Yu-Chou Shih

Screen-Printable Silver Pastes with Nanosized Glass Frits for Silicon Solar Cells

In this study, silver pastes containing different sizes of glass frits were employed to form the front-side electrodes in order to examine the size effect of the glass frits on the interfacial microstructures between the front-side silver contacts and the n-type silicon emitter layers, which will subsequently affect the electrical performance of silicon solar cells. The interfacial microstructures at the Ag/Si interfaces were investigated by advanced electron microscopy techniques. The transfer length method (TLM) was used to measure the specific contact resistivity of silver electrodes screen-printed on the n-type silicon substrates. The particle size of the glass frits was found to strongly affect the interfacial microstructures and therefore resulted in different specific contact resistivities (ρc) of the fabricated silver ohmic contacts. Nanosized glass frits showed excellent etching ability during engineered thermal treatments. The samples made with silver pastes containing microsized glass frits showed a thick residual glass layer at the Ag/Si interface, while the silver paste with nanosized glass frits was found to form an interface with less glass residue, which led to lower resistance after otherwise identical processing.

Study of optimal filler size for high performance polymer-filler composite optical reflectors

Polymer-based diffuse reflectors are cost-effective and high performance alternatives to metallic reflectors and multilayered dielectric mirrors in optical and photonic modules. In this study, we optimized the reflectance of high performance polymer-based reflectors by controlling filler particle size. We use a simple analytical model to investigate the critical size of inorganic fillers required to obtain the highest reflectance with a lower filler volume fraction and reflector thickness. The key model predictions show good agreement with our experimental data using Al2O3-silicone and TiO2-silicone composite reflectors. Our results demonstrate for the first time that for inorganic fillers, the effect of filler size on reflectance is non-monotonic, and a critical filler size ranging from one submicron to several microns provides the maximum reflectance. We observed a 50% difference in optical reflectance between 1 micron and 35 micron Al2O3 at 0.1 volume fraction. A significant improvement in reflectance for polymer-filler reflectors can be realized by utilizing optimized filler size, in order to increase the performance-price ratio for optical and photonic devices.

Role of Transparent Die Attach Adhesives for Enhancing Lumen Output of Midpower LED Emitters With Standard MESA Structure

The role of die attach adhesives (DAAs) in influencing the lumen output of midpower (30-150-mA input power) blue and white light emitting diodes (LEDs), which currently dominate the LED backlighting and general solid-state lighting, is studied. It is reported for the first time that a replacement of conventional silver-filled epoxy DAA by an optically transparent DAA can lead to a significant enhancement of light output as high as 13% and 21% increase for midpower blue and white LEDs, respectively. The mechanism for the observed enhancement is discussed in this paper.

Optical Interaction Between LED Backside Reflectors and Die Attach Adhesives

Adding optical reflectors to the backside of GaN-based light-emitting diode (LED) chip has been adopted for enhancing the light output. The optical effectiveness of backside reflectors (BRs) in enhancing light output is investigated by studying the light output of packaged mid-power blue and white LED emitters based on BR-based and BR-free chips. It is found that the effectiveness is strongly dependent on the optical properties of die attach adhesive (DAA) materials. When epoxy-silver DAA is applied, the difference in light output between the gold BR-based and BR-free blue LED chips is about 9% points at 40% coverage. However, with optically clear or reflective DAAs, BR-free chips show higher light output than the gold BR-based chips. For the optically clear DAA with 40% coverage, a 20% point enhancement of the output power of blue LED chips is obtained by the high transmittance of the DAA and high reflectance of the silver plating on the leadframe. With the optically reflective DAA, the enhancement for blue LED chips is ~22% points due to the high reflectance of the DAA. This letter suggests a cost-effective method for enhancing the light output of mid-power LED emitters based on relatively low-cost BR-free LED chips by using optically clear or reflective DAAs depending on the applications and with a low fillet coverage for both materials.

Effect of Thinning Encapsulant Layer on Junction and Phosphor Temperature of White Light-Emitting Diodes

Thinning of an encapsulant layer for light-emitting diodes (LEDs) is a current major technological trend. The influence of encapsulant thickness on the junction temperature as well as the maximum phosphor temperature for blue and white LEDs is studied systematically for the first time. Using the finite-element method and forward voltage method, it is demonstrated that, in contrast to common belief, a thinned encapsulant layer for relatively low-power LED emitters with an input electrical power of 0.5 W will lead to an increase in the junction temperature for the white LEDs fabricated using different methods, including the phosphor volume distributed in-cup method and chip-coating method, as well as for monochromatic blue LEDs. It is also demonstrated that, different from the effect of encapsulant thickness on the junction temperature, a reduction in the encapsulant thickness results in a decrease in the maximum phosphor temperature for the in-cup white LEDs and an increase in the phosphor temperature for the chip phosphor-coating white LEDs. For example, the largest phosphor temperature drop of the in-cup white LED can be as high as 5 °C. The effect of thermal conductivity of leadframe and phosphor concentration under different encapsulant thicknesses is also indicated.

Effectiveness of Polymer Composite-Induced Passive Radiation Cooling in Thermal Management of LED Emitters and Modules: Impact on Hotspot Elimination

Effectiveness of polymer composite-induced passive radiation cooling in thermal management of LED emitters and modules is elucidated by numerical simulations coupled with key experimental observations. Specifically, various polymer-filler composites coated on the board of LED emitters and modules are investigated for their effectiveness in reducing the board as well as junction temperature. It is demonstrated that a maximum temperature drop of 11 °C can be achieved for the single chip-on-board LED emitter with 10-W input power. Moreover, for the linear LED module, a reduction of 14.53 °C is observed in the peak junction temperature of LED emitters with 1-W input power in the light bar. The polymer composite coating is also demonstrated to significantly boost the uniformity of temperature distribution and to reduce the risk of hotspot. Some of the key simulation results are further examined to be consistent with analytical modeling. Significant implications of the present polymer composite-induced passive radiation cooling results to the challenging thermal design of electronic devices with limited space are also discussed.

Quantum Dot Based Enhancement or Elimination of Color Filters for Liquid Crystal Display

The feasibility of quantum dot (QD)-based silicone composites as suitable candidates, in terms of down conversion efficiency, for the replacement or the enhancement of color filters in high-brightness liquid crystal displays is investigated for the first time. It is demonstrated that the replacement of color filters (CFs) with a full color conversion QD composites can lead to an 86.16% enhancement in energy efficiency over the conventional LCD with phosphor-converted white light-emitting diodes and RGB CFs as backlighting. A potential second-type technology utilizing QD-on-CF for color generation is also investigated, and it is found that the energy efficiency enhancement can reach 86.86%. In addition to QDs, the state-of-art red and green phosphors are also investigated for the purpose of enhancement of color filters in LCD displays in terms of down conversion efficiency, and the result shows a 105.78% enhancement in overall down conversion efficiency. The energy efficiency of QD-based color conversion composites can be further improved by optimization of QD dispersion in the resin, which makes them more suitable replacement for color filters in LCDs due to some intrinsic limitations in phosphor materials.

Novel Ceramic Additives for Screen-Printable Silicon Solar Cell Metallization

The interfacial structure between front-side silver electrodes and n-type silicon emitters plays a very crucial role for the electrical and mechanical properties of silicon solar cells. Studies show that the residual glass layers at the Ag/Si interfaces will significantly increase the contact resistance, and this subsequently leads to a decrease in the overall efficiency of the silicon solar cells. In this work, silver-coated nano-sized non-glass frits using an electroless plating method were employed to improve the interfacial conductivity. Transfer length method was applied to evaluate the electrical performance of the samples made with different ceramic additives. For samples made with nano-sized silver-coated ceramic additives, the improvement of conductivity was found to be about 22% compared to additives with the same compositions with no surface treatment. The results indicate that the silver layer on the surface of ceramic additives provides a conducting channel within the residual insulating layer and therefore reduces overall electrical resistance.

Nano-sized glass frits with surface treatment for silicon solar cells

The interfacial structure between silver electrodes and n-type silicon emitters plays a very crucial role for the electrical and mechanical properties of silicon solar cells. Studies show that the residual glass frits layers at the Ag/Si interfaces will hugely increase the contact resistance and this subsequently leads to a decrease in the overall efficiency of the silicon solar cells. In this work, silver-coated glass frits were employed to improve the interfacial conductivity. Transfer length method (TLM) was applied to evaluate the electrical performance of samples made by different glass frits. For samples made with silver-coated glass frits, the improvement of conductivity was found to be about 15% compared to glass frits without any surface treatment. The results indicate that the silver layer on the surface of glass frits provide a conducting channel within the residual insulating layer and therefore reduce overall resistance.

Liquid encapsulation for monochromatic LED emitter packages: Enhancement of thermal-optical performance and reliability

A special liquid encapsulant and the liquid encapsulation process are developed for enhancing passive cooling for the packaging of monochromatic LED emitters. It was observed that the liquid encapsulation process is superior than the conventional solid silicone encapsulation process in terms of the junction temperature for all the packaged emitters of various wavelengths using the liquid encapsulant. An enhanced reliability in terms of lifetime is thus expected for the emitters encapsulated with the liquid encapsulant. Moreover, an enhanced light output is also demonstrated for the emitters packaged with the liquid encapsulant, which can be attributed to be a result of the improved heat dissipation by convection and conduction in the upward direction through the liquid as well as in the downward direction through the contact area between the liquid and the reflective cup.