Liming Ding

Color tuning and efficiency enhancement through interfacial energy transfer and thickness control in light-emitting diodes

We report a possible interfacial energy transfer phenomenon in double-layer light-emitting diodes (LEDs) consisting of a poly(p-phenylene vinylene) (PPV) hole-transport layer and an emitting polymer layer containing either of two blue emitting polymers (I and II). The polymers in film form have almost the same photoluminescence (PL) spectra though they have different band gaps as determined from their absorption and PL spectra. This is due to intermolecular∕intramolecular energy transfer in polymer II films since the latter structure contains two differing oligomeric chromophores regularly interspersed by long nonconjugated blocks. In double-layer LEDs, PPV functions not only as a hole-transport layer which improves hole injection but also as a chromophore participating in the tuning of the LED emission color through interfacial energy transfer. The significant spectral overlap between the absorption spectrum of PPV and the emission spectra of I or II supports this energy transfer. The presence of PPV can...

Light emitting poly(para-phenylenevinylene-alt-3-tert butyl-meta-phenylenevinylenes

Abstract Poly(para-phenylenevinylene-alt-3-tert-butyl-meta-phenylenevinylene) was synthesized by Wittig condensation with varying ratio of cis and trans olefin bonds. The luminescent properties of the unisomerized and trans-isomerized polymers (1 and 2, respectively) were compared. Nearly identical emission spectra were observed for both polymers in dilute solutions, but the emission intensity of 1 is lower. This is attributed to the cis vinylic moieties in 1, which favor nonradiative excited state decay processes by limiting formation of a planar excited state. Double layer LEDs with the configuration ITO/PPV/(1 or 2)/Ca–Al were fabricated. Substantially better efficiencies and luminances were observed for devices made with polymer 2 as the emissive layer, by comparison to 1. The results show that the elimination of cis olefin defects in these polymers is important for the optimization of LEDs.

Chemically tuning the optoelectronic properties of terphenylene-containing block copolymers†

A series of partially conjugated polymers containing terphenylene linked by vinylene units were synthesized by Wittig condensation polymerization of the appropriate diphosphonium salts and the dialdehyde monomer. meta-Phenylene, para-phenylene, 1,3,4-oxadiazole- 2,5-diyl-1,4-phenylene, 2,5-dimethoxy-1,4-phenylene and 9,10-anthrylene units were incorporated into the vinylene blocks to control the band gaps. The effect of molecular architecture on optoelectronic and thermal properties of the polymers was investigated. The emission of the copolymers can be tuned by changing the nature of the vinylene blocks to show violet, blue, green and green-yellow. Double-layer LEDs with ITO/PEDOT/polymer/Ca/Al structures were fabricated and, in parallel with the photoluminescence results, the change of emission color was also shown in the electroluminescence spectra.