10.1117/2.1200706.0772
Organic light-emitting devices
and displays offer high contrast
and energy recycling
Chung-Chih Wu
New device structures achieve better visibility in bright surroundings
and can be integrated with solar cells to extend battery lifetimes.
Organic light-emitting devices (OLEDs) are becoming an important next-generation display technology due to advantages
including rapid response and vivid color. Both conventional
bottom-emitting and top-emitting OLEDs are usually composed of a reflective back electrode, organic layers, and a semitransparent electrode through which light exits.1, 2 OLEDs reflect
ambient light off their back electrode. Such reflection seriously
degrades the contrast of an OLED display when it is used outdoors or under bright lights, and thus the sharpness and colors
of displayed images.
Polarizers or filter films laminated on the surface of the
display panel can reduce reflection of ambient illumination.3
Using such contrast-enhancement films, however, substantially
reduces the power efficiency of the displays (to below ∼40% of
the device efficiency). The films also add complexity to a display’s design and add to the cost of fabrication. We would like
to make OLED structures that offer low-ambient-light reflection
yet retain as much emission as possible. This would allow use to
enhance the display contrast with less impact on cost and efficiency.
A few high-contrast OLED structures have been reported that
reduce ambient-light reflection and thus the contrast ratio of
OLED displays.3–6 However, these all deal with bottom-emitting
OLEDs and may not be readily adapted for those that emit from
the top. (Top-emitters have some technical advantages, such as
larger aperture ratios, over bottom-emitting devices for highperformance active-matrix OLED displays.) Furthermore, these
previous structures all required the insertion of extra layers into
the active region of devices (i.e., between the anode and the
cathode). Thus one must be concerned about the layers’ optical
as well as electrical characteristics such as carrier injection and
Figure 1. (a) Structure of a top-emitting organic LED (OLED); (b)
cross section of a low-reflection top-emitting active-matrix OLED
(AMOLED); (c) photos of (left) a conventional bottom-emitting
AMOLED display and (right) a low-reflection top-emitting AMOLED
display.
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10.1117/2.1200706.0772 Page 2/3
transport. Recently, we reported a high-contrast OLED structure that exploits the optical characteristics of the electrodes and
anti-reflection coatings deposited outside the active region of devices alone, thus reducing the impact on electrical characteristics
and device complexity (see Figure 1).7 We have implemented
Figure 3. (a) The schematic diagram showing the structure of the
implemented integration of a poly-Si thin-film transistor (TFT), an
amorphous-Si:H p-i-n solar cell, and a top-emitting OLED; (b) (left) a
photo showing the fabricated sample under testing, in which the small
bright spot is emission of the OLED driven by the TFT; (center) a closer
view of the pixel area and the layout; (right) an optical micrograph of
one complete pixel. ITO: indium tin oxide.
Figure 2. (a) Structure of a stack that incorporates an OLED and a solar cell. (b) Photos of the OLED and solar-cell stack and a conventional
bottom-emitting OLED. (c) Current-versus-voltage characteristics of
the solar cell in dark and under air mass (AM) 1.5 illumination. The
inset of (c) shows the spectral response of the solar cell under external
illumination.
this structure in top-emitting OLEDs. The devices produced offer better efficiency than conventional OLEDs laminated with
polarizers, and are compatible with active-matrix backplanes
(i.e., the thin-film transistor driving circuits). We also demonstrated active-matrix OLED (AMOLED) displays that incorporate these high-contrast top-emitting OLEDs and thus have
improved readability in brightly lit environments.
With all previous contrast-enhancement approaches, incident
photons have been simply absorbed and wasted. Moreover, a
significant portion of the internal OLED emission fails to escape
the device and is also wasted. This wasted energy can be recycled by placing a solar cell in the back of an OLED (see Figure
2) that can both reduce the reflection and provide useful electrical power. The portion of internal OLED emission that cannot emerge is also absorbed by the solar cell and converted.
This feature is attractive for mobile electronics, which are typically limited by power usage considerations. With such energy
recycling, the devices themselves would add a self-powering
function as well as achieve higher system-level efficiency. Both
attributes could contribute to longer battery lifetimes for mobile
electronics.
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10.1117/2.1200706.0772 Page 3/3
Recently, we reported integrating OLEDs and solar cells. The
resulting structure not only exhibits a contrast superior to the
conventional polarizer approach but also is capable of recycling
both incident ambient illumination and internally-generated
OLED emission (see Figure 3).8 Luminous ambient-light reflection as low as ∼1% (superior to that achieved with polarizers)
can be achieved without compromising the electroluminescent
efficiency for high-contrast display applications. By inserting the
solar cell between the driving low-temperature polycrystallineSi (LTPS) thin-film transistor and the pixel OLED, we also
demonstrate the energy-recycling pixel structure for AMOLED
displays (see Figure 3).9
In summary, careful design of the optical structures and
integration of solar cells allow us to construct OLEDs for highperformance display applications. These are high-contrast highaperture-ratio energy-recycling self-powering devices that are
compatible with active-matrix backplanes.
References
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J. Brown, High-efficiency top-emitting organic light-emitting devices, Appl. Phys. Lett.
81, p. 3921, 2002.
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9. C.-Y. Yang, T.-Y. Cho, Y.-Y. Chen, C.-J. Yang, C.-Y. Meng, C.-H. Yang, P.-C. Yang,
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Author Information
Chung-Chih Wu
Department of Electrical Engineering
Graduate Institutes of Electro-optical Engineering
and Graduate Institute of Electronics Engineering
National Taiwan University
Taipei, Taiwan
http://www2.ee.ntu.edu.tw/∼oled/
Chung-Chih Wu received his PhD in electrical engineering from
Princeton University. He is currently a full professor of Electrical Engineering at the Graduate Institute of Electro-optical Engineering, and the Graduate Institute of Electronics Engineering
at National Taiwan University. His research interests include organic semiconductors for optoelectronic and electronic devices,
displays, and lighting technologies. He has served on committees for conferences including the International Display Manufacturing Conference (IMDC), the Conference on Lasers and
Electro-Optics (CLEO), as well as the annual meetings of the
IEEE Lasers and Electro-Optics Society (LEOS), the Society for
Information Display (SID), and the Optical Society of America
(OSA).
c 2007 SPIE—The International Society for Optical Engineering