Sang-Wook Park

Silicon Quantum Dots in a Dielectric Matrix for All-Silicon Tandem Solar Cells

We report work progress on the growth of Si quantum dots in different matrices for future photovoltaic applications. The work reported here seeks to engineer a wide-bandgap silicon-based thin-film material by using quantum confinement in silicon quantum dots and to utilize this in complete thin-film silicon-based tandem cell, without the constraints of lattice matching, but which nonetheless gives an enhanced efficiency through the increased spectral collection efficiency. Coherent-sized quantum dots, dispersed in a matrix of silicon carbide, nitride, or oxide, were fabricated by precipitation of Si-rich material deposited by reactive sputtering or PECVD. Bandgap opening of Si QDs in nitride is more blue-shifted than that of Si QD in oxide, while clear evidence of quantum confinement in Si quantum dots in carbide was hard to obtain, probably due to many surface and defect states. The PL decay shows that the lifetimes vary from 10 to 70 microseconds for diameter of 3.4 nm dot with increasing detection wavelength.

Thermal characterization of high performance MCP with silicon spacer having low thermal impedance

This paper presents the thermal optimization of a thermally enhanced FBGA MCP (multichip package). Because of insufficient heat path from top chip to PCB, heat is trapped between chips as shown in computer simulations. To solve the heat trap problem, we designed and developed a thermally enhanced MCP with a silicon spacer, which was applied to a 144 FBGA MCP. An experiment was carried out to verify the thermal characteristics of the thermally enhanced MCP, concerning structural variation and replacement of the adhesive. The experimental results showed that the thermal resistance of the MCP is improved and the low thermal conductive adhesive material is an important heat path barrier.

Study of silicon quantum dot p-n or p-i-n junction devices on c-Si substrate

The tandem stack of cells is one of the promising approaches for using a full solar spectrum and improving solar cell performance. By restricting the dimensions of silicon to less than Bohr radius of bulk crystalline silicon (~5 nm), quantum confinement causes its effective bandgap to increase. Therefore silicon quantum dot superlattice can be a good candidate for realizing all silicon tandem solar cells. In this work, silicon quantum dot heteroface and p-i-n homojunction devices on crystalline silicon wafers have been fabricated to understand the electrical properties of these junctions. The conduction mechanisms were determined by analyzing the temperature dependence of the current-voltage characteristics. We have experimentally investigated the material properties of silicon (Si) quantum dot (Si QD) superlattices and fabricated the device as a first step towards silicon based tandem cells. This study indicates the silicon quantum dots can be a good candidate for all-silicon tandem solar cells.

Toward a High Efficiency Siliocn Solar Cells-Simplified Cell Processing using Paste Contained Phosphorous Compounds

In this study, we present the result of preliminary investigations on the use of selective phosphorous doping and contact opening process in crystalline silicon solar cells. Typical emitter sheet resistance used in a screen-printing metallization process is 30-50 Ohm/sq. Screen printing drastically affects the design of the emitter: it must be very highly doped to decrease the high-contact resistance and not very shallow so that it is not perforated during paste firing, which would short-circuit the junction. Therefore we made improvement involves making separate diffusions for the different regions since the requirements are so different: a heavily doped and thick region under the contacts, a thin and lowly doped region under the passivating layer. Furthermore we opened the metal contact area to make a narrow grid lines simultaneously. As a result we could increase fill factor and reduce contact resistance by industrial process

Fabrication and optimization of wafer level SAW filter package using laser via drilling

Wafer level surface acoustic wave (SAW) filter package, 0.8times0.6 mm2, is drilled by laser via process. Via formation for interconnection is based on smaller package manufacture. LT (LiTaO3) which is base material of SAW filter is difficult to drill a small via by RIE (Reactive Ion Etching) because the RIE gets a very small etch rate and has wafer broken issue by thermal heating. So, our previous review is focused on the sand blasting process for via formation. However, there is limitation on via size and via depth and makes a leakage path at interface of Cu and via side wall by roughness and micro crack after electroplating. So, study on the laser via technology for meeting a design rule requests below 30 um via hole size and over 110 um depth. That gives a good solution of via formation for our package. However, the laser via makes the secondary issues such as debris and via shape, which disturb the electroplating of via inside. Overcoming these troubles and removing debris of via inside by new cleaning method are applied to general etching steps. Finally, we make a 0.8times0.6 mm2 SAW filter using laser via with etching steps compared with it using normal laser process. The result of electrical performance and laser process for wafer level SAW filter package is also discussed.

A Study on Wire Ball/Pad Open Failure Mechanism of a Multi-Stack Package (MSP) under High Temperature Storage (HTS) Condition

The mechanism of a wire ball/pad open failure at a gold wire and bonding pad interface of a multi-stack package (MSP) under high temperature storage (HTS) condition, 150 degC, is studied. Failure analysis using FE-SEM (field emission) and FIB-SEM (focused ion beam) was conducted. The analysis revealed that the main factors that contribute to a ball/pad failure were the tensile (pull-off) stress imposed on the gold wire and the bond weakening process due to metallic diffusion and corrosion. By preparing altered MSP samples and conducting verification HTS tests, it was found that the tensile stress was due to the thermal expansion of the protective encapsulant applied at the wirebonding region. Further failure analysis using FIB-SEM, AES, and TOF-SIMS indicated that the bonding strength between the gold wire and pad degraded due to the Kirkendall void resulting from metallic diffusion at high temperature and the IMC corrosion due to ion impurity

Advanced Manufacturing Concepts for Crystalline Silicon Solar Cells: Phosphorous Doping and Contact Opening Process

In this work, we report on the last investigation of POP (selective Phosphorous doping and contact Opening Process) in crystalline silicon solar cells. For the industrial solar cells, it must be very highly doped to decrease the high-contact resistance and not very shallow so that it is not perforated during paste firing, which would short-circuit the junction. We made improvement involves making separate diffusions for the different regions since the requirements are so different: a heavily doped and thick region under the contacts, a thin and lowly doped region under the passivating layer. Furthermore we opened the metal contact area to make a narrow grid lines simultaneously. As a result we could increase fill factor and reduce contact resistance by industrial process.

Study on boron doped silicon quantum dots superlattices for all-silicon tandem solar cells

Optimized chemical structure was proposed for boron-doped Si quantum dots superlattices. Boron-doped Si quantum dots superlattices were then synthesized by a co-sputtering technique, and characterized for their promising application in all-Si tandem solar cells. The formation of Si quantum dots was confirmed by transmission electron microscopy. The effect of boron dopant concentration on the Si crystallization was investigated. Lateral resistivity of Si quantum dots superlattices was studied. The results showed that boron addition does not affect the quantum dot size, but significantly reduces the resistivity of Si quantum dots superlattices.

Design Analysis of Crystalline Silicon Solar Cell Using 1-Dimensional Modelling

The simulation program for solar cells, PC1D, was briefly reviewed and the device modeling of a multicrystalline Si solar cell using the program was carried out to understand the internal operating principles. The effects of design parameters on the light absorption and the quantum efficiency were investigated and strategies to reduce carrier recombination, such as back surface field and surface passivation, were also characterized with the numerical simulation. In every step of the process, efficiency improvements for the key performance characteristics of the model device were determined and compared with the properties of the solar cell, whose efficiency (20.3%) has been confirmed as the highest in multicrystalline Si devices. In this simulation work, it was found that the conversion efficiency of the prototype model (13.6%) can be increased up to 20.7% after the optimization of design parameters.

High thermal performance FBGA MCP with a silicon spacer

This paper presents the high thermal performance FBGA MCP (2 chips) using a silicon spacer. When many devices are integrated in a package, they consume much power and raise the junction temperature, so it is important for the package to have high thermal performance. Hence, we designed and developed a high thermal performance FBGA MCP with a silicon spacer that makes additional heat paths. With the silicon spacer, the thermal resistance of junction to ambient is improved by about 15% and the junction to junction is reduced by about 50%. To guarantee the package reliability of the package with the silicon spacer, we performed the reliability test in JEDEC level 3 condition and the package passed the test. The electrical reliability of the package was verified by measuring the diode voltage of the thermal test chip.