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-ethylhexyloxy)phenyl)fl uorene (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 fl uorescence yields and good thermal stability. Light-emitting diodes (LEDs) using PPF-SO25, PPF-SO15-BT1, and PPF-SO15-DHTBT1 as emission polymers have maximum effi ciencies LE max = 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 effi cient 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 effi ciency, CRI, and color stability.

Organic emitters for solid state lighting

“…6) [24]. In this case white light stability has been attributed to the fact that the three copolymers have very similar chemical structures and do not show tendency to crystallization and phase separation.…”

Altering the emission properties of conjugated polymers

“…Additional opportunities are provided by other CP classes. A good example here is provided by the blue, green and red light‐emitting CPs based on 9,9‐bis(4‐(2‐ethylhexyloxy)phenyl)fluorene containing a dibenzothiophene‐ S,S ‐dioxide unit with additional benzothiadiazole or 4,7‐di(4‐hexylthien‐2‐yl)‐benzothiadiazole moieties . Besides good thermal stability, these polymers show high FL yields, rendering them promising candidates for OLED fabrication.…”

Highly efficient non-doped single-layer blue organic light-emitting diodes based on light-emitting conjugated polymers containing trifluoren-2-ylamine and dibenzothiophene-S,S-dioxide

“…Previously, time-of-flight (TOF) mobility measurements revealed that thermally stable SO-based derivatives have high electron mobility of 10 À4 -10 À3 cm 2 V À1 S À1 [21,22]. Due to the excellent electron-inject/transport property of SO units, recently, Yu et al reported highly efficient double-layer blue OLEDs with CE max of 7 cd/A through utilizing polyfluorene containing SO units and poly(N-vinylcarbazole) (PVK) as an EML and HTL, respectively [20].…”

“…On the other hand, it is also difficult to inject electrons from the cathode in blue OLEDs due to the large energy band gaps of the fluorene-based emitters. To enhance the electron injection and transport, with an introduction of electron-withdrawing components, for example, dibenzothiophene-S,S-dioxide (SO) units, into the polymers or oligomers is proven to be effective [16][17][18][19][20]. Previously, time-of-flight (TOF) mobility measurements revealed that thermally stable SO-based derivatives have high electron mobility of 10 À4 -10 À3 cm 2 V À1 S À1 [21,22].…”