Bright Luminescence from Nontoxic CsCu2X3 (X
= Cl, Br, I) Rachel Roccanova,1‡ Aymen Yangui,1‡ Gijun Seo,2 Tielyr D. Creason,1 Yuntao Wu,3,4 Do Young
Kim,2 Mao-Hua Du,5 Bayrammurad Saparov1*
1Department of Chemistry and Biochemistry, University of Oklahoma, 101 Stephenson Parkway,
Norman, OK 73019, USA
Ave, Tulsa, OK 74106, USA
3Synthetic Crystal Research Center, Shanghai Institute of Ceramics, Chinese Academy of
Sciences, Shanghai, 201800, China
4Scintillation Materials Research Center and Department of Materials Science and Engineering,
University of Tennessee, Knoxville, Tennessee, 37996, USA
5Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN
37831, USA
EXPERIMANTAL METHODS
Reactants. Chemicals utilized in this study were used as purchased: (i) copper (I) chloride,
99.99%, Acros Orcganics; (ii) copper(I) bromide, 99.999%, Aldrich; (iii) copper iodide, 99.9%,
Aldrich; (iv) cesium chloride, 99.99%, Acros Organics; (v) cesium bromide, 99.9%, Acros
Organics; (vi) cesium iodide, 99.999%, Acros Organics.
Synthesis of CsCu2X3 (X = Cl, Br, I). Crystalline ingots were prepared using a 1:2 stoichiometric
ratio of CsX to CuX (X = I, Br, Cl) ground in an agate mortar, pelletized and sealed under dynamic
vacuum in quartz ampules. Pelletized samples were annealed at 410 °C for 48 hours and slowly
cooled over 20 hours to room temperature resulting in polycrystalline ingots.
Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) measurements were performed on
a Rigaku MiniFlex600 system equipped with a Dtex detector using a Ni-filtered Cu-Kα radiation
source. All scans were performed at room temperature from the 5-90 (2θ) range, with a step size
of 0.2. All data were corrected for the amorphous background of the glass slides used during
collection and fitted using the Pawley method through Rigaku’s PDXL2 software package. To
check the air stability, samples were left in ambient air for more than two months with periodic
PXRD measurements using the same condition mentioned above.
Thermal Analysis. Simultaneous thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) measurements were carried out on a TA Instruments SDT650 unit.
Measurements were performed using 90 µL alumina crucibles on 8-10 mg samples under a 100
mL/min flow of dry nitrogen in the 100–575 °C range with 5 °C/min heating rate.
Optical Measurements. Room temperature diffuse reflectance spectra of polycrystalline powder
of CsCu2X3 of were measured using a high-resolution PerkinElmer LAMBDA 750 UV−vis−NIR
spectrometer equipped with a 100 mm InGaAs integrating sphere attachment. The diffuse
3
reflectance data were converted to pseudoabsorption spectra according to the the Kubelka-Munk
equation:1 (F(R) = α/S = (1-R)2/(2R), where R is the reflectance, α is the absorption coefficient, S
is the scattering coefficient.
measurements were performed at ambient temperature, on polycrystalline powder samples, using
a HORIBA Jobin Yvon Fluorolog-3 spectrofluorometer equipped with a Xenon lamp and Quanta-
φ integrating sphere. PLQY data were analyzed using the two-curve method in a varied range from
280 – 800 nm using the imbedded QY software in the Horiba-Jobin Yvon software.
Time resolved photoluminescence (TRPL) measurements were done on polycrystalline powder
samples using a HORIBA Jobin Yvon Fluorolog-3 spectrofluorometer equipped with a time-
correlated single photon counting module. HORIBA Jobin Yvon NanoLEDs (pulsed light-emitting
diodes) were used as the excitation source. The duration of the light pulse was shorter than 2 ns.
Temperature and power dependence PL spectra were measured using a Princeton Instruments
PIXIS-eXcelon silicon CCD. The excitation wavelength was the 325 nm (3.815 eV) line of a He-
Cd laser (Kimmon Electric HeCd dual-wavelength laser; model: IK552R-F). The samples were
placed in a helium bath cryostat, and the measurements were performed between 4 and 295 K.
In depth structural analysis. Isolation of the copper halide tetrahedra as ribbons can be seen most
prominently down the b and c axis (as shown in Figure 1(b-c)) where the Cs+ cations fill in the
channels separating [Cu2X3]- “nanowires.” which greatly impacts the observed luminescence in
this family. The Cu-X bond distances in the [Cu2X3]- chains vary from 2.272 to 2.490 Å for
CsCu2Cl3, 2.427 to 2.571 Å for CsCu2Br3, and 2.604 to 2.703 Å for CsCu2I3, following the
expected trend based on the increasing halide ionic radii going down the group. Noticeable
distortions of the CuX4 tetrahedra are evident from the tetrahedral angles (X-Cu-X) of 102.45 to
4
119.31º, 106.41 to 116.48º, and 107.1 to 114.24º for X = Cl, Br, and I, respectively. Interestingly,
a trend of decreasing tetrahedral distortions going down the group is observed in this series.
In octahedral systems, connectivity and magnitude of distortion are known to affect the band
structure, emission properties, and defect formation within a perovskite lattice and are evaluated
using the bond lengths and angles between the metals and halides and the volume of the individual
octahedra.2 Deviation from ideal octahedral geometry results in an increase in distortion within a
perovskite system and has been shown to negatively affect the overlap between the orbitals of the
metal and halides resulting in wider band gaps and blue shifting the onset of absorption,3 as well
as decreasing the PLQY and lifetime due to an increase in the reduced mass of excitons within the
system.4 Among the quantitative methods used to determine the magnitude of distortion within a
perovskite system, the variation in octahedral distance (Δd),5 angle (σ2 oct),6 and the overall
octahedral elongation (<λoct>)6 have been used in particular to relate increase in distortion with
increased Stokes-shifts, full width at half maximums (FWHMs), and broad white-light emission
caused by the self-trapping of carriers resulting from strong exciton-phonon coupling.7-10 Such
broadband luminescence is commonly seen in alkali halides,10-11 hybrid organic-inorganic
materials,9, 12-19 and recently in all-inorganic metal halides such as A3M2I9 (A = Cs, Rb; M = Bi,
Sb),20 CsZnCl2I,21 (C8NH12)4Bi0.57Sb0.43Br7,22 and Cs2AgInCl6.23 To correlate the optical properties
of the CsCu2X3 systems with that of other octahedral systems, attempts were made to relate the
calculated tetrahedral distortion to the optical properties reported below. The effect of tetrahedral
distortion on optoelectronic properties in solid-state structures have been rarely studied with no
reports comparing the magnitude of distortion to the observed properties, like in many octahedral
systems. Robinson et al. were the first to propose quantitative tetrahedral distortion parameters
that directly correlated to their proposed octahedral parameters σ2 oct and λoct, allowing for the
5
comparison of the amount of distortion present in completely different polyhedra-based systems.6
Using this method, we calculated the σ2 tet and <λtet> values for CsCu2X3 and found that the
distortion from both σ2 tet and <λtet> decrease from CsCu2Cl3 to CsCu2I3. Typically four parameters,
variation in angle (σ2, ΔθXMX) and bond distance (<λ>, Δd) have been used to quantify octahedral
distortion through the differences in bond distances and angles, respectively.6, 24 In an attempt to
relate the observed luminescence of CsCu2X3 to the structural distortions observed in these
compounds, we performed similar distortion analysis adopted for the tetrahedral geometry. The
tetrahedral angle variance (σ2 tet) and the average tetrahedral elongation <λtet> are given by the
following equations:6
=1
where σ2 tet is deviation in bond angle of the system, θtet is the individual tetrahedral angles between
the center metal and each adjacent ligand, <λtet> is the overall octahedral elongation, li is the
measured distance between the metal center and each ligand, and l0 is the ideal bond distance
determined from the ionic radii.
Only octahedral Δd and ΔθXMX relationships have been previously reported, however, they can
be adapted for tetrahedral systems via the following equations:
= 1
2
where Δd is one fourth of the summation of each difference of each individual bond distance
(dn) the average bond distance (d) of the tetrahedra in question squared, and ΔθXMX is one sixth of
the summation of the absolute value of the difference of each individual angle (θXMX(n)) and the
average angle of the tetrahedra in question ⟨⟩ squared. The results of these analyses are
summarized in Table S2. Confirming the noticeable trend observed for bond angles, σ2 tet, <λtet>,
Δd, and ΔθXMX all suggest decreasing tetrahedral distortion from CsCu2Cl3to CsCu2Br3 to
CsCu2I3This trend is opposite of what would be expected if one only considers the increase in
atomic radii from Cl to I. However, this opposite trend has been previously reported in the
tetrahedral-based (SC(NH2))2CdX2 system,25 and 2D octahedral-based system
((CH3)2NC6H4NH3)PbX4 (X = Br, I).26 For CsCu2X3, as shown in Table 1, the Stokes-shift
increases from 208 to 249 nm and the FWHM of the broadband emission increases from 102 and
200 nm, going from for CsCu2Cl3, to CsCu2I3, which is typically due to increasing distortion within
octahedral systems.7-10, 12-18
Device fabrication. LED fabrication was attempted based on CsCu2X3 (Figure S11). In order to
fabricate LEDs, a CsCu2I3 was used as a yellow additive in an 1,3-Bis(N-carbazolyl)benzene
(mCP) host layer. LEDs were fabricated on patterned indium tin oxide (ITO) glass substrates. An
ITO was used as a transparent bottom anode. The substrates were first cleaned with acetone and
isopropanol in an ultrasonic cleaner and subsequently rinsed with de-ionized water, blown dry with
N2 gas, and treated with UV ozone. A MoO3 and a 4,4′-Cyclohexylidenebis[N,N-bis(4-
methylphenyl)benzenamine] (TAPC) were used as a hole injection layer and a hole transport layer,
respectively. CsCu2I3 (10 vol. %) doped in mCP host was used as a yellow emission layer. A 1,3,5-
7
tri (m-pyrid-3-yl-phenyl) - benzene (TmTyPB) and a LiF were used as an electron transport layer
and the electron injection layer, respectively. An Al was used as a top reflective cathode. All layers
were deposited sequentially by vacuum thermal evaporation at a pressure of 10-6 Torr. The area of
the device was 4 mm2.
Computational Methods. Density functional theory calculations were performed using the VASP
code.27 The interaction between ions and electrons was described by projector augmented wave
method.28 The valence wavefunctions were expanded in a plane-wave basis with a cut-off energy
of 369 eV. All atoms were relaxed to minimize the Feynman-Hellmann forces to below 0.02 eV/Å.
The electronic band structure and the density of states were calculated based on
Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional.29 The band gap was further
corrected by the hybrid PBE0 functional.30
8
Figure S1. Room temperature powder X-ray diffraction (PXRD) patterns (black lines) for (a)
CsCu2Cl3, (b) CsCu2Br3, (c) CsCu2I3. Pawley fits of the data are shown in red and difference
plots shown in blue. In CsCu2Cl3 samples, a CsCl impurity peak (marked with a green asterisk)
is noticeable.
Figure S2. Room temperature powder X-ray diffraction (PXRD) patterns (black lines) for (a)
CsCu2Cl1.5Br1.5, and (b) CsCu2Br1.5I1.5. In our CsCu2Cl1.5Br1.5 sample, unassigned impurity peaks
totaling 5.4% based on intensity ratios are marked by green asterisks.
10
Figure S3. (a-c) PXRD refined lattice parameters, and (d) unit cell volume shown as a function of
halide content as alloying progresses from CsCu2Cl3 (Cl) to CsCu2Br3 (Br) to CsCu2I3 (I).
11
Figure S4. PXRD patterns of (a) CsCu2Cl3, (b) CsCu2Br3, and (c) CsCu2I3 left out in ambient
conditions and measured over a period of 2 months.
12
of (a) CsCu2Cl3, (b) CsCu2Br3, and (c) CsCu2I3.
13
Figure S6. (a) Kubelka-Munk plots of diffuse reflectance for CsCu2X3 and mixed-halide alloys
with (b-c) estimated band gap fittings.
14
Figure S7. (a) Band structure and (b) density-of-states (DOS) plots for CsCu2Cl3 calculated
using the DFT-PBE method. Note that the band gap is underestimated.
15
Figure S8. Normalized PLE spectra of CsCu2I3 measured for different emission wavelengths.
16
Figure S9. Power dependence of PL spectra for CsCu2X3 measured at 4 K under 325 nm
excitation. Black curves shows the fitting using y ~ Pk, with y, P, and k are the PL intensity, the
excitation power, and the refinement coefficient, respectively.31
17
Figure S10. Temperature dependence of PL for (a) CsCu2Cl1.5Br1.5, (b) CsCu2Br3, (c)
CsCu2Br1.5I1.5, and (d) CsCu2I3 under 325 nm irradiation. CsCu2Cl3 is shown in the main text in
Figure 4.
10-6
10-4
10-2
100
102
0.2
0.4
0.6
0.8
1.0
(c)
PeakEL=554nm
FWHM=136nm
Figure S11. Light-emitting diode (LED) fabricated using CsCu2I3 as an additive in a mCP host.
(a) Luminance-current-voltage (LIV) characteristics, (b) quantum efficiency (inset: the device
structure), and (c) electroluminescence (EL) spectrum (inset: a photo image of the LED
operating at 8 V) of the LED.
19
Table S1. Summary of lattice constants from Pawley fits of the PXRD data for CsCu2X3.
Composition a (Å) b (Å) c (Å) V (Å3)
CsCu2Cl3 9.4925(2) 11.8780(2) 5.5935(2) 630.67(2)
CsCu2Cl1.5Br1.5 9.5541(9) 12.2059(11) 5.7218(5) 667.27(11)
CsCu2Br3 9.866(2) 12.348(7) 5.816(5) 708.6(5)
CsCu2Br1.5I1.5 9.9424(7) 12.9562(9) 5.9935(4) 772.06(9)
CsCu2I3 10.545(2) 13.173(9) 6.099(9) 847.4(1)
20
Table S2. Selected interatomic distances (Å) and angles (°) in CsCu2X3 based on the
crystallographic data reported in literature.32-34
Label Distance (Å) Label Angle (°)
CsCu2Cl3
Cl2 2.272(8) Cl1-Cu-Cl3 107.33(5)
Cl3 2.273(0) Cl1-Cu-Cl4 109.49(2)
Cl4 2.490(4) Cl2-Cu-Cl3 109.49(2)
Br2 2.427(0) Br1-Cu-Br3 108.15(5)
Br3 2.427(0) Br1-Cu-Br4 108.61(0)
Br4 2.571(4) Br2-Cu-Br3 108.61(0)
I2 2.604(2) I1-Cu-I3 107.10(2)
I3 2.604(2) I1-Cu-I4 108.91(0)
I4 2.703(0) I2-Cu-I3 114.24(4)
21
Table S3. Results of the tetrahedral distortion evaluation showing a linear decrease in distortion
with increase in halogen size.
CsCu2Cl3 CsCu2Br3 CsCu2I3
<λtet> 0.9785 0.9537 0.8984
Δd 21.67 x 10-4 13.59 x 10-4 3.05 x 10-4
ΔθXMX 68.84 x 10-4 19.44 x 10-4 14.37 x 10-4
Table S4. Summary of the time-resolved PL refinement results for CsCu2X3.
Sample CsCu2Cl3 CsCu2Cl1.5Br1.5 CsCu2Br3 CsCu2Br1.5I1.5 CsCu2I3
Excitation
(nm)
Emission
(nm)
527 587 533 584 576
I0 152.5 ± 0.6 85 ± 0.5 49.9 ± 0.27 15.2 ± 0.5 35.2 ± 0.7
A1 8.8 1022 ± 6.4
102
(ns) 2.1 ± 0.04 1.6 ± 0.02 1.9 ± 0.01 72.2 ± 1.7 62± 2
A2 6.4 104 ± 5.7
102
356 ± 197
(ns) 13.8 ± 0.6 15.1 ± 1.1 18 ± 0.01 26.6 ± 0.7 126.5 ± 20
Table S5. Temperature-dependent PL refinement for CsCu2X3.
Sample CsCu2Cl3 CsCu2Cl1.5Br1.5 CsCu2Br3 CsCu2Br1.5I1.5 CsCu2I3
(meV.K-1) 0.05 ± 0.01 0.02 ± 0.008 0.3 ± 0.04 0.18 ± 0.04 027 ± 0.01
(meV.K-1) 471 ± 13 712 ± 32 512 ± 62 910 ± 42 617 ± 22
(meV) 6.4 ± 0.3 6.2 ± 0.6 5.1 ± 0.2 11.5 ± 0.2 4.3± 0.3
22
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