Journal of Nanomaterials

Journal of Nanomaterials / 2014 / Article
!A Letter to Editor for this article has been published. To view the article details, please click the ‘Letter to the Editor’ tab above.
Special Issue

Low-Dimensional Nanostructures for Optoelectronic Applications

View this Special Issue

Research Article | Open Access

Volume 2014 |Article ID 289752 | https://doi.org/10.1155/2014/289752

Jun Wang, Weizhi Li, Chong Wang, "Improving Light Outcoupling Efficiency for OLEDs with Microlens Array Fabricated on Transparent Substrate", Journal of Nanomaterials, vol. 2014, Article ID 289752, 6 pages, 2014. https://doi.org/10.1155/2014/289752

Improving Light Outcoupling Efficiency for OLEDs with Microlens Array Fabricated on Transparent Substrate

Academic Editor: Wen Lei
Received23 Oct 2013
Accepted23 Dec 2013
Published12 Jan 2014

Abstract

Low light outcoupling efficiency restricts the wide application of organic light-emitting diodes in solid state light market although the internal quantum efficiency of the device could reach near to 100%. In order to improve the output efficiency, different kinds of microlens array on the substrate emission surface were designed and simulated using light tracing method. Simulation results indicate that the microlens array on the substrate could efficiently improve the light output efficiency and an enhancement of 1.8 could be obtained with optimized microlens structure design. The microlens array with semicircle shape using polymer material was fabricated on glass substrate by a facile approach. Finally, the organic device with microlens array substrate was manufactured and the light output of the device with surface microlens structure could increase to 1.64 times comparing with the device without microlens.

1. Introduction

Due to high power efficiency, simple fabrication processes, and wide-color covering, organic light-emitting diodes (OLEDs) are the focus of attentions in applications of display and solid state lighting [1, 2]. Because of the difference among the refractive indices of the substrate, anode, organic materials, and the air, two wave guiding phenomena become obvious in the anode/organic layers and substrate/air layers of OLEDs [3]. Only 20% of the total emitted light from the organic layers can escape into the front side of substrate to form useful emission [4, 5]. Therefore the greatest potential for enhancing OLEDs external efficiency is to improve the light outcoupling from substrate.

Many different approaches had been done with the aim of optimizing the light outcoupling for OLEDs, including low-index grids [6, 7], periodic corrugated structure [8], Bragg mirrors [9, 10], buckling patterns [11], photonic crystals [12, 13], antireflection coatings [14], and monolayer of SiO2 microparticles [15]. Those methods focused on changing the contact surface of glass substrate with organic device, which were also complicated to achieve them.

Different kinds of microlens array on the substrate surface have also been employed in some researches and showed obvious enhancement in device efficiency. The microlens array using prepolymer NOA65 material was fabricated on the substrate which was prepatterned by microcontact printing of hydrophobic self-assembled monolayers, and the light outcoupling efficiency was improved by 24.5% without any apparent color change [16]. Spherical microlens pattern using photoresist material was designed on the backside of OLEDs substrate by conventional etching method to minimize the total internal reflection loss at substrate/air interface, and an enhancement of 1.65 could be obtained with high refractive index glass as substrate [17]. Flexible microlens arrays using a mixture of polydimethylsiloxane prepolymer by breath figure method were poured on organic substrate surface and a 34% of external quantum efficiency enhancement for OLEDs [18]. An irregular hemispherical microlens system made of flat polyethylene terephthalate film with microcavity (stacks of SiO2 and SiNx layers) was used for organic light-emitting devices and the external out-coupling factor of the devise increases by a factor of ~1.8 with wide viewing angles [19]. The numerical simulations of microlens array could also be used effectively in the infrared region with the ray tracing simulation method [2022].

In this paper, a facile approach is proposed to enhance the light outcoupling for organic devices using traditional photolithography method. Transparent substrate with microlens array is simulated with ray trace software. The photoresist is spun coating on the substrate to form microlense array and then the light outcoupling results of organic device are discussed with normal and the modified substrate. With respect to many previously reported studies on microlense fabrication method, here the easy achieved method using polymer photoresist is proposed, and by which ~85% enhancement of the light extraction for OLED is obtained.

2. Simulation

The mechanism of outcoupling enhancement with microlens array on substrate for organic device was proposed in Figure 1. Some light emitting from organic material and transmitting to glass-air interface would be reflected to glass because of the mismatch of refractive index and the incident angle higher than critical angle. With microlens array modification of the glass-air interface, the light emitting with large incident angle could be extracted out because the surface of glass was changed by the array. A portion of the light may reflect at the interface and back to the organic layer, but that portion could reflect at the metal organic interface and reach the glass-air interface again. Therefore, more light is extracted out due to morphology changing by the microlens array. The geometric structure and fill factor of the microlens array would affect the outcoupling of emission light.

In order to identify the influence of geometric structure and fill factor of the lens array to the light outcoupling efficiency, three-dimensional models of organic device with the structure shown in Figure 1 were setup and simulated with ray tracing method using TracePro software package. The structure of organic device was glass substrate ( mm)/ITO ( mm)/multiorganic layers ( mm)/aluminum cathode ( mm). For simplicity, the metal cathode layer was regarded as ideal reflecting layer, and the absorption parameter of substrate material was set to zero. Refractive index of glass substrate was 1.5 [23, 24], which was very near to that of the AZ series photoresist used to form microlens. Five kinds of microlens with different geometrical parameters were designed and simulated, and those parameters were listed in Table 1. The design of microlens size is dependent on some reported microlens results and the dimension of the microlens we fabricated. Types 1, 2, and 3 microlens arrays were different in the contact angle with substrate; types 1, 4, and 5 microlens array were different in the fill factor at the output plane surface of substrate.


DeviceMicrolens typesRadius (µm)Gap (µm)Hight (µm)Contact angle (°)

1Type 150 μm5 μm50 μm90
2Type 250 μm5 μm25 μm60
3Type 350 μm5 μm13.4 μm30
4Type 430 μm5 μm30 μm90
5*Type 550 μm5 μm50 μm90

 *In device 5, microlens arrangement was similar to honeycomb lattice; the other four types were orthorhombic pattern.

Those organic devices with different substrate and same organic device structure were simulated with ray tracing method. Illuminance maps of the substrate output surface are shown in Figures 2(a), 2(b), and 2(c), corresponding to devices 1, 3, and 4, respectively. The maximum illuminance value is 442 lux for device 1, which is much higher than that of device 3 (321 lux). The light outcoupling efficiencies of different contact angle organic devices (types 1, 2, and 3) microlens array on substrate are 38.7%, 34.8%, and 31.4%, respectively. As the contact angle of microlens with substrate is increased, light outcoupling efficiency is improved. But the microlens array with 90° contact angle is uneasy to fabricate in the realizing process, and feasible angle should be considered combining with the manufacture method. Comparing the microlens with same contact angle and different fill factor (types 1, 4, and 5), the light outcoupling efficiencies of device are 38.7%, 38.5%, and 30.3%, respectively. The microlens fill factor on the substrate of type 1 is near to type 5, just the pattern arrangement difference, so that the outcoupling efficiencies of two devices are very comparative. For the device with lower fill factor (57.7%, device 5), the light outcoupling efficiency became obvious down to 30.3%. It’s apparent that higher fill factor microlens array design would be the better choice to promote the light outcoupling. The illuminance value and uniformity of three organic devices could be obtained from Figure 2. The heterogeneity of light intensity in the substrate surface center (30 mm 30 mm) is caused by the quantitative limitation of light ray used in the simulation process. Comparing with the output efficiency of 21.5% for normal glass substrate, adding microlens array in the glass substrate could efficiently improve the light outcoupling and an enhancement of ~1.8 could be obtained from the simulation results with optimized microlens array structure on the substrate.

3. Microlens Fabrication and Device Performance

Before using photoresist as microlens material on the glass substrate surface, the transmittance character of the material in visible spectra area should be identified. The photoresist was spun coating on the glass substrate to form a unique thin film. Then the transmittance of photoresist film with glass substrate in visible region was tested by ultraviolet spectrometer UV7000 and shown in Figure 3. The average transmittance of the film (including substrate) at 400 nm ~ 700 nm region is over 95%, so that it’s suitable for photoresist to act as microlens array.

The organic device with multiorganic layers structure was fabricated on the glass substrate with microlens array and the concise process was shown in Figure 4. In the figure, the transparent substrate Figure 4(a) with ITO film was spun coating a thick layer of photoresist Figure 4(b), and then the photoresist pattern Figure 4(c) formed on the substrate using normal photolithography process and photomask. To achieve hemispherical type lens array Figure 4(d), the substrate was postbaked in oven at 140°C temperature for 30 mins. At last, multiorganic layers and metal cathode were evaporated on the substrate in vacuum chamber Figure 4(e). The scan electron microscope image of the microlens was described in Figure 4(f), which indicated that microlens array with semicircle shape has been formed. The top view photo and 3-dimension image of the microlens array were also shown in the inset of Figure 3. The height and diameter of microlens are ~2 μm and 5 μm, respectively, and the gap between the microlens is ~0.5 μm. The size of the microlens array is decreased equal proportion with the former simulated microlens type 1, because the thick film (50 μm) material would absorb the emission light obviously and also thick microlens with hemispherical section is uneasy to control.

In order to identify the improvement of light outcoupling performance of substrate with microlens array, organic devices with microlens array covered substrate and normal substrate were prepared. Using same white LED diode as light source, the illumination images of the LED through normal glass substrate and the substrate with microlens array are inserted in Figure 5. It’s obvious that both the luminance and the emission area of the substrate with microlens array (b) are higher than the normal substrate (a). OLEDs were also fabricated on both substrates with the device structure ITO/NPB (40 nm)/CBP: (tpbi)2Ir(acac) (2 wt%, 30 nm)/BCP (10 nm)/Alq3 (40 nm)/LiF (1 nm)/Al (100 nm). The luminance-voltage characteristics of two organic devices are presented in Figure 5. The luminance value of the organic device with microlens array on substrate is about 1.3 times than that with normal glass substrate at the same driving voltage. A ~30% of maximum luminance enhancement for OLEDs could be obtained from the microlens array covered substrate.

Not only the luminance but also the emission area would increase from the test result shown in the inset of Figure 5. The organic devices using microlens array substrate and normal substrate (50 mm 50 mm, the center organic emission area 30 mm 30 mm) were fabricated with the former structure, and luminance in the center line luminance of both devices was demonstrated in Figure 6. The luminance value of device with microlens array on the substrate was obvious higher than that of device without microlens, and also the emission area extended to the margin of the glass substrate. The luminance integral area of the device covered with microlens array is 1.28 times larger than that of normal substrate device. So the total luminance integral square area of with microlens device is ~1.64 times larger than that of without microlens. The microlens array on substrate surface could efficiently improve the light outcoupling from OLEDs.

4. Conclusion

In summary, a facile approach of fabricating microlens array on substrate is introduced to enhance the light outcoupling for organic devices using traditional photolithography method. Different kinds of microlens array are simulated with ray trace software and optimized microlens array model with an enhancement of 80% light output efficiency that is obtained from the simulation results. The microlens array with hemispherical type section and high uniform shape could be easily fabricated using photoresist material on glass substrate. Organic devices were fabricated with the microlens array covered substrate and normal glass substrate. The maximum luminance of organic device with microlens array is ~1.3 times higher than that without microlens array covered substrate and the light output intensity is about 1.64 times comparing with the device without microlens.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work is supported by National Science Foundation of China (NSFC) via Grant no. 61006036, “the Fundamental Research Funds for the Central Universities” (ZYGX2011X012), and Program for New Century Excellent Talents in University (NCET-10-0299).

References

  1. T. H. Han, Y. Lee, M. R. Choi et al., “Extremely efficient flexible organic light-emitting diodes with modified graphene anode,” Nature Photonics, vol. 6, no. 2, pp. 105–110, 2012. View at: Publisher Site | Google Scholar
  2. W. J. Hyun, S. H. Im, O. O. Park, and B. D. Chin, “Corrugated structure through a spin-coating process for enhanced light extraction from organic light-emitting diodes,” Organic Electronics, vol. 13, no. 4, pp. 579–585, 2012. View at: Publisher Site | Google Scholar
  3. Y. H. Ho, K. Y. Chen, K. Y. Peng, M. C. Tsai, W. C. Tian, and P. K. Wei, “Enhanced light out-coupling of organic light-emitting diode using metallic nanomesh electrodes and microlens array,” Optics Express, vol. 21, no. 7, pp. 8535–8543, 2013. View at: Publisher Site | Google Scholar
  4. S. W. Liu, J. X. Wang, Y. Divayana et al., “An efficient non-Lambertian organic light-emitting diode using imprinted submicron-size zinc oxide pillar arrays,” Applied Physics Letters, vol. 102, Article ID 053305, 2013. View at: Publisher Site | Google Scholar
  5. Z. B. Wang, M. G. Helander, J. Qiu et al., “Unlocking the full potential of organic light-emitting diodes on flexible plastic,” Nature Photonics, vol. 5, no. 12, pp. 753–757, 2011. View at: Publisher Site | Google Scholar
  6. Y. Sun and S. R. Forrest, “Enhanced light out-coupling of organic light-emitting devices using embedded low-index grids,” Nature Photonics, vol. 2, no. 8, pp. 483–487, 2008. View at: Publisher Site | Google Scholar
  7. T. W. Koh, J. M. Choi, S. Lee, and S. Yoo, “Optical outcoupling enhancement in organic light-emitting diodes: highly conductive polymer as a low-index layer on microstructured ITO electrodes,” Advanced Materials, vol. 22, no. 16, pp. 1849–1853, 2010. View at: Publisher Site | Google Scholar
  8. Y. G. Bi, J. Feng, Y. F. Li et al., “Enhanced efficiency of organic light-emitting devices with metallic electrodes by integrating periodically corrugated structure,” Applied Physics Letters, vol. 100, Article ID 053304, 2012. View at: Publisher Site | Google Scholar
  9. S.-H. Cho, Y.-W. Song, J.-G. Lee et al., “Weak-microcavity organic light-emitting diodes with improved light out-coupling,” Optics Express, vol. 16, no. 17, pp. 12632–12639, 2008. View at: Google Scholar
  10. C.-L. Lin, T.-Y. Cho, C.-H. Chang, and C.-C. Wu, “Enhancing light outcoupling of organic light-emitting devices by locating emitters around the second antinode of the reflective metal electrode,” Applied Physics Letters, vol. 88, Article ID 081114, 2006. View at: Publisher Site | Google Scholar
  11. W. H. Koo, S. M. Jeong, F. Araoka et al., “Light extraction from organic light-emitting diodes enhanced by spontaneously formed buckles,” Nature Photonics, vol. 4, no. 4, pp. 222–226, 2010. View at: Publisher Site | Google Scholar
  12. Y. R. Do, Y. C. Kim, Y. W. Song et al., “Enhanced light extraction from organic light-emitting diodes with 2D SiO2/SiNx photonic crystals,” Advanced Materials, vol. 15, no. 14, pp. 1214–1218, 2003. View at: Publisher Site | Google Scholar
  13. W. Xu and Y. Li, “The effect of anisotropy on light extraction of organic light-emitting diodes with photonic crystal structure,” Journal of Nanomaterials, vol. 2013, Article ID 969120, 6 pages, 2013. View at: Publisher Site | Google Scholar
  14. K. Saxena, D. S. Mehta, V. K. Rai, R. Srivastava, G. Chauhan, and M. N. Kamalasanan, “Implementation of anti-reflection coating to enhance light out-coupling in organic light-emitting devices,” Journal of Luminescence, vol. 128, no. 3, pp. 525–530, 2008. View at: Publisher Site | Google Scholar
  15. T. Bocksrocker, J. Hoffmann, C. Eschenbaum et al., “Micro-spherically textured organic light emitting diodes: a simple way towards highly increased light extraction,” Organic Electronics, vol. 14, pp. 396–401, 2013. View at: Google Scholar
  16. W. K. Huang, W. S. Wang, H. C. Kan, and F. C. Chen, “Enhanced light out-coupling efficiency of OLEDs with self-organized microlens arrays,” SID Symposium Digest of Technical Papers, vol. 37, no. 1, pp. 961–963, 2006. View at: Publisher Site | Google Scholar
  17. H. J. Peng, Y. L. Ho, C. F. Qiu, M. Wong, and H. S. Kwok, “Coupling efficiency enhancement of organic light emitting devices with refractive microlens array on high index glass substrate,” SID Symposium Digest of Technical Papers, vol. 35, no. 1, pp. 158–161, 2004. View at: Publisher Site | Google Scholar
  18. F. Galeotti, W. Mróz, G. U. Scavia, and C. Botta, “Microlens arrays for light extraction enhancement in organic light-emitting diodes: a facile approach,” Organic Electronics, vol. 14, pp. 212–218, 2013. View at: Google Scholar
  19. J. Lim, S. S. Oh, D. Y. Kim et al., “Enhanced out-coupling factor of microcavity organic light-emitting devices with irregular microlens array,” Optics Express, vol. 14, no. 14, pp. 6564–6571, 2006. View at: Publisher Site | Google Scholar
  20. N. Guo, W. D. Hu, X. S. Chen et al., “Investigation of radiation collection by InSb infrared focal-plane arrays with micro-optic structures,” Journal of Electronic Materials, vol. 42, pp. 3181–3185, 2013. View at: Google Scholar
  21. N. Guo, W. D. Hu, X. S. Chen, C. Meng, Y. Q. Lv, and W. Lu, “Optimization of microlenses for InSb infrared focal-plane arrays,” Journal of Electronic Materials, vol. 40, no. 8, pp. 1647–1650, 2011. View at: Publisher Site | Google Scholar
  22. N. Guo, W. D. Hu, X. S. Chen et al., “Optimization for mid-wavelength InSb infrared focal plane arrays under front-side illumination,” Optical and Quantum Electronics, vol. 45, pp. 673–679, 2013. View at: Google Scholar
  23. H. Peng, Y. L. Ho, X.-J. Yu, M. Wong, and H.-S. Kwok, “Coupling efficiency enhancement in organic light-emitting devices using microlens array—theory and experiment,” IEEE Journal of Display Technology, vol. 1, no. 2, pp. 278–282, 2005. View at: Publisher Site | Google Scholar
  24. S. Mladenovski, K. Neyts, D. Pavicic, A. Werner, and C. Othe, “Exceptionally efficient organic light emitting devices using high refractive index substrates,” Optics Express, vol. 17, no. 9, pp. 7562–7570, 2009. View at: Publisher Site | Google Scholar

Copyright © 2014 Jun Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


More related articles

 PDF Download Citation Citation
 Download other formatsMore
 Order printed copiesOrder
Views3383
Downloads1575
Citations

Related articles

We are committed to sharing findings related to COVID-19 as quickly as possible. We will be providing unlimited waivers of publication charges for accepted research articles as well as case reports and case series related to COVID-19. Review articles are excluded from this waiver policy. Sign up here as a reviewer to help fast-track new submissions.