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A method based on the variation of the area ratio of the base surface of microlenses to the light-emitting surface of the planar OLED device has been demonstrated. This method can evaluate the performance of microlens arrays with a desired geometrical shape on the improvement of the luminance efficiency for the planar light emitting devices. The maximum enhancement of the luminance efficiency of the devices in the studied microlens arrays is 56%. It is also found that the improvement of the luminance efficiency of the devices increases linearly with decreasing base length of the microlens array.
©2004 Optical Society of America
1. Introduction
For the next-generation light source and the backlight for flat panel display, energy-saving, environmental protection, low thickness, and light weight are the well-known requirements. To achieve theses requirements, light-emitting diodes (LEDs), electroluminescent (EL) devices, and organic light-emitting diodes (OLEDs) are among the best candidates. For the planar light sources, however, LEDs are not economically favorable due to the high cost of the light modules. The EL devices do not have sufficient light intensity for general lighting purpose. Therefore, OLEDs seem to be one of the most potent candidates as the next-generation planar light source.
To increase the output power efficiency of these devices, two methods have been extensively studied either by the increase of internal quantum efficiency of the devices or by the improvement of output coupling efficiency. The internal quantum efficiency is generally enhanced by adjusting thin-film structures of the devices. But the increase of internal quantum efficiency is very limited. Since the refractive indices of glasses and plastics are approximately 1.5, which is higher than air, only about 20% to 30% of the emitting light from OLED or EL devices can propagate into air. Therefore, there is a large space for improvement on the output coupling efficiency of the devices and many extensive researches are taking place on this topic.
In improving the output coupling efficiency, many techniques based on destroying waveguiding phenomena have been studied [1–11]. Among these technologies, the preparation of macro-lenses [6–8] and microlens arrays [9–11] is simple and reliable in processing. However, the macro-lens cannot fulfill some criteria for the next-generation light sources, such as flexibility and light weight, as concluded by extensive and completed studies shown in the literatures. On the other hand, for the applications of microlens arrays on self-emitting devices, little work has been done on systematical analysis [9–11].
In this study, a simple method will be proposed to evaluate the performance of the microlens array with a desired geometrical shape for the improvement of the luminance efficiency for planar OLED devices. This evaluation method is a revolutionary method since it is a new method aimed for microlens as opposed to for macro-lenses. There are methods for evaluating individual macro-lens due to its large lens area and strong focused beam, but for microlenses individual lens measurement is difficult. So a method to evaluate the luminance efficiency of microlenses through the measurement of microlenses patterned area is required. The influence of the base length of square-based microlenses on the enhancement of the luminance efficiency will be discussed. The individual microlens luminance efficiency will be deduced from the proposed measurement method.
2. Experimental methods
The experimental process flow is depicted in Fig. 1. The square-based photoresist plates are firstly made through phtotlithography process, then these plates are treated with thermal reflowing process to transform these plates into the shape of microlens array on the substrate. Subsequently, by using electroforming and UV forming techniques, the replica of microlens array on PMMA films are made. The base length (L) of microlenses in this study is between 100 and 190 µm. The gap (d) between two neighboring microlenses is changed in the range from 11 to 68 µm. Finally, PMMA films with or without microlens array are attached on the planar OLED device by using refractive-index matched oil in order to measure their luminance.
In order to measure and analyze each set of microlens arrays with the same geometrical shape, the base length and height of the microlenses are kept constant, only the gap between two neighboring microlenses is changed. The current density for the planar organic light-emitting device is driven with a set value at the luminance of the device with 500 cd/m2. For comparison, all the microlens arrays are attached in turn on the same OLED device. Therefore, driving currents, driving voltages, and active areas of the devices are the same in this study.
The base length and gap of the duplicated microlens array are measured with a scanning electron microscope (Hitach, S3500). The luminance of the device is measured with a chroma meter (Minolta, CS-100). For the angular dependence measurement, the device is fixed on a goniometer stage, the luminance is then measured at different tilting angles from 0° to 75°.
3. Principle
The area ratio (Rarea), which is the ratio of the total area of base surface of microlenses (Amicrolenses) to the total light-emitting area of the device (Adevice), is expressed as
As the gap approaches to zero, the area ratio approaches to one. In other words, the surface of the device will be fully occupied by microlenses.
The luminance perpendicular to the device surface without the microlens array is maintained at a value B0 when the device is driven with a set current density. The luminance of the device with microlens array (Bmicrolenses) is measured by driving it with the same current density. It is well-known that the luminance efficiency (E) [12] of a planar OLED device is proportional to its luminance (B) and active area (A), but is inversely proportional to its driving current (I), in terms of cd/A, as illustrated below
In this study, all the driving currents, driving voltages, and active areas of the device are kept the same. Therefore, from Eq. (2), the luminance ratio of the device with microlenses to that without microlenses should be equal to the ratio of their luminance efficiencies,
where Emicrolenses and E0 are the power efficiencies of the devices with and without the microlens array, respectively.
Since the area ratio is changed only by varying the value of the gap between two neighboring microlenses for each set of experiments, the geometrical shape of microlenses will keep the same for each set of experiments. Therefore, the performance of each microlens array on the luminance efficiency will not be affected by changing the area ratio.
4. Results and discussion
A duplicated microlens array, made from the ultraviolet hardened epoxy, on a PMMA film is shown in Fig. 2. When using a green OLED device and keeping B0 equal to 500 cd/m2, the luminance ratio of the devices with microlens array to those without microlenses is found to be larger with increasing area ratio, as shown in Fig. 3. The result is similar to that obtained by Peng et al. [11]. The maximal improvement of the luminance efficiency is 56% in this study when the base length of the microlens and the gap between two neighboring microlenses are 100 and 11 µm, respectively. Though the base length of the microlens used in this study is between 100 and 190 µm and is much larger than the diameter of microlenses used in other literatures (diameter of 5~20 µm), the improvement of the luminance efficiency in this study is better than that obtained in those literatures [9–11]. For example, Möller et al. used the microlens array with a diameter of 10 µm and obtained an 50% improvement of the output coupling efficiency of OLED device [9], Choi et al. enhanced the output coupling efficiency of micro LEDs by 23% using the microlens with a diameter of 20 µm [10], and Peng et al. increase the luminance efficiency of OLED devices by up to 20% using the microlenses with diameters of 5 to 20 µm [11]. The reason for the better performance of microlens arrays used in this study may be due to a more perfect spherical geometry, which is easy to make by using thermal reflow and UV forming techniques.
Fig. 3. The relationship between the luminance ratio and area ratio for microlens arrays with different base lengths, where B0 is 500 cd/m2.
Analyzing data shown in Fig. 3 using linear regression, a proportional relationship between luminance ratio and the area ratio can be formulated,
where k1 is the slope in these curves and k2 is a constant for each set of microlens arrays, as illustrated in Table 1.
Table 1. The values of k1 and k2 at different base lengths of microlens array
If the area ratio approaches zero, there will be no microlenses on the PMMA film, so that the luminance B and the constant k2 should equal to B0 and one, respectively. However, all the values of k2 are smaller than one and k2 decreases as the base length of microlenses increases in our results. It could be possibly due to the scattering by two additional interfaces (oil/OLED and oil/PMMA) and the absorption by the refractive-index matched oil. It also could be due to the absorption either by microlens array itself or by thin epoxy film formed on the PMMA after UV forming procedure.
In an ideal case, if both the absorption of microlens-forming material and the interface scattering approach to zero (k 2=1), and the PMMA film is fully covered with microlenses, i.e., d=0 and Rarea=1, the slope k1 can be obtained by rearranging Eqs. (3) and (4), as
This means that k1, the slope in the luminance ratio versus area ratio curves, is actually the performance of improving the luminance efficiency of the planar OLED devices by the microlens arrays.
Figure 4 shows that k1 increases linearly with decreasing base length of microlenses. Both the values of k1 and k2 are listed in Table 1. That is, the smaller the base length of microlenses, the more the enhancement of the luminance efficiency of the device. Analyzing data shown in Fig. 4 using linear regression, a proportional relationship between k1 and the base length of microlenses can be formulated,
From Eqs (5) and (6), the maximum improvement of luminance efficiency can possibly reach 108% when the base length of the microlenses approaches 1 µm and there is no gap between two neighboring microlenses.
The angular-dependent luminance of the OLED device without and with microlenses at different spacings is shown in Fig. 5, which indicates that all the devices are not a Lambertian emission. The total output power of the OLED devices with microlenses can be obtained by integration of the luminance intensity times sinθ over θ, where θ is the tilting angle. The output power is increased by 20% to 26% as the area ratio increases from 35% to 81% from our calculation. Thus the output power efficiency could be enhanced by 26% when the microlens array with a base length of 100 µm attached on the OLED device.
Fig. 5. The far-field luminance pattern of a planar OLED without and with microlenses at different spacings. (The base length of microlenses is 100 µm)
5. Conclusions
A simple method has been demonstrated to evaluate the performance of luminance-efficiency enhancement for microlens arrays attached on the planar OLED devices. The maximum improvement of luminance efficiency is 56% in this study. The enhancement of the luminance efficiency is more pronounced when the base length of microlens is smaller. Additionally, the physical meaning of the slopes of the luminance ratio versus area ratio curves is the performance of the microlens array on the improvement of luminance efficiency for the planar OLED devices. It is also shown that the luminance efficiency of the devices could possibly be enhanced by more than 100% by attachment of microlens arrays on the devices. However, the power efficiency could only be increased by 26% when using microlenses with a base length of 100 µm.
Acknowledgments
The authors gratefully acknowledge the financial support from the National Science Council of the Republic of China under the project No. NSC 93-2218-E-259-001 and from the Nanotechnology Research Center of ITRI under the project No. A331XS9T10.
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