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The Next Small Thing A problem found in one lab is solved in another. Organic light-emitting diodes increasingly show promise as lighting sources that are more efficient, more cost effective and more flexible than solid-state LED technology. Despite the potential, the technology faces some hurdles. A University of Tennessee researcher, with the help of colleagues and instruments at Oak Ridge National Laboratory's nanoscience center, has taken steps toward overcoming one of those hurdles by employing two emerging technologies—spintronics and nanotechnology. Spintronics exploits both the quantum spin states and charge states of electrons.
Bin Hu, assistant professor in UT's Materials Science and Engineering department, was one of more than 300 users hosted by ORNL's nanoscience center in 2007 during the center's second year of operation. In his project, Hu worked with researchers at the center and in ORNL's microscopy group to test and analyze the OLED device he had developed at UT. OLEDs, made up of layers of a polymer and organic compounds, are easy and affordable to make in large quantities using a simple process similar to an inkjet printer. They can also be made with large, flexible sheets, opening the door to a host of potential applications that include portable electronic newspapers, large but inexpensive white lighting panels and big-screen televisions and projectors. One of the barriers to deploying the technology is the inefficiency that arises when voltage is applied to organic compounds to produce light. The process generates excitons as singlets and triplets with the ratio of 1:3. An exciton is a bound state of a pair of particles that results when a photon kicks an electron out of orbit, leaving a positively charged hole, which binds with the electron when they meet in an organic semiconducting molecule. An exciton has slightly less energy than the unbound electron and hole. The challenge for Hu is that triplet excitons, at 75% of the population, produce heat that limits the efficiency of OLED devices. Changing the relative ratio between singlet and triplet excitons thus becomes a critical issue in improving OLED efficiency. The ability to fine-tune electrons—that is, to change the degree to which each electron's spin is aligned with a given direction, or spin polarization—is an important element of spintronics that might offer a way to boost the efficiency of OLEDs. Unfortunately, applying a thin magnetic electrode coating to deliver polarized electrons to these OLED devices has typically produced dismal results. "For example," Hu says, "if you supply 100 electrons to the OLED, only five electrons have an oriented polarization and 95 electrons are left at random." After repeated attempts at developing a workable magnetic coating, Hu and UT student Yue Wu tried applying the ferromagnetic element cobalt as the electrode coating to inject charge carriers with spin polarization in both the typical thin film form and also as nanoparticles. The nanoparticles demonstrated a significant improvement of polarization efficiency from 5% to 60%. This improvement of spin polarization efficiency has boosted overall OLED efficiency by 20%. To understand why the nanoparticles worked better than the cobalt film, Hu turned to ORNL's nanoscience center. There, Hu used ORNL instruments and expertise to characterize the magnetic, spin-dependent transport and structural properties of the OLED devices fabricated in his UT lab. ORNL researchers played a role in helping Hu analyze and better understand the spin-dependent transport across the ferromagnetic nanodot and organic polymer interfaces. Their research results were recently published in Physical Review B. Hu says the spaghetti-like structure of the polymer chains that make up OLEDs create a difficult surface to which a magnetic electrode film can adhere. Nanoparticles, on the other hand, nestle inside the chaotic surface structure, enabling a more uniform response. In addition, although typically a conductivity mismatch exists between the magnetic material and the polymer semiconductor, partial oxidization of the nanodot surfaces helped overcome this barrier, paving the way for more efficient transfer of electrons. The end result: a brighter, more efficient light, as well as a potential host of electronics applications.—Larisa Brass
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