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Towards organic light-emitting diode microdisplays with sub-pixel patterning
Organic Electronics 11 (2010) 57–61
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Towards organic light-emitting diode microdisplays with sub-pixel patterning Fabian Ventsch, Malte C. Gather, Klaus Meerholz * Department of Chemistry, University of Cologne, Luxemburgerstr. 116, D-50939 Cologne, Germany
a r t i c l e
i n f o
Article history: Received 21 July 2009 Received in revised form 25 September 2009 Accepted 26 September 2009 Available online 12 October 2009 Keywords: OLED Top-emitter Microdisplay
a b s t r a c t We report on solution processed top-emitting organic light-emitting diodes (TEOLEDs) that are compatible with TFT- and CMOS-substrates, thus being applicable in full-color microdisplays. In this device geometry TEOLEDs offer highly tunable and controllable color-coordinates. Compared to the standard sRGB-colorspace the color gamut is increased by 18%. The CIE (Commission internationale de l’éclairage) color coordinates are more saturated in the red and green spectral region and comparable in the blue region. Brightness levels of 1000 cd/m2 are reached at voltages below 7 V for all colors which is within the operating voltage range of CMOS-backplanes. The polymeric emitters are crosslinkable and can be patterned in a UV-lithography process. The achieved minimum feature size is suitable for high resolution microdisplays. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction For several years the interest in organic light-emitting diodes (OLEDs) has shifted from rather simple devices such as monochrome displays to complex full-color active-matrix displays. In this context, increasing amounts of money are invested in the commercialization of displays based on OLEDtechnology. As a preliminary highlight, this trend resulted in the implementation of an 11 in. OLED-TV (XEL-1) by Sony in 2007. The main effort in this field is to increase display sizes. However, there is also an ever increasing demand for mobile TV and video equipment. Here it is necessary to decrease pixel size in order to build very small displays (typically below 1.5 in. diagonal) with high resolution. These socalled microdisplays are the key components in video goggles. By use of magnifying optics an enlarged image of the display is projected directly into the human eye. Today an overwhelming share of the display market is held by liquid crystal (LC)-technology and hence any new technology has to compete against it. In LC-displays white light from a backlight is converted into red, green and blue * Corresponding author. E-mail address: [email protected] (K. Meerholz). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.09.026
(RGB) by color filters laminated on top of the LC-cell. The gap between filter and LC-cells leads to partial illumination of adjacent sub-pixels and, therefore, to a loss in color gamut. This effect becomes worse with decreasing pixel size and hence puts a fundamental limit to the resolution of microdisplays. Underwood et al. [1] established a microdisplay in which the LC-cell was replaced by a white-emitting OLED (see Fig. 1a). This reduces the power consumption and simplifies the fabrication process of the microdisplays but does not solve the problem of the limited color gamut. Here, we propose a process for the fabrication of a high resolution microdisplay in which red-, green- and blue-emitting OLEDs form the sub-pixels of the display. To achieve this, thin films of RGB-emitting materials will be deposited from solution and be structured via direct UVlithography in order to define the color of corresponding sub-pixels (see Fig. 1b). The used electroluminescent materials are oxetane-functionalized and can be crosslinked through cationic ring opening polymerisation. In the presence of a photo acid generator (PAG) this reaction can be initiated by UV-light giving the material photoresist-like properties [2,3]. As light and color are generated in the same layer, crosstalk between sub-pixels is eliminated and a large color gamut is expected. The pixel size in a microdisplay
58
F. Ventsch et al. / Organic Electronics 11 (2010) 57–61
Fig. 1. (a) Schematic of a commercial OLED-based microdisplay using a polymeric white emitter and color filters. (b) RGB sub-pixel approach proposed here.
with VGA resolution (640 480 pixels) amounts to a few micrometers. We have already demonstrated that direct lithographic patterning of electroluminescent polymers yields minimum feature sizes down to 2 lm [3]. In contrast to direct lithography of solution processed OLED materials, vacuum deposition of small molecules through shadow masks is not a suitable option for the fabrication of microdisplays: The quality of the shadow masks suffers from material being deposited onto it over time. This leads to a gradual reduction of the actual pixel size and to an enhanced formation of disruptive particles on the OLED. For the small pixels of microdisplays, the required OLED quality is only ensured for very few evaporation procedures per shadow mask [4]. It is obvious that top-emitting OLEDs (TEOLEDs) are favorable over bottom-emitting ones (BEOLEDs) when working with opaque complementary metal oxide semiconductor (CMOS) backplanes. These active transistor backplanes are necessary to display complex content. There are two possible geometries for TEOLEDs: Inverted OLEDs with a transparent top-electrode [5–7] as well as cavity TEOLEDs [8–10]. The latter allow for better control of the emission characteristics due to the layer-thicknessdependent microcavity effect. Especially since many polymeric emitters show a broad emission spectrum (see Fig. 3c), the control of both, spectral width and peak wavelength, is beneficial to achieve primary color coordinates suitable for display applications. In general, a cavity TEOLED consists of a metallic mirror which is usually the anode of the OLED, a stack of organic layers, and a semi-transparent metallic cathode. In this Fabry–Perot-cavity-like geometry, light is reflected between the two metallic layers. Light with wavelengths matching the criterion for constructive interference after reflection at each metal layer is amplified and emitted through the semi-transparent mirror. Light with other wavelengths is weakened or even suppressed from being coupled out [11–13]. As a consequence, the cavity thickness determines the emitted peak wavelength, and the cavity quality defines the spectral width.
capable of efficiently injecting holes to the electroluminescent copolymers used in this work (EHOMO = 5.4 eV [15]). This problem can be circumvented by the addition of a molybdenum oxide (MoO3)-layer (5.3 eV [16]). When used in a cavity structure, MoO3 is preferable to the commonly used 3,4-poly(ethylendioxythiophen)-poly(styrolsulfonat) (PEDOT) for transparency reasons. We investigated TEOLEDs on glass substrates with a highly reflecting silver (Ag) anode due to our standard OLED processing. However, silver is not suitable in standard CMOS fabrication process. Thus, to demonstrate that the results are of relevance for a true CMOS process, we investigated in addition TEOLEDs containing an aluminum (Al)/(TiN)/(MoO3) anode on a CMOS dummy substrate. The silver anode, the HIL (MoO3) and the cathode layers (barium and silver) were vacuum deposited in a Leybold Univex 450 chamber. The emitting polymers were spincoated from solution in nitrogen atmosphere. Crosslinking was initiated by exposure to UV-light. A softcuring step at 90 °C on a hotplate is essential to allow chain propagation and promote further crosslinking. In crosslinked areas the polymer film becomes insoluble. Finally, the film is rinsed with toluene to remove non-exposed material. The subpixels in the future microdisplays will be developed in the same manner. Here, the entire layer is crosslinked to emulate the future conditions in microdisplays. At last, a postbaking step at 200 °C is applied. For comparison of current–voltage–luminescence (J–V–L) curves and for color point characterization, OLEDs have been investigated in the top-emitting as well as in the bottom-emitting geometry. The cavity TEOLEDs consisted of the following layer stack:
Glass=Ag ð150 nmÞ=MoO3 ð30 nmÞ==XR ð90 nmÞ or XG ð70 nmÞ or XB ð35 nmÞ==Ba ð4 nmÞ=Ag ð20 nmÞ: Using an Al mirror instead of an Ag mirror will change the optical characteristics of the cavity for a given layer thickness due to the difference in reflectivity. However, this can be compensated by re-adjusting the thickness of the emitting layer, thus, leading to identical spectral output. To study the influence on the OLED performance we fabricated a red-emitting device with Al mirror (CMOS dummy). The entire layer stack of the CMOS dummy was:
Si=Al ð100 nmÞ=TiN ð10 nmÞ=MoO3 ð30 nmÞ== XR ð100 nmÞ==Ba ð4 nmÞ=Ag ð20 nmÞ: The sequence of the BEOLED stack was:
Glass=ITO ð160 nmÞ=MoO3 ð30 nmÞ==XR ð80 nmÞ or XG ð80 nmÞ or XB ð70 nmÞ==Ba ð4 nmÞ=Ag ð150 nmÞ: Each diode has an active area of 7.9 mm2. 3. Results and discussion
2. Experimental
3.1. Device performance
On CMOS surfaces, titanium nitride (TiN) is usually used as hole-injection layer (HIL). Due to its rather high work function of about 5 eV vs. vacuum [14], TiN is not
In Fig. 2 current density, luminance and current efficiencies are presented for TEOLEDs. Brightness levels of 1000 cd/m2 for red and blue and 10,000 cd/m2 for green
59
were achieved at voltages of <7 V and a current density of <40 mA/cm2. Comparing the efficiencies of the TEOLEDs and BEOLEDs, similar values are achieved for the red and the green emitter (see Table 1). In this case the losses due to absorption at the semi-transparent cathode are compensated by the light amplification in forward direction due to microcavity effects. The decrease in efficiency of the blue emitter in the TEOLED case can be attributed to differences in optical and electrical device performance of TEOLEDs and BEOLEDs: the emission width of the polymeric blue emitter is broad with a significant amount of emission in the green region of the spectrum (see Fig. 3). In the BEOLED geometry this fraction of green leads to sky-blue emission. The layer thickness (70 nm) in this case is chosen to achieve optimum electrical performance (balancing efficiency and operating voltage). Deep blue emission with corresponding color coordinates is obtained in the TEOLED geometry at layer thicknesses of 35 nm (first maximum of emission) and 140 nm (second maximum). This is contrary to the optimum layer thickness in terms of efficiency and operating voltage. High leakage currents for the thin layers (35 nm) and high driving voltages for the thick layers (140 nm) are observed. Besides reducing the electrical performance, the spectral blue-shift leads to a decrease in current-efficiency as the brightness is corrected by the
2
a
Brightness [cd/m ]
2
Current Density [mA/cm ]
F. Ventsch et al. / Organic Electronics 11 (2010) 57–61
1200
10
4
800
10
3
400
10
2
10
1
0
4
5
6
7
8
9
10
Voltage [V]
Efficiency [cd/A]
b8 7 6 5 1 0 0
10 20 30 40 2 Current Density [mA/cm ]
50
0
52
54
0
53
0
0 57
0 49
0 40
47
0
48
0
0.2
60 6 0 62 10 69 0 0
59
0.4 0
Y
0
58
0
50
0.6
56
0
55
0
0.8
51
a
0
Fig. 2. (a) Current density (closed symbols) and brightness (open symbols) versus voltage, for red (triangles = Ag/MoO3-anode, stars = Al/ TiN/MoO3-anode), green (circles) and blue (squares) TEOLEDs (b) Current efficiency versus voltage (symbols see (a)). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Normalized Intensity
b
1.0 0.8 0.6 0.4 0.2 0.0 400 450 500 550 600 650 700 750 800
Wavelength [nm]
c
1.0
Normalized Intensity
0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 X
0.8 0.6 0.4 0.2 0.0 400 450 500 550 600 650 700 750 800
Wavelength [nm]
Fig. 3. (a) Color coordinates of TEOLEDs (squares) BEOLED (circles) and sRGB color space for CRTs (triangles). EL spectra of blue (squares), green (circles), red (triangles = Ag/MoO3-anode, stars = Al/TiN/MoO3-anode) TEOLEDs (b) and BEOLEDs (c) and Etfos simulations (solid lines).
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F. Ventsch et al. / Organic Electronics 11 (2010) 57–61
Table 1 Efficiency and color coordinates for RGB emitters in TEOLED and BEOLED geometry. XRAl
Mirror
XRAg
Mirror
XGAg
Mirror
XBAg
Mirror
TEOLED Layer thickness [nm] gmax [cd/A] (CIE-values)
100 1.8 (0.69; 0.31)
90 1.6 (0.68; 0.32)
70 7.2 (0.29; 0.66)
35 0.7 (0.14; 0.10)
BEOLED Layer thickness [nm] gmax [cd/A] (CIE-values)
/ /
80 1.2 (0.65; 0.34)
80 9.4 (0.28; 0.58)
70 2.0 (0.18; 0.26)
sensitivity of the human eye. In order to distinguish the fractions of eye-sensitivity correction and electrical device performance, we normalized the device efficiency by the overlap integral of the EL-spectrum and the eye-sensitivity curve and then compared these ‘‘quasi quantum efficiencies”. It turns out that 32% of the efficiency decrease can be attributed to the color shift to deeper blue. The remaining percentage (68%) of efficiency decrease has thus to be due to electrical device performance. Comparing the red-emitting TEOLED device featuring an Ag/MoO3 anode with the CMOS dummy (Al/TiN/ MoO3-anode), reduced current- and light output at a given voltage are observed in the latter case. This is mainly due to a decrease in electric field resulting from the enlarged layer thickness. The current efficiency is slightly increased while kmax is red-shifted. The increase in efficiency is probably due to an enlarged internal quantum efficiency due to a better charge carrier balance in the CMOS dummy device. Optical simulations show that the expected output is even reduced by a factor of 0.45 (see Supplementary data). With respect to the electrical characteristics of green- and blueemitting materials on a CMOS dummy device, we expect similar results. 3.2. Color characteristics In Figs. 3b and 3c the electroluminance spectra of bottom- and top-emitting OLEDs are shown. The experimental spectra were confirmed by optical simulations using Fluxim Etfos 1.3 [17]. Assuming an exciton density that is maximum at the HIL/EML interface and decays exponentially towards the cathode with a 1/e width of 20 nm [18], simulation and experimental results are in excellent agreement. Due to microcavity effects, narrowband emission is achieved in case of the TEOLED devices. In comparison to the BEOLEDs, this results in a higher color purity and, therefore, better primary CIE-values (see also Fig. 3a). Especially for the blue emitter much better color coordinates are achieved. Overall, our TEOLEDs achieve better values than required by the sRGB color space which defines the limits for computer monitors and HDTVs. In particular, the red and green emission is more saturated than required. In this case the slightly red-shifted emission of the CMOS dummy device compared with our standard TEOLED has no significant effect on the color coordinate. The blue nearly meets the sRGB specifications. By a comparison of the areas of the color triangles [19], we conclude that the color gamut is increased by 18% (see Fig. 3a). Calculated from the primary color coordinates, the fraction of emission required from each of the RGB TEOLEDs to obtain white emission, i.e. CIE(x, y) = (0.33; 0.33) is
(RGB) = (0.26; 0.61; 0.13). The corresponding maximum current efficiency for white light emission is 4.9 cd/A at a brightness level of 1000 cd/m2. This corresponds to a value of 5 cd/A achieved in BEOLED geometry [3], i.e. the properties are fully conserved. 4. Summary In conclusion, we investigated solution processed crosslinkable electroluminescent materials in top- and bottomemitting OLEDs. We showed the advantages of TEOLEDs over BEOLEDs in terms of color tuning. We also demonstrated that our TEOLEDs are suitable for high color gamut and high brightness displays. Due to their photoresist like properties, they can be directly patterned via UV-lithography and hence will allow for high resolution microdisplays. The next step will be to process and investigate the TEOLED stack and arrays of RGB sub-pixels on CMOS-backplanes for microdisplays. We already demonstrated, that on a CMOS dummy substrate the electrical and optical characteristics are essentially preserved. Acknowledgement We thank Herbert Metzner (University of Cologne) and the technical workshop for technical support, EU/Hypoled for funding and U. Vogel and D. Kreye (both Fraunhofer IPMS, Dresden) for fruitful discussions and for the supply with CMOS dummy wafers. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.orgel.2009.09. 026. References [1] I. Underwood, G. Kelly, R. Woodburn, P-OLED microdisplays, in: IEEE LEOS Annual Meeting Conference Proceedings, vol. 1–2, 2006, p. 438. [2] C.D. Muller, A. Falcou, N. Reckefuss, M. Rojahn, V. Wiederhirn, P. Rudati, H. Frohne, O. Nuyken, H. Becker, K. Meerholz, Multi-colour organic light-emitting displays by solution processing Nature 421 (2003) 829. [3] M.C. Gather, A. Koehnen, A. Falcou, H. Becker, K. Meerholz, Solutionprocessed full-color polymer organic light-emitting diode displays fabricated by direct photolithography, Adv. Funct. Mater. 17 (2007) 191. [4] J.W. Hamer, A. Yamamoto, G. Rajeswaran, S.A. Van Slyke, Mass production of full-color AMOLED displays, SID Dig. 39 (2005) 1902. [5] G. Parthasarathy, G. Gu, S.R. Forrest, A full-color transparent metalfree stacked organic light emitting device with simplified pixel biasing, Adv. Mater. 11 (1999) 907. [6] H.K. Kim, D.G. Kim, K.S. Lee, M.S. Huh, S.H. Jeong, K.I. Kim, T.Y. Seong, Plasma damage-free sputtering of indium tin oxide cathode layers
F. Ventsch et al. / Organic Electronics 11 (2010) 57–61
[7]
[8]
[9]
[10]
[11]
[12]
for top-emitting organic light-emitting diodes, Appl. Phys. Lett. 86 (2005) 183503. Y.Q. Li, L.W. Tan, X.T. Hao, K.S. Ong, F.R. Zhu, L.S. Hung, Flexible topemitting electroluminescent devices on polyethylene terephthalate substrates, Appl. Phys. Lett. 86 (2005) 153508. Q. Huang, K. Walzer, M. Pfeiffer, K. Leo, M. Hofmann, T. Stubinger, Performance improvement of top-emitting organic light-emitting diodes by an organic capping layer: an experimental study, J. Appl. Phys. 100 (2006) 064507. C.J. Lee, R.B. Pode, D.G. Moon, J.L. Han, N.H. Park, S.H. Baik, S.S. Ju, On the problem of microcavity effects on the top emitting OLED with semitransparent metal cathode, Phys. Status Solidi A Appl. Res. 201 (2004) 1022. H.J. Peng, J.X. Sun, X.L. Zhu, X.M. Yu, M. Wong, H.S. Kwok, Highefficiency microcavity top-emitting organic light-emitting diodes using silver anode, Appl. Phys. Lett. 88 (2006) 073517. A. Dodabalapur, L.J. Rothberg, T.M. Miller, Color variation with electroluminescent organic semiconductors in multimode resonant cavities, Appl. Phys. Lett. 65 (1994) 2308. A. Dodabalapur, L.J. Rothberg, R.H. Jordan, T.M. Miller, R.E. Slusher, J.M. Phillips, Physics and applications of organic microcavity light emitting diodes, J. Appl. Phys. 80 (1996) 6954.
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[13] J.H. Hou, J. Wu, Z.Y. Xie, L.X. Wang, Tuning the emissive colour of top-emitting organic light-emitting diodes by using exterior multilayer films, J. Phys. D: Appl. Phys. 42 (2009) 035107. [14] J. Westlinder, G. Sjoblom, J. Olsson, Variable work function in MOS capacitors utilizing nitrogen-controlled TiNx gate electrodes, Microelectr. Eng. 75 (2004) 389. [15] M.C. Gather, F. Ventsch, K. Meerholz, Embedding organic lightemitting diodes into channel waveguide structures, Adv. Mater. 20 (2008) 1966. [16] H.J. Bolink, E. Coronado, J. Orozco, M. Sessolo, Efficient polymer lightemitting diode using air-stable metal oxides as electrodes, Adv. Mater. 21 (2009) 79. [17] A.G. Fluxim, Dr. Beat Ruhstaller, Dorfstrasse 7, CH-8835 Feusisberg, Switzerland. [18] A. Koehnen, M.C. Gather, N. Riegel, P. Zacharias, K. Meerholz, Enhanced efficiency of multilayer organic light-emitting diodes with a low-refractive index hole-transport layer: an effect of improved outcoupling?, Appl Phys. Lett. 91 (2007) 113501. [19] R.J. Xie, N. Hirosaki, T. Takeda, Wide color gamut backlight for liquid crystal displays using three-band phosphorconverted white light-emitting diodes, Appl. Phys. Exp. 2 (2009) 022401.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Towards organic light-emitting diode microdisplays with sub-pixel patterning Fabian Ventsch, Malte C. Gather, Klaus Meerholz * Department of Chemistry, University of Cologne, Luxemburgerstr. 116, D-50939 Cologne, Germany
a r t i c l e
i n f o
Article history: Received 21 July 2009 Received in revised form 25 September 2009 Accepted 26 September 2009 Available online 12 October 2009 Keywords: OLED Top-emitter Microdisplay
a b s t r a c t We report on solution processed top-emitting organic light-emitting diodes (TEOLEDs) that are compatible with TFT- and CMOS-substrates, thus being applicable in full-color microdisplays. In this device geometry TEOLEDs offer highly tunable and controllable color-coordinates. Compared to the standard sRGB-colorspace the color gamut is increased by 18%. The CIE (Commission internationale de l’éclairage) color coordinates are more saturated in the red and green spectral region and comparable in the blue region. Brightness levels of 1000 cd/m2 are reached at voltages below 7 V for all colors which is within the operating voltage range of CMOS-backplanes. The polymeric emitters are crosslinkable and can be patterned in a UV-lithography process. The achieved minimum feature size is suitable for high resolution microdisplays. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction For several years the interest in organic light-emitting diodes (OLEDs) has shifted from rather simple devices such as monochrome displays to complex full-color active-matrix displays. In this context, increasing amounts of money are invested in the commercialization of displays based on OLEDtechnology. As a preliminary highlight, this trend resulted in the implementation of an 11 in. OLED-TV (XEL-1) by Sony in 2007. The main effort in this field is to increase display sizes. However, there is also an ever increasing demand for mobile TV and video equipment. Here it is necessary to decrease pixel size in order to build very small displays (typically below 1.5 in. diagonal) with high resolution. These socalled microdisplays are the key components in video goggles. By use of magnifying optics an enlarged image of the display is projected directly into the human eye. Today an overwhelming share of the display market is held by liquid crystal (LC)-technology and hence any new technology has to compete against it. In LC-displays white light from a backlight is converted into red, green and blue * Corresponding author. E-mail address: [email protected] (K. Meerholz). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.09.026
(RGB) by color filters laminated on top of the LC-cell. The gap between filter and LC-cells leads to partial illumination of adjacent sub-pixels and, therefore, to a loss in color gamut. This effect becomes worse with decreasing pixel size and hence puts a fundamental limit to the resolution of microdisplays. Underwood et al. [1] established a microdisplay in which the LC-cell was replaced by a white-emitting OLED (see Fig. 1a). This reduces the power consumption and simplifies the fabrication process of the microdisplays but does not solve the problem of the limited color gamut. Here, we propose a process for the fabrication of a high resolution microdisplay in which red-, green- and blue-emitting OLEDs form the sub-pixels of the display. To achieve this, thin films of RGB-emitting materials will be deposited from solution and be structured via direct UVlithography in order to define the color of corresponding sub-pixels (see Fig. 1b). The used electroluminescent materials are oxetane-functionalized and can be crosslinked through cationic ring opening polymerisation. In the presence of a photo acid generator (PAG) this reaction can be initiated by UV-light giving the material photoresist-like properties [2,3]. As light and color are generated in the same layer, crosstalk between sub-pixels is eliminated and a large color gamut is expected. The pixel size in a microdisplay
58
F. Ventsch et al. / Organic Electronics 11 (2010) 57–61
Fig. 1. (a) Schematic of a commercial OLED-based microdisplay using a polymeric white emitter and color filters. (b) RGB sub-pixel approach proposed here.
with VGA resolution (640 480 pixels) amounts to a few micrometers. We have already demonstrated that direct lithographic patterning of electroluminescent polymers yields minimum feature sizes down to 2 lm [3]. In contrast to direct lithography of solution processed OLED materials, vacuum deposition of small molecules through shadow masks is not a suitable option for the fabrication of microdisplays: The quality of the shadow masks suffers from material being deposited onto it over time. This leads to a gradual reduction of the actual pixel size and to an enhanced formation of disruptive particles on the OLED. For the small pixels of microdisplays, the required OLED quality is only ensured for very few evaporation procedures per shadow mask [4]. It is obvious that top-emitting OLEDs (TEOLEDs) are favorable over bottom-emitting ones (BEOLEDs) when working with opaque complementary metal oxide semiconductor (CMOS) backplanes. These active transistor backplanes are necessary to display complex content. There are two possible geometries for TEOLEDs: Inverted OLEDs with a transparent top-electrode [5–7] as well as cavity TEOLEDs [8–10]. The latter allow for better control of the emission characteristics due to the layer-thicknessdependent microcavity effect. Especially since many polymeric emitters show a broad emission spectrum (see Fig. 3c), the control of both, spectral width and peak wavelength, is beneficial to achieve primary color coordinates suitable for display applications. In general, a cavity TEOLED consists of a metallic mirror which is usually the anode of the OLED, a stack of organic layers, and a semi-transparent metallic cathode. In this Fabry–Perot-cavity-like geometry, light is reflected between the two metallic layers. Light with wavelengths matching the criterion for constructive interference after reflection at each metal layer is amplified and emitted through the semi-transparent mirror. Light with other wavelengths is weakened or even suppressed from being coupled out [11–13]. As a consequence, the cavity thickness determines the emitted peak wavelength, and the cavity quality defines the spectral width.
capable of efficiently injecting holes to the electroluminescent copolymers used in this work (EHOMO = 5.4 eV [15]). This problem can be circumvented by the addition of a molybdenum oxide (MoO3)-layer (5.3 eV [16]). When used in a cavity structure, MoO3 is preferable to the commonly used 3,4-poly(ethylendioxythiophen)-poly(styrolsulfonat) (PEDOT) for transparency reasons. We investigated TEOLEDs on glass substrates with a highly reflecting silver (Ag) anode due to our standard OLED processing. However, silver is not suitable in standard CMOS fabrication process. Thus, to demonstrate that the results are of relevance for a true CMOS process, we investigated in addition TEOLEDs containing an aluminum (Al)/(TiN)/(MoO3) anode on a CMOS dummy substrate. The silver anode, the HIL (MoO3) and the cathode layers (barium and silver) were vacuum deposited in a Leybold Univex 450 chamber. The emitting polymers were spincoated from solution in nitrogen atmosphere. Crosslinking was initiated by exposure to UV-light. A softcuring step at 90 °C on a hotplate is essential to allow chain propagation and promote further crosslinking. In crosslinked areas the polymer film becomes insoluble. Finally, the film is rinsed with toluene to remove non-exposed material. The subpixels in the future microdisplays will be developed in the same manner. Here, the entire layer is crosslinked to emulate the future conditions in microdisplays. At last, a postbaking step at 200 °C is applied. For comparison of current–voltage–luminescence (J–V–L) curves and for color point characterization, OLEDs have been investigated in the top-emitting as well as in the bottom-emitting geometry. The cavity TEOLEDs consisted of the following layer stack:
Glass=Ag ð150 nmÞ=MoO3 ð30 nmÞ==XR ð90 nmÞ or XG ð70 nmÞ or XB ð35 nmÞ==Ba ð4 nmÞ=Ag ð20 nmÞ: Using an Al mirror instead of an Ag mirror will change the optical characteristics of the cavity for a given layer thickness due to the difference in reflectivity. However, this can be compensated by re-adjusting the thickness of the emitting layer, thus, leading to identical spectral output. To study the influence on the OLED performance we fabricated a red-emitting device with Al mirror (CMOS dummy). The entire layer stack of the CMOS dummy was:
Si=Al ð100 nmÞ=TiN ð10 nmÞ=MoO3 ð30 nmÞ== XR ð100 nmÞ==Ba ð4 nmÞ=Ag ð20 nmÞ: The sequence of the BEOLED stack was:
Glass=ITO ð160 nmÞ=MoO3 ð30 nmÞ==XR ð80 nmÞ or XG ð80 nmÞ or XB ð70 nmÞ==Ba ð4 nmÞ=Ag ð150 nmÞ: Each diode has an active area of 7.9 mm2. 3. Results and discussion
2. Experimental
3.1. Device performance
On CMOS surfaces, titanium nitride (TiN) is usually used as hole-injection layer (HIL). Due to its rather high work function of about 5 eV vs. vacuum [14], TiN is not
In Fig. 2 current density, luminance and current efficiencies are presented for TEOLEDs. Brightness levels of 1000 cd/m2 for red and blue and 10,000 cd/m2 for green
59
were achieved at voltages of <7 V and a current density of <40 mA/cm2. Comparing the efficiencies of the TEOLEDs and BEOLEDs, similar values are achieved for the red and the green emitter (see Table 1). In this case the losses due to absorption at the semi-transparent cathode are compensated by the light amplification in forward direction due to microcavity effects. The decrease in efficiency of the blue emitter in the TEOLED case can be attributed to differences in optical and electrical device performance of TEOLEDs and BEOLEDs: the emission width of the polymeric blue emitter is broad with a significant amount of emission in the green region of the spectrum (see Fig. 3). In the BEOLED geometry this fraction of green leads to sky-blue emission. The layer thickness (70 nm) in this case is chosen to achieve optimum electrical performance (balancing efficiency and operating voltage). Deep blue emission with corresponding color coordinates is obtained in the TEOLED geometry at layer thicknesses of 35 nm (first maximum of emission) and 140 nm (second maximum). This is contrary to the optimum layer thickness in terms of efficiency and operating voltage. High leakage currents for the thin layers (35 nm) and high driving voltages for the thick layers (140 nm) are observed. Besides reducing the electrical performance, the spectral blue-shift leads to a decrease in current-efficiency as the brightness is corrected by the
2
a
Brightness [cd/m ]
2
Current Density [mA/cm ]
F. Ventsch et al. / Organic Electronics 11 (2010) 57–61
1200
10
4
800
10
3
400
10
2
10
1
0
4
5
6
7
8
9
10
Voltage [V]
Efficiency [cd/A]
b8 7 6 5 1 0 0
10 20 30 40 2 Current Density [mA/cm ]
50
0
52
54
0
53
0
0 57
0 49
0 40
47
0
48
0
0.2
60 6 0 62 10 69 0 0
59
0.4 0
Y
0
58
0
50
0.6
56
0
55
0
0.8
51
a
0
Fig. 2. (a) Current density (closed symbols) and brightness (open symbols) versus voltage, for red (triangles = Ag/MoO3-anode, stars = Al/ TiN/MoO3-anode), green (circles) and blue (squares) TEOLEDs (b) Current efficiency versus voltage (symbols see (a)). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Normalized Intensity
b
1.0 0.8 0.6 0.4 0.2 0.0 400 450 500 550 600 650 700 750 800
Wavelength [nm]
c
1.0
Normalized Intensity
0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 X
0.8 0.6 0.4 0.2 0.0 400 450 500 550 600 650 700 750 800
Wavelength [nm]
Fig. 3. (a) Color coordinates of TEOLEDs (squares) BEOLED (circles) and sRGB color space for CRTs (triangles). EL spectra of blue (squares), green (circles), red (triangles = Ag/MoO3-anode, stars = Al/TiN/MoO3-anode) TEOLEDs (b) and BEOLEDs (c) and Etfos simulations (solid lines).
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F. Ventsch et al. / Organic Electronics 11 (2010) 57–61
Table 1 Efficiency and color coordinates for RGB emitters in TEOLED and BEOLED geometry. XRAl
Mirror
XRAg
Mirror
XGAg
Mirror
XBAg
Mirror
TEOLED Layer thickness [nm] gmax [cd/A] (CIE-values)
100 1.8 (0.69; 0.31)
90 1.6 (0.68; 0.32)
70 7.2 (0.29; 0.66)
35 0.7 (0.14; 0.10)
BEOLED Layer thickness [nm] gmax [cd/A] (CIE-values)
/ /
80 1.2 (0.65; 0.34)
80 9.4 (0.28; 0.58)
70 2.0 (0.18; 0.26)
sensitivity of the human eye. In order to distinguish the fractions of eye-sensitivity correction and electrical device performance, we normalized the device efficiency by the overlap integral of the EL-spectrum and the eye-sensitivity curve and then compared these ‘‘quasi quantum efficiencies”. It turns out that 32% of the efficiency decrease can be attributed to the color shift to deeper blue. The remaining percentage (68%) of efficiency decrease has thus to be due to electrical device performance. Comparing the red-emitting TEOLED device featuring an Ag/MoO3 anode with the CMOS dummy (Al/TiN/ MoO3-anode), reduced current- and light output at a given voltage are observed in the latter case. This is mainly due to a decrease in electric field resulting from the enlarged layer thickness. The current efficiency is slightly increased while kmax is red-shifted. The increase in efficiency is probably due to an enlarged internal quantum efficiency due to a better charge carrier balance in the CMOS dummy device. Optical simulations show that the expected output is even reduced by a factor of 0.45 (see Supplementary data). With respect to the electrical characteristics of green- and blueemitting materials on a CMOS dummy device, we expect similar results. 3.2. Color characteristics In Figs. 3b and 3c the electroluminance spectra of bottom- and top-emitting OLEDs are shown. The experimental spectra were confirmed by optical simulations using Fluxim Etfos 1.3 [17]. Assuming an exciton density that is maximum at the HIL/EML interface and decays exponentially towards the cathode with a 1/e width of 20 nm [18], simulation and experimental results are in excellent agreement. Due to microcavity effects, narrowband emission is achieved in case of the TEOLED devices. In comparison to the BEOLEDs, this results in a higher color purity and, therefore, better primary CIE-values (see also Fig. 3a). Especially for the blue emitter much better color coordinates are achieved. Overall, our TEOLEDs achieve better values than required by the sRGB color space which defines the limits for computer monitors and HDTVs. In particular, the red and green emission is more saturated than required. In this case the slightly red-shifted emission of the CMOS dummy device compared with our standard TEOLED has no significant effect on the color coordinate. The blue nearly meets the sRGB specifications. By a comparison of the areas of the color triangles [19], we conclude that the color gamut is increased by 18% (see Fig. 3a). Calculated from the primary color coordinates, the fraction of emission required from each of the RGB TEOLEDs to obtain white emission, i.e. CIE(x, y) = (0.33; 0.33) is
(RGB) = (0.26; 0.61; 0.13). The corresponding maximum current efficiency for white light emission is 4.9 cd/A at a brightness level of 1000 cd/m2. This corresponds to a value of 5 cd/A achieved in BEOLED geometry [3], i.e. the properties are fully conserved. 4. Summary In conclusion, we investigated solution processed crosslinkable electroluminescent materials in top- and bottomemitting OLEDs. We showed the advantages of TEOLEDs over BEOLEDs in terms of color tuning. We also demonstrated that our TEOLEDs are suitable for high color gamut and high brightness displays. Due to their photoresist like properties, they can be directly patterned via UV-lithography and hence will allow for high resolution microdisplays. The next step will be to process and investigate the TEOLED stack and arrays of RGB sub-pixels on CMOS-backplanes for microdisplays. We already demonstrated, that on a CMOS dummy substrate the electrical and optical characteristics are essentially preserved. Acknowledgement We thank Herbert Metzner (University of Cologne) and the technical workshop for technical support, EU/Hypoled for funding and U. Vogel and D. Kreye (both Fraunhofer IPMS, Dresden) for fruitful discussions and for the supply with CMOS dummy wafers. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.orgel.2009.09. 026. References [1] I. Underwood, G. Kelly, R. Woodburn, P-OLED microdisplays, in: IEEE LEOS Annual Meeting Conference Proceedings, vol. 1–2, 2006, p. 438. [2] C.D. Muller, A. Falcou, N. Reckefuss, M. Rojahn, V. Wiederhirn, P. Rudati, H. Frohne, O. Nuyken, H. Becker, K. Meerholz, Multi-colour organic light-emitting displays by solution processing Nature 421 (2003) 829. [3] M.C. Gather, A. Koehnen, A. Falcou, H. Becker, K. Meerholz, Solutionprocessed full-color polymer organic light-emitting diode displays fabricated by direct photolithography, Adv. Funct. Mater. 17 (2007) 191. [4] J.W. Hamer, A. Yamamoto, G. Rajeswaran, S.A. Van Slyke, Mass production of full-color AMOLED displays, SID Dig. 39 (2005) 1902. [5] G. Parthasarathy, G. Gu, S.R. Forrest, A full-color transparent metalfree stacked organic light emitting device with simplified pixel biasing, Adv. Mater. 11 (1999) 907. [6] H.K. Kim, D.G. Kim, K.S. Lee, M.S. Huh, S.H. Jeong, K.I. Kim, T.Y. Seong, Plasma damage-free sputtering of indium tin oxide cathode layers
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