Efficient and chromaticity stable green and white organic light-emitting devices with organic-inorganic hybrid materials

Article abstract of DOI:10.1039/d0ra02122a

Efficient inverted bottom emissive organic light emitting diodes (IBOLEDs) with tin dioxide and/or Cd-doped SnO₂nanoparticles as an electron injection layer at the indium tin oxide cathode:electron transport layer interface have been fabricated. The SnO₂NPs promote electron injection efficiently because their conduction band (?3.6 eV) lies between the work function (W_f) of ITO (4.8 eV) and the LUMO of the electron-transporting molecule (?3.32 eV), leading to enhanced efficiency at low voltage. The 2.0percent SnO₂NPs (25 nm) with Ir(ddsi)₂(acac) emissive material (SnO₂NPs/ITO) have an enhanced current efficiency (η_c, cd A^?1) of 52.3/24.3, power efficiency (η_p, lm W^?1) of 10.9/3.4, external quantum efficiency (η_ex, percent) of 16.4/7.5 and luminance (L, cd m^?2) of 28?182/1982. A device with a 2.0percent Cd-doped SnO₂layer shows higherη_c(60.6 cd A^?1),η_p(15.4 lm W^?1),η_ex(18.3percent) andL(26?858 cd m^?2) than SnO₂devices or control devices. White light emission was harvested from a mixture of Cd-SnO₂NPs and homoleptic blue phosphor Ir(tsi)₃; the combination of blue emission (λ_EL= 428 nm) from Ir(tsi)₃and defect emission from Cd-SnO₂NPs (λ_EL= 568 nm) gives an intense white light with CIE of (0.31, 0.30) and CCT of 6961 K. The white light emission [CIE of (0.34, 0.35) and CCT of 5188 K] from colloid hybrid electrolyte BMIMBF₄-SnO₂is also discussed.

Full text of DOI:10.1039/d0ra02122a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Efficient and chromaticity stable green and white
organic light-emitting devices with organic–
inorganic hybrid materials†
Cite this: RSC Adv., 2020, 10, 21206
Venugopal Thanikachalam,
and Jayaraman Jayabharathi
Balu Seransenguttuvan
*
Efficient inverted bottom emissive organic light emitting diodes (IBOLEDs) with tin dioxide and/or Cd-
doped SnO nanoparticles as an electron injection layer at the indium tin oxide cathode:electron
transport layer interface have been fabricated. The SnO NPs promote electron injection efficiently
because their conduction band (ꢀ3.6 eV) lies between the work function (W ) of ITO (4.8 eV) and the
2
2
f
LUMO of the electron-transporting molecule (ꢀ3.32 eV), leading to enhanced efficiency at low voltage.
The 2.0% SnO
2
NPs (25 nm) with Ir(ddsi)
2
2
(acac) emissive material (SnO NPs/ITO) have an enhanced
ꢀ1
ꢀ1
current efficiency (h
c
, cd A ) of 52.3/24.3, power efficiency (h
p
, lm W ) of 10.9/3.4, external quantum
ꢀ2
efficiency (hex, %) of 16.4/7.5 and luminance (L, cd m ) of 28 182/1982. A device with a 2.0% Cd-doped
ꢀ
1
ꢀ1
ꢀ2
SnO
devices or control devices. White light emission was harvested from a mixture of Cd–SnO
homoleptic blue phosphor Ir(tsi)
; the combination of blue emission (lEL ¼ 428 nm) from Ir(tsi)
defect emission from Cd–SnO
NPs (lEL ¼ 568 nm) gives an intense white light with CIE of (0.31, 0.30)
and CCT of 6961 K. The white light emission [CIE of (0.34, 0.35) and CCT of 5188 K] from colloid hybrid
electrolyte BMIMBF –SnO is also discussed.
2
layer shows higher h
c
(60.6 cd A ), h
p
(15.4 lm W ), hex (18.3%) and L (26 858 cd m ) than SnO
NPs and
and
2
2
Received 6th March 2020
Accepted 21st May 2020
3
3
2
DOI: 10.1039/d0ra02122a
rsc.li/rsc-advances
4
2
electron injection layer (EIL). However, these methods require
a high annealing temperature (300 C) for better electron
1
. Introduction
ꢁ
Organic light-emitting diodes (OLEDs) have been attractive for transportation. ZnO nanoparticles (NPs) have been used as an
2
0–25
ꢁ
use in displays and solid-state lighting due to their exibility, efficient EIL
with a low process temperature (˂100 C), but
1
–3
26
color contrast, color purity, low power-consumption, etc.
Inverted bottom emissive OLEDs (IBOLEDs) with a bottom decided to use SnO
cathode are more advantageous than conventional OLEDs due gap and low sensitivity towards UV light.
to their minimized voltage drop. However, efficient electron
White light emission is a hot topic of research because white
ZnO is unstable under UV illumination. Therefore, it was
2
2
and Cd-doped SnO NPs with a wide band
27,28
injection is a current issue in IBOLEDs due to the lack of suit- light emitting devices (WOLEDs) would drastically reduce
able cathode materials. Because of its high optical transparency global electricity consumption. Organic dyes, lanthanide-doped
2
9–31
32
and electrical conductivity, ITO is used as a cathode in IBO- semiconducting NPs, quantum dots,
DNA, nano dye
33
34
LEDs, but its high work function (4.8 eV) limits electron injec- polymer conjugates, core–shell materials and ZnO NPs with
tion from ITO to an electron transporting layer (ETL). Electron graphene and GaN are used for white light emission. Near-UV
injection has been enhanced by incorporation of a metallic layer chips with a blend of red, green and blue phosphors or
a combination of yellow and blue emitters have been used for
cesium or rubidium. However, these methods lead to oper- white light generation because of the high CCT (correlated color
35
4
5
with a low work function and doped ETL with lithium,
6–8
9
36,37
ational instability by oxidation of the metal or diffusion of the temperature) of these LEDs.
Though blue light emission
10,11
38–40
metal dopant
and they need a high-cost vacuum process. from SnO
2
NPs has been widely studied,
2
doping of SnO by
These issues can be overcome by solution processable methods metal ions with various ionic radii, different structures, elec-
1
2
13–15
using conjugated polymers and n-type metal oxides,
oxide,
zinc tronegativities and valency differences leads to additional
16,17
18
19
41
titanium dioxide and zirconium dioxide as an defect emission. Cadmium has been used as a dopant due to
4
+
its lower valency and larger ionic radius than Sn ions and it
generates various defects responsible for visible emission.
Herein, we report efficient IBOLEDs using SnO and Cd–SnO -
Department of Chemistry, Annamalai University, Annamalainagar 608 002,
Tamilnadu, India. E-mail: jtchalam2005@yahoo.co.in
2
2
NPs as EILs with a green phosphorescent emissive layer. The
fabricated device (2% SnO NPs/ITO) exhibits a maximum
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra02122a
1
2
21206 | RSC Adv., 2020, 10, 21206–21221
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
ꢀ
1
current efficiency (h
c
, cd A ) of 52.3/24.3, power efficiency (h
p
,
2.2. Iridium(III)-bis-(E)-1-(2,3-dihydrobenzo[b][1,4]dioxin-6-
ꢀ
1
lm W ) of 10.9/3.4, external quantum efficiency (h , %) of 16.4/ yl)-4,5-diphenyl-2-styryl-1H-imidazole acetylacetonate
.5 and luminance (L, cd m ) of 28 182/1982. Also, a device [Ir(ddsi) (acac)]
ex
ꢀ2
7
2
with a 2.0% Cd-doped SnO layer shows a higher h (60.6 cd
2
c
A mixture of ddsi (2.2 mmol) and iridium(III) chloride trihydrate
ꢀ1
ꢀ1
ꢀ2
A
), h
SnO devices or control devices. The white emission was har-
vested from a mixture of Cd–SnO NPs and homoleptic blue
phosphor Ir(tsi)
; the combination of blue emission (lEL ¼ 428
nm) from Ir(tsi) and defect emission from Cd–SnO NPs (lEL
68 nm) gives an intense white light (CIE (0.31, 0.30); CCT ¼
961 K).
p
(15.4 lm W ), hex (18.3%) and L (26 858 cd m ) than
(
1 mmol) in 2-ethoxyethanol : H
C) and the formed dimer (1 mmol) was reuxed (120 C; N
2
O (3 : 1) was reuxed (N
2
; 120
2
ꢁ
ꢁ
2
)
2
2 3
with K CO (2.5 mmol) and acetyl acetone (2.2 mmol) in 2-
3
46
ethoxyethanol (5 ml). The green-colored acetylacetonate
3
2
¼
iridium complex was characterized by spectral techniques
5
6
1
(
Scheme 1). H NMR (400 MHz, CDCl ): d (ppm): 1.21 (d, 2H),
3
1.47–1.72 (m, 2H), 3.39 (m, 2H), 4.36 (s, 4H), 4.44 (d, 2H), 5.88
(
6
m, 2H), 5.98 (m, 2H), 6.2 (d, 2H), 6.49 (m, 2H), 7.11–7.14 (m,
1
3
H), 7.21–7.26 (m, 12H), 7.42 (m, 8H). C NMR (100 MHz,
): d (ppm): 26.18, 44.02, 46.82, 64.31, 105.49, 105.72,
13.26, 115.82, 121.86, 126.45, 127.02, 128.09, 128.79, 132.01,
2. Experimental
CDCl
1
1
3
2.1. (E)-1-(2,3-Dihydrobenzo[b][1,4]dioxin-6-yl)-4,5-diphenyl-
2-styryl-1H-imidazole (ddsi)
+
34.31, 134.94, 147.54. MS: m/z. 1207.40 [M ]. Calcd 1207.31.
A mixture of benzil (5 mmol), (E)-3-(4-uorophenyl)acryl-
aldehyde (5 mmol), 1,4-benzodioxane-6-amine (6 mmol) and
2
(
.3. (E)-1-(2,3-Dihydrobenzo[b][1,4]dioxin-5-yl)-2-(4-
ammonium acetate (61 mmol) was reuxed (ethanol; 12 h; N
stream). The reaction mixture was chilled and the yellow solid
2
triuoromethyl)styryl)-1H-imidazole (tsi)
stream). H NMR (400 A mixture of (E)-3-(4-triuoromethylphenyl)acrylaldehyde (1
MHz, CDCl ): d (ppm): 4.36 (s, 4H), 6.7 (m, 3H), 6.99 (s, 2H), mmol), glyoxal (1 mmol), 2,3-dihydro-1,4-benzodioxane-6-
42
1
of ddsi was separated (ethanol; 12 h; N
2
3
1
3
7
.14–7.22 (m, 5H), 7.30–7.32 (m, 6H), 7.48 (d, 4H). C NMR (100 amine (1 mmol) and ammonium acetate (1 mmol) in ethanol
MHz, CDCl ): d (ppm): 64.3, 112.8, 114.5, 114.6, 121.9, 123, was reuxed (12 h; N ). The mixture was cooled, ltered and
25.4, 126.4, 127.5, 128.8, 129.3, 131.9, 133.1, 133.4, 135.2, 137, column chromatographed (9 : 1 benzene:ethyl acetate). Yield:
3
2
1
1
6
2
+
ꢀ1
1
37.0, 147.6. MS: m/z. 458.55 [M ]. Calcd 458.35. FT-IR (cm ): 71%. H NMR (400 MHz, CDCl
90, 762, 846, 935, 1006, 1018, 1230, 1515, 1606, 1620, 1650, 3H), 6.99 (s, 2H), 7.23–7.3 (m, 3H), 7.40–7.5 (m, 3H). C NMR
850, 2960, 3070, 3420. (100 MHz, CDCl ): d (ppm): 64.3, 112.8, 114.5, 115.63, 115.77,
3
): d (ppm): 4.39 (s, 4H), 6.7 (m,
1
3
3
Scheme 1 Synthetic route for emissive materials.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 21206–21221 | 21207

View Article Online
RSC Advances
Paper
˚
1
1
(
21.96, 124.28, 125.19, 125.48, 126.79, 130.26, 133.45, 137.00, 1.5406 A with a tube current of 30 mA at 40 kV. The energy
+
37.61, 138.51, 147.68. MS: m/z. 372.34 [M ] calcd 372.21. FT-IR dispersive X-ray spectra (EDS) were recorded with a JEOL JSM-
cm ): 698, 832, 886, 945, 1027, 1156, 1228, 1505, 1600, 1632, 5610 scanning electron microscope (SEM) equipped with
ꢀ1
1
644, 2860, 3426.
a back electron (BE) detector. The sample was placed on an
adhesive carbon slice supported on copper stubs and coated
with 10 nm thick gold using a JEOL JFC-1600 auto ne coater
prior to measurement. TEM analysis was carried out using
a JEOL JEM 2100 high-resolution transmission electron micro-
scope (HR-TEM) with 200 kV voltage. The uorescence lifetime
was measured in nanoseconds with a time correlated single
photon counting (TCSPC) spectrometer and Horiba Fluorocube-
2.4. fac-Tris[2-(4-triuoromethylstyryl)-1-(2,3-dihydrobenzo
2
1
[
[
b][1,4]dioxin-8-yl)-1H-imidazolynato-C ,N ]iridium(III)
Ir(tsi)
3
]
A mixture of tsi (7.83 mmol), iridium(III) trisacetylacetonate
1.56 mmol) and glycerol (9 ml) was reuxed under an N stream
for four days. Aer cooling, the reaction mixture was extracted
with dichloromethane and the separated fac-isomer Ir(dsi) was
(
2
01-NL lifetime system with a nano LED (pulsed diode excitation
3
1
source) as the excitation source and TBX-PS as the detector. The
UV-vis absorption and uorescence spectra were recorded with
a PerkinElmer Lambda 35 spectrophotometer and a Perki-
nElmer LS55 spectrouorometer, respectively. The absolute
puried by hexane washing (Scheme 1). H NMR (400 MHz,
CDCl ): d (ppm): 4.36 (s, 6H), 4.44 (d, 3H), 5.06 (t, 3H), 5.21 (t,
3
3
H), 5.88–5.98 (m, 6H), 6.19–6.2 (m, 3H), 6.49–6.53 (m, 4H), 7.23
1
3
(m, 6H), 7.40 (m, 6H). C NMR (100 MHz, CDCl
3
): d (ppm):
5
1
1
6.05, 59.02, 64.33, 103.48, 105.72, 111.76, 114.38, 121.39,
23.36, 124.2, 125.1, 126.7, 127.00, 128.9, 129.5, 130.2, 132.00,
+
38.22, 147.56. MS: m/z. 1310.30 [M ]; calcd 1310.19.
2.5. 1-Butyl-3-methylimidazolium tetrauoroborate
4
[BMIMBF ]
About 10.8 g of n-bromobutane and 8.2 g of 1-methyimidizole
ꢁ
were reuxed under stirring (70 C; 48 h). The reaction mixture
was washed with ethyl acetate to eliminate the unreacted
reagents and the solvent was evaporated to get the pale yellow
42
liquid of 1-butyl-3-methylimidazolium bromide (BMIMBr).
About 6.57 g of BMIMBr and 3.29 g of NaBF were reuxed in
4
acetone under stirring at low temperature (10 h). The reaction
ꢁ
mixture was extracted with dichloromethane and dried (80 C; 2
42
1
h). H NMR (300 MHz, CDCl
H), 1.33 (m, 2H), 1.77 (m, 2H), 3.73 (m, 2H). C NMR (76 MHz,
CDCl ): d (ppm): 13.89, 20.60, 33.22, 44.45, 54.06, 122.85,
3
): d (ppm): 0.9 (s, 1H), 0.96 (m,
1
3
3
3
123.00, 137.07.
2
.6. Nanocrystalline SnO and Cd-doped SnO
2
2
To a reaction mixture of tin tetrachloride pentahydrate (SnCl4-
5H O) without or with cadmium chloride, moderate PEG in
$
2
distilled water and aq. NH
stirring (pH 9). The milky colloidal solution was subjected to
stirring for 3 h followed by centrifugation and the formed SnO
3
were added drop by drop under
2
and/or Cd-doped SnO
2
was washed with distilled water, dried
ꢁ
ꢁ
(
80 C; 12 h) and calcinated (400 C; 2 h; heating rate
ꢁ
0 C min ) to form SnO and or pale yellow colored Cd-doped
2
ꢀ1
1
SnO solid.
2
2
.7. BMIMBF –Cd–SnO colloidal hybrid electrolyte
4 2
About, 0.1 mmol of cationic IL BMIMBF was added to Cd–SnO
4
2
in CHCl dispersion and sonicated for 2 h. The formed light
3
yellow viscous BMIMBF –Cd–SnO colloidal hybrid electrolyte
4
2
was used to generate white emission.
2
.8. Characterization
The powder X-ray diffractogram (XRD) was recorded with
a PANalytical X'Pert PRO diffractometer using Cu Ka rays at Cd–SnO
Fig. 1 XRD pattern of SnO
2 2 4
(a), Cd-doped SnO (b) and HMIMBF –
2
(c).
21208 | RSC Adv., 2020, 10, 21206–21221
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
quantum yield (PLQY) of the emissive material was determined doped SnO
2
and BMIMBF
4
–Cd–SnO
2
hybrid electrolyte nano-
in dichloromethane with a 0.5 M H SO solution of quinine crystals were found to be 4.8, 2.5 and 18.25 nm and 220.5, 423.3
2
4
2
ꢀ1
44
(
0.54) as reference using a uorescence spectrometer Model- and 57.98 m g , respectively.
F7100
and
using
the
equation:
The SEM images of pristine SnO , Cd-doped SnO2 and
2
BMIMBF –Cd–SnO nanomaterials reveal agglomeration of
4 2
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Iunk
Istd
Astd
h
unk
f
¼ fstd
, where funk and fstd are the
unk
nanoparticles (Fig. 2). The energy dispersive X-ray spectra
conrm the presence of constituent elements. The observed
carbon peaks are due to the carbon stub on which the samples
were dispersed and the absence of other elements reveals the
purity of the nanomaterial. The EDX spectra of cationic
Aunk
h
std
radiative quantum yield of the sample and standard; Iunk and
I
std are the integrated emission intensities of sample and
standard, respectively; Aunk and Astd are the absorbance of
sample and standard, respectively; and hunk and hstd are the
refractive index of sample and standard solution, respectively.
The solid-state quantum yield was measured on a quartz plate
with an integrating sphere.
BMIMBF –Cd–SnO show the presence of carbon, nitrogen,
4
2
cadmium and tin, which conrms the binding of BMIMBF4
with Cd–SnO
2
. The HR-TEM images of pristine SnO
2
, Cd-doped
SnO and BMIMBF
2
4
–Cd–SnO hybrid electrolyte conrm the
2
nanoparticulate character. The measured average particle sizes
from the TEM image are larger than those deduced from the
XRD results because the nanoparticles associate among them-
selves (Fig. 3). TEM images at higher magnication display the
lattice fringes and the observed interplanar distance is 0.34 nm
2
.9. Fabrication of OLED
Green OLEDs with a conguration of ITO/x wt% SnO (x ¼ 0.5,
2
1.0, 1.5 or 2.0%) or x wt% Cd–SnO (2.0%)/3TPYMB (40 nm)/
2
2 3
CBP:Ir(ddsi) (acac)/MoO (10 nm)/Au(100 nm) were fabricated,
where ITO and Au are used as the cathode and anode conduc-
tors, respectively. Tris[3-(3-pyridyl)mesityl]borane (3TPYMB),
Ir(ddsi) (acac) and MoO are used as the electron transport
2 3
layer, emissive layer and barrier-reducing hole injection/
corresponding to the (1 1 0) plane of rutile SnO (Fig. 3). The
2
2 2 4 2
SAED of pristine SnO , Cd-doped SnO and BMIMBF –Cd–SnO
hybrid electrolyte shows bright concentric circles belonging to
the planes (1 1 0), (1 0 1), (2 1 1) and (1 1 2), conrming the
polycrystalline tetragonal structure (Fig. 3). X-ray photoelectron
spectroscopy (XPS) was used to characterize the composition of
pristine SnO and Cd-doped SnO nano-material and the spec-
electron-blocking layer, respectively. Ir(ddsi) (acac) doped 4,4-
2
N,N-dicarbazolbiphenyl (CBP) with a doping ratio of 8 wt% (40
nm) was used as a green-emitting layer. All layers were depos-
ited on an ITO plate by a thermal evaporation unit with a glove
box under optimized evaporation rates. The thickness was
monitored using a quartz crystal digital thickness monitor. The
current density–voltage and light intensity of the OLEDs were
measured using a Keithley 2400 source measuring unit. The EL
spectra of the devices were carried out in an ambient atmo-
sphere without further encapsulation.
2
2
trum shows the presence of the constituent elements (Fig. 4).
The carbon peak is attributed to the residual carbon from the
sample and adventitious hydrocarbon from the XPS instru-
ments and it is not indicated. Compared to pristine SnO
binding energies of Sn 3d5/2 and 3d3/2 in Cd-doped SnO
2
, the
NPs
2
decreased from 487.0 to 486.2 eV and from 495.3 to 494.5 eV,
respectively, which may be ascribed to there being insufficient
45
oxygen. The symmetric Sn 3d peaks and XRD results conrm
2.10. Computational details
4+
the presence of Sn ions. A slight shi in the binding energy of
The optimized geometry, highest occupied molecular orbital the O 1s transition peak from 531.0 to 529.2 eV was observed for
HOMO) and lowest unoccupied molecular orbital (LUMO) Cd–SnO . The binding energies of the Cd 3d5/2 and Cd 3d3/2
contour map of Ir(ddsi) (acac) and Ir(tsi) were studied with the peaks at 406.3 eV and 412.6 eV, respectively, conrm the pres-
(
2
2
3
43
2+
46
Guassian-09 package [DFT/B3LYP/6-31G (d, p)].
2
ence of Cd in Cd–SnO nanoparticles.
3
. Results and discussion
3
.2. Optical properties
The absorption at 363.6 nm (pristine SnO
doped SnO ) may be attributed to the band gap transition and
and HMIMBF –Cd–SnO nanoparticles. All diffraction peaks are indicates the existence of highly crystalline SnO . The band
3.1. Morphological properties
2
) and 410.6 nm (Cd-
Fig. 1 shows the XRD patterns of pristine SnO
2
, Cd-doped SnO
2
2
4
2
2
assigned to the tetragonal rutile phases of tin oxide (JCPDS no. gaps of pristine SnO and Cd-doped SnO2 can be estimated
2
2
˚
˚
4
1-1445, a ¼ 4.740 A, c ¼ 3.190 A). The high-intensity peaks from a plot of (ahn) versus photon energy as 3.41 and 3.02 eV,
ꢁ
ꢁ
ꢁ
47
around 26.0 , 33.0 and 53.0 correspond to the (1 1 0), (1 0 1), respectively. This reduction in band gap compared with bulk
and (2 1 1) planes of tetragonal SnO , respectively. The absence SnO (3.62 eV) may be attributed to the formation of impurity
of any other unassigned peak indicates the purity of the bands created by oxygen vacancies/defects.
synthesized crystalline SnO , Cd-doped SnO and BMIMBF of BMIMBF –Cd–SnO at 351 nm is due to the p–p* transition
Cd–SnO NPs. The broadened peaks of Cd-doped SnO and of the imidazolium ring and a weak tail above 400 nm is
BMIMBF –Cd–SnO imply that the cadmium dopant increases attributed to the presence of different associated structures of
2
2
48,49
The absorption
2
2
4
–
4
2
2
2
4
2
the concentration of lattice imperfections due to a decrease in the imidazolium ring formed due to electrostatic interaction
49
internal micro strain within the matrix and conrmed the among the p-orbitals. The observed tail peak can be attributed
44
presence of small crystallites. Using Scherrer's formula, the to scattering phenomena induced by inorganic nanoparticles.
2 4 2
average crystallite size and surface area of pristine SnO , Cd- The absorption spectrum of BMIMBF –Cd–SnO dispersion is
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 21206–21221 | 21209

View Article Online
RSC Advances
Paper
2 2 4 2
Fig. 2 SEM image with EDX of pristine SnO (a), Cd-doped SnO (b) and BMIMBF –Cd SnO (c).
dominated by the strong absorption of Cd–SnO
2
NPs that over- shied to 492 nm for Cd-doped SnO
2
and the yellow emission at
whelms the contribution from BMIMBF
optical absorption of BMIMBF –Cd–SnO
4
. The improvement in 560 nm is red-shied to 572 nm. Doping with cadmium
in the visible region is enhances the yellow emission and suppresses the green emis-
4
2
associated with the synergistic effect of the two main compo- sion due to an increase in the concentration of bulk oxygen
nents of Cd–SnO and BMIMBF (Fig. 5). vacancies at the donor level because of oxygen non-
The emission spectra of Cd-doped SnO NPs and pristine stoichiometry created by the replacement of Sn ions by Cd
2
4
4
+
2+
2
2
+
SnO (380, 492 and 572 nm) are shown in Fig. 5. The spectra are ions. As the ionic radius of the dopant Cd (99 pm) is larger
broad and asymmetric with a peak centered at 504 nm. than that of Sn (83 pm), substitution of Sn ions by Cd ions
Deconvolution of this broad PL peak reveals three peaks at 384, is unlikely. Hence, the other possibility is the existence of Cd
04 and 560 nm. The band at 384 nm is ascribed to band gap ion in the inter-lattice position. The variation in the oxidation
2
4
+
4+
2+
2
+
5
emission and the peaks at ꢂ504 (2.46 eV) and 560 nm (2.2 eV) states of tin and cadmium suggests the creation of oxygen
are composed of green and yellow emissions, respectively. The defects. The interstitial based cadmium ions and the oxygen
green and yellow emissions are attributed to the recombination defects create impurity energy levels which are responsible for
of excited electrons trapped in the donor level created by bulk the observed emission. The intensity of Cd-doped SnO2 is
oxygen vacancies with holes in the acceptor level formed by enhanced over pristine SnO due to the high concentration of
2
surface oxygen vacancies coordinated with tin atoms at surface oxygen vacancies as well as singly ionized oxygen
50,51
different angles.
4
The yellow emission originates from the vacancies because of its smaller size. The pure BMIMBF ionic
recombination of trapped electrons in the singly ionized surface liquid exhibits an emission band centered around 368 nm with
52
bridging oxygen vacancies with holes in the valence band
a shoulder on 423 nm and the band prole extends beyond
(
Scheme 1). The green emission at 504 nm of SnO NPs is blue- 500 nm when excited at 340 nm. The decay of bare BMIMBF
2
4
21210 | RSC Adv., 2020, 10, 21206–21221
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
2 2 4 2
Fig. 3 TEM images and SAED of SnO (a), Cd–doped SnO (b) and BMIMBF –Cd–SnO (c).
and BMIMBF
4
–Cd–SnO
2
shows a triexponential function: the Ir(ddsi)
2
(acac) and 258 nm for Ir(tsi)
3
are assigned to a spin-
major component has a lifetime varying between 462 and 578 ps allowed ligand-centered transitions. The other two bands of
90%) with two other minor components of 3.0 ns (7%) and 8.3 Ir(ddsi) (acac) (358 and 376 nm) and Ir(tsi) (343 and 380 nm)
(
2
3
ns (3%) (Fig. 6). The average lifetime depends upon the conrmed the spin-allowed MLCT transitions: ground state to
1
permanent time of the carriers with the number of de-excitation singlet excited state [ MLCT ) S
0
] and triplet excited state
]. Both transitions originate from the interaction of
–Cd– the ligand with the iridium center of Ir(ddsi) . The intensity of the
transition is closest to the MLCT ) S transition,
transition is strongly
3
pathways. The energy transfer between BMIMBF
may be responsible for increasing the lifetime of BMIMBF
SnO . As the only photoluminescent component of BMIMBF
Cd–SnO is the imidazolium moiety, the modication in PL which shows that the MLCT ) S
decay was induced in BMIMBF , which reveals charge transfer symmetry allowed by spin–orbit coupling.
4
and Cd–SnO
2
[ MLCT ) S
0
4
2
3
1
2
4
–
MLCT ) S
0
0
3
2
0
5
3–60
The spin–orbit
4
40
between BMIMBF and Cd–SnO (Table 1).
coupling was enhanced by the closeness of p–p* and MLCT and
the heavy-atom effect of the iridium(III) complexes. The two
4
2
3
.3. Characterization of emissive materials
Fig. 5 shows the UV-vis absorption (labs) spectra of the organo-
metallic complexes, Ir(ddsi) (acac) and Ir(tsi) in CH Cl . The
phosphors Ir(ddsi)
in solution and solid. The broad emissive spectra of Ir(ddsi)
show green emission at 513 and 502 nm and Ir(tsi) shows blue
2
(acac) and Ir(tsi)
3
show strong luminescence
2
(acac)
3
2
3
2
2
emission at 433 and 440 nm (Fig. 5). The broad spectra reveal that
intense absorptions in the ultraviolet region at 254 nm for
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 21206–21221 | 21211

View Article Online
RSC Advances
Paper
2 2
Fig. 4 X-ray photoelectron spectra of pristine SnO and Cd-doped SnO .
3
3
the excited triplet states of Ir(ddsi)
2
(acac) and Ir(tsi)
3
possess
MLCT/ p–p* to S
0
0
whereas a shoulder peak with lower intensity
3
62–64
a predominantly MLCT character. Generally, the phosphores- derived from the n ¼ 0 to n ¼ 1 electronic transition.
The gain
3
3
ꢀs
n
cence spectra from ligand-centered p–p* and MLCT states have
e S
in intensity is expressed as I0/n ¼
Rhys factor, correlated to the intensities of vibration from the
, where S is the Huang–
47–50
a vibronic and broad shape.
The absence of vibronic struc-
n!
tured emission from organometallic complexes conrmed the
MLCT nature of the emission and it was further conrmed by the
phosphorescence lifetimes of 2.3 and 1.8 ms, respectively (Fig. 6).
MuDQ2
3
3
excited MLCT/ p–p* state to the ground state (S ), S ¼
u is the vibrational frequency, M is the reduced mass, and DQ is
;
0
2
h
The wave function (F) of the triplet state (F
T
) is a mixture of F
(p–p*) + bF (MLCT) [a and
b are normalized coefficients, F (p–p*) and F (MLCT) are the
T
6
1
the displacement of potential energy surfaces between the S and
0
(
p–p*) and F
T
(MLCT);
F
T
¼ aF
T
T
excited states. The structural distortion upon excitation is
explained by the ratio of the height of the two emission peaks
T
T
3
3
wave functions of (p–p*) and (MLCT) excited states, respec-
ꢀ
ꢁ
3
I0/1
I0/0
tively: when a > b, the triplet state is dominated by p–p*; when
b > a, the triplet state is dominated by the MLCT excited state].
S ¼
.
3
From the radiative lifetime and PL quantum yield of
The observed two peaks of Ir(ddsi)
Ir(tsi) (433 and 440 nm) may attributed to the Franck–Condon
electronic transition from the vibrational level of the triplet state
2
(acac) (513 and 502 nm) and
Ir(ddsi) (acac) (s ¼ 2.3 ms; F ¼ 0.82) and Ir(tsi) (s ¼ 1.8 ms; F ¼
2
3
3
0
.98), radiative (k ) and non-radiative (k ) decay rate constants
r nr
3
3
have been calculated from F ¼ F {k /(k + k )}, k ¼ F/s, k ¼
ISC
r
r
nr
r
nr
(
0
MLCT/ p–p*) to the ground state (S ) (Fig. 5). The peak with
ꢀ1
0
(1/s) ꢀ (F/s) and s ¼ (k
r
+ knr
)
(F is quantum yield; s is lifetime;
dominant intensity stemmed from the n ¼ 0 to n ¼ 0 transition of
21212 | RSC Adv., 2020, 10, 21206–21221
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Fig. 5 (a) Diffuse reflectance spectra; (b) emission spectra of pristine SnO
2
and Cd-doped SnO
and BMIMBF
2
; (c) Frank–Condon electronic transition of Ir(tsi)
–Cd–SnO
3
and Ir(ddsi)
2
(acac); and (d) UV and PL spectra of Ir(tsi)
3
2
, Ir(ddsi) (acac), BMIMBF
4
4
2
.
6
5–69
and FISC is intersystem-crossing yield).
The rate constants chelate disposition and the Ir–N distance lies between 2.08 and
8
ꢀ1
˚
reveal that radiative emission [3.6 ꢃ 10 s for Ir(ddsi) (acac); 2.10 A. The cyclo-metalated carbon atoms C(12) and C(21) are
2
8
ꢀ1
5.4 ꢃ 10
s
for Ir(tsi) ] is predominant over non-radiative mutually cis around the iridium atom and the Ir–C distance lies
3
8
ꢀ1
8
ꢀ1
˚
transition [0.07 ꢃ 10 s for Ir(ddsi) (acac); 0.02 ꢃ 10 s for between 2.04 and 2.08 A. Due to the stronger Ir–C interaction of
2
Ir(tsi)
3
]. From DFT [B3LYP/6-31G(d,p)] analysis, it was shown the ddsi ligand, Ir–C bonds orient at the trans disposition.
is predominantly Electron-rich phenyl fragments of Ir(ddsi) (acac) show a trans
that the HOMO in homoleptic Ir(tsi)
3
2
distributed over N-side coupling whereas the LUMO is localized effect; thus the trans-C,C geometry is thermodynamically of
on one of the C^N ligands of the iridium complex (Fig. 6). The higher energy and is kinetically more labile (transphobic) which
˚
heteroleptic Ir(ddsi)
hedral geometry around the iridium atom with two cyclo- relative to the Ir–N bond length (Ir–Nav ¼ 2.06 A).
metalated ddsi ligands and one ancillary acetylacetonate trochemical stability of the complexes was conrmed by
acac) ligand. The ddsi ligand adopts an eclipsed conguration a reversible one-electron oxidation wave (Fig. 6). The HOMO
2
(acac) complex exhibits a distorted octa- is conrmed by the shorter Ir–C bond length (Ir–Cav ¼ 2.04 A)
˚
57,59
The elec-
(
2 3
and two nitrogen atoms N(5) and N(7) reside at the trans-N,N energy [5.08 eV for Ir(ddsi) (acac) and 5.31 eV for Ir(tsi) ] is
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 21206–21221 | 21213

View Article Online
RSC Advances
Paper
2 3 2
Fig. 6 (a) Fortier molecular orbital contour map of Ir(ddsi) (acac) and Ir(tsi) . (b) Cyclic voltammogram. (c) DSC and TGA curve of Ir(ddsi) (acac)
and Ir(tsi) . (d) Lifetime decay curve.
3
Table 1 Optical and thermal properties of Ir(tsi)
Parameters Ir(tsi)
3
and Ir(ddsi)
2
(acac)
2
3
.18 eV for Ir(tsi) ] is calculated by subtracting the optical band
70
(acac) gap energy from EHOMO [ELUMO ¼ EHOMO ꢀ 1239/lonset]. To
3
Ir(ddsi)
2
determine their suitability for device fabrication, thermal
characterization was undertaken by TGA analysis (Fig. 6). The
ꢁ ꢁ
high values of T /T [436/178 C for Ir(ddsi) (acac); 431/188 C
d g 2
for Ir(tsi)
Ir(tsi) exhibit excellent thermal properties and could be sub-
jected to vacuum-evaporation without decomposition.
could form morphologically stable amorphous lms on thermal
evaporation, which would reduce the phase separation on
heating and improve the lifetime of the devices.
l
ab (nm)
258, 343, 380
433, 440
431/188
0.98
254, 358, 376
502, 513
436/178
0.82
lem (nm)
ꢁ
T
f
d
( C)
3 2
] reveal that the emissive materials Ir(ddsi) (acac) and
HOMO/LUMO (eV)
(eV)
s (ns)
ꢀ5.31/ꢀ2.18
ꢀ5.08/ꢀ2.22
3
E
g
ꢀ3.13
2.86
71–73
They
1.8
5.4
0.02
2.3
3.6
0.07
8
ꢀ1
k
k
r
ꢃ 10 (s
)
8
ꢀ1
nr ꢃ 10 (s
)
determined from the oxidation potential and the ferrocenium/ 3.4. Electroluminescent studies
ferrocene redox couple energy [EHOMO (eV) ¼ ꢀ(E + 4.8)]
ox
The thicknesses of 4.8 nm sized SnO NP layers spin-coated
from 0.5, 1.0, 1.5 and 2.0 wt% solutions were 11, 13, 20 and
2
whereas the LUMO energy [2.22 eV for Ir(ddsi) (acac) and
2
21214 | RSC Adv., 2020, 10, 21206–21221
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Fig. 7 (a) Exciton dissociation SnO
2
:Ir(ddsi)
2
(acac) interface and non-radiative energy transfer from Ir(ddsi)
2
(acac) to SnO
2
NPs and (b) lifetime
decay of devices.
2
5 nm, respectively. The root-mean-square (RMS) roughness of devices VI (0% SnO ) and VII [0% Ir(ddsi) (acac)] (Fig. 8). Among
2
2
the SnO
2
NP layers spin-coated on ITO substrates from 0.5, 1.0, devices II–V, device IV (with 2.0% SnO
2
; 25 nm) exhibited the
ꢀ
1
1
.5 and 2.0 wt% solutions are 2.96, 2.74, 2.51 and 2.13 nm, maximum (SnO
respectively (Fig. 2). The RMS roughness was much smoother 24.3), power efficiency (h
than that of ITO (4.14 nm) and it decreased with increasing quantum efficiency (hex, %: 16.4/7.5) and luminance (L, cd m
2
NPs/ITO) current efficiency (h
c
, cd A : 52.3/
ꢀ
1
p
, lm W : 10.9/3.4), external
ꢀ2
:
concentration of SnO
2
NPs. Owing to the low-temperature (100 28 182/1982). The reference devices VI (0% SnO
2
ꢀ2
NPs) and VII
ꢁ
C) thermal treatment, SnO NPs lm retained an amorphous [Alq (EML)] show luminances (L) of 1982 cd m at 19.0 V and
state, as shown in the XRD pattern. Hence, SnO NPs can be 2102 cd m at 22.0 V, respectively. The enhanced efficiency
2
3
ꢀ
2
2
used as a functional layer at the interface between ITO and the with increasing SnO2 NP thickness may due to the fact that
ETL of OLEDs (Fig. 8). The lifetime of the lm increased with an increasing the uniform surface coverage will increase the elec-
increasing thickness of the SnO
2
NP layer (1.28 ns at 0%; 1.73 ns tron injection through improved energy level matching at the
74
ꢀ
at 1.0%; 2.20 ns at 1.5% and 2.67 ns at 2.0%) (Fig. 7). The ITO:3TPYMB interface. As more electrons are injected, the e –
+
2
2.0 wt% SnO NPs block exciton quenching efficiently by surface h balance is improved, leading to enhanced efficiencies over
quenching or non-radiative energy transfer quenching mecha- those of other devices.
nisms (Fig. 7). The electroluminescent performances of the
The electron injection barrier (1.48 eV) from ITO to the
fabricated devices using EIL SnO NPs with thicknesses of 2.96 ELUMO of 3TPYMB is reduced (0.28 eV) to 1.2 eV using an SnO2
2
(
0.5 wt%), 2.74 (1.0 wt%), 2.51 (1.5 wt%), and 2.13 nm (2.0 wt%) NP layer. The device with 2.0 wt% SnO NPs shows enhanced
2
ꢀ
1
ꢀ1
were studied. The efficiencies of the devices fabricated by the efficiencies: h
combination of SnO
Ir(ddsi) (acac) (devices I–IV) increased relative to the reference (without an SnO
c
, cd A of 115%; h
p
, lm W of 221%; hex, % of
767 times higher than the control device
layer – device V). The device with 2.0 wt% SnO
ꢀ2
2 L
NPs as EILs and green emissive material 119; h , cd m
2
2
2
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 21206–21221 | 21215

View Article Online
RSC Advances
Paper
Fig. 8 Device performances: (a) current density and voltage; (b) current efficiency and current density; (c) power efficiency and current density;
d) external quantum efficiency and current density; and (e) normalized electroluminescent spectra of devices (I–V) (inset: CIE – V).
(
ꢀ
1
NPs shows enhanced efficiencies: h , cd A of 396%; h , lm smoother surface morphology of the SnO lm in a thicker lm.
W
c
ꢀ2
p
2
ꢀ
1
of 3966%; h , % of 466; h , cd m 766.9 times higher than The normalized EL spectra of devices I–V show emission at
ex L
the control device (Alq – device VI; see Table 2 and Fig. 8). As 504 nm for IBOLEDs without a change in the shape of the
3
more electrons are injected into the ETL in the devices with an emission peak, since the layer is highly transparent (Fig. 8). The
SnO
thus efficiencies are higher than those of the control device. because the layer is highly transparent and amorphous.
Comparing the performances of IBOLEDs with different SnO We have also fabricated devices using Cd–SnO as an elec-
layer thicknesses, a higher efficiency has been harvested in the tron injection layer. The device with a 2.0% Cd-doped SnO
2 2
NP layer, electron–hole recombination is enhanced and SnO NP layer scarcely affects the EL spectra of IBOLEDs
2
2
2
ꢀ1
device with a thicker SnO2 lm. This is attributed to the layer shows higher current efficiency h (60.6 cd A ) and power
c
21216 | RSC Adv., 2020, 10, 21206–21221
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Table 2 Summary of device efficiencies of SnO
2 2
(I–IV) and Cd–SnO (V) with control devices
a
b
c
e
f
g
h
i
Device (% SnO
SnO (V))
2
(I–IV) & Cd–
L (cd
ɳ
ex
ɳ
c
(cd
ɳ
W
p
(lm
)
Z
(%)
ɳp(VI/VII)
Z
(%)
ɳex(VI/VII)
Z c
(%)
ɳ
(VI/VII)
ZɳL(VI/VII)
(%)
ꢀ
2
ꢀ
ꢀ1
2
V
1000 (V) m
)
(%)
A )
d
I (0.5%)d
I (1.0%)
16.8
16.6
16.6
15.1
13.3
19.0
22.0
14215
15710
16631
28182
30858
1982
14.8
15.9
16.0
16.4
18.3
7.5
43.0
45.2
50.4
52.3
60.6
24.3
2.2
7.5
121/241
132/249
200/364
221/396
352/432
—
97/412
112/450
113/454
119/466
144/533
—
77/1287
82/1326
107/1526
115/1587
144/533
—
617/576
693/647
739/691
767/767
1255/1178
—
7.9
10.2
10.9
15.4
3.4
d
III (1.5%)d
IV (2.0%d)
V (2.0%)
Control device VI (0%)
Control device (VII)
j
k
2102
3.1
2.89
—
—
—
—
a
b
c
d
e
i
f
i
Brightness. Maximum external quantum efficiency. Luminous efficiency. Power efficiency.
Z
h
¼ (h
p
– h
p
/h
p
) ꢃ 100.
Z
3
h
¼ (hex ‒ hex/hex) ꢃ
p
ex
g
i
h
i
i
j
1
00. Z c
h
¼ [h
c
ꢀ h
c
/h
c
) ꢃ 100.
Z
nL ¼ (L ꢀ L/L) ꢃ 100. Values for devices with NPs. ITO/SnO
2
(2.0%)/Alq
3
(100 nm)/MoO
(10 nm)/Al (100 nm).
k
VII [Alq
3
(EML)].
Fig. 9 Device configuration with energy level diagram of HyLEDs.
ꢀ
1
efficiency h
). These higher efficiencies reveal that the Cd-doped SnO
in combination with Ir(ddsi) (acac) makes electron injection mance. The Cd-doped SnO
more efficient through the improved energy level matching at due to the improved energy level matching at the ITO:Cd-doped
the ITO:Ir(ddsi) (acac) interface. The current density and SnO :Ir(ddsi) (acac) interface. As more electrons are injected,
luminance increase as the concentration of Cd-doped SnO effective electron–hole recombination leads to higher efficien-
p
(15.4 lm W ) than those of devices I–IV (Fig. 8 and the ECB of Cd-doped SnO
2
and promotes electron injection from
, leading to enhanced device perfor-
layer injects electrons efficiently
9
2
layer ITO to the ECB of Cd–SnO
2
2
2
2
2
2
2
increases, because the total thickness of the device increases. As cies than that of the control device. Holes are injected from the
the thickness of the Cd-doped SnO layer increases to 4.0%, the Au anode coated with MoO3 HIL into the highest occupied
2
current density and luminance increase slightly. This may be molecular orbital (HOMO) of Ir(ddsi)
due to the fact that the injection and transport of electrons injected from the Cd-doped SnO –EIL-coated ITO cathode into
become better than those of organic molecules, as the surface the LUMO of Ir(ddsi) (acac). The CB of Cd-doped SnO is situ-
coverage of Cd-doped SnO on ITO becomes better with an ated higher than the LUMO of the emissive material Ir(ddsi) (-
increase in the Cd-doped SnO A systematic acac), which leads to activation-less electron injection from Cd–
study for device optimization by controlling the thickness and SnO into Ir(ddsi) (acac). The deeper VB of Cd-doped SnO
morphology of Cd-doped SnO is under way. Therefore the use results in an efficient hole-blocking functionality, improving
2
(acac) and electrons are
2
2
2
2
2
75–77
2
layer thickness.
2
2
2
2
78
of a Cd-doped SnO2 layer in an optoelectronic device is of efficiencies (Table 2). The efficiencies of SnO and Cd-doped
2
current interest owing to the advantages of processability at low SnO
2
based devices have been compared with those of various
78,79
temperature, surface roughness and photostability. The device recently reported electron injection layers (Table S1†)
which
2 2
shows that SnO and Cd-doped SnO based devices are the best
, lm W of 221/396%; hex, % of 119/ in terms of power and current efficiencies, and we believe that
with 2.0 wt% Cd-doped SnO
2
shows enhanced efficiencies: h
c
,
ꢀ
1
ꢀ1
cd A of 115/1587%; h
p
ꢀ
2
4
66; h
devices (VI/V).
The band gap energy of CdO is much lower (2.24 eV) than performance.
that of SnO (4.1 eV) and this may be the reason for the effective
The EL spectrum of Cd-doped SnO (l ¼ 380 nm) shows
L
, cd m 767/767 times higher than those of control SnO
2 2
and Cd-doped SnO NPs should be adopted as good
potential candidates for future displays as well as for device
2
2
ex
2
+
performances of Cd doped SnO
2
as an electron injection a broad yellow emission centered at 580 nm which is similar to
material. The lowering of the band gap results in a lowering of the PL spectra with a red-shi in the emission peak position due
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 21206–21221 | 21217

View Article Online
RSC Advances
Paper
4 2
Fig. 10 White light generation from BMIMBF –Cd–SnO colloidal hybrid electrolyte.
to increase in excitation wavelength. In addition to the broad photoexcitation of BMIMBF
4 2 4
–Cd–SnO , BMIMBF ionic liquid
yellow emission at 580 nm, weak blue emission at 401 nm also show emission at 450 nm may attributed to associated form of
81
2ꢀ
appears in the EL spectra. This blue emission may be attributed BMIMBF
4 2
ionic liquid. In Cd-doped SnO NPs, some O ions
to tin interstitials and non-stoichiometric oxides, which escape from host lattice, leading to the formation of oxygen
80
encouraged us to check the white emission. In order to obtain vacancies. This oxygen vacancy centre can trap an electron,
81
white emission, Cd-doped SnO NPs are mixed with homoleptic resulting in the formation of defect states. The valence band
2
blue phosphor Ir(tsi) (1 : 4). The blended phosphors are excited (VB), defect states and conduction band (CB) are the three
3
2
+
by a 380 nm LED chip driven at an applied voltage of 2.8 V and energy levels for Cd–SnO NPs. Aer the addition of Cd to the
.5 mA current. The recorded EL spectrum shows yellow emis- host lattice, the negative charge of the Cd ion has to be
2
2
+
3
sion at 568 nm with weak blue emission at 428 nm originating compensated for somewhere in the lattice in the form of an
2
+
from blue phosphor Ir(tsi)
and yellow phosphors leads to white emission with CIE (0.31, metastable V
.30) and CCT ¼ 6961 K. White emission is classied as warm or electrons from nearby tin vacancies to present the form of a tin–
cold in terms of CCT (correlated color temperature) values, oxygen vacancy pair and luminescence intensity is increased
3
. The combined emission of blue oxygen vacancy. By incorporating Cd ions into the SnO
2
host,
0
centers are created in situ, which will share
0
81
calculated using the McCamy empirical formula: CCT ¼ compared to pristine SnO due to the simultaneous formation
2
2
+
x ꢀ xc
y ꢀ yc
3
2
of V0 centers. However, aer introducing Cd ions into the
ꢀ
449n + 352n ꢀ 6823n + 5520.33, where n ¼
is the
host, non-radiative recombination can also occur easily due to
the enhanced probability of the interaction with nearby Cd
2
+
negative inverse slope of the line, x ¼ 0.332 and y ¼ 0.186, and
c
c
2
+
the obtained CCT value is 6961 K. Generally, a CCT value greater
than 5000 K implies a cold white light used for commercial
lighting purposes and less than 5000 K indicates a warm white
light used for home applications. In the present study, the
CCT value is 6961 K, which represents cold white light emission.
ions, originating from the continuous solubility of Cd ions in
the SnO host. The surface effects can strongly inuence the
optical properties of these SnO nanoparticles. Because of the
large surface-to-volume ratio of SnO2 nanoparticles, much
unsaturated bonding of the surface atoms is expected at the
surface of the particles. So, the surface will play an important
role in the quenching of the luminescence. Recombination of
holes in this centre with electrons in the CB leads to trap
2
2
47
2
Thus, Cd-doped SnO nanoparticles could be a better replace-
ment for the metal activator doped yellow phosphors used for
white OLED fabrication.
81
emissions at 492 nm (green) and at 572 nm (yellow). Photo-
excitation of the BMIMBF –Cd–SnO hybrid electrolyte
exhibits strong emission at 450 nm, 492 nm and 572 nm. A
3
.5. White light emission from BMIMBF
electrolyte
We have synthesised hybrid colloidal electrolyte namely,
BMIMBF –Cd–SnO to generate white light emission upon
4 2
–Cd–SnO colloidal
4
2
combination of blue emission from BMIMBF and green and
4
4
2
21218 | RSC Adv., 2020, 10, 21206–21221
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
yellow emissions from Cd–SnO
light with CIE (0.34, 0.35) and CCT ¼ 5188 K (Fig. 10).
2
NPs generates intense white
8 T. Xiong, F. Wang, X. Qiao and D. Ma, Appl. Phys. Lett., 2008,
92, 263305–263308.
9
J. H. Lee, P. S. Wang, H. D. Park, C. I. Wu and J. J. Kim, Org.
Electron., 2011, 12, 1763–1767.
4
. Conclusion
1
1
1
0 B. W. D'Andrade, S. R. Forrest and A. B. Chwang, Appl. Phys.
Lett., 2003, 83, 3858–3860.
1 G. Parthasarathy, C. Shen, A. Kahn and S. R. Forrest, J. Appl.
Phys., 2001, 89, 4986–4992.
2 C. Zhong, S. Liu, F. Huang, H. Wu and Y. Cao, Chem. Mater.,
2011, 23, 4870–4876.
Enhanced efficiencies have been harvested from IBOLEDs using
electron injection layer SnO NPs and/or Cd–SnO NPs coated at
the ITO:ETL interface. The 2.0% SnO NPs (25 nm) with
Ir(ddsi) (acac) emissive material enhance efficiencies (SnO /
2
2
2
2
2
ꢀ
1
ITO), current efficiency (h , cd A : 52.3/24.3), power efficiency
(h , lm W : 10.9/3.4), external quantum efficiency (hex, %: 16.4/
p
.5) and luminance (L, cd m : 28 182/1982). The device with
c
ꢀ1
13 M. Sessolo and H. J. Bolink, Adv. Mater., 2011, 23, 1829–1845.
14 Y. Vaynzof, D. Kabra, T. J. K. Brenner, H. Sirringhaus and
R. H. Friend, Isr. J. Chem., 2012, 52, 496–517.
15 M. G. Panthani and B. A. Korgel, Annu. Rev. Chem. Biomol.
Eng., 2012, 3, 287–311.
ꢀ2
7
ꢀ1
2 c p
a 2.0% Cd-doped SnO layer shows a higher h (60.6 cd A ), h
ꢀ
1
ꢀ2
(
15.4 lmW ), hex (18.3%) and L (26 858 cd m ) at 2.36 V than
SnO devices or control devices. White light has been generated
from a mixture of Cd–SnO NPs and homoleptic blue phosphor
Ir(tsi) . A combination of blue emission (l ¼ 428 nm) from
2
1
1
1
6 D. Kabra, L. P. Lu, M. H. Song, H. J. Snaith and R. H. Friend,
Adv. Mater., 2010, 22, 3194–3198.
7 J. W. Ryan, E. Palomares and E. Mart ´ı nez-Ferrero, J. Mater.
Chem., 2011, 21, 4774–4777.
8 H. J. Bolink, E. Coronado, D. Repetto, M. Sessolo,
E. M. Barea, J. Bisquert, G. Garcia-Belmonte, J. Prochazka
and L. Kavan, Adv. Funct. Mater., 2008, 18, 145–150.
9 N. Tokmoldin, N. Griffiths, D. D. C. Bradley and S. A. Haque,
Adv. Mater., 2009, 21, 3475–3478.
2
3
EL
Ir(tsi) and defect emission from Cd–SnO NPs (l ¼ 568 nm)
3
2
EL
gives intense white light with CIE (0.31, 0.30) and CCT ¼ 6961 K.
Upon photo-excitation, BMIMBF –Cd–SnO gives white emis-
4
2
sion with CIE (0.34, 0.35) and CCT ¼ 5188 K by the combination
of blue and yellow emissions.
1
2
2
Conflicts of interest
0 T. Chiba, Y. J. Pu, M. Hirasawa, A. Masuhara, H. Sasabe and
J. Kido, ACS Appl. Mater. Interfaces, 2012, 4, 6104–6108.
1 T. Chiba, Y. J. Pu, H. Sasabe, J. Kido and Y. Yang, J. Mater.
Chem., 2012, 22, 22769–22773.
There are no conicts to declare.
Acknowledgements
22 S. Y. Ryu, S. H. Kim, C. S. Kim, S. Jo and J. Y. Lee, Curr. Appl.
Phys., 2012, 12, 1378–1380.
This research was supported by the DST (Department of Science
and Technology – EMR/2014/000094, F. No. SR/S1/1C-73/2010,
F. No. SR/S1/1C-07/2007), DRDO (Defence Research and Devel-
opment Organization – 213/MAT/10-11), CSIR (Council of
Scientic and Industrial Research – No. 01/(2707)/13EMR-II),
UGC (University Grant Commission – 6-21/2008, F. No. 30-71/
2
2
2
3 H. Lee, I. Park, J. Kwak, D. Y. Yoon and C. Lee, Appl. Phys.
Lett., 2010, 96, 153306–153309.
4 L. Qian, Y. Zheng, K. R. Choudhury, D. Bera, F. So, J. Xue and
P. H. Holloway, Nano Today, 2010, 5, 384–389.
5 J. Kwak, W. K. Bae, D. Lee, I. Park, J. Lim, M. Park, H. Cho,
H. Woo, D. Y. Yoon, K. Char, S. Lee and C. Lee, Nano Lett.,
2004(SR)) and DST-Nano Mission (SR/NM/NS-1001/2016).
2012, 12, 2362–2366.
26 T. Kuwabara, C. Tamai, Y. Omura, T. Yamaguchi, T. Taima
and K. Takahashi, Org. Electron., 2013, 14, 649–656.
References
2
7 A. Kay and M. Gr ¨a tzel, Chem. Mater., 2002, 14, 2930–2935.
1
C. W. Han, K. M. Kim, S. J. Bae, H. S. Choi, J. M. Lee, 28 J. M. Caruge, J. E. Halpert, V. Wood, V. Bulovi ´c and
T. S. Kim, Y. H. Tak, S. Y. Cha and B. C. Ahn, SID Int. -
Symp. Dig. Tech. Pap., 2012, 43, 279–281.
R. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer,
B. L u¨ ssem and K. Leo, Nature, 2009, 459, 234–238.
K. Yamae, H. Tsuji, V. Kittichungchit, Y. Matsuhisa,
M. G. Bawendi, Nat. Photon., 2008, 2, 247–250.
29 S. Sadhu, T. Sen and A. Patra, Chem. Phys. Lett., 2007, 440,
121–124.
30 S. C. Vanithakumari and K. K. Nanda, Adv. Mater., 2009, 21,
3581–3584.
2
3
S. Hayashi, N. Ide and Y. Komoda, SID Int. - Symp. Dig. 31 F. Wang, Y. H. Chen, C. Y. Liu and D. G. Ma, Chem. Commun.,
Tech. Pap., 2012, 43, 694–697. 2011, 47, 3502–3504.
T. Y. Chu, J. F. Chen and C. H. Chen, Jpn. J. Appl. Phys., 2006, 32 K. S. Sanju, P. P. Neelakandan and D. Ramaiah, Chem.
5, 4948–4950.
Commun., 2011, 47, 1288–1290.
X. Zhou, M. Pfeiffer, J. S. Huang, J. Blochwitz-Nimoth, 33 V. Vohra, G. Calzaferri, S. Destri, M. Pasini, W. Porzio and
4
5
4
D. S. Qin, A. Werner, J. Drechsel, B. Maenning and K. Leo,
Appl. Phys. Lett., 2002, 81, 922–924.
T. Y. Chu, J. F. Chen, S. Y. Chen, C. J. Chen and C. H. Chen,
Appl. Phys. Lett., 2006, 89, 053503–053508.
S. Y. Chen, T. Y. Chu, J. F. Chen, C. Y. Su and C. H. Chen,
Appl. Phys. Lett., 2006, 89, 053518–053521.
C. Botta, ACS Nano, 2010, 4, 1409–1416.
34 X. Wang, W. Li and K. Sun, J. Mater. Chem., 2011, 21, 8558–
8565.
35 P. Kumar, L. S. Panchakarla, S. V. Bhat, U. Maitra,
K. S. Subrahmanyam and C. N. R. Rao, Nanotechnology,
2010, 21, 385701–385706.
6
7
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 21206–21221 | 21219

View Article Online
RSC Advances
Paper
3
3
3
3
4
4
4
4
6 S. Ye, F. Xiao, Y. X. Pan, Y. Y. Ma and Q. Y. Zhang, Mater. Sci. 53 B. X. Mi, P. F. Wang, M. W. Liu, H. L. Kwong, N. B. Wong,
Eng. Rep., 2010, 71, 1–34. C. S. Lee and S. T. Lee, Chem. Mater., 2003, 15, 3148–3151.
7 C. H. Huang and T. M. Chen, Opt. Express, 2010, 18, 5089– 54 J. D. Priest, G. Y. Zheng, N. Goswami, D. M. Eichhorn,
5
099.
8 A. Kar, S. Kundu and A. Patra, J. Phys. Chem. C, 2011, 115,
18–124.
9 Z. Lia, X. Lia, X. Zhanga and Y. Qian, J. Cryst. Growth, 2006,
91, 258–261.
C. Woods and D. P. Rillema, Inorg. Chem., 2000, 39, 1955–
1963.
55 J. Jayabharathi, V. Thanikachalam, K. Saravanan and
N. Srinivasan, J. Fluoresc., 2011, 21, 507–519.
56 K. Saravanan, N. Srinivasan, V. Thanikachalam and
J. Jayabharathi, J. Fluoresc., 2011, 21, 65–80.
57 J. Jayabharathi, V. Thanikachalam, N. Srinivasan and
K. Saravanan, J. Fluoresc., 2011, 21, 596–606.
1
2
0 P. P. Sahay, R. K. Mishra, S. N. Pandey, S. Jha and
M. Shamsuddin, Curr. Appl. Phys., 2013, 13, 479–486.
1 M. G. Tecedor, D. Maestre, A. Cremades and J. Piqueras, J.
Mater. Chem. C, 2016, 4, 5709–5716.
58 J. Jayabharathi, V. Thanikachalam and K. Saravanan, J.
Photochem. Photobiol., A, 2009, 208, 13–20.
2 G. Godinho, A. Walsh and G. W. Watson, J. Phys. Chem. C,
2
009, 113, 439–448.
59 S. Lamansky, P. Djurovich, D. Murphy, F. Abdel Razzaq,
H. F. Lee, C. Adachi, P. E. Burrows, S. R. Forrest and
M. E. Thompson, J. Am. Chem. Soc., 2001, 123, 4304–4312.
3 (a) M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, 60 M. G. Colombo, A. Hauser and H. U. Gudel, Inorg. Chem.,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, 1993, 32, 3088–3092.
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, 61 S. Okada, K. Okinaka, H. Iwawaki, M. Furugori,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery,
J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd,
M. Hashimoto, T. Mukaide, J. Kamatani, S. Igawa,
A. Tsuboyama, T. Takiguchi and K. Ueno, Dalton Trans.,
2005, 9, 1583–1590.
E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, 62 K. C. Tang, K. L. Liu and I. C. Chen, Chem. Phys. Lett., 2004,
J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, 386, 437–441.
S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, 63 (a) D. S. Mcclure, J. Chem. Phys., 1949, 17, 905–913; (b)
M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
W. L. Leong, P. S. Lee, S. G. Mhaisalkar, T. P. Chen and
A. Dodabalapur, Appl. Phys. Lett., 2007, 90, 042906–042910.
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, 64 H. Bassler and B. Schweitzer, Acc. Chem. Res., 1999, 32, 173–
R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, 182.
P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, 65 N. J. Turro, V. Ramamurthy and J. C. Scaiano,
O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and ChemPhysChem, 2012, 88, 1033–1036.
D. J. Fox, Gaussian, Inc., Wallingford., CT, 2009, vol. 121, 66 C. K. Moon, K. H. Kim, J. W. Lee and J. J. Kim, Chem. Mater.,
pp. 150–166; (b) T. Lu and F. Chen, J. Comput. Chem., 2012,
3, 580–592.
4 S. Gnanam and V. Rajendran, J. Sol-Gel Sci. Technol., 2010,
3, 555–559.
5 X. Liu, J. Iqbal, Z. Wu, B. He and R. Yu, J. Phys. Chem. C, 2010,
14, 4790–4796.
2015, 27, 2767–2769.
3
67 H. Jou, Y. X. Lin, S. H. Peng, C. J. Li, Y. M. Yang, C. L. Chin,
J. J. Shyue, S. S. Sun, M. Lee, C. T. Chen, M. C. Liu,
C. C. Chen, G. Y. Chen, J. H. Wu, C. H. Li, C. F. Sung,
M. J. Lee and J. P. Hu, Adv. Mater., 2014, 26, 555–562.
68 R. S. Kesarkar, W. Mr ´o z, M. Penconi, M. Pasini, S. Destri,
M. Cazzaniga, D. Ceresoli, P. R. Mussini, C. Baldoli,
U. Giovanella and A. Bossi, Angew. Chem., 2016, 128, 2764–
2768.
4
4
4
5
1
6 V. Vasanthia, M. Kottaisamy, K. Anitha and
V. Ramakrishnan, Mater. Sci. Semicond. Process., 2018, 85,
141–149.
4
4
7 R. Mariappan, V. Ponnuswamy, P. Suresh, R. Suresh, 69 L. S. Cui, Y. Liu, X. Y. Liu, Z. Q. Jiang and L. S. Liao, ACS Appl.
M. Ragavendar and C. Sankar, Mater. Sci. Semicond.
Process., 2013, 16, 825–832.
8 V. Kumar, V. Kumar, S. Som, J. H. Neethling, E. Olivier,
O. M. Ntwaeaborwa and H. C. Swart, Nanotechnology, 2014,
Mater. Interfaces, 2015, 7, 11007–11014.
70 (a) J. Liu, Z. Zeng, X. Cao, G. Lu, L. H. Wang, Q. L. Fan,
W. Huang and H. Zhang, Small, 2012, 8, 3517–3522; (b)
Z. Wang, Y. Feng, S. Zhang, Y. Gao, Z. Gao, Y. Chen,
X. Zhang, P. Lu, B. Yang, P. Chen, Y. Mab and S. Liuc,
Phys. Chem. Chem. Phys., 2014, 16, 20772–20779.
25, 135701–135710.
4
9 G. Zhang, C. Xie, S. Zhang, S. Zhang and Y. Xiong, J. Phys.
Chem. C, 2014, 118, 18097–18109.
71 M. Lepeltier, F. M. Savary, B. Graff, J. Lalev ´e e, D. Gigmes and
F. Dumur, Synth. Met., 2015, 199, 139–146.
5
0 J. D. Prades, J. Arbiol, A. Cirera, J. R. Morante, M. Avella,
L. Zanotti, E. Comini, G. Faglia and G. Sberveglieri, Sens. 72 X. Yang, X. Xu, J. S. Dang, G. Zhou, C. L. Ho and W. Y. Wong,
Actuators, B, 2007, 126, 6–12. Inorg. Chem., 2016, 55, 1720–1727.
1 S. Samson and C. G. Fonstad, J. Appl. Phys., 1973, 44, 4618– 73 D. R. Martir, A. K. Bansal, V. D. Mascio, D. B. Cordes,
5
5
4621.
A. F. Henwood, A. M. Z. Slawin, P. C. J. Kamer, L. M. Sarti,
A. Perteg ´a s, H. J. Bolink, I. D. W. Samuel and
E. Z. Colman, Inorg. Chem. Front., 2016, 3, 218–235.
2 S. Lettieri, M. Causa, A. Setaro, F. Trani, V. Barone, D. Ninno
and P. Maddalena, J. Chem. Phys., 2008, 129, 244710–244714.
21220 | RSC Adv., 2020, 10, 21206–21221
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
7
7
7
7
7
7
4 J. B. Baxter and C. A. Schmuttenmaer, J. Phys. Chem. B, 2006, 80 S. S. Changa, S. O. Yoona and H. J. Park, Ceram. Int., 2005,
10, 25229–25239. 31, 405–410.
5 S. K. Hau, H. L. Yip, N. S. Baek, J. Zou, K. O'Malley and 81 (a) J. Jhon, M. Vequizo, S. Kamimura, T. Ohno and
1
A. K. Y. Jen, Appl. Phys. Lett., 2008, 92, 253301–253304.
6 B. S. Ong, C. Li, Y. Li, Y. Wu and R. Loutfy, J. Am. Chem. Soc.,
A. Yamakata, Phys. Chem. Chem. Phys., 2018, 20, 3241–
3248; (b) A. Mele, C. D. Tran and S. H. P. Lacerda, Angew.
Chem., Int. Ed., 2003, 42, 4364–4366; (c) H. Hamaguchi and
R. Ozawa, Adv. Chem. Phys., 2005, 131, 85–104; (d)
H. Tokuda, K. Hayamizu, K. Ishii, M. A. B. H. Susan and
M. Watanabe, J. Phys. Chem. B, 2004, 108, 16593–16600; (e)
C. S. McCamy, Color Res. Appl., 1992, 17, 142–144.
2
007, 129, 2750–2751.
7 J. B. Baxter and C. A. Schmuttenmaer, J. Phys. Chem. B, 2006,
10, 25229–25239.
1
8 S. Stolz, Y. Zhang, U. Lemmer, G. H. Sosa and H. Aziz, ACS
Appl. Mater. Interfaces, 2017, 9, 2776–2785.
9 J. Jayabharathi, S. Panimozhi, V. Thanikachalam,
A. Prabhakaran and P. Jeeva, RSC Adv., 2018, 8, 8402–8411.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 21206–21221 | 21221