Chuan Feng Shih

Efficiency improvement of blended poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 solar cells by nanoimprinting

This work demonstrates the effects of nanoimprinting on poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (P3HT:PCBM)-blended organic solar cells at room temperature. Textured Si wafer was used as a stamp. Nanoimprinting significantly increased the open-circuit voltage, the short-circuit current, and the fill factor, increasing the power conversion efficiency by ∼50%. The fill factor contributed most to the cell efficiency. Upon nanoimprinting, not only the surface structure but also the applied pressure contributed to the performance of the device. The origin of the hydrostatic pressure-induced efficiency improvement was also investigated. The proposed approach has potential to be applied in the future to improve the efficiency of various organic solar cells.

Band Offsets of InN/GaN Interface

In this paper, we report on the band discontinuities of the wurtzite-InN/GaN interface. X-ray photoemission spectroscopy studies reveal that the offset ratios of conduction bands and valence bands are approximately 80 and 20%, respectively. The valence band offset (0.5 eV) is close to the theoretical value determined on the basis of the density functional theory from first principle that was reported by Wei and Zunger [Appl. Phys. Lett. 69 (1996) 2719]. The photoluminescence signals of InN/GaN quantum wells were also studied. The luminescence of the wells showed a 60 meV quantum confinement shift from the bulk InN signal. The finite potential well model of quantum mechanics is used to show that this shift supports the above results.

In-rich In1−xGaxN films by metalorganic vapor phase epitaxy

Single crystalline In1−xGaxN films containing high In content (70%–100%) were grown by metalorganic vapor phase epitaxy. A linear relation was observed between the lattice constants and gas phase Ga∕In ratios. The surface morphology changed from pyramid for InN to more planar ones for the InGaN alloys with increasing Ga content. The electron mobility decreased rapidly from 1200cm2∕Vs for InN to less than 100cm2∕Vs for In0.7Ga0.3N, with a carrier concentration of low- 1019cm−3 for all the as-grown films. Using photoluminescence a single emission peak was observed at 1.4–1.6μm for the In-rich InGaN with decreasing wavelengths up to below 20% of Ga. Two peaks were observed for the In0.80Ga0.20N, however, indicating possible phase separation. The x-ray photoelectron spectroscopic measurement showed shifts to higher binding energies for both In and Ga with increasing Ga content. The estimated alloy composition, however, depended sensitively on the sputtering conditions of the samples.

Effect of nitrogen doping on the electron field emission properties of chemical vapor deposited diamond films

Abstract Nitrogen-doped diamond films were successfully synthesized using a urea/methanol saturated solution as a nitrogen source. Scanning electron microscopy (SEM) and Raman spectroscopy revealed that the morphology and quality of the diamond films were not altered due to nitrogen doping. However, increasing the urea/methanol ratio in gas mixture markedly influences the field emission properties of diamond films, which were optimized when the films were grown using H 2 /CH 4 /urea=300:18:6 sccm. The electron field emission of the films can be turned on at a low field as 5.0 V μm −1 and the emission current density as large as J e =1019 μA cm −2 can be attained at 21.6 V μm −1 applied field. The effective work function is φ e =0.034 eV.

Concentration dependence of carrier localization in InN epilayers

The authors studied the concentration dependence of carrier localization in InN epilayers using time-resolved photoluminescence (PL). Based on the emission-energy dependence of the PL decays and the PL quenching in thermalization, the localization energy of carriers in InN is found to increase with carrier concentration. The dependence of carrier concentration on the localization energy of carriers in InN can be explained by a model based on the transition between free electrons in the conduction band and localized holes in the deeper tail states. They suggest that carrier localization originates from the potential fluctuations of randomly located impurities.

Electron transport in In-rich InxGa1−xN films

We have performed electrical transport measurements on metal-organic vapor phase epitaxy grown In-rich InxGa1−xN (x=1, 0.98, and 0.92) films. Within the experimental error, the electron density in InGaN films is temperature independent over a wide temperature range (4K⩽T⩽285K). Therefore, InxGa1−xN (0.92⩽x⩽1) films can be regarded as degenerate semiconductor systems. The experimental results demonstrate that electron transport in In-rich InxGa1−xN (x=1, 0.98, and 0.92) films is metalliclike. This is supported by the temperature dependence of the density, resistivity, and mobility which is similar to that of a metal. We suggest that over the whole measuring temperature range residue imperfection scattering limits the electron mobility in In-rich InxGa1−xN (x=1, 0.98, and 0.92) films.

AlGaN films grown on (0001) sapphire by a two-step method

A two-step growth method, commonly used for GaN on sapphire, was applied to grow high-quality Al0.2Ga0.8N on sapphire. Comparing to the one grown on a low-temperature grown AlN buffer layer, the decomposition, recrystallization, and islands coalescence processes of the two-step growth increased the surface flatness, the crystal quality, the electrical property, suppressed the phase separation, and released the biaxial tensile strain. A 2.0μm thick high-quality crack-free nearly GaN-free Al0.2Ga0.8N epilayer was obtained.

Schottky behavior at InN–GaN interface

In this work, GaN Schottky diodes were fabricated by depositing InN on GaN surfaces. The junction between these two materials exhibits strong rectifying behavior. The barrier heights were determined to be 1.25 eV, 1.06 eV, and 1.41 eV by current-voltage, current-voltage-temperature, and capacitance-voltage methods, respectively. These values exceed those of any other metal∕GaN Schottky barriers. Therefore, the conduction-band offset between InN and GaN should not be smaller than the barrier heights obtained here.

Application of modified transmission line model to measure p-type GaN contact

This work presented a procedure for extending the modified transmission line model to measure non-ohmic contact. This method was applied to the p-type GaN contact with the resulting sheet resistance similar to that determined by the Hall measurement. The voltage–current density (V–J) curve obtained using this procedure was also similar to that by directly analyzing the current–voltage curve of a light-emitting diode. Both results revealed the validity of this procedure. Rather than yielding a specific contact resistance for an ohmic contact, this procedure yielded a V–J curve to describe the non-ohmic contact characteristics. Similarly, this procedure could also extend the linear transmission line model to the analysis of non-ohmic contacts.

Incorporation of potassium at CuPc/ C60 interface for photovoltaic application

This letter demonstrates the effect of potassium (K) doping at the donor–acceptor interface of CuPc / C 60 -based organic solar cells. Power conversion efficient (PCE) doubled when a little K was doped into the CuPc / C 60 interface and the device was postannealed ( 75 ° C , 10 min). Changes in binding energies, depletion capacitance, and mobilities of electrons and holes were investigated. The marked improvement of PCE arose mostly from the increase in short-circuit current, owing to the modified charge transfer process.