Researchers from the University of Michigan developed metal-free phosphorescent OLED emitters. The idea is that if the emitter molecules cannot vibrate, they cannot release energy and light and so more energy is converted into light. At first they tried creating a stiff lattice (crystalize the emitters) - this achieved 55% light conversion (better than the 25% of regular fluorescent OLEDs, but not as good as the 100% achieved by heavy metal doping).
But this method cannot be adopted for commercial OLEDs easily, and so the second method they tried is to tweaking the organic molecules so that they form structural bonds with a transparent polymer (they attach "like magnets"). This is an easier process, but it achieved only 24% efficiency - similar to a regular fluorescent OLEDs. But they are working on ways to improve this. The important point is that they demonstrated that increasing the intermolecular bonding strength could efficiently suppress the vibrational loss of the phosphorescent light.
An interesting side effect is that if water is applied to the emitter, it dissolves the connections between the emitter and the polymers - and so it reverts back to fluorescent light. This can perhaps be used to develop a water sensor (which will change color or brightness in the presence of water).
In past years, several research groups and companies are trying to develop more efficient fluorescent emitters, trying to achieve PHOLED-like efficiency without the heavy metals. Professor Chihaya Adachi at Kyushu University is developing TADF (Thermally Activated Delayed Fluorescence) emitters that seem promising. TADF based OLEDs are also being commercialized by Cynora (based on copper). Reseachers from the Universities of Bonn, Regensburg, Utah and the MIT also developed a new method to enable phosphorescence OLEDs without any heavy atoms at room temperature.