EQE optimization For a Top-Emitting OLED
The external quantum efficiency (EQE) described the number of outcoupled photons per injected charges and is defined by the 4 factors charge balance, singlet-triplet generation efficiency, radiative quantum efficiency and light outcoupling efficiency:
In an OLED stack, only part of the emitted light can escape the device, this problem is caused by a high refractive index in organic layers. Most of the light is lost to three loss channels ( for a top-emitting OLED ):
- guided modes ( total internal reflection )
- absorption losses ( absorption in layers and at electrodes interface )
- surface-plasmon-polariton ( evanescent coupling mode )
In this study, we optimize the outcoupling efficiency by changing layers material, emitter properties and electrode material.
- Emitter characterization with PHELOS measurement and SETFOS fitting
- Parameters of the simulation
- Impact of layers refractive index
- Impact of dipole distribution
- Impact of dipole orientation
- Impact of electrode material and thickness
1. Emitter characterization with PHELOS measurement and SETFOS fitting
To optimize the light extraction efficiency, we have to know some properties about the OLED, like layer thicknesses, emitter position, distribution, orientation and emission spectrum. By using two products Phelos and Setfos, we can get this information.
Here is our reference stack in figure 1 which is a top-emitting OLED, we will change different parameters in Setfos to see their impact on the outcoupling efficiency.
Figure 1: Reference stack, top emitting OLED
2. Parameters of the simulation
For the case of a bottom-emitting OLED ( figure 2 ), to escape the device light has to go through the thin transparent ITO electrode and the glass substrate. As the organic layer has a high refractive index about 1.8, much higher than the glass (about 1.5) and the air (n=1), there are two total internal reflections in a bottom-emitting device. Light can escape the device only if it is emitted within the escape cone which is defined by materials refractive index, the incidence angle of the emission is defined by the dipole orientation.
Figure 2: Bottom emitting OLED
For the case of a top-emitting OLED ( figure 1 ), light can be reflected by the top electrode and by the bottom electrode, the constructive interference condition between emitted light and reflected light depends on the wavelength, the incidence angle of the emission which depends on the dipole orientation, the ETL, HTL thicknesses, the dipole position and the layers refractive index.
For a top-emitting OLED usually a capping layer (CPL) is used to increase the outcoupling efficiency by tuning the CPL thickness. The CPL thickness also influences the emitted color at different angles, as shown in figure 3.
3. Impact of layers refractive index
According to a publication by Salehi et al. ("Manipulating refractive index in organic light emitting diodes"), layers refractive index can be reduced by pore inclusion using oblique angle deposition leading to increased light extraction efficiency in OLED devices. For the simulation with Setfos we can use different ETL and HTL materials with different refractive indices to investigate the impact on the outcoupling efficiency.
To do the optimization of extraction efficiency, we use the emission module and the sweep mode. By defining the sweep range, we will get result for each case in the range. Figure 4 shows a plot of the outcoupling efficiency I_OC versus HTL and ETL thicknesses, we can see four maxima. We focus on the first and the second maximum in the following.
Figure 4: Outcoupling efficiency versus ETL and HTL thicknesses
In the next step we optimize the CPL thickness to further improve the first and the second maximum.
Figure 5 depicts simulation results for different cases from the reference stack to the stack where ETL and HTL have lower refractive index material (about 1.65), the outcoupling efficiency I_OC is equal to 26% for the first maximum in the reference stack.
Figure 5: Mode contribution for ETL and HTL change by material with n~1.65
- By decreasing the refractive index of both ETL and HTL materials from 1.8 to 1.65, we increase the outcoupling efficiency I_OC by 31% (compared to the reference stack) in the first maximum and by 25% in the second maximum.
- By decreasing the refractive index, we decrease considerably the guided mode plus evanescent coupling mode I_GM+EC by 21% for the first maximum and by 30% for the second maximum.
- The absorption losses I_AL increase for the second maximum, this is why I_OC does not increase a lot whereas I_GM+EC decreases considerably for the second maximum.
According to the publication by Salehi et al., "It has been shown that the refractive index of Alq3 can be tuned from 1.75 to 1.2 using OAD (oblique angle deposition)". We can therefore further increase the outcoupling efficiency (up to 150% compared to the reference case ) by using even lower ETL and HTL refractive index ( n=1.2 ). The question is whether such low-index material can be produced.
Figure 6: Mode contribution with lower ETL and HTL refractive index
In the next step we investigate the influence of the refractive index of the EML. But it is not the same story because the outcoupling efficiency is higher for high EML index, as seen in figure 7.
Figure 7: Mode contribution for lower EML refractive index (n_ETL=n_HTL~1.65)
4. Impact of dipole distribution
Constructive interference of reflection at the electrode interface depends on the wavelength, the incidence angle, layers refractive index, layers thicknesses but also on the dipole distribution and position. We consider the low-n (n=1.65) stack with isotropic emission and with optimized layer thicknesses for dipoles with Dirac distribution at position 0, i.e. at the ETL interface. We now change the dipole position from 0 to 1 by steps of 0.1, it changes the outcoupled flux by 22%. These changes are however only because the outcoupling efficiency had been optimized for dipole position 0.
Figure 8: Mode contribution versus dipole position for layer thicknesses optimized for dipole at position 0
If we optimize layer thicknesses for each dipole position, we find that the outcoupling efficiency does not change ( figure 9 ).
5. Impact of dipole orientation
Emitters can be regarded as dipoles which emit light into a certain direction. In the following we will study the impact of dipole orientation on the outcoupling efficiency. Figure 10 shows two cases of dipoles in an EML layer, a perpendicular dipole which emits light inside the plane of the emitting layer and a parallel dipole which emits light mostly into the escape cone, the parallel dipole is therefore more efficient.
Figure 10: Two extreme cases of dipole, the perpendicular one and the parallel one
In Setfos, we quantify dipole orientation by a number between 0 and 1. Hereby 0 means all dipoles are parallel and 1 means all dipoles are perpendicular. We consider the low-n stack with dipole position 0.5 (in the center of the EML) and change the dipole orientation from 0 to 1 by steps of 0.1. For each case we optimize layer thicknesses for the first maximum. The result ( figure 11 ) shows that parallel dipoles are most efficient with a small contribution of guided mode plus evanescent mode because light is almost completely emitted in the escape cone. Between the parallel dipole and the perpendicular one, the optimized layer thicknesses change considerably from 50 nm to 140 nm.
6. Impact of electrode material and thickness
We will now consider the low n stack with Dirac distribution at position 0.5, and this time with dipole orientation equal to 0.18 (the result from the fitting). We change top and bottom electrode material between silver (Ag) and aluminum (Al) to study the impact of the electrodes material. Figure 12 shows the difference in absorption and reflection between the two materials. We notice that silver reflects more and absorbs less than aluminum for wavelengths over 420 nm.
Figure 12: Reflectance (solid lines) and absorbance (dashed lines) of Ag and Al versus wavelength
In figure 13 we plot the comparison of four cases on the first maximum and on the second maximum contribution of each mode. It shows that silver as top electrode induces less absorption losses, and the best case is the case with silver as top electrode and aluminum as bottom electrode.
Figure 13: Mode contribution for four combinations of electrodes material
We can also see the impact of the top electrode thickness. In figure 14 we plot the comparison between cases with top electrode thicknesses of 20, 10 and 5 nm. There is no big impact on the outcoupling efficiency, so we have to consider electrical effects first and not use a too thin electrode. There is no impact of the bottom electrode thickness because it is a very thick layer. Optically, the bottom electrode is like a mirror which reflects light .
Figure 14: Mode contribution for different top electrode thicknesses
In figure 15 we show a summary of the parameter changes employed in our study to increase the outcoupling efficiency. In the reference stack we already use Ag as top electrode. We can demonstrate an increase of the EQE from 26 % to 51 % by optical optimization! We can increase even more if we get lower ETL and HTL index material.
Figure 15: Mode contribution from the reference stack to low n stack ( n~1.65 ) with parallel dipole.
- We used Phelos measurement and Setfos fitting to determine the emitter orientation, emitter position and emission spectrum.
- We used Setfos to optimize the EQE for HTL, ETL and electrodes material, the dipole orientation and position should be known from Setfos fitting to optimize layer thicknesses.
- In order to obtain the optimum EQE in OLEDs, we have to use low-index ETL and HTL, parallel oriented emitters and silver as top electrode.