Sung-Nien Hsieh

High-performance polymer light-emitting diodes utilizing modified Al cathode

We report an increase of electroluminescence (EL) efficiency by two orders of magnitude for poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) based polymer light-emitting diodes (PLED) while employing Al as the cathode with an ultrathin layer of poly(ethylene oxide) (PEO). EL efficiencies of MEH-PPV PLEDs biased at 10 mA were found to be 0.017cd∕A at 31cd∕m2, 1.50cd∕A at 2515cd∕m2, and 4.96cd∕A at 8416cd∕m2 for applying Al, PEO∕Al, and PEO∕LiF∕Al as the device cathodes, respectively. The significant improvement in the device performance is attributed to the promotion of minority carrier injection (electrons), where the threshold of the injection can be characterized through the deviation of Fowler–Nordhiem tunneling prediction.

Organic oxide/Al composite cathode in efficient polymer light-emitting diodes

This work presents the fabrication of efficient polymer light-emitting diodes (PLEDs) by thermally evaporating a salt-free neutral organic-oxide buffer layer onto the surface of the electroluminescent film in a vacuum before the device cathode, made of Al—rather than the low work function metals, such as Ca or Ba—is deposited. The electroluminescence (EL) efficiency of phenyl-substituted poly(para-phenylene vinylene) copolymer-based PLEDs with an organic oxide/Al composite cathode, reaches 8.86cd∕A, which is markedly higher than those, 5.26cd∕A and 0.11cd∕A, of devices with Ca∕Al and Al cathodes, respectively. The device performance is improved by the formation of a specific organic oxide/Al complex at the cathode interface during the deposition of Al, facilitating the injection of electrons and eliminating the metal-induced quenching sites of luminescence in the EL layer near the recombination region.

Self-assembled tetraoctylammonium bromide as an electron-injection layer for cathode-independent high-efficiency polymer light-emitting diodes

Tetraoctylammonium bromide (TOAB), a general kind of quaternary ammonium salt, was spin-coated onto the surface of a green-emissive poly(9,9-dialkylfluorene) derivative (G-PF) to fabricate cathode-independent polymer light-emitting diodes (PLEDs). The electroluminescence efficiencies were 15.4, 11.4, and 9.1 cd A−1 for TOAB with Al, Ag, and Au as the cathode, respectively, which are better than that of the device with Ca/Al as the cathode (6.1 cd A−1). The molecular nanomorphologies of TOAB deposited on G-PF were investigated using synchrotron X-ray diffraction. Results show that TOAB molecules nucleated on the hydrophobic G-PF surface and self-assembled into a highly ordered lamellar structure during the spin-coating process. This unique structure produces suitable molecular dipoles between N+ and Br−, significantly improving the electron-injection ability from stable metals to G-PF. The direction of the molecular dipole between N+ and Br− can be reversed by using a hydrophilic ZnO for producing an efficient electron injection layer in an inverted device. The self-assembled molecules of TOAB create an anisotropic dipole on hydrophilic or hydrophobic surfaces, making them a potentially efficient electron-injection layer.

High-brightness top-emissive polymer light-emitting diodes utilizing organic oxide/Al∕Ag composite cathode

This work presents the fabrication of high-brightness (over 30000cd∕m2) top-emissive polymer light-emitting diodes (PLEDs) using a hybrid semitransparent cathode capable of efficient injection of electrons. The composite cathode is comprised of the organic oxide/Al complex as the injection buffer layer covered by a thin Ag overlayer. The anode is made of Ag:Ag2O coated on the glass substrate. The electroluminescence (EL) efficiency of 8.9cd∕A for phenyl-substituted poly(para-phenylene vinylene) copolymer based top-emissive PLED markedly exceeds that of 4.3cd∕A for the control device with the bottom-emissive configuration. The high performance is attributed to the balanced injection of charge carriers and the effective extraction of EL emission from the top cathode. The optical microcavity effect significantly promotes the EL emission in the direction along the surface normal.

Improved Performance of Top-Emissive Polymer Light-Emitting Device with Semitransparent Ag Cathode with the Aid of Au Nanoparticles

An efficient top-emissive polymer light-emitting diode (T-PLED) employing Ag as a semitransparent cathode is achieved by adding Au nanoparticles to a phenyl-substituted poly(para-phenylene vinylene) copolymer (HY-PPV). The efficiency of the T-PLED with Au nanoparticles is 3.4 cd/A, which is much higher than that of the T-PLED without such particles (0.15 cd/A). The superior performance of the device is attributed to the balance between the electron and hole currents induced by the Au nanoparticles, which increases the probability of hole–electron recombination. Furthermore, the devices stability in air can be significantly improved because of the use of stable electrodes.