Tae-Soo Kim

Light-soaking effects and capacitance profiling in Cu(In,Ga)Se2 thin-film solar cells with chemical-bath-deposited ZnS buffer layers

We fabricated Cu(In,Ga)Se2 (CIGS) solar cells with chemical-bath deposited (CBD) ZnS buffer layers with different deposition times. The conversion efficiency and the fill factor of the CIGS solar cells reveal a strong dependence on the deposition time of CBD-ZnS films. In order to understand the detailed relationship between the heterojunction structure and the electronic properties of CIGS solar cells with different deposition times of CBD-ZnS films, capacitance-voltage (C-V) profiling measurements with additional laser illumination were performed. The light-soaking effects on CIGS solar cells with a CBD-ZnS buffer layer were investigated in detail using current density-voltage (J-V) and C-V measurements with several different lasers with different emission wavelengths. After light-soaking, the conversion efficiency changed significantly and the double diode feature in J-V curves disappeared. We explain that the major reason for the improvement of efficiency by light-soaking is due to the fact that negatively charged and highly defective vacancies in the CIGS absorber near the interface of CBD-ZnS/CIGS were formed and became neutral due to carriers generated by ultra-violet absorption in the buffer layer.

A Method to Obtain Auger Recombination Coefficient in an InGaN-Based Blue Light-Emitting Diode*

We propose and demonstrate to derive the Auger recombination coefficient by fitting efficiency–current and carrier lifetime–current curves simultaneously, which can minimize the uncertainty of fitting results. The obtained Auger recombination coefficient is cm6s−1 in the present sample, which contributes slightly to efficiency droop effect.

Experimental separation of injection and radiative efficiencies in InGaN/GaN light emitting diodes

The carrier injection efficiency (CIE) and the radiative efficiency (RE) are experimentally determined in order to clarify the origin of the efficiency droop in blue-emitting GaN light emitting diodes. The difference in the shape of RE curves and the external quantum efficiency (EQE) curves shows the CIE is a function of the injection current, while the RE curves show the droop behavior to a certain degree. The experimentally determined CIE is significantly lower than unity and decreases with the current density, indicating that imperfect carrier injection has strong effects on the efficiency droop. Through our analysis, we conclude that both an intrinsic component, such as Auger recombination, and current leakage component exist and make notable contributions to the total EQE droop. These two components can be quantitatively separated through our proposed method. In addition, the result above is also comparatively investigated with the result of time-resolved electroluminescence (TREL) spectroscopies. The obtained CIE and RE consistently explain the behavior of TREL at various current density levels.

Surface-plasmon enhanced Raman scattering of DNA molecules on regular arrays of modified gold nanoparticles

We performed surface-plasmon enhanced Raman scattering (SERS) studies of a DNA monolayer on chemically modified Au nanoparticle arrays. Drastic enhancement of Raman signal from DNA molecules was observed on the optimized templates that were prepared by using electron-beam lithography followed by chemical modification. We observed that Raman signal of DNA molecules was enhanced when the surface plasmons in the closely-spaced Au nanoparticles were resonantly excited. Our results suggest that the artificial generation of spatially controlled hot spots for SERS can be achieved by this technique.