Abstract
White polymer phosphorescent light-emitting diodes (WPLEDs) have been fabricated employing poly(ethylene glycol) dimethyl ether blended in the single active layer to enhance the emission efficiency. The devices have a simple sandwich architecture of ITO/poly((3,4-ethylenedioxythiophene): poly(styrenesulfonate)/emissive layer/CsF/Al. The emissive layer uses a blend of poly(9-vinylcarbazole), 1,3-bis[(4-tert- butylphenyl)-1,3,4-oxidiazolyl]phenylene, two or three phosphorescent dopants with complementary colors. The addition of poly(ethylene glycol) dimethyl ether enhances electron injection, transport, and the balance of densities of electrons and holes. The measured current efficiency in the front viewing direction is 17.5 cd/A at 1800 cd/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for the two complementary WPLEDs, and 35.7 cd/A at 3000 cd/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for the three complementary color WPLEDs. The current efficiencies remain high even at brightness levels up to 30,000 cd/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The high current efficiency is ascribed to the improved electron injection ability from the metal cathode, the enhanced charge carrier transport ability and the enhanced red emitting intensity by blending with PEG-DME. Also the low roll-off of the current efficiency was due to the lower triplet-polaron annihilation and the electric field-induced triplet exciton quenching by increased charge carrier transport in unipolar device and broadened recombination zone. The improved charge carrier injection at the interface and the enhanced charge carrier transport were resulted from specific interfacial interactions between PEG-DME and aluminum and higher electric field by blending with PEG-DME.
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