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Illumination & Displays

Transparent organic LEDs for new lighting applications

The inclusion of a capping layer above the cathode can enhance the transmittance and efficiency of a device.
7 November 2013, SPIE Newsroom. DOI: 10.1117/2.1201310.005197

Organic LEDs (OLEDs) are light sources that do not produce excessive heat and do not contain toxic materials.1 The light emitted from OLEDs is UV-free and can therefore be used to illuminate biological substances without causing harmful effects. OLEDs are ultra-thin (about 150nm), have very low weight, and can be tailored to many formats. A specific type of OLED—a so-called transparent OLED (TOLED)—is bidirectional, i.e., it emits light from its top and bottom surfaces (see Figure 1).2–4 It is vital that all components in a TOLED are transparent so that the simultaneous top and bottom light emission can be achieved.

To ensure transparency of TOLEDs, the thickness of their cathodes should not exceed 20nm. Transparent cathodes in TOLEDs usually consist of a multilayer metallic thin film.3, 4 However, limited light transmission through the cathode means that the efficiency of top emission is lower than that of the bottom emission. As such, the asymmetric structure of the light propagation causes different electroluminescence (EL) spectra for the top and bottom emissions from TOLEDs.


Figure 1. A schematic illustration of a typical transparent organic LED (TOLED). The capping layer and transparent cathode are key components in the device.

To address the problem of asymmetry in the EL spectra, we designed a TOLED that uses a capping layer (CL), with a unique optical function.2 The use of the CL does not disturb the electrical characteristics of the device and therefore obviates the need to alter the stack structure. Our CL is an optical dielectric that can enhance the transmittance of the device and the top-emission efficiency. Including the CL on the cathode produces two interfaces, i.e., air/CL and CL/cathode.


Figure 2. Photographs of a white TOLED. (a) The device is switched off and is completely transparent. (b) The device is operated with a low current and is partly transparent. (c) The device is operated at a saturation current level and emits white light. The effective emission area of the TOLED is 78×78mm2.

Figure 3. (a) The concept of using a TOLED to make a tunable color temperature OLED. (b) Photographs of the blue OLED and the orange TOLED that were optically mixed to produce the white OLED.

Due to the complexity of multilayer interference, it is difficult to establish a straightforward optical principle that achieves the EL spectral matching of bottom and top emissions in our TOLED. We achieve high transmittance and high top-emission efficiency by adjusting the CL thickness so that we created destructive interference between the light components that are reflected at the two interfaces (Eair/CL and ECL/cathode). In this condition, generated light travels toward the top side of the TOLED without being diminished by the reflected components of Eair/CL and ECL/cathode. We can achieve high total efficiency (sum of the top and bottom efficiencies) by adjusting the CL thickness so that constructive interference takes place between Eair/CL and ECL/cathode. The constructively interfered light component therefore adds to the bottom-traveling component to boost the efficiency. However, it is difficult to simultaneously enhance the total and top emissions because the interference conditions for achieving high transmittance and high total efficiency are not the same. We observe that as the CL thickness is changed the peak EL intensity varies, but the peak wavelengths remain constant. The measured intensity ratio can therefore be used to achieve spectral matching. In our experiments, we obtain spectral matching at the CL thickness that corresponds to the maximum total emission.

We created a white TOLED (see Figure 2) by combining emissive layers of phosphorescent red, phosphorescent green, and fluorescent blue. When no current is applied to the device, the panel is transparent and similar to an ordinary window. When the current is slightly above the operating level, the TOLED generates noticeable light intensity, but is still somewhat transparent. At high current levels, the TOLED is no longer transparent and emits white light in both directions.

We are also able to make OLEDs with tunable color temperatures. We modulate the white characteristic (color temperature) by optically mixing a reddish-orange with blue, and varying their relative intensities. This mixing is achieved by vertically stacking two independent OLEDs, the top one being a TOLED. The efficiency of a blue OLED is relatively low, and therefore it is better to place this OLED in the lower portion and the orange OLED in the upper portion. We are able to tune our device within a temperature range of 1500–10,000K, which covers the full Planckian locus (the path of color temperature on a chromaticity diagram). Our approach (see Figure 3) does not require fine patterning and is therefore easy to implement.

The unique features of TOLEDs can be used to provide several new lighting formats. For example, a TOLED can be used as an ordinary window when it is light and as a luminary when it is dark outside. We have shown that capping layers can be used to raise the performance of current TOLEDs. We are now working towards integrating interactive functionalities, such as atmospheric and soothing lighting conditions, into future TOLEDs.

Financial support for this work came from the Electronics and Telecommunications Research Institute (ETRI) Internal Research Fund of the Ministry of Science, ICT and Future Planning, and from the Ministry of Knowledge and Economy/Korea Evaluation Institute of Industrial Technology.


Jaehyun Moon, Jin Woo Huh, Chul Wong Joo, Jun-Han Han, Jonghee Lee, Hye Yong Chu, Jeong-Ik Lee
Electronics and Telecommunications Research Institute (ETRI)
Daejeon, South Korea

Jaehyun Moon obtained a materials science and engineering PhD from Carnegie Mellon University. His current research interests include flexible electronics, functional nanomaterials, and OLEDs.

Jin Woo Huh received her PhD in electrical engineering from Korea University. She currently researches transparent OLEDs and light extraction techniques.

Chul Wong Joo has an MS in polymer science and engineering of organic electronic devices from Dankook University, Korea. His research interests include device architectures in OLED devices and next-generation displays.

Jun-Han Han received his MS in electronic engineering from Hanyang University, Korea. His research topics include OLED lighting applications and display circuits.

Jonghee Lee has a PhD in chemistry from the Korea Advanced Institute of Science and Technology. His main research interests are solution process-based OLEDs and their device structures.

Hye Yong Chu gained her PhD in information display from Kyung Hee University, Korea. She studies novel device architectures in OLED and leads the Next-Generation Display Department at ETRI.

Jeong-Ik Lee received his PhD in chemistry from the Korea Advanced Institute of Science and Technology. His main research interests are OLED materials and devices. He leads the OLED Research Center at ETRI.


References:
1. B. W. D'Andrade, S. Forrest, White organic light-emitting devices for solid-state lighting, Adv. Mater. 16, p. 1585-1595, 2004.
2. V. Bulvoic, G. Gu, P. E. Burrow, M. E. Thomson, S. Forrest, Transparent light-emitting devices, Nature 380, p. 26-29, 1996.
3. J. W. Huh, J. Moon, J. W. Lee, D.-H. Cho, J.-W. Shin, J.-H. Han, J. Hwang, C. W. Joo, H. Y. Chu, J.-I. Lee, The optical effects of capping layers on the performance of transparent organic light emitting diodes, IEEE Photonics J. 4, p. 39-47, 2012.
4. B. J. Chen, X. W. Sun, S. C. Tan, Transparent organic light-emitting devices with LiF/Mg:Ag cathode, Opt. Express 13, p. 937-941, 2005.