Organic light emitting diodes (OLEDs) are the most promising candidates for transparent displays ands lighting applications, primarily owing to their very thin form factors and simple device structure, which typically consists of a transparent bottom electrode, organic layers, and a metallic top electrode. By replacing the top metallic electrode with a transparent electrode such as very thin metallic films or transparent conductive oxides (TCOs), it is possible to fabricate transparent OLEDs (TrOLEDs) in a relatively simple way.[1-4] To avoid potential sputter damage to underlying organic layers that can happen during deposition of the TCO layers, a transparent top electrode is often made using a thin metal film covered with an optical capping layer.[2-5] Adopting this approach, many researchers have already developed TrOLEDs having high transmittance.[2-4] However, in order to secure the practical viability of TrOLEDs, one must consider not only the transmittance but also the power and external quantum efficiency. Since TrOLEDs emit light in both bottom (forward) and top (backward) directions, the amount of light reaching an observer is far smaller than that of an opaque control device for the same amount of current, rendering the development of efficiency-enhancing scheme a top-priority agenda in TrOLEDs. In this regard, it would be beneficial if one can adopt various technologies developed for improving the efficiency of conventional non-transparent OLEDs such as outcoupling-enhancement schemes that extract light trapped in a substrate, or organic/ITO layers due to total internal reflections.[6, 7] However, one should note that adopting those light-extraction enhancing schemes developed for conventional OLEDs, for use with TrOLEDs often severely compromises the esthetic advantage of TrOLEDs, which is their essential, differentiating feature desired for many of their potential applications. For example, various light extraction techniques suggested to date include a micro-lens array attached on the back side of a glass substrate, embedded micro-structure with materials having a high or a low refractive index,[9, 10] and embedded sub-wavelength nanostructures such as photonic crystals.[11-15] In spite of their effectiveness in improving the light extraction efficiency of OLEDs, the mechanisms underlying most of these light extraction techniques fundamentally involve an optical scattering process. Therefore, those light extraction structures, if applied in TrOLEDs, would cause the scattering of incident light projected onto the devices, causing an optical blur or haze, and thus severely limiting their applicability.
In this work, we demonstrate a method to improve the light extraction efficiency in TrOLEDs with virtually no compromise in their optical clarity. An periodically-perforated tungsten trioxide (WO3) layer previously proven effective in opaque OLEDs was adopted and optimized to extract the waveguide modes and surface plasmon modes used in TrOLEDs. With a proper design based on an optical model relevant to the TrOLEDs under study, we demonstrate a relative enhancement of external quantum efficiency (EQE) by approximately 34% for the bottom side emission and by 79% for the top side emission. Furthermore, we demonstrate the proposed structure has little influence on the diffuse or haze characteristics of the TrOLEDs, simultaneously realizing the commonly conflicting goals of increasing the efficiency of TrOLEDs, and maintaining their optical clarity.
The schematic structures of the suggested nanostructured TrOLED, and their planar control devices, are shown in Figure 1a and b, respectively. Note that a perforated WO3 layer with a thickness of 25 nm is fabricated onto a planar ITO layer. The order of layers is important to prevent the formation of cracks and sharp edges in the ITO layer that might appear if the ITO were deposited on the perforated nanostructure. For the fabrication of an periodically perforated WO3 layer, we utilized a colloidal lithography technique using polystyrene nanospheres. SEM images of the newly fabricated, perforated WO3 layer are shown in Figure 1c. The organic layers and the metal layers deposited onto the perforated WO3 layer have a periodic, corrugated structure.
The waveguide modes and the surface plasmon modes trapped in OLEDs can be extracted if the period of the corrugated nanostructure satisfies a Bragg scattering condition. For a hexagonal grating structure, the Bragg scattering condition in a normal emission is as follows:
where kguide is the wave vector of the waveguide mode or the surface plasmon mode, m and n are integers, and Λ is the period of the hexagonal grating structure, which corresponds to the spatial period of the perforated WO3 layer in the proposed nanostructured TrOLEDs. The kguide at each wavelength can be obtained from the dispersion relation of the waveguide modes and the surface plasmon modes formed within the planar TrOLED structure shown in Figure 1b. The results of calculations combining the dispersion relationship obtained using a transfer matrix method and Equation (1), are shown in Figure 2a. In addition, the calculated electric field intensity profiles of the waveguide modes and the surface plasmon modes, are presented in Figure 2b for 600 nm, the main wavelength of 4-dicyanomethylene-2-methyl-6-(p-dimenthyaminostyryl)-4H-pyran (DCM), the emitting material used in the fabricated TrOLED devices. From the calculated results shown in Figure 2a and b, one can identify that two TE waveguide modes (TE0 and TE1), two TM waveguide modes (TM2 and TM1), and one surface plasmon mode (TM0) are formed in the planar TrOLED structure. From the results of the optical calculation, we fabricated a TrOLED with a hexagonal nanostructure of period 330 nm into the device. The outcoupling enhancement of each waveguide mode and the surface plasmon mode was thus expected to occur at wavelengths of 438 nm (TE1), 528 nm (TE0), 461 nm (TM2), 516 nm (TM1), and 646 nm (TM0).
Figure 2c shows the electroluminescence (EL) spectra of the planar TrOLED, and the nanostructured TrOLED, from the bottom (glass side) of the device in normal emission (at emission angle 0°). Figure 2d shows EL spectra of both devices from the top side (cathode side) of the device in normal emission. In order to distinguish the enhancement of each waveguide mode and surface plasmon mode, we obtained the enhancement ratio of the EL spectra from emission of both sides as a function of the wavelength, as shown in Figure 2e. For both the bottom side emission and the top side emission, emission enhancement peaks were observed at wavelengths around 510 nm and 650 nm. These wavelengths correspond to the wavelengths at which outcoupling enhancement of the TM1 waveguide mode and the surface plasmon mode (TM0), was expected to occur from the above calculation. On the other hand, outcoupling of the TE0 waveguide mode was not observed in the measured EL spectra. As shown in the calculated electric-field-intensity profiles of the waveguide modes, and the surface plasmon mode presented in Figure 2b, the electric field of the TE0 waveguide mode was mostly confined within the ITO layer while the electric field of the TM1 waveguide mode and the surface plasmon mode (TM0) were largely confined near the surface of the silver (Ag) cathode. Because the TrOLED with the perforated WO3 layer has a planar ITO layer, little scattering of the TE0 waveguide mode was expected. It is thought that the TM1 waveguide mode and the surface plasmon mode (TM0) were more efficiently scattered by the corrugated Ag cathode layer than by the TE0 waveguide mode. On the other hand, the main emission wavelength of the fabricated TrOLEDs was around 600 nm. Thus, the outcoupling of the surface plasmon mode (TM0), which occurred near 650 nm (near the main emission wavelength), could effectively bring the practical enhancement of the EL intensity. However, EL emission at wavelengths near 510 nm, where the outcoupling of the TM1 waveguide mode occurred, was basically insignificant in the fabricated TrOLEDs. The enhancement of the EL intensity shown in Figure 2c and d mostly resulted in enhanced the outcoupling of the surface plasmon mode (TM0).
Figure 3a shows the current density (J)–voltage (V) characteristics of the devices fabricated in this work. The J–V characteristics of both devices turned out to be similar, indicating that the introduction of the perforated WO3 layer does not alter the electrical integrity of the TrOLEDs under study. This is partly due to a good hole-injecting property of WO3 layers.[16, 17] Insertion of a layer having an insulating character or that with mismatched energy levels would have caused an increase in the operating voltage of the OLEDs. A work function of the WO3 measured by ultraviolet photoemission spectroscopy (UPS) was analyzed and found to be 5.1 eV. The embedded WO3 layer would be suitable to inject the hole from the ITO layer (that has a work function of 4.5 eV) to the organic layers. Figure 3b and c show the EQE and the power efficiency of the planar TrOLED, and the nanostructured TrOLED, for bottom and top emissions, respectively. When only the bottom emission was taken into account, the EQE of the TrOLED with the perforated WO3 layer was around 34% higher than that of the planar TrOLED, at a current density of 100 mA/cm2. Meanwhile, when only the top emission was considered, the EQE increased by around 79% at a current density of 100 mA/cm2. There are two possible reasons for the increased EQE. One is the enhanced outcoupling of the waveguide modes and the surface plasmon mode, and the other is the altered charge injection balance caused by the embedded, perforated WO3 layer. However, the J-V characteristic of the nanostructured TrOLED was similar to that of the planar TrOLED. Also, as shown in Figure 2c and d, emission enhancement was only observed at the specific wavelengths at which the outcoupling of the waveguide modes and surface plasmon mode occurred. This was not observed at other wavelengths. Considering these results, it would be reasonable to conclude that the improvement in the EQE was mostly caused by the enhanced outcoupling. The enhancement in EQE was reflected also in the power efficiency. The relative increment was around 29% for bottom emission and 75% for top emission.
When one embeds the light extraction structure into TrOLEDs, one should carefully monitor its influence on the transmittance or haze characteristics of the device. The measured direct transmittance (TDirect) spectra of the fabricated devices are shown in Figure 4a. One can confirm that the TDirect of the nanostructured TrOLED was similar to that of the planar TrOLED, except that there were two minima at wavelengths around 510 nm and 650 nm. These wavelengths correspond to the above calculated wavelengths at which outcoupling of the TM1 waveguide mode and the surface plasmon mode (TM0) was expected to occur. When the incident light was projected onto the nanostructured TrOLED, some of the incident light at these wavelengths was coupled to the TM1 and TM0 waveguide modes and then either absorbed or scattered out again into the surroundings. The latter is possible because the re-scattering out of the waveguide modes also occurs under the same Bragg condition. In either of the cases, direct transmission with a dip in those particular wavelengths is expected to occur, being consistent with our experimental observations. Other than such features in the transmittance spectra, the spectra of the total transmittance (TTotal, the sum of the diffuse transmittance and the TDirect) of the fabricated devices shown in Figure 4b and c are virtually equivalent to those of their respective TDirect spectra. This indicates that there is no significant optical diffusion and haziness occurring in either of the devices. The optical clarity of the nanostructured OLEDs also can be seen from the photographs of the fabricated nanostructured OLEDs shown in Figure 4d. The letters on paper placed on the backside of the device are clearly shown without any blurring or haziness.
From these results, we can conclude that the suggested light extraction method of inserting a periodically perforated WO3 layer into TrOLEDs, effectively improves the outcoupling efficiency without causing optical haze in the proposed devices. This should be a clear advantage over other outcoupling enhancement methods where the scattering can occur over a broad spectral range, and thus the overall direct transmittance or visual clarity can be significantly compromised. On the other hand, one may recall that methods using a periodic nanostructure could have a serious problem: a dependency of emission characteristics on the emission angle and/or the azimuthal angle. However, the periodic nanostructures in the proposed TrOLEDs are prepared by colloidal lithography, which tends to form a plurality of domains distributed spatially over a scale of approximately 10-μm with a random orientation as shown in Figure 1c. This long-range, randomized characteristic significantly reduces the angular dependency of emission characteristics while the periodicity within each domain can still enhance the outcoupling. In particular, the angular dependency on the azimuthal angle disappears almost completely. (See Figure S1 in the Supporting Information.)
We proposed the use of a nanostructured TrOLED incorporating a periodically perforated WO3 layer in order to improve the outcoupling efficiency of TrOLEDs. The insertion of the periodic nanostructure efficiently increased the EQE and the power efficiency of the TrOLEDs due to enhanced outcoupling of the waveguide modes and the surface plasmon modes. While most other outcoupling-enhancement structures are expected to compromise the optical clarity when they are applied to TrOLEDs, the suggested nanostructured TrOLEDs exhibited similar transmittance with that of planar TrOLEDs with the exception of a slight decrease at specific wavelengths. Most importantly, there was virtually no influence on the diffusion or haze characteristics, a critical feature to consider in TrOLED applications. We believe that the suggested method of inserting a periodically perforated WO3 layer into a TrOLED is a practical, useful approach to enhance the outcoupling efficiency of TrOLEDs.