Space charge and polarization effects upon doping organic light-emitting diodes with pyran-containing donor-acceptor molecules

Doping of organic light-emitting devices using highly luminescent molecules is a well-established strategy to improve brightness, efficiency and lifetime or to adjust emission color and color purity. Laser dyes from the pyran-containing donor-acceptor family are particularly interesting for emission in the yellow and red spectral domain. In solution, these polar dyes show marked solvatochromic shifts in the absorption and emission. The fluorescence quantum efficiency also depends strongly on solvent polarity, which suggests the occurrence of twisted excited states. In thin solid films such as the ones used in the fabrication of organic light-emitting diodes, additional effects have to be considered. When pyran derivatives are used as doping molecules in a Tris(8-hydroxyquinolinato)aluminium (Alq(3)) host matrix, the emission color is highly dependent on doping concentration. This is attributed to polarization effects induced by the doping molecules themselves. For some pyran derivatives, such as 4-(dicyanomethylene)-2-methyl-6-{2[(4-diphenylamino) phenyl]ethenyl}-4H-pyran (DCM-TPA), the electroluminescence spectrum is not pure and shows a green Alq(3) shoulder, even at high concentrations. We have shown that these effects depend on the position of the frontier orbital energy levels with respect to those of the host. Charge carrier trapping by the doping molecules influences the charge density distribution. At high current densities. this can lead to a shift of the recombination zone out of the doped region. To impede charge recombination processes taking place in the undoped host matrix, charge blocking layers efficiently confine the recombination zone and give rise to increased efficiency.