Red, green, and blue light-emitting polyfluorenes containing a dibenzothiophene-S,S-dioxide unit and efficient high-color-rendering-index white-light-emitting diodes made therefrom

Abstract

A series of blue (B), green (G) and red (R) light-emitting, 9,9-bis(4-(2-ethyl-hexyloxy)phenyl)fluorene (PPF) based polymers containing a dibenzothiophene-S,S-dioxide (SO) unit (PPF-SO polymer), with an additional benzothiadiazole (BT) unit (PPF-SO-BT polymer) or a 4,7-di(4-hexylthien-2-yl)- benzothiadiazole (DHTBT) unit (PPF-SO-DHTBT polymer) are synthesized. These polymers exhibit high fluorescence yields and good thermal stability. Light-emitting diodes (LEDs) using PPF-SO25, PPF-SO15-BT1, and PPF-SO15-DHTBT1 as emission polymers have maximum efficiencies LEmax = 7.0, 17.6 and 6.1 cd A-1 with CIE coordinates (0.15, 0.17), (0.37, 0.56) and (0.62, 0.36), respectively. 1D distributed feedback lasers using PPF-SO30 as the gain medium are demonstrated, with a wavelength tuning range 467 to 487 nm and low pump energy thresholds (≥18 nJ). Blending different ratios of B (PPF-SO), G (PPF-SO-BT) and R (PPF-SO-DHTBT) polymers allows highly efficient white polymer light-emitting diodes (WPLEDs) to be realized. The optimized devices have an attractive color temperature close to 4700 K and an excellent color rendering index (CRI) ≥90. They are relatively stable, with the emission color remaining almost unchanged when the current densities increase from 20 to 260 mA cm-2. The use of these polymers enables WPLEDs with a superior trade-off between device efficiency, CRI, and color stability. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.