Novel Organic Molecules for High-Efficiency Blue Organic ElectroLuminescence
Investigating Organization
Pacific Northwest National Laboratory
Principal Investigator(s)
Paul Burrows
Subcontractor
None
Funding Source
Building Technologies Program/NETL
Award
DOE Share: $2,400,000
Contract Period
10/01/04 - 10/31/07
This project explores using state-of-the-art phosphorescent organic light emitters to dramatically increase the power efficiency of blue organic light emitting devices by incorporating them in novel, electron-transporting host layers. Blue is thought by many to be the color that limits the efficacy of white OLED devices, as well as full-color organic light emitting displays. Typically, organic phosphors are doped into a conductive host matrix and emission results from energy transfer from the host to the triplet state of the phosphor. Development of efficient blue OLEDs based on this technology has been particularly challenging because the host material must exhibit triplet level emission ≤450 nm without sacrificing charge transporting properties. Current host materials do not meet these requirements because there is a tradeoff between increasing the bandgap of the material and decreasing the p-aromatic system, which adversely affects charge transport properties. Deeper blue phosphors have only been demonstrated in insulating, wide bandgap host materials with charge transport occurring via hopping between adjacent dopant molecules. This leads to high voltage and, therefore, less efficient devices.
An alternative route for achieving blue shifted emission energies is to replace the nitrogen heteroatoms with phosphorus. For example, aromatic diphosphine oxides are stable compounds that exhibit electroluminescence in the ultraviolet spectral region (335 nm for one example already tested) while extended electronic states in the phosphorus atom give rise to good electron transport at low voltages. Thus, it is possible to widen the bandgap without eliminating the aromatic backbone of the molecule, which makes these materials excellent hosts for high-efficiency blue phosphors, as well as longer wavelength OLEDs.