Thermally Stable Copper(II)-Doped Cesium Lead Halide Perovskite

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Thermally Stable Copper (II) Doped Cesium Lead Halide Perovskite Quantum Dots with a Strong Blue Emission Chenghao Bi, Shixun Wang, Qiang Li, Stephen V Kershaw, Jianjun Tian, and Andrey L. Rogach J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00290 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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The Journal of Physical Chemistry Letters

Thermally Stable Copper (II) Doped Cesium Lead Halide Perovskite Quantum Dots with a Strong Blue Emission

Chenghao Bi,† Shixun Wang,† Qiang Li,† Stephen V. Kershaw,‡ Jianjun Tian†* and Andrey L. Rogach‡ *

†Institute

for Advanced Materials and Technology, University of Science

and Technology Beijing, 100083, China. ‡Department

of Materials Science and Engineering, and Centre for

Functional Photonics (CFP), City University of Hong Kong, Kowloon, Hong Kong SAR

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

Abstract All-inorganic perovskite quantum dots (QDs) have emerged as potentially promising materials for lighting and displays, but their poor thermal stability restricts their practical application. In addition, optical characteristics of the blue-emitting CsPbX3 QDs lag behind their red and green emitting counterparts. Herein, we addressed these two issues by doping divalent Cu2+ ions into the perovskite lattice to form CsPb1-xCuxX3 QDs. Extended X-ray absorption fine structure (EXAFS) measurements reveal that doping smaller Cu2+ guest ions induces a lattice contraction and eliminates halide vacancies, which leads to an increased lattice formation energy and improved short-range order of the doped perovskite QDs. This results in the improvement of both the thermal stability and the optical performance of CsPb1-xCux(Br/Cl)3 QDs, which exhibit bright blue photoluminescence at 450-460 nm, with a high quantum yield of over 80%. The CsPb1-xCuxX3 QD films maintain stable luminescence performance even when annealed at temperatures of over 250 °C. Keywords: lead halide perovskite nanocrystals, copper doping, improved thermal stability, blue-emitting materials, lattice engineering

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Lead halide perovskite quantum dots (QDs), and in particular all-inorganic cesium lead halide (CsPbX3, X = Cl, Br and I) nanoparticles have attracted a lot of attention recently,1-5

owing

to

their

outstanding

properties,

such

as

tunable

and

narrow-linewidth photoluminescence (PL) through the entire visible spectrum, high defect tolerance and large photon absorption cross-sections.2,

6-8

CsPbX3 QDs have

been considered as potential candidates for a variety of applications such as solar cells,9-11 lasers,3,

12, 13

photodetectors,14-16 and light-emitting diodes (LEDs).17-26 To

date, green-emitting CsPbBr3 QDs and red-emitting CsPbI3 QDs with PL quantum yields (QY) of more than 90% have been successfully synthesized,27,

28

and the

external quantum efficiencies (EQE) of green and red LEDs based on these materials have risen to over 10%.25, 26, 29 It is equally important to produce high-performance blue-emitting CsPbX3 QDs, which are usually obtained by tuning the proportion of mixed halide anions, namely Br- and Cl-. 2 However, the PL QY of such mixed-halide CsPb(Br/Cl)3 QDs is typically less than 40%, due to the lattice mismatch.20 Another strategy towards achieving blue emission color is to shift the PL peak of the entirely bromide-based CsPbBr3 QDs by reducing their size and controlling the shape based on quantum confinement effects, such as making 2D CsPbBr3 nanoplatelets with a PLQY of 54%.4 We recently reported blue-emitting core/shell CsPbBr3/amorphous CsPbBrx QDs with a high PL QY of over 80%, which however suffered from poor thermal stability due to the metastable amorphous shell.30 For practical LED applications, the temperature of the device often exceeds 60C under continuous operations,5 making the thermal stability of perovskite QD emitters an important practical issue. Another reported synthetic strategy aimed at enhancing the formation energies and modulating the kinetics of exciton relaxation of perovskite QDs has been the doping of smaller divalent cations into the host lattice of CsPbX3 QDs in place of Pb2+ cations.31 Mn2+ doping of CsPbCl3 perovskite QDs has been realized by a direct synthetic method, leading to the inclusion of the characteristic emission of Mn2+ in the 3



resulting dual-color emitting particles, as well as the enhancement of the overall PL QY from 5% to 54%.32-35 Stam et al. reported CsPbMxPb1-xBr3 (M = Sn2+, Cd2+, and Zn2+) QDs made by post-synthetic cation exchange, through the partial replacement of Pb2+ cations by the mentioned divalent cations.36 Particles with a blue emission at 479 nm and PL QY of 62% have been demonstrated, while we note that emission at this wavelength corresponds not to pure blue but to a sky blue light. Although introducing dopants into perovskite QDs could improve their luminescence, the blue-emitting QDs (450~460 nm) still have much lower PL QY (~60%).36, 37 Copper, the transition metal element with a small ionic radius (73 pm for Cu2+, as compared to 119 pm for Pb2+) is another potential dopant for perovskite QDs. Doping Cu2+ into the host lattices of the traditional II-VI and III-V QDs has been successful in achieving modulation of their optical and electronic performance.38-40 Herein, we demonstrate a successful doping of CsPbX3 QDs to partly substitute Pb2+ with Cu2+, which resulted in the formation of CsPb1-xCuxX3 (X=Br, Br/Cl) QDs with improved PL QY and thermal stability. Benefiting from doping the Cu2+ ions, the mixed halide (Br/Cl) perovskite QDs showed blue emission in the range of 450-460 nm, with a narrow PL linewidth and PL QY of over 80%. Increased lattice binding energy and the improved short-range order of the lattice are identified as factors improving both the thermal stability and PL QYs of the Cu-doped QDs. The demonstrated ability of doping the divalent copper ions into CsPbX3 QDs is a powerful strategy to obtain blue-emitting perovskite QDs with remarkable thermal stability, which may open additional possibilities for a range of applications in optoelectronic devices, such as LEDs, lasers and sensors.

As schematically illustrated in Figure 1a, in the case of successful doping Cu2+ ions would occupy the Pb2+ ion site, leading to the contraction of the octahedra through replacement of some of the larger Pb2+ ions (119 pm) by smaller Cu2+ ions (73 pm). Such a contraction of the octahedra causes contraction of the Cu-X bond length with a 4


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higher formation energy than that of the Pb-X bond, further resulting in a more stable lattice and much-improved thermal stability as shown in Figure 1b.31,

41

We have

carried out first-principle calculations based on density functional theory (DFT) for the formation energy of CsCuBr3, CsPbBr3 and CsPb1-xCuxBr3, with results shown in Table S1, and the absolute value of the formation energy was indeed observed to rise with incorporation of Cu2+. For instance, the formation energies are -14.2 eV for CsCuBr3, -6.82 eV for CsPb0.93Cu0.07Br3 and -6.56 eV for CsPbBr3, which corroborates that CsPb1-xCuxBr3 QDs are more energetically stable than their counterparts without Cu2+. Moreover, the contraction of the octahedra increases the interaction between Pb and X orbits, thus the energy band structure of the CsPbX3 QDs is changed, which is in agreement with the data shown in Figure 1c-d and Figure S1. The calculation also shows that doping divalent Cu2+ induces the increase of the bandgap of CsPbX3, which results in an optical blue-shift. At the same time, halide vacancies and distorted Pb-X octahedra in the lattice of CsPbX3 could destroy the short-range order of the lattice and act as trapping centers for charge carriers, leading to poor PLQYs for the band-edge emission. On the one hand, partial substitution of Pb2+ with a smaller B-site cation can reduce the extent of Pb-X octahedral distortion.31 On the other hand, the introduction of smaller B-site cation can increase the defect formation energy, which eliminates the distortion of the octahedral by suppressing the formation of the halide vacancies. We therefore hypothesized that doping Cu2+ into the CsPbX3 lattice could improve the order of the local coordination environment of Pb and enhance the short-range order of the lattice; in the case it does not introduce new recombination channels, it could be beneficial for the radiative recombination and increase PLQY of the doped perovskite nanocrystals.

5



Figure 1. (a) Schematic diagram illustrating the possible mechanism of the formation of Cu-doped CsPbX3 QDs (CsPb1-xCuxX3). Incorporation of smaller divalent Cu2+ cations results in the contraction of the octahedra. (b) Schematic diagram of the suggested mechanism responsible for the increase of the thermal stability of Cu-doped perovskite QDs. DFT calculated electronic band structures for the (c) CsPbBr3 and (d) CsPb1-xCuxBr3 QDs.

The x value in CsPb1-xCuxBr3 QDs has been tailored by varying the molar feed ratio of PbBr2 and CuBr2 from 1:0 to 1:3; the values of x for the actual incorporated copper element in the CsPb1-xCuxBr3 QDs was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES), as given in Figure S2. Figure 2 and Figure S3 show transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of the CsPb1-xCuxBr3 QDs with different x values ranging from 0 to 0.23. As shown in Figure 2a-d, clear lattice fringes throughout the particles were observed, indicating that both CsPbBr3 QDs and CsPb1-xCuxBr3 QDs are highly crystalline. The HRTEM images show that both CsPbBr3 QDs and CsPb1-xCuxBr3 QDs possessed the orthorhombic crystal structure corresponding to the (100) plane. Crystalline lattice constants for the (100) planes were calculated based on the HRTEM and fast Fourier transform (FFT) images. The lattice constants decreased from 0.580 nm to 0.561 nm as the x value increased from 0.04 to 0.23, which is in a 6


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good agreement with the lattice contraction owing to the smaller Cu2+ ions which substitute the larger Pb2+ ions. Figure 2e shows X-ray diffraction (XRD) patterns of the CsPb1-xCuxBr3 QDs, indicating their orthorhombic crystalline structure similar to that of the CsPbBr3 QDs. The diffraction peaks can be indexed to the planes of orthorhombic CsPbBr3 (JCPDS No. PDF#18-0364). Notably, XRD patterns of CsPbBr3 and CsPb0.12Cu0.88Br3 QDs show a distinct split of the diffraction peak at 2θ equal to 14.9°and 15.1°, which can be attributed to poor short-range order due to the octahedral distortion and partially distorted lattice planes.42 As shown in Figure 2f, the peak position for the (200) reflex shifts monotonically to higher angles with increasing x value, which is consistent with the HRTEM and XRD refinement results (Table S2), indicating gradually smaller lattice parameters with increasing x values. Hence, we infer that doping small-sized divalent copper ions into the cesium lead bromide perovskite lattice indeed results in a lattice contraction, thus offering the possibility to improve the formation energy of the lattice. We notice that XRD patterns of CsPb0.88Cu0.12Br3 and CsPb0.77Cu0.23Br3 QDs show the diffraction peak of the impurity phase of CsCuBr3 (JCPDS No. PDF#48-1219), which correlates with an appearance of some irregular-shaped nanoparticles as seen in the HRTEM images of these samples (Figure S3b and Figure 2d). Indeed, some additional CsCuBr3 phase may be formed due to the presence of an excessive amount of CuBr2 additive. As shown in Figure S4, both CsPbBr3 and CsPb0.93Cu0.07Br3 QDs exhibited XPS peaks of Cs, Pb, Br, C. However, two additional peaks in CsPb0.93Cu0.07Br3 QDs appeared at 933 and 953 eV, which can be ascribed to the Cu 2d signals of Cu2+, proving that Cu2+ ions have been doped into CsPbBr3 QDs. Furthermore, the high-resolution XPS spectra (Figure S4d) revealed that the binding energy of Pb2+ 4f 5/2 and Pb2+ 4f 7/2 in CsPb0.93Cu0.07Br3 QDs decreased in comparison to that of CsPbBr3 QDs, which is caused by the lattice contraction due to the incorporation of Cu2+ ions in CsPbBr3 QDs. Quantitative XPS analysis indicates a Cu/Pb ratio of 7/93 for the CsPb0.93Cu0.07Br3 QDs, which is consistent with the ICP-OES. The TEM and HRTEM images, and XRD patterns of CsPb(Br/Cl)3 and CsPb0.93Cu0.07(Br/Cl)3 are shown in Figure S5 and Figure S6. The lattice shrinkage can also be observed in 7



CsPb0.93Cu0.07(Br/Cl)3 QDs according to the experimental results, which is in good agreement with our assumption that the introduction of Cu2+ can lead to a lattice contraction.

Figure 2. TEM images of CsPb1-xCuxBr3 QDs with different values of x, namely 0 (a1), 0.05 (b1), 0.07 (c1), and 0.23 (d1). Frames (a2) - (d2) show HRTEM images of the corresponding samples. The inset reveals the corresponding FFT spectra. (e) XRD patterns of the CsPb1-xCuxBr3 QDs with different values of x; frame (f) provides the enlarged-view comparison of the (200) peak position. Black clover symbols in (e) label the peaks corresponding to the Cu-related impurity.

The UV−vis absorption spectra of CsPb1-xCuxBr3 QDs with different x values are shown in Figure 3a. The first excitonic peak of CsPb1-xCuxBr3 QDs shifts to higher energy with increasing Cu2+ concentration, indicating the larger bandgap due to the improved lattice contraction. In addition, the weaker Urbach tails in the 8


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CsPb1-xCuxBr3 QD absorption spectra reveal that doping Cu2+ can indeed contribute to the better crystallinity of perovskite QDs and thus a sharp absorption edge.43, 44 The PL spectra reveal that the emission peak of CsPb1-xCuxBr3 QDs experiences a blue-shift from 517 to 499 nm (Figure 3b) with increasing x value. Notably, an additional PL peak of CsPb0.77Cu0.23Br3 QDs at 430 nm can be observed in Figure 3b, which may be due to the emission of Cu2+ in the host at the high doping level.

45

Generally, the blue-shifted optical spectra can be attributed to the contraction of the host perovskite, leading to a stronger interaction between Pb and Br orbitals. The lower conduction band of the lead halide perovskites is composed of the Br 4p-orbitals and Pb 6p-orbitals. We carried out a first-principle theoretical calculation on the electronic structures and density of states by Vienna Ab-initio Simulation Package (VASP) code as shown in Figure S1. The results for CsPb1-xCuxBr3 perovskites demonstrate the extended bandgap and the higher conduction-band minimum, indicating that the experimental results are in good agreement with the theoretical calculation.

9



Figure 3. (a) Optical absorption, (b) PL spectra, (c) PL QYs and (d) time-resolved PL decays (shown by dots; three-exponential fitting curves are given by solid lines) of CsPb1-xCuxBr3 QDs with different x values, as indicated on the frames. The inset in (b) shows a photograph of the green-emitting CsPb0.93Cu0.07Br3 QDs under the 365 nm excitation. (e) and (f) Optical 10


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absorption and PL spectra of CsPb(Br/Cl)3 QDs and CsPb0.93Cu0.07(Br/Cl)3 QDs, respectively; the insets show photographs of these solutions under 365 nm excitation. (g) Time-resolved PL decays (shown by dots) of the CsPb(Br/Cl)3 QDs and CsPb0.93Cu0.07(Br/Cl)3 QDs; the three-exponential fitting is given by solid lines of the same color. (h) PL QYs of thCsPb1-xCux(Br/Cl)3 QDs compared with the PL QY values for blue-emitting perovskite QDs reported in the literature.

The PL QY values provided in Figure 3c show that the emission increases from 85% for the undoped CsPbBr3 QDs to 95% for CsPb0.93Cu0.07Br3 QDs, and then gradually decreases to 43% for CsPb0.77Cu0.23Br3 QDs. The time-resolved PL decays of the samples are given in Figure 3d, with the fitting average lifetimes (τave) provided in Table S3. It can be seen that the trend of the average and radiative decay rates of CsPb1-xCuxBr3 is highly consistent with the variation of PL QYs. We notice that the PL QY of the CsPb1-xCuxBr3 QDs increases upon changing x value from 0 to 0.07, accompanied by the slower PL kinetics (longer average lifetimes and faster radiative decay rates). The CsPb0.93Cu0.07Br3 QDs exhibit the fastest radiative decay rates (0.24 ns-1) and the highest PLQY (95%). Typically, in our work, the PL QYs of QDs dropped abruptly when the x value was more than 0.07, accompanied by the decrease of the radiative decay rates (from 0.24 ns-1 to 0.18 ns-1). These results suggest that the Cu2+ doping favors the suppression of defects recombination, but the excessive Cu2+ ions would act as additional non-radiative relaxation channels, leading to the decrease of the radiative decay rates. We further synthesized mixed-anion CsPb1-xCux(Br/Cl)3 QDs with a Pb/Cu ratio of 0.93/0.07 in an attempt to obtain bright emitting material with a truly blue PL color. From

the

UV-vis

absorption

and

PL

spectra

of

CsPb(Br/Cl)3

and

CsPb0.93Cu0.07(Br/Cl)3 QDs (Figure 3e and 3f), we conclude that the introduction of Cu2+ ions leads to a blue-shift of both the UV absorption peak (from 448 nm to 444 nm) and the PL emission peak (from 466 nm to 453nm). The absorption spectrum of CsPb(Br/Cl)3 QDs near the edge is flat, but the CsPb0.93Cu0.07(Br/Cl)3 QDs shows a distinct first excitonic peak, which can be related to the high crystallinity of the CsPb0.93Cu0.07(Br/Cl)3 QDs.46 Cu ions as dopants in the host perovskite matrix have a strong preference for octahedral coordination with halide ions due to the shorter Cu-X 11



bond length and high formational energy, which can effectively improve the extent of reaction and the crystallinity of the CsPb1-xCux(Br/Cl)3 QDs. In addition, Stokes shift of CsPb0.93Cu0.07(Br/Cl)3 (0.06 eV) is smaller than that of the CsPb(Br/Cl)3 QDs (0.11 eV), suggesting that the CsPb0.93Cu0.07(Br/Cl)3 have fewer interband traps than that of the CsPb(Br/Cl)3 QDs due to the doping of Cu2+. Importantly, CsPb0.93Cu0.07(Br/Cl)3 QDs with an emission peak at 453 nm (which corresponds to the truly blue emission) show a remarkably high PL QY of 80%, which is much higher than that of the undoped CsPb(Br/Cl)3 sample (PL QY of 23%), and to the best of our knowledge constitutes a record PL efficiency for Br/Cl mixed halide perovskite QDs emitting at 450-460 nm. A further blue-shift of the PL peak could be achieved by changing the ratio of Pb/Cu to 0.91/0.09; the resulting CsPb0.91Cu0.09(Br/Cl)3 QDs exhibited a PL peak at 446 nm (Figure S7) while still maintaining a high PLQY of 78%. Time-resolved PL decays of the CsPb(Br/Cl)3 and CsPb0.93Cu0.07(Br/Cl)3 QDs are shown in Figure 3g. The CsPb0.93Cu0.07(Br/Cl)3 QDs show much longer average lifetimes and faster radiative decay rates than those of the CsPb(Br/Cl)3 QDs, owing to the introduction of Cu2+. The PL QYs of the whole range of CsPb1-xCux(Br/Cl)3 QDs (with values of x varying from 0.07 to 0.12) synthesized in this work are summarized and compared with previous studies on blue-emitting perovskite QDs in Figure 3h. It can be observed that the PL QYs of the CsPb0.93Cu0.07(Br/Cl)3 QDs synthesized in this work are much higher than that of all other previous reports. 30, 37, 47-49

EXAFS spectra have been measured to analyze the local coordination environment and the local order around the center atoms of QDs with and without the copper dopant; they are presented in Figure 4. Figure 4a and b display the k3-weighted Pb LIII-edge EXAFS spectra and the corresponding Fourier transforms of the scattering from CsPbBr3 and CsPb0.93Cu0.07Br3 QDs. Both samples show a periodic amplitude (see Figure 4a), indicating that the Cu-doped samples still maintain a distinct short-range order which is similar to the undoped sample. This implies that the Cu2+ doping does not affect the short-range order of the perovskite lattice. In the R space 12


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(Figure 4b), the asymmetric peak for CsPbBr3 QDs could be attributed to the Pb-Br octahedral distortion. Contrary to that, CsPb0.93Cu0.07Br3 QDs show a symmetric peak in the R space, indicating that the incorporation of Cu2+ can suppress the octahedral distortion and improve the short-range order of the lattice. Furthermore, the CsPb0.93Cu0.07Br3 QDs show a stronger amplitude in the R space than compared with that of the CsPbBr3 QDs, suggesting the improved order of the local coordination environment around the Pb. To further understand the doping-induced variation of the short-range order of the lattice, we performed a single-shell fit of both EXAFS spectra over the R range of 1.8−3.2 Å. There are four parameters in the model: the coordination number (CN), the energy shift (ΔE0), the adjustment of the half path length (ΔR), and the mean-square relative displacement of absorber and backscatter atoms (σ2).

50

The related fitting

results of the EXAFS spectra are summarized in Table 1. The R-factors of both samples are far less than 0.02, corroborating the fitting results. The distances between Pb and Br have been determined as 3.01 Å for CsPbBr3 and 2.92 Å for CsPb0.93Cu0.07Br3, respectively, showing that the Cu2+ doping indeed causes the shorter Pb-Br bond and the lattice contraction. Hence, the formation energy of the perovskite lattice will be increased by doping copper, which is consistent with the DFT calculation results (Table S1), and the thermal stability of the CsPbX3 perovskite QDs also will be enhanced. The CN of Pb with Br atoms is 6.0 in the CsPb0.93Cu0.07Br3 QDs, which is the optimum CN (6) of the CsPbX3 perovskite structure. In contrast, the CN of CsPbBr3 QDs is 5.0, which is less than the optimum CN of the classic perovskite structure. This observation demonstrates that Cu2+ doping can tremendously reduce the number of Br vacancies. σ2 reflects the mean-square disorder in the distribution of interatomic distances.50 A smaller σ2 (0.012 Å2) is observed for the CsPb0.93Cu0.07Br3 QDs, which is different from the CsPbBr3 QDs (0.015 Å2), indicating that Cu2+ doping could improve the order of the perovskite lattice. Taken as a whole, the EXAFS data provide a strong evidence that the Cu2+ doping can inhibit formation of structural defects and improve the order of 13



the perovskite lattice. The improvement of PL QY can be associated with improved short-range order and decreased halide vacancies of the lattice. However, at the other extreme, excessive Cu2+ doping would cause stronger disorder of the lattice such as the lower CN and weaker amplitude, which results in decreased PL QY (see Figure S8a, b and Table S4). To further prove the location of Cu2+ in the doped CsPbX3 structure, the k2-weighted Cu K-edge EXAFS and the corresponding Fourier transforms of CsPb0.93Cu0.07Br3 are shown in Figure 4c, d and Figure S8c. As seen in the X-ray absorption near-edge structure (XANES) spectra (Figure 4c), a relatively symmetric white line near 8990 eV is observed, which is a typical spectral feature of Cu2+ in an octahedral coordination. The peak located at about 1.5 Å (the distance not phase corrected) in the R space (Figure 4d) indicates that the interatomic distance between the absorbing and surrounding atoms (Cu-Br bond) is 1.5 Å, which is much shorter than that of the Pb-Br bond (3.0 Å). The doping-induced significant change of the local environments of Pb and Cs, and the octahedral coordination of Cu2+ ions verified that isovalent substitution of Cu for Pb had occurred.

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Figure 4. Local structure characterizations of CsPbBr3 and CsPb0.93Cu0.07Br3 QDs. (a) k3-weighted Pb LIII-edge EXAFS and (b) corresponding Fourier transforms (FTs) of CsPbBr3 and CsPb0.93Cu0.07Br3 QDs. (c) Cu K-edge XANES spectrum of CsPb0.93Cu0.07Br3 QDs. (d) k2-weighted Cu K-edge EXAFS Fourier transforms (FT) of CsPb0.93Cu0.07Br3 QDs.

Table 1. Results of the fit performed on the Pb LIII-edge k3-weighted EXAFS spectra for CsPbBr3 and CsPb0.93Cu0.07Br3 QDs. Sample

shell

CNa

ΔE0(eV)b

CsPbBr3

Pb-Br

5.0 ± 1.8

2.4 ± 2.2

CsPb0.93Cu0.07Br3 Pb-Br 6.0 ± 0.6

4.6 ± 3.4

aThe

σ2(Å2)c

R(Å)d

0.015 ± 0.002 0.012 ± 0.001

3.01 ± 0.03 2.92 ± 0.02

R-facto r 0.004 0.001

fitted coordination number (CN). bThe energy shift. cThe mean-square relative

displacement of the absorber and backscatter atoms. dThe distance between the absorber and backscatter atoms. To study the thermal stability of the Cu-doped and undoped perovskite samples, the XRD-derived structural studies and the PL QY estimations for the CsPbBr3 QDs and CsPb0.93Cu0.07Br3 QDs deposited on the glass slides and heated to different 15



temperatures (70 °C, 120 °C, 150 °C, 200 °C, 250 °C) ambient atmosphere have been carried out. As shown in Figure 5a, the CsPbBr3 QDs could only retain their original orthorhombic crystal structure after annealing at no more than 120 °C. When the annealing temperature was raised to 150 °C, new diffraction peaks at 11.6° 24.1°, 35.4° and 47.1° emerged, which correspond to the diffraction of (002), (112), (212) and (206) planes of the CsPb2Br5 phase, formed due to the structural rearrangement of the perovskites according to the previous report.51 When the annealing temperature was increased to 250 °C, the enhanced diffraction peaks of CsPb2Br5 at 11.7°and 33.3° became evident. In contrast, CsPb0.93Cu0.07Br3 QDs were able to retain the orthorhombic crystal structure for annealing temperatures of up to 250 °C (Figure 5b), evidenced by an absence of any diffraction peaks corresponding to the CsPb2Br5 phase. Furthermore, as seen from Figure S9, a much weaker decline of the PL emission is observed for the annealed CsPb0.93Cu0.07Br3 QDs, as their PL QY still remains as high as 35% after annealing at 250 °C, while the PL QY of the CsPbBr3 QDs strongly diminishes to only 1%. These data suggest that the weak thermal stability of CsPbBr3 QDs is related to the formation of the impurity phase of CsPb2Br5, which the transformation from CsPbBr3 to CsPb2Br5 is attributed to a two-step process, starting from the decomposition of the CsPbBr3 at high temperature (CsPbBr3 → PbBr2 + CsBr), then following by the reaction of PbBr2 with CsPbBr3 to form the CsPb2Br5 phase.

52, 53

The improved thermal stability of the copper doped

CsPbX3 QDs may be related to the following factors. As we discussed above, doping of Cu2+ ions into the CsPbX3 lattice would induce the octahedral contraction; the Cu-X bond length is shorter than that of Pb − X, which favors the increase of the formation energy of the lattice, as supported by our first-principles calculation and EXAFS results. The value of the formation energy for CsPb0.93Cu0.07Br3 (-6.82 eV) was increased by 0.26 eV with respect to CsPbBr3 (-6.56 eV). Thus, CsPb0.93Cu0.07X3 QDs cannot easily decompose or react with any PbBr2 due to the higher formation energy and more stable crystal structure. Figure 5c-d shows the photographs of the blue-emitting QD films, and the corresponding PL QYs of the CsPb(Br/Cl)3 QDs and CsPb0.93Cu0.07(Br/Cl)3 QDs annealed at different temperatures. The PL QYs of the 16


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CsPb0.93Cu0.07(Br/Cl)3 QDs decreases only slightly (from 53% to 40%) after annealing at 250 °C, while the PL QY of the CsPb(Br/Cl)3 QDs dropped from 11% to nearly zero. Also, the CsPb1-xCuxX3 QDs exhibit excellent long-term stability as shown in Figure S10. CsPb0.93Cu0.07Br3 QDs and CsPb0.93Cu0.07(Br/Cl)3 QDs solution maintain a nearly constant PL QYs with a value of 90% and 80% after storage for 30 days under ambient conditions with the humidity of 60% at 25 °C. (Figure S10a) However, the PL QYs of undoped ones showed strongly diminishes to nearly zero after 30 days. Also, we further verified the stability of the CsPb1-xCuxX3 QDs films prepared by spin-coating on glass substrates under ambient conditions. (Figure S10b) The solid-state thin films based Cu-doped QDs show the excellent long-term stability, which still retain the initial PL QYs after storage for 16 days. In contrast, the PL QYs of undoped QDs films undergo the striking degradation to be zero after 16 days. These results demonstrate the potential for the practical application. At last, the photoluminescence and stability properties for blue-emitting perovskite QDs were summarized in Table S6.

54, 55

Our CsPb1-xCux(Br/Cl)3 QDs exhibited high PLQY,

good air stability in the ambient condition and specific excellent thermal stability.

Figure 5. XRD patterns of perovskite films based on (a) CsPbBr3 QDs and (b) 17



CsPb0.93Cu0.07Br3 QDs after annealing at different temperatures, as indicated on frames. Black clover symbols and the corresponding crystals planes indicate the position of peaks corresponding to the crystallographic structure of CsPb2Br5. (c) Photographs showing different emission colors and (d) corresponding PL QYs of CsPb0.93Cu0.07(Br/Cl)3 QDs (in red) and CsPb(Br/Cl)3 QDs (in black) deposited as thin films on glass slides, as a function of the annealing temperature.

Using the hot-injection method, we successfully synthesized copper doped CsPb1-xCuxX3 (X = Br, Cl, or Br/Cl) QDs with a strong PL emission in the green and blue spectral range, and superior thermal stability. The incorporation of smaller divalent Cu2+ ions leads to the overall contraction of the perovskite lattice. Such a lattice contraction increases the interaction between Pb and X orbitals, resulting in the change of the energy band structure of the CsPbX3 QDs, as demonstrated by first-principle calculations, EXAFS and XANES. Reduced octahedral distortion and fewer halide vacancies could be achieved by introducing Cu2+ into CsPbX3, which enhanced the short-range order and PL QY of the QDs. Additionally, the shorter bond length improves the formation energy of perovskite lattices, resulting in the significant improvement of the thermal stability of Cu-doped QDs. The CsPb0.93Cu0.07(Br/Cl)3 QDs show a truly blue emission in the wavelength range of 450-460 nm, with a remarkably high PL QY of over 80%. In addition, after annealing at 250 °C under ambient conditions, the doped QD film still maintain a PL QY of 40%, which was 75% of the initial value (53%). The Cu(II) doping approach demonstrated here opens up new possibilities for the synthesis of high-performance bright blue-emitting perovskite QDs with excellent thermal stability, thus providing a novel platform for stable perovskite-based blue-emitting optoelectronic devices.

ASSOCIATED CONTENTS Supporting Information. The experimental methods, calculated density of states for 18


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CsPbBr3 and CsPb0.93Cu0.07Br3 QDs (Figure S1), the x values for the CsPb1-xCuxBr3 QDs determined by ICP-OES (Figure S2), TEM images of the CsPb1-xCuxBr3 QDs (Figure S3), XPS spectra for the CsPbBr3 and CsPb0.93Cu0.07Br3 QDs (Figure S4), TEM and HRTEM images of the CsPb(Br/Cl)3 QDs and CsPb0.93Cu0.07(Br/Cl)3 QDs (Figure S5) XRD patterns of the CsPb(Br/Cl)3 and CsPb0.93Cu0.07(Br/Cl)3 QDs (Figure S6), optical absorption and PL spectra of the CsPb0.91Cu0.09(BrCl)3 QDs and evolution of PL spectra for the CsPb1-xCux(Br/Cl)3 QDs with the different x values (Figure S7), k3-weighted Pb LIII-edge EXAFS of CsPbBr3 and CsPb0.88Cu0.12Br3 QDs (Figure S8), thermal stability of Cu-doped CsPb0.93Cu0.07Br3 QDs and bare CsPbBr3 QDs (Figure S9), calculated formation energies for CsPbBr3 QDs doped with different Cu2+ contents by using first-principle calculations based on DFT (Table S1), the lattice constants of CsPb1-xCuxPbBr3 QDs as a function of x values, estimated from XRD patterns (Table S2), summary of time-resolved PL fitting average lifetimes, PL QYs and radiative decay rates of CsPb1-xCuxBr3 QDs with different x values (Table S3), results of the fits performed on the Pb LIII-edge k3-weighted EXAFS spectra for CsPbBr3 and CsPb0.88Cu0.12Br3 QDs (Table S4), summary of optical properties, including emission peak positions, PL FWHM and PL QYs of the CsPbX3 and Cu-doped CsPbX3 QDs (Table S5). AUTHOR INFORMATION *Corresponding authors: [email protected] 19



[email protected] ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (51774034, 51772026), National Key Research and Development Program of China (2017YFE0119700), Beijing Natural Science Foundation (2182039), National Natural Science Foundation of China/Research Grants Council Joint Research Scheme (N_CityU108/17), and the Talent Introduction Plan of Overseas Top Ranking Professors by the State Administration of Foreign Expert Affairs (MSBJLG040). We thank the staff at the 1W2B beamline at the Beijing Synchrotron Radiation Facility for EXAFS measurements. Notes The authors declare no competing financial interests.

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Thermally Stable Copper(II)-Doped Cesium Lead Halide Perovskite

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