Exploiting a Dual-Fluorescence Process in Fluorene-Dibenzothiophene-S,S-dioxideCo-Polymers to Give Efficient Single Polymer LEDs with Broadened Emission

A description of the synthesis of random (9,9-dioctylfluorene-2,7-diyl)–(dibenzothiophene-S,S-dioxide-3,7-diyl) co-polymers (p(F-S)x) by palladium-catalyzed Suzuki cross-coupling polymerization where the feed ratio of the latter is varied from 2 to 30 mol % (i.e., x = 2–30) is given. Polymer light emitting devices are fabricated with the configuration indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid)/p(F–S)x/Ba/Al. The device external quantum efficiency increased as the ratio of the S co-monomer was increased, up to a maximum of 1.3% at 100 mA cm−2 for p(F-S)30 and a brightness of 3 770 cd m−2 (at 10 V). The S units impart improved electron injection, more balanced mobilities, and markedly improved device performance compared to poly(9,9-dioctylfluorene) under similar conditions. These co-polymers display broad emission, observed as greenish-white light, which arises from dual fluorescence, viz. both local excited states and charge transfer states. Utilizing dual emission can reduce problems associated with Forster energy transfer from high-energy to-low energy excited states.