Jae-Phil Shim

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.

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.

Improved photovoltaic effects in InGaN-based multiple quantum well solar cell with graphene on indium tin oxide nanodot nodes for transparent and current spreading electrode

We implemented graphene network on indium tin oxide (ITO) nanodots to a transparent and current-spreading electrode in InGaN-based solar cell to improve power conversion efficiency. The external quantum efficiency and short circuit current density (Jsc) of solar cells with graphene network on ITO nanodots were enhanced compared to those of solar cells with ITO and bare graphene film. The increase of the power conversion efficiency is attributed to the high transmittance, internal light-scattering effect, and effective carrier absorption of ITO nanodots.

Fabrication of a vertically-stacked passive-matrix micro-LED array structure for a dual color display

We report a color tunable display consisting of two passive-matrix micro-LED array chips. The device has combined vertically stacked blue and green passive-matrix LED array chips sandwiched by a transparent bonding material. We demonstrate that vertically stacked blue and green micro-pixels are independently controllable with operation of four color modes. Moreover, the color of each pixel is tunable in the entire wavelength from the blue to green region (450 nm - 540 nm) by applying pulse-width-modulation bias voltage. This study is meaningful in that a dual color micro-LED array with a vertically stacked subpixel structure is realized.

Monolithic integration of AlGaInP-based red and InGaN-based green LEDs via adhesive bonding for multicolor emission

In general, to realize full color, inorganic light-emitting diodes (LEDs) are diced from respective red-green-blue (RGB) wafers consisting of inorganic crystalline semiconductors. Although this conventional method can realize full color, it is limited when applied to microdisplays requiring high resolution. Designing a structure emitting various colors by integrating both AlGaInP-based and InGaN-based LEDs onto one substrate could be a solution to achieve full color with high resolution. Herein, we introduce adhesive bonding and a chemical wet etching process to monolithically integrate two materials with different bandgap energies for green and red light emission. We successfully transferred AlGaInP-based red LED film onto InGaN-based green LEDs without any cracks or void areas and then separated the green and red subpixel LEDs in a lateral direction; the dual color LEDs integrated by the bonding technique were tunable from the green to red color regions (530–630 nm) as intended. In addition, we studied vertically stacked subpixel LEDs by deeply analyzing their light absorption and the interaction between the top and bottom pixels to achieve ultra-high resolution.