G Model
CCLET 5205 No. of Pages 5
B. Yang et al. / Chinese Chemical Letters xxx (2019) xxx–xxx
3
The electrochemical properties of DCAF and TCAF were
investigated by cyclic voltammetry (CV) in Fig. 1. The LUMO
energy levels of DCAF and TCAF were calculated to be -4.17 eV and
devices
(ppy)
E
and
F
was ITO/MoO
3
(1 nm)/CBP (45 nm)/CBP:Ir
2
(acac) (8%, 15 nm)/TPBi (40 nm)/TCAF/Al (120 nm) (Fig. S8,
Table S3 in Supporting information). And device B possessed
different TCAF thickness. Fig. 3 presented the J–V–L, and LE–L
characteristics with the main parameters showed in Table 2.
Observed from J–V–L graph of device B (Figs. 2 and 3), when 0.5 nm
thick TCAF as EIM, device B displayed good performance. But when
the TCAF thickness increased to 1.0 nm and 1.5 nm in devices E and
F, the Von increased to 7.3 V and 8.6 V, respectively. Their Lmax
sharply decreased from 31,549, 2005 to 209 cd/m , respectively.
The Lmax of device B was 15.7 and 150.9 times than that of devices E
and F, respectively.
-
-
4.52 eV, respectively, according to the empirical equation LUMO =
[4.4 + Ere] [62–64] as listed in Table 1. The corresponding highest
occupied molecular orbital (HOMO) energy level were calculated
to be -6.14 eV, -6.13 eV (Fig. S4 in Supporting information),
respectively. In Fig. 1b, the LUMO energy levels of DCAF and TCAF
delocalized over the entire molecule including carbonyl, dicyano-
methylene. While both HOMO energy levels delocalized on the CAF
backbone. The thermogravimetric analysis (TGA) exhibited that
2
ꢀ
ꢀ
DCAF and TCAF decomposed at 247 C and 261 C with 5% weight
loss, respectively (Fig. S5 in Supporting information). It is indicated
that DCAF and TCAF are stable enough and potentially suitable for
electronic devices.
Following the increased voltage, the current density of device B
reached a high level as 17.9 mA/cm which was 8 times than that of
devices E and F (2.0 and 1.7 mA/cm ), respectively. Meanwhile, the
2
2
In order to disclose the electron injection ability of DCAF and
TCAF, two primary devices were fabricated using DCAF and TCAF as
the electron injection layer (EIL) with 0.5 nm thickness and TPBi as
the ETL with 40 nm thickness. The corresponding configurations
corresponding LEmax measured on devices B, E, and F were
recorded as 62.34, 19.86, and 2.79 cd/A, respectively. As shown in
Fig. 3b and Table 2, when the thickness of TCAF increased, the LE–L
curve slumped to a quite lower level and sharply decreased
efficiency. It was indicated that the thicknesses of TCAF deter-
mined the electron injection ability. It was explained that charges
could tunnel through the CAFs with only 0.5 nm thickness rather
than thicker CAFs [70]. Device B exhibited a very good stability
with the LE keeping at level of 57.8 cd/A which was 92.7% of the
were designed as ITO/MoO
3
(1 nm) [65] / CBP (45 nm) [66] / CBP:
Ir(acac) (15 nm) [67] / TPBi (40 nm) [66] / CAFs (0.5 nm) / Al
120 nm) showed in Fig. 2a and b, and Table S1 (Supporting
(
(
ppy)
2
information). Fig. 2d-f and Table 2 showed the current density–
voltage–luminance (J–V–L) and luminance efficiency–luminance
2
(LE–L) characteristics and main parameters. Devices A and B with
LEmax (62.34 cd/A) when luminance intensity was 4900 cd/m .
0
.5 nm thickness CAFs as the EIL exhibited different turn-on
However, when the thickness of TCAF increased from 0.5 to 1 nm
and 1.5 nm, the luminance curve immediately decreased bellow
250 cd/m . As shown in the Fig. 4c, the best PEmax (21.47 lm/W) of
voltage (Von) (Von is defined as the voltage by luminance of 1 cd/
m ). Obviously, the high Von of device A sharply decreased 0.54
2
2
times from 9.4 V to a lower Von as 5.0 V of device B. Especially, the
maximum luminescence (Lmax) intensity of device Breached as
device B was 16.8 and 34.6 times than that of device F (4.64 lm/W)
and device G (0.62 lm/W), respectively. Fig. S12 (Supporting
information) showed the EL spectra of device B and E driven at
14 V with the green light emission peaks at ꢁ523 nm. Accordingly,
the EQE of device B (16.56%) was 3.1 and 22.4 times than that of
device E (5.29%) and device F (0.74%), respectively. It was indicated
that device parameters based on TCAF as EIM largely depended on
the special thickness of 0.5 nm of interfacial modification layer for
better performance.
2
2
high as 31,549 cd/m which is 6.8 times than the Lmax (4672 cd/m )
of device A. Beside of that, the current density of device A and
device B were 0.54, 7.1 mA/cm2 at 11 V, respectively. Meanwhile,
the corresponding maximum luminance efficiency (LEmax) mea-
sured on device B was up to 62.34 cd/A, almost 9 times than that of
device A (6.65 cd/A), the LEmax of device B is obviously greater than
that of device A in contrast to the performance of standard device
with Cs
2
CO
3
as EIM (71.4 cd/A, Table S6 in Supporting information).
As presented in Fig. S13 (Supporting information), the atomic
force microscope (AFM) exhibited topographic images and phase
This phenomenon [51,68,69] could be explained by the deeper
LUMO and stronger electronic affinity of TCAF with four cyano
groups inducing interface dipole effect with potential charge
transfer at TPBi/TCAF/Al interfaces.
images (5
m
m  5
mm) of the vapor-deposited TCAF films under
vacuum with gradually increased thickness from 0.5 nm, 1 nm to
1.5 nm on bare Si substrates. All the surface morphologies of three
films possessed similar roughness and uniform surface without
obvious grain dispersion. The morphologies were conducive to the
interfacial modification and electron injection.
At the same luminance intensity of 4000 cd/m2 (Fig. 2e), the
efficiencies could maintain 88.6%, 91.7% of LEmax (62.34 cd/A) when
the LE of devices A and B reached 5.89 and 53.88 cd/A, respectively.
As shown in Table 2, the maximum power efficiency (PEmax) of
device B (21.74 lm/W) was much higher than that of device A
In order to further investigate the performance of OLEDs with
TCAF as EIM, another series of devices were constructed by
regulating the thicknesses of TPBi as ETL (Fig. 4). In contrast to
device B with optimal thickness of TCAF (0.5 nm) and TPBi (40 nm),
three more devices (G, H, and I) were fabricated by changing the
thicknesses (30, 50, and 60 nm) of TPBi as ETL (Fig. S9, Table S4 in
Supporting information). As shown in Fig. 4a and Table 2, when the
thickness of TPBi was 30 nm, device G displayed quite similar and
(
1.42 lm/W), which is almost 14 times than that of device A. The
largest EQE belonged to device B as high as 16.56%. Furthermore, as
showed in Fig. 2g and Table S2 (Supporting information), two
electron-only devices (C and D) were fabricated and device D
displayed larger current density than that of device C with Al. It
was indicated that the lower driving voltage improved EL
characteristics with higher LE, which demonstrating the better
electron injection ability of TCAF than that of DCAF.
Compared with device B, two more devices (E and F) have also
been fabricated with 1.0 nm and 1.5 nm thick TCAF as EIM to
further investigate the relationship between electron injection
ability and thickness of TCAF (Fig. 3). The general configuration of
2
slightly smaller Von of 4.7 V, higher Lmax of 34,504 cd/m , smaller
LEmax of 54.67 cd/A, and decreased PEmax of 20.90 lm/W than that of
device B. However, when the thickness of TPBi increased to 50 nm
and 60 nm, devices H and I presented remarkably decreased
parameters including Von, Lmax, and PEmax (Table 2) except for LEmax
and EQE. As depicted in Fig. 4b and Table 2, these two parameters
of devices G, B, H, and I kept at the same level as 54.67, 62.34, 56.95,
and 58.66 cd/A and 14.53, 16.56, 15.25 and 15.71% with gradually
increased thicknesses of TPBi. The Lmax of device G reached
Table 1
Electrochemical and thermal characteristics of DCAF and TCAF.
2
2
3
4,504 cd/m larger than the value of devices B (31,549 cd/m ) and
Samples
LUMOcv (eV)
HOMOcv (eV)
Gapcv (eV)
T
g
(oC)
2
H (23,986 cd/m ). And it was also almost 5 times than the value of
device I (6568 cd/m ). At 5000 cd/m , LE decreased to be 87.63,
2.60, 84.84, and 77.35% in contrast to their LEmax while LE
DCAF
TCAF
À4.17
À4.52
À6.14
À6.13
1.97
1.6 1
247
261
2
2
9