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Optoelectronics & Communications

Effect of coated magnetic nanocolloids on hybrid organic LEDs

An ‘insulating’ organic shell between inorganic magnetic nanoparticles and an organic semiconductor matrix enhances the efficiency of advanced light technologies in several ways.
16 December 2010, SPIE Newsroom. DOI: 10.1117/2.1201012.003314

Flexible opto-elelectronic devices are of great interest for sensors, solar-energy harvesting, lighting, and other applications. By the same token, fabricating better-performing devices requires substantial processing optimization and materials design. Organic semiconductors1 and inorganic nanoparticles2 are both key players in this field, and each is far from being developed. An alternative approach is to combine them to take advantage of the interplay between their properties to improve the efficiency of hybrid opto-electronic devices. One way of doing this is by blending metal nanoparticles and semiconductor quantum dots. Our own interest is more the effect of magnetic nanoparticles on organic semiconductors, and specifically the role of the interface between inorganic magnetic nanoparticles and their organic matrix.3In this context, we are investigating electric and luminous properties of hybrid organic-inorganic LEDs: see Figure 1(A). The LEDs are based on a commercial conjugated polymer—super-yellow, see Figure 1(B)4—blended with magnetic nanoparticles—iron platinum (FePt), see Figure 1(C)—that we synthesized in-house using thermal decomposition.5


Figure 1.(A) Organic LED (OLED) structure. (B) Chemical structure of ‘super-yellow.’ (C) Transmission-electron-microscope image of iron platinum (FePt) nanoparticles. ITO: Indium tin oxide. R, R': Side chains. x, y, z: Denote repeat units.

The FePt nanoparticles have a diameter of approximately 5nm as observed by transmission-electron microscopy. This finding is in agreement with x-ray diffraction data that also indicates a face-centered cubic crystalline structure consistent with the product usually obtained by solution synthesis. Without post-treatment, the nanoparticles are superparamagnetic, with a blocking temperature below room temperature as observed using a superconducting quantum-interference device (better known as SQUID) magnetometer. Finally, we applied a ligand-exchange protocol to alter both the molecular structure of the organic shell and its bonding to the surface of the nanocrystals while preserving solution processability and miscibility. We note that, at room temperature, the organic coatings we have considered in this study have very little influence on the properties of the magnetic nanoparticles.

We increased the concentration of nanoparticles in the polymer matrix up to 1% by weight, and investigated the photophysical properties of both super-yellow pristine and blended thin films using steady-state optical spectroscopy and time-resolved photoluminescence. The data displayed very little dependence on the concentration of the magnetic nanoparticles. In contrast, and despite the small concentration of nanoparticles dispersed in super-yellow, we observed a clear enhancement of device performance in the case of some of the blended films compared with the pristine structure. This increase in efficiency arises from lower drive voltage and higher luminance (see Figure 2).


Figure 2.Current density versus voltage of pristine super-yellow (black line) and super-yellow:FePt (99:1 weight percent) for nanoparticles with organic shell I (green line) and II (red line). (inset) Typical OLED electroluminescence spectrum.

When dealing with colloidal nanoparticles, one should not overlook the potential impact of the organic ligands (molecules) that, in solution, dynamically bond to the surface of the nanocrystals. This bonding leads to a balance between bonded and diluted (i.e., free) molecules in solution. Similar behavior is expected in polymer solutions. Free ligands could then have more impact on device performance than those bonding to inorganic nanoparticles and, thus, more influence than the inorganic nanoparticles themselves. We conclude that control experiments always need to be carried out with free ligands dispersed in the polymer matrix. We calculated a ligand concentration based on the expected density of ligands bonded to the surface of the nanocrystals. In the systems described here, the blends of ligands and super-yellow behaved like pristine polymer thin films.

Still, and most importantly, we note that not all ligand-nanoparticle systems we investigated resulted in increased device efficiency. In contrast with work done using metal or semiconductor nanoparticles, in which conjugated ligands promote charge transport, all organic shells we were interested in were made of insulating molecules. Consequently, our findings imply that the interplay between the organic shell and the inorganic nanoparticles can be designed to tune the mobility and trap density of charge carriers in hybrid devices, which in turn helps to enhance device efficiency.

In summary, we have undertaken collaborative research on plastic-like materials for flexible electronics. Our current work combining material and device fabrication and characterization has helped us to better understand the interplay between device performance and the nanocolloid architecture relevant to lighting and display devices. Indeed, magnetic nanocolloids appear to be a promising strategy for improving the efficiency of opto-electronic devices. As a next step, we plan to make further use of the magnetic properties of our nanomaterials to design more versatile and efficient hybrid organic-inorganic LEDs.


Salvatore Gambino, Shu Chen, Ifor Samuel, Pascal André
School of Physics and Astronomy Organic Semiconductor Centre, SUPA, University of St Andrews
St Andrews, UK

Pascal André leads a research group at St Andrews and is a visiting scientist at the RIKEN research institute as a fellow of the Canon Foundation in Europe. His interests include nanoparticle design, interfacial phenomena, and molecular engineering with a view to understanding and controlling the properties of hybrid organic-inorganic nanocolloids for biomedicine and optoelectronics.

Pascal André
Chemistry and Materials Physics Building, RIKEN
Wako, Japan

Pascal André leads a research group at St Andrews and is a visiting scientist at the RIKEN research institute as a fellow of the Canon Foundation in Europe. His interests include nanoparticle design, interfacial phenomena, and molecular engineering with a view to understanding and controlling the properties of hybrid organic-inorganic nanocolloids for biomedicine and optoelectronics.