Yunzhang Wang

Alternating‐current light‐emitting devices based on conjugated polymers

Most polymer electroluminescent devices to date are represented as tunnel diodes and operate under direct‐current (dc) driving field. Here we report the fabrication of symmetrically configured alternating‐current (ac) light‐emitting (SCALE) devices based on conjugated polymers. The new devices consist of an emissive polymer layer sandwiched between two redox polymer layers. This configuration enables the SCALE devices to work under both forward and reverse dc bias as well as in ac modes. The nearly ohmic electrode/redox polymer contacts improve the charge injection efficiency significantly and make the SCALE device operation insensitive to electrode work functions. Symmetric operation supports the key role of redox polymer/emissive polymer interface states.

Application of polyaniline (emeraldine base, EB) in polymer light-emitting devices

Abstract We report the fabrication of several multilayer light-emitting-diode (LED) devices based on a novel conjugated polymer, poly(2,5-dihexadecanoxy phenylene vinylene pyridyl vinylene) (PPV-PPy V), involving the use of polyaniline (emeraldine base, EB) as an insulating layer between the emissive polymer layer and the electrodes. In all the above devices with various configurations (‘3-layers’, ‘4-layers’-1’, ‘4-layers-2’ and ‘5-layers’), only the symmetrically configured a.c. light-emitting (SCALE) (‘5-layers’) device shows the capability of operating in both forward and reverse bias modes and in an a.c. mode. The SCALE devices have a typical turn-on voltage of about 4–6 V and work well under both forward and reverse bias modes. It is important to note that the total resistance ( R= V/I +) of the four devices at any given applied potential decreases as the number of insulating polymer layers increases, suggesting that the nature of the electrode/polymer interface plays a critical role in determining the characteristics of the devices.

EXCIPLEX EMISSION IN BILAYER POLYMER LIGHT-EMITTING DEVICES

Photoluminescent and electroluminescent studies of bilayer heterojunctions formed from a poly(pyridyl vinylene phenylene vinylene) (PPyVPV) derivative and poly(vinyl carbazole) (PVK) show an emission peak which cannot be ascribed to either the PPyVPV derivative or PVK layer. Through studies of absorption and photoluminescence excitation (PLE) spectra we demonstrate that the additional feature results from an exciplex at the bilayer interface. The photoluminescence efficiency of the exciplex is greater than 20%. The electroluminescence spectrum from the bilayer devices is entirely due to exciplex emission, with internal efficiencies initially achieved exceeding 0.1%.

Blue electroluminescent devices based on soluble poly(p‐pyridine)

We have fabricated unilayer electroluminescent devices from soluble poly(p‐pyridine) (PPy). The solubility of PPy in weak acids allows direct spin casting of the polymer films. The electroluminescence spectrum peaks at 2.5 eV (497 nm) corresponding to white light weighted towards the blue end of the spectrum. The photoluminescence spectrum peaks at 2.35 eV (530 nm). The operating voltages of the devices ranged from 4 to 12 V with current densities of 6 to 8 mA/mm2. We compare our devices with similar blue emitting devices based on poly(p‐phenylene).

Light-emitting devices based on pyridine-containing conjugated polymers

We report the fabrication of light-emitting devices based on several pyridine-containing conjugated polymers and copolymers in various device configurations. The high electron affinity of pyridine based polymers enables the use of relatively stable metals such as Al or even ITO as electron injecting contacts. Taking advantages of the better electron transport properties of the pyridine-containing polymers, we fabricate bilayer devices utilizing poly(9-vinyl carbazole) (PVK) as hole transporting/electron blocking polymer, which improves the device efficiency and brightness significantly due to the charge confinement and exciplex emission at the PVK/emitting polymer interface. The incorporation of conducting polyaniline network electrode to PVK reduces the device turn on voltage significantly while maintaining the high efficiency. The control of the aggregate formation in the polymer films by blending with insulating host polymers opens up the possibility of making voltage-controlled multi-color light-emitting devices.

Poly (p-pyridine) - and poly (p-pyridyl vinylene) -based polymers: their photophysics and application to SCALE devices

Abstract Photophysics and light-emitting device applications of poly ( p -pyridine) - and poly( p -pyridyl vinylene)-based polymers are presented. Extensive time-resolved (ps to ms) photoluminescence, stimulated emission and photoinduced absorption studies of solutions, powders and films demonstrate that the primary photoexcitation of these polymers is an intrachain singlet exciton. The presence of (n,π * ) states leads to enhanced intersystem crossing to triplet excitons for the powder form, while aggregate formation plays a key role in the films. Polarons are important at longer times. These polymers were used to fabricate ‘conventional’ polymer light-emitting diodes. In addition, these polymers were used to demonstrate a novel light-emitting structure, the symmetrically configured a.c. light-emitting (SCALE) device. These new devices have potential advantages in their use with high workfunction electrodes, such as gold, and also in their a.c. operation.

MULTICOLOR MULTILAYER LIGHT-EMITTING DEVICES BASED ON PYRIDINE-CONTAINING CONJUGATED POLYMERS AND PARA-SEXIPHENYL OLIGOMER

There is increased interest in developing multicolor light-emitting devices. We report here the fabrication and study of multilayer color-variable/white light-emitting devices based on pyridine-containing conjugated polymers and para-sexiphenyl (6P) oligomer. Voltage-dependent multicolor emission was observed in both bilayer and trilayer configurations. The emission colors of single devices cover a wide range of visible spectra whose Commission International de I’Eclairage (CIE) color coordinates vary from blue to white to green with increasing voltages. The color coordinate traverses along a straight line in the CIE chromaticity diagram. The role of interfaces in the operation of devices is discussed.

Exciplex emission from bilayers of poly(vinyl carbazole) and pyridine based conjugated copolymers

We present photoluminescence and electroluminescence studies of bilayers and blends formed from poly(vinyl carbazole) (PVK) and poly(pyridyl vinylene phenylene vinylene) (PPyVPV) copolymer derivatives. Bilayers of PVK and the PPyVPV copolymers have a photoluminescence emission which cannot be assigned to either the photoluminescence of PVK or the PPyVPV layer. The blends of the two polymers show a similar new photoluminescence emission for a large range of concentrations. Absorption and photoluminescence excitation spectra confirm that the additional feature is an excited state species which results from an exciplex at the polymer/polymer interface. Bilayer light-emitting devices utilizing the PPyVPV copolymers show an electroluminescence spectrum consistent with emission from the exciplex. The efficiency of the bilayer devices as compared to single layer devices increases by over three orders of magnitude due to the exciplex formation and the elimination of exciton formation near the luminescence quenchi...

Color variable bipolar/ac light-emitting devices based on conjugated polymers

There is increased interest in developing color variable light emitting devices. We report here the fabrication of color variable bipolar/ac light-emitting devices based on conjugated polymers. The devices consist of blends of pyridine-phenylene and thiophene-phenylene based copolymers sandwiched between the emeraldine base form and the sulfonated form of polyaniline. ITO and Al are used as electrodes. The devices operate under either polarity of driving voltage with different colors of light being emitted, red under forward bias, and green under reverse bias. The relative fast time response allows the rapid switching of colors and ac operation.

Application of aluminum, copper and gold electrodes in a.c. polymer light-emitting devices

Abstract Symmetrically configured a.c. light-emitting (SCALE) ‘5-layer’ devices having the configuration M/EB/P/EB/ITO, where M = Al, Cu or Au, EB = polyaniline (emeraldine base), P = poly(2,5-dihexadecanoxy phenylene vinylene pyridyl vinylene) or PPV-PPyV, and ITO = indium-tin oxide glass, show electroluminescent properties in both forward and reverse bias modes. In the absence of emeraldine base, in the case of aluminum and copper, electroluminescence is observed only in the forward biasmode; in the case of gold no electroluminescence is observed in either forward or reverse bias modes. The electrical properties of the ‘5-layer’ devices (M = Al, Cu) are most surprising since their total resistance at a given applied voltage is significantly less than that of the corresponding devices in which the two polyaniline insulator layers are not present.