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Letter
Direct-Indirect Nature of the Bandgap in Lead-Free Perovskite Nanocrystals Yuhai Zhang, Jun Yin, Manas R. Parida, Ghada H. Ahmed, Jun Pan, Osman M. Bakr, Jean-Luc Bredas, and Omar F. Mohammed J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 23 Jun 2017 Downloaded from http://pubs.acs.org on June 24, 2017
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Direct-Indirect Nature of the Bandgap in Lead-Free Perovskite Nanocrystals Yuhai Zhang,† Jun Yin,† Manas R. Parida,† Ghada H. Ahmed,† Jun Pan,† Osman M. Bakr,† Jean-Luc Brédas,†# Omar F. Mohammed*,† †King Abdullah University of Science and Technology, KAUST Solar Center, Division of Physical Sciences and Engineering, Thuwal 23955-6900, Kingdom of Saudi Arabia #New permanent address: School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA
AUTHOR INFORMATION Corresponding Author
[email protected]
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ABSTRACT
With record efficiencies achieved in lead halide perovskite-based photovoltaics, urgency has shifted towards finding alternative materials that are stable and less-toxic. Bismuth-based perovskite materials are currently one of the most promising candidates amongst those alternatives. However, the band structures of these materials, including the nature of the bandgaps, remain elusive due to extremely low photoluminescence quantum yield (PLQY) and scattering issues in their thin-film form. Here, we reveal the specific nature of the material’s electronic transitions by realizing monodisperse colloidal nanocrystals (NCs) of hexagonal-phase Cs3Bi2X9 perovskites, which afford well-resolved PL features. Interestingly, the PL profile exhibits a dual-spectral feature at room temperature with comparable intensities, based on which we propose an exciton recombination process involving both indirect and direct transitions simultaneously – an observation further supported by temperature dependent and density functional theory (DFT) calculations. Our findings provide experimental and theoretical insights into the nature of the bandgaps in bismuth halide materials–essential information for assessing their viability in solar cells and optoelectronics. TOC GRAPHICS
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A search for lead-free perovskite materials (Cu, Sn, Bi, etc) has been motivated by the intrinsic toxicity and instability of lead-based perovskites, which hinder their utilization in actual applications.1-6 In particular, polycrystalline bismuth halide materials have attracted considerable attention because of their long-term stability.7-9 For example, solar cells based on (CH3NH3)3Bi2I9 perovskite have been demonstrated to remain stable under ambient conditions for more than 10 weeks.10-11 Despite this progress, the limited efficiency (< 1%) of bismuth halide perovskites in photovoltaics has greatly hindered their practical application.12-14 A key to improving the conversion efficiency is to understand the nature of their bandgaps.15 Recently, increased attention has focused on the discussion of indirect transitions involved in the exciton recombination process.16 However, the recombination pathways remain unclear. Herein, we present the first report of the optical characterization of lead-free perovskite NCs of hexagonal-phase Cs3Bi2I9 with a narrow size distribution and good colloidal stability, which allows us to explore the nature of their bandgap. Through the use of temperature-dependent photo-luminescence spectroscopy, transient absorption spectroscopy, and DFT, we demonstrate the co-existence of direct-indirect transitions involved in the charge recombination process in excited Cs3Bi2I9 NCs. The Cs3Bi2I9 NCs were synthesized using a hot-injection method.2 Briefly, a Cs-oleate precursor was injected into a BiI3 precursor at a moderate temperature (~100 °C) under ambient conditions (see the Supporting Information for further details). An instantaneous color change from yellow to deep orange indicated the formation of Cs3Bi2I9 NCs (Figure S1). A transmission electron microscopy (TEM) image reveals the high uniformity of the NCs, and the size histogram shows a narrow distribution with a standard deviation of less than 10% (Figure 1a). A
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high-resolution TEM image points to the single-crystal nature of the particles and their hexagonal morphology (Figure 1b). The X-ray diffraction pattern suggests a hexagonal crystallographic phase with well-developed (006) facets, which belongs to the space group P63/mmc(194) (Figure 1c), as previously reported.13 The NCs can be easily dispersed in nonpolar solvents, such as toluene. It is worth noting that the use of a nanoparticle colloid instead of a thin film can greatly alleviate problems related to scattering, resulting in high reproducibility during optical measurements (Figure S2). Figure 1d shows the steady-state absorption and PL spectra of the Cs3Bi2I9 NCs in toluene. Intriguingly, we observed two spikes in the PL profile at room temperature, centered at 580 nm (2.14 eV) and 605 nm (2.05 eV). This is an unusual feature because for most semiconductor materials, emission spectra with fine structures are observed only at liquid-helium temperature.17 It is possible that these two emissions stem from different exciton recombination pathways, involving direct and indirect bandgap transitions simultaneously. For instance, GaP, WS 2, and WSe2, all of which have a direct-indirect bandgap nature, present PL profiles with a double-peak feature at room temperature.18-20 Moreover, we observed a long tail in both the absorption and PL spectra. These optical characteristics indicate that an indirect transition may participate in optical transitions.21 Broadly similar characteristics of a long tail and multiple spikes were also observed in two other bismuth halides NCs, namely, Cs3Bi2Cl9 and Cs3Bi2Br9 (Figure 1, e and f).
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Figure 1. (a) Transmission electron microscopy (TEM) image of Cs3Bi2I9 NCs; the inset shows the narrow size distribution histogram of the particles, which indicates an average diameter of 18 nm. (b) High-resolution TEM image of a single Cs3Bi2I9 nanocrystal, showing clear lattice fringes corresponding to (100) facets. (c) X-ray diffraction pattern of the asprepared Cs3Bi2I9 NCs, indexed to a hexagonal crystalline phase with Inorganic Crystal Structure Data (ICSD) Coll. Code.162078,1447. (d-f) Normalized absorption and PL spectra of Cs3Bi2X9 (X=I, Cl, Br) NCs colloidal; the emission was recorded at room temperature under excitation with a Xe lamp at 470 nm, 340 nm, and 370 nm, respectively.
One conventional way to determine whether an indirect bandgap transition is present is to fit the absorption spectrum using a Tauc plot.22 Figure 2a shows two Tauc plots for the Cs3Bi2I9 NCs generated based on the assumptions of direct and indirect transitions. We find that the Tauc plot fit under the indirect transition assumption yields a bandgap value (2.15 eV) that is close to the experimental result (2.14 eV). To shed more light on the nature of the bandgap in Cs3Bi2I9 NCS, we performed temperaturedependent absorption and PL measurements ranging from 25 °C to 95 °C (Figure 2c, Figure S3, 5 ACS Paragon Plus Environment
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S11). Theoretically, the radiative recombination of excitons through a direct bandgap transition decreases with increasing temperature because of thermal quenching.23-24 In stark contrast to this behavior, recombination through an indirect bandgap transition can be greatly enhanced by increasing the temperature because it requires additional momentum compensation from phonons to obey momentum conservation.16,
25-26
Indeed, we observed different variations in the two
peaks of the PL spectrum as well as a red shift in absorption at high temperatures (Figure S11). As shown in Figure 2b, the typical PL profile can be divided into three Gaussian peaks centered at 580 nm, 605 nm, and 635 nm (Figure 2b). As the temperature increases, the intensity of the 580-nm emission gradually decreases, while that of the 605-nm emission significantly increases (Figure S3). This strongly suggests that the 605-nm emission originates from phonon-assisted indirect transitions during the exciton recombination process, whereas the 580-nm emission stems from direct transitions. To further confirm the origin of PL, power dependent PL was conducted in the range of 37~740 µW/cm2 (Figure S10). The linear relationship between PL intensity (580, 605, and 635 nm) and excitation power indicates that the PL originates from exciton recombination, instead of defects.
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Figure 2. (a) Normalized Tauc plots for the typical absorption spectra of Cs3Bi2I9 NCs under assumptions of an indirect bandgap (red) and a direct bandgap (black). (b) Deconvolution of the PL profile of Cs3Bi2I9 NCs at room temperature, showing three peaks centered at 580 nm, 605 nm, and 635 nm. (c) Typical PL profiles recorded at different temperatures (black for 25 °C, red for 95 °C); the inset shows the variation in the ratio of the intensity of the 605-nm peak over that of the 580-nm peak. (d) DFT band structure and (e) density of states of Cs3Bi2I9 crystals; the direct and indirect transitions are denoted by red and cyan arrows, respectively. (f) Proposed recombination pathways in Cs3Bi2I9 NCs. It is worth noting that a rise in temperature may induce irreversible particle segregation or phase transitions in materials,27 particularly in a colloidal system, which can affect PL spectra. To rule out this possibility, we examined the reversibility of the PL profiles during the heating and cooling processes. We observed a complete recovery of the PL profiles after cooling to room temperature (Figure S4). In another control experiment to exclude the possibility of aggregation, we recorded the PL spectra of different particle concentrations to examine the concentration
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dependence of PL. We observed essentially identical PL profiles, indicating that the dual-spectral characteristic is an intrinsic optical property of Cs3Bi2I9 NCs (Figure S5). According to Kasha’s rule, PL is most likely generated by exciton recombination from the conduction band minimum (CBM) to the valence band maximum (VBM).28 In a typical indirectbandgap material such as silicon, the strength of the indirect transition dominates, and therefore, only PL from indirect transitions can be observed.25 Interestingly, in the Cs3Bi2I9 NCs, we observed both types of transitions at room temperature with comparable intensities. To understand the origin of the direct transition, we investigated the band structure of Cs3Bi2I9 using density functional theory29 (DFT) (Figure 2, d and e, Figure S6). The Heyd-Scuseria-Ernzerhof functional with spin-orbit coupling was used to calculate the electronic bands and density of states. The DFT calculations indicate the coexistence of a low-lying indirect bandgap of 2.10 eV from the Γ point to the K point and a slightly larger direct gap of 2.23 eV at the Γ point; both are close to the experimental values (2.05 eV and 2.14 eV). The energy difference between these two transitions is only 130 meV, indicating that it can be compensated by thermal fluctuations at room temperature. Moreover, the large DOS at Γ point enhanced the possibility of direct transitions. Therefore, Kasha’s rule is relaxed and the emission peak from the direct bandgap transition can be readily observed. It should be noted that the thermal energies required by indirect and direct transitions are 18 meV and 130 meV (Figure S7 and Figure 2d), respectively, while the thermal energy at room temperature is 26 meV. Therefore, indirect transition should be more sensitive to temperature, as shown in temperature-dependent PL experiment. Based on the discussions above, we propose the following recombination pathways for excitons in Cs3Bi2I9 NCs (Figure 2f). Upon excitation, electrons in the valence band are promoted to the conduction band. Such absorption process involves direct bandgap transition
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since we observed a considerable intrinsic absorption coefficient at 488 nm (1.6×104 cm-1).4 According to Kasha’s rule, the excited electrons relax to the CBM and begin to recombine with holes in the VBM. With the assistance of phonons, indirect bandgap transitions occur, accompanied by the generation of an emission band at 605 nm. Also, vertical transitions related to the direct bandgap compete in the recombination process, giving rise to the emission at 580 nm. It should be noted that the low PLQY of this material can also be due to the presence of nonradiative recombination pathways such as native defects, which can be favored by the indirect bandgap.14 Indeed, we observed an extremely low quantum yield (0.017 %) from the Cs3Bi2I9 NCs (Figure S8).12
Figure 3. (a) Nanosecond TA spectra of Cs3Bi2I9 NCs under 532-nm excitation, showing strong excited-state absorption (ESA) at 460 nm and ground-state bleaching (GSB) at 486 nm. (b) Temporal evolution of the GSB and ESA profiles which fitted using a bi-exponential function.
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Since an indirect transition usually exhibits a much longer lifetime than a direct transition,16, 25 we used nanosecond transient absorption (ns-TA) spectroscopy to probe the ground-state bleaching (GSB) recovery and excited-state absorption (ESA) of the Cs3Bi2I9 NCs (Figure 3; the TA experimental setup is detailed elsewhere).30-31 The recovery profiles could be well fitted using a bi-exponential function, yielding two lifetimes of 1 ns and 9 ns for GSB and two lifetimes of 2 ns and 8 ns for ESA. The long and short components were assigned to indirect and direct transitions, respectively. We argue that such short lifetimes are likely caused by the intrinsically strong exciton binding energy (~300 meV) in Cs3Bi2I9 NCs13 (Figure S9, no quantum confinement was observed with 3-nm NCs) and by the high pump fluence used in these experiments, as observed in the WS2 system.19 In conclusion, we have reported a facile method of synthesizing uniform lead-free Cs3Bi2I9 nanocrystals. The bandgap of these Cs3Bi2I9 NCs is proposed to have a dual direct-indirect transition nature, based on the observation of double-spectral features in the PL profile. This dual nature was confirmed by temperature-dependent emission and transient absorption spectra. DFT calculations show good agreement with the experimental data in terms of the bandgaps and the energy difference between the direct and indirect transitions. Our findings provide new insight for understanding and deciphering the nature of the bandgaps in bismuth halide perovskites, which can enhance their utility in low-cost photovoltaic and light-emitting devices with longterm stability and reduced toxicity.
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ASSOCIATED CONTENT
AUTHOR INFORMATION Corresponding Authors *O.F.M: Email:
[email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by King Abdullah University of Science and Technology (KAUST). Supporting Information. Experimental details for synthesis, optical characterizations, TEM, XRD, and PLQY measurements. REFERENCES (1) Cortecchia, D.; Dewi, H. A.; Yin, J.; Bruno, A.; Chen, S.; Baikie, T.; Boix, P. P.; Grätzel, M.; Mhaisalkar, S.; Soci, C. Lead-Free MA2CuClxBr4–x Hybrid Perovskites. Inorg. Chem. 2016, 55, 1044-1052. (2) Jellicoe, T. C.; Richter, J. M.; Glass, H. F.; Tabachnyk, M.; Brady, R.; Dutton, S. E.; Rao, A.; Friend, R. H.; Credgington, D.; Greenham, N. C.; Bohm, M. L. Synthesis and Optical Properties of Lead-Free Cesium Tin Halide Perovskite Nanocrystals. J. Am. Chem. Soc. 2016, 138, 29412944. (3) McMeekin, D. P.; Sadoughi, G.; Rehman, W.; Eperon, G. E.; Saliba, M.; Hörantner, M. T.; Haghighirad, A.; Sakai, N.; Korte, L.; Rech, B. A Mixed-Cation Lead Mixed-Halide Perovskite Absorber for Tandem Solar Cells. Science 2016, 351, 151-155.
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