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Top-emitting organic light-emitting diodes

Simone Hofmann, Michael Thomschke, Björn Lüssem, and Karl Leo

Open Access Open Access

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.

©2011 Optical Society of America

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Figures (7)
Fig. 1
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.

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Fig. 2
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.

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Fig. 3
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]. 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], Proc. of SPIE 7617, 761716 (2010). Image courtesy of SPIE.

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Fig. 4
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], Applied Physics Letters 97, 253305 (2010). Copyright 2010, American Institute of Physics.

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Fig. 5
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] 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.

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Fig. 6
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], SID Digest 11 (2011). Image courtesy of SID.

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Fig. 7
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], Journal of Applied Physics 104, 104502, (2008). Copyright 2008, American Institute of Physics.

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Tables (1)

Tables Icon

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).

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Equations (10)

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F W H M = λ 2 2 L cav × 1 − R t R b π R t R b 4 .
I ( λ , θ ) = T t [ 1 + R b + 2 R b cos ( − ϕ b + 4 π n z cos ( θ org , EML ) λ ) ] ( 1 − R b R t ) 2 + 4 R b R t sin 2 ( Δ ϕ 2 ) I 0 ( λ )
Δ ϕ = − ϕ b − ϕ t + ∑ i 4 π n i d i cos ( θ org , i ) λ
I ( λ ) = T t [ 1 + R b + 2 R b cos ( 4 π n z λ ) ] 1 + R b R t − 2 R b R t cos ( 4 π L cav λ ) I 0 ( λ ) .
G = T t [ ( 1 + R b ) 2 − 4 R b cos 2 ( 2 π z λ ) ] ( 1 − R t R b ) 2 + 4 R t R b sin 2 ( 2 π L cav λ ) τ cav τ .
Δ λ θ = ∑ θ ( ∑ i 4 π d i λ n i [ cos θ org , i − 1 ] + Δ ϕ t + Δ ϕ b ) .
F = ( 1 − q ) + q ∫ 0 ∞ K ( u ) d u ,
q = Γ r Γ r + Γ n r ,
K = 1 3 K T M ν + 2 3 ( K T M h + K T E h ) .
E Q E = γ χ ∫ λ q F ( λ ) q F ( λ ) + 1 − q η out ( λ ) I 0 ( λ ) d λ ,
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