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We investigate the improvement in efficiency of organic light emitting diodes/displays (OLEDs) by embedding a typical OLED structure within a metallic patch grating resonator. A patch grating resonator is similar to the more familiar Fabry-Perot resonator, except that one mirror of the resonator is a metallic patch grating with a pitch ~ λ/2 that reduces lateral propagation of radiative emission. FDTD simulations of the proposed structure indicate a potential 71% increase in emitted power over that of a reference OLED structure, and an additional 5% gain from adding an ITO spacer adjacent to the metallic electrode layer (for a total 76% increase). Implementation of this structure requires little to no modification of the OLED manufacturing process.
©2010 Optical Society of America
1. Introduction
The efficiency of organic light emitting diodes/displays (OLEDs) is limited, in part, due to incomplete light extraction from the active, light-emitting layer. Loss mechanisms include total internal reflection (TIR) at the glass/air interface, coupling to dielectric waveguide modes of the organic layers, and dissipation into the metal contacts [1–3]. We propose embedding an OLED structure within a directive antenna, formed by a patch grating resonator, in order to reduce these loss pathways.
Fabry-Perot resonators [4–6], photonic bandgap structures [7–9], and metamaterials [10] have been used to increase the directionality of transmission from antennas (i.e. directive antennas) in the microwave region. Fabry-Perot resonators have also been used to improve the color purity and spatial distribution of light emitted from microcavity light-emitting diodes [11–13]. Others have suggested using optical antennas to control the emission properties of light [14–16]. Considerable work has been performed using metal grating structures to improve the outcoupling efficiency of OLEDs [17–19], primarily via directed scattering of surface plasmon modes of the corrugated metal surface.
Here we propose and analyze a directive antenna structure that restricts the lateral propagation of light within an OLED through the replacement of one mirror of a Fabry-Perot resonator with a thin metallic patch grating with a pitch of ~ λ/2 embedded into the ITO anode layer. FDTD simulations of the proposed structure indicate a potential 71% increase in emitted power over a reference OLED structure, and an additional 5% gain from adding an ITO spacer layer adjacent to a reflective cathode to place the emitting layer a distance λ/4 above the ground plane. To facilitate understanding of the mechanisms for increased efficiency of the proposed structure, we begin by evaluating simulation results for a single dipole placed above a reflective ground plane and systematically increase the complexity of the system until the proposed structure is attained. Simulations were performed with Lumerical FDTD Solutions.
2. Optimization of radiative emission from a free-space dipole
An emitting dipole placed at a distance d 1 from a metallic reflector is schematically shown in Fig. 1. The power radiated to the far field is a sinusoidal function of the distance of the dipole from the metallic reflector [1, 20] due to the interference between the dipole and its image. Simulation results are shown in Fig. 2 using an aluminum ground plane; power is calculated by integrating Poynting vector across a line. As expected, the power radiated to the far field is essentially sinusoidal with local maxima at distances that are close to odd multiples of λ/4.
Fig. 1. Illustration of an electric dipole positioned a distance d 1 above a ground plane. The dipole image is also shown.
The reflector/ground plane can be viewed as a mirror that provides an image of the dipole emitter at a distance 2d 1 from the dipole as depicted in Fig. 1. The effect of such an image is similar to a dual antenna transmitter where the directionality of emission increases with separation 2d 1. If the dipole is placed at an odd multiple of λ/4 away from the mirror, the reflected electric fields will be in phase with the directly emitted fields (in the far field) and the emitted power will quadruple at small viewing angles, as verified in the simulation results of Fig. 2. Furthermore, there is a slight narrowing of the angular distribution of the emission profile at a separation of ~ λ/4, and a greater narrowing at a separation of ~ 3λ/4. Angular radiation profiles are shown in Fig. 2.
Fig. 2. (left) Average emitted power from an x-oriented dipole versus separation distance d 1 from a ground plane (green), normalized to the power from a free space dipole (blue). Here, λ=500 nm and the ground plane is a 100 nm thick layer of aluminum. (right) Irradiance versus emission angle for separation distances d 1 corresponding to maxima in the response curves. Blue line is for a free-space dipole; green line is for d 1 = λ/4; and red line is for d 1 = 3λ/4.
3. Adding a partial reflector to increase directionality of dipole emission
By adding a partial reflector opposite the reflective ground plane, additional dipole images are created and the directionality of the emitted radiation is increased. This is a common (asymmetric) Fabry-Perot resonator, which has been used in a variety of electromagnetic devices for spectral and/or spatial narrowing including antennas [4–6, 21] and OLEDs [12, 13]. Furthermore, by using a patch grating with a pitch of λ/2 as the partial reflector, as shown in Fig. 3, the lateral propagation of radiative energy within the structure may be inhibited and the directionality of emission further increased. Here, the patch grating is constructed in a 50 nm thick layer of Al.
Fig. 3. Illustration of an electric dipole placed within a resonator with a patch grating reflector.
Radiated power from the dipole in a patch grating resonator is shown in Fig. 4. The separation between the ground plane and the dipole (d 1) is equal to the separation between the dipole and the patch grating (d 2). Again, the separations of maximal output power are odd multiples of λ/4, with a maximum increase in power of nearly 10× over an isolated dipole.
The increase in directionality via the patch grating is shown in Fig. 4 at the optimal separation d 1 = d 2 = λ/4. Compared to a single reflector, the on-axis irradiance is increased more than 3× due to the patch grating, while the overall emission is increased more than 2×. The trade-off for this increase in emission is the greater sensitivity in placement of the emitter between the ground plane and patch grating.
Fig. 4. (left) Effect of patch grating on average emitted power from an x-oriented dipole versus distance from a ground plane, normalized to the power from a free space dipole where λ=500 nm. The patch width W = 180 nm, and the patch pitch Λ=250 nm. The dipole is vertically centered between a 100 nm thick aluminum ground plane and a patch grating so that d 1 = d 2. Green line is without the patch; red line is with the patch. (right) Effect of patch grating on the directionality of emission from an x-oriented dipole, normalized to the power from a free space dipole, where d 1 = d 2 = λ/4.
4. Effect of the resonator on an OLED
Figure 5 shows the structures used to assess the effectiveness of the patch grating resonator on OLED light emission. The reference structure is based upon the OLED device detailed in [1], with an adjustment in the thickness of the ITO and glass layers to optimize the emitted power. A metallic grating embedded in the ITO layer of the reference structure increases the power emitted from a dipole located within the active layer of the OLED. For purposes of these simulations, the dipole was assumed to be vertically centered in the Alq3 layer of the OLED.
Fig. 5. Illustration of a reference OLED structure (left) and the reference structure with a patch grating embedded into the upper ITO layer (right). An optional ITO spacer layer is indicated in the resonator structure.
Figure 6 shows the emitted power as a function of distance d 2 of the patch grating from the dipole. Using a refractive index of 2.0 for ITO, the optimal patch separation for the OLED structure is Λ=125 nm. As shown, the emitted power is quite sensitive to the distance between the dipole and the grating, typical of a Fabry-Perot resonator. Nevertheless, a 71% increase in emitted power is obtained at a dipole to patch separation 225 nm.
Fig. 6. (left) Average emitted power vs. patch layer distance normalized to the power from a reference OLED structure. Blue line is the reference OLED; green line is the OLED with patch grating (d 1 = 40 nm); and red line is OLED with patch grating and 20 nm ITO spacer (d 1 = 60 nm). (right) Irradiance versus emission angle normalized to the irradiance at the peak emission angle of the reference OLED. In both graphs, λ=500 nm, the patch length W=80 nm, and the patch pitch Λ = 125 nm.
Figure 6 also shows the emitted power as a function of emission angle for patch placements corresponding to the emission peaks. Comparing cross-sections of the electric field distributions between the reference and patch grating resonator OLEDs shows the suppression of waveguiding modes in the organic layers and narrowing of the emission profile in the glass layer, as shown in Fig. 7. This latter effect allows greater light coupling from the glass layer due to the increase in emitted power below the critical angle. Also note the significant decrease in radiation trapped in the ITO layer.
Fig. 7. Representative electric field distributions (E 2) for the patch-grating resonator (top) and reference (bottom) OLEDs. The ITO and glass regions are indicated by the dashed lines. The color scale is logarithmic.
5. Inserting a spacer to increase output power
In the prototype OLED structure, the optical path distance d 1 (see Fig. 5) between the dipole (centered within the active layer) and the metallic substrate does not correspond to an odd multiple of λ/4. To correct this sub-optimal condition, a spacer layer of ITO may be added over the metallic substrate (note that we are neglecting the detrimental effect of this layer on the electrical properties of the device).
FDTD simulation results shown in Fig. 6 indicate a further 15% increase in output power by adding an ITO spacer layer of 20 nm to the OLED stack (for a total of 86% increase over the reference OLED). Also apparent from Fig. 6, the introduction of the 20 nm ITO spacer layer results in a decrease of approximately 15 nm in the optimal distance d 2 between the dipole and the patch grating layer.
The results so far have been obtained for an emitting x-oriented dipole vertically centered (z = 0) within the active (Alq3) layer of the OLED structure which, for purposes of simulation, has been assumed to be the position of electron-hole recombination. However, electron-hole recombination occurs throughout the active layer, necessitating consideration of the effect of dipole position on overall output power. Further, in a real device, the effective molecular dipole orientation would be random; in our 2-D simulations, this can be accounted for by also considering the radiative efficiency of a z-oriented dipole. However, a z-oriented dipole does not efficiently radiate from the planar structure (i.e. the emitted power is significantly less than that from the x-orientation, even with the patch grating structure), so we consider its contribution negligible and focus on the x-orientation.
In Fig. 8, we consider the effects of dipole position on the emitted power. We see a strong dependence, where dipoles placed close to the groundplane are nearly quenched. We also see that the influence of the spacer is to reduce this effect and to position the maximum output for dipole position in the center of the active layer.
Fig. 8. Average relative emitted power of a dipole as a function of dipole position z, normalized to the maximum emitted power of the reference OLED with vertically centered dipole (z = 0, without a patch grating) where λ=500 nm, the patch length W = 80 nm, and pitch Λ= 125 nm. The patch grating position corresponds to optimal values obtained from Fig. 6. Blue line is the reference OLED; blue dashed line is the reference OLED with 20 nm ITO spacer; green line is the OLED with patch grating (d 1 + d 2=265 nm); and red line is OLED with patch grating and 20 nm ITO spacer (d 1 + d 2=270 nm).
Averaged over all possible dipole positions within the active layer, the patch grating improves the output power by nearly 71%. The spacer layer and patch grating combination improves the output power by 76% over the reference OLED structure, despite the fact that the patch grating without the spacer layer has a higher, but off-centered, peak. The shifting of this peak is the source of the improvement attained with the spacer layer.
6. Conclusions
We have shown that the emitted power and directionality of an OLED structure can be increased by embedding the OLED within a patch grating resonator structure. Specifically, a partially reflective patch grating of 50 nm thickness is embedded into the ITO layer. This metal grating structure can be simple to implement (via imprinting or interference lithography methods) and is incorporated directly as part of the glass/ITO substrate, requiring little to no modification of the remaining OLED manufacturing steps.
Our 2D simulation results suggest that with a patch grating with lateral pitch of λ/2, a 71% improvement in extracted light power can be obtained. Further improvement can be made by spacing the emitting layer from the back contact. Similar results are expected in the full 3D case.
Acknowledgement
This research was sponsored in part by a grant from Corning.
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