Japanese Journal of Applied Physics Vol. 46, No. 1, 2007, pp. 182 186 #2007 The Japan Society of Applied Physics Emiflective Display with Integration of Reflective Liquid Crystal Display and Organic Light Emitting Diode Bo-Ru YANG, Kang-Hung LIU, and Han-Ping D. SHIEH 1 Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan 30010, Republic of China 1 Department of Photonics and Display Institute, National Chiao Tung University, Hsinchu, Taiwan 30010, Republic of China (Received August 18, 2006; accepted October 3, 2006; published online January 10, 2007) A novel emi-flective display which integrates a reflective liquid crystal display (R-LCD) and an organic light emitting diode (OLED) was demonstrated, whose OLED achieved a gain factor of 8 in contrast ratio (CR) compared with the conventional OLED. Under the high light ambience, the R-LCD is sustained with the CR of 10 : 1 at the viewing angle between 55 ; while in the dim ambience, the OLED is operated with the CR of 5000 : 1 at 50. By replacing the backlight system with OLED, emi-flective display has the benefits of lighter weight (<90%), thinner form factor (<40%), and lower power consumption (<2%, under sunlight) compared with the conventional LCD; therefore, to be very applicable for mobile products. [DOI: 10.1143/JJAP.46.182] KEYWORDS: emi-flective, reflective, transflective, liquid crystal display, organic light emitting diode 1. Introduction A conventional transmissive liquid crystal display (T- LCD) has the characteristics of good color saturation and high contrast ratio except operating under the high light ambience. As for mobile applications, T-LCDs consume high power owing to the need of backlight. On the contrary, R-LCDs consume much less power. In addition, due to using the ambient light as the light source, R-LCDs have the merits of thin form factor, light weight, and good sunlight readability. Nevertheless, R-LCDs cannot be operated under the dim ambience. To compensate the deficiency of the R- LCDs, a transflective LCD composed of a T-LCD and an R-LCD has been developed, as shown in Fig. 1. 1) A transflective LCD can show clear images at most of ambience. Nonetheless, the thickness cannot be largely reduced due to the need of backlight, and the aperture ratio is lowered due to the pixel being divided into the reflective and the transmissive regions. Replacing the backlight module with an OLED is a plausible method to further reduce the volume and the weight of the transflective LCD. The combination of an OLED and a R-LCD was first proposed by Lueder in 2000. 2) Afterwards, the hybrid device structures, 3 6) the pixel and the driving circuit design, 7) and the optical performance 8) were reported. These proposed stacked devices, as shown in Fig. 1, can exhibit high-quality image at most of the ambience. Furthermore, the stacked devices have much lighter weight, smaller form factor, and higher aperture ratio compared with those of the conventional transflective LCDs due to replacing the bulky backlight system with an OLED. However, the OLED and the R-LCD were stacked rather than integrated, which made the air gap come into being. The air gap reduces the contrast ratio of the OLED and the R-LCD. To further resolve this issue, we report on the integration of the OLED and the R-LCD in a sequential process. This paper neologizes the integrated structure as an emiflective display because the integrated structure is composed of the emissive device OLED and the reflective E-mail address: ybr.eo93g@nctu.edu.tw 182 device R-LCD, which follows the example of transflective display with the combination of the transmissive and the reflective LCDs. 2. Proposed Structure The proposed emi-flective display configuration comprises a reflective electrode, the organic layers, a transparent electrode, a thin-film encapsulation (TFE) layer, a lower electrode of LCD, a lower alignment layer, an LC layer, an upper alignment layer, an upper electrode of the LCD on the top glass, and a circular polarizer, as shown in Fig. 2. It should be noted that the proposed emi-flective display device has only two substrates; besides, there is no air gap inside the cell. The fabrication process of the integrated device can be carried out by passivating the OLED with the TFE, and then depositing the ITO and polyimide on the TFE layer, followed by sandwiching the prepared top-substrate, and finally injecting the liquid crystal. To drive the R-LCD and OLED of the integrated display respectively, we designed an active matrix driving circuit, as shown in Fig. 2. 7) The TFT backplane is positioned at the lower substrate of OLED. The point A in Fig. 2 is connected from the lower electrode of the LCD to the backplane through the pre-formed contact hole. As the switch (SW) is open, the circuit operates as a conventional LCD driving circuit; when SW is closed, the LCD is set to be normally white and the OLED is operated. One of the major issues of integrating the OLED and R- LCD is the passivation of the OLED. In order to protect the OLED from the thermal damage during the fabrication, the polyimide shall be able to be processed in low temperature (<120 C). Besides, the OLED shall be well-protected from the intrusion of polyimide. To simplify the fabrication, the proposed TFE was replaced with a glass lid passivation which can also provide an air-gap-free integrated device with the OLED and the R-LCD. By integrating the individual OLED and R-LCD, the air gap between the two glass lids is eliminated, hence, not only the parallax effect can be decreased, but also the contrast ratio and the viewing angle can be increased. The degradation of contrast ratio is ascribed to the undesired reflection
Fig. 1. The configurations of conventional transflective and stacked emi-flective displays. Fig. 2. The proposed configuration and driving circuit of emi-flective display. caused by total internal reflection, interface scattering, and the partially reflection from the incident light. The undesired reflected light has different phase and wavelength with the desired reflected light, causing the lowered brightness in the bright state and the light leakage in the dark state. To reduce the undesired reflection, inserting an index matching layer between the two glass lids was commonly adopted; however, this makes the separation between OLED and R-LCD thicker which leads to a more serious parallax. By eliminating the glass lid and the air gap, our designed structure reduced the interfaces and thickness of the hybrid device, thus, improved the undesired reflection and parallax simultaneously. 3. Device Fabrication and Measurement The emi-flective display device is carried out by covering a prepared LCD on the top-emission OLED, which is the alternative device structure for demonstrating the air-gap free emi-flective device as shown in Fig. 3. In this 183 structure, the integration was achieved without putting the OLEDs in the high temperature process which would damage the OLEDs, and also eliminated the air gap. In addition, we chose the LC material as twist nematic type, the hole injection material as CF x, the hole transporting material as N,N 0 -bis-(1-naphthyl)-n,n 0 -diphenyl-1,1 0 -biphenl-4,4 0 -diamine (NPB), the emitting material as tris(8-hydroxyquinoline) aluminum complex (Alq 3 ), the electron injection material as LiF, and the transparent metal as aluminum. The OLED device structure is: indium tin oxide (ITO)/NPB (20 nm)/alq 3 (150 nm)/lif (1 nm)/al (15 nm). Moreover, to identify the different reflecting abilities of the transparent cathode (region A), which represents the glare reflector, and the diffusive reflector (region B), the integrated device is divided into two regions. The diffusive light is more suitable to simulate the sunlight ambience than collimated light. As a result, the diffusive light is adopted as the ambient light source in the measurement, which is based on the conoscopic method. 10)
Integrated R-LCD (glare reflector, A) Integrated R-LCD (diffusive reflector, B) (c) Fig. 3. Schematic diagram of fabricated emi-flective display structure and its iso-contrast contour diagrams operated in reflection mode with glare reflector and (c) diffusive reflector. The measuring mechanism can be described as follows: First, the diffusive ambient light source is reflected by the sample. Second, the light with the same inclination propagates in parallel to enter the lens system. Third, the parallel rays will be converged to the detector. Finally the iso-contrast contour is formed and stored by the data processor. In order to measure the optical performance of the device under the different light ambiences, the transmission (OLED) mode is measured under the 30 nits light ambience, and the reflection (R-LCD) mode is measured under the 600 nits light ambience. 4. Experimental Results After measured the devices by conoscope, the different reflecting images of the diffusive and the glare reflector were obtained. The iso-contrast contour diagrams are shown in Figs. 3 and 3(c), where the contrast ratio of B-region is higher than that of A-region owing to the reflective ability of the diffusive reflector is superior to that of the transparent cathode. The comparison between A- and B-regions concludes that using the top-emission OLED as the emissive part, the transparent electrode should be textured as a diffusive reflector to achieve high reflective ability. By this modification, the aperture ratio of the emi-flective display can be 100% which is much higher than conventional transflective displays. However, it is difficult to form a diffusive bumpy structure on the cathode, since the transparent cathode is very thin and tend to be damaged. Therefore, the best approach to form a diffusive transflector on the transparent cathode is to laminate a TFE layer on the cathode first, and then to form the diffusive structure on the TFE layer. To test the performances of the original, stacked, and integrated devices, the iso-contrast contour diagrams of each configuration of device were measured and shown in Fig. 4. The contrast ratio decreased to one-eighth of that of the original R-LCD after stacking the R-LCD on the OLED due to the air gap effect. Nevertheless, after integration, the contrast ratio of the reflective mode was a factor of 6 higher 184 than that of the stacked device. This result demonstrated that the air gap effect degrades the functions of the R-LCD part of the emi-flective display greatly. On the other hand, Figs. 4(d) and 4(e) show that after stacking the OLED with the R-LCD, the contrast ratio of the OLED in the dim ambience does not decrease, but be enhanced. These results were due to the aid of R-LCD cell acting as a light valve to minimize the effect of the ambient light; hence, the contrast ratio is enhanced. Though the viewing angle of the OLED became narrowed due to the confinement of the R-LCD placed above, the contrast ratio could be boosted to 5000 : 1 at 50 owing to the aid of the upper R-LCD. It should be noted that the contrast ratio after integration was enhanced to a factor of 8 higher than that of the original and stacked OLED. 5. Discussion The contrast ratio and the viewing angle performance of the hybrid display are significantly enhanced by eliminating the air gap between the OLED and the R-LCD. Comparing the integrated hybrid device with the stacked one, the contrast ratios are improved to a factor of 6 and 8 in the reflective and the emissive modes, respectively. Moreover, comparing the reflective mode of the integrated hybrid device with that of the reported stacked device, 2) the former still yields a higher viewing angle performance than the latter, as shown in Table I. In addition, comparing the measurement result with others, 8) the contrast ratios of 10 : 1 in the integrated structure and 2:1 in the stacked structure at the viewing angle of 55 in the reflective mode were obtained, respectively, which demonstrated the enhancement after eliminating the air-gap. 6. Conclusions We demonstrated the integration of the OLED and the R- LCD through which the contrast ratio of the OLED achieved a gain factor of 8 comparing to the original and stacked OLEDs. Though the viewing angle of the OLED was suppressed by the upper liquid crystal layer, the contrast
(c) (d) (e) (f) Fig. 4. Measured iso-contrast contour diagrams of different configuration: original, stacked, and (c) integrated R-LCD; (d) original, (e) stacked, and (f) integrated OLED. Table I. The viewing angle performance comparison of the different hybrid structures. (reflection mode) (transmission mode) Light source of the measurement setup Stacked structure 2Þ Stacked structure 8Þ Integrated structure 7:1at 25 2:1at 50 10 : 1 at 55 Not available Not available 5000 : 1 at 55 Not available Collimated Diffusive ratio of the OLED was boosted to 5000 : 1 at the viewing angle between 50 by being combined with the R-LCD, which is very applicable for mobile products. On the other hand, the R-LCD after being integrated with the OLED sustained the contrast ratio of 10 : 1 at the viewing angle between 55 which can compensate the power-consuming issue of the OLED in the bright ambience. Moreover, emiflective display has the benefits of lighter weight (<90%), thinner thickness (<40%), and lower power consumption (<2%, under sunlight) in comparison with the conventional LCDs for the laptop products. 10) Therefore, emi-flective display is a promising technique in mobile applications. 185 Acknowledgements The authors would like to express their appreciation to Chuan-Wei Hsu, Hsing-Lung Wang, Shih-Nan Lee, Wen- Sheng Wang, Professor Fang-Chung Chen, and professor Chin-Hsin Chen for their valuable discussion and technical assistance. This work is partially supported by MOE ATU Program Aim for the Top University #95W803. 1) X. Zhu, Z. Ge, T. X. Wu, and S. T. Wu: J. Disp. Technol. 1 (2005) 15. 2) E. Lueder and M. Randler: SID Int. Symp. Dig. Tech. Pap. 2 (2000) 1025.
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