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ABSTRACT: Over the last decade, polymer solar cells (PSCs) have attracted a lot of attention and highest power conversion efficiencies (PCE) are now close to 10%. Here we employ an optical structure - the microlens array (MLA) - to increase light absorption inside the active layer, and PCE of PSCs increased even for optimized devices. Normal incident light rays are refracted at the MLA and travel longer optical paths inside the active layers. Two PSC systems - poly(3-hexylthiophene-2,5-diyl):(6,6)-phenyl C61 butyric acid methyl ester (P3HT:PCBM) and poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:(6,6)-phenyl C71 butyric acid methyl ester (PCDTBT:PCBM) - were investigated. In the P3HT:PCBM system, MLA increased the absorption, absolute external quantum efficiency, and the PCE of an optimized device by ∼4.3%. In the PCDTBT:PCBM system, MLA increased the absorption, absolute external quantum efficiency, and PCE by more than 10%. In addition, simulations incorporating optical parameters of all structural layers were performed and they support the enhancement of absorption in the active layer with the assistance of MLA. Our results show that utilizing MLA is an effective strategy to further increase light absorption in PSCs, in which optical losses account for ∼40% of total losses. MLA also does not pose materials processing challenges to the active layers since it is on the other side of the transparent substrate.
Physical Chemistry Chemical Physics 02/2013; 15(12):4297-302. · 3.57 Impact Factor
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ABSTRACT: A novel architecture has been employed to fabricate transparent electrodes with high conductivity and high optical transmittance at high incident angles. Soft lithography is used to fabricate polymer grating patterns onto which thin metallic films are deposited. Etching removes excess metal leaving tall walls of metal. Polymer encapsulation of the structure both protects the metal and minimizes diffraction. Transmission is dependent upon the height of the walls and encapsulation and varies from 60% to 80% for structures with heights of 1400 nm to 300 nm. In encapsulated structures, very little distortion is visible (either parallel to or perpendicular to standing walls) even at viewing angles 60° from the normal. Diffraction is at characterized through measurement of intensity for zeroth through third order diffraction spots. Encapsulation is shown to significantly reduce diffraction. Measurements are supported by optical simulations.
Optics Express 01/2013; 21(2):2393-401. · 3.59 Impact Factor
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ABSTRACT: Thin microporous films were formed by dropcasting a toluene solution containing various ratios of polystyrene:polyethylene glycol blends on a glass substrate, with OLEDs on the ITO that coated the opposite side of that substrate. We demonstrate for the first time that such easily-fabricated films with surface and bulk micropores in the index-matching polystyrene can serve as random microlens-like arrays to improve forward OLED light extraction by up to ~60%. A theoretical interpretation of the angular emission profile of the device, considering the geometrical change at the substrate/air interface and the scattering by the pores within the films, was established in excellent agreement with the experiments. The use of such blended thin films provides an economical method, independent of the OLED fabrication technique, for improving the outcoupling efficiency.
Optics Express 11/2011; 19 Suppl 6:A1272-80. · 3.59 Impact Factor
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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 mm(2), i.e., much larger than the ~3 × 3 mm(2) OLED pixel, the electroluminescence (EL) in the forward direction is enhanced by ~100%. Similarly, a 19 × 25 mm(2) μLA enhances the EL extracted from a 3 × 3 array of 2 × 2 mm(2) 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.
Optics Express 07/2011; 19 Suppl 4:A786-92. · 3.59 Impact Factor
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Advanced Materials 06/2011; 23(21):2469-73. · 13.88 Impact Factor
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Advanced Materials 11/2010; 23(1):112-6. · 13.88 Impact Factor
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ABSTRACT: This paper reports on the effect of nanometer-scale photonic crystal structures on the enhancement of the light extraction in GaN light-emitting diodes. Photonic crystals with hole or pillar-patterned structures with lattice constants of 460, 600, 750, and 920 nm are fabricated on indium-doped tin oxide (ITO) electrodes and/or p-GaN layers using laser holography and reactive ion etching. It is found that the light extraction efficiency depends strongly on the distance between the photonic crystal and the active layer, as well as the lattice constant for both structures. Photonic crystal light-emitting diodes (LEDs) with a lattice constant of 750 nm and hole depths of 260 nm in the ITO layer show an increase in light extraction of up to 32%, compared to conventional LEDs, without degradation in the electrical properties while a maximum enhancement of 26% is obtained from the device with a lattice constant of 460 nm and pillar heights of 60 nm on the p-GaN layer. The dependence of the extraction efficiency on the lattice constant is also calculated using a 3-D finite-difference time-domain method and compared with experimental results.
IEEE Journal of Quantum Electronics 10/2010; · 1.88 Impact Factor
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ABSTRACT: Ordered and free-standing metallic nanowires were fabricated by e-beam deposition on patterned polymer templates made by interference lithography. The dimensions of the nanowires can be controlled through adjustment of deposition conditions and polymer templates. Grain size, polarized optical transmission and electrical resistivity were measured with ordered and free-standing nanowires.
Nanotechnology 05/2010; 21(21):215301. · 3.98 Impact Factor
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ABSTRACT: We report the fabrication and characterization of woodpile photonic crystals with up to 12 layers through titania nanoparticle infiltration of a polymer template made by soft lithography. Because the complicated alignment in the conventional layer-by-layer fabrication associated with diamondlike symmetry is replaced by a simple 90° alignment, the fabricated photonic crystal has semicrystalline phase. However, the crystal performs similarly to a perfectly aligned crystal for the light propagation integrated from the surface normal to 30° at the main photonic band gap.
Applied Physics Letters 05/2010; 96(19):193303-193303-3. · 3.84 Impact Factor
