Single-component light-emitting electrochemical cell fabricated from cationic polyfluorene: Effect of film morphology on device performance

Planar light-emitting electrochemical cells (LECs) were prepared by drop casting the conjugated polyelectrolyte poly(9,9-bis[ 6 ′ -(N,N,N,-trimethylammonium)hexyl]fluorene-co-alt-1,4-phenylene) bromide ( PFN + Br − ) onto substrates patterned with micrometer-sized interelectrode (Au) gaps. When operated at room temperature (RT), such LECs exhibit a gap-size-dependent turn-on voltage for light emission, but when operated at 140 ° C , the turn-on voltage is essentially gap-size independent and close to the band gap of PFN + Br − . This temperature dependence is consistent with operation below and above the melting temperature ( T m = 130 ° C ) of PFN + Br − and is consequently a signature of anion mobility. The RT morphology of PFN + Br − is dependent on the film preparation process: slow evaporation of the solvent via drop casting produces a partially crystalline film, while a fast evaporation via spin casting produces a glassy film. The glassy spin-cast film can, however, be transformed into a partially crystalline film by slow cooling ( 0.5 ° C ∕ min ) from an elevated temperature ( 180 ° C ) to RT. The photoluminescence (PL) from a partially crystalline film is slightly blueshifted and its vibronic structure better resolved than the PL from a glassy spin-cast film.