Top-emitting organic light-emitting diodes

doi:10.1364/OE.19.0A1250

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Top-emitting organic light-emitting diodes

Simone Hofmann, Michael Thomschke, Björn Lüssem, and Karl Leo »View Author Affiliations

Optics Express, Vol. 19, Issue S6, pp. A1250-A1264 (2011)
http://dx.doi.org/10.1364/OE.19.0A1250

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Abstract

We review top-emitting organic light-emitting diodes (OLEDs), which are beneficial for lighting and display applications, where nontransparent substrates are used. The optical effects of the microcavity structure as well as the loss mechanisms are discussed. Outcoupling techniques and the work on white top-emitting OLEDs are summarized. We discuss the power dissipation spectra for a monochrome and a white top-emitting OLED and give quantitative reports on the loss channels. Furthermore, the development of inverted top-emitting OLEDs is described.

1. Introduction

Since the first demonstration of electroluminescence from an organic heterojunction by Tang and VanSlyke in 1987 [1

C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes,” Appl. Phys. Lett. 51(12), 913–915 (1987). [CrossRef]

], the scientific and industrial interest in organic light-emitting diodes (OLEDs) has steadily increased. The properties of OLEDs, such as area-emission, high electron to photon conversion efficiency, wide viewing angle, low operating voltage, fast switching, vivid colors, light weight, small thickness, color tunability and dimmability are important advantages for lighting and display applications. The possibility of processing on flexible substrates makes OLEDs even suitable for roll-to-roll production. In 2008, Sony has started producing the first commercial OLED-TV XEL-1. A number of cell- and smart-phones from Nokia and Samsung are nowadays available with an OLED display. First white OLED tiles for lighting are sold by Philips, OSRAM and others.

Active matrix flat panel OLED displays benefit from a top-emitting OLED (TOLED) structure [2

J.-S. Yoo, S.-H. Jung, Y.-C. Kim, S.-C. Byun, J.-M. Kim, N.-B. Choi, S.-Y. Yoon, C.-D. Kim, Y.-K. Hwang, and I.-J. Chung, “Highly Flexible AM-OLED Display With Integrated Gate Driver Using Amorphous Silicon TFT on Ultrathin Metal Foil,” J. Disp. Tech. 6, 565–570 (2010). [CrossRef]

, 3

S.-K. Hong, J.-H. Sim, I.-G. Seo, K.-C. Kim, S.-I. Bae, H.-Y. Lee, N.-Y. Lee, and J. Jang, “New Pixel Design on Emitting Area for High Resolution Active-Matrix Organic Light-Emitting Diode Displays,” J. Disp. Tech. 6, 601–606 (2010). [CrossRef]

], emitting light away from the substrate and the backplane electric circuit, which increases the aperture ratio of the display. Figure 1 displays the structure of a bottom- and a top-emitting OLED. For bottom-emitting OLEDs, light is emitted through a transparent bottom contact, which is usually tin-doped indium oxide (ITO). As top contact, a thick, highly reflective layer (approx. 100 nm) of aluminum or silver is applied. The organic layers in between consist of a hole and electron transport layer (HTL, ETL), a hole and electron blocking layer (HBL, EBL) and the emission layer (EML). Blochwitz et al. [4

J. Blochwitz, M. Pfeiffer, T. Fritz, and K. Leo, “Low voltage organic light emitting diodes featuring doped ph-thalocyanine as hole transport material,” Appl. Phys. Lett. 73(6), 729 (1998). [CrossRef]

] showed that doping of transport layers increases the conductivity and improves the injection of charges. The voltage drop for a thick doped layer (ca. 50–300 nm) becomes negligible, and the transport layers can be used to adjust the position of the emission zone, which strongly influences the optical properties of the device [5

L. H. Smith, J. a E. Wasey, and W. L. Barnes, “Light outcoupling efficiency of top-emitting organic light-emitting diodes,” Appl. Phys. Lett. 84, 2986 (2004). [CrossRef]

, 6

R. Meerheim, M. Furno, S. Hofmann, B. Lüssem, and K. Leo, “Quantification of energy loss mechanisms in organic light-emitting diodes,” Appl. Phys. Lett. 97, 253305 (2010). [CrossRef]

, 7

C. J. Lee, R. B. Pode, D. G. Moon, J. I. Han, N. H. Park, S. H. Baik, and S. S. Ju, “On the problem of microcavity effects on the top emitting OLED with semitransparent metal cathode,” Phys. Stat. Sol. 201, 1022–1028 (2004). [CrossRef]

]. Instead of doped transport layers, the introduction of a thin layer of LiF (ca. 0.5 nm) [8

G. E. Jabbour, B. Kippelen, N. R. Armstrong, and N. Peyghambarian, “Aluminum based cathode structure for enhanced electron injection in electroluminescent organic devices,” Appl. Phys. Lett. 73, 1185 (1998). [CrossRef]

, 9

L. S. Hung, C. W. Tang, M. G. Mason, P. Raychaudhuri, and J. Madathil, “Application of an ultrathin LiF/Al bilayer in organic surface-emitting diodes,” Appl. Phys. Lett. 78, 544 (2001). [CrossRef]

] and metal oxides (Ag₂O [10

C.-W. Chen, P.-Y. Hsieh, H.-H. Chiang, C.-L. Lin, H.-M. Wu, and C.-C. Wu, “Top-emitting organic light-emitting devices using surface-modified Ag anode,” Appl. Phys. Lett. 83, 5127 (2003). [CrossRef]

, 11

S. Chen, R. Song, J. Wang, Z. Zhao, Z. Jie, Y. Zhao, B. Quan, W. Huang, and S. Liu, “Improved performances in top-emitting organic light-emitting diodes based on a semiconductor zinc oxide buffer layer,” J. Luminescen. 128, 1143–1147 (2008). [CrossRef]

], MoO₃, and V₂O₅ [12

X. Zhu, J. Sun, X. Yu, M. Wong, and H.-S. Kwok, “Investigation of Al- and Ag-Based Top-Emitting Organic Light-Emitting Diodes with Metal Oxides as Hole-Injection Layer,” Jap. J. Appl. Phys. 46, 1033–1036 (2007). [CrossRef]

]) are also well established to improve the electron and hole injection. The blocking layers are intrinsic layers (ca. 10 nm thick) confining charges and excitons in the emission layer.

Fig. 1 Comparison of general bottom- (a) and top-emitting (b) OLED structure. The emission direction is defined by a transparent bottom contact and a reflective top contact (bottom emission) or a highly reflective bottom contact and a semitransparent top contact (top emission). The transposition of the layers leads to an inverted OLED structure (c). HTL = hole transport layer, EBL = electron blocking layer, EML = emission layer, HBL = hole blocking layer, ETL = electron transport layer, CL = capping layer.

In top-emitting configuration, a thick, highly reflective bottom contact is used and a semi-transparent contact is deposited on top of the organic layers. Having two highly reflective contacts, TOLEDs exhibit usually strong microcavity effects like spectral narrowing and a spectral shift of the emission peak with the viewing angle, resulting in a strong dependency on the color (blue shift) [5

L. H. Smith, J. a E. Wasey, and W. L. Barnes, “Light outcoupling efficiency of top-emitting organic light-emitting diodes,” Appl. Phys. Lett. 84, 2986 (2004). [CrossRef]

, 13

C.-C. Wu, C.-L. Lin, P.-Y. Hsieh, and H.-H. Chiang, “Methodology for optimizing viewing characteristics of top-emitting organic light-emitting devices,” Appl. Phys. Lett. 84, 3966 (2004). [CrossRef]

In 1996, Bulović et al. [14

V. Bulović, G. Gu, P. E. Burrows, S. R. Forrest, and M. E. Thompson, “Transparent light-emitting devices,” Nature 380, 6569 (1996). [CrossRef]

] started investigating OLEDs with a sputtered ITO contact on the top side, but the efficiencies were very low (around 0.1%) due to the damage of the organic layers underneath. Thus, a lot of work has been done optimizing the sputtering process [15

L.-S. Hung and J. Madathilb, “Radiation damage and transmission enhancement in surface-emitting organic light-emitting diodes,” Thin Solid Films 410, 101–106 (2002). [CrossRef]

, 16

C.-H. Chung, Y.-W. Ko, Y.-H. Kim, C.-Y. Sohn, H. Y. Chu, and J. H. Lee, “Improvement in performance of transparent organic light-emitting diodes with increasing sputtering power in the deposition of indium tin oxide cathode,” Appl. Phys. Lett. 86, 093504 (2005). [CrossRef]

, 17

H.-K. Kim, K.-S. Lee, and J. H. Kwon, “Transparent indium zinc oxide top cathode prepared by plasma damage-free sputtering for top-emitting organic light-emitting diodes,” Appl. Phys. Lett. 88, 012103 (2006). [CrossRef]

, 18

S. Han, X. Feng, Z. H. Lu, D. Johnson, and R. Wood, “Transparent-cathode for top-emission organic light-emitting diodes,” Appl. Phys. Lett. 82, 2715 (2003). [CrossRef]

]. Furthermore, the introduction of a buffer layer has been intensively investigated [19

G. Parthasarathy, P. E. Burrows, V. Khalfin, V. G. Kozlov, and S. R. Forrest, “A metal-free cathode for organic semiconductor devices,” Appl. Phys. Lett. 72, 2138 (1998). [CrossRef]

, 20

G. Parthasarathy, C. Adachi, P. E. Burrows, and S. R. Forrest, “High-efficiency transparent organic light-emitting devices,” Appl. Phys. Lett. 76, 2128 (2000). [CrossRef]

, 21

A. Yamamori, S. Hayashi, T. Koyama, and Y. Taniguchi, “Transparent organic light-emitting diodes using metal acethylacetonate complexes as an electron injective buffer layer,” Appl. Phys. Lett. 78, 3343 (2001). [CrossRef]

]. However, in terms of performance, the devices could not compete with their bottom-emitting counterparts. In 2001, comparable performances for monochrome TOLEDs were reached by thermal evaporation of thin metal layers [9

, 22

H. Riel, S. Karg, T. Beierlein, B. Ruhstaller, and W. Rieß, “Phosphorescent top-emitting organic light-emitting devices with improved light outcoupling,” Appl. Phys. Lett. 82, 466 (2003). [CrossRef]

] and the introduction of a dielectric capping layer (CL) [23

Q. Huang, K. Walzer, M. Pfeiffer, V. Lyssenko, G. He, and K. Leo, “Highly efficient top emitting organic light-emitting diodes with organic outcoupling enhancement layers,” Appl. Phys. Lett. 88, (2006). [CrossRef]

,24

H. Riel, S. Karg, T. Beierlein, W. Rieß, and K. Neyts, “Tuning the emission characteristics of top-emitting organic light-emitting devices by means of a dielectric capping layer: An experimental and theoretical study,” J. Appl. Phys. 94, 5290 (2003). [CrossRef]

]. In 2010, an external quantum efficiency (EQE) of 29% [25

S. Hofmann, M. Thomschke, P. Freitag, M. Furno, B. Lüssem, and K. Leo, “Top-emitting organic light-emitting diodes: Influence of cavity design,” Appl. Phys. Lett. 97, 253308 (2010). [CrossRef]

], even exceeding the efficiency of a comparable bottom-emitting OLED, was reached for a phosphorescent red TOLED by a carefully optimized device design.

However, top-emitting OLEDs have to fulfill certain market requirements, besides low manufacturing costs and a long device lifetime. A low onset voltage and a high electron to photon conversion efficiency, as well as an effective light outcoupling are preconditions for high brightness and high efficiency. For applications requiring a spotlight a more forward emitting light source is needed, whereas for soft lighting a lambertian light source with an angular independent emission spectrum is desired. Concerning white light emission, a color rendering index (CRI) above 75 and appropriate Commission Internationale de l’Eclairage color coordinates (CIE) are essential.

In this review, the development of top-emitting organic light-emitting diodes is briefly discussed. In Section 2, the optical properties and influence of the microcavity are presented. In Section 3, strategies for increasing the light outcoupling are discussed. The development of white TOLEDs is reported in Section 4. We will focus on inverted top-emitting OLEDs in Section 5. Finally, the potential and prospects of TOLEDs are summarized in the last section.

2. Microcavity

The so-called microcavity effects, like spectral narrowing, spectral shift of the emitted light with increasing viewing angle, and emission enhancement due to the Purcell effect can be described considering the OLED as a Fabry-Pérot resonator. A deeper understanding including the light loss mechanisms can be obtained by modeling the power dissipation by an emissive dipole in a multilayer structure.

2.1. Fabry-Pérot resonator

For a basic understanding, the top-emitting OLED can be regarded as a Fabry-Pérot resonator, where the anode and cathode are the parallel mirrors. Consequential, the emitted (outcoupled) spectrum exhibits a full width at half maximum (FWHM) [26

E. F. Schubert, N. E. J. Hunt, R. J. Malik, D. L. Sivco, A. Y. Cho, and G. J. Zydzik, “Highly Efficient Light-Emitting Diodes with Microcavities,” Science 265, 12 (1994). [CrossRef]

] of

F W H M = \frac{λ^{2}}{2 L_{cav}} \times \frac{1 - \sqrt{R_{t} R_{b}}}{π \sqrt[4]{R_{t} R_{b}}} .

(1)

Here λ denotes the peak emission wavelength, L_cav = nd (n the refractive index, d = cavity thickness) the optical cavity thickness, and R_t and R_b the reflectivity of the top and bottom contact, respectively. The FWHM of the spectrum is decreased, which means that the spectral width is narrowed, when increasing the cavity length or increasing the reflectivity of the contacts. Assuming a reflectivity of R_t = 0.5, R_b = 0.9, n = 1.7, and a cavity length d of 100 nm, a FWHM of 60 nm (240 nm) for a peak wavelength of 400 nm (800 nm) is obtained. One can see that the design of a white top-emitting OLED, which emits over the visible wavelength regime (380 nm to 780 nm) is challenging.

The spectral emission intensity I(λ, θ) from the microcavity in dependence of the wavelength and the emission angle θ can be calculated by [27

D. G. Deppe, C. Lei, C. C. Lin, and D. L. Huffaker, “Spontaneous Emission from Planar Microstructures,” J. Mod. Opt. 41, 325 (1994). [CrossRef]

]

I (λ, θ) = \frac{T_{t} [1 + R_{b} + 2 \sqrt{R_{b}} \cos (- ϕ_{b} + \frac{4 π n z \cos (θ_{org, EML})}{λ})]}{{(1 - \sqrt{R_{b} R_{t}})}^{2} + 4 \sqrt{R_{b} R_{t}} {sin}^{2} (\frac{Δ ϕ}{2})} I_{0} (λ)

(2)

where T and R are the transmittance and the reflectivity of contacts, ϕ_b the phase shift at the bottom contact, z the distance from the emitter to the highly reflecting mirror. The factor I ₀ is the emission of the radiating molecules. After one cycle of the light wave, the phase shift Δϕ is given by

Δ ϕ = - ϕ_{b} - ϕ_{t} + \sum_{i} \frac{4 π n_{i} d_{i} cos (θ_{org, i})}{λ}

(3)

with the resonance condition Δϕ = 2π m, where m is the mode index and d_i the thicknesses and n_i the refractive indices of all organic layers within the cavity. It can be seen from Eq. (2) and Eq. (3) that the radiated intensity I is dependent on the emission angle θ, which is directly connected by Snell’s law to the propagation directions θ_org,i of the light within the organic layers, and further to the angle θ_org,EML in the emitting layer. It is obvious that for white light emission based on red-green-blue subcolor stacking, it is not possible to achieve the maximum emission intensity for each subcolor using a fixed resonant cavity structure.

Considering only forward emission, Eq. (2) reads [28

J.-H. Lee, K.-Y. Chen, C.-C. Hsiao, H.-C. Chen, C.-H. Chang, Y.-W. Kiang, and C. C. Yang, “Radiation Simulations of Top-Emitting Organic Light-Emitting Devices With Two- and Three-Microcavity Structures,” J. Disp. Tech. 2, 130 (2006). [CrossRef]

]

I (λ) = \frac{T_{t} [1 + R_{b} + 2 \sqrt{R_{b}} cos (\frac{4 π n z}{λ})]}{1 + R_{b} R_{t} - 2 \sqrt{R_{b} R_{t}} cos (\frac{4 π L_{cav}}{λ})} I_{0} (λ) .

(4)

Figure 2 shows the emitted spectrum calculated using Eq. (4) for forward emission for two cavity structures. The photoluminescence (PL) of the red phosphorescent emitter Iridium(III)bis(2-methyldibenzo-[f,h]chinoxalin)(acetylacetonat) (Ir(MDQ)₂(acac)) doped with 10 wt% into the matrix N,N’-Di(naphthalen-1-yl)-N,N’-diphenyl-benzidine (α – NPD) with a peak wavelength of 610 nm is set to be the emission of the radiating molecules I ₀. The reflectivities of the bottom and top contacts are 90% and 50%. The transmittance of the top contact is set to 20%. The refractive index of the organic material is assumed to be 1.7. To achieve constructive interference, the cavity length L_cav is designed by multiples of λ/2. We compare two cavity lengths λ/2 and λ. The distance z from the emitting molecules to the bottom contact is set to 70 nm and 180 nm, respectively.

Fig. 2 Simulation of the emitted spectrum of two cavity structures for forward emission according to Eq. (4) and the photoluminescence spectrum of the red emitter Ir(MDQ)₂(acac) doped with 10 wt% in α – NPD. The high reflectivity of the contacts and the increase of cavity length lead to spectral narrowing.

For both cavity structures it can be seen that the spectra are narrowed compared to the PL-spectrum. Furthermore, the peak emission is shifted to higher wavelengths, which is due to the fact, that the cavity length and the position of the molecules within the device are not optimized.

To take the Purcell effect into account, the emission enhancement factor G relative to free space emission at a wavelength λ is given by [29

Q. Wang, Z. Deng, and D. Ma, “Realization of high efficiency microcavity top-emitting organic light-emitting diodes with highly saturated colors and negligible angular dependence,” Appl. Phys. Lett. 94, 233306 (2009). [CrossRef]

, 26

E. F. Schubert, N. E. J. Hunt, R. J. Malik, D. L. Sivco, A. Y. Cho, and G. J. Zydzik, “Highly Efficient Light-Emitting Diodes with Microcavities,” Science 265, 12 (1994). [CrossRef]

]

G = \frac{T_{t} [{(1 + \sqrt{R_{b}})}^{2} - 4 \sqrt{R_{b}} {cos}^{2} (\frac{2 π z}{λ})]}{{(1 - \sqrt{R_{t} R_{b}})}^{2} + 4 \sqrt{R_{t} R_{b}} {sin}^{2} (\frac{2 π L_{cav}}{λ})} \frac{τ_{cav}}{τ} .

(5)

The ratio τ_cav /τ is the relation of the exciton lifetime in the cavity and the lifetime in an infinite medium. From Eq. (2), it can be concluded that the spectral emission might exhibit a shift Δλ_θ of the peak wavelength with increasing viewing angle θ, which can be estimated by [31

A. B. Djurišić and A. D. Rakić, “Organic microcavity light-emitting diodes with metal mirrors: dependence of the emission wavelength on the viewing angle.,” Appl. Opt. 41, 7650 (2002). [CrossRef]

, 30

A.W. Lu and A.D. Rakić, “Design of microcavity organic light emitting diodes with optimized electrical and optical performance,” Appl. Opt. 48, 2282 (2009). [CrossRef] [PubMed]

]

Δ λ_{θ} = \sum_{θ} (\sum_{i} \frac{4 π d_{i}}{λ} n_{i} [cos θ_{org, i} - 1] + Δ ϕ_{t} + Δ ϕ_{b}) .

(6)

Additionally to the choice of materials, the following issues were found to have strong impact on the device efficiency: the cavity length [32

X. Zhu, J. Sun, X. Yu, M. Wong, and H.-S. Kwok, “High-Performance Top-Emitting White Organic Light-Emitting Devices,” Jap. J. Appl. Phys. 46, 4054–4058 (2007). [CrossRef]

, 28

], the distance of the emitter molecules to the bottom contact [32

X. Zhu, J. Sun, X. Yu, M. Wong, and H.-S. Kwok, “High-Performance Top-Emitting White Organic Light-Emitting Devices,” Jap. J. Appl. Phys. 46, 4054–4058 (2007). [CrossRef]

, 33

X. W. Chen, W. C. H. Choy, S. He, and P. Chui, “Comprehensive analysis and optimal design of top-emitting organic light-emitting devices,” J. Appl. Phys. 101, 113107 (2007). [CrossRef]

, 28

, 34

D.-S. Leem, S.-Y. Kim, J.-H. Lee, and J.-J. Kim, “High efficiency p-i-n top-emitting organic light-emitting diodes with a nearly Lambertian emission pattern,” J. Appl. Phys. 106, 063114 (2009). [CrossRef]

], the top contact thickness [28

, 33

X. W. Chen, W. C. H. Choy, S. He, and P. Chui, “Comprehensive analysis and optimal design of top-emitting organic light-emitting devices,” J. Appl. Phys. 101, 113107 (2007). [CrossRef]

], and the thickness of the capping layer [23

, 33

X. W. Chen, W. C. H. Choy, S. He, and P. Chui, “Comprehensive analysis and optimal design of top-emitting organic light-emitting devices,” J. Appl. Phys. 101, 113107 (2007). [CrossRef]

, 28

The contact thickness and the capping layer thickness can be varied without affecting the electrical properties of the OLED. Using doped charge transport layers [4

], it is also possible to adjust the cavity length and the distance of emitter molecules to the bottom contact without any loss in driving voltage.

The current efficiency is defined as the luminance in forward direction divided by the applied current density, whereas the external quantum efficiency and the luminous efficacy are depending on the emission of the OLED into the forward half-sphere. It is obvious that due to the microcavity effects these three efficiencies can not be maximized at the same time. The optimization in terms of external quantum efficiency [13

,25

S. Hofmann, M. Thomschke, P. Freitag, M. Furno, B. Lüssem, and K. Leo, “Top-emitting organic light-emitting diodes: Influence of cavity design,” Appl. Phys. Lett. 97, 253308 (2010). [CrossRef]

] exhibits a strong dependency on the emitted spectra with the viewing angle, accompanied by a superlambertian emission behavior. The maximum luminance is obtained for a viewing angle in the range of 30° to 60°. An optimization of the current efficiency is linked to maximum luminance at 0°. It will be discussed in Section 3 that the approach of a scattering capping layer [35

T. W. Canzler, S. Murano, D. Pavicic, O. Fadhel, C. Rothe, A. Haldi, M. Hofmann, and Q. Huang, “Efficiency Enhancement in White PIN OLEDs by Simple Internal Outcoupling Methods,” SID Digest 11, 975–978 (2011). [CrossRef]

] and a microlens foil [36

C. J. Yang, S. H. Liu, H. H. Hsieh, C. C. Liu, T. Y. Cho, and C. C. Wu, “Microcavity top-emitting organic light-emitting devices integrated with microlens arrays: Simultaneous enhancement of quantum efficiency, cd/A efficiency, color performances, and image resolution,” Appl. Phys. Lett. 91, 253508 (2007). [CrossRef]

] are promising to obtain high efficiencies and color stability at the same time.

2.2. Power dissipation

To obtain quantitative statements about the loss mechanism, a more sophisticated model is required. It is well established to treat the emissive molecules embedded in the active layer as forced damped harmonic oscillators [37

W. L. Barnes, “Fluorescence near interfaces: the role of photonic mode density,” J. of Mod. Opt. 45, 661–699 (1998). [CrossRef]

] to calculate the generated power. The generated power by dipoles within a microcavity normalized to the power that would be emitted in an infinite medium is given by

F = (1 - q) + q \int_{0}^{\infty} K (u) d u,

(7)

q = \frac{Γ_{r}}{Γ_{r} + Γ_{n r}},

(8)

where the factor q is the internal quantum efficiency of the emitting dipoles, given by the intrinsic radiative and non-radiative decay rates Γ _r and Γ _nr . The variable u represents the in-plane component of the normalized wavevector for waves propagating in the emitting medium. The in-plane wavevector is normalized with respect to propagation in the emitting layer.

The term K(u) denotes the power dissipation density per unit du and can be calculated for randomly oriented dipoles as

K = \frac{1}{3} K_{T M_{ν}} + \frac{2}{3} (K_{T M_{h}} + K_{T E_{h}}) .

(9)

The three components of the total power density are K_TMv for vertical dipoles coupling to transverse magnetic (TM) waves, K_TMh , and K_TEh for the horizontal dipoles coupling to TM and transverse electric (TE) waves, respectively. To obtain the power dissipation spectrum for a top-emitting OLED, the emission through a usually nitrogen filled cavity and the encapsulation glass have to be considered to calculate the far-field radiation K_out for the emission into air. Further details can be found in Ref. [38

M. Furno, R. Meerheim, M. Thomschke, S. Hofmann, B. Lüssem, and K. Leo, “Outcoupling efficiency in small-molecule OLEDs: from theory to experiment,” Proc. SPIE 7617, 761716 (2010). [CrossRef]

, 39

K. G. Sullivan and D. G. Hall, “Enhancement and inhibition of electromagnetic radiation in plane-layered media. I.Plane-wave spectrum approach to modeling classical effects,” J. Opt. Soc. Am. B 14, 1149 (1997). [CrossRef]

]. By conversion of the power spectrum per unit normalized in-plane wavevector into a power spectrum per unit solid angle, a comparison to spectro-goniometer measurements is possible.

The external quantum efficiency (EQE) can be expressed by [6

R. Meerheim, M. Furno, S. Hofmann, B. Lüssem, and K. Leo, “Quantification of energy loss mechanisms in organic light-emitting diodes,” Appl. Phys. Lett. 97, 253305 (2010). [CrossRef]

]

E Q E = γ χ \int_{λ} \frac{q F (λ)}{q F (λ) + 1 - q} η_{out} (λ) I_{0} (λ) d λ,

(10)

where γ is the electrical efficiency, χ is the singlet/triplet ratio of the emitter species, and η_out is the outcoupling efficiency. The EQE is further depending on the normalized emission of the radiating molecules I ₀(λ), which applies to ∫ _λ I ₀(λ)d λ = 1.

Figure 3 shows the power dissipation spectrum for a red phosphorescent top-emitting OLED with capping layer (layer architecture like in Fig. 1 (b)) as a function of free-space wavelength and normalized in-plane wavevector. The in-plane wavevector is normalized with respect to propagation in the emitting layer. The detailed structure can be found in Ref. [38

M. Furno, R. Meerheim, M. Thomschke, S. Hofmann, B. Lüssem, and K. Leo, “Outcoupling efficiency in small-molecule OLEDs: from theory to experiment,” Proc. SPIE 7617, 761716 (2010). [CrossRef]

]. Up to a wavevector of u < 0.57 light can radiate in the far field. Waveguided (wg) and surface plasmon polariton (SPP) modes are present at higher wavevectors.

Fig. 3 Power dissipation spectrum for a red phosphorescent top-emitting OLED as a function of free-space wavelength and normalized in-plane wavevector in the emitting layer. The structure of the top-emitting OLED can be found in Ref. [38

M. Furno, R. Meerheim, M. Thomschke, S. Hofmann, B. Lüssem, and K. Leo, “Outcoupling efficiency in small-molecule OLEDs: from theory to experiment,” Proc. SPIE 7617, 761716 (2010). [CrossRef]

]. Up to a wavevector of u < 0.57 light can radiate in the far field, at higher wavevectors waveguided and plasmonic modes can be observed. Reprinted with permission from Furno et al. [38

M. Furno, R. Meerheim, M. Thomschke, S. Hofmann, B. Lüssem, and K. Leo, “Outcoupling efficiency in small-molecule OLEDs: from theory to experiment,” Proc. SPIE 7617, 761716 (2010). [CrossRef]

], Proc. of SPIE 7617, 761716 (2010). Image courtesy of SPIE.

Furno et al. [38

M. Furno, R. Meerheim, M. Thomschke, S. Hofmann, B. Lüssem, and K. Leo, “Outcoupling efficiency in small-molecule OLEDs: from theory to experiment,” Proc. SPIE 7617, 761716 (2010). [CrossRef]

] calculated an outcoupling efficiency of 19.9% for a red top emitting OLED with capping layer. By fine-tuning of the cavity, 30% can be obtained [6

R. Meerheim, M. Furno, S. Hofmann, B. Lüssem, and K. Leo, “Quantification of energy loss mechanisms in organic light-emitting diodes,” Appl. Phys. Lett. 97, 253305 (2010). [CrossRef]

]. Similar values are found by Smith et al. [5

L. H. Smith, J. a E. Wasey, and W. L. Barnes, “Light outcoupling efficiency of top-emitting organic light-emitting diodes,” Appl. Phys. Lett. 84, 2986 (2004). [CrossRef]

] for a green fluorescent emitter and an Al/ITO cathode. It is demonstrated, that the dominating losses are (wavelength dependent) waveguided modes and surface plasmon modes at the anode and cathode. Electrical deficit and absorption are minor losses. Chen et al. [33

X. W. Chen, W. C. H. Choy, S. He, and P. Chui, “Comprehensive analysis and optimal design of top-emitting organic light-emitting devices,” J. Appl. Phys. 101, 113107 (2007). [CrossRef]

] showed that with an optimized cavity thickness, position of the emission zone, and the capping layer, an outcoupling efficiency of 52% for a Gaussian shaped intrinsic spectrum with a peak at 530 nm and a width of 80 nm can be realized.

To optimize the OLED further, the loss mechanisms have to be studied in detail. In Fig. 4 different loss channels for a series of red phosporescent top-emitting OLEDs with varying ETL thickness is validated against experimental data. Using Eq. (10) Meerheim et al. [6

R. Meerheim, M. Furno, S. Hofmann, B. Lüssem, and K. Leo, “Quantification of energy loss mechanisms in organic light-emitting diodes,” Appl. Phys. Lett. 97, 253305 (2010). [CrossRef]

] obtained an internal quantum efficiency q of 0.84 and an electrical efficiency γ of 0.89 assuming a singlet/triplet ratio χ = 1. The highest EQE of 27% at 176 cd/m² brightness (0.74 mA/cm²) is obtained for a first order cavity.

Fig. 4 Measured EQE at 0.74 mA/cm² of red top-emitting OLEDs as a function of the ETL thickness and comparison to simulation results. The figure also shows the distribution of all loss channels in the devices. Waveguided and plasmonic losses are not distinguished due to the complex modal cavity structure. Reprinted with permission from Meerheim et al. [6

R. Meerheim, M. Furno, S. Hofmann, B. Lüssem, and K. Leo, “Quantification of energy loss mechanisms in organic light-emitting diodes,” Appl. Phys. Lett. 97, 253305 (2010). [CrossRef]

3. Outcoupling

Phosphorescent emitters can reach internal quantum efficiencies close to 100%. [40

C. Adachi, M. Baldo, M. E. Thompson, and S. R. Forrest, “Nearly 100% internal phosphorescence efficiency in an organic light-emitting device,” J. Appl. Phys. 90, 5048 (2001). [CrossRef]

, 6

R. Meerheim, M. Furno, S. Hofmann, B. Lüssem, and K. Leo, “Quantification of energy loss mechanisms in organic light-emitting diodes,” Appl. Phys. Lett. 97, 253305 (2010). [CrossRef]

]. Using transport layers and blockers, it can be assured that the majority of charge carriers reach the EML and form excitons. However, the measured external quantum efficiencies are lower, meaning that most of the light is trapped inside the organic thin film structure. With regard to the loss mechanisms, we are discussing the techniques which are known to enhance the light outcoupling in top-emitting OLEDs.

The use of ITO as top-contact instead of a metal layer improves outcoupling, because of the high transmission of ITO. The challenge is to apply the ITO on top without damaging the organic layers underneath. However, Kanno et al. [41

H. Kanno, Y. Sun, and S. R. Forrest, “High-efficiency top-emissive white-light-emitting organic electrophosphorescent devices,” Appl. Phys. Lett. 86, 263502 (2005). [CrossRef]

] showed in 2005 by doping the electron transport layer with Li efficiencies up to 10.5% (9.8 lm/W) for a white device.

Using thin metal layers, Hung [9

], Riel [22

] and later Huang [23

] demonstrated that an additional dielectric capping layer can enhance the outcoupling efficiency. The reflectivity of the top contact is reduced and hence the microcavity effects are weakened. Due to the microcavity effects explained in Section 2, an optimization of the external quantum efficiency is accompanied by a strong color shift with the viewing angle [33

X. W. Chen, W. C. H. Choy, S. He, and P. Chui, “Comprehensive analysis and optimal design of top-emitting organic light-emitting devices,” J. Appl. Phys. 101, 113107 (2007). [CrossRef]

, 25

S. Hofmann, M. Thomschke, P. Freitag, M. Furno, B. Lüssem, and K. Leo, “Top-emitting organic light-emitting diodes: Influence of cavity design,” Appl. Phys. Lett. 97, 253308 (2010). [CrossRef]

]. Using a light scattering capping layer the emission becomes stable with viewing angle and the efficiency can be kept high [35

To extract waveguided modes, a scattering foil can be applied as shown by Yang et al. [36

] using a hexagonal PDMS microlens array, which is placed on a micrometer-thick parylene passivation layer on top of the OLED. In 2009, the same group [42

C.-C. Liu, S.-H. Liu, K.-C. Tien, M.-H. Hsu, H.-W. Chang, C.-K. Chang, C.-J. Yang, and C.-C. Wu, “Microcavity top-emitting organic light-emitting devices integrated with diffusers for simultaneous enhancement of efficiencies and viewing characteristics,” Appl. Phys. Lett. 94, 103302 (2009). [CrossRef]

] showed efficiencies up to 42 cd/A and 8.4% for a green top-emitting OLED. The main advantage is that these layer scatters the light, which makes the emission spectrum as well as the color stable in dependence of the viewing angle. However, this addresses mostly modes in the capping and in the passivation layer, so the improvement is rather small.

The total internal reflection at the interface organic/air or metal/air due to the change of refractive index is the most obvious reason for light trapping inside the device. Usually the organic materials exhibit a refractive index of about 1.7. Smith et al. [43

L. Smith and W. Barnes, “Using a low-index host layer to increase emission from organic light-emitting diode structures,” Org. Electron. 7, 490 (2006). [CrossRef]

] demonstrated by optical simulations that a 2.6 fold increase in outcoupling efficiency may be achieved by a decrease of the refractive index to 1.

A way to couple out surface plasmon modes in a top-emitting structure is introduced by Wedge et al. [44

S. Wedge, A. Giannattasio, and W. Barnes, “Surface plasmon–polariton mediated emission of light from top-emitting organic light-emitting diode type structures,” Org. Electron. 8, 136–147 (2007). [CrossRef]

], using a corrugated dielectric layer on top of the metal contact. Although the emission intensity is increased by two orders of magnitude compared to a similar planar structure, the emission enhancement could only be demonstrated under optical excitation and not for a complete device. Bragg scattering of surface plasmon modes is demonstrated by Feng et al. [45

J. Feng, T. Okamoto, and S. Kawata, “Highly directional emission via coupled surface-plasmon tunneling from electroluminescence in organic light-emitting devices,” Appl. Phys. Lett. 87, 241109 (2005). [CrossRef]

]. They use a two dimensional corrugated silicon substrate and observe a highly directional emission due to the excitation and coupling of surface plasmon modes to the far field at the corrugated silver top contact. Unfortunately, no efficiency values are reported and highly directional emission is not desirable concerning lighting applications.

Wang et al. [46

Q. Wang, Z. Deng, and D. Ma, “Highly efficient inverted top-emitting organic light-emitting diodes using a lead monoxide electron injection layer,” Opt. Express 17, 17269–17278 (2009). [CrossRef] [PubMed]

] claimed that a self nano-aggregated bathocuproine (BCP) film on top of the cathode might as well scatter plasmonic modes. Another interesting method to extract surface plasmon polariton modes is shown by An et al. [47

K. H. An, M. Shtein, and K. P. Pipe, “Surface plasmon mediated energy transfer of electrically-pumped excitons,” Opt. Express 18, 4041 (2010). [CrossRef] [PubMed]

] applying a down-conversion layer as capping layer on a transparent OLED. Here the plasmon modes can couple to the emitter molecules and enhance the emission. A 6.5-fold enhancement of light emission is shown. However, external quantum efficiency values are missing. In 2010, Tien et al. [48

K. C. Tien, M. S. Lin, Y. H. Lin, C.-H. Tsai, M. H. Shiu, M. C. Wei, H. C. Cheng, C. L. Lin, H. W. Lin, and C. C. Wu, “Utilizing surface plasmon polariton mediated energy transfer for tunable double-emitting organic light-emitting devices,” Org. Electron. 11, 397–406 (2010). [CrossRef]

] have used this effect on transparent OLEDs, but the obtained efficiencies in the range of 5% are rather low.

Since only TM polarized light can couple to surface plasmons, one can expect a reduction of plasmon losses by lowering the amount of vertical dipoles. Frischeisen et al. [49

J. Frischeisen, D. Yokoyama, A. Endo, C. Adachi, and W. Brütting, “Increased light outcoupling efficiency in dye-doped small molecule organic light-emitting diodes with horizontally oriented emitters,” Org. Electron. 12, 817 (2011). [CrossRef]

] demonstrated for bottom-emitting OLEDs that the outcoupling efficiency is indeed increased. The use of preferential horizontal oriented emitters might be beneficial for top-emitting OLEDs as well.

4. White emission

As seen in Section 2, a broad white emission from a top-emitting OLED is very challenging. The microcavity effects such as spectral narrowing and spectral shift (blue shift) with increasing viewing angle impede the device design, because a high efficiency does not accompany an angular stable emission. However, with the introduction of a dielectric capping layer and a careful choice of layer thicknesses, an angular stable emission and a good efficiency of 13.3 lm/W can be achieved for a normal structure [50

P. Freitag, S. Reineke, S. Olthof, M. Furno, B. Lüssem, and K. Leo, “White top-emitting organic light-emitting diodes with forward directed emission and high color quality,” Org. Electronics 11, 1676–1682 (2010). [CrossRef]

] as well as for an inverted structure [51

M. Thomschke, R. Nitsche, M. Furno, and K. Leo, “Optimized efficiency and angular emission characteristics of white top-emitting organic electroluminescent diodes,” Appl. Phys. Lett 94, 083303 (2009). [CrossRef]

]. Here, the emitters are stacked in a horizontal thin film structure between the contacts.

Recently, Canzler et al. [35

] reported outstanding efficiencies of 36.5 lm/W using a tandem OLED and a scattering capping layer.

Chen et al. [59

S. Chen and H.-S. Kwok, “Top-emitting white organic light-emitting diodes with a color conversion cap layer,” Org. Electron. 12, 677–681 (2011). [CrossRef]

] demonstrated an efficiency up to 8.7 lm/W using the approach of a down-conversion material as capping layer. In 2008, Ji et al. [56

W. Ji, L. Zhang, R. Gao, L. Zhang, W. Xie, H. Zhang, and B. Li, “Top-emitting white organic light-emitting devices with down-conversion phosphors: theory and experiment,” Opt. Express 16, 15489–94 (2008). [CrossRef] [PubMed]

] could already show this effect, but the efficiency was very low. The down-conversion material absorbs a part of the blue light of the underlying OLED and re-emits at longer wavelengths. The outcoupling is positively influenced, because the emission zone of the conversion material is not placed inside a strong microcavity.

Due to the spectral narrowing of the emission, top-emitting OLEDs usually have a poor color rendering index. The Energy Star program of the U.S. Environmental Protection Agency and the U.S. Department of Energy requires a minimum CRI of 75 for indoor solid state lighting [60

“ENERGY STAR ® Program Requirements for Solid State Lighting Luminaires Eligibility Criteria – Version 1.1,” 1–23 (2008) http://www.energystar.gov.

]. The devices of Canzler [35

] and Freitag [50

] have a CRI of 75 and 77, respectively. Furthermore, a good CRI of 84 has been reached by Ji et al. [61

W. Ji, J. Zhao, Z. Sun, and W. Xie, “High-color-rendering flexible top-emitting warm-white organic light emitting diode with a transparent multilayer cathode,” Org. Electron. 12, 1137–1141 (2011). [CrossRef]

] in 2011, using a transparent multilayer cathode. Ma et al. [62

J. Ma, X. Piao, J. Liu, L. Zhang, T. Zhang, M. Liu, T. Li, W. Xie, and H. Cui, “Optical simulation and optimization of ITO-free top-emitting white organic light-emitting devices for lighting or display,” Org. Electron. 12, 923–935 (2011). [CrossRef]

] showed CRI values up to 91 applying a 1D metallic–dielectric photonic crystal anode, but unfortunately efficiency values are not reported.

Table 1 summarizes the efficiencies of white top-emitting OLEDs for the last years and Fig. 5 shows the corresponding CIE color coordinates. Although more than 10 lm/W have been realized, which is comparable to the efficacy of an incandescent lamp [63

S. Pimputkar, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Prospects for LED lighting,” Nature Photonics 3, 180–182 (2009). [CrossRef]

], the adjustment of the CIE color coordinates into the 7-step chromaticity quadrangles is rather difficult. To fulfill the Energy Star requirements for solid state lighting, the color coordinates of the white OLED have to fall into one of these eight quadrangles.

Fig. 5 CIE color coordinates for normal direction of the denoted publications in Table 1 on white top-emitting OLEDs. The dotted line represents the Planckian radiator, whereas A and E are the warm white point and point of equal energy, respectively. According to the Energy Star requirements [60

“ENERGY STAR ® Program Requirements for Solid State Lighting Luminaires Eligibility Criteria – Version 1.1,” 1–23 (2008) http://www.energystar.gov.

] for solid state lighting the color coordinates of the white OLED have to fall into one of the 7-step chromaticity quadrangles to fulfill luminaire requirements.

Table 1 Efficiencies of white top-emitting OLEDs sorted by publication year. The devices are compared by their performance (LE = luminous efficacy, EQE = external quantum effi-ciency, CE = current efficiency).

group	year	efficiency			comment
group	year	LE [lm/W]	EQE [%]	CE [cd/A]	comment

Feng [52 T. Feng, T. A. Ali, E. S. Ramakrshnan, R. Campos, and W. E. Howard, “Structure and characterization of a white up-emitting OLED on silicon for microdisplays,” Proc. SPIE 4105, 30–36 (2001). [CrossRef] ]	2001	1.9	1.8	4.9	at 1000 cd/m², no capping layer
Hsu [53 S.-F. Hsu, C.-C. Lee, S.-W. Hwang, and C. H. Chen, “Highly efficient top-emitting white organic electroluminescent devices,” Appl. Phys. Lett. 86, 253508 (2005). [CrossRef] ]	2005	9.6		22.2	at 20 mA/cm², index matching SnO₂ capping layer, angular stable
Kanno [41 H. Kanno, Y. Sun, and S. R. Forrest, “High-efficiency top-emissive white-light-emitting organic electrophosphorescent devices,” Appl. Phys. Lett. 86, 263502 (2005). [CrossRef] ]	2005	9.8	10.5		peak efficiency, ITO cathode, no capping layer, angular stable
Lin [54 S.-J. Lin, H.-Y. Ueng, and F.-S. Juang, “Effects of Thickness of Organic and Multilayer Anode on Luminance Efficiency in Top-Emission Organic Light Emitting Diodes,” Jap. J. Appl.Phys. 45, 3717–3720 (2006). [CrossRef] ]	2006			1.1	peak efficiency, no capping layer
Zhu [32 X. Zhu, J. Sun, X. Yu, M. Wong, and H.-S. Kwok, “High-Performance Top-Emitting White Organic Light-Emitting Devices,” Jap. J. Appl. Phys. 46, 4054–4058 (2007). [CrossRef] ]	2007	12.8	9.1	18.8	peak efficiencies, angular stable
Lee [55 M. Lee and M. Tseng, “Efficient, long-life and Lambertian source of top-emitting white OLEDs using low-reflectivity molybdenum anode and co-doping technology,” Curr. Appl. Phys. 8, 616–619 (2008). [CrossRef] ]	2008			4.6	at 3000 cd/m², angular stable
Ji [56 W. Ji, L. Zhang, R. Gao, L. Zhang, W. Xie, H. Zhang, and B. Li, “Top-emitting white organic light-emitting devices with down-conversion phosphors: theory and experiment,” Opt. Express 16, 15489–94 (2008). [CrossRef] [PubMed] ]	2008			1	peak efficiency, down-conversion capping layer
Thomschke [51 M. Thomschke, R. Nitsche, M. Furno, and K. Leo, “Optimized efficiency and angular emission characteristics of white top-emitting organic electroluminescent diodes,” Appl. Phys. Lett 94, 083303 (2009). [CrossRef] ]	2009	13.3	7.8	26.7	at 5.4 mA/cm², angular stable, inverted structure
Freitag [50 P. Freitag, S. Reineke, S. Olthof, M. Furno, B. Lüssem, and K. Leo, “White top-emitting organic light-emitting diodes with forward directed emission and high color quality,” Org. Electronics 11, 1676–1682 (2010). [CrossRef] ]	2010	13.3	4.9	18	at 1000 cd/m², angular stable
Wang [57 Q. Wang, Z. Deng, J. Chen, and D. Ma, “Realization of blue, green, and white inverted microcavity top-emitting organic light-emitting devices based on the same emitting layer,” Opt. Lett. 35, 462–464 (2010). [CrossRef] [PubMed] ]	2010			5.6	at 10V, angular stable, inverted structure
Xie [58 G. Xie, Z. Zhang, Q. Xue, S. Zhang, L. Zhao, Y. Luo, P. Chen, B. Quan, Y. Zhao, and S. Liu, “Highly efficient top-emitting white organic light-emitting diodes with improved contrast and reduced angular dependence for active matrix displays,” Org. Electron. 11, 2055–2059 (2010). [CrossRef] ]	2010	17.6		27.7	at 1000 cd/m², Cu anode, angular stable
Canzler [35 T. W. Canzler, S. Murano, D. Pavicic, O. Fadhel, C. Rothe, A. Haldi, M. Hofmann, and Q. Huang, “Efficiency Enhancement in White PIN OLEDs by Simple Internal Outcoupling Methods,” SID Digest 11, 975–978 (2011). [CrossRef] ]	2011	36.5	28.8		at 1000 cd/m², tandem structure, scattering capping layer
Chen [59 S. Chen and H.-S. Kwok, “Top-emitting white organic light-emitting diodes with a color conversion cap layer,” Org. Electron. 12, 677–681 (2011). [CrossRef] ]	2011	8.7		17.7	peak efficiency, down-conversion capping layer

Furthermore, the luminous efficacy is still not as high as for white bottom-emitting diodes reaching 30 lm/W on a flat glass substrate and even 124 lm/W on high index glass using an outcoupling sphere [64

S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459, 234 (2009). [CrossRef] [PubMed]

Figure 6 shows the calculated radiated power spectrum per unit normalized in-plane wavevector at the emitter location and at a wavelength of 475 nm for a white top-emitting OLED with capping layer in Ref. [50

]. Power components with in-plane wavevector up to 0.565 can escape the optical structure and can eventually radiate in the far field. The TE0 guided mode is the main loss channel for TE-polarized light. For TM-polarized light two strong plasmonic modes SPP1 and SPP0 are observed. The outcoupling efficiency is calculated to be 25% at a wavelength of 475nm: 18% of the internally radiated power is lost into waveguided modes, 50% is lost into surface plasmon modes and 7% are absorption losses. The plasmonic losses increase over 75% of the radiated power at a wavelength 600 nm. The key issue to achieve highly efficient white top-emitting OLEDs is therefore the suppression and/or extraction of plasmonic and waveguided modes as described in Section 3.

Fig. 6 Calculated radiated power spectrum per unit normalized in-plane wavevector at wavelength of 475 nm for the top-emitting OLED with capping layer in Ref. [50

]. Power components with in-plane wavevector up to 0.565 can escape the optical structure and can eventually radiate in the far field. The main loss modes are indicated on the diagram: TE-polarized waveguided mode TE0 and surface plasmon polariton modes SPP1 and SPP0. Reprinted with permission from Freitag et al. [65

P. Freitag, S. Hofmann, M. Furno, T. C. Rosenow, B. Lüssem, S. Reineke, S. Mogck, T. Wanski, C. May, and K. Leo, “Novel Approaches for OLED Lighting,” SID Digest 11, (2011).

], SID Digest 11 (2011). Image courtesy of SID.

5. Inverted structure

Recently, Chen et al. [66

S. Chen, L. Deng, J. Xie, L. Peng, L. Xie, Q. Fan, and W. Huang, “Recent Developments in Top-Emitting Organic Light-Emitting Diodes,” Adv. Mater. 22, 5227–5239 (2010). [CrossRef] [PubMed]

] published a review on top-emitting OLEDs focusing on display application, but without considering inverted TOLEDs (Fig. 1 (c)). Low cost display driver circuits are preferentially based on n-channel a-Si TFT technology. Thus, display applications require OLED structures with an inverted layer sequence.

One of the first top-emitting OLEDs demonstrated in 1997 had an inverted structure [67

V. Bulović, P. Tian, P. E. Burrows, M. R. Gokhale, S. R. Forrest, and M. E. Thompson, “A surface-emitting vacuum-deposited organic light emitting device,” Appl. Phys. Lett. 70, 2954–2954 (1997). [CrossRef]

]. This means that the cathode is located on the substrate, followed by the ETL (cf. Figure 1 (c)) and so forth. Thus, the finalizing anode layer has to be made of semitransparent metal or conductive oxides like ITO [68

T. Dobbertin, O. Werner, J. Meyer, A. Kammoun, D. Schneider, T. Riedl, E. Becker, H.-H. Johannes, and W. Kowalsky, “Inverted top-emitting organic light-emitting diodes using sputter-deposited anodes,” Appl. Phys. Lett. 82, 284–286 (2003). [CrossRef]

] for light outcoupling. A complete device transfer of a noninverted bottom-emitting OLED to another substrate via lamination has been demonstrated to fabricate inverted top-emitting OLEDs as well [69

K.-H. Kim, S.-Y. Huh, S.-M. Seo, and H. H. Lee, “Inverted top-emitting organic light-emitting diodes by whole device transfer,” Org. Electr. 9, 1118–1121 (2008). [CrossRef]

The major problem of all reported inverted OLED devices is the high driving voltage compared to their non-inverted counterparts. In most cases, this effect is attributed to a poor electron injection from the bottom cathode [70

C.-W. Chen, C.-L. Lin, and C.-C. Wu, “An effective cathode structure for inverted top-emitting organic light-emitting devices,” Appl. Phys. Lett. 85, 2469–2471 (2004). [CrossRef]

] due to a sharp cathode-ETL interface. Here, noninverted top-emitting OLEDs benefit from the effect of metal penetration [71

W. Song, S. K. So, J. Moulder, Y. Qiu, Y. Zhu, and L. Cao, “Study on the interaction between Ag and tris(8-hydroxyquinoline) aluminum using x-ray photoelectron spectroscopy,” Surf. Interface Anal. 32, 70–73 (2001). [CrossRef]

] into organics when depositing the cathode onto the organic layers. This leads to different energy level alignments comparing metal-organic and organic-metal interfaces [72

S. Scholz, Q. Huang, M. Thomschke, S. Olthof, P. Sebastian, K. Walzer, K. Leo, S. Oswald, C. Corten, and D. Kuckling, “Self-doping and partial oxidation of metal-on-organic interfaces for organic semiconductor devices studied by chemical analysis techniques,” J. Appl. Phys. 104, 104502 (2008). [CrossRef]

Several approaches like ZnS nanoparticles [73

H. Lee, I. Park, J. Kwak, D. Y. Yoon, and C. Lee, “Improvement of electron injection in inverted bottom-emission blue phosphorescent organic light emitting diodes using zinc oxide nanoparticles,” Appl. Phys. Lett. 96, 153306 (2010). [CrossRef]

], a pentacene interlayer [74

C. Yun, H. Cho, H. Kang, Y. Mi Lee, Y. Park, and S. Yoo, “Electron injection via pentacene thin films for efficient inverted organic light-emitting diodes,” Appl. Phys. Lett. 95, 053301 (2009). [CrossRef]

], or a Tris(8-hydroxyquinolinato) aluminum + LiF + Al tri-layer [70

C.-W. Chen, C.-L. Lin, and C.-C. Wu, “An effective cathode structure for inverted top-emitting organic light-emitting devices,” Appl. Phys. Lett. 85, 2469–2471 (2004). [CrossRef]

] have been proposed to enhance electron injection in inverted OLEDs. Alkali metal doped injection and transport layers [75

X. Zhou, M. Pfeiffer, J. S. Huang, J. Blochwitz-Nimoth, D. S. Qin, A. Werner, J. Drechsel, B. Maennig, and K. Leo, “Low-voltage inverted transparent vacuum deposited organic light-emitting diodes using electrical doping,” Appl. Phys. Lett. 81, 922–924 (2002). [CrossRef]

], as well as MgO, PbO and WO₃ buffer layers [76

H. W. Choi, S. Y. Kim, W.-K. Kim, and J.-L. Lee, “Enhancement of electron injection in inverted top-emitting organic light-emitting diodes using an insulating magnesium oxide buffer layer,” Appl. Phys. Lett. 87, 082102 (2005). [CrossRef]

, 46

, 77

J. Meyer, T. Winkler, S. Hamwi, S. Schmale, H. H. Johannes, T. Weimann, P. Hinze, W. Kowalsky, and T. Riedl, “Transparent Inverted Organic Light-Emitting Diodes with a Tungsten Oxide Buffer Layer,” Adv. Mater. 20, 3839–3843 (2008). [CrossRef]

] or oxygen doping [78

K. Hong, K. Kim, and J.-L. Lee, “Enhancement of electrical property by oxygen doping to copper phthalocyanine in inverted top emitting organic light emitting diodes,” Appl. Phys. Lett. 95, 213307 (2009). [CrossRef]

] are techniques that have also been shown to improve the device performance. Wang et al. [46

] reached efficiencies up to 16.6 lm/W (33.8 cd/A) for a green emitter. In case of doped charge transport layers, the energetic differences between metal-organic and organic-metal interfaces can be neglected. It can be shown that the injection is not the dominating factor in this case [79

M. Pfeiffer, S. R. Forrest, X. Zhou, and K. Leo, “A low drive voltage, transparent, metal-free n-i-p electrophosphorescent light emitting diode,” Org. Electr. 4, 21–26 (2003). [CrossRef]

]. Therefore, as the voltage drops only across the intrinsic layers, the transport across the organic layers and their interfaces with the corresponding energetic barriers have to be considered as the reason for the increased voltages as depicted in Figure 7. Recently, thermal annealing has been found to improve the driving voltages drastically [81

P.-S. Wang, I.-W. Wu, and C.-I. Wu, “Enhancement of current injection in inverted organic light emitting diodes with thermal annealing,” J. Appl. Phys. 108, 103714 (2010). [CrossRef]

, 80

M. Thomschke, S. Hofmann, S. Olthof, M. Anderson, H. Kleemann, M. Schober, B. Lüssem, and K. Leo, “Improvement of voltage and charge balance in inverted top-emitting organic electroluminescent diodes comprising doped transport layers by thermal annealing,” Appl. Phys. Lett. 98, 083304 (2011). [CrossRef]

]. Hereby, material diffusion processes, activated chemical reactions as well as morphological changes can be the origin of the annealing effects and thus be responsible for the differences of inverted and non-inverted device performances.

Fig. 7 Comparison of current-luminance-voltage characteristics of non-inverted and the equivalent inverted top-emitting OLED. Square symbols refer to the normal structure, circular symbols to the inverted structure. The I–V curves diverge up to 2V difference. Reprinted with permission from Scholz et al. [72

White top-emitting OLEDs with inverted layer sequence have also been reported. Making use of an index matching capping layer for light outcoupling, 5.6 cd/A [57

Q. Wang, Z. Deng, J. Chen, and D. Ma, “Realization of blue, green, and white inverted microcavity top-emitting organic light-emitting devices based on the same emitting layer,” Opt. Lett. 35, 462–464 (2010). [CrossRef] [PubMed]

] and 26.7 cd/A (13.3 lm/W) [51

] have been reached.

6. Conclusion

Recent development of top-emitting OLEDs including basic optical microcavity theory, the calculation of power dissipation, white light emission and inverted structures have been discussed. The main loss mechanisms are identified and strategies for outcoupling are presented.

It is shown that top-emitting OLEDs and inverted top-emitting OLEDs are promising candidates for solid state lighting and display applications. However, further work on light extraction techniques on waveguided modes and surface plasmon modes is required to reach high outcoupling efficiencies.

Acknowledgments

This work was funded by the BMBF with support code 13N11060 (project acronym “R2Flex”). Dr. Mauro Furno, Novaled AG, Dresden, is acknowledged for fruitful discussions.

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36.	C. J. Yang, S. H. Liu, H. H. Hsieh, C. C. Liu, T. Y. Cho, and C. C. Wu, “Microcavity top-emitting organic light-emitting devices integrated with microlens arrays: Simultaneous enhancement of quantum efficiency, cd/A efficiency, color performances, and image resolution,” Appl. Phys. Lett. 91, 253508 (2007). [CrossRef]
37.	W. L. Barnes, “Fluorescence near interfaces: the role of photonic mode density,” J. of Mod. Opt. 45, 661–699 (1998). [CrossRef]
38.	M. Furno, R. Meerheim, M. Thomschke, S. Hofmann, B. Lüssem, and K. Leo, “Outcoupling efficiency in small-molecule OLEDs: from theory to experiment,” Proc. SPIE 7617, 761716 (2010). [CrossRef]
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50.	P. Freitag, S. Reineke, S. Olthof, M. Furno, B. Lüssem, and K. Leo, “White top-emitting organic light-emitting diodes with forward directed emission and high color quality,” Org. Electronics 11, 1676–1682 (2010). [CrossRef]
51.	M. Thomschke, R. Nitsche, M. Furno, and K. Leo, “Optimized efficiency and angular emission characteristics of white top-emitting organic electroluminescent diodes,” Appl. Phys. Lett 94, 083303 (2009). [CrossRef]
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56.	W. Ji, L. Zhang, R. Gao, L. Zhang, W. Xie, H. Zhang, and B. Li, “Top-emitting white organic light-emitting devices with down-conversion phosphors: theory and experiment,” Opt. Express 16, 15489–94 (2008). [CrossRef] [PubMed]
57.	Q. Wang, Z. Deng, J. Chen, and D. Ma, “Realization of blue, green, and white inverted microcavity top-emitting organic light-emitting devices based on the same emitting layer,” Opt. Lett. 35, 462–464 (2010). [CrossRef] [PubMed]
58.	G. Xie, Z. Zhang, Q. Xue, S. Zhang, L. Zhao, Y. Luo, P. Chen, B. Quan, Y. Zhao, and S. Liu, “Highly efficient top-emitting white organic light-emitting diodes with improved contrast and reduced angular dependence for active matrix displays,” Org. Electron. 11, 2055–2059 (2010). [CrossRef]
59.	S. Chen and H.-S. Kwok, “Top-emitting white organic light-emitting diodes with a color conversion cap layer,” Org. Electron. 12, 677–681 (2011). [CrossRef]
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61.	W. Ji, J. Zhao, Z. Sun, and W. Xie, “High-color-rendering flexible top-emitting warm-white organic light emitting diode with a transparent multilayer cathode,” Org. Electron. 12, 1137–1141 (2011). [CrossRef]
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72.	S. Scholz, Q. Huang, M. Thomschke, S. Olthof, P. Sebastian, K. Walzer, K. Leo, S. Oswald, C. Corten, and D. Kuckling, “Self-doping and partial oxidation of metal-on-organic interfaces for organic semiconductor devices studied by chemical analysis techniques,” J. Appl. Phys. 104, 104502 (2008). [CrossRef]
73.	H. Lee, I. Park, J. Kwak, D. Y. Yoon, and C. Lee, “Improvement of electron injection in inverted bottom-emission blue phosphorescent organic light emitting diodes using zinc oxide nanoparticles,” Appl. Phys. Lett. 96, 153306 (2010). [CrossRef]
74.	C. Yun, H. Cho, H. Kang, Y. Mi Lee, Y. Park, and S. Yoo, “Electron injection via pentacene thin films for efficient inverted organic light-emitting diodes,” Appl. Phys. Lett. 95, 053301 (2009). [CrossRef]
75.	X. Zhou, M. Pfeiffer, J. S. Huang, J. Blochwitz-Nimoth, D. S. Qin, A. Werner, J. Drechsel, B. Maennig, and K. Leo, “Low-voltage inverted transparent vacuum deposited organic light-emitting diodes using electrical doping,” Appl. Phys. Lett. 81, 922–924 (2002). [CrossRef]
76.	H. W. Choi, S. Y. Kim, W.-K. Kim, and J.-L. Lee, “Enhancement of electron injection in inverted top-emitting organic light-emitting diodes using an insulating magnesium oxide buffer layer,” Appl. Phys. Lett. 87, 082102 (2005). [CrossRef]
77.	J. Meyer, T. Winkler, S. Hamwi, S. Schmale, H. H. Johannes, T. Weimann, P. Hinze, W. Kowalsky, and T. Riedl, “Transparent Inverted Organic Light-Emitting Diodes with a Tungsten Oxide Buffer Layer,” Adv. Mater. 20, 3839–3843 (2008). [CrossRef]
78.	K. Hong, K. Kim, and J.-L. Lee, “Enhancement of electrical property by oxygen doping to copper phthalocyanine in inverted top emitting organic light emitting diodes,” Appl. Phys. Lett. 95, 213307 (2009). [CrossRef]
79.	M. Pfeiffer, S. R. Forrest, X. Zhou, and K. Leo, “A low drive voltage, transparent, metal-free n-i-p electrophosphorescent light emitting diode,” Org. Electr. 4, 21–26 (2003). [CrossRef]
80.	M. Thomschke, S. Hofmann, S. Olthof, M. Anderson, H. Kleemann, M. Schober, B. Lüssem, and K. Leo, “Improvement of voltage and charge balance in inverted top-emitting organic electroluminescent diodes comprising doped transport layers by thermal annealing,” Appl. Phys. Lett. 98, 083304 (2011). [CrossRef]
81.	P.-S. Wang, I.-W. Wu, and C.-I. Wu, “Enhancement of current injection in inverted organic light emitting diodes with thermal annealing,” J. Appl. Phys. 108, 103714 (2010). [CrossRef]

OCIS Codes
(160.4890) Materials : Organic materials
(220.0220) Optical design and fabrication : Optical design and fabrication
(230.3670) Optical devices : Light-emitting diodes
(250.0250) Optoelectronics : Optoelectronics
(310.6860) Thin films : Thin films, optical properties
(310.6845) Thin films : Thin film devices and applications

ToC Category:
Light-Emitting Diodes

History
Original Manuscript: August 1, 2011
Manuscript Accepted: October 8, 2011
Published: November 7, 2011

Virtual Issues
Organic Light-Emitting Diodes (2011) Optics Express

Citation
Simone Hofmann, Michael Thomschke, Björn Lüssem, and Karl Leo, "Top-emitting organic light-emitting diodes," Opt. Express 19, A1250-A1264 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-S6-A1250

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References

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G. Parthasarathy, C. Adachi, P. E. Burrows, and S. R. Forrest, “High-efficiency transparent organic light-emitting devices,” Appl. Phys. Lett.76, 2128 (2000). [CrossRef]
A. Yamamori, S. Hayashi, T. Koyama, and Y. Taniguchi, “Transparent organic light-emitting diodes using metal acethylacetonate complexes as an electron injective buffer layer,” Appl. Phys. Lett.78, 3343 (2001). [CrossRef]
H. Riel, S. Karg, T. Beierlein, B. Ruhstaller, and W. Rieß, “Phosphorescent top-emitting organic light-emitting devices with improved light outcoupling,” Appl. Phys. Lett.82, 466 (2003). [CrossRef]
Q. Huang, K. Walzer, M. Pfeiffer, V. Lyssenko, G. He, and K. Leo, “Highly efficient top emitting organic light-emitting diodes with organic outcoupling enhancement layers,” Appl. Phys. Lett.88, (2006). [CrossRef]
H. Riel, S. Karg, T. Beierlein, W. Rieß, and K. Neyts, “Tuning the emission characteristics of top-emitting organic light-emitting devices by means of a dielectric capping layer: An experimental and theoretical study,” J. Appl. Phys.94, 5290 (2003). [CrossRef]
S. Hofmann, M. Thomschke, P. Freitag, M. Furno, B. Lüssem, and K. Leo, “Top-emitting organic light-emitting diodes: Influence of cavity design,” Appl. Phys. Lett.97, 253308 (2010). [CrossRef]
E. F. Schubert, N. E. J. Hunt, R. J. Malik, D. L. Sivco, A. Y. Cho, and G. J. Zydzik, “Highly Efficient Light-Emitting Diodes with Microcavities,” Science265, 12 (1994). [CrossRef]
D. G. Deppe, C. Lei, C. C. Lin, and D. L. Huffaker, “Spontaneous Emission from Planar Microstructures,” J. Mod. Opt.41, 325 (1994). [CrossRef]
J.-H. Lee, K.-Y. Chen, C.-C. Hsiao, H.-C. Chen, C.-H. Chang, Y.-W. Kiang, and C. C. Yang, “Radiation Simulations of Top-Emitting Organic Light-Emitting Devices With Two- and Three-Microcavity Structures,” J. Disp. Tech.2, 130 (2006). [CrossRef]
Q. Wang, Z. Deng, and D. Ma, “Realization of high efficiency microcavity top-emitting organic light-emitting diodes with highly saturated colors and negligible angular dependence,” Appl. Phys. Lett.94, 233306 (2009). [CrossRef]
A.W. Lu and A.D. Rakić, “Design of microcavity organic light emitting diodes with optimized electrical and optical performance,” Appl. Opt.48, 2282 (2009). [CrossRef] [PubMed]
A. B. Djurišić and A. D. Rakić, “Organic microcavity light-emitting diodes with metal mirrors: dependence of the emission wavelength on the viewing angle.,” Appl. Opt.41, 7650 (2002). [CrossRef]
X. Zhu, J. Sun, X. Yu, M. Wong, and H.-S. Kwok, “High-Performance Top-Emitting White Organic Light-Emitting Devices,” Jap. J. Appl. Phys.46, 4054–4058 (2007). [CrossRef]
X. W. Chen, W. C. H. Choy, S. He, and P. Chui, “Comprehensive analysis and optimal design of top-emitting organic light-emitting devices,” J. Appl. Phys.101, 113107 (2007). [CrossRef]
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Appl. Opt.

Appl. Phys. Lett

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