Kun-Yu Lai

Periodic Si Nanopillar Arrays Fabricated by Colloidal Lithography and Catalytic Etching for Broadband and Omnidirectional Elimination of Fresnel Reflection

Periodic Si nanopillar arrays (NPAs) were fabricated by the colloidal lithography combined with catalytic etching. By varying the size of colloidal crystals using oxygen plasma etching, Si NPAs with desirable diameter and fill factor could be obtained. The Fresnel reflection can be eliminated effectively over broadband regions by NPAs; i.e., the wavelength-averaged specular reflectance is decreased to 0.70% at wavelengths of 200-1900 nm. The reflectance is reduced greatly for the incident angles up to 70 degrees for both s- and p-polarized light. These excellent antireflection performances are attributed to light trapping effect and very low effective refractive indices, which can be modified by the fill factor of Si in the NPA layers.

Effect of indium fluctuation on the photovoltaic characteristics of InGaN/GaN multiple quantum well solar cells

Severe In fluctuation was observed in In0.3Ga0.7N/GaN multiple quantum well solar cells using scanning transmission electron microscopy and energy dispersive x-ray spectroscopy. The high In content and fluctuation lead to low fill factor (FF) of 30% and energy conversion efficiency (η) of 0.48% under the illumination of AM 1.5G. As the temperature was increased from 250 to 300 K, FF and η were substantially enhanced. This strong temperature-dependent enhancement is attributed to the additional contribution to the photocurrents by the thermally activated carriers, which are originally trapped in the shallow quantum wells resulting from the inhomogeneous In distribution.Severe In fluctuation was observed in In0.3Ga0.7N/GaN multiple quantum well solar cells using scanning transmission electron microscopy and energy dispersive x-ray spectroscopy. The high In content and fluctuation lead to low fill factor (FF) of 30% and energy conversion efficiency (η) of 0.48% under the illumination of AM 1.5G. As the temperature was increased from 250 to 300 K, FF and η were substantially enhanced. This strong temperature-dependent enhancement is attributed to the additional contribution to the photocurrents by the thermally activated carriers, which are originally trapped in the shallow quantum wells resulting from the inhomogeneous In distribution.

Nanowire arrays with controlled structure profiles for maximizing optical collection efficiency

The nanowire array (NWA) layers with controlled structure profiles fabricated by maskless galvanic wet etching on Si substrates are found to exhibit extremely low specular reflectance (<0.1%) in the wavelengths of 200–850 nm. The significantly suppressed reflection is accompanied with other favorable antireflection (AR) properties, including omnidirectionality and polarization-insensitivity. The NWA layers are also effective in suppressing the undesired diffuse reflection. These excellent AR performances benefit from the rough interfaces between air/NWA layers and NWA layers/substrate and the decreased nanowire densities, providing the gradient of effective refractive indices. The Raman intensities of Si NWAs were enhanced by up to 400 times as compared with the signal of the polished Si, confirming that the NWA layers enhance both insertion and extraction efficiencies of light. This study provides an insight into the interaction between light and nanostructures, and should contribute to the structural optimization of various optoelectronic devices.

Subwavelength Si nanowire arrays for self-cleaning antireflection coatings

Galvanic wet etching was adopted to fabricate Si nanowire arrays (NWAs) as a near-perfect subwavelength structure (SWS), which is an optically effective gradient-index antireflection (AR) surface and also exhibits super-hydrophobicity with an extremely high water contact angle (159°). Fresnel reflection and diffuse reflection over the broad spectrum can be eliminated by a Si NWA AR coating. Moreover, Si NWA SWSs show polarization-independent and omnidirectional AR properties. The wavelength-averaged specular and diffuse reflectance of Si NWA SWSs are as low as 0.06% and 2.51%, respectively. The effects of the surface profile of this biomimetic SWS on the AR and super-hydrophobic properties were investigated systematically.

Periodic Si nanopillar arrays by anodic aluminum oxide template and catalytic etching for broadband and omnidirectional light harvesting

Large-area, periodic Si nanopillar arrays (NPAs) with the periodicity of 100 nm and the diameter of 60 nm were fabricated by metal-assisted chemical etching with anodic aluminum oxide as a patterning mask. The 100-nm-periodicity NPAs serve an antireflection function especially at the wavelengths of 200~400 nm, where the reflectance is decreased to be almost tenth of the value of the polished Si (from 62.9% to 7.9%). These NPAs show very low reflectance for broadband wavelengths and omnidirectional light incidence, attributed to the small periodicity and the stepped refractive index of NPA layers. The experimental results are confirmed by theoretical calculations. Raman scattering intensity was also found to be significantly increased with Si NPAs. The introduction of this industrial-scale self-assembly methodology for light

Photon management in nanostructured solar cells

The unique geometry and intriguing physical properties of nanostructure-based solar cells gives them great potential to achieve the goals of cost-effectiveness and high-efficiency. With nanostructured solar cells it is expected to be possible to break the Shockley–Queisser limit. This potential has driven widespread research and development in photon management to enhance light absorption over the past decade. However, efficiency is not proportional to light absorption. Nowadays, researchers are starting to address this issue. A thorough understanding of the advantages and the scope of the application of each photon management scheme is critical to finding a breakthrough for this predicament. In this review, we present the theorems and describe recent progresses in primary photon management schemes for nanostructures, including antireflection, light scattering, and resonance (e.g., metallic resonance, dielectric resonance, and photonic crystals). The antireflection effect allows more light to enter the solar cell. Light scattering enhances the interaction between the light and the nanostructure, extending the light propagation paths in the devices. Resonance effects can redirect and precisely confine the light to the region where efficient photoelectric conversion efficiency occurs. Finally, we discuss the challenges of nanostructured solar cells, and indicate potential routes to overcome the performance-limiting problems.

Hierarchical structures consisting of SiO2 nanorods and p-GaN microdomes for efficiently harvesting solar energy for InGaN quantum well photovoltaic cells.

We experimentally and theoretically demonstrated the hierarchical structure of SiO(2) nanorod arrays/p-GaN microdomes as a light harvesting scheme for InGaN-based multiple quantum well solar cells. The combination of nano- and micro-structures leads to increased internal multiple reflection and provides an intermediate refractive index between air and GaN. Cells with the hierarchical structure exhibit improved short-circuit current densities and fill factors, rendering a 1.47 fold efficiency enhancement as compared to planar cells.

An efficient broadband and omnidirectional light-harvesting scheme employing a hierarchical structure based on a ZnO nanorod/Si3N4-coated Si microgroove on 5-inch single crystalline Si solar cells

We employ a ZnO nanorod/Si(3)N(4)-coated Si microgroove-based hierarchical structure (HS) for a light-harvesting scheme in 5 inch single crystalline Si solar cells. ZnO nanorods and Si microgrooves were fabricated by a simple and scalable aqueous process. The excellent light-harvesting characteristics of the HS, such as broadband working ranges and omnidirectionality have been demonstrated using external quantum efficiencies and reflectance measurements. The solar cells with the hierarchical surface exhibit excellent photovoltaic characteristics, i.e., a short-circuit current (J(SC)) of 38.45 mA cm(-2), open-circuit voltage of 609 mV and conversion efficiency of 14.04%. As incident angles increase from 0° to 60°, only 5.3% J(SC) loss is achieved by employing the hierarchical surface, demonstrating the enhanced omnidirectional photovoltaic performances, also confirmed by the theoretical analysis. A viable scheme for broadband and omnidirectional light harvesting using the HS employing microscale/nanoscale surface textures on single crystalline Si solar cells has been demonstrated.

Enhanced light-extraction from hierarchical surfaces consisting of p-GaN microdomes and SiO2 nanorods for GaN-based light-emitting diodes

We report an efficient light-extraction scheme employing the hierarchical structure, p-GaN microdomes/SiO2 nanorods (NRs), on GaN light-emitting diodes (LEDs). Compared with the flat LED, the LEDs with hierarchical surfaces exhibits a light-output improvement of 36.8%. The considerable enhancement in light-extraction efficiency is attributed to the multiple tilted surfaces of microdomes and the graded refractive indexes provided by the SiO2 NRs, reducing total internal reflection and Fresnel reflection. The enhanced optical performances are supported by the finite-difference time-domain analysis. Advances in light extraction scheme employing hierarchical structures demonstrated here pave the way to solid-state lighting technology.

Solar energy harvesting scheme using syringe-like ZnO nanorod arrays for InGaN/GaN multiple quantum well solar cells

Syringe-like ZnO nanorod arrays (NRAs) synthesized by a hydrothermal method were applied as the light-harvesting layer on InGaN-based multiple quantum well (MQW) solar cells. Theoretical calculations show that the NRAs with an abrupt shrinkage of tip diameter can further suppress surface reflectance in comparison with the flat NRAs. InGaN-based MQW solar cells with the syringe-like NRAs exhibit greatly improved conversion efficiencies by 36%. These results are attributed to the improved flatness of the refractive index profile at the air/device interface, which results in enhanced light trapping effect on the device surface.