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Soft holographic interference lithography microlens for enhanced organic light emitting diode light extraction

Joong-Mok Park, Zhengqing Gan, Wai Y. Leung, Rui Liu, Zhuo Ye, Kristen Constant, Joseph Shinar, Ruth Shinar, and Kai-Ming Ho

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Abstract

Very uniform 2 μm-pitch square microlens arrays (μLAs), embossed on the blank glass side of an indium-tin-oxide (ITO)-coated 1.1 mm-thick glass, are used to enhance light extraction from organic light-emitting diodes (OLEDs) by ~100%, significantly higher than enhancements reported previously. The array design and size relative to the OLED pixel size appear to be responsible for this enhancement. The arrays are fabricated by very economical soft lithography imprinting of a polydimethylsiloxane (PDMS) mold (itself obtained from a Ni master stamp that is generated from holographic interference lithography of a photoresist) on a UV-curable polyurethane drop placed on the glass. Green and blue OLEDs are then fabricated on the ITO to complete the device. When the μLA is ~15 × 15 mm2, i.e., much larger than the ~3 × 3 mm2 OLED pixel, the electroluminescence (EL) in the forward direction is enhanced by ~100%. Similarly, a 19 × 25 mm2 μLA enhances the EL extracted from a 3 × 3 array of 2 × 2 mm2 OLED pixels by 96%. Simulations that include the effects of absorption in the organic and ITO layers are in accordance with the experimental results and indicate that a thinner 0.7 mm thick glass would yield a ~140% enhancement.

©2011 Optical Society of America

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Figures (4)
Fig. 1
Fig. 1 (a) Schematic of μLA fabrication on glass. A master template is covered with PDMS. The PDMS is removed from the master and then pressed against a PU drop on another glass substrate. The PDMS is lifted off and the PU microlens array remains on the glass substrate. (b) SEM image of the 2D patterns of photoresist and (c) the resulting PU microlens array.

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Fig. 2
Fig. 2 Images of two OLED arrays with (a) green emitting Alq3 and (b) blue emitting DPVBi. The left side pixels in each image are under a microlens array and the right ones are reference pixels. The surrounding (rim) lines are the epoxy sealant used for OLED encapsulation. (c) EL spectra of the Alq3-based OLED with a PU microlens array measured with different apertures of an integrating sphere. (d) EL spectra of a DPVBi-based OLED with microlenses measured with a 25 mm diameter integrating sphere aperature. The black lines in (c) and (d) are the reference spectra of nominally identical OLED pixels without the microlenses.

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Fig. 3
Fig. 3 Schematic of light extraction model with microlenses: (a) left: extraction enhancement by the microlens array due to incidence angles change that reduces the total internal reflection inside the glass; right: light waveguiding in the glass in the absence of the microlenses. (b,c) Forward EL intensity of an OLED pixel with the microlens array that is the sum of the slightly diffused emission at the pixel area and the extra emission due to the microlens. (d) Forward intensity of an OLED pixel without microlenses.

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Fig. 4
Fig. 4 The EL spectra of the 9 pixels without (open circles) and with (solid squares) the 19 × 25 mm2 PU μLA, and the EL spectrum of the 9 pixels with the μLA (open triangles), obtained from an integrating sphere with 25 mm aperture. All devices were driven at 8 V. Each OLED pixel is 2 × 2 mm2, and adjacent pixels are separated by a 2 mm gap.

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Tables (1)

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Table 1 (a)–(c) Calculated ηext Enhancements with Various μLA Areas, Glass Thickness, and Integrating Sphere Apertures a

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Equations (1)

Equations on this page are rendered with MathJax. Learn more.

η e x t ~ ( 1 − cos θ c ) ~ 1 2 n o r g 2 ,
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