Mikiko Mori

Correlation between Device Performance and Defects in GaInN-Based Solar Cells

We investigated the correlation between the device performance and defects, such as V-shaped pits and threading dislocations, in GaInN-based solar cells. To realize high-performance GaInN-based solar cells with a high open-circuit voltage and fill factor, it is essential to realize a low pit density of less than 107 cm-2. In this study, we were unable to observe clear evidence of any effect of the threading dislocation density in the GaN underlying layer.

Concentrating Properties of Nitride-Based Solar Cells Using Different Electrodes

We investigated the concentration properties of GaInN-based solar cells using different window electrodes. A significant difference was observed between the concentrating properties of the window electrode structures. It was clearly found that indium tin oxide (ITO) is suitable as an electrode. The short-circuit current density, open-circuit voltage, fill factor, and energy conversion efficiency of nitride-based solar cells fabricated using an ITO electrode were 7.1×102 mA/cm2, 2.2 V, 79%, and 4.0%, respectively, under an air mass filter of 1.5G at 300 suns and at room temperature.

Introduction of the moth-eye patterned sapphire substrate technology for cost-effective high-performance LEDs

The improvement of the performance and the reduction of the cost for LEDs using the moth-eye patterned sapphire substrate (MPSS) were investigated. TEM and CL observation results clearly show that the MPSS can provide a thinner GaN template of equal or higher quality than the micron scale patterned sapphire substrate (PSS) since the MPSS only has a submicron scale structure. A 3-μm-thick high quality GaN template on the MPSS with a dislocation density of 1.9×108 cm-2 has been demonstrated. The LEDs on MPSS with a 600 nm pitch show the highest light output among the evaluated LEDs on various types of substrates as it is 1.52 times higher than that on flat sapphire substrate (FSS) and 1.18 times higher than that on PSS. The pitch dependence of the light output improvement is in excellent agreement with that of the transmittance at GaN/sapphire interface simulated by the rigorous coupled wave analysis. As a result of comparisons for the GaN templates and LEDs on MPSS, PSS and FSS, it can be concluded that MPSS provides the most cost effective solution for high performance LED.

Concentrating properties of nitride-based solar cells using GaInN/GaInN superlattices

We investigated the concentrating properties of nitride based solar cells at light intensities of up to 200 suns at room temperature. The devices were GaInN-based solar cells with a GaInN/GaInN superlattice active layer on a freestanding GaN substrate. The conversion efficiency of these solar cells increased with increasing of concentration up to 200 suns. We obtained the solar cells with a pit-free structure and up to 3.4% conversion efficiency by irradiating concentrated sunlight with intensities of up to 200 suns. The short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency were 510 mA/cm2, 1.9 V, 70%, and 3.4%, respectively, under an air mass 1.5G at 200 suns and room temperature. We also discuss the relationship between crystal quality and solar cell performance.

Indium-zinc oxide transparent electrode for nitride-based light-emitting diodes

The basic properties of indium-zinc oxide (IZO) were investigated from the view point of the potential of light-emitting diodes (LEDs) for nanostructured transparent contact. The resistivity and contact resistance to p-GaN were obtained to be 2.5×10-4 Ωcm and 9.4×10-4 Ωcm2, respectively, which are comparable to those of indium-tin oxide (ITO). The light output of the LED with the moth-eye IZO was 10 % and 40 % higher than that of the LED with the moth-eye ITO and that of the LED without the moth-eye structure, respectively.

Fabrication of high-efficiency LED using moth-eye structure

To realize high-efficiency light-emitting diodes (LEDs), it is essential to increase light extraction efficiency. The moth-eye structure, consisting of periodic cones with a submicron-scale pitch on a surface/interface, is known to enhance the light extraction efficiency of light-emitting diodes. Previously, nitride-based flip-chip blue LEDs with the moth-eye structure on the backside of the substrate were demonstrated to exhibit high light extraction efficiency. In this report, face-up blue LEDs with a double moth-eye structure are described. One moth-eye structure was formed on the sapphire substrate and the other structure was formed on the ITO contact, where the pitch of the cones in both structures was 500 nm. The patterning of cones with such a small pitch was carried out by a low-energy electron-beam lithography technique. The output powers of mounted LEDs without resin encapsulation were measured using an integrated sphere. For reference, face-up LEDs without a moth-eye structure and with a single moth-eye structure on an ITO contact were also examined. The single moth-eye and double moth-eye LEDs have 1.2 and 1.4 times higher output power than the non-moth-eye LED, receptively.