Inorganic Cesium Bismuth Halide Perovskite ...

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Authors: Keli Han, Bin Yang, Junsheng Chen, Feng Hong, Xin Mao,. Kaibo Zheng, Songqiu Yang, Yajuan Li, Tõnu Pullerits, and. Weiqiao Deng. This manuscript ...
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Accepted Article Title: Lead-Free, All-Inorganic Cesium Bismuth Halide Perovskite Nanocrystals with Long-Term Stability Authors: Keli Han, Bin Yang, Junsheng Chen, Feng Hong, Xin Mao, Kaibo Zheng, Songqiu Yang, Yajuan Li, Tõnu Pullerits, and Weiqiao Deng This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201704739 Angew. Chem. 10.1002/ange.201704739 Link to VoR: http://dx.doi.org/10.1002/anie.201704739 http://dx.doi.org/10.1002/ange.201704739

10.1002/anie.201704739

Angewandte Chemie International Edition

COMMUNICATION Lead-Free, Air-Stable All-Inorganic Cesium Bismuth Halide Perovskite Nanocrystals Bin Yang, Junsheng Chen, Feng Hong, Xin Mao, Kaibo Zheng, Songqiu Yang, Yajuan Li, Tõnu Pullerits, Weiqiao Deng and Keli Han* analogs.[23] Consequently, the all-inorganic Cs-based perovskites usually exhibit better stability as well as less defect density compared with organic MA-based perovskite NCs.[23-26] Moreover, it has been reported that the Cs based all-inorganic Bi-perovskite bulk materials are more crystalline, with higher PLQE and superior optoelectronic performance compared to the organic-inorganic hybrid conterparts.[27] However, to the best of our knowledge, there is no report about all-inorganic Bi based perovskite NCs. Herein, we present a facile approach to synthesize allinorganic Cs3Bi2X9 (X= Cl, Br, I) perovskite NCs at room temperature. The PL spectra of perovskite NCs series cover a wide wavelength range from 400 to 560 nm with different halide composition. The typical ligand free Cs3Bi2Br9 NCs exhibited blue emission at 468 nm, with full width at half maximum (FWHM) of 40 nm. Ligand free Cs3Bi2Br9 NCs show low PLQE (0.2%), due to the fast trapping process. Such trap-states can be further passivated by adding oleic acid (OA) leading to a drastic increasing of PLQE (4.5%). Although the PLQE value is still much lower than the blue emitted lead-based perovskite NCs (CsPb(Br0.5Cl0.5)3, PL peak: 455 nm, PLQE: 37%),[28] we find that the Cs3Bi2Br9 NCs exhibit high air-stability for over 30 days.

Abstract: Lead-based perovskite nanocrystals (NCs) have demonstrated outstanding optical properties and cheap synthesis methods conferring them a tremendous potential in the field of optoelectronic devices. However, two critical problems are still unresolved and hindering their commercial applications: one is the fact of being lead-based and the other is the poor stability. Here, we report a lead-free all-inorganic perovskite Cs3Bi2X9 (X= Cl, Br, I) NCs with emission wavelength ranging from 400 to 560 nm synthesized by a facile room temperature reaction. The ligand free Cs3Bi2Br9 NCs exhibits blue emission with photoluminescence quantum efficiency (PLQE) about 0.2%. The PLQE can be increased to 4.5 % when extra surfactant (oleic acid) is added during the synthesis processes. We determine that this improvement stems from passivation of fast trapping process (2~20 ps). Notably, the trap-states can also be passivated in humid condition and the NCs exhibited high stability towards air exposure exceeding 30 days.

Recently, lead-based perovskite nanocrystals (NCs) with formula APbX3 (A: Cs, CH3NH3 (MA), X=Cl, Br, I) have received broad attention for their excellent optical and electronic properties.[1-19] The perovskite nanostructures have high PL quantum efficiency (PLQE),[6,7,9,10] large absorption crosssection,[11,12] low-threshold for lasing,[13,14,15] tunable PL emission in the whole UV-Vis range due to the quantum confinement.[16] However, the toxic of lead-based materials pose a threat to the environment and humankind.[17] Furthermore,the application of perovskite NCs is still limited by their instability in air.[18,19] Thus, searching for low toxic and highly stable perovskite NCs is on the cutting-edge of the research. The search for the lead-free perovskite NCs initiated with replacing Pb with less toxic tin (Sn) element in perovskite structrues.[20] Unfortunately, the Sn-based perovskite NCs are unstable with low PLQE about 0.14%.[20] Bismuth-based perovskite with lower toxicity than lead is promising to constitute perovskite materials.[21] Bi3+ produce a layered form of the vacancy-ordered perovskites, which can beviewed as a tripling of the traditional perovskite unit cell with only two-thirds of the octahedral positions fully occupied (Figure 1a). More recently, organo-Bi perovskite NCs MA3Bi2X9 (Cl, Br, I) have been synthesized.[22] In general, the organic cations in the perovskites are more prone to decompose compared to the inorganic

[a]

[b]

[c]

Figure 1. (a) Cs3Bi2Br9 unit cell. (b) XRD patterns of Cs3Bi2Br9 NCs. (c) TEM image of Cs3Bi2Br9 NCs. Insert: The selected area electron diffraction pattern of Cs3Bi2Br9 NCs. (d)-(e) HRTEM images of Cs3Bi2Br9 NCs. (f) Size distribution histogram of Cs3Bi2Br9 NCs.

The Cs3Bi2Br9 NCs were synthesized in a one-step reaction. We used dimethyl sulfoxide (DMSO) as the solvents to dissolve CsBr and BiBr3 to form precursor solution. Isopropanol was used as the anti-solventto precipitate NCs. After that, a yellow-green colloidal solution was formed, indicating the nucleation of the Cs3Bi2Br9 NCs, as shown in Figure. S1a (details in the SI). The crystal lattice of Cs3Bi2Br9 consists of metal halide octahedral layers with the voids between the layers filled with Cs+, as shown in Figure 1a.[29,30] In the unit cell one Cs+ group is surrounded by six metal halide octahedral (for details of structural data see Table S1).[29,30] X-ray diffraction (XRD) measurements (Figure. 1b) show that the sample exhibits trigonal P3m1 symmetry.[29,30] Figure. 1c shows transmission electron microscopy (TEM) images of as-synthesized NCs. The NCs exhibited quasi-spherical shape with an average diameter of 6 nm and a size distribution of about 2 nm (Figure. 1f). A highresolution TEM image of Cs3Bi2Br9 NCs (Figure. 1d, 1e) reveals a high crystallinity with a lattice spacing of 0.33 nm, corresponding to that of (003) plane in Cs 3Bi2Br9 trigonal structure (0.328 nm).[30,31] Additionally, the selected area

B. Yang, F. Hong, X. Mao, S. Yang, Y. Li, A. Li, Prof. Dr.W. Deng, Prof. Dr.K. Han State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, P. R. China. E-mail: [email protected] B. Yang, F. Hong, X. Mao, Y, Li University of the Chinese Academy of sciences, Beijing 100039, P. R. China. J. Chen, K. Zheng,Prof. Dr.T. Pullerits Department of Chemical Physics and NanoLund, Chemical Center, Lund University, P.O. Box 124, 22100 Lund, Sweden. Supporting information for this article is given via a link at the end of the document

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Angewandte Chemie International Edition

COMMUNICATION electron diffraction pattern in insert of Figure. 1c confirms high crystallinity of the Cs3Bi2Br9 perovskite NCs. Next we studied the steady-state optical properties of the Cs3Bi2Br9 NCs. As shown in Figure 2a, a long sub-band gap tail up to 700 nm has been observed in the absorption spectrum, which will be discussed in details below. The absorption spectrum shows pronounced band edge exciton peak at 439 nm (Figure. 2a). Compared with the bulk single crystal, the exciton peak of NCs is blue-shifted by 20 nm,[31] suggesting the quantum confinement effect. The exciton peak is similar to the thin film sample (440 nm),[32] which indicates large exciton binding energy. The PL spectrum exhibits an emission peak around 468nm for the ligand free NCs at the red side of the absorption edge with a Stokes shift of 175meV and shows a FWHM of 40 nm (Figure. 2a), which is similar to that in MA3Bi2Br9 NCs (FWHM: 62 nm).[22] In addition, such band edge emission of our NCs cannot be detected in the single crystal at room temperature,[30] and low PL with 5 peaks (463, 468, 473, 481, and 492 nm) were observed in bulk thin films.[32] The single PL peak with narrow FWHM further confirmed the high quality and narrow size distribution of the NCs. The PLQE of as-obtained ligand free NCs was 0.2%. However, the PLQE can be significantly increased to 4.5 % with extra surfactant (OA) added (for details see SI). We also noted that the PL spectra of OA capped NCs show blue-shift to about 460 nm, and the FWHM broadens to about 45 nm (Figure S6). Analyses of the temperature dependent PL provides the exciton binding energy of OA capped NCs of 67±5 meV (SI, Figure S8), which is about

one order of magnitude higher than the binding energy in bulk single crystals.[30] Notably, the long absorption tail is greatly suppressed by OA addition (Figure. 2a). To reveal photo-physics of the low PLQE of ligand free NCs and the improved PLQE of OA capped NCs, we first carried out time-resolved PL (TR-PL) measurement on both ligand free and OA capped NCs by using time correlated single photon counting (TCSPC). The PL decays were well fitted by a tri-exponential function with a short-lived component (τ1 15 ns). (see Figure. 2b and Table S2). Here the contribution of the ultra-long lived component is negligible with relative amplitudes (RAs)3 ns). The ultrafast trapping transfer process is responsible for the low PLQE of the NCs. The trap states can be partially passivated by OA enhancing PL emission from the NCs.

Cs3Bi2I9is hard to detect). The PLQE of OA capped Cs3Bi2(Br0.5Cl0.5)9 NCs can be increased to 0.3%, whereas OA have negligible influence on other halide composition (details in SI). TR-PL results (Figure S12, Table S4) show that high RAs of trap-related processes may lead to the low PLQE.

Figure 4.(a) The PLQE value of ligand free Cs3Bi2Br9 NCs measured under different storing time in air. (b) XRD spectra of as-synthesized Cs3Bi2Br9 NCs (black), Cs3Bi2Br9 NCs stored in open air for 15 days before (blue) and after annealing (red). (c) PLQE values of ligand free NCs stored in various air humidity conditions of 30-50%, 50-70% and 70-90% for 48 hours.(d) TR-PL kinetics of as-synthesized ligand free Cs3Bi2Br9 NCs and after being stored in open air for 22 days.

The stability of perovskite NCs is critical for their long-term operation. To evaluate the natural stability of Cs3Bi2Br9 NCs (without ligand capped), the PLQE was recorded under ambient moisture conditions (RH=30–50%, without light). It should be noted that the NCs are aggregated after storing in open air for 24 hours, and we re-dispersed the NCs by ultrasonication for 5 minutes before PLQE measurement. Only a slight red-shift of the PL spectra occurs during 15 days of exposer, and the FWHM stays almost the same (Figure S13). Interestingly, Figure. 4a shows that the PLQE of Cs3Bi2Br9 NCs increased when stored in ambient conditions for one month showing a maximum value after 22 days. XRD patterns of Cs3Bi2Br9 NCs stored for 10 days in air are shown in Figure. 4b. The XRD (blue line) shows an additional peaks close to the (001) peak at 9.3°. Notably, the peak disappeared when annealing (100 ℃ ) was carried out to the aged Cs3Bi2Br9 NCs (red line in Figure. 4b), and the patterns were identical to as-synthesized NCs (black line). Cs3Bi2Br9 NCs stored in ambient air for 30 days (the XRD pattern shows an additional peaks close to the (003) peak at 29.2°) has similar pattern as after 10 days (see Figure. S14). This is probably due to the adsorption of water on the surface of Cs3Bi2Br9 NCs followed by the formation of a perovskite hydrate.[38,39] The enhancement of PLQE is probably due to the perovskite hydrate that can passivate surface trap-states.[38,39] In order to validate this hypothesis we first measured the PLQE under different humidity conditions. In particular, the assynthesized NCs were divided into three groups and stored under humidity condition: 30-50%, 50-70% and 70-90% for 48 hours, respectively. Figure. 4c shows that the PLQE is lowest under 30-50% humidity (close to ambient humidity) and it grows with increase of the humidity. Next,we compared the PL decay

Figure 3. (a) Photographs of as-obtained colloidal Cs3Bi2X9 (X= Cl, Cl0.5Br0.5, Br, Br0.5I0.5, I). (b) XRD patterns of NCs containing pure and mixed halides. (c) Steady-state absorption and PL spectra of NCs containing pure and mixed halides.

Bi-perovskite NCs with other halide composition (Cs3Bi2X9, X = Cl, Br0.5Cl0.5, Br0.5I0.5, I) can be synthesized by the same one-pot method where various Cs halides and Bi halides has been used as precursors for injection (for details see SI.). The as-synthesized Cs3Bi2X9 (X = Cl, Br0.5Cl0.5, Br, Br0.5I0.5, I) NCs exhibit color changes from light blue to dark red when the halide is varied from Cl to I as shown in Figure. 3a. The XRD pattern (Figure. 3b) show that the I, Br, Cl based NCs have preferred orientation and iodine ions produce a hexagonal space group p63/mmc (Table S3, Figure.S11).[35-37] Furthermore, we studied the absorption and PL properties of as-synthesized Cs3Bi2X9 perovskite NCs, shown in Figure. 3c. Absorbance measurements of NCs revealed the exciton peaks range from 380 to 510 nm, while the PL peaks ranging from 400 to 560 nm. Compared with above-mentioned Cs3Bi2Br9 NCs, the PLQEs of other samples are much lower (0.09% for Cs3Bi2Cl9 and 0.08% for Cs3Bi2(Br0.5Cl0.5)9, while emission from Cs3Bi2(Br0.5I0.5)9 and

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COMMUNICATION of the as-synthesized NCs and NCs stored in air for 22 days. It is obvious that the RAs of band-edge PL increased after 22 days (Figure. 4d). Ligand free Cs3Bi2Cl9 and Cs3Bi2(Br0.5Cl0.5)9 NCs show similar properties (Figure. S15). The OA capped Cs3Bi2Br9 NCs also show good moisture stability (Figure S16). The photo-stability of ligand free Cs3Bi2Br9 NCs was also studied under the UV light illumination (370 nm, 1.3 mW/cm2). As shown in Figure. S17, the Cs3Bi2Br9 NCs showed higher stability, with only 23% decrease of PL intensity after 400 min continuous illumination. In summary, we report on a facile and scalable route for the synthesis of lead-free, Cs3Bi2X9 (X= Cl, Br, I) perovskite NCs. TR-PL and TA measurements were used to study the excited state dynamics of the as-synthesized Cs3Bi2Br9 NCs. It is found that the fast trapping process (2~20 ps) lead to a low PLQE of ligand free Cs3Bi2Br9 NCs (0.2%) and the trapping process can be passivated by OA, which can enhance the PLQE up to 4.5%. Notably, Cs3Bi2Br9 NCs exhibited superior air-stability exceeding 30 days.

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We are grateful to the National Basic Research Program of China (2013CB834604), the National Natural Science Foundation of China (Grant No: 21533010 and 21321091), LLC grant via Laserlab-Europe EU-H2020 654148, Swedish Research Council, KAW foundation and NPRP grant # NPRP7227-1-034 from the Qatar National Research Fund (a member of Qatar Foundation) for their financial support.

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Keywords:perovskite•nanocrystal•bismuth• ligand• photoluminescence

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COMMUNICATION COMMUNICATION Bin Yang, Junsheng Chen, Feng Hong, Xin Mao, Kaibo Zheng, Songqiu Yang, Yajuan Li, Tõnu Pullerits, Wei-Qiao Deng, and Ke-Li Han* Page 1 – Page 4

Lead-Free, Air-stable All-Inorganic Cesium Bismuth Halide Perovskite Nanocrystals

Lead-free, all-inorganic perovskite Cs3Bi2X9 (X= Cl, Br, I) nanocrystals (NCs) are synthesized. The ligand capped Cs3Bi2Br9 NCs exhibits blue emission with photoluminescence quantum efficiency up to 4.5 % and exhibits high stability for over one month.

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