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A pixel partition scheme assisted with patterned or center-hollowed microlens-array films (MAFs) was proposed to improve the optical characteristics and electrical properties of organic light-emitting diodes (OLEDs). In our optical simulation results, a pixel of 1 × 1 mm2 with a center-hollowed MAF has a 42% luminance enhancement; however, after dividing the large pixel into ten by ten smaller pixels of 100 × 100 μm2, the partitioned units with a corresponding center-hollowed MAF can have a 104% luminance enhancement under the same total active area and the same optical power of organic emitters. Furthermore, a significant 127% luminance enhancement by the introduction of a high-refractive-index substrate can be obtained.
©2010 Optical Society of America
1. Introduction
A fluorescent lamp typically has power efficiency (the luminous efficacy of a source) of 80 lm/W; however, mercury vapor raises toxic and environmental concerns [1]. Solid-state light sources, i.e., organic light-emitting diodes (OLEDs) and inorganic light-emitting diodes (LEDs), are both free from these concerns and are environmentally friendly. In particular, OLEDs have several advantages over LEDs, such as being large-area, thin, and diffusive light sources; having flexible and transparent substrates; and being able to operate at a low temperature. However, planar OLEDs suffer from low light-extraction efficiency; hence, surface-texturing techniques for the extraction of the substrate mode [2–11], internal out-coupling structures for the extraction of the organic/ITO mode [12–14], and high-refractive-index substrates [10,15,16] are often applied. Moreover, the power efficiency of an OLED reaches 90 lm/W by combining a high-refractive-index substrate with periodic pyramidal out-coupling structures [10]. A microlens-array film (MAF) attachment is often regarded as a reliable and easily fabricated surface-texturing approach. In our previous investigations, both the relative luminance and relative luminous power achieved by applying single-period or regular MAFs increase with a higher fill factor and higher height ratio [2,8,9]. Nevertheless, high-efficiency regular MAFs usually cause serious image deterioration. Therefore, in contrast with a “fully centered” MAF [17], we proposed a “center-hollowed” MAF [7] to avoid the light-trapping phenomenon occurring at the edge of the microlens and to obtain better image quality and higher efficiency simultaneously.
For lighting purposes, large total-emitting-area OLEDs are needed for generating sufficient luminous flux, e.g., 5,000 lm, for a typical office luminary or lighting fixture [1]. However, luminance and current density strongly drooped when the active area of an OLED was increased [18]. This electrical falloff can be attributed to the spread of current leakage on the ITO surface, which accounts for the voltage drop as the current transports along the active area. Furthermore, large-area OLEDs also suffered from joule heating-induced brightness inhomogeneity [19]. Besides, a lager pixel more likely encounters a catastrophic short-circuit defect during fabrication; therefore, a series-connected small pixelated OLED architecture can be fault tolerant and scalable to a large total-emitting area [20]. We have discovered that a larger active area with a regular MAF results in higher luminance enhancement [7]. However, large-area OLEDs suffer from several disadvantages as mentioned above [18–20]. On the other hand, a smaller active area with a center-hollowed MAF leads to higher luminance enhancement [6]. Furthermore, relative luminance at normal direction is shown in Fig. 1 by applying a center-hollowed MAF versus a regular MAF with varying pixel sizes. In summary, center-hollowed MAFs are more efficient with a small-area OLED for both optical and electrical characteristics. Therefore, we decided to combine many small-area OLEDs with center-hollowed MAFs to replace single large-area OLEDs with regular MAFs.
Fig. 1 Relative luminance at normal direction by applying center-hollowed MAFs vs. regular MAFs with varying pixel sizes.
2. Prerequisite
As long as bare OLED devices have the same total active area and luminous power given by the organic emitter, the luminance at normal direction and the luminous power between the original single large pixel (1 × 1 mm2) and the small partitioned pixel units (i.e., four units of 500 × 500 μm2, sixteen units of 250 × 250 μm2, twenty-five units of 200 × 200 μm2, and one-hundred units of 100 × 100 μm2) are supposed to be equal. In Table 1 , the relative luminance at normal direction and the relative luminous power are normalized to the un-partitioned single 1 × 1 mm2 pixel; the result satisfies our prerequisite. In other words, the optical characteristics were almost unaffected by the pixel partition scheme.
Table 1. Simulated relative luminance at normal direction and relative luminous power of the partitioned pixel units with the spacing of one row of microlenses, all under the same total emissive area and total power
The parameters of the OLED structure were reflective cathodes of 86% reflectivity/organic emitter (120 nm)/ITO (100 nm)/glass (thickness of 0.7 mm and area of 10 × 10 mm2)/MAF; the cathode reflectivity of the Al/Alq3 interface was calculated by a transfer matrix method [21]. Also, the area and thickness of the attached MAF were 5 × 5 mm2 and 100 μm, respectively; the microlenses of the MAF were 50 μm in diameter, 25 μm in height, and rectangularly arranged with a period of 53 μm in both rows and columns. For simplicity and generality, the emitter of the isotropic emission was assumed rather than calculated apodization [11]. The 3D Monte Carlo based ray-tracing simulation [11] was employed, and Fresnel loss was also considered.
3. Results and discussion
The optical characteristics of partitioned pixel units attached with either regular or center-hollowed MAFs having the spacing of one, two, and three rows of microlenses in between the units were evaluated. Figure 2 shows the pixel partition configuration with center-hollowed MAFs having the spacing of one row of microlenses.
Fig. 2 Partitioned pixel units with center-hollowed MAFs have spacing of one row of microlenses (red circles). The white squares without microlenses represent emissive units of (a) 500 × 500 μm2, (b) 250 × 250 μm2, (c) 200 × 200 μm2, and (d) 100 × 100 μm2.
3.1 Angular luminous intensity
The angular luminous intensity shown in Figs. 3 and 4 are all normalized to luminous intensity at normal direction of the un-partitioned 1000 × 1000 μm2 pixel. From Fig. 3, the viewing-angle characteristic of the un-partitioned pixel without the MAF is close to a Lambertian emitter. The relative luminous intensity at normal direction of the OLED with the center-hollowed MAF was lower than that with the regular MAF.
Fig. 4 Angular relative luminous intensity of partitioned pixel units are equal to (a) 500 × 500 μm2, (b) 250 × 250 μm2, (c) 200 × 200 μm2, and (d) 100 × 100 μm2, respectively.
As we divided the large pixel into sixteen parts, the relative luminous intensity curve of the partitioned pixels with center-hollowed MAFs and regular MAFs begin to split, as shown in Fig. 4(b). If we further divide the pixel into one hundred units and insert three rows of microlenses in between the units, the separation of the two groups will become apparent, as shown in Fig. 4(d). Furthermore, the partitioned units with center-hollowed MAFs all had better viewing-angle characteristics than the un-partitioned pixel without the MAF and better relative luminous intensity at normal direction than that of the partitioned pixel units with the regular MAF. However, the valley in the angular relative luminous intensity plot resulted in a small, relative luminous power decrease shown in Section 3.3. That valley resulted from the neighboring center-hollowed region seen by the emissive units. Thus, more rows of microlenses in between partitioned units let that valley move to a larger viewing angle; larger units make a wider valley. However, each 500 × 500 μm2 unit encountered only two center-hollowed regions in rows and columns; thus, no sharp valley was observed.
3.2 Luminance at normal direction
Considering the relative luminance at normal direction (normalized to that of 1000 × 1000 μm2 pixel of either n sub = 1.52 or 1.72), the regular MAFs showed enhancement ranging from 53% to 59% for the refractive index of the substrate, n sub = 1.52, and from 77% to 85% for n sub = 1.72, which was irrelevant to the area of the units and the number of rows of microlenses in between the units. This is because for regular MAFs, the microlenses on top of the total emissive area are constant; thus, the drawback of lens-edge-trapping [7] is also about constant. However, the center-hollowed MAF showed as much as a 104% enhancement for n sub = 1.52 and a 127% enhancement for n sub = 1.72. As the area of the single unit shrinks logarithmically, the relative luminance of an OLED with a center-hollowed MAF will exhibit an approximate linear increment; as the rows of the microlenses in between the units increase, the relative luminance of an OLED with a center-hollowed MAF will also increase, as shown in Fig. 5 . Besides, the upper bound for n sub = 1.52, i.e., 115% of the relative luminance enhancement by inserting rows of microlenses in between the units of 100 × 100 μm2 and the center-hollowed MAF, can be inferred from Fig. 1. In summary, the pixel partition scheme with a center-hollowed MAF did boost the relative luminance much higher than that with a regular MAF.
Fig. 5 Relative luminance at normal direction of the units attached with center-hollowed MAFs or regular MAFs vs. the area of a single unit with varying the spacing of rows of microlenses; the refractive indices of the substrates are (a) 1.52 and (b) 1.72.
3.3 Luminous power
Considering the relative luminous power (normalized to that of 1000 × 1000 μm2 pixel of either n sub = 1.52 or 1.72), the un-partitioned OLED with a center-hollowed MAF was shown to be higher than that with a regular MAF. The partitioned units with a regular MAF also showed little dependence with the area of the single unit, while the partitioned units with a center-hollowed MAF showed linear dependence with the logarithmical shrinkage of the area due to the valleys in Fig. 4. Furthermore, in Fig. 6 , as the number of microlens rows in between the units increased, the relative luminous power of the OLED with the center-hollowed MAF decreased a little, but that with the regular MAF was almost unaltered. Power is the summation of the sinθ-multiplied angular luminous intensity. Because from Fig. 4 the large-angle intensity distribution for the center-hollowed MAF varied little compared to the small-angle intensity distribution, the relative luminous power enhancement was negligibly affected by the area of each unit and the number of rows of microlenses in between the units. Furthermore, the lower bounds of the relative luminous power enhancement for the center-hollowed and regular MAF are close, i.e., around 50% for n sub = 1.52 and 90% for n sub = 1.72. Also, the simulated out-coupling efficiencies of the planar substrate with the second anti-node configuration were 24% and 20% for the regular-and high-refractive-index substrates, respectively [16].
Fig. 6 Relative luminous power of units attached with center-hollowed MAFs or regular MAFs vs. the area of a single unit with varying the spacing of rows of microlenses; the refractive indices of the substrates are (a) 1.52 and (b) 1.72.
4. Conclusion
The advantageous trend of a center-hollowed MAF over a regular MAF in the relative luminance of an OLED at normal direction can be explained as follows. Within the region above the emissive units, each smaller partitioned unit leads the rays to be emitted more toward normal direction. Moreover, in between the region above the partitioned units, more rows of microlenses can direct the off-axis rays toward normal direction. In addition to improving electrical characteristics, i.e., better current spreading, lower joule heating, and fault tolerance [18–20], smaller partitioned units and center-hollowed MAFs with more rows of microlenses in between the units can obtain a higher relative luminance at normal direction of as much as 104%. Besides, the high-refractive-index substrate can enhance extra of more than 23% due to less Fresnel loss at the ITO/substrate interface. Such enhancement is valid for even larger sizes of OLEDs. These characteristics can be applied to the directional illumination applications, e.g., signage, street lights, and desk lamps. Combining these techniques reveals the promising future of energy efficient and long-lifetime organic solid-state lighting.
Acknowledgements
The authors gratefully acknowledge the financial support of the National Science Council of China under projects NSC-98-2221-E-002-035, NSC-99-2221-E-002-140, NSC 98-2221-E-002-038-MY3, NSC 98-2221-E-259-003, and NSC 98-2622-E-259-001-CC3 and the NTU under the Aim for the Top University Project.
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