A Diels-Alder crosslinkable host polymer for improved PLED performance: The impact on solution processed doped device and multilayer device performance

We report on the synthesis of a polyfluorene derivative, PFO(X), with furan pendant groups capable of Diels-Alder crosslinking with a maleimide containing small molecule passive crosslinker (PC) and a maleimide containing red emitting donor-acceptor-donor dopant molecule, bE-BTD(X). It was initially intended that a blend of these three components would afford a system where the dopant concentration could be increased to the point where complete energy transfer from the host polymer to the emissive dopant would be achieved. Because such systems often suffer from quenching and shifts in emission maxima indicative of emitter aggregation, it was hypothesized that crosslinking the emissive dopant with the host polymer would lead to de-aggregation of the dopant emitter. In thin films of PFO(X) and bE-BTD(X), a 16 nm bathochromic shift is observed in the emission maximum when the dopant concentration is increased from 1% to 8%, suggesting that the dopant is aggregating. In similar films where PC is included and the film is heated to affect crosslinking, a comparable 16 nm shift in the emission maximum is observed indicating that aggregation is still occurring and not affected by the heating step. Similar decreases in luminance are observed independent of whether the heating step is included. Not unexpectedly, however, crosslinking does afford an insoluble network that allows for the subsequent solution deposition of additional layers. When an electron transport layer (ETL) is used in PFO(X)/PC devices, increases of 190% and 490% are observed in luminance and luminous efficiency, respectively, relative to devices without an ETL indicating that this Diels-Alder crosslinkable system is amenable to multilayer deposition by solution methods. When bE-BTD(X) is included as the dopant emitter, similar increases in luminance and luminous efficiency are observed with the ETL included compared to devices where this layer is omitted.