Molecular-scale interface engineering for polymer light-emitting diodes

Abstract

Achieving balanced electron-hole injection and perfect recombination of the charge carriers is central to the design of efficient polymer light-emitting diodes (LEDs). A number of approaches have focused on modification of the injection contacts, for example by incorporating an additional conducting-polymer layer at the indium-tin oxide (ITO) anode. Recently, the layer-by-layer polyelectrolyte deposition route has been developed for the fabrication of ultrathin polymer layers. Using this route, we previously incorporated ultrathin (<100 A) charge-injection interfacial layers in polymer LEDs. Here we show how molecular-scale engineering of these interlayers to form stepped and graded electronic profiles can lead to remarkably efficient single-layer polymer LEDs. These devices exhibit nearly balanced injection, near-perfect recombination, and greatly reduced pre-turn-on leakage currents. A green-emitting LED comprising a poly(p-phenylene vinylene) derivative sandwiched between a calcium cathode and the modified ITO anode yields an external forward efficiency of 6.0 per cent (estimated internal efficiency, 15-20 per cent) at a luminance of 1,600 candelas per m2 at 5 V.

Spin-dependent polaron recombination in conjugated polymers

“…Nonetheless, a large number of observations of much higher quantum efficiencies have been reported in OLEDs made from conjugated polymers, including values as high as 83%. [1][2][3][4][5][6] A considerable amount of theoretical work focused on understanding the mechanisms underlying exciton formation in conjugated polymers has also been reported. [7][8][9][10][11] So far, however, a commonly agreed theoretical picture is not available (for a review, see Ref.…”

Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes

“…External quantum efficiencies (EQEs) of 30% have been reported recently [1][2][3][4][5][6] ; this is considered to be the light-outcoupling efficiency limit of phosphorescent OLEDs under isotropically oriented transition dipole moments 4 . It has long been recognized that orienting the transition dipole moment of an emitter along the horizontal direction (parallel to the substrate) can enhance the outcoupling efficiency beyond that achieved under isotropic orientation, as demonstrated in polymer-based and vacuumevaporated fluorescent molecule-based OLEDs [7][8][9][10][11][12][13] . Nonetheless, the orientation of the transition dipole moments of iridium complexes used as phosphorescent emitters in efficient OLEDs is typically considered to be isotropic because they are near-globular and small enough to have configurational diversity in their orientational states.…”

Solution-processed multilayer small-molecule light-emitting devices with high-efficiency white-light emission

“…To achieve the multilayer structures by solution processing, research efforts have focused on p-conjugated polymers that afford a robust hydrophobic layer, on which a hydrophilic layer can be deposited from orthogonal solvents, such as water or water/alcohol mixture [6][7][8][9][10] . In situ cross-linking reactions have also been explored to afford covalently bound structures that are highly resistant to processing solvents [11][12][13][14][15][16][17][18][19] .…”