Shui-Hsiang Su

Enhancing Efficiency of Organic Light-Emitting Diodes Using Lithium-Doped Electron Transport Layer

In this work, we study the electrical and optical properties of organic light-emitting diodes (OLEDs) with a lithium (Li)-doped tris(8-hydroxyquinolino)-aluminum (Alq3) electron transport layer (ETL). When the Li:Alq3 doping ratio in a Li-doped Alq3 ETL is 2:1, the luminous efficiency of OLEDs is 5.25 cd/A; that of an OLED without a Li-doped Alq3 ETL is only 0.7 cd/A. X-ray photoelectron (XPS) and UV–vis absorption spectra of Li-doped Alq3 films reveal that the Li-doped Alq3 ETL has an improved electron conductivity. However, heavy Li doping in a Li-doped Alq3 ETL reduces the optoelectric performance of OLEDs. Excess Li atoms or cations quench light-emitting excitons in an Alq3 emitting layer (EML). Additionally, annealing improves the morphological stability of Li-doped Alq3 films. An OLED, comprising a Li-doped Alq3 ETL, requires no extrinsic thin LiF film, meeting commercial requirements, improving reproducibility and ensuring uniformality in a large area.

Increasing the Fill Factor and Power Conversion Efficiency of Polymer Photovoltaic Cell Using V2O5/CuPc as a Buffer Layer

In this study, we investigated the high fill factor (FF) and high power conversion efficiency (PCE) of organic photovoltaic (OPV) cells using a dual hole-transporting layer (HTL) of vanadium oxide (V2O5)/copper phthalocyanine (CuPc) as a buffer layer. The OPV cell configuration consists of indium tin oxide (ITO)/V2O5/CuPc/poly(3-hexylthiophene) (P3HT):phenyl C61-butyric acid methylester (PCBM)/LiF/Al. The FF and PCE of the device with a V2O5/CuPc buffer layer are approximately threefold and tenfold, respectively, greater than those of a conventional device without the buffer layer. The FF and PCE of the device are 62.2 and 2.11%, respectively, under a simulated AM1.5G illumination of 100 mW/cm2. The increased PCE and FF of the device are obtained using a V2O5/CuPc buffer layer, which is attributed to the stepwise hole-transporting configuration and the increased optical absorption from the anode.

Temperature Effect on the Optoelectronic Properties of GaN-based Light-Emitting Diodes with ITO p-Contacts

In this study, indium-tin-oxide (ITO) films were used as p-contacts in GaN-based light-emitting diodes (LEDs). The dependence of the optoelectronic performance of LEDs on various annealing temperatures of ITO films was studied. At an annealing temperature of 600°C the transmittance of ITO films can reach 98.6% at a wavelength of 470 nm. The specific contact resistance characterized by the circular transmission line model is as low as 7.73 X 10 -3 Ω cm 2 . Furthermore, GaN-based LEDs employing an ITO p-contact annealed at 600°C have high luminescence intensities of 243.2 mcd at a 20 mA injection current. Under the same injection current, the proposed LED also has the largest electroluminescence intensity with the ITO p-contact annealed at 600°C.

Efficiency Enhancement of Field-Emission Organic Light Emitting Diodes Using a Dynode Structure

Highly efficient field-emission organic light emitting diodes (FEOLEDs) using a dynode structure were successfully fabricated. The inorganic phosphor used to emit light in a field-emission diode (FED) was replaced with an organic electroluminescent (EL) light emitting layer in a FEOLED. A dynode structure is proposed to be used in the FEOLED and its electron amplification ability can enhance the efficiency of a FEOLED. With the dynode-FEOLED structure, the current density is largely improved from 3.5 to 8.5 mA/cm 2 under the same electric field (2.66 V/μm) of the FEOLED and dynode-FEOLED. Bright and uniform emission images are obtained in the dynode-FEOLED. Compared with the FEOLED, the luminance of dynode-FEOLED improves greatly, reaching an eightfold rise to 850 cd/m 2 from 115 cd/m 2 at the same anode voltage (400 V). The dynode-FEOLED also overcomes the emission uniformity problem in the field-emission cathodes consisting of screen-printing carbon nanotubes.

Field-Emission Organic Light-Emitting Device Using Oxide-Coated Cathode as Electron Source

While examining the optical and electrical properties of field-emission organic light-emitting devices with oxide-coated cathodes as electron sources, this investigation elucidates the mechanism of emission from organic thin films as emission layers. Experimental results indicate that the emission characteristics depended strongly on the device structure. Green emission was observed in a single-layered Alq 3 film. Green emission was also observed from the Alq 3 layer of the multilayered m-MTDATA/NPB/Alq 3 structure. However, the emission efficiency from the multilayered m-MTDATA/NPB/Alq 3 structure significantly exceeded that of the single-layered structure. Electroluminescence is the most likely emission mechanism in these devices.

Improving the Luminous Efficiency of Red Organic Light-Emitting Diodes Using a Carrier Balance Layer

This work proposes a carrier balance layer (CBL) to yield red organic light-emitting diodes (OLEDs) with high luminous efficiency. In the structure of OLEDs, the CBL (l,3,5-tris(N-phenylbenzimidazol-2,yl)benzene, TPBi) is inserted into the emitting layer, 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetra-methy-1julolidyl-9-enyl)-4H-pyran (DCJTB)-doped tris(8-hydroxyquinolino)-aluminum (Alq 3 ). Experimental results demonstrated that the luminous efficiency of the OLED with a CBL was 4.63 cd/A 30% higher than that of the typical OLED without a CBL. Inserting CBL into an emitting layer is suggested to rearrange and enlarge the recombination zone. The improvement in efficiency is attributed to the enlargement of the recombination zone in which more excitons were generated and recombined. Sufficient electron-hole recombination improved luminous efficiency.

Reduction of ohmic contact on p-GaN with surface treatment using Cl2 inductively coupled plasma

A reduction of Ni/Au ohmic contact on p-type GaN is obtained with surface treatment on GaN films using Cl2 inductively coupled plasma (ICP). X-ray photoelectron spectroscopy (XPS) shows the modifying of the surface atomic ratio after the Cl2 ICP treatment. The atomic ratio of nitrogen to gallium becomes larger after the Cl2 ICP treats samples for 5 s. It suggests that GaClx and/or GaOx is formed and then removed in the boiling HCl solution. The Ga vacancies at the p-type GaN surface are therefore produced and act as acceptors for holes. It leads to the reduction of the contact resistance through the decrease of the resistivity for the conduction of holes.

Efficiency enhancement of solution-processed inverted organic solar cells with a carbon-nanotube-doped active layer

Solution-processed titanium-doped ZnO (TZO) is synthesized by the sol–gel method to be the electron-transporting layer (ETL) in an inverted organic solar cell (IOSC). Carbon nanotubes (CNTs) are doped into an active layer of poly(3-hexylthiophene):[6,6]-phenyl C 61 butyric acid methyl ester (P3HT:PCBM). The addition of CNTs in the P3HT:PCBM composite increases the conjugation length of P3HT:PCBM:CNTs, which simultaneously enhances the capacity of the composite to absorb solar energy radiation. Vanadium oxide (V2O5) was spin-coated onto the active layer to be a hole-transporting layer (HTL). The power conversion efficiency (PCE) results indicate that the V2O5 nanobelt structure possesses better phase separation and provides a more efficient surface area for the P3HT:PCBM:CNT active layer to increase photocurrent. The optimized IOSCs exhibited an open circuit voltage (Voc), a short-circuit current density (Jsc), a fill factor (FF), and a PCE of 0.55 V, 6.50 mA/cm2, 58.34%, and 2.20%, respectively, under simulated AM1.5G illumination of 100 mW/cm2.

Nitride-Based Blue-Light-Emitting Diodes with ITO-Covered Ni ∕ Au Mesh p-Contacts

This investigation presents a novel indium-tin oxide (ITO) covered Ni/Au mesh p-contact to improve the light transmittance and output efficiency of InGaN/GaN multiquantum well light-emitting diodes. The presented ITO-covered Ni/Au mesh p-contact blocks less light than the conventional Ni/Au film p-contact but makes a good ohmic contact. Experimental results indicate that to light with a wavelength of 470 nm, the ITO-covered Ni/Au mesh p-contact exhibits 94% transmittance and 12-mW output power at a 20-mA injection current. In contrast, at the same wavelength the conventionally adopted Ni/Au film p-contact exhibits 72% light transmittance and 8.12-mW output power at a 20-mA injection current.

All-solution-processed inverted organic solar cell with a stacked hole-transporting layer

In this study, inverted organic solar cells (IOSCs) have been fabricated and characterized. A sol–gel zinc oxide (ZnO) film is used as a hole-blocking layer (HBL). Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and copper phthalocyanine (CuPc) are used as a hole-transporting layer (HTL). The HBL, active layer, and HTL films are fabricated by spin-coating technique. The anode is fabricated from Ag nanoparticles by drop titration using a Pasteur burette. Experimental results show that the PEDOT:PSS/CuPc stacked HTL provides a stepwise hole-transporting energy diagram configuration, which subsequently increases the charge carrier transporting capability and extracts holes from the active layer to the anode. The characteristics of the IOSCs were optimized and exhibited an open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE) of 0.53 V, 6.13 mA/cm2, 37.53%, and 1.24%, respectively, under simulated AM1.5G illumination of 100 mW/cm2. Hence, a solution process is feasible for fabricating low-cost and large-area solar energy devices.