Elsevier

Optik

Volume 212, June 2020, 164604
Optik

Original research article
Improvement in light out-coupling efficiency of OLED by using high fill factor parabola curve microlens arrays

https://doi.org/10.1016/j.ijleo.2020.164604Get rights and content

Highlights

  • The influence of the parameters of the spherical and aspheric MLAs on the luminance efficiencywe were investigated. High fill factor parabola curve MLAs was introduced to improve the light out-coupling efficiency of OLED.

  • Geometric ray tracing was employed to investigate and optimize the tructure parameters of spherical and aspheric MLAs, such as the curved profile, the base shape, the fill factor, the aspect ratio (the ratio of height to diameter), and the material refractive index of the MLAs.

  • The experimental results are consistent with the trend of the simulation results, and show that the maximum light out-coupling efficiency of OLED improvement is 49 %.

Abstract

Aspheric microlens arrays (MLAs) are introduced on glass substrate to improve the light out-coupling efficiency (LOCE) of organic light-emitting diode (OLED). Geometric ray tracing was employed to simulate the improved effects and optimize the structure parameters of the MLAs pattern. The simulation results show that when the parameters of MLAs as follows, the curved profile is parabola, the base shape is hexagon, the fill factor is 1, the aspect ratio is 0.6, and the material refractive index should be match to that of the substrate of the OLED, which can improve LOCE maximum.

Poly-dimethyl-siloxane (PDMS) parabola curve MLAs films ware fabricated by maskless lithography technique and template replication. The experimental results show that the MLAs can improve the LOCE and the trends of the experimental results agree with the simulation results. A maximum increase of 49 % in the coupling efficiency was achieved.

Introduction

OLED has requirements such as energy-saving, environmental protection, low thickness and light weight; which seen to be one of the most potent candidates as the nest generation planar light source. A shortcoming of OLED is almost 80 % of the generated light in the device is lost due to wave guiding and total internal reflection in the glass substrate [1,2]. In improving the light out-coupling efficiency (LOCE), many techniques based on destroying wave guiding phenomena or reducing total internal reflection has been studied. These techniques include use of micro porous or scatterting polymer [[3], [4], [5]], insertion of a thin silica aerogel layer of very low refractive index [6], and use of periodic structures placed in the optically active layer to introduce Bragg scattering normal to the substrate plane [7], use of micro pyramids or micro-V-shape grooves [[8], [9], [10]], use of nanoparticles or photonic structure [[11], [12], [13], [14], [15]], and so on. However, these methods are often accompanied by undesirable attributes such as an angle dependent electroluminescent spectrum, angular variations in emission intensity, or complex and expensive fabrication processes.

A method that avoids many of these shortcomings is use MLAs to the back side of the glass substrate. The fabrication of MLAs is simple and reliable in processing, can be easily applied to large area substrates, and the MLAs without any color shift when by applied in OLED. In order to optimize the coupling enhancement factor, literatures have study the details concerning the impact of geometrical structure of the spherical MLAs [[16], [17], [18], [19], [20], [21], [22]]. However, little study has been undertaken on analysis of the effect of aspherical MLAs parameters on improving the LOCE of OLED.

In this study, we investigate the influence of the parameters of the spherical and aspheric MLAs on the luminance efficiency by geometric ray tracing method, such as the curved profile, the base shape, the fill factor, the aspect ratio (the ratio of height to diameter), and the material refractive index of the MLAs. Maskless lithography technique and replica molding technique were used to fabricate the poly-dimethyl-siloxane (PDMS) MLAs films. Finally, the PDMS MLAs films were applied to OLED panels and the out-coupling efficiency were measured.

Section snippets

Simulation

In order to analyze the influence of the structure parameters of the MLAs to the LOCE, three-dimensional model of OLED, as Fig. 1(a) and the OLED with MLAs, as Fig. 1(b), were setup and simulated with ray tracing method using TracePro software. The size of the OLED is 1.5*1.5 mm, the metal cathode is extremely negative, the thickness of organic light-emitting layer PPV, PEDOT and the ITO anode are all 100 nm, they refractive index are 1.865, 1.502 and 1.8, respectively. The thickness of glass

Experimental

MLAs have been applied more and more in the optical and lighting systems in recent years, such as in diffusers, optical communication, sensing application, image systems and illumination. Traditional methods for making MLAs, such as reflow, grayscale lithography, reaction ion etching, etc., difficult to produce MLAs which have high fill-factor and aspherical surface profile. Our group presented a novel method to fabricate high fill-factor aspheric microlens array by digital maskless

Results and discussion

Fig. 6 are the magnifying image and 3D image of the PDMS parabola curve MLAs films, which fabricated just as Fig. 5 process sketched. The radius of the micrlens is 20 microns, the aspect ratio is 0.6, the base shape is (a) circle, (b) square and (c) hexagon.

Fig. 7 is the OLED with (the right part) and without (the left part) microlens arrays film.To determine the influence of the microlens array on the improvement of the luminance efficiency of planar OLED devices, two different arrangements

Conclusion

The influence of the parameters of the Aspheric MLAs on the LOCE of the OLED is analyzed by ray tracing simulation software. The simulation results show that the improved LOCE is related to the fill factor, the material refractive index and the aspect ratio of the MLAs, and so on. The LOCE improvement of OLED is increase with the increase of the fill factor of the MLAs. The refractive index of the material of MLAs must be matched with the refractive index of the OLED substrate, so that the LOCE

Acknowledgements

This work is supported by Foundation of Key Laboratory of Nondestructive Testing (Nanchang Hangkong University), Ministry of Education (Grant No. EW201908087), Key research and development program of Jiangxi Province (Grant No. 20171BBE50012), and PhD Foundation of Nanchang Hangkong University (Grant No. EA201708389)

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