Min-De Yang

Improvement of conversion efficiency for multi-junction solar cells by incorporation of Au nanoclusters.

We studied the photoluminescence (PL) and photovoltaic current-voltage characteristics of the three-junction InGaP/InGaAs/Ge solar cells by depositing Au nanoclusters on the cell surface. The increases of the PL intensity and short-circuit current after incorporation of Au nanoclusters are evident. An increase of 15.3% in energy conversion efficiency (from 19.6 to 22.6%) is obtained for the three-junction solar cells in which Au nanoclusters have been incorporated. We suggest that the increased light trapping due to radiative scattering from Au nanoclusters is responsible for improving the performance of the three-junction solar cells.

Time-resolved photoluminescence in anodic aluminum oxide membranes

We have investigated the time-resolved photoluminescence (PL) from anodic aluminum oxide membranes (AAOMs). The PL intensity shows a nonexponential decay, which can be fitted by a stretch-exponential function. Based on the carrier lifetime and the PL emission-energy dependence of the PL decay time, we suggest that the F+-center is responsible for the blue PL in AAOMs. The temperature dependence of the PL lifetime has been explained using a model based on thermally activated nonradiative recombination.

Visible luminescence properties of (Ga1−xZnx)(N1−xOx) solid solution (x = 0.22)

Temperature-dependent photoluminescence (PL) and time-resolved photoluminescence (TRPL) are measured for the (Ga1−xZnx)(N1−xOx) solid solution with x = 0.22 to study its luminescence properties. PL result shows that the material exhibits visible luminescence at around 1.87 eV (663 nm) with a broad emission band even at room temperature. The origin of luminescence mechanism can be attributed to the radiative recombination of the electrons bound to donors and the holes bound to acceptors. The investigation from the integrated PL intensity and TRPL as a function of temperature indicates that the activation energy for thermalizing the electrons bound to a donor dominates the luminescence behavior in the (Ga1−xZnx)(N1−xOx) solid solution.

Density-dependent energy relaxation of hot electrons in InN epilayers

This work investigates the dependence of the hot-electron energy relaxation in InN epilayers on electron density. From the high-energy tail of photoluminescence, the electron temperature of the hot electrons was determined. Acoustic phonons have an important role in the energy relaxation of the hot electrons. The density-dependent electron energy loss rate in InN can be explained by a combination of longitudinal optical and acoustic phonon emissions. A slowing of energy loss rate at high electron densities was observed and attributed to piezoelectric coupling to acoustic phonons.

Structural and optical characteristics of γ-In 2 Se 3 Nanorods grown on Si substrates

This study attempted to grow single-phase γ-In2Se3 nanorods on Si (111) substrates by metal-organic chemical vapor deposition (MOCVD). High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) confirmed that the In2Se3 nanorods are singularly crystallized in the γ phase. The photoluminescence of γ-In2Se3 nanorods at 15 K was referred to as free and bound exciton emissions. The bandgap energy of γ-In2Se3 nanorods at room temperature was determined to be ∼1.99 eV, obtained from optical absorption.

Improvement of Material Quality of Multijunction Solar Cells by Rapid Thermal Annealing

We studied the CW and time-resolved photoluminescence (PL), and spectral response of three-junction InGaP/InGaAs/Ge solar cells following rapid thermal annealing (RTA). The improvement of material quality in the InGaP active layer after RTA is evident from the PL and spectral response. If the annealing temperature is 300 °C, the 10 K PL intensity is maximum, which increases by about a factor of 30 compared with that of the untreated sample. We suggest that the removal of the phosphorus-vacancy-related complexes is responsible for improvement of the material quality after RTA. The photocurrent of the cell also increases following RTA if the incident photon energy is greater than ~1.8 eV.

Optical studies of InN epilayers on Si substrates with different buffer layers

We have investigated the photoluminescence (PL) and time-resolved PL from the InN epilayers grown on Si substrates with different buffer layers. The narrowest value of the full width at half maximum of the PL peak is 52 meV with the AlN/AlGaN/GaN triple buffer layer, which is better than previous reports on similar InN epilayers on Si substrates. Based on the emission-energy dependence of the PL decays, the localization energy of carriers is also the least for the InN with a triple buffer layer. According to the x-ray diffraction measurements, we suggest that the reduced lattice mismatch between the InN epilayer and the top buffer layer is responsible for improvement of sample quality using the buffer-layer technique.

Dependence of Biasing Voltage and Illumination Power on the Built-in Electric Field of InGaP Solar Cells

The electroreflectance spectra of InGaP solar cells at various biasing voltages and illumination levels were studied. The Franz–Keldysh oscillations were observed and the electric field at the junction of solar cells can be extracted. The measured electric field decreases with increasing biasing voltage and illumination level. The theoretical electric fields as a function of the biasing voltage and the photocurrent are calculated on the basis of the depletion approximation in the junction theory and the photovoltaic effect, respectively.

Measuring the Junction Temperature of GaInP/GaInAs/Ge Multijunction Solar Cells Using Photoluminescence

The junction temperatures of the three individual subcells of InGaP/InGaAs/Ge solar cells were measured using photoluminescence (PL) with three different excitation lasers. With the illumination of an extra xenon–mercury lamp, the linear relationship between the PL energy and the illumination level is clearly observed and advantageously used for deriving the junction temperature. Using the Varshni relationship between the PL peak energy and the heat-sink temperature allows us to determine the junction temperature in each subcell.

Improving performance of InGaN/GaN light-emitting diodes and GaAs solar cells using luminescent gold nanoclusters

We studied the optoelectronic properties of the InGaN/GaN multiple-quantum-well light emitting diodes (LEDs) and singlejunction GaAs solar cells by introducing the luminescent Au nanoclusters. The electroluminescence intensity for InGaN/GaN LEDs increases after incorporation of the luminescent Au nanoclusters. An increase of 15.4% in energy conversion efficiency is obtained for the GaAs solar cells in which the luminescent Au nanoclusters have been incorporated. We suggest that the increased light coupling due to radiative scattering from nanoclusters is responsible for improving the performance of the LEDs and solar cells.