Dong-Seon Lee

Au nanoparticle-decorated graphene electrodes for GaN-based optoelectronic devices

We studied GaN-based optoelectronic devices such as light-emitting diodes (LEDs) and solar cells (SCs) with graphene electrodes. A decoration of Au nanoparticles (NPs) on multi-layer graphene films improved the electrical conductivity and modified the work function of the graphene films. The Au NP-decorated graphene film enhanced the current injection and electroluminescence of GaN-based LEDs through low contact resistance and improved the power conversion efficiency of GaN-based SCs through additional light absorption and energy band alignment. Our study will enhance the understanding of the role of Au NP-decorated graphene electrodes for GaN-based optoelectronic device applications.

Morphology development of GaN nanowires using a pulsed-mode MOCVD growth technique

In this paper, we demonstrate a scalable process for the precise position-controlled selective growth of GaN nanowire arrays by metalorganic chemical vapor deposition (MOCVD) using a pulsed-mode growth technique. The location, orientation, length, and diameter of each GaN nanowire are controlled via pulsed-mode growth parameters such as growth temperature and precursor injection and interruption durations. The diameter and length of each GaN nanowire are in the ranges of more than 240 nm and 250–1250 nm, respectively, with different vertical-to-lateral aspect ratios that depend on the growth temperature. Also, it is found that a higher growth temperature helps increase the vertical growth rate and reduces the lateral growth rate of GaN nanowire arrays. Furthermore, in the case of longer TMGa injection duration, the Ga-rich region allows the higher lateral growth rate of GaN nanostructures, which leads to a transition in the morphology from nanowires to a thin film, while in the case of longer NH3 injection duration, the surface morphology changes from nanowires to pyramidal structures. In addition, the surface structure can also be controlled by varying the precursor interruption duration. Finally, we report and discuss a growth model for GaN nanowire arrays under pulsed-mode MOCVD growth.

Improved Efficiency by Using Transparent Contact Layers in InGaN-Based p-i-n Solar Cells

InGaN/GaN p-i-n solar cells were fabricated either without a current spreading layer or with ITO or Ni/Au spreading layers. A 10.8% indium composition was confirmed within an i-InGaN layer using X-ray diffraction. I-V characteristics were measured at AM1.5 conditions, with solar cell parameters being obtained based on I-V curves in all cases. Current spreading layers produced strong effects on efficiency. The solar cell with the ITO current spreading layer showed the best results, i.e., a short circuit current density of 0.644 mA/cm2, an open circuit voltage of 2.0 V, a fill factor of 79.5%, a peak external quantum efficiency of 74.1%, and a conversion efficiency of 1.0%.

InGaN-Based p–i–n Solar Cells with Graphene Electrodes

InGaN-based p–i–n solar cells with graphene electrodes were fabricated and compared with solar cells using indium tin oxide (ITO) electrodes. In particular, we analyzed the properties of graphene film by means of high-resolution transmission electron microscopic (HRTEM) and Raman spectroscopy, also comparing optical properties with those of ITO, conventionally used as transparent electrodes. The solar cells using graphene revealed a short circuit current density of 0.83 mA/cm2, an open circuit voltage of 2.0 V, a fill factor of 75.2%, and conversion efficiency of 1.2%, comparable to the performance of solar cells using ITO.

A self-assembled Ag nanoparticle agglomeration process on graphene for enhanced light output in GaN-based LEDs

We introduce Ag nanoparticles fabricated by a self-assembled agglomeration process in order to enhance the electrical properties, adhesive strength, and reliability of the graphene spreading layer in inorganic-based optoelectronic devices. Here, we fabricated InGaN/GaN multi-quantum-well (MQW) blue LEDs having various current spreading layers: graphene only, graphene with Ag nanoparticles covering the surface, and graphene with Ag nanoparticles only in selectively patterned micro-circles. Although the Ag nanoparticles were found to act as an additional current path that increases the current spreading, optical properties such as transmittance also need to be considered when the Ag nanoparticles are combined with graphene. As a result, LEDs having a graphene spreading layer with Ag nanoparticles formed in selectively patterned micro-circles displayed more uniform and stable light emission and 1.7 times higher light output power than graphene only LEDs.

Thin Ni film on graphene current spreading layer for GaN-based blue and ultra-violet light-emitting diodes

We introduced a thin nickel (Ni) film onto graphene as a current spreading layer for GaN-based blue and ultraviolet (UV) light emitting diodes (LEDs). The thin Ni film was confirmed to improve the electrical properties of the graphene by reducing the sheet and contact resistances. The advantages of Ni on graphene were more remarkable in UV LEDs, in which the operation voltage was reduced from 13.2 V for graphene alone to 7.1 V. As a result, UV LEDs with Ni on graphene showed a uniform and reliable light emission, at ∼83% of electroluminescence of indium tin oxide.

Size-controlled InGaN/GaN nanorod array fabrication and optical characterization

We demonstrate a cost-effective top-down approach for fabricating InGaN/GaN nanorod arrays using a wet treatment process in a KOH solution. The average diameter of the as-etched nanorods was effectively reduced from 420 nm to 180 nm. The spatial strain distribution was then investigated by measuring the high-resolution cathodoluminescence directly on top of the nanorods. The smaller nanorods showed a higher internal quantum efficiency and lower potential fluctuation, which can subsequently be exploited for high-efficiency photonic devices.

Effect of indium composition on carrier escape in InGaN/GaN multiple quantum well solar cells

The influence of indium composition on carrier escape was studied considering recombination in InGaN/GaN multiple quantum well solar cells with indium compositions of 17% and 25%. Competition between tunneling and recombination turned out to act as a crucial role for the short-circuit current density (Jsc) and fill factor (FF). To enhance the Jsc and the FF, the tunneling-dominant carrier decay rather than recombination is required in the operating range of the solar cells which is possible by optimizing the band structures for a shorter tunneling time and by improving the crystalline quality for a longer recombination time.

Improved Photovoltaic Effects of a Vertical-Type InGaN/GaN Multiple Quantum Well Solar Cell

We investigated the photovoltaic performance of InGaN/GaN multiple quantum well (MQW) solar cells by comparing vertical-type and conventional lateral-type solar cells. We found that both bottom reflector and front surface texturing of vertical-type InGaN/GaN MQW solar cells enhanced light absorption by 45%, leading to an enhancement of the short circuit current density (JSC) by 1.6 times, compared to that of a lateral-type structure. For the vertical-type InGaN/GaN solar cell, Ag was used for bottom reflectors and pyramid textured surfaces were formed by KOH etching after a lift-off process, whereas lateral-type structures were fabricated on sapphire substrates having smooth surfaces. As a result, the vertical InGaN/GaN MQW solar cells showed a high fill factor of 80.0% and conversion efficiency of 2.3%; in contrast, the conventional lateral structure produced a fill factor of 77.6% and a conversion efficiency of 1.4%.

Efficiency Improvement in InGaN-Based Solar Cells by Indium Tin Oxide Nano Dots Covered with ITO Films

InGaN based MQW solar cells have been fabricated with 4 different transparent top electrode structures: (1)- ITO 200 nm, (2)-ITO nano dots only, (3)-ITO nano dots on ITO 50 nm and (4)-ITO nano dots on ITO 100 nm. The solar cell with the ITO 50 nm on ITO nano dots under AM 1.5 conditions showed the best results: 2.3 V for V(oc), 0.69 mA/cm(2) for J(sc), 41.8% for peak EQE, and 0.91% for conversion efficiency. Efficiency improvement was possible due to the decreased reflectance achieved by the ITO nano dots covered with an ITO film with optimized thickness.