Supplementary information:
1. Light Out-Coupling Efficiency of PVK- and SimCP2-based OLEDs- An Account of
Isotropic Refractive Index Simulation.
To clarify the issue of different refractive index of active layer, we start from
utilizing the plane-wave model to calculate the light out-coupling efficiency of
devices with isotropic thin films. We assume that both PVK-based and SimCP2-based
devices are isotropic in dipole orientation. In our simulation, the following values of
refractive index (n) and absorption coefficient (k) in non-active layers are used: nair =
1, nglass = 1.52, nITO = 2.04~1.74 , nPEDOT = 1.56~1.53, kPEDOT = 0.0025~0.01, nAl =
0.38~1.48, and kAl = 4.08~6.98 for λ = 400~700 nm. For simple comparison, we
merely consider the single refractive index nPVK = 1.7 and nSimCP2 = 1.9, respectively,
for wavelength λ= 400~700 nm. With this simulation, we can calculate the ratio of
two important optical modes in experimental luminous flux data, air and substrate
modes. As shown in Figure. 1, the air modes are 17.8 and 24.8 % for PVK- and
SimCP2-based devices, respectively, and the substrate modes are 23.7 and 27.3% for
PVK- and SimCP2-based devices, respectively. With higher refractive index in active
layer, there would be higher light out-coupling efficiency (air mode) and the substrate
mode. From the simple plane-wave calculation, PVK-based devices show lower light
out-coupling efficiency than SimCP2-based devices. Therefore, the difference of
refractive index in the active layers alone cannot explain experimental results, i.e.,
without BEF, PVK devices have higher total luminous flux of 2.31 lm than 1.91 lm of
SimCP2 devices.
2. Light Out-Coupling Efficiency of PVK- and SimCP2-based OLEDs- An Account of
Anisotropic Refractive Index Simulation.
The plausible reason for such discrepancy is optical anisotropic effect and it is caused
by different dipole orientations. In general, the anisotropic film, which mostly
happens in polymer materials (see Figure 2), has different refractive index in different
directions.
The difference of refractive index means the optical anisotropy of the thin film
material showing different ordinary (no or nTE) and extraordinary (ne or nTM) refractive
indexes because of different distribution in dipole orientation. The maximum
difference of refractive index (no and ne) occurs at the in-plane and perpendicular
direction. There are several reports about the optical anisotropy in polymeric thin
films and polymer-based devices.1-5 This means that the light characteristic, the
transverse electric (TE) or transverse magnetic (TM) mode, would be altered through
the anisotropic environment because of its dipole orientations. Wan et al. have found
that the optical anisotropic effect have great impacts on polymer light-emitting
devices.4,5 They considered interference effects into anisotropic optoelectronic
devices by both experiment and simulation. The quantum efficiency and angular
distribution of emission in anisotropic case is more different than those in isotropic
case. Campoy-Quiles et al. have measured optical anisotropy in PVK thin film with
the refractive index of nTE = 1.8 and nTM=1.68 at λ = 500 nm.3 Hence, we attribute
such optical anisotropy to the orientation distribution of the oscillating electric dipoles
in the SimCP2 or PVK hosted active layers. As previously reported by Kim et al., 6 the
oscillating electric dipoles are categorized into either in-plane anisotropic or isotropic
orientation and they are different in polymeric or small molecular materials.
Flämmich et al. proposed a model to prove that the different dipole orientations have
great influence on light out-coupling efficiency.7
From our experimental data, with and without BEF, the total luminous flux is
2.98 and 2.31 lm for PVK-based devices and 3.12 and 1.91 lm for SimCP2-based
devices. Without BEF, the light out-coupling in PVK-based devices is greater than
that of SimCP2-based devices, however, the outcomes are reversed with BEF attached
to the devices. In general the dipoles of PVK tend to be in-palne anisotopic, whereas
the orientations of small molecule SimCP2 are more isotropic. The more in-plane
dipoles in the active layer it is, the higher the light out-coupling is and hence the
device efficiency becomes higher. Accordingly, we incorporate in-plane anisotropic
dipole into our optical simulation model for polymeric PVK system, as described in
Equation 5. For the small molecule material SimCP2, it is more like an isotropic
system and Equation 4 is suitable for the optical simulation herein.
As shown in Figure 3, we compare three different models in calculating the
surface light out-coupling efficiency for PVK-based and SimCP2-based devices. For
ray-optics model, assuming n around 1.7 for most π-conjugation organic compounds,
the light out-coupling efficiency is 26.1% for PVK-based device and 19.1% for
SimCP2-based device. This is an opposite trend of light out-coupling efficiency when
compared with the simulation results without the consideration of anisotropoic
refractive index of polymeric materials (see results shown in Figure 1). However,
using ray-optics method to calculate light out-coupling efficiency, it only considers
the difference in refractive index of each layer without considering optical
interference and surface plasmonic effects. Therefore, we also applied more precise
plane-wave model to calculate light out-coupling efficiency. The plane-wave model
has been tested and verified by other research groups before.8-10 Here, the refractive
index of non-active layer and thickness of each layer have been shown earlier in this
paper. For λ = 400~700 nm, the refractive index n (absorption coefficient k) of active
layer is 1.8~1.68 (0.11~0.0028) and 2~1.75 (0.11~0.0028) for PVK and SimCP2,
respectively.
From more precise plane-wave prediction (Equation (4) for SimCP2
and Equation (5) for PVK system), the light out-coupling efficiency has been revised
to 36% for PVK-based devices and 26.2% for SimCP2-based devices, of which some
emission is outside the surface-escape-cone.6 In both simulation models, the light
out-coupling efficiency of in-plane anisotropy (polymer-based device) system is both
37% [(26.1-19.1)/19.1 or (36-26.2)/26.2)] better than that of the isotropy (small
molecule-based device) system. Including anisotropic effect of material refractive
index, we have acquired simulation results that are in accordant with the experimental
observation.
Furthermore, we have also compared our simulation results to the recently
reported data by Danz et al.11 Our predictions of light out-coupling efficiency were
quite similar to their simulation results, i.e., 38% for polymer-based device and 27.4%
for small molecule-based device, a similar 39% ((38-27.4)/27.4) higher of the light
out-coupling efficiency for in-plane anisotropic than isotropic system, as indicated in
Figure 3.
References
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2
J. W. M. Prest and D. J. Luca, Journal of Applied Physics 51, 5170 (1980).
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W. M. V. Wan, N. C. Greenham, and R. H. Friend, Journal of Applied Physics
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Figure Captions:
FIG 1. Calculated light out-coupling efficiency and substrate mode of electric
oscillating dipole with isotropic orientation used plane-wave model with single
refractive index of n = 1.7 and 1.9 in place of variant refractive index.
FIG 2. Schematic illustration of dipole orientation of small molecular material
(isotropy) and polymeric material (in-plane anisotropy).
FIG 3. Calculated light out-coupling efficiency of electric oscillating dipole with
in-plane (blue lines) and isotropic (red lines) orientation used ray-optics and
plane-wave models. We also compared our results to the Danz’s plane-wave
method.11
Mode Ratio (%)
80
70
60
50
40
30
20
10
0
Air mode
Substrate mode
n=1.7 (PVK) n=1.9 (SimCP2)
Fig. 1
Small molecule based-device :
Polymer based-device :
Isotropic orientation
In-plane orientation
z
y
y
x
x
Extraction Efficiency (%)
Fig. 2
60 Dipole orientation
55
In-plane anisotropic
50
Isotropic
45
Danz's Plane-wave
40
Plane-wave
35
30 Ray-optics
25
20
15
10
5
0
Various Models
Fig. 3