (see the Supporting Information, SI, for synthesis details). Then, the mixture was heated up to 140 °C and benzoyl chloride was swiftly injected to start nucleation and growth of the NCs. The composition of the Cs2Ag1-xNaxInCl6 NCs was modulated by systematically changing the Na/Ag precursors ratio from 0 to 1, while keeping the molar ratio of the Bi precursor fixed at 0.5% with respect to In. All syntheses were carried out in air.

The elemental analysis, performed via both X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) energy dispersive spectroscopy (EDS), revealed that the composition of the NC samples followed closely the Na/Ag precursors molar ratios employed in the syntheses (Table S1 and Figures S1 and S2). Furthermore, the XPS analysis confirmed that all the elements were in the expected oxidation states (i.e. Cs+, Ag+, Na+, In3+, Cl-) (Figure S1). Interestingly, Cs/In ratios measured by XPS are higher than those detected by SEM-EDS, suggesting that our DP NCs have a Cs-rich surface (Table S1), presumably due to a combination of Cs(oleate) and CsCl terminations. To measure the amount of Bi in our samples, which was below the detection limit of both XPS and SEM-EDS characterizations, we employed inductively coupled plasma mass spectrometry (ICP-MS) analysis, which provided Bi/In ratios ranging from 0.22% to 1%, consistent with the amount of Bi precursor employed in our syntheses (Table S1). The samples were characterized by X-ray diffraction (XRD) which indicated that the NCs crystallized in a cubic Fm-3m double perovskite structure, compatible with that of bulk Cs2AgInCl6 (ICSD number 244519, a=10.481 Å), without the presence of secondary phases (Figure 1a).

Figure 1. (a) XRD patterns of Bi-doped Cs2Ag1-xNaxInCl6 NCs with the corresponding reflections of bulk Cs2AgInCl6 (ICSD number 244519). (b-e) TEM micrographs of Bi-doped Cs2Ag1-xNaxInCl6 NCs. The scale bars are 50nm. (f) High resolution TEM image of the Cs2.4Ag0.4Na0.6InCl5.6 NCs sample (x=0.6) with the fast Fourier transform (FFT) from one of the NCs in its [001] zone axis given in inset. (g) Energy dispersive X-ray (EDX) elemental maps showing the distribution of Cs, Ag, Na, In and Cl in the NCs.


Page 2 of 7

Page 3 of 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters As extracted from the experimental XRD patterns, the increase in the Na content led to a small lattice expansion, with the lattice parameters increasing from 10.50 Å for Cs2AgInCl6 NCs, up to 10.54 Å for Cs2NaInCl6 NCs, the latter matching with the bulk value reported in the literature.27–29 These results suggest that Na+ cations randomly substitutes Ag+ cations at the B+ sites of the DP structure. This is not surprising considering that Cs2AgInCl6 and Cs2NaInCl6 materials have the same crystal structure and a low lattice mismatch (0.6%). Also, similarly to what observed by Luo et al.,27 the intensity of the (111) peak at ~14°, which is directly related to the Na/Ag composition through the dispersion factor of the Na, Ag and In atoms, systematically increased with the increase in the Na content, further suggesting the formation of alloyed Cs2Ag1-xNaxCl6 NCs. Transmission electron microscopy (TEM) micrographs of Cs2Ag1xNaxInCl6 NCs evidenced the formation of NCs with a cuboidal shape and a mean size of 9-10nm when employing an amount of Na up to 70 % (x = 0.7) and around 12-13nm when further increasing the Na content (i.e. 0.8 ≤ x ≤ 1) (Figure 1b-e and Figure S3). On the other hand, control experiments, performed at different reaction temperatures (i.e. 120°C, 160°C and 180°C) resulted in polydisperse NC samples (with the presence of secondary phases when working at 120°C) (Figure S4). The HRTEM analysis of a representative sample, that is Cs2Ag0.4Na0.6InCl6 NCs, revealed that the NCs were monocrystalline, with no evident structural defects (Figure 1f and Figures S5 and S6). The cubes were oriented in their respective [001] zone axes and were faceted by the {001} type planes (Figure 1f). The electron diffraction pattern was also indexed according to the DP structure of Cs2AgInCl6 (ICSD 244519) (Figure S7), while

according to the EDX elemental maps all elements were uniformly distributed within the NCs (Figure 1g). In order to investigate the electronic structure of our complex systems, and to understand the effects of both Bi-doping and of alloying Cs2AgInCl6 and Cs2NaInCl6 materials, we performed DFT calculations. It is known that bulk Cs2AgInCl6 and Cs2NaInCl6 band structures exhibit a parity forbidden direct band gap at the  point, while Cs2AgBiCl6 presents an indirect band gap.16 As a result, the lowest edge-to-edge transition for all systems has a negligible oscillator strength, indicating a propensity to low PL emission efficiencies, which is in contrast with the observed ΦPL-values found in bulk alloyed Ag/Na DP doped with Bi (ΦPL~85%).27 In our computational models, we decided to reproduce the experimental observations by starting from a bulk Cs2NaInCl6 cubic cell, which was then doped with one Bi3+ ion (replacing one In3+ ion) and with an increasing amount of Ag+ ions (replacing Na+ ions). This was done to help us in rationalizing the role of both Ag and Bi in the electronic structure of our DP system. We also decided to choose a large 2x2x2 supercell ensuring that: (1) the main electronic information is always stored at the  point, and (2) that in this supercell the Bi doping is 3%, not far from the experimental value (0.5%). All calculations were done with the CP2k package30 (see SI for further details). We started our analysis by looking at the density of states (DOS) of the Cs2NaInCl6 system. As shown in other works16, this system presents states at the conduction band minimum (CBM) that are mostly localized on the 5s orbitals of the In3+ and states at the valence band maximum (VBM) that are localized on the 3p orbitals of chlorine (Figure 2a).

Figure 2. (a) Electronic structure of Cs2NaInCl6, Cs2NaInCl6 doped with 1 Ag+ and 1 Bi3+ ions, and Cs2AgInCl6 computed at the DFT/PBE level of theory31 at the  point of a 2x2x2 supercell. Each orbital is represented in real space and decomposed according to each atom type. (b) CBM and VBM orbital plots in real space. The pure undoped materials present delocalized wavefunctions for electron and holes (left and right panels), while the doped system exhibits localization of the wavefunction (central panels). (c) Trend of the oscillator strength for the BiCl6  AgCl6 transition as a function of the percentage of Na ions around the Bi ion. Here the 0% corresponds to a Cs2NaInCl6 2x2x2 supercell in which: (1) one InCl6 octahedron at the center of the supercell is replaced by a BiCl6 one, and (2) the BiCl6 octahedron is, in turn, completely surrounded by 6 AgCl6 octahedra, i.e. the maximum number of octahedra that can be placed around a given octahedron in the DP lattice. The increasing percentage of Na in the plot corresponds then to a combination of: 4 AgCl6 and 2 NaCl6 octahedra surrounding BiCl6 (33%), 2 AgCl6 – 4 NaCl6 for 66%, 1 AgCl6 – 5 NaCl6 (83%) and 6 NaCl6 (100%).



Besides being parity forbidden, the lowest band transition exhibits also a strong charge-transfer character due to a large difference in electronegativity between Na and Cl ions. As a result, the transition dipole moment for this edge-to-edge transition is negligible. By replacing one Na+ and one In3+ with one Ag+ and one Bi3+ ions at the center of the supercell, respectively, we noticed two main changes in the electronic structure. The first is that the two topmost VB edge states were now composed by heavily mixed configuration of Ag 5d and 3p Cl orbitals. More specifically, these were  antibonding orbitals between 5𝑑𝑧2 and 5𝑑𝑥2 ― 𝑦2 of Ag with the 3p orbitals of Clions. Both these orbitals were strongly localized in the AgCl6 octahedron, with some contribution from the neighboring BiCl6 octahedron, as revealed by the orbital plot in Figure 2b (middle panel), indicating that Na ions acted as an electronic barrier to Ag ions that prevented the hole wavefunction to be delocalized at the VB edge. Such a hole wavefunction delocalization, on the other hand, existed in pure Cs2NaInCl6 as shown in Figure 2b (left panel). The second important change was the formation, in the CB, of three states localized on 6p Bi3+ orbitals, which were mixed with the 3p orbitals of Cl. At the relativistic scalar level of theory, these states lied at about 1.0 eV above the CB edge, although we could expect them to move near the CBM by adding the spin-orbit contribution. While we could not compute the spin-orbit coupling for this large cell, a calculation on a single [BiCl6]3- octahedron revealed a lowering of the energy of the 6p orbitals by about 0.75 eV upon addition of the spin-orbit coupling term at a similar level of theory (see SI for details). Since the 6p orbitals of Bi have a different parity than the 5s orbitals of In, their mixing was negligible, and the orbitals became strongly localized on the Bi center. Thus, we could conclude that in

a Na-rich DP system, both Ag and Bi act as centers that can localize carriers (holes and electrons, respectively) at the band edges. We expected that when these two ions were connected spatially to each other by a Cl ion, the DP system could efficiently emit light via trapped exciton (TE) emission. To verify this, we have computed the oscillator strength for the TE transition, i.e. BiCl6→AgCl6, in nearby connected octahedra and found that the most intense value occurs when the 6pz orbital of Bi was aligned on the same axis with the 5𝑑𝑧2 orbital of Ag. Conversely, the dipole transition elements from the 6px and 6py orbitals of Bi to the 5𝑑𝑧2 of Ag were found to be weak. We also computed the oscillator strength of the lowest electronic transition as a function of the number of Ag cations surrounding the Bi dopant. In other words, we took the Cs2NaInCl6 supercell doped with one Bi ion and kept replacing the first neighboring Na ions with Ag ones. Overall, the trend showed an increase of the dipole transition matrix elements at increased amount of Na ions surrounding the Bi dopant, with a maximum being at 66% of Na (Figure 2c). Apparently, an increase of Na ions could reduce the delocalization of the VBM states on the 5d of Ag ions, increasing the spatial overlap between CB and VB, and thus augmenting the oscillator strength. We stress that such increase depended only on the VB edge states because the CB is dominated by the more localized Bi, whose charge localization was unaffected by the number of surrounding Ag ions. This TE emission picture is also consistent with the fact that such DP system exhibits efficient PL even at very low concentration of Bi ions:27 photogenerated carriers can always recombine in the Bi-Ag centers, provided they are transported to those centers within the emission lifetime.

Figure 3. (a) Optical absorption (continuous lines) and PL (dashed lines) spectra of Bi-doped CsNaxAg1-xInCl6 NCs at increasing x from x=0 (red bottom curve) to x=1 (blue top curve) with 0.1 steps. The spectra have been shifted vertically for clarity. (b) Respective PL excitation (PLE) spectra collected at the PL maximum. All the PLE spectra are fitted with two Gaussian curves (red and blue dashed lines) corresponding to the spectra of pure CsAgInCl6 NCs (red bottom curve) and CsNa0.9Ag0.1InCl6 NCs (blue top curve). (c) Relative weight of the fitted PLE components with increasing x. (d) Optical absorption and PL peak energies and (e) respective Stokes shift (ΔSS) upon increasing the Na content. (f) Schematic depiction of the energy levels involved in the photophysics of Bi-doped CsNaxAg1-xInCl6 NCs. (g) PL Quantum Yield (ΦPL) at increasing Na content.


Page 4 of 7

Page 5 of 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters These theoretical predictions were corroborated by the experimental observation of the optical properties of our DP NCs. In Figure 3a we report the optical absorption and PL spectra of Bidoped Cs2Ag1-xNaxInCl6 NCs with increasing Na content. The pure Bi-doped Cs2AgInCl6 NCs showed a well-defined excitonic absorption peak at 3.55 eV. This was in stark contrast with the calculated DFT/PBE of 1.0 eV presented in Figure 2a. We would like to highlight here that there is a debate in the literature on the exact band gap of this material, with values spanning from 1.0 eV with semi-local DFT functionals (like in this work) up to 2.5 eV with the more accurate HSE06 functional.16,32 Some DFT works report also bandgaps of 3.3 eV, however these are obtained by tuning the amount of exact Hartree-Fock exchange in the HSE06 functional that has the effect of opening the gap “empirically”.32 We thus computed the optical spectrum at the DFT/PBE level in the supercell and realized that the first optically active (and intense) transitions appeared at about 0.7 eV above the edge-to-edge transition (Figure S8). Combining together the above observations, we could conclude that the bandgap was indeed at about 2.5 eV, featuring an optically inactive transition, whereas the excitonic feature observed experimentally at 3.55 eV likely stemmed from upper-lying optically active states. The broad (FWHM~780 meV) intragap PL band at 2 eV, corresponding to a Stokes shift as large as 1.55 eV, was consistent with the above description and with previous reports on Bi-doped Cs2Ag1-xNaxInCl6 powders, but also with the TE picture emerging from the DFT modelling, as we will see below. Upon partial replacement of Ag with Na in the NCs, at increasing x the PL energy and linewidth remained essentially unvaried, whilst the absorption peak shifted towards higher energies and exhibited significant broadening for 0.3 ≤ x ≤ 0.7. Also, for x ≥ 0.8, the NCs in solution tended to aggregate into larger particles, resulting in a dominant light scattering contribution that complicated the spectral analysis. In order to overcome this issue, we acquired the PLE spectra at the PL maximum of each NC sample (Figure 3b). A PLE measurement for Cs2NaInCl6 NCs was not possible as these systems are not emissive. All acquired PLE spectra matched closely the respective absorption spectra confirming the broader excitation linewidth of the NCs with mixed Ag-Na composition. By using a linear combination of pure Bi-doped Cs2AgInCl6 and Bi-doped Cs2Ag0.1Na0.9InCl6 NCs (considered as representative for the pure Cs2NaInCl6 system based on the close rebalance between the respective absorption peak energy) PLE contributions, we could reproduce all PLE spectra. The relative weight of the two contributions are reported in Figure 3c as a function of the Na content, showing the formation of alloyed DP NCs for intermediate x-values and their final conversion into Cs2NaInCl6 NCs. As a result of this effect, the absorption energy (and consistently the respective PLE maximum) shifted essentially linearly, with increasing x, from 3.4 eV to 3.9 eV (Figure 3d). Importantly, such an evolution of the NC band gap with the Na content occurred with no variation of the PL peak energy (Figure 3d), resulting in the progressive increase of the Stokes shift from ~1.5 eV (x=0.1) to 1.9 eV (x=0.9) (Figure 3e). This confirmed that the PL did not directly involve band-edge carriers and corroborated its origin from the radiative recombination of a TE with the photo-hole localized into an Ag+-related intragap state above the VBM, and the photoelectron trapped into a Bi3+-state just below the CBM (Figure 3f). While the presence of Bi dopants

was essential in order to achieve any PL emission from Cs2Ag1xNaxInCl6 NCs, as confirmed by our control experiments (Figure S9), the substitution of Ag with Na had profound effects on the emission efficiency of our NCs. As shown in Figure 3g, upon increasing the Na content up to 0.6, ΦPL increased significantly from ~6% to ~22% and then dropped back to ~6% for larger Na contents. This trend was in qualitative agreement with the theoretically predicted behavior of the oscillator strength vs. x, resulting from the stronger localization of the hole wavefunction close to the Ag-center. However, the observed ~3.5-fold enhancement in ΦPL for x= 0.6 with respect to Bi-doped Cs2AgInCl6 NCs (x=0) was quantitatively larger than what expected considering exclusively the concomitant effect of Na on the respective radiative decay rate (~1.5 times larger in Bi-doped Cs2Ag0.3Na0.7InCl6 NCs with respect to the pure Bidoped Cs2AgInCl6 NCs) at constant nonradiative losses. This, therefore, suggested that Na played a dominant role also in suppressing nonradiative decay channels in the NCS, as demonstrated experimentally below. Time-resolved PL experiments enabled us to shed light on the recombination dynamics of our DP NCs and to clarify the effect of Na on the nonradiative processes affecting their PL efficiency. In Figure 4a we report the contour plot of the spectrally-resolved PL intensity as a function of time for a representative sample of Bidoped Cs2Ag0.5Na0.5InCl6 NCs. Data for the whole set of NCs are reported in Figure S10. All samples presented a broad PL spectrum with homogeneous lifetime extending to several seconds, indicating the decay of one main emissive species (the weak intensity drop at ~1.85 eV in Figure 4a is due to the instrument response). This further supported the picture of a broad PL linewidth resulting from the decay of a manifold of TEs with slightly different energies due to the distribution of Bi3+ and Ag+ sites within the NC energy gap. The PL decay curves of Bi-doped Cs2Ag1xNaxInCl6 NCs collected at the corresponding emission maxima are reported in Figure 4b. As highlighted in Figure 4c, all curves showed a rapid intensity drop within the first ~5 ns (comparable to the excitation pulse width used for the experiments). We ascribed such a component to fast nonradiative losses likely due to trapping in lattice defects, which rendered a subpopulation, fD, of our DP NCs non-emissive (or ‘dark’, D). The relative weight of such a fast decay, ID ∝ fD, could thus be used to estimate the fraction, fB=1-fD, of emissive (or ‘bright’, B) NCs in the ensemble. In agreement with our picture, fB followed a bell-shaped trend (Figure 4d) resembling the evolution of ΦPL with the Na content (Figure 3f), thus indicating that the PL efficiency was mostly determined by fast nonradiative processes that were effectively suppressed by suitable insertion of Na ions. Based on this picture, we ascribed the longer-lived tail of the PL decay to the recombination of the TE in bright NCs. With increasing x, such a decay progressively slowed down, as quantified by the effective PL lifetimes extracted as the time after which the PL dropped by a factor of 1/e (after excluding the initial nonradiative portion, Figure 4b). This suggested that Na substitution removed also less efficient nonradiative pathways that competed with the radiative decay of TE. Because of the presence of such additional channels, it was, however, not trivial to directly link the PL kinetics of TE to the effects of Na on the radiative rate predicted by the DFT calculations.



Figure 4. (a) Contour plot of the PL spectra of Bi-doped CsNa0.5Ag0.5InCl6 NCs. The PL peak energy is indicated with a dashed white line. (b) Normalized time-resolved PL decay traces of Bi-doped CsNaxAg1-xInCl6 NCs at increasing x, from x=0 (red) to x=0.9 (blue) with 0.1 steps. Inset: Effective lifetime τSLOW - extracted as the time after which the PL intensity drops by a factor of 1/e - of the slow component of the PL decay traces reported in the main panel vs. the Na content, x. (c) PL traces in the first 200 ns of the decay for the same DP NCs as in ‘b’, highlighting the evolution with the Na content of the fast nonradiative component, ID, due to the fraction, fD, of non-emissive ‘dark’ (D) NCs in the ensemble. (d) Fraction, fB=1-fD, of emissive ‘bright’ (B) NCs vs. x. All measurements were conducted in air at room temperature using an excitation energy of 3.5eV. In conclusion, we have reported the colloidal synthesis of Bidoped Cs2Ag1-xNaxInCl6 NCs having a double perovskite crystal structure, in which the relative amount of Na could be tuned from 0 to 1. Our DFT calculations revealed that in Na-rich conditions the Ag+ ions in the DP structure give rise to localized AgCl6 energy levels just above the VBM, whereas Bi3+ dopants introduce BiCl6 states which localize just below the CBM. Therefore, when the Na content is high enough, an increase in the PL of the system can be ascribed to a trapped emission between states localized in the BiCl6 and AgCl6 octahedra, respectively. This model is in accordance with our optical measurements, according to which the PLQY of the DP NCs increased with the amount of Na up to a maximum of ~22% for the Cs2Ag0.4Na0.6InCl6 stoichiometry and then dropped when further increasing the Na content. Also, the presence of Bi dopants inside the DP structure was found to be essential in order to observe any PL emission. The peculiar optical properties of our NC system, and in particular their large Stoke-shifted PL, make them promising for solar harvesting (such as solar concentrators) or lighting devices.27,33

ASSOCIATED CONTENT

[email protected], [email protected], [email protected], [email protected], [email protected]

ORCID Federico Locardi: 0000-0002-1794-8282 Ivan Infante: 0000-0003-3467-9376 Sergio Brovelli: 0000-0002-5993-855X Luca De Trizio: 0000-0002-1514-6358 Liberato Manna: 0000-0003-4386-7985 Mirko Prato: 0000-0002-2188-8059 Valerio Pinchetti: 0000-0003-3792-3661 Maurizio Ferretti: 0000-0003-0709-3281 Matteo Luca Zaffalon: 0000-0002-1016-6413

Author Contributions Locardi F. and Sartori E. contributed equally to this work.

Supporting Information. Experimental details, XPS and SEM-EDS analyses, TEM and HRTEM images, Syntheses performed at 120°, 160°C and 180°C, SAED pattern, Computed absorption spectra for Cs2AgInCl6 and Cs2NaInCl6, Synthesis without Bi, Contour plots of PL spectra.

Notes

AUTHOR INFORMATION

We would like to thank Lea Pasquale, Filippo Drago and Simone Lauciello for carrying out the XPS, ICP-MS and SEM-EDS measurements, respectively. I.I. acknowledges the Netherlands Organization of Scientific Research (NWO) through the Innovational Research Incentive (Vidi) Scheme (Grant No. 723.013.002). The

Corresponding Authors

The authors declare no competing financial interest.

ACKNOWLEDGMENT


Page 6 of 7

Page 7 of 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters computational work was carried out on the Dutch national einfrastructure with the support of the SURF Cooperative. We also acknowledge funding from the programme for research and Innovation Horizon 2020 (2014-2020) under the Marie Skłodowska-Curie Grant Agreement COMPASS No. 691185.

REFERENCES (1) Snaith, H. J. Present Status and Future Prospects of Perovskite Photovoltaics. Nat. Mater. 2018, 17, 372–376. (2) Akkerman, Q. A.; Rainò, G.; Kovalenko, M. V.; Manna, L. Genesis, Challenges and Opportunities for Colloidal Lead Halide Perovskite Nanocrystals Nat. Mater. 2018, 17, 394–405. (3) Shamsi, J.; Urban, A. S.; Imran, M.; De Trizio, L.; Manna, L. Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties. Chem. Rev. 2019, 119, 3296– 3348. (4) Ghosh, S.; Pradhan, B. Lead-Free Metal Halide Perovskite Nanocrystals: Challenges, Applications, and Future Aspects. ChemNanoMat 2019, 5, 300–312. (5) Fu, H. Review of Lead-Free Halide Perovskites as LightAbsorbers for Photovoltaic Applications: From Materials to Solar Cells. Sol. Energy Mater. Sol. Cells 2019, 193, 107–132. (6) Giustino, F.; Snaith, H. J. Toward Lead-Free Perovskite Solar Cells. ACS Energy Lett. 2016, 1, 1233–1240. (7) Bekenstein, Y.; Dahl, J. C.; Huang, J.; Osowiecki, W. T.; Swabeck, J. K.; Chan, E. M.; Yang, P.; Alivisatos, A. P. The Making and Breaking of Lead-Free Double Perovskite Nanocrystals of Cesium SilverBismuth Halide Compositions. Nano Lett. 2018, 18, 3502–3508. (8) Creutz, S. E.; Crites, E. N.; De Siena, M. C.; Gamelin, D. R. Colloidal Nanocrystals of Lead-Free Double-Perovskite (Elpasolite) Semiconductors: Synthesis and Anion Exchange to Access New Materials. Nano Lett. 2018, 18, 1118–1123. (9) Liu, Y.; Jing, Y.; Zhao, J.; Liu, Q.; Xia, Z. Design Optimization of Lead-Free Perovskite Cs2AgInCl6:Bi Nanocrystals with 11.4% Photoluminescence Quantum Yield. Chem. Mater. 2019, 31, 3333–3339. (10) Dahl, J. C.; Osowiecki, W. T.; Cai, Y.; Swabeck, J. K.; Bekenstein, Y.; Asta, M.; Chan, E. M.; Alivisatos, A. P. Probing the Stability and Band Gaps of Cs2AgInCl6 and Cs2AgSbCl6 Lead-Free Double Perovskite Nanocrystals. Chem. Mater. 2019, 31, 3134–3143. (11) Breternitz, J.; Levcenko, S.; Hempel, H.; Gurieva, G.; Franz, A.; Hoser, A.; Schorr, S. Mechanochemical Synthesis of the Lead-Free Double Perovskite Cs2[AgIn]Br6 and Its Optical Properties. J. Phys. Energy 2019, 1, 025003. (12) Yang, B.; Hong, F.; Chen, J.; Tang, Y.; Yang, L.; Sang, Y.; Xia, X.; Guo, J.; He, H.; Yang, S.; et al. Colloidal Synthesis and Charge-Carrier Dynamics of Cs2AgSb1−yBiyX6 (X: Br, Cl; 0 ≤y ≤1) Double Perovskite Nanocrystals. Angew. Chemie - Int. Ed. 2019, 58, 2278–2283. (13) Deng, W.; Deng, Z. Y.; He, J.; Wang, M.; Chen, Z. X.; Wei, S. H.; Feng, H. J. Synthesis of Cs2AgSbCl6 and Improved Optoelectronic Properties of Cs2AgSbCl6/TiO2 Heterostructure Driven by the Interface Effect for Lead-Free Double Perovskites Solar Cells. Appl. Phys. Lett. 2017, 111, 2–7. (14) Tran, T. T.; Panella, J. R.; Chamorro, J. R.; Morey, J. R.; McQueen, T. M. Designing Indirect–Direct Bandgap Transitions in Double Perovskites. Mater. Horiz. 2017, 4, 688–693. (15) Du, K. Z.; Meng, W.; Wang, X.; Yan, Y.; Mitzi, D. B. Bandgap Engineering of Lead-Free Double Perovskite Cs2AgBiBr6 through Trivalent Metal Alloying. Angew. Chemie - Int. Ed. 2017, 56, 8158–8162. (16) Meng, W.; Wang, X.; Xiao, Z.; Wang, J.; Mitzi, D. B.; Yan, Y. Parity-Forbidden Transitions and Their Impact on the Optical Absorption Properties of Lead-Free Metal Halide Perovskites and Double Perovskites. J. Phys. Chem. Lett. 2017, 8, 2999–3007. (17) Luo, J.; Li, S.; Wu, H.; Zhou, Y.; Li, Y.; Liu, J.; Li, J.; Li, K.; Yi, F.; Niu, G.; et al. Cs2AgInCl6 Double Perovskite Single Crystals: Parity Forbidden Transitions and Their Application for Sensitive and Fast UV Photodetectors. ACS Photonics 2018, 5, 398–405.

(18) Locardi, F.; Cirignano, M.; Baranov, D.; Dang, Z.; Prato, M.; Drago, F.; Ferretti, M.; Pinchetti, V.; Fanciulli, M.; Brovelli, S.; et al. Colloidal Synthesis of Double Perovskite Cs2AgInCl6 and Mn-Doped Cs2AgInCl6 Nanocrystals. J. Am. Chem. Soc. 2018, 140, 12989–12995. (19) Volonakis, G.; Haghighirad, A. A.; Milot, R. L.; Sio, W. H.; Filip, M. R.; Wenger, B.; Johnston, M. B.; Herz, L. M.; Snaith, H. J.; Giustino, F. Cs2InAgCl6: A New Lead-Free Halide Double Perovskite with Direct Band Gap. J. Phys. Chem. Lett. 2017, 8, 772–778. (20) Karmakar, A.; Dodd, M. S.; Agnihotri, S.; Ravera, E.; Michaelis, V. K. Cu(II)-Doped Cs2SbAgCl6 Double Perovskite: A Lead-Free, LowBandgap Material. Chem. Mater. 2018, 30, 8280–8290. (21) Nandha K., N.; Nag, A. Synthesis and Luminescence of MnDoped Cs2AgInCl6 Perovskites. Chem. Commun. 2018, 54, 5205–5208. (22) Zhou, J.; Rong, X.; Zhang, P.; Molokeev, M. S.; Wei, P.; Liu, Q.; Zhang, X.; Xia, Z. Manipulation of Bi3+/In3+ Transmutation and Mn2+Doping Effect on the Structure and Optical Properties of Double Perovskite Cs2NaBi1-xInxCl6. Adv. Opt. Mater. 2019, 7, 1–9. (23) Majher, J. D.; Gray, M. B.; Strom, T. A.; Woodward, P. M. Cs2NaBiCl6:Mn2+ - A New Orange-Red Halide Double Perovskite Phosphor. Chem. Mater. 2019, 31, 1738–1744. (24) Chen, N.; Cai, T.; Li, W.; Hills-Kimball, K.; Yang, H.; Que, M.; Nagaoka, Y.; Liu, Z.; Yang, D.; Dong, A.; et al. Yb- and Mn-Doped Lead-Free Double-Perovskite Cs2AgBiX6 (X = Cl-, Br-) Nanocrystals. ACS Appl. Mater. Interfaces 2019, 6, 16855-16863. (25) Lee, W.; Hong, S.; Kim, S. Colloidal Synthesis of Lead-Free Silver-Indium Double-Perovskite Cs2AgInCl6 Nanocrystals and Their Doping with Lanthanide Ions. J. Phys. Chem. C 2019, 123, 2665–2672. (26) Khalfin, S.; Bekenstein, Y. Advances in Lead-Free Double Perovskite Nanocrystals, Engineering Band-Gaps and Enhancing Stability Through Composition Tunabilty. Nanoscale 2019, 11, 8665–8679. (27) Luo, J.; Wang, X.; Li, S.; Liu, J.; Guo, Y.; Niu, G.; Yao, L.; Fu, Y.; Gao, L.; Dong, Q.; et al. Efficient and Stable Emission of Warm-White Light from Lead-Free Halide Double Perovskites. Nature 2018, 563, 541–545. (28) Yurtaeva, S. V.; Usachev, A. E.; Yablokov, Y. V. Spin-Spin Interactions of Ce3+ Ions in the Elpasolite Crystal Cs2NaInCl6. Phys. Solid State 1997, 39, 1075–1080. (29) Morss, L. R.; Siegal, M.; Stenger, L.; Edelstein, N. Preparation of Cubic Chloro Complex Compounds of Trivalent Metals: Cs2NaMCl6. Inorg. Chem. 1970, 9, 1771–1775. (30) Hutter, J.; Iannuzzi, M.; Schiffmann, F.; Vandevondele, J. Cp2k: Atomistic Simulations of Condensed Matter Systems. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2014, 4, 15–25. (31) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868. (32) Zhou, J.; Xia, Z.; Molokeev, M. S.; Zhang, X.; Peng, D.; Liu, Q. Composition Design, Optical Gap and Stability Investigations of Lead-Free Halide Double Perovskite Cs2AgInCl6. J. Mater. Chem. A 2017, 5, 15031– 15037. (33) Meinardi, F.; Colombo, A.; Velizhanin, K.A.; Simonutti, R.; Lorenzon, M.; Beverina, L.; Viswanatha, R.; Klimov, V.I.; Brovelli, S. Largearea Luminescent Solar Concentrators Based on ‘Stokes-Shift-Engineered’ Nanocrystals in a Mass-Polymerized PMMA Matrix. Nat. Photonics, 2014, 8, 392–399.


Emissive Bi-Doped Double Perovskite Cs2Ag1âxNaxInCl6

Recommend Documents

Emissive Bi-Doped Double Perovskite Cs2Ag1âxNaxInCl6