C.H. Chang

Oblique electron-beam evaporation of distinctive indium-tin-oxide nanorods for enhanced light extraction from InGaN/GaN light emitting diodes

This paper presents a novel and mass-producible technique to fabricate indium-tin-oxide (ITO) nanorods which serve as an omnidirectional transparent conductive layer (TCL) for InGaN/GaN light emitting diodes (LEDs). The characteristic nanorods, prepared by oblique electron-beam evaporation in a nitrogen ambient, demonstrate high optical transmittance (T>90%) for the wavelength range of 450nm to 900nm. The light output power of a packaged InGaN/GaN LED with the incorporated nanorod layer is increased by 35.1% at an injection current of 350mA, compared to that of a conventional LED. Calculations based on a finite difference time domain (FDTD) method suggest that the extraction enhancement factor can be further improved by increasing the thickness of the nanorod layer, indicating great potential to enhance the luminous intensity of solid-state lighting devices using ITO nanorod structures.

Improved optical transmission and current matching of a triple-junction solar cell utilizing sub-wavelength structures

Sub-wavelength antireflective structures are fabricated on a silicon nitride passivation layer of a Ga₀.₅In₀.₅P/GaAs/Ge triple-junction solar cell using polystyrene nanosphere lithography followed by anisotropic etching. The fabricated structures enhance optical transmission in the ultraviolet wavelength range, compared to a conventional single-layer antireflective coating (ARC). The transmission improvement contributes to an enhanced photocurrent, which is also verified by the external quantum efficiency characterization of the fabricated solar cells. Under one-sun illumination, the short-circuit current of a cell with sub-wavelength structures is enhanced by 46.1% and 3.4% due to much improved optical transmission and current matching, compared to cells without an ARC and with a conventional SiN(x) ARC, respectively. Further optimizations of the sub-wavelength structures including the periodicity and etching depth are conducted by performing comprehensive calculations based on a rigorous couple-wave analysis method.

Boosted Gain of the Differential Amplifier Using the Second Gate of the Dual-Gate a-IGZO TFTs

A dual-gate amorphous InGaZnO4 (a-IGZO) thin-film transistor (TFT) has two gate electrodes. The primary gate electrode is at the bottom, and the other is on the top. The drain current of the TFT can be controlled by both the bottom and top gates. This phenomenon provides great flexibility for the circuit design. In this letter, we propose the differential amplifier circuit using the top gate of the dual-gate a-IGZO TFT as an input for positive feedback to boost the gain. It is experimentally verified that the gain of the differential amplifier circuit is increased three times larger than those without the feedback loop.

A Metal-Insulator-Semiconductor Solar Cell With High Open-Circuit Voltage Using a Stacking Structure

A stacking metal-insulator-semiconductor (MIS) solar cell structure, which integrates an n-type MIS solar cell with a p-type MIS solar cell, is proposed to effectively enlarge the open-circuit voltage (<i>V</i>_oc). The measured <i>V</i>_oc is up to 0.71 V under simulated air mass 1.5 illumination (100 mW/cm²). This <i>V</i>_oc is larger than those of the n-type or p-type MIS solar cells with or without surface passivation. In this letter, we successfully demonstrate the feasibility of the <i>V</i>_oc enhancement of MIS solar cells by using a stacking structure.

Indium-tin-oxide nanowhiskers crystalline silicon photovoltaics combining micro- and nano-scale surface textures

In this work, we present a solution that employs combined micro- and nano-scale surface textures to increase light harvesting in the near infrared for crystalline silicon photovoltaics, and discuss the associated antireflection and scattering mechanisms. The combined surface textures are achieved by uniformly depositing a layer of indium-tin-oxide nanowhiskers on passivated, micro-grooved silicon solar cells using electron-beam evaporation. The nanowhiskers facilitate optical transmission in the near-infrared, which is optically equivalent to a stack of two dielectric thin-films with step- and graded- refractive index profiles. The ITO nanowhiskers provide broadband anti-reflective properties (R<5%) in the wavelength range of 350-1100nm. In comparison with conventional Si solar cell, the combined surface texture solar cell shows higher external quantum efficiency (EQE) in the range of 700-1100nm. Moreover, the ITO nano-whisker coating Si solar cell shows a high total efficiency increase of 1.1% (from 16.08% to17.18%). Furthermore, the nano-whiskers also provide strong forward scattering for ultraviolet and visible light, favorable in thin-wafer silicon photovoltaics to increase the optical absorption path.

Biomimetic antireflection coating for efficiency enhancement of triple-junction solar cells utilizing nanosphere lithography

In this work, we demonstrate a thorough device design, fabrication, characterization, and analysis of biomimetic antireflective structures implemented on a Ga0.5In0.5P/GaAs/Ge triple-junction solar cell. The sub-wavelength structures are fabricated on a silicon nitride passivation layer using polystyrene nanosphere lithography followed by anisotropic etching. Both the reflectance spectroscopy and external quantum efficiency measurements confirm the improved optical absorption in the ultraviolet/blue and near-infrared wavelengths. Hence, the conversion efficiency of cell with sub-wavelength structure (SWS) is enhanced by 46.1% and 3.4% due to much improved current-matching, compare to cells without an ARC and SL ARC, respectively. We further employ RCWA method to analyze the reflectance by tuning structural factors such as periodicity or height of the SWS.

Enhanced angular response of power conversion efficiency for silicon solar cells utilizing a uniformly distributed nano-whisker medium

In the research of photovoltaic devices, eliminating Fresnel reflection loss is a critical issue on the way to pursue higher efficiency. To maximize the power conversion efficiency, dielectric antireflective coating shows a cost-effective approach, but not enough to absorb broadband solar radiation effectively. Recently, the functional nanostructure shows high potential to be an omnidirectional antireflective coating for the photovoltaic devices. Here we demonstrate Indium-Tin-Oxide (ITO) nano-whiskers, grown by the self-catalyst vapor-liquid-solid (VLS) mechanisms on the textured Si substrate. The ITO nano-whiskers can provide broadband anti-reflective properties (R<5%) in the wavelength range of 350–1100nm. In comparison with conventional Si solar cell, the ITO nano-whiskers coating solar cell shows higher external quantum efficiency (EQE) in the range of 700–1100nm. Moreover, the ITO nano-whisker coating Si solar cell shows a high total efficiency increase of 1.1% (from 16.08% to17.18%). The angular response of the conversion efficiency also increases from 7% at the normal incidence to more than 15% for incident angles over 70°.

Novel Indium-Tin-Oxide nano-whiskers for enhanced transmission of surface-textured silicon photovoltaic cells

Conductive Indium-Tin-Oxide (ITO) nano-whiskers were deposited on surface-textured Si solar cells using glancing-angle electron-beam deposition. With different deposited time, the ITO nano-structured layer exhibit tunable thickness, which can be related to the surface reflectance. The optimized nano-whisker surface demonstrates a broadband anti-reflective properties (R≪5%), better than the traditional Si3N4 antireflection coating. Current-voltage and quantum efficiency analyses with the measured reflectivity data show enhanced optical transmission in the long wavelength range from 700nm to 1000nm, corresponding to a conversion efficiency improvement from 13.93% to 14.37%.

High efficiency gallium arsenide solar cells using Indium-Tin-Oxide nano-columns

An array of conductive Indium-Tin-Oxide (ITO) nano-columns is deposited on GaAs solar cells using the oblique-angle electron-beam deposition method. Calculations based on a rigorous coupled-wave analysis method show that such ITO nano-columns offer superior angular and spectral anti-reflective (AR) properties. The optical characteristics of the ITO nano-columns are described. The conversion efficiency of GaAs solar cells with ITO nano-columns as the AR coating is increased by 23%, mainly limited by the quantum efficiency at the wavelengths below 600nm.

Characteristics of MIS solar cells using sputtering SiO 2 insulating layers

In this study, the characteristics of both p-type and n-type Metal-Insulator-Semiconductor (MIS) solar cells with sputtering SiO2 insulating layers fabricated by radio-frequency (RF) magnetron sputtering are investigated. The characteristics of MIS solar cells are considerably affected by the thickness of the SiO2 insulating layer and a hydrogen (H2) annealing process. Moreover, the performance of MIS solar cells with sputtering SiO2 insulating layer can be greatly enhanced by depositing a SiNx passivation layer on top. It suggests that a sputtering SiO2 insulating layer is a good alternative insulating layer for MIS solar cells.