Patrick Guo-Qiang Lo

Si nanopillar array optimization on Si thin films for solar energy harvesting

In this letter, Si thin film (800 nm thick) with nanopillar array decorated surface is studied via simulation for its solar energy absorption characteristics. It is found that the light absorption is significantly enhanced due to the adding of the Si nanopillar (SiNP) array to the Si thin film. The absorption characteristics of the SiNP structure would be approximately optimum (especially at ∼2.5 eV, the high energy density region in the solar spectrum) when the periodicity of SiNP array is set as ∼500 nm, which can be explained when comparing the incident light wavelength with the periodicity of SiNP array.

Design guidelines of periodic Si nanowire arrays for solar cell application

In this letter, optimum periodic Si nanowire (SiNW) arrays are designed via simulation for solar cell application, in terms of the structural parameters, e.g., the array periodicity (P) and SiNW diameter (D). It is found that the more efficient light absorption compared to that of the Si thin film with the same thickness could be realized when P is between 250 and 1200 nm. Further, the ratio of D to P should be >0.5 (or more specifically ∼0.8) for the optimized solar energy harvesting. The underlying physics is also discussed in this work.

Low power 50 Gb/s silicon traveling wave Mach-Zehnder modulator near 1300 nm

A silicon traveling-wave Mach-Zehnder modulator near 1300 nm is demonstrated to operate at 50 Gb/s with a differential 2 Vpp signal at 0 V reverse bias, achieving a 800 fJ/bit power consumption.

Efficient and broadband polarization rotator using horizontal slot waveguide for silicon photonics

We report an efficient and low-loss polarization rotator based on mode evolution using horizontal slot waveguide. The device is fabricated using complementary metal–oxide–semiconductor compatible processes, which allows monolithic integration with active drive electronics and other photonic components. A rotator fabricated with 100 μm transition length provides a high extinction ratio >14 dB for both transverse-magnetic (TM)-transverse-electric (TE) and TE-TM rotation. The excess loss of the device is <1 dB for both rotations as etching of the bottom Si waveguide is prevented. The device also exhibits a uniform rotation response over C+L band wavelength range of 1530-1600 nm.

Novel silicon nanohemisphere-array solar cells with enhanced performance.

8 Recent studies on Si nanostructures, such as Si nanowire (NW)/nanopillar (NP), Si nanocone, and Si nanopore arrays, have demonstrated their excellent antirefl ection properties, which provide opportunities for Si-based solar cells to enhance the light absorption by incorporating these nanostructures. [ 1 ] Unlike the antirefl ection coating (ARC) or surface plasmon-based light absorption enhancement, [ 2 ] there is no parasitic light absorption loss for the Si nanostructureincorporated systems as the nanostructures themselves are active absorbers. However, it is noted that high aspect ratios are required for the above-mentioned Si nanostructures to effi ciently trap light, which makes the conformal deposition of transparent electrodes challenging. [ 3 ] The high aspect ratio is also associated with large surface area and thus high surface defects are expected, which can result in intensive surface recombination of photogenerated carriers. [ 4 ] These impacts lead to poor photogenerated carrier collection, and thus low power conversion effi ciency (PCE) of the solar cells can be expected as reported previously. [ 5 ]

Design High-Efficiency Si Nanopillar-Array-Textured Thin-Film Solar Cell

In this letter, the electrical performance in terms of power conversion efficiency (PCE) of Si nanopillar (SiNP)-textured thin-film solar cell is investigated via simulation, where the SiNP physical dimensions were predetermined for optimized solar-energy absorption. Several key factors affecting PCE of the solar cell, e.g., the impact of Si quality in terms of minority carrier lifetime, emitter doping level, and emitter junction depth, is highlighted, and a practical guideline to design high-efficiency SiNP-textured thin-film solar cell is thus provided. The PCE of the structure under investigation can be significantly boosted, compared with the to-date reported Si thin-film solar cell.

Polarization splitter using horizontal slot waveguide

In this work, compact and efficient polarization splitters using horizontal slotted waveguides are presented. The device is fabricated using Complementary Metal-Oxide-Semiconductor (CMOS) compatible processes, which allows monolithic integration with active drive electronics and other photonic components. The splitter is built using two parallel horizontal slotted waveguides and achieves a TM-TM transmission coefficient higher than -0.2dB over a bandwidth of 1550-1600nm for a small coupling length of 12μm. The presented device is readily implemented in polarization diversity circuits.

Boosting Short-Circuit Current With Rationally Designed Periodic Si Nanopillar Surface Texturing for Solar Cells

Large-scale rationally designed periodic Si nanopillar (SiNP) arrays of varying diameters and heights have been fabricated by a top-down method. The impacts of the structural parameters (e.g., diameter/periodicity/height) on the reflectance and absorption of the SiNP array have been extensively studied, and the results are in agreement with our theoretical prediction of Li et al. Owing to the notably enhanced light absorption of the optimized SiNP array, a short-circuit current density Jsc of 34.3 mA/cm2 was obtained on an axial p-n SiNP array surface-textured solar cell, which is the highest to date among reported Si nanowire/SiNP-based solar cells. Jsc is significantly boosted compared to that of the untextured solar cell (18.1 mA/cm2), which implies that the SiNP array is a promising texturing technology for thin-film photovoltaic application.

Si Nanopillar Array Surface-Textured Thin-Film Solar Cell With Radial p-n Junction

The electrical characteristics of a Si nanopillar array surface-textured thin-film solar cell with radial p-n junction are studied in this letter. Several key factors affecting the power conversion efficiency (PCE) of the solar cell, e.g., surface recombination velocity, the shell thickness, back surface field, and minority carrier diffusion length, are highlighted. The radial p-n junction provides better PCE than the axial p-n junction at same given conditions (i.e., doping concentration, minority carrier diffusion length, etc.), indicating that the radial p-n junction is preferred in improving the cells performance.

A compact bi-wavelength polarization splitting grating coupler fabricated in a 220 nm SOI platform.

We experimentally demonstrate a polarization splitting grating coupler that is operational near 1310 nm and 1550 nm in a silicon-on-insulator platform, using the same fiber angle for both wavelength bands. At 1550 nm, the device has an insertion loss of 7.1 dB and a 1.5-dB transmission window of 35 nm. At 1310 nm, the insertion loss and 1.5-dB transmission window are 8.2 dB and 18 nm, respectively. Polarization isolation at 1550 nm is 24 dB. This is the first experimental demonstration of a bi-wavelength polarization-splitting grating coupler.