Original research articleImprovement in light out-coupling efficiency of OLED by using high fill factor parabola curve microlens arrays
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)
References (23)
- et al.
Enhanced light-outcoupling in organic light-emitting diodes through a coated scattering layer based on porous polymer films
Org. Electron.
(2017) - et al.
Light extraction of flexible OLEDs based on transparent polyimide substrates with 3-D photonic structure
Org. Electron.
(2017) - et al.
High-speed repliction of light-extraction structure with thermal roller nanoimprinting
Microelectron. Eng.
(2015) - et al.
Simulation for light extraction efficiency of OLEDs with spheroidal microlenses in hexagonal array
Opt. Commun.
(2018) - et al.
Fabrication of high fiff-factor aspheric microlens array by digital maskless lithography
Optik
(2017) - et al.
Angular dependence of the emission from a conjugated polymer light-emitting diode: implications for efficiency calculations
Adv. Mater.
(1994) - et al.
Surface plasmon mediated emission from organic light‐emitting diodes
Adv. Mater.
(2002) - et al.
Enhanced outcoupling in organic light-emitting diodes via a high-index contrast scattering layer
ACS Photonics
(2015) - et al.
Microporous polymer films for enhancing light extraction of white-light organic light-emitting diodes
Org. Electron.
(2018) - et al.
Doubling coupling-out efficiency in organic light-emitting devices using a thin silica aerogel layer
Adv. Mater.
(2001)
Organic light-emitting diodes: solving efficiency-stability tradeoff in top-emitting organic light-emitting devices by employing periodically corrugated metallic cathode
Adv. Mater.
Cited by (14)
Highly enhanced light extraction for organic light emitting diodes by self-assembly microlens-array films
2023, Journal of LuminescenceSimulated and experimental analyses of the thermal dissipation of organic light-emitting diodes
2023, OptikCitation Excerpt :Organic light-emitting diodes (OLEDs) are considered as one of the most promising products in the 21st century due to their simple manufacturing process, rich and cheap raw materials, self-illuminating, fast response time, and ultrathin and flexible characteristics [1–4].
Improved out-coupling efficiency of organic light-emitting diodes using micro-sized perovskite crystalline template
2022, Organic ElectronicsCitation Excerpt :The total transmittance, parallel transmittance and haze in both films are about 92%, 45%, and 50%, respectively. Seeing that the haze is similar in both films, the improvement in the light extraction efficiency of the devices can be largely attributed to the fill factor of the macrostructure (crystals), and not the total light scattering effect of the film [8,12,22]. Fig. 5 shows the fill factor of the templates.
Light extraction efficiency of nanostructures on OLED prepared using nanoimprinting and thermal expansion
2022, Applied Surface ScienceCitation Excerpt :A light outcoupling layer can be fabricated on the exterior of OLEDs to enhance light extraction through optical scattering. Random corrugated structures [3,26,27], nanoparticle-embedded layers [18,28-30], microporous films [5,31,32], and microlens arrays [7,33-35] are examples of optical scattering structures. The microlens arrays can be manufactured by molding or embossing from master lens arrays, or by vacuum deposition combined with an additional process, such as photolithography.
Effect of the Ag thickness on the Light Extraction Efficiency of the OLED with an ITO/Ag/ITO Anode
2022, Journal of Semiconductor Technology and Science