Color-tunable mixed photoluminescence emission from Alq3 organic layer in metal-Alq3-metal surface plasmon structure
© Chen et al.; licensee Springer. 2014
Received: 29 June 2014
Accepted: 30 September 2014
Published: 13 October 2014
This work reports the color-tunable mixed photoluminescence (PL) emission from an Alq3 organic layer in an Au-Alq3-Au plasmonic structure through the combination of organic fluorescence emission and another form of emission that is enabled by the surface plasmons in the plasmonic structure. The emission wavelength of the latter depends on the Alq3 thickness and can be tuned within the Alq3 fluorescent spectra. Therefore, a two-color broadband, color-tunable mixed PL structure was obtained. Obvious changes in the Commission Internationale d’Eclairage (CIE) coordinates and the corresponding emission colors of Au-Alq3-Au samples clearly varied with the Alq3 thickness (90, 130, and 156 nm).
In recent years, organic light-emitting diodes (OLED) have been attracting considerable attention for various illumination applications because they exhibit excellent properties that traditional light sources do not, including high brightness, large size, transparency, and flexibility. OLED have been considered to be potential next generation of light sources [1–3]. To realize full color and white OLED, various color mixing structures, including multiple dopant emissive layers and multiple emissive layers, and down conversion in the optical microcavity, have been utilized with various degrees of success [4–10]. Some methods for generating white color emission from OLED have been developed, such as the method of partial energy transfer in which the OLED materials are doped with fluorescent/phosphorescent dyes [11–20]. The mixing of the EL from the host molecules with excimer/exciplex emissions has also been found to yield white emission [21–23]. Other methods involve stacking red-, green- and blue-emissive components on a charge generation layer (CGL) [24–29]. Recently, metal-dielectric-metal (MDM) surface plasmon (SP) structures, which are similar to planar optical microcavities, have attracted a great deal of attention because they have various potential applications in optoelectronic devices [30–34].
In an MDM structure with semi-infinitely thick metals, if the thickness of the dielectric is smaller than the SP penetration depth at the metal/dielectric interface, then the SPs on both interfaces of the dielectric layer can interact with each other and split into two hybridized SP modes: an odd SP with an anti-symmetric magnetic field distribution and an even SP with a symmetrical magnetic field distribution . The dispersion curve of the odd SP can cross the light line in air and shift up (or down) in energy when the dielectric thickness is reduced (or increased), indicating that the odd SP is not only radiative when one of the two semi-infinite thick metal layers is reduced to a finite and semi-transparent layer but that its energy is also tunable by variation of the dielectric thickness. Therefore, if the dielectric layer in the MDM structure is replaced with an organic emitter layer with a thickness of the order of an optical wavelength, then owing to near-field optics and the Purcell effect, the organic excitons can preferentially recombine into the odd SP that is confined in this metal-organic emitter layer-metal structure, and the radiative odd SP subsequently couples out of the device to photons in the air. This property has been investigated experimentally [32, 33].
However, the application of this light emission has so far received little attention. In this work, a color-tunable mixing method, using a single organic light-emitting layer that is embedded between two metal layers of finite thickness is proposed. When one of the two metals is of finite thickness and is semi-transparent, the organic fluorescence still occurs and serves as a constant source of light. Further light emission is produced through the odd SP, and the wavelength of this emission is tunable by variation of the thickness of the organic layer. Therefore, color-tunable mixed emission from an MDM structure with finite thickness metal layers can easily be realized.
Design and fabrication of sample
Results and discussion
Color-tunable mixed PL emission was achieved by insertion of an emitting layer between two metal layers of finite thickness to form an MDM structure. The PL emission is produced by the combination of two kinds of emission. The first is the organic fluorescent emission; the second is enabled by the odd SP in the MDM structure, and its wavelength is tunable, resulting in a color-tunable mixed emission. This emission process provides a feasible approach to generating two-primary white light.
The authors would like to thank the Ministry of Science and Technology and the Bureau of Energy, Ministry of Economic Affairs of Taiwan, for financially supporting this research. Ted Knoy is appreciated for his editorial assistance.
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