What is the coolest thing about the strides made in producing a spin-polarized, light-emitting diode (spin OLED)? Efficiency: spin OLEDs can theoretically produce twice as much light for the same power consumption and generate a range of colors from a single diode.
Organic spin valves consist of an organic spacer layer sandwiched between two spin-polarized ferromagnetic electrodes (one of cobalt, the other of lanthanum strontium manganese oxide). Modifying them to emit light as an LED does—opening the door to a more energy efficient light source—has seemed the natural extension of the technique since it was first demonstrated in 2004. Until now, though, the technical details have frustrated researchers.
First, they replaced the material of the first demonstrated spacer layer—aluminum tris(8-hydroxyquinoline), or Alq3—with a new organic polymer, deuterated poly(dioctyloxy)phenyl vinylene (D-DOO-PPV for short). D-DOO-PPV is a π-conjugated polymer; replacing hydrogen atoms near the molecular backbone with deuterium increases light-emission efficiency.
Photo: Tho D. Nguyen, University of Utah
Second, they modified the cobalt electrode, applying a lithium fluoride monolayer. As Vardeny describes it, this allows negatively charged electrons to be injected into the gate on one side of the valve while positive positron “holes” are injected through the other side.
The authors frankly acknowledge that there’s still a lot of work to do on the 300 μm x 300 μm x 40 nm prototype (photo at left) before spin OLEDs become everyday commodities. The current model works only in the cold: Output starts to fall off around 200 K (-73 C) and stops at about 240 K (-33 C). And the prototype emits only a single (orange) wavelength, though in theory many colors are possible.