Xiaohui Yang

Improved efficiency of molecular organic EL devices based on super molecular structure

Abstract A novel aluminum chelate, tris(5-hydroxymethyl-8-quinolinolato)aluminum (AlOq) as the emitter has been developed. The device (ITO/PVK/AlOq/Al) emitting green light has a luminance of more than 1000 cd/m2. The highest quantum efficiency of light-emitting devices is about 1.44 lm/W, better by three times than that of similar structure device made with Alq as the emitter duo to the formation of super molecular structure in AlOq thin films.

Color-variable electroluminescence from poly(p-phenylene vinylene) derivatives

Photoluminescent and electroluminescent properties of poly(2-methoxy-5-nonyloxy-p-phenylene vinylene) (MN-PPV), one of the soluble poly(p-phenylene vinylene) (PPV) derivatives is synthesized through a dehydrochlorination reaction. The EL peak at 576 nm with a low threshold voltage in the monolayer structure is independent of the bias. When the Alq 3 layer is very thin (≤10 nm) in the bilayer structure of ITO/MN-PPV/Alq 3 /Al, the intrinsic emission only from MN-PPV is obtained. When the Alq 3 layer gets thicker (∼30 nm), the intrinsic emission from Alq 3 at 520 nm will increase gradually with the bias, while emission from MN-PPV decreases. Based on the energy levels and field-assisted tunneling at the surface, we consider that this color-variable EL is ascribed to the redistribution of the electric field on the devices and field-assisted charge crossing through the triangular energy barrier at high biases.

Electric field induced quenching of photoluminescence of poly(N-vinylarbaole) (PVK) doped with dyes

Abstract In this paper, electric field-induced quenching of poly(N-vinylarbaole) (PVK) doped with dyes was studied. The electric field-induced quenching results from the field-induced annihilation of the emitting species. Our researches show that the effect of electric field on photoluminescence is different for different excitation wavelengths. The difference is attributed to the generation and recombination processes of exciton.

Photoluminescence and electroluminescence properties of a new PPV derivative containing oxadiazole segments

Abstract The synthesis of a new PPV derivative (poly[(2,5–diphenylene–1,3,4–oxadiazole)–4,4′vinylene] (O-PPV) oligomer) and the investigation of the absorption and photoluminescence (PL) properties of O-PPV in the forms of solution and solid-state film are reported. Our results showed that the PL in the case of solid-state originates from an excimer or interchain charge transfer complex. To understand the nature of carrier transport, two types of devices: ITO/PPV/O-PPV/Al and ITO/O-PPV/Tb(AcAc) 3 Phen/Al were fabricated. Their electroluminescence result indicates that O-PPV has both hole and electron transport abilities. As a result, the EL efficiency of the single layer O-PPV device is higher compared with that of the single layer PPV due to the balance of charge injection and transport. The electron transport property is attributed to the property of excimers.

Interchain charge-transfer states in poly[(2,5-diphenylene-1,3,4-oxadiazole)-4,4′-vinylene] (O-PPV) oligomer

Abstract We synthesized the soluble poly[(2,5-diphenylene-1,3,4-oxadiazole)-4,4′-vinylene] (O-PPV) oligomer. The photoluminescence (PL) properties are investigated using absorption and selective excitation measurements for different O-PPV concentrations in solid-state O-PPV/PEO blends, as well as comparing PL spectra in dilute solution and solid state pristine O-PPV. The results show that solid-state O-PPV emission arises from interchain charge-transfer states due to interchain effects. In the O-PPV/PEO blends PL spectra, we find that the PL spectra excited at 300 nm are different from those at 350 nm. We think that this is directly associated with the properties of intrachain excited states in conjugated polymers.

Blue photo- and electroluminescence from a novel poly (phenelene vinylene) derivative containing oxadiazole segment in main and side chains

Abstract Optical and electroluminescent properties of a novel PPV derivative (OPPV) which contains oxadiazole segments in main and side chains were studied. Both the single-layer EL device and the triple-layer EL devices fabricated with OPPV as the active layer emitted blue light from OPPV with a peak of spectrum at around 460 nm. The device structure: ITO/PVK+OPPV(2:1 weight ratio)/OXD(40 nm)/Alq(20 nm)/Al, was fabricated, which showed much better performance than the single-layer EL device and the triple-layer EL device without PVK in the emitter material.

A novel oligomer poly (phenylene vinylene) derivative containing oxadiazole segment

A novel oligomer PPV derivative (OPPV) is reported, which is designed to contain the oxadiazole function segment in the main chain. Photoluminescence of the OPPV thin film originates from aggregated states. In a double layer device consisting of poly phenylene vinylene (PPV) and OPPV, emission only from PPV layer was observed and the efficiency of the device was improved by about one order of magnitude compared with the single layer PPV device, this indicates the electron transport characteristic of OPPV. In the OPPV/Tb complex device, OPPV functions as the hole transport layer. A possible explanation is proposed.

Effect of TPD:CuPc LB films as electron blocking layer on EL emission

In organic thin film electroluminescence device with ITO/PPV/TPD:CuPc/Alq:DCMl/Al structure, the TPD:CuPc Langmuir-Blodgett (LB) film as electron-blocking layer was inserted between PPV and Alq:DCMl layer. A device without TPD:CuPc layer or a device with monolayer of TPD:CuPc which was applied at higher DC voltage exhibited EL emission from both the PPV and Alq:DCMl layers. Whereas a device with monolayer of TPD:CuPc layer which was applied at lower voltage or a device with double layers of TPD:CuPc had an EL emission only from Alq:DCMl layer implying that the TPD:CuPc layer can effectively block the electrons and the Alq:DCMl electron transport layer worked as the emitting layer. Hence, we can spatially control the recombination zone and obtain different EL emission by adjusting the thickness of carrier blocking layer.