In a bottom emission OLED, because the refractive index of organic layers and ITO (n = 1.6–2.0) are higher than that of glass substrate (n = 1.5) and the thickness of organic layers and ITO are much thinner than that of glass substrate, extraction efficiency can be derived from classical optics as 1/2n_Org², approximately [55]. Here, n_Org represents the refractive index of the organic layer. A simple calculation indicates that extraction efficiency is about 20% with n_Org = 1.6. However, when we look closer at the layer structure of the organic layers and ITO, we can see that these layers are as thin as the wavelength of the visible range. Hence, we may regard it as a Fabry-Perot cavity and the emission intensity in the normal direction can be described by the following equation [56]: (1) | E out up ( λ ) | 2 = | E in ( λ ) | 2 × T 2 × [ 1 + R 1 + 2 R 1   cos ( 4 π x λ ) ] 1 + R 1 R 2 − 2 R 1 R 2 cos ( 4 π L λ )

Here, | E out up ( λ ) | 2 is the output intensity, |E_in (λ)|² is the free-space EL intensity, x is the effective distance between recombination zone and the reflective cathode, and R₁ is the reflectivity of the cathode. Also, R₂ and T₂ are the effective reflectivity and transmittivity of the ITO anode side, respectively. L is the total optical length (including anode, organic layers, and cathode) of the cavity. When the electrode materials are chosen (i.e., fixing R₁, T₁, R₂, and T₂ values), one can choose an x value at the numerator to maximize the output intensity. Then, one can minimize the denominator by changing the L value. Note that x also affects L. Taking a two-layer OLED (consisting only of HTL and ETL sandwiched by ITO and metal cathode) as an example, one should first tune the ETL thickness, and then adjust the HTL and ITO values to obtain the optimized output intensity. Considering the electrical properties, the relative ITO and HTL thickness can be further varied. Note that Equation (1) is wavelength dependent. That means the optimized device structure for one color may be not suitable for other colors, hence for display applications with red, green, and blue primaries, one may need three different device structures to optimize the optical characteristics. Typically, there are some common layers in red, green, and blue OLEDs to reduce the complexity during device fabrication. For white light applications, device design is more difficult, because the emission spectrum typically covers a wide spectral range [57]. Sometimes, tradeoffs among different colors are necessary to obtain the highest efficiency and good color coordinates. Based on the transfer matrix calculation, the optimized extraction efficiency of a bottom-emission OLED can be as high as 26.4% [58]. Although this value is higher than that calculated from classical optics, it reveals that the light extraction from the device is one of the important bottlenecks for OLED efficiency.

Compared to bottom-emission OLEDs, extraction efficiency was improved by 20.8% within a 120° viewing cone in a top-emission OLED due to the removal of the glass substrate mode, provided that the organic layers are the same [59]. As shown in Figure 3(b), a top-emission OLED uses the reflective metal as the anode and a transparent (or semitransparent) cathode. However, there are some material and fabrication issues. One of the criteria for the anode material is a high workfunction. Au exhibits a high workfunction, but a Au thin film looks yellowish, because it absorbs the blue light which may be not suitable for full color applicationa. The workfunction of Ti is also high enough, however, conductivity in this case is low and the optical absorption is high, which is not suitable for the anode reflector. A Ag thin film after oxidation with ozone treatment is an alternative choice which provides a high workfunction [60]. For the cathode material, ITO is a straightforward choice, as shown in Figure 3(c)[61]. However, ion bombardment during ITO sputtering may result in the damage to the organic layer underneath. A lower deposition rate is typically required for obtaining better electrical and optical properties [62]. Besides, the workfunction of ITO is high, which is not suitable for electron injection, so typically an EIL for facilitating electron injection, together with protection of the organic layers from ion bombardment is needed [63,64]. Besides, ITO, an alternative is a thin metal film (such as Ag, Sm, and Ca/Mg), as shown in Figure 3(d). Conductivity of metal is about two orders of magnitude higher than that of ITO. This means that the cathode thickness can be reduced by a hundred times when replacing transparent ITO by thin metal film [65]. It can be expected that the reflection from the semitransparent metal cathode is higher, which results in a stronger microcavity effect. By suitable adjustment of the cavity conditions, one may achieve a higher luminance at a certain viewing direction (ex: normal direction) than in a bottom-emission OLED. However, emission luminance and spectrum shift with different viewing direction due to such a strong cavity effect. Typically, a dielectric layer is added on top of the semitransparent one to adjust the transmission, reflection, and affect the cavity length of the cathode for optimizing the optical intensity and selecting the desired wavelength, as shown in Figure 3(e)[66]. Another cavity at the anode side was also proposed for providing an independent control for the optical parameters in the anode side [67].

A sub-micron grating structure is used to diffract the light from the waveguiding mode to the external mode. By fabricating an OLED on a 1-D grating structure, one can extract the waveguiding light in the organic and ITO out of the device due to Bragg scattering [68]. Such a corrugated structure can be also fabricated on organic layers with a suitable fabrication process [69]. Not only is extraction efficiency improved, but the extracted light is polarization dependent. To quantitatively illustrate the emission wavelength at different viewing angles (θ) to the normal of the substrate, one may use the Bragg scattering equation: (2) k 0   sin   θ = ± k g ± m 2 π Λwhere k₀ and k_g are the wavevectors in the air and guided (ITO/organic) modes, respectively, m is an integer, and Λ is the grating pitch. A 2-D grating (or photonic crystal) can be also used for improving extraction efficiency [70]. The operating principle of this work is also based on sub-micron diffraction. Besides, the holes on the glass substrate have been filled up with SiNx to obtain a smooth surface for OLED fabrication. However, when the distance between the corrugated structure and the OLED is too far, the resulting extraction efficiency is not so obvious. Increases in the extraction efficiency of over 50% and 80% was achieved experimentally and theoretically, respectively, because the periodic modulation converts the guided waves into external leaky waves. Planarization of devices after fabrication of the periodic structure is one of important issues in this configuration for good device performance, which can be accomplished by using polymer materials in a wet process, since the polymer materials will be spin-coated on the surface and hence a flat surface can be achieved. Also, micro- and nano-structures can be obtained through some chemical methods. In 2004, Fichet et al. used self-organized structures by spin coating a polymer layer on the top of a 2-D periodical pattern [71]. More than a two-fold increase in efficiency was observed. The pattern consists of hydrophobic and hydrophilic parts, which results in phase separation in certain regiona. The pitch is around 4 μm which is limited by the microcontact printing process. Höfler et al. in 2006 showed the possibility of creating planar surfaces with refractive index change, rather than the corrugated structure which increases the difficulties in fabrication [72]. In this study, a five-fold improvement was achieved for the normal direction. By introducing a photorefractive unit into the hole-transporting polymer, refractive index changes can be achieved with Δn = 0.006 at 540 nm.

Surface plasmon resonance (SPR) is another mechanism which can effectively couple the light in organic mode [73]. Although metals are typically opaque, light can transmit though a metal film with nanostructure due to the SPR. By inserting anodic aluminum oxide (AAO) nanoporous films between the ITO transparent electrode and glass substrate, a 50% increase in extraction efficiency was observed [74]. The pore size of AAO is 300 nm, and the cell size is around 400–450 nm. It is interesting that although the metallic hole structure is so thick, it still exhibits a strong extraction efficiency enhancement. In 2007, Chiu et al. demonstrated a multi-layer structure upon a 1-D grating to further enhance the light intensity by six times [75,76]. Here, the periodical grating provided an additional in-plane wave vector, and wavevector matching condition of in-plane wavevector (k_g) and surface plasmon wavevector (k_sp) can be obtained. There are two possible surface plasmon modes at the metal/air and metal/organic interfaces, i.e., k_{sp(metal/air)} and k_{sp(metal/organic)} respectively, which can be expressed as: (3) k g = k organic   sin   θ = ω c ɛ organic ɛ metal ɛ organic   +   ɛ metal ± m 2 π Λ = k s p ( metal / organic ) (4) k g = k 0   sin   θ = ω c ɛ metal 1   +   ɛ metal ± m 2 π Λ = k s p ( metal / air )where k_organic is the wavevector in organic material. ω, c, ɛ_Alq3, ɛ_m are angular frequency, light velocity, relative permittivity of organic emitters and metal, respectively. After designing the layer structures, the PL signals can be as high as 4 and 6 times the original one at certain viewing angles. In this case, tris-(8-hydroxyquinoline) aluminum (Alq3) thin film was employed as the organic emitter which exhibited a broadband emission from 489 to 598 nm centered at 532 nm with the planar structure. Due to the diffraction of the nanostructure, the PL signals were viewing-angle dependent, which changed the emission color from red to blue within 20° viewing angles. Figure 4 shows the photos of EL emission from an OLED with corrugated structure in the upper right region. When such a device was lit, the upper left corner (with corrugated structure) was colorful while the other region (planar structure) emitted a uniform color. The corrugated structure is a 1-D Au grating inserted between the ITO anode and the organic layers. The cathode was a thin metal and this image was taken for the top-side emission.

Gu et al. proposed to etch the substrate with truncated cones and fabricate the OLED on the plateau region [77]. Hence, waveguiding light will be reflected by the cone and possibly redirected out of the device. Up to 5.7 and 1.9-fold improvements in extraction efficiency were demonstrated in simulation and experiment, respectively. The increase of the extraction efficiency comes from the light reflection mechanism. However, the active area decreases due to the shaping of the substrate. TIR from high to low refractive index is the root cause of the limitation of the extraction efficiency. That means one may engineer the refractive index value to increase the extraction efficiency. The simplest way to eliminate the glass substrate mode of an OLED is to remove the glass substrate. However, this is impossible because the OLED is too thin and some mechanical support is needed. An alternative way is to insert a very low refractive index material (such as a silica aerogel layer with n = 1.03) between the OLED and the glass substrate [78]. Photons emitted from OLED pass though the aerogel and glass substrate to the air with low, high, and low refractive indexes, respectively. There is no TIR effect and hence one can remove the glass substrate mode with an 80% increase in extraction efficiency. However, the silicon aerogel would be washed out during a wet process, such as ITO etching, which limits its applications. With a non-planar structure on the glass/air interface, one may also reduce the TIR and couple out the glass substrate mode. Monolayers of silica micro-spheres were used as the scattering medium which were put on either sides of the substrate, but not between the OLED or glass substrate [79]. These couple out the light from the substrate mode, rather than the organic mode. Strong scattering light was found from the non-pixel region, which means: (1) it can effectively couple the light out, and (2) image blurring may be a problem when used for display applications. The scattering effect happens between two media with distinct refractive indexes. Although scattering can effectively reduce TIR, it impedes the light outcoupling when the scattering effect is too strong. By controlling the thickness and composition of scattering layer, one may achieve a maximum enhancement of 40% [80,81]. In 2007, Lin et al. demonstrated OLED extraction efficiency improvement and light redirection by using a commercial diffuser and brightness enhancement film (BEF) attachment [82]. The diffuser consists of micron range micro-spheres which can effectively scatter the light. Luminance increases for every direction in the diffuser case. On the other hand, BEF is a prism sheet which redirects light with +/–40° to the normal direction. Hence, the light intensity increases most in the normal direction.

Similar to the packaging process of a semiconductor LED, a hemispheric dome made of epoxy effectively couples out and redirects light. The attachment of a small OLED on a big hemispheric lens can also effectively couple out the waveguiding mode [83]. However, such a non-planar structure is too bulky. Microlense array film (MAF) was used for the same propose [84]. Figure 5 shows the photos with the MAF attached on a bottom-emission OLED with white-light emission. The MAF was attached on the upper-right corner. One can see a clear enhancement in extraction efficiency in the microlense region of the MAF. Enhancement of extraction efficiency at different viewing angles is 28.3, 54.0, and 49.5% at 0°, 30°, and 60° to the substrate normal. However, due to the light ray redirection, it also resulted in image blurring for display applications. Using a micro-cylinder may result in lower blur [85]. It is also possible to combine the microcavity effect and microlense structure [86]. SiNx/SiOx provides a high reflectivity and hence the cavity effect is strong. In such a microcavity, the color shift at different viewing angle is typically serious, since the effective cavity length varies with different viewing angles. With the attachment of the microlense array, which effectively redirects the light, the spectral shift with different viewing angles is reduced a lot. At the same time, the extraction efficiency improvement is still as high as 80%. There are different methods to fabricate the micro- and nano-structures. In 2004, Wei et al. proposed the use of a thermal reflow of the photoresist (PR) to obtain the hemispheric shape, followed by electroforming and UV-forming to transfer the pattern onto the epoxy film, as shown in Figure 6 [87]. At first, PR was patterned on a 4-inch Si-wafer by lithography. After a high temperature treatment of the wafer over glass transition temperature of PR, the dome-shape was formed, as shown in Figure 6(a), where the dark region was occupied by the PR dome. Then, the pattern was transferred reversely to the polydimethylsiloxane (PDMS) mold by a thermal curing process [Figure 6(b)]. Finally, PDMS was then used to duplicate the epoxy film by UV-curing. Figure 6(c) demonstrate the final epoxy film. Also, in 2006, Sun proposed to use imprint lithography to fabricate microlense array attached on the white OLED, which results in a maximum external quantum efficiency of 14.3% at 900 cd/m², power efficiency of 21.6 lm/W at 220 cd/m², and a color rendering index of 87 [88].

When we look more closely to the intensity profile of the OLED, we can observe that although the microlense couples out the light which is originally trapped inside the substrate due to the total internal reflection between the interface of the glass substrate and the air, the corrugated structure of the microlens also impedes the light outcoupling with the photon emission smaller than the critical angle. For coupling out the light smaller than the critical angle, the microlenses are partially removed, which is called “hollow structure” [89]. By removing the microlenses in the emitting layer region, the luminous efficiency can be greatly improved.

In 2007, Chen et al. showed experimentally that a pyramidal array light-enhancing layer (pyramidal ALEL) on an OLED panel enhanced the luminance efficiency with a gain factor of 2.03 [90]. Not only was the extraction efficiency discussed, but the Moire effect was also discussed. Since the pyramid has a square base-shape, different alignments (x and y displacement, and rotational) result in different effects. Not only micro-scale structures, but nano-structures can also be used to effectively increase the light extraction. In 2007, Cheng et al. showed that a polydimethylsiloxane (PDMS) film with micro-mesh surface increases the external quantum efficiency [91]. The micromesh was fabricated from a AAO template. Although AAO is in nano-scale range, it bundles and links with one another to form a micro-scale mesh. Extraction efficiency can be increased by 46% with this mesh.

It is also possible to extract organic and substrate modes at the same time. A low index grid (LIG) was inserted between the ITO anode and organic layers to extract the organic modes, while microlense array was attached outside the glass substrate to extract the substrate waveguiding mode [92]. Compared with the planar OLED, 1.32-, 1.68-, and 2.3-fold enhancement in extraction efficiency were observed for the cases of with LIG, with microlenses, and with LIG+microlenses, respectively. Another way to extract the organic mode is to use a high-index substrate. In 2009, Reineke, et al. demonstrated a OLED with efficiency comparable to a fluorescent tube in the journal Nature [93]. The main objective of this research was to engineer device substrates and output light coupling. In a traditional glass substrate with a low refractive index of 1.5, device efficiency with PIN structure reaches 30 lm/W at 1,000 cd/m². For the same device structure fabricated on a high refractive index glass substrate, device efficiency is improved to 40 lm/W since matched refractive indices of glass substrate and organic layer can extract light trapped within the organic layer to glass substrate. Using a patterned surface of a high refractive index glass substrate or a refractive index matched half-sphere, one can further extract light from the glass mode, thus leading to about 2- and 3-fold efficiency improvement to 90 and 124 lm/W, respectively.

One may also note that although extraction efficiency can be greatly improved by those corrugated structures and this is suitable for lighting applications, it results however in image blur for display applications at the same time. In a real OLED display, a circular polarizer film is attached to the glass substrate for reducing the electrode reflection and hence improving the contrast ratio [94]. However, with the introduction of microstructures, it is not possible to attach such a film. Hence, for display applications, some absorption layer should be added inside an OLED or placed underneath a transparent OLED for reducing the ambient reflection and maintaining the contrast ratio [95–98].