Optimization of the Electrochemically Generated Luminescence of Polyfluorene Films

Electrochemically generated chemiluminescence (ECL) from solid-state poly­(9,9′-octylfluorene) films was obtained by the successive electrochemical injection of polarons with opposite sign (annihilation ECL) in double potential steps. The processes involved in the recombination of polarons, the quenching of the electrochemically generated excitons, and the emission stability have been also analyzed. A systematic study at various electrochemical potentials has been carried out to determine optimal conditions to obtain a maximum ECL intensity, particularly with potentials of 0.4 V beyond the doping onsets (for both p- and n-doping). Results show that while the ECL emission is slightly affected by the potential of the emission, it is strongly dependent on the charging potential. Under large charging potential, the electrochemically generated excitons formed in the emission step get quenched by an excess of polarons and bipolarons that are detrimental to ECL. The maximum emission is obtained when the applied potentials are 0.4 V over the doping onset of the polymer conjugated chain. After partial depletion of the excess of polarons, the emission coming from the recombination of balanced polarons of opposite sign is observed. The stability of ECL emission in the optimal range was determined in continuous cathodic–anodic pulses. A rapid loss of the intensity in the initial stages is observed due to the dissolution of low molecular weight polymer fragments that become soluble upon electrochemical doping. The emission spectrum remains stable during the complete lifetime of the polymer. The spectral shape of PFO films indicates that the emission occurs mainly from the β-phase domains in which the electrochemical charge is preferentially injected.