Yiren Chen

Realization of a High-Performance GaN UV Detector by Nanoplasmonic Enhancement

Realization of highly responsivity UV detectors is a critical challenge for accelerating the application of UV detectors. Exploiting nanoplasmonic enhancement, Ag nanoparticles have been formed on the GaN surface and the responsivity of the GaN UV detector has been enhanced about 30 times compared with that without Ag nanoparticles.

Influence of threading dislocations on GaN-based metal-semiconductor-metal ultraviolet photodetectors

The influence of threading dislocations on the properties of GaN-based metal-semiconductor-metal (MSM) ultraviolet photodetectors was investigated. It was found that screw dislocations had a strong influence on the dark current of the photodetectors, while edge dislocations had the predominant effect on their responsivity. The dark current increased as the screw dislocation density increased due to their lowering of the Schottky barrier height. However, the responsivity of the photodetectors decreased with increasing edge dislocation density because of the dangling bonds along those edge dislocation lines which enhance the recombination of photogenerated electron-hole pairs. The results suggest that reducing both the screw and edge dislocation densities is an effective way to improve the photoelectric property of GaN-based MSM ultraviolet photodetectors.

High response organic ultraviolet photodetector based on blend of 4,4 ', 4 ''-tri-(2-methylphenyl phenylamino) triphenylaine and tris-(8-hydroxyquinoline) gallium

The authors demonstrate high response organic ultraviolet (UV) photodetector (PD) using 4,4′,4″-tri-(2-methylphenyl phenylamino) triphenylaine (m-MTDATA) and tris-(8-hydroxyquinoline) gallium (Gaq3) to act as the electron donor and acceptor, respectively. The m-MTDATA:Gaq3 blend device shows a photocurrent of 405μA∕cm−2 at −8V, corresponding to a response of 338mA∕W under an illumination of 365nm UV light with an intensity of 1.2mW∕cm2. The high response is attributed to the enhanced dissociation of geminate hole-electron pairs in the distributed heterojunction of the blend and suppression of radiative decay. Photophysics of the PD involved is also discussed in terms of the performance and device structures.

Effect of asymmetric Schottky barrier on GaN-based metal-semiconductor-metal ultraviolet detector

An asymmetric Schottky barrier metal-semiconductor-metal (MSM) ultraviolet (UV) detector with Ni/GaN/Au structure was designed and the effect of the asymmetric Schottky barrier on the detector response was investigated. This detector had response at 0 V bias and increased responsivity when a positive bias was applied to the Ni/GaN contact; however, the internal gain disappeared when a negative bias was applied to this point. This contrasts with a symmetric Ni/GaN/Ni Schottky barrier MSM UV detector which had no internal gain under positive/negative bias and almost no response at 0 V bias. The improved performance of the asymmetric Schottky barrier detector was because of the lower work function of Au causing reduction of Schottky barrier and hence enhancing a hole-accumulating and trapping process, which resulted in internal gain.

Cascade-energy-level alignment based organic photovoltaic cells by utilizing copper phthalocyanine as bipolar carrier transporting layer

We demonstrate a cascade-energy-level alignment based organic photovoltaic cell by using stacking three materials with appropriate energy levels. A cell with a structure of ITO/4,4′,4″-tris[N,(3-methylphenyl)-N-phenylamino]-triphenylamine (m-MTDATA)/copper phthalocyanine (CuPc)/fullerene (C60)/4,4′-N,N′-dicarubreneazolebiphenyl (BCP)/LiF/Al was shown to have a power efficiency enhancement in more than 30% over that of a standard reference cell (ITO/CuPc/C60/BCP/LiF/Al), which has only one exciton-dissociation interface. The efficiency improvement was mainly ascribed to the ingenious cascade-energy-level alignment and the application of the bipolar carrier transporting property.

Very high-efficiency organic light-emitting diodes based on cyclometallated rhenium (I) complex

A phosphorescent (Ph) cyclometallated rhenium (I) (ReI) complex was synthesized by reacting 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline with pentacarbonylbromorhenium in refluxing toluene solutions. The precipitates were easily sublimed to obtain a pure electrically neutral carbonyl diamine ReI complex, (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)Re(CO)3Br. The Re complex was used as an orange emitting dopant in 4,4′-N,N′-dicarbazole-biphenyl host to fabricate Ph organic light-emitting diodes (PhOLEDs). The maximum electroluminescence (EL) efficiency and luminance of 21.8cd∕A and 8315cd∕cm2 were harvested, respectively, which were the highest EL results among the PhOLEDs based on ReI complexes. The improvements of the EL performances could be ascribed to the synergistic effects of together incorporation of two reciprocally repulsive phenyl and methyl groups on the backbone of 1,10-phenanthroline molecule of the ligand.

Double wavelength ultraviolet light sensitive organic photodetector

The authors demonstrate an organic ultraviolet (UV) photodetector (PD) device in which 1,3,5-tris(3-methylphenyl-phenylamino)-triphenyamine and 1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene were used as the electron donor and acceptor, respectively. The PD diode offers responses of 75.2 and 22.5 mA/W as the 365 and 330 nm UV light with 1.0 mW/cm2 intensities illuminate the PD diode through anode and cathode sides, respectively. It is interesting that only the planar heterojunction structure diode can provide the special response feature while bulk-heterojunction device could not do it. The working mechanism of the PD diode was also discussed in detail.

Improved performance of GaN metal-semiconductor-metal ultraviolet detectors by depositing SiO2 nanoparticles on a GaN surface

GaN metal-semiconductor-metal (MSM) ultraviolet detectors were investigated by depositing different density of SiO2 nanoparticles (SNPs) on the GaN. It was shown that the dark current of the detectors with SNPs was more than one order of magnitude lower than that without SNPs and the peak responsivity was enhanced after deposition of the SNPs. Atomic force microscopy observations indicated that the SNPs usually formed at the termination of screw and mixed dislocations, and further current-voltage measurements showed that the leakage of the Schottky contact for the GaN MSM detector decreased with deposited the SNPs. Moreover, the leakage obeyed the Frenkel–Poole emission model, which meant that the mechanism for improving the performance is the SNPs passivation of the dislocations followed by the reduction in the dark current.GaN metal-semiconductor-metal (MSM) ultraviolet detectors were investigated by depositing different density of SiO2 nanoparticles (SNPs) on the GaN. It was shown that the dark current of the detectors with SNPs was more than one order of magnitude lower than that without SNPs and the peak responsivity was enhanced after deposition of the SNPs. Atomic force microscopy observations indicated that the SNPs usually formed at the termination of screw and mixed dislocations, and further current-voltage measurements showed that the leakage of the Schottky contact for the GaN MSM detector decreased with deposited the SNPs. Moreover, the leakage obeyed the Frenkel–Poole emission model, which meant that the mechanism for improving the performance is the SNPs passivation of the dislocations followed by the reduction in the dark current.

Highly efficient green organic light-emitting diodes from single exciplex emission

Spectral single and stable green exciplex emission was demonstrated from organic light-emitting diodes (OLEDs) with 4,4′,4″-tris[3-methylphenyl(phenyl)amino] triphenylamine and 4,7-diphenyl-1,10-phenanthroline that function as electron donor (D) and acceptor (A), respectively. As 8-hydroxyquinoline aluminum (Alq3) was attached to the acceptor layer, electroluminescent (EL) properties of the two exciplex-type OLEDs with D/A-bilayer and D:A mixture layer configurations were markedly improved, i.e., a peak current efficiency of 7.6cd∕A at 2.38mA∕cm2 in three-layer device and a maximum luminance of 6620cd∕m2 at 8.7V in blend layer device were obtained, respectively, without changing the peak position (535nm) and the shape of EL spectrum. Discussion is given on the harvest of the pure green exciplex emission and enhancement of luminance which is obtained by inserting Alq3 layer.

Tunable Dipole Surface Plasmon Resonances of Silver Nanoparticles by Cladding Dielectric Layers

The tunability of surface plasmon resonance can enable the highest degree of localised surface plasmon enhancement to be achieved, based on the emitting or absorbing wavelength. In this article, tunable dipole surface plasmon resonances of Ag nanoparticles (NPs) are realized by modification of the SiO2 dielectric layer thicknesses. SiO2 layers both beneath and over the Ag NPs affected the resonance wavelengths of local surface plasmons (LSPs). By adjusting the SiO2 thickness beneath the Ag NPs from 5 nm to 20 nm, the dipole surface plasmon resonances shifted from 470 nm to 410 nm. Meanwhile, after sandwiching the Ag NPs by growing SiO2 before NPs fabrication and then overcoating the NPs with various SiO2 thicknesses from 5 nm to 20 nm, the dipole surface plasmon resonances changed from 450 nm to 490 nm. The SiO2 cladding dielectric layer can tune the Ag NP surface charge, leading to a change in the effective permittivity of the surrounding medium, and thus to a blueshift or redshift of the resonance wavelength. Also, the quadrupole plasmon resonances were suppressed by the SiO2 cladding layer because the dielectric SiO2 can suppress level splitting of surface plasmon resonances caused by the Ag NP coupling effect.