Efficient Light Harvesting/Extraction Schemes Employing Structure Designs from Microscale to Nanoscale for InGaN/GaN Devices
Date Issued
2012
Date
2012
Author(s)
Ho, Cheng-Han
Abstract
In this thesis, we will firstly focus on InGaN/GaN solar cells, and secondly we move to GaN/InGaN light emitting diodes. The final is our conclusion.
First, SiO2 nanorod arrays (NRAs) are fabricated on InGaN-based multiple quantum well (MQW) solar cells using self-assembled Ag nanoparticles as the etching mask and subsequent reactive ion etching. The SiO2 NRAs effectively suppress the undesired surface reflections over the wavelengths from 330 to 570 nm, which is attributed to the light-trapping effect and the improved mismatch of refractive index at the air/MQW device interface. Under the air mass 1.5 global illumination, the conversion efficiency of the solar cell is enhanced by ~21 % largely due to increased short-circuit current from 0.71 to 0.76 mA/cm2. The enhanced device performances by the optical absorption improvement are supported by the simulation analysis as well.
Second, InGaN-based multiple quantum well (MQW) solar cells (SCs) employing the p-GaN microdome were demonstrated to significantly boost the conversion efficiency by 102 %. The improvements in short-circuit current density (Jsc, from 0.43 to 0.54 mA/cm2) and fill factor (from 44 % to 72 %) using the p-GaN microdome are attributed to enhanced light absorption due to surface reflection suppression. The concept of microdome directly grown during SC epitaxial growth preserving mechanical robustness and wafer-scale uniformity proves a promising way in promoting the photovoltaic performances of SCs without any additional process.
Third, the hierarchical structure of SiO2 nanorod arrays/p-GaN microdomes was applied as a light harvesting scheme on InGaN-based multiple quantum well solar cells. Using self-assembled Ag nanoparticles as the etching mask and subsequent reactive ion etching, SiO2 NRAs were fabricated upon the p-GaN microdomes. Due to the light trapping effect of the roughness and the improved match of refractive index by SiO2 nanorod arrays, the undesired Fresnel reflections are effectively suppressed. Cells with the hierarchical surfaces exhibit excellent photovoltaic performances including enhanced short-circuit current densities and fill factor, and the measured conversion efficiency is enhanced by 1.47-fold. The improved light absorption in device is consistent with the finite-difference time-domain analysis.
Finally, we report the enhanced light extraction efficiency of the hierarchical structure, SiO2 nanorods/p-GaN microdomes, fabricating on InGaN/GaN LEDs. Compared with conventional flat LEDs, the light output intensity of bare microdome LED presents an improvement of 16.7 % at 20 mA, yet it boosts to 36.8 % for SiO2 NRA/p-GaN microdome LED. The results are attributed to the scattering effect and the effective refraction indexes of the textured structures that reduce the total internal reflection, contributing to the most light extraction. The enhanced optical performances are supported by the improved light output power calculated by finite-difference time-domain analysis.
Subjects
Solar cell (SC)
InGaN/GaN
Reactive ion etching (RIE)
Nanorod
Antireflection
Light harvesting
Microdome
Light-emitting diode (LED)
Internal/External quantum efficiency (IQE/EQE)
Light extraction efficiency
SDGs
Type
thesis
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