Hemispherical Microlens Array Research Articles

Novel multilayered hot embossing process for fabricating a microstructure pattern on various polymer substrates

This paper describes a novel, multilayered hot embossing method for fabricating a microstructure pattern on various polymer substrates. The method involves a multilayered hot embossing process with three polymer substrates (polycarbonate, polymethyl methacrylate, and low-density polyethylene), three pieces of microstructured molds, and graphene-based heaters. Only one hot embossing step is required in which a microstructure pattern is directly fabricated onto the three polymer substrates. The total processing cycle time is less than that of the traditional hot embossing process. In this study, thin graphene-based heaters were fabricated and tested. A facility that integrates graphene-based heating and multilayered hot embossing was designed and implemented. Under the proper processing conditions, a hemispherical microlens array and a microcylinder array pattern were successfully fabricated on the three polymer substrates. Finally, the uniformity of embossing temperature and the replication quality of the microstructure patterns were measured and analyzed. The technique shows great potential for the rapid and flexible fabrication of microstructure patterns on various polymer substrates.

Light extraction enhancement and directional control of scintillator by using microlens arrays.

The total internal reflection restricts light extraction efficiency of scintillator, leading to reduced detection efficiency and signal-to-noise ratio in the field of scintillator-based radiation detection system. This research presents the method of applying microlens arrays to improve the light extraction efficiency as well as achieve directional control of emission for scintillators. For BGO (Bi4Ge3O12) scintillator covered with PMMA (polymethyl-methacrylate) hemispherical microlens array, the 2.59-fold in particular angle (θem = 45°) and overall 1.94-fold angle-integrated enhancement ratios have been obtained. Furthermore, we analyze and optimize some parameters of microlens arrays such as the packing arrangement, duty ratio, size, refractive index, and shape. As a result, when the refractive index of microlens is slightly larger than that of scintillator, a maximum 6.23-fold angle-integrated enhancements can be achieved. It can be concluded that the microlens array covered on scintillator has considerable value for practical applications on radiation detection.

Efficient, Nonintrusive Outcoupling in Organic Light Emitting Devices Using Embedded Microlens Arrays

We demonstrate efficient light extraction from the active region of bottom-emitting organic light emitting devices (OLEDs) using a high refractive index, nondiffractive hemispherical microlens array located between the transparent anode and embedded in the low refractive index glass substrate (n = 1.5). The subelectrode microlens array (SEMLA) results in a maximum external quantum efficiency of 70 ± 4% for green phosphorescent OLEDs (PHOLEDs). Furthermore, the wavelength- and viewing-angle-independent light extraction structure results in white PHOLED external efficiencies of 50 ± 3%. The SEMLA light extraction structure is nonintrusive; that is, it lies completely outside of the OLED structure. Since this design has no effect on the image resolution, it is compatible with applications for both displays and white light illumination, with no dependence on molecular transition dipole orientation and the active organic layers (and hence diode electrical characteristics) used in the PHOLED. Finally, due to th...

Improvement of light outcoupling efficiency of organic light-emitting diodes utilizing microlens array fabricated using poly oxymethylene mold

ABSTRACTThis paper reports a new method for fabrication of microlens arrays using a polyoxymethylene (POM) mold with concave-type cylindrical holes prepared by mechanical-micro milling without photolithography and wet etching processes. Microlens arrays were fabricated by a replica molding process using the concave-type POM mold. A polymethylmethacrylate (PMMA) thin film was deposited onto the POM mold and a concave PMMA template was prepared by blowing a nitrogen gas stream onto the thin PMMA film suspended on the micro POM mold. Microlens arrays were fabricated by pouring ultraviolet (UV)-epoxy onto the concave PMMA template and curing the UV-epoxy under UV light. Two types of microlens arrays, one with a hemispherical structure and one with a truncated-cone-type structure, were fabricated. After attaching the hemispherical microlens array onto organic light-emitting diodes (OLEDs), the external quantum efficiency (EQE) of the OLEDs was enhanced by approximately 56.5%. This work provides a valuable method for easy fabrication of microlens arrays with diameters of several hundred micrometers.

A novel and rapid fabrication method for a high fill factor hexagonal microlens array using thermal reflow and repeating spin coating

This paper presents an innovative process for the fabrication of a high fill factor microlens array using thermal reflow and repeating spin coating. The repeated spin coating method creates a hexagonal microlens array after the thermal reflow processes as an alternative to the expensive gray-mask and longtime electroplating method. The first, the round base and hexagonally arranged array, precisely define the bottom shape of the liquid photoresist, and a tall hemispherical microlens array with a small radius of curvature is fabricated during the photoresist thermal reflow process. The second, the surplus photoresist, is then rotated at high velocity using a spinner. The micro-plano-convex microlens molds are then finished. The polydimethylsiloxane (PDMS) microlens array is fabricated using a replica molding process. The experimental results show that a high fill factor hexagonal PDMS microlens array is successfully fabricated. The fabricated microlens array has good surface roughness. The proposed fabrication method facilitates mass production and gives increased luminance efficiency and luminance power efficiency to produce a high fill factor microlens array that is suitable for use in the light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs).

Molding hemispherical microlens arrays on flexible substrates for highly efficient inverted quantum dot light emitting diodes

Microlens arrays were introduced into a QLED and the EQE of the device was enhanced by a factor of 64%.

Novel fabrication method of microlens arrays with High OLED outcoupling efficiency

We presented a novel fabrication method of pyramidal and hemispherical polymethylmethacrylate (PMMA) microlens arrays to improve the outcoupling efficiency. Pyramidal microlens arrays were fabricated by replica molding processes using concave-pyramidal silicon molds prepared by the wet etching method. Concave-hemispherical PMMA thin film was used as a template for fabrication of the hemispherical microlens array. The concave-hemispherical PMMA template was prepared by blowing a N2 gas stream onto the thin PMMA film suspended on a silicon pedestal. A PMMA microlens arrays with hemispherical structure were fabricated by a replica molding process. The outcoupling efficiency of the hemispherical microlens array was greater than that of the pyramidal microlens array. The outcoupling efficiency of hemispherical microlens arrays with a higher contact angle was larger than that of those with lower contact angle. This indicates that, for the hemispherical microlens with larger contact angle, more light can be extracted from the OLEDs due to the decrease in the incident angle of the light at the interface between an air and a hemispherical microlens arrays. After attaching a hemispherical microlens array with contact angle of 50.4° onto the OLEDs, the luminance was enhanced by approximately 117%.

Close-packed hemispherical microlens arrays for light extraction enhancement in organic light-emitting devices

Close-packed hemispherical microlens arrays have been fabricated using a soft lithography method and used to enhance the light extraction efficiency in organic light-emitting devices (OLEDs). A close-packed monolayer of polystyrene microspheres was used to produce a soft template of polydimethylsiloxane for fabricating microlens arrays from a photopolymerizable optical adhesive. The microlens contact angle and array fill-factor increase with the polystyrene microsphere size, reaching (85°±5°) and (85±3)% for 100μm size microspheres, respectively. Attaching such a large-area (2cm in diameter) microlens array on the glass substrate of an OLED leads to up to 70% efficiency enhancement, which also shows dependencies on the area and detailed structure of the OLED.

Enhancing light extraction in organic light-emitting devices via hemispherical microlens arrays fabricated by soft lithography

We demonstrate enhanced light extraction in organic light-emitting devices (OLEDs) by using microlens arrays fabricated by a soft lithography technique. A large-area and close-packed polystyrene (PS) monolayer was formed on the SiO2 substrate using a convective-capillary assembly method, and a polydimethylsiloxane polymer was used to obtain a concave template, from which microlens arrays were fabricated from a photopolymerizable transparent optical adhesive. The microlens contact angle and array fill factor both depend on the size of PS microspheres, and nearly close-packed, hemispherical microlens arrays with microlens contact angle of (85 ± 5 deg) and array fill factor of (85 ± 3)% were obtained with 100-μm PS microspheres. The enhancement in the light-extraction efficiency in OLEDs when such fabricated microlens arrays were attached to the light-emitting surface depends on the contact angle of microlens, device size, and detailed multilayer structure of the OLED. For a large-area (12 × 12 mm) fluorescent OLED with a near close-packed hemispherical microlens array, a maximum enhancement of (70 ± 7)% in the light-extraction efficiency was achieved.

Close-Packed Hemispherical Microlens Array from Two-Dimensional Ordered Polymeric Microspheres

This paper describes a facile, reproducible soft-lithography-based method for fabricating hexagonally close-packed microlens arrays by templating the surface of a colloidal monolayer, which is formed by spin-casting monodisperse polystyrene microspheres. The relief structure of colloidal monolayers has successfully generated PDMS elastomers with hexagonal arrays of hemispherical air voids. Closely packed hemispherical microlens arrays were imprinted on ultraviolet-curable photopolymers which are bound on glass substrates. Atomic force microscopy measurements showed that each spherical hole of the PDMS molds is 103 nm deep and the replicated microlens is 95 nm in height with narrow size distribution and good reproducibility. Without a multistep engineering process, this method might provide a reliable route to fabricate embossed thin layers ranging from nanometer to micrometers by controlling the size of polymer microspheres over a centimeter scale area.

A new microlens array fabrication method using UV proximity printing

This paper presents a new microlens array fabrication method that controls the printing gap in the UV lithography process. This method can precisely control the microlens array geometric profile array in the fabrication process without thermal reflow. The proximity printing bends the UV light away from the aperture edges and produces a certain exposure in the photoresist outside the aperture edges due to the diffraction effects. This causes the photoresist bottom in two adjacent patterns to link together after development. The fabricated microlens diameter has the same size as the pitch distance between the two apertures. For example, a 120 µm aperture pitch distance will generate a 120 µm diameter microlens. The proximity gap between the mask and photoresist should be two times the aperture pitch distance or greater to generate an accurate microlens array. The experimental results showed that photoresist microlens arrays could be formed automatically without using the thermal reflow process when the printing gap ranged from 240 µm to 840 µm. A hemispherical microlens array with a 120 µm diameter and various geometric profiles can be fabricated using this method.