Jianzhuo Zhu

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.

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.

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.

High efficiency electrophosphorescence device using a thin cleaving layer in an Ir-complex doped emitter layer

The authors demonstrate a considerable increase in current efficiency of fac-tris(2-phenylpyridine) iridium doped phosphorescent organic green-light emitting diode in which a thin 4,7 dipheny-1,10-phenanthroline (Bphen) layer acts as a cleaving layer. As 4nm Bphen layer divides the emitting layer (EML) into two sub-EMLs, a maximum current efficiency of 53cd∕A (corresponding to external efficiency quantum of 15%) is obtained, which is higher for 2.3 folds than that of the device without it, especially the current efficiency increases 64% over the reference device at a luminance of 40000cd∕m2. The increases are demonstrated to the high electron mobility and special energy level alignment of Bphen with 4,4′-N,N′-dicarbazole-biphenyl host. The efficiency improvement attributes to a higher exciton formation probability in the recombination zone and better balance of the carrier injection. The detail enhancement mechanism of the efficiency is also discussed.

Transverse evolution of a plasma channel in air induced by a femtosecond laser.

We investigate the evolution of filamentation in air by using a longitudinal diffraction method and a plasma fluorescence imaging technique. The diameter of a single filament in which the intensity is clamped increases as the energy of the pump light pulse increases, until multiple filaments appear.

Very high open-circuit voltage ultraviolet photovoltaic diode with its application in optical encoder field

We demonstrate a very high voltage based organic photovoltaic (PV) diode by stacking two ultraviolet (UV) sensitized organic PV diodes. It shows an open-circuit voltage (Voc) of 4.34 V under 365 nm UV irradiation with intensity of 2 mW/cm2. Due to the especially high Voc the PV diode device could be integrated in electronic logic device, as a result a high or a low potential states can be harvested directly under the UV irradiation or not, respectively. The difference between high and low potential states is great and the temporal response is fast, which make it an attractive and promising candidate in application of optical encoder.

Low efficiency roll off at high current densities in Ir-complex based electrophosphorescence diode with exciton diffusing and fluorescence compensating layers

We demonstrate a fac-tris(2-phenylpyridine) iridium-based electrophosphorescent organic green-light emitting diode with a considerably reduced current-efficiency roll off at high current density. Such a low roll off of efficiency was achieved by inserting nondoped 4,4′-N,N′-dicarbazole-biphenyl (CBP) layer and a tris-(8-hydroxy-quinoline) aluminum (Alq3) layer between the emitting and electron-transporting layers to diffuse excitons from the emitting layer. The Alq3 layer is found to contribute as a complementary green fluorescent emitter at high current density. Thus, only a small decrease from 20.7to16.7cd∕A was detected in current efficiency when the current density increases from 3.9to100mA∕cm2. A high current efficiency of 12.6cd∕A was achieved even at 350mA∕cm2, indicating that the efficiency roll off was drastically reduced when compared with the device without CBP and Alq3 layers.

A very high efficiency electrophosphorescent device doped with short triplet lifetime phosphor using multi-recombination zones

We have demonstrated a high-efficiency electrophosphorescent device doped with bis(2-phenyl-benzoimidazole) iridium (III) acetylacetonate with a triplet lifetime in the range of nanoseconds. Then the doped mixed host was sandwiched by two doped single-host layers. Multi-recombination zones were formed in the emission layer. Such a structure could increase the proportion of recombined carriers to injected carriers and extend the recombination zone. As a result, a maximum external quantum efficiency of 18.6% was attained due to the presence of multi-recombination zones. The mechanism of improving the electroluminescence performance was also discussed.